The role of macroalgae in structuring a New England fouling community and the implications for floating dock management to ameliorate invasive species

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While much of this work has focused on sessile invertebrates, on hard substrates in the marine environment, invertebrates and macroalgae, organisms from two different kingdoms, can compete for the same limiting resource, space. In rocky subtidal habitats, research suggests that algae may exclude invertebrates or impact invertebrate post-settlement mortality and, to investigate this in fouling communities, I conducted manipulative experiments on the sides of floating docks. In three out for four experiments, macroalgae did not exclude invertebrates but did alter invertebrate community composition, communities with algae having more native species, mainly molluscs, whereas communities without algae were dominated by invasive species, specifically colonial ascidians. In one experiment, macroalgae also appeared to facilitate invertebrate settlement in the early stages of community assembly, mediated by both algae structure and natural chemical cues. If macroalgae presence in fouling communities can shift the balance of invertebrate assemblages towards those containing native species, this suggests a possible role for macroalgae in resisting invertebrate invasions. Both floating docks and marinas could thus be managed to enhance autotroph persistence and more studies in invasion biology could investigate facilitation, indirect effects, and interactions between organisms from different taxonomic groups. Ascidians competition floating docks fouling communities invertebrates macroalgae invasive species Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Fouling communities are assemblages of sessile plants and animals that form on artificial structures in the marine environment and are considered, ‘from a biological point of view, as an accident, and of very recent origin (WHOI 1952).’ Nonetheless, these communities, when they form on the side of floating docks, are easily accessible to researchers, and the small size, short generation time, and sessile nature of organisms make them readily amenable to experimental manipulations and monitoring through time. Fouling communities have thus become popular model systems that have contributed much to our understanding of basic ecological principles in the marine realm (Ferrario et al. 2020 ; Jewett et al. 2022 ), from early studies documenting community establishment and development (Osman 1977 ; Sutherland 1974 , 1981 ; Sutherland and Karlson 1977 ) to more recent ones focusing on invasion biology (Lord 2017 ; Sorte et al. 2010 ; Stachowicz et al. 2002 ; Stachowicz and Byrnes, 2006 ). Investigating the diversity-invasibility relationship has been particularly popular (Beshai et al. 2023 ; Marraffini and Geller 2015 ; Stachowicz et al. 2002 ; Stachowicz and Byrnes 2006 ), as these systems are space limited, heavily influenced by competition, and often composed of a high number of invasive species (Lord 2017 ). All of these studies, however, have focused on sessile invertebrates (Gartner et al. 2016 ; Glasby et al. 2007 ; Karlson and Osman 2012; Ruiz et al. 2009 ) but these are not the only taxa in these systems, and macroalgae can also be found on the sunlit sides of floating docks. How sessile macroalgae and invertebrates, organisms from two different kingdoms, compete for space in a resource limited environment is a major distinction between marine and terrestrial realms and it is surprising that this has not been previously investigated. Here I investigate potential mechanisms for this and consider implications for invasion biology. As manmade structures continue to be added to the marine environment facilitating species invasions (Bulleri and Chapman 2010 ; Glasby and Connell 1999 ; Glasby et al. 2007 ), any mechanisms discerned can help us inform management of floating docks in marinas and help mitigate the spread of non-native species. Species invasions have naturally occurred throughout time (Elton 2020 ; Mooney and Cleland 2001 ; Vermeij 1991 ; Webb 1991 ), but the rate at which man is transporting species around the world today is unprecedented (Drake et al. 1989 ; Ricciardi 2007 ). Consequently, there is a lag in our understanding of, and response to, species invasions, with implications for biodiversity, ecosystem services, and biosecurity (Ricciardi et al. 2021 ). Indeed, in the marine environment, species invasions are contributing to a third and current stage of coastal ecosystem collapse (Jackson 2001 ). Invasive species are transported in ballast water (Bailey 2015 ; Carlton and Geller 1993 ) and on boat hulls (Bax et al. 2003 ; Lambert and Lambert 1998 ), and can easily expand their ranges when marine vessels dock in new ports. Manmade floating docks provide an optimum structure to which marine invaders can ‘jump ship’ (Glasby and Connell 1999 ; Carlton and Geller 1993 ; Ruiz et al. 2000 ) and these new habitats are well known to support fouling communities that are often primarily composed of non-native invertebrate species (Bax et al. 2002 ; Lambert and Lambert 2003 ; Ruiz et al. 2009 ), specifically prolific invaders such as ascidians (Aldred and Clare 2014 ; Dijkstra et al. 2007 ; Lambert and Lambert 2003 ; Lindeyer and Gittenberger 2011 ). However, on the sunlit sides of floating docks in the Gulf of Maine fouling communities, algae also occur, but their role in structuring fouling communities has not been investigated. On subtidal rocky reefs in the Gulf of Maine, macroalgae and invertebrates also coexist but tend to be partitioned based on substrate angle with macroalgae typically occupy horizontal, sunlit surfaces while animals dominate vertical walls and underhangs (Miller and Etter 2008 , 2011 ; Sebens 1985 ; Witman and Dayton 2001 ). Invertebrates are, however, able to persist on horizontal surfaces if algae are excluded by shading, suggesting algae presence may exclude invertebrates or impact invertebrate post-settlement mortality (Miller and Etter 2008 ). Indeed, algae can inhibit invertebrates through competition for space, physical disturbance, harboring of small predators, and allelopathy. Algae can pre-empt space (Connell et al. 1997 ) and physically or chemically interfere with invertebrates. Physically, algae fronds can scour the substratum reducing invertebrate recruitment (Connell 2003 ; Duggins et al. 1990 ), and they can abrade (Box and Mumby 2007 ; River and Edmunds 2001 ; Titlyanov et al. 2007 ) or smother (Hughes 1989 ) adults, impacting invertebrate assemblages beneath the algal canopy (Kennelly 1989 ). Algae can also overgrow (Coyer et al. 1993 ; Davis and White 1994 ; Jompa and McCook 2002 ; Lewis 1986 ; Young and Chia 1984 ) and dislodge invertebrates (Witman 1987 ). Algal fronds can alter water flow, sedimentation, and light intensity, which can, in turn, negatively impact invertebrate recruitment, feeding, and growth (Duggins et al. 1990 ). Algae also provide habitat for micropredators (Osman and Whitlatch 2004 ; Stachowicz and Whitlatch 2005 ) and mesograzers (Duffy and Hay 2000 ) that prey on invertebrate recruits. Chemically, algae can release allelopathic compounds that directly result in the mortality of invertebrate recruits and adults (Rasher and Hay 2010 ; Rasher et al. 2011 ) and substances that alter the microbial activity of corals, inhibiting coral growth and survival (Haas et al. 2011 ; Smith et al. 2006 ; Thurber et al. 2012 ; Vermeij et al. 2009 ). Algae may also act as vectors for disease (Nugues et al. 2004 ; Sweet et al. 2013 ). If algae can inhibit invertebrates on rocky reefs, can they also inhibit them on the sunlit sides of floating docks and, if so, does this have implications for species invasions? I used manipulative experiments, hung off the sides of floating docks, to explore if algae influence invertebrate settlement, recruitment, or adult survivorship. Potential mechanisms of exclusion were also investigated via plastic algal mimics and modified (trimmed) natural algae which allowed chemical characteristics and algal structure to be manipulated respectively. Specifically, I asked 1) Do macroalgae exclude sessile invertebrates, and 2) How might macroalgae exclude sessile invertebrates? I seek to extend the fouling community model from its focus on sessile invertebrates to include autotrophs, with the aim of increasing our understanding of invasions in the marine realm to help inform management of floating dock systems, information that is crucial as both marine urban sprawl (Firth et al. 2016 ; Todd et al. 2019 ) and species invasions (Drake et al. 1989 ; Ricciardi et al. 2021 ) continue at unprecedented rates. Methods Study site I conducted four experiments in a New England fouling community to investigate if macroalgae exclude sessile invertebrates on the shallow sunlit sides of floating docks, as well as potential mechanisms for this. The experiments were carried out at a depth of 1 m on polycarbonate settlement plates suspended vertically off floating docks at Dorchester Yacht Club, Boston, Massachusetts, (42.305556°N, 71.046111°W). The fouling community was composed of sessile algae and invertebrates typical of this region, including red and green algae, sponges, bryozoans, polychaetes, molluscs, barnacles, and colonial and solitary ascidians (Table S1 and Wagstaff ( 2024 )) for a full species list). All species of algae in this system were native, while the invertebrate assemblage was a mix of native and invasive species (Table S1 ). Experiments Assembly. To test if macroalgae influence the formation of invertebrate assemblages during the establishment of natural communities, fouling communities were allowed to form on settlement plates in the presence or absence of algae. Polycarbonate settlement plates (15x15 cm) were attached to polycarbonate boards and submerged 1 m below the water surface. Four replicates of each treatment were deployed, and arranged in a Latin square. Treatments were: 1) natural community, where assemblages were allowed to form without any manipulation, and 2) algae removal, where algae were carefully removed with or forceps weekly in summer, bimonthly in fall and spring, and once a month in winter. The removal schedule was based on seasonal algae growth, which is fastest in summer and slowest in winter. I expected that algae would limit invertebrates in the natural community treatment and there would be a greater diversity and abundance of invertebrate species found in the removal treatment. This experiment was conducted over 18 months, from April 2012 to October 2013. At the end of the experiment settlement plates were collected, all algae and arborescent invertebrates trimmed to enable primary space occupiers to be identified, and communities photographed using an Olympus Stylus Tough 8010 camera. Digital photographs were later enlarged on a screen and percent cover and species richness of space occupying sessile invertebrates quantified using 200 random points overlaid on the central 12x12 cm area of a settlement plate to avoid edge effects. Displacement . To test if macroalgae can displace established sessile invertebrates in fouling communities, the percent cover of macroalgae and invertebrates were manipulated using small tiles that were then assembled to form larger plates, depending upon the experimental treatment. Firstly, small 3x3 cm polycarbonate tiles were pre-seeded with either algae (tiles were placed on the sides of floating docks when algal recruitment was high) or invertebrates (tiles were placed underneath floating docks where low light levels prevented algal growth). Once seeded, 3x3 cm tiles, 25 in total, were then pieced together to create the larger 15x15 cm starting communities (Fig. 1 ). For example, to create a starting community of approximately 50% algae and approximately 50% invertebrates, 12 (or 13) 3x3 cm tiles with pre-seeded with algae and 13 (or 12) 3x3 cm tiles pre-seeded with invertebrates were randomly assembled to create a larger 15x15 cm plate (Fig. 1 , Table S2). This was achieved by gluing threaded polycarbonate rods into blind holes on the underside of the 3x3 cm tiles (i.e., the rod did not penetrate the upper settling surface) and then passing these rods through holes in a 17x17 cm, 0.5 cm thick, polycarbonate base plate and securing with polycarbonate nuts on the underside (Fig. 1 ). The four starting communities, or treatments, in this experiment were: 1) algae (50% algae tiles and 50% blank tiles) to test if invertebrate assemblages could form in the presence of established algae; 2) inverts (50% invertebrate tiles and 50% blank tiles) to test if incoming algae could still form and limit invertebrates; 3) inverts and algae (50% invertebrate tiles and 50% algae) to test if adult algae could displace adult invertebrates; and 4) inverts and algae removal (50% invertebrate tiles and 50% blank tiles with continual algal removal) as a control to determine if invertebrate mortality was influenced by colonization of macroalgae in treatment 2 (Table S2). I expected that established algae would limit invertebrate settlers and adults in treatments 1 and 3 respectively, depressing invertebrate abundance and diversity, and that incoming algae (treatment 2) would also have the same effect. Lastly, I expected that invertebrates would persist when algae were removed in treatment 4. Algae were removed from the control treatment in the same way and frequency as previously described for the Assembly experiment. The duration of the experiment, number and arrangement of replicates, data collection, and response variables were also the same. Recruits. The presence or absence of algae was manipulated to test if invertebrate recruits, defined as newly settled individuals that have colonized plates, were affected by established algae. In this experiment, modifications were made to algae to identify potential mechanisms of exclusion. To eliminate whiplash, algal fronds were cut 1 cm above their bases to remove the distal thalli but keep other algal properties such as allelochemical composition and some micropredator habitat intact. Conversely to maintain algae structure and thus potential whiplash but to eliminate allelochemicals, a plastic structural mimic was used. As in the previous experiment, 3x3 cm tiles containing either normal algae, cut algae, plastic algal mimics, or no algae, were combined with 3x3 cm invertebrate recruit tiles to create starting communities. Twenty-five 3x3 cm tiles were assembled into larger 15x15 cm fouling communities depending on the experimental treatment. Starting communities, or treatments, were 1) algae normal (50% intact algae tiles and 50% invertebrate recruit tiles) to test if algae inhibit invertebrate recruits; 2) algae cut (50% modified algae tiles with distal thalli removed and 50% invertebrate recruit tiles) to test if invertebrate recruits can persist if some of the physical effects of algae, such as whiplash, are removed; 3) algae mimic (50% plastic algae mimic tiles and 50% invertebrate recruit tiles) to assess if the presence of the distal thalli (relative to treatment 2 where the distal thalli is removed) or the absence of allelochemicals might affect invertebrate recruits; and 4) no algae (50% blank tiles with algae absent and continually removed and 50% invertebrate recruit tiles) (control) (Table S2). I expected algae to inhibit recruits and that percent cover and diversity of invertebrates would be lowest in treatment 1 where normal algae is present, intermediate in treatments 2 and 3 where some of the characteristics of algae are eliminated, and highest in treatment 4 where no algae are present. The cutting and removal of algae in treatments 2 and 4 was performed twice a week. Distal thalli were cut 1 cm above the base using scissors and algae removal was as described in previous experiments. The algae present in the system at this time, Ulva intestinalis , was mimicked using green ‘Easter Grass’ which was attached to tiles using Loctite Marine Epoxy. The experiment ran for five weeks, from June 26th th until July 31 st , 2012. This experiment was shorter than the previous two, as the aim was to tease out the influence of algae on early ontogenetic stages of invertebrates. The number and arrangement of replicates, data collection, and response variables were as above in Assembly methods. Settlers. To test if and how the presence of algae influenced the settlement of invertebrate larvae, where settlers are defined as incoming larvae that settle on plates, I conducted an experiment similar to the Recruits experiment described above, but the algae tiles (normal, cut, or mimicked) were arrayed with blank tiles to allow invertebrate settlement (Fig. 2 ). Starting communities, or treatments, were 1) algae normal (50% intact algae tiles and 50% blank tiles); 2) algae cut (50% modified algae tiles (distal thalli removed with scissors) and 50% blank tiles); 3) algae mimic (50% plastic algae mimic tiles and 50% blank tiles); and 4) no algae (all blank tiles, 100%, with algae absent and continually removed) (control). As in the Recruits experiment, I expected algae to inhibit settlers and that percent cover and diversity of invertebrates would be lowest in treatment 1 where normal algae is present, intermediate in treatments 2 and 3 where some of the characteristics of algae are eliminated, and highest in treatment 4 where no algae are present. The distal thalli were cut in the algae cut treatment, and algae removed from the no algae treatment as in the Recruits experiment. Green polyethylene bags were cut into the shape of Ulva linza fronds (the algae present in the system at the time of the experiment) and attached to tiles using Loctite Marine Epoxy (Fig. 2 ). The experiment ran for 5 weeks, from August 8th to September 11th, 2012. The number and arrangement of replicates, data collection, and response variables were as above in the Assembly experiment. Statistical analyses Experiments were analyzed as single factor MANOVAs. Response variables were percent cover of primary space occupying sessile invertebrates and diversity of space occupying sessile invertebrates. Percent cover data was transformed to logits, ln[p/(1-p)], to homogenize variances where p is the proportion of sessile inverts + 0.025 (0.025 was added to avoid proportions of 0 or 1). Diversity was calculated using Shannon’s diversity index. MANOVAs were followed up with univariate ANOVAs to test for the contribution of each dependent variable using a Bonferroni-corrected α of 0.025. Post-hoc pairwise comparison tests, i.e., Tukey’s HSD, were carried out when ANOVA results revealed significant differences among treatments. Normality was tested using the Shapiro-Wilk test. Homogeneity of variance was tested using Levene’s median test. Compositional differences among treatments of primary space occupying sessile invertebrates were compared using PERMANOVA and non-metric multi-dimensional scaling (NMDS), both based on Bray-Curtis distances. Multivariate dispersion was tested using PERMDISP, which is a multivariate analog of Levene's test for homogeneity of variances. The contribution of individual species to the differences between groups was assessed using similarity percentage, or SIMPER. All analyses were carried out in R version 3.0.2 (R Core Team 2013) using the Vegan package (Oksanen et al. 2012). Results Assembly. In the Assembly experiment, which tested if macroalgae influenced the formation of invertebrate assemblages when natural communities are allowed to establish, the presence of algae did not negatively impact invertebrates, contrary to what I expected. Neither percent cover nor diversity of space occupying sessile invertebrates differed between treatments (Figs. 3 a and e, Tables 1 and 2 ) but treatments did influence community composition (Pseudo-F = 2.15, p = 0.03; Fig. 4 a, Table 3 ). In the natural community treatment, where both algae and invertebrate assemblages were allowed to form, there were more native Mytilus edulis, Crepidula plana , and algae/mud/tube complex, as well as the invasive Ostrea edulis (Table S3). In the algae removal treatment, there were more invasive colonial ascidians including Botryllus schlosseri , Botrylloides violaceus , and Diplosoma listerianum. Thus, the algae removal treatment was dominated by invasive species whereas the natural community supported more native species. Across treatments native barnacles and invasive solitary ascidians— Ascidiella aspersa , Ciona intestinalis , and Styela clava —did not vary (Table S3). These observations were supported by SIMPER. As species diversity did not differ between treatments and neither did species identities (Table S3), differing species abundances account for differences in community composition. Table 1 Results of MANOVAs for Assembly , Displacement , Recruits , and Settlers experiments Source df Wilks Approx F Pr(> F) Assembly Treatment Residuals 1 6 0.96 0.10 0.904 Displacement Treatment Residuals 3 12 0.65 0.89 0.522 Recruits Treatment Residuals 3 12 0.90 0.20 0.974 Settlers Treatment Residuals 3 12 0.02 69.89 < 0.001* * Significant p-value Table 2 Results of univariate ANOVAs for percent cover and diversity of primary space occupying sessile invertebrates for the Assembly , Displacement , Recruits , and S ettlers experiments Percent cover Diversity Source df SS F P df SS F P Assembly Treatment Residuals 1 6 0.96 0.10 0.904 1 6 0.005 0.43 0.06 0.809 Displacement Treatment Residuals 3 12 0.65 0.89 0.522 3 12 0.12 0.38 1.23 0.342 Recruits Treatment Residuals 3 12 0.90 0.20 0.974 3 12 0.004 0.11 0.14 0.934 Settlers Treatment Residuals 3 12 0.02 726.60 < 0.001* 3 12 0.82 0.50 6.49 0.007* * Significant p-value Table 3 Results of PERMANOVA, using the Bray-Curtis distance metric, for community composition of primary space occupying sessile invertebrates for the Assembly , Displacement , Recruits , and Settlers experiments Source df SS Pseudo-F P Assembly Treatment Residuals 1 6 0.22 0.54 2.147 0.03 Displacement Treatment Residuals 3 12 0.822 1.01 3.27 0.001 Recruits Treatment Residuals 3 12 0.18 1.40 0.52 0.802 Settlers Treatment Residuals 3 12 2.84 0.24 46.76 0.001* * Significant p-value Displacement. In the Displacement experiment, which tested if algae could displace established sessile invertebrates, percent cover and diversity did not differ between treatments (Figs. 3 b and f, Tables 1 and 2 ) but community composition did (Pseudo-F = 3.27, p = 0.001; Fig. 4 b, Table 3 ), similar to the findings from the Assembly experiment. Composition in the algae removal treatment differed from the other three treatments where algae was allowed to be present in the system (algae, inverts, inverts and algae), all of which overlapped in the NMDS plot (Fig. 4 b). As in the Assembly experiment, communities with algae contained more native M. edulis, C. plana , algae/mud/tube complex, and the invasive oyster O. edulis . Higher abundances of invasive colonial ascidians were found in the algae removal treatment (Table S3). These findings are consistent with the results from the previous experiment. Across treatments, barnacles and solitary ascidians did not vary (Table S3). Again, all observations were supported by SIMPER. The native sponge, Halichondria panacea , and the native encrusting bryozoan, Electra pilosa , were also found in treatments with algae. The abundances of these, however, were not high enough to influence statistical results (Table S3) but does support the other findings in this and the previous experiment that algae appear to support native species. Recruits. Modified algae ( U. intestinalis )—both plastic mimics and algae with the distal thalli removed—used to test if algae negatively impacted invertebrate recruits as well as potential mechanisms for this, did not influence percent cover (Fig. 3 c, Tables 1 and 2 ), diversity (Fig. 3 g, Tables 1 and 2 ), or community composition (Fig. 4 c, Table 3 ). Only invasive colonial ascidians B. schlosseri and B. violaceus occurred in these communities and their abundances did not vary across treatments (Table S3). Interestingly, the encrusting bryozoan, Electra pilosa , a native invertebrate species, was again only found in communities with natural algae (treatments 1 and 2), albeit in very small numbers. Settlers. The Settlers experiment tested if algae, U. linza , negatively influenced invertebrate settlement and potential mechanisms for this. Treatments did alter percent cover (F = 726.60, p < 0.001; Fig. 3 d, Tables 1 and 2 ) and diversity (F = 6.49, p = 0.007; Fig. 3 h, Tables 1 and 2 ), but results were opposite to those expected i.e., algae did not negatively impact invertebrates but instead appeared to facilitate them. Diversity of invertebrates was higher in all treatments with algae, whether normal, cut or mimicked, and decreased slightly in the no algae treatment (Fig. 3 h), suggesting a role for the structure of algae in facilitating invertebrate settlement and colonization. Results were slightly different for percent cover of invertebrates. Here, percent cover was greatest in the algae normal and algae cut (distal thalli removed) treatments, intermediate in the algae mimic treatment, and least in the no algae treatment where algae were removed (Fig. 3 d). This suggests that invertebrate larvae prefer to settle on some structure (Treatments 1, 2 and 3), but with a preference for natural algae (Treatments 1 and 2), regardless of whether the distal thalli have been removed or not. The differing results between experiments could be due to the various experimental time frames (18 months for Assembly and Displacement experiments versus 5 weeks), with this experiment focusing on early stages of invertebrate assemblage formation, i.e., algae facilitating the settlement of invertebrate larvae, and the Assembly and Displacement representing climax communities. Community composition also differed among treatments (Pseudo-F = 46.76, p = 0.001; Fig. 4 d, Table 3 ) and, as in the Assembly and Displacement experiments, the treatment without algae was different from all of the treatments with algae, regardless of whether algae were normal, cut or mimicked (Fig. 4 d). However, unlike in the Assembly and Displacement experiments, all communities were dominated by invasive colonial ascidians, with their differing abundances accounting for differences in community composition (Table S3), observations that were supported by SIMPER. This difference could again be due to the differing time frames of the experiments, with native species not having enough time to colonize in this experiment or perhaps their larvae were not present in the water column during the short time period when this experiment was conducted. One native species, C. plana , was found only in communities with natural algae (algae normal and algae cut) but did not occur in high enough abundances to influence results, but does suggest that algae support more native species. Discussion Here I showed that in a New England fouling community, the presence of algae affected community composition, specifically the balance between native and invasive species, and that, in the early stages of community assembly, some algae species ( U. linza ) also appeared to facilitate the settlement of invertebrates. These findings add to our incomplete and lagging understanding of invasion resistance in the marine realm and may have implications for maintaining populations of autotrophs, informing management of both floating dock systems and the environments in which they are found. Algae facilitated invertebrate settlement In the Settlers experiment, U. linza appeared to facilitate invertebrate colonization by providing both structure and natural cues. Algae structure may facilitate invertebrate settlement by creating small eddies that increase the deposition of recruits (Sebens 1983 ) and algae biofilms and chemical cues may further attract invertebrate larvae (Crisp 1974 ; Dobretsov 1999 ; Hadfield and Paul 2001 ; Pawlik 1992 ; Tebben et al. 2015 ). There was some preference for natural over mimicked algae and this may reflect an overall preference of invertebrate larvae to settle into natural algae, higher rates of recruitment once settled, or greater colonization success once recruited. Some coral species have also been shown to settle preferentially into communities with natural rather than mimicked algae (Diaz-Pulido et al. 2010 ; Nugues and Szmant 2006 ). It was not expected that algae would facilitate invertebrates, instead it was expected that they would inhibit them. These unexpected results might be due to the types of algae present in fouling communities as opposed to other systems. The dominant species in this study were ephemeral green algae compared to large brown leathery macrophytes (see Steneck and Dethier 1994 ) on rocky reefs, which are known to negatively impact invertebrates (Box and Mumby 2007 ; Connell 2003 ; River and Edmunds 2001 ; Witman 1987 ). Furthermore, temperate algae species also do not produce as many defensive chemicals as their tropical counterparts (Norris and Fenical 1982 ; Bolser and Hay 1996 ; Hay 1996 ) and studies examining the allelopathic effects of algae on invertebrates have focused on tropical reefs (Rasher and Hay 2010 ; Rasher et al. 2011 ). In the Gulf of Maine there is some evidence that algal exudates negatively impact polychaete larvae (Warkus et al. 2011), but it is unclear whether this negative effect of algae on invertebrates is restricted to this taxon or this ontogenetic stage. The results from the short-term Settlers experiment, that treatments with algae had a higher percent cover and diversity of invertebrates, contrasts the longer-term Assembly and Displacement experiments, where the presence or absence of algae did not influence invertebrate cover or diversity. This could be due to the differing time frames of the experiments, with the Settlers experiment teasing out what happens in the initial stages of invertebrate assemblage formation, i.e., that algae facilitate invertebrate colonization and, in general, this ecological process tends to be common in the early stages of community assembly (Clements 1916 ; Connell and Slatyer 1977 ; Dean 1981 ; Odum 1969 ). It should be cautioned, however, that only one species of algae, Ulva linza , was used in this experiment, and thus any inferences about algae facilitating invertebrate settlement are restricted to this species. Algae affected community composition of invertebrates In the Assembly and Displacement experiments, algae affected community composition, treatments with algae having more native invertebrate species than treatments without algae. Thus, algae might confer some invasion resistance by maintaining native invertebrate assemblages and subsequently limiting the abundance of invasive species. How might algae do this? One striking observation from the Assembly and Displacement experiments is that communities with algae were associated with greater abundances of molluscan fauna i.e., the native blue mussel, M. edulis , and the native slipper snail, C. plana . Settlement of blue mussels into algae is well known (Bayne 1964 ; 1976 ; Dobretsov and Wahl 2001 ) and can be induced by waterborne algal compounds (Dobretsov and Wahl 2001 ). Similarly, C. fornicata , a congener of C. plana has also been shown to metamorphose in response to chemical cues from coralline algae (Taris et al. 2010 ). As a general rule, the greater the diversity of algae in an assemblage, the greater the diversity of molluscs (Azevedo 2008), and this relationship appears to be mediated by algal functional form, (Chemello and Milazzo 2002 ; Duarte et al. 2015 ), and functional identity of algae does affect meiofauna diversity in intertidal algae (Dijkstra et al. 2017 ; Torres et al. 2015 ; Veiga et al. 2016 ). Regardless of the mechanism by which algae confer invasion resistance, understanding the importance of macroalgae as a microhabitat for small-sized species, which generally occupy the lowest trophic levels, can contribute to a better knowledge of the dynamics of these ecosystems and of the species that they harbor (Queiroz and Dias 2014 ). If the strong biotic interactions between native algae and native invertebrates hinder the infiltration of invasive species (here that algae attract native invertebrates thus limiting invasive invertebrates), this is consistent with the biotic resistance hypothesis of species invasions (Beaury et al. 2020 ; Maron and Vila 2001), where invasion success is negatively correlated with species richness (Levine and D’Antonio 1999; Shea and Chesson 2002 ). In algae communities, the functional diversity of algae has been shown to protect against invasion by other algae species (Britton-Simmons 2006 ) and, in invertebrate communities, increased species richness significantly decreased invasion success, because species-rich communities more completely and efficiently used available space, the limiting resource in fouling communities (Stachowicz et al. 1999 ). However, no studies have investigated how algae and invertebrates together might resist invasion. Indeed, studies investigating the biotic resistance hypothesis, tend to focus on interactions between species in the same trophic level or that of consumer-resource and not between organisms of different kingdoms. Furthermore, facilitation is only incorporated into invasion biology when discussing how native species can facilitate non-native species (Cavieres 2021 ) and not how native species can facilitate other native species and thus limit invasions via indirect effects. Thus, both facilitation (Bruno et al. 2003 ) and indirect effects (White et al. 2006 ) need to be better incorporated into this field. Disturbance is also well-known to facilitate invasive marine invertebrates (Altman and Whitlatch 2007 ), possibly due to their ability to colonize open space (Stachowicz et al. 2002 ; Tyrrell and Byers 2007 ; Kremer et al. 2010 ; Janiak et al. 2013), and this may also explain why invertebrates were more abundant in removal treatments, that they were more successfully able settle into the open space provided when algae was removed than native species were. Management of floating docks for macroalgae If algae do facilitate and maintain native species assemblages in floating dock systems, then ensuring the persistent of these taxa is important. Unfortunately, manmade structures introduce shade to the marine environment, reducing plant growth, productivity, and survival (Burdick and Short 1999 ; Dyson and Yocom 2015 ). In eelgrass beds, one management strategy has been to construct docks out of materials that will allow sufficient light penetration for photosynthesis, such as fiberglass grating in single-family docks in Florida (Landry et al. 2008 ) and glass prisms in ferry terminals in Washington State (Blanton et al. 2002 ). As macroalgae in rocky habitats also show a decrease in biomass and percent cover with shade from manmade structures (Pardal-Souza et al. 2017 ), macroalgae might also benefit from docks made of similar materials (Fig. 5 ). Other suggestions include orientating the sides of docks towards the sun (Burdick and Short, 1999 ), using rugose textured materials on the side of docks to allow increased surface area for algae to grow (Dyson and Yocom 2015 ; Logan et al. 2022 ), transplanting algae on to manmade structures (Firth et al. 2016 ; Marzinelli et al. 2009 ; Susini et al. 2007 ), and the addition of structures such as seawall stairs, vegetation baskets, and vertical gardens (Dyson and Yocom 2015 ) (Fig. 5 ). While it is becoming increasingly popular to design manmade structures with the natural environment in mind (Dafforn et al. 2015 ), the impacts of floating docks on macroalgae are still largely understudied (Lambert et al. 2023 ), and ecological designs for docks, piers, and overwater structures must address the effects of shading on vegetation (Dyson and Yocom 2015 ). Marine urban sprawl in general needs to be managed more (Dafforn et al. 2015 ; Firth et al. 2016 ), and invasive species along in coastal and estuarine environments should also be managed to the same extent as overexploitation, pollution, and climate change (Williams and Grosholz 2008 ). As fouling communities predominantly occur in harbors and marinas, these locations should also be managed to promote healthy algae populations. These systems tend to be characterized by increased suspended sediments (Iannuzzi et al. 1996 ) that reduce light penetration and thus photosynthesis by algae (Airoldi 2003), and are highly polluted with associated reductions in biodiversity (Ferrario et al. 2020 ; Kenworthy et al. 2018 ), including many species of brown and red algae (Fowles et al. 2018 ). Marinas also have reduced water flow, trapping sediments inside, as well as increased turbidity, temperature and pH (Rivero et al. 2013 ). These marked environmental changes have ecological consequences and should be a primary consideration during the planning process (Rivero et al. 2013 ). Conclusions Worldwide, macroalgae are under threat from sedimentation, loss of habitat, invasive species, pollution, hypoxia, and acidification (Brodie et al. 2014 ; Mineur et al. 2014 ; Schiel 2009 ; Steneck et al. 2002 ; Walker and Kendrick 1998 ). Any factor that reduces algal cover thus has the potential to shift floating dock invertebrate communities towards those dominated by invasive species, which in turn increases the risk of invasion to nearby native habitats (Glasby and Connell 1999 ; Lambert and Lambert 2003 ; Ruiz et al. 2009 ; Simkanin et al. 2012 ). Managing floating docks (Blanton et al. 2002 ; Landry et al. 2008 ; Dyson and Yocom 2015 ) and marinas (Rivero et al. 2013 ) to enhance macroalgae growth might be one way to reduce fouling community dominance by invasive species. As well as hosting a diversity of native fauna, algae highly productive and provide multiple ecosystem services and need to be managed accordingly (Duffy et al. 2019 ; Harley et al. 2012 ; Mineur et al. 2014 ; Smale et al. 2013 ; Strong et al. 2015 ). Declarations Funding Funding was provided by the Doctoral Dissertation Research Grant Program from University of Massachusetts Boston and the Lerner-Gray Fund for Marine Research from the American Museum of Natural History. Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions MW designed the study, conducted the experiments, analyzed the results, and wrote the manuscript. Acknowledgements Thank you to the two anonymous reviewers who helped shape and refine this manuscript as well as to R. Etter who assisted with many aspects of the work. Field assistance was provided by E. Franck, D. Katzmark, and S. 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Supplementary Files SupplementaryInformationBioInv.docx Cite Share Download PDF Status: Published Journal Publication published 20 Apr, 2026 Read the published version in Biological Invasions → Version 1 posted Reviewers agreed at journal 13 Oct, 2025 Reviewers invited by journal 13 Oct, 2025 Editor invited by journal 01 Oct, 2025 Editor assigned by journal 25 Sep, 2025 First submitted to journal 24 Sep, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7706424","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":528693275,"identity":"e2c96e92-64c3-4c93-8b87-0b1bbb228edb","order_by":0,"name":"Martine Wagstaff","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYHADxgcfPgApAyCWIKS0AUTyMDAbzpxBspbZPMRo0W0/+/wxT02tvT37YcZm2zY7e3MG5oO3efBoMTuTbtjMc+x4Yg9PMmNzblty4s4GtmRrvFoOpDE257AdS+BhyD/+OLftQILBAR4zabxazj8Davl3zJ6H/zFjs2XbAXuDA/zf8Gu5kQZyTw1jjwTQYYxtBxg3HOBhI6DlGePsv30HEntuPGZs7DkH9Eszm7HlHLwOS2P4OONbnT17fzJjw48yYIixNz+88QaPFig4jMRmJqwcBOqIUzYKRsEoGAUjEwAAxtZNsp1BDssAAAAASUVORK5CYII=","orcid":"https://orcid.org/0009-0003-5499-5648","institution":"University of California Santa Barbara","correspondingAuthor":true,"prefix":"","firstName":"Martine","middleName":"","lastName":"Wagstaff","suffix":""}],"badges":[],"createdAt":"2025-09-24 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15:33:45","extension":"html","order_by":18,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":232737,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/9f98f03a4780169760df4e36.html"},{"id":94470748,"identity":"aeffbf87-adc9-4869-933b-6e9a2e68b4c7","added_by":"auto","created_at":"2025-10-27 15:33:43","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":154832,"visible":true,"origin":"","legend":"\u003cp\u003eStarting community manipulations. Small 3 x 3 cm tiles pre-seeded with algae or invertebrates were assembled into a 15 x 15 starting community. This figure represents the ‘algae and invertebrates’ treatment in the Displacement experiment. This diagram is a schematic only, and multiple species were present on the tiles (illustration credits: Martine Wagstaff)\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/033d9a87d1645656fe269631.jpeg"},{"id":94470747,"identity":"c85c38b6-41d1-4eeb-908f-f2b1c363d3dd","added_by":"auto","created_at":"2025-10-27 15:33:42","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":532177,"visible":true,"origin":"","legend":"\u003cp\u003eTreatments in the \u003cem\u003eSettlers\u003c/em\u003eexperiment through time. The figure shows the manipulations for the algae cut and the algae mimic treatment. At the end of the experiment, day 35, the algae normal and algae cut treatments had high space coverage by colonial ascidians, the algae mimic treatment had intermediate coverage, and the no algae treatment had very little to no coverage (photo credits: Martine Wagstaff)\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/e1e6ba56c51225c4aeb79207.jpeg"},{"id":94470755,"identity":"8ba20128-7169-4750-8749-dd71ad514d00","added_by":"auto","created_at":"2025-10-27 15:33:45","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":132417,"visible":true,"origin":"","legend":"\u003cp\u003eCentroid plots showing percent cover and diversity respectively of primary space occupying sessile invertebrates, for all treatments in the a and e) \u003cem\u003eAssembly\u003c/em\u003e, b and f) \u003cem\u003eDisplacement\u003c/em\u003e, c and g) \u003cem\u003eRecruits\u003c/em\u003e, and d and h) \u003cem\u003eSettlers\u003c/em\u003eexperiments. Percent cover and diversity were not affected by the treatments in the \u003cem\u003eAssembly\u003c/em\u003e, \u003cem\u003eDisplacement\u003c/em\u003e, and \u003cem\u003eRecruits\u003c/em\u003e experiments, but were affected by the treatments in the \u003cem\u003eSettlers\u003c/em\u003e experiment. Upper case letters in plots d and h denote the different groupings revealed by Tukey’s HSD test. Error bars represent one standard error\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/45119fdc8d947cf301ca82bc.jpeg"},{"id":94470752,"identity":"456dee12-bbc4-42c3-ab02-b987c81c6a54","added_by":"auto","created_at":"2025-10-27 15:33:44","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":100451,"visible":true,"origin":"","legend":"\u003cp\u003eNMDS ordination plots of community replicates separated by treatments using Bray-Curtis dissimilarities for space occupying sessile invertebrate communities for the a) \u003cem\u003eAssembly\u003c/em\u003e, b) \u003cem\u003eDisplacement\u003c/em\u003e, c) \u003cem\u003eRecruits\u003c/em\u003e, and d) \u003cem\u003eSettlers\u003c/em\u003eexperiments. Treatments affected community composition in the \u003cem\u003eAssembly\u003c/em\u003e, \u003cem\u003eDisplacement\u003c/em\u003e, and \u003cem\u003eSettlers\u003c/em\u003e experiments but not in the \u003cem\u003eRecruits\u003c/em\u003e experiment\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/190c496056ea42bae0c70a46.jpeg"},{"id":94470649,"identity":"92a6a6ef-d193-4559-a7a8-18d34a06d6fd","added_by":"auto","created_at":"2025-10-27 15:32:51","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":287028,"visible":true,"origin":"","legend":"\u003cp\u003eRecommendations for floating dock management to encourage macroalgae growth (illustration credit: Martine Wagstaff)\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/fddef0357de6276ef0e1b954.png"},{"id":107929238,"identity":"b8d30d7b-bbd7-40ab-983c-d14063d58342","added_by":"auto","created_at":"2026-04-27 16:14:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1692595,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/d5a906af-baa5-4cf1-8630-fd36e625b31d.pdf"},{"id":94470761,"identity":"5ee6e36d-c1b7-45c4-86fd-68475c98013e","added_by":"auto","created_at":"2025-10-27 15:33:47","extension":"docx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":33176,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformationBioInv.docx","url":"https://assets-eu.researchsquare.com/files/rs-7706424/v1/58d46407ba74cae2ca32e17d.docx"}],"financialInterests":"","formattedTitle":"The role of macroalgae in structuring a New England fouling community and the implications for floating dock management to ameliorate invasive species","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFouling communities are assemblages of sessile plants and animals that form on artificial structures in the marine environment and are considered, \u0026lsquo;from a biological point of view, as an accident, and of very recent origin (WHOI 1952).\u0026rsquo; Nonetheless, these communities, when they form on the side of floating docks, are easily accessible to researchers, and the small size, short generation time, and sessile nature of organisms make them readily amenable to experimental manipulations and monitoring through time. Fouling communities have thus become popular model systems that have contributed much to our understanding of basic ecological principles in the marine realm (Ferrario et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Jewett et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), from early studies documenting community establishment and development (Osman \u003cspan citationid=\"CR84\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Sutherland \u003cspan citationid=\"CR113\" class=\"CitationRef\"\u003e1974\u003c/span\u003e, \u003cspan citationid=\"CR114\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Sutherland and Karlson \u003cspan citationid=\"CR115\" class=\"CitationRef\"\u003e1977\u003c/span\u003e) to more recent ones focusing on invasion biology (Lord \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Sorte et al. \u003cspan citationid=\"CR104\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Stachowicz et al. \u003cspan citationid=\"CR106\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Stachowicz and Byrnes, \u003cspan citationid=\"CR105\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Investigating the diversity-invasibility relationship has been particularly popular (Beshai et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Marraffini and Geller \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Stachowicz et al. \u003cspan citationid=\"CR106\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Stachowicz and Byrnes \u003cspan citationid=\"CR105\" class=\"CitationRef\"\u003e2006\u003c/span\u003e), as these systems are space limited, heavily influenced by competition, and often composed of a high number of invasive species (Lord \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). All of these studies, however, have focused on sessile invertebrates (Gartner et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Glasby et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Karlson and Osman 2012; Ruiz et al. \u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e2009\u003c/span\u003e) but these are not the only taxa in these systems, and macroalgae can also be found on the sunlit sides of floating docks. How sessile macroalgae and invertebrates, organisms from two different kingdoms, compete for space in a resource limited environment is a major distinction between marine and terrestrial realms and it is surprising that this has not been previously investigated. Here I investigate potential mechanisms for this and consider implications for invasion biology. As manmade structures continue to be added to the marine environment facilitating species invasions (Bulleri and Chapman \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Glasby and Connell \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Glasby et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), any mechanisms discerned can help us inform management of floating docks in marinas and help mitigate the spread of non-native species.\u003c/p\u003e\u003cp\u003eSpecies invasions have naturally occurred throughout time (Elton \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Mooney and Cleland \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Vermeij \u003cspan citationid=\"CR125\" class=\"CitationRef\"\u003e1991\u003c/span\u003e; Webb \u003cspan citationid=\"CR130\" class=\"CitationRef\"\u003e1991\u003c/span\u003e), but the rate at which man is transporting species around the world today is unprecedented (Drake et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Ricciardi \u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). Consequently, there is a lag in our understanding of, and response to, species invasions, with implications for biodiversity, ecosystem services, and biosecurity (Ricciardi et al. \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Indeed, in the marine environment, species invasions are contributing to a third and current stage of coastal ecosystem collapse (Jackson \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Invasive species are transported in ballast water (Bailey \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Carlton and Geller \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1993\u003c/span\u003e) and on boat hulls (Bax et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Lambert and Lambert \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), and can easily expand their ranges when marine vessels dock in new ports. Manmade floating docks provide an optimum structure to which marine invaders can \u0026lsquo;jump ship\u0026rsquo; (Glasby and Connell \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Carlton and Geller \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Ruiz et al. \u003cspan citationid=\"CR96\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) and these new habitats are well known to support fouling communities that are often primarily composed of non-native invertebrate species (Bax et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Lambert and Lambert \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Ruiz et al. \u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e2009\u003c/span\u003e), specifically prolific invaders such as ascidians (Aldred and Clare \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Dijkstra et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Lambert and Lambert \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Lindeyer and Gittenberger \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). However, on the sunlit sides of floating docks in the Gulf of Maine fouling communities, algae also occur, but their role in structuring fouling communities has not been investigated.\u003c/p\u003e\u003cp\u003eOn subtidal rocky reefs in the Gulf of Maine, macroalgae and invertebrates also coexist but tend to be partitioned based on substrate angle with macroalgae typically occupy horizontal, sunlit surfaces while animals dominate vertical walls and underhangs (Miller and Etter \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2008\u003c/span\u003e, \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Sebens \u003cspan citationid=\"CR99\" class=\"CitationRef\"\u003e1985\u003c/span\u003e; Witman and Dayton \u003cspan citationid=\"CR133\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Invertebrates are, however, able to persist on horizontal surfaces if algae are excluded by shading, suggesting algae presence may exclude invertebrates or impact invertebrate post-settlement mortality (Miller and Etter \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). Indeed, algae can inhibit invertebrates through competition for space, physical disturbance, harboring of small predators, and allelopathy. Algae can pre-empt space (Connell et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1997\u003c/span\u003e) and physically or chemically interfere with invertebrates. Physically, algae fronds can scour the substratum reducing invertebrate recruitment (Connell \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Duggins et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1990\u003c/span\u003e), and they can abrade (Box and Mumby \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; River and Edmunds \u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Titlyanov et al. \u003cspan citationid=\"CR120\" class=\"CitationRef\"\u003e2007\u003c/span\u003e) or smother (Hughes \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e1989\u003c/span\u003e) adults, impacting invertebrate assemblages beneath the algal canopy (Kennelly \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). Algae can also overgrow (Coyer et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1993\u003c/span\u003e; Davis and White \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Jompa and McCook \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Lewis \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e1986\u003c/span\u003e; Young and Chia \u003cspan citationid=\"CR137\" class=\"CitationRef\"\u003e1984\u003c/span\u003e) and dislodge invertebrates (Witman \u003cspan citationid=\"CR135\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Algal fronds can alter water flow, sedimentation, and light intensity, which can, in turn, negatively impact invertebrate recruitment, feeding, and growth (Duggins et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). Algae also provide habitat for micropredators (Osman and Whitlatch \u003cspan citationid=\"CR85\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Stachowicz and Whitlatch \u003cspan citationid=\"CR108\" class=\"CitationRef\"\u003e2005\u003c/span\u003e) and mesograzers (Duffy and Hay \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2000\u003c/span\u003e) that prey on invertebrate recruits. Chemically, algae can release allelopathic compounds that directly result in the mortality of invertebrate recruits and adults (Rasher and Hay \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Rasher et al. \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2011\u003c/span\u003e) and substances that alter the microbial activity of corals, inhibiting coral growth and survival (Haas et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Smith et al. \u003cspan citationid=\"CR103\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Thurber et al. \u003cspan citationid=\"CR119\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Vermeij et al. \u003cspan citationid=\"CR126\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Algae may also act as vectors for disease (Nugues et al. \u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Sweet et al. \u003cspan citationid=\"CR116\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIf algae can inhibit invertebrates on rocky reefs, can they also inhibit them on the sunlit sides of floating docks and, if so, does this have implications for species invasions? I used manipulative experiments, hung off the sides of floating docks, to explore if algae influence invertebrate settlement, recruitment, or adult survivorship. Potential mechanisms of exclusion were also investigated via plastic algal mimics and modified (trimmed) natural algae which allowed chemical characteristics and algal structure to be manipulated respectively. Specifically, I asked 1) Do macroalgae exclude sessile invertebrates, and 2) How might macroalgae exclude sessile invertebrates? I seek to extend the fouling community model from its focus on sessile invertebrates to include autotrophs, with the aim of increasing our understanding of invasions in the marine realm to help inform management of floating dock systems, information that is crucial as both marine urban sprawl (Firth et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Todd et al. \u003cspan citationid=\"CR121\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) and species invasions (Drake et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Ricciardi et al. \u003cspan citationid=\"CR92\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) continue at unprecedented rates.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eStudy site\u003c/p\u003e\u003cp\u003eI conducted four experiments in a New England fouling community to investigate if macroalgae exclude sessile invertebrates on the shallow sunlit sides of floating docks, as well as potential mechanisms for this. The experiments were carried out at a depth of 1 m on polycarbonate settlement plates suspended vertically off floating docks at Dorchester Yacht Club, Boston, Massachusetts, (42.305556\u0026deg;N, 71.046111\u0026deg;W). The fouling community was composed of sessile algae and invertebrates typical of this region, including red and green algae, sponges, bryozoans, polychaetes, molluscs, barnacles, and colonial and solitary ascidians (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e and Wagstaff (\u003cspan citationid=\"CR127\" class=\"CitationRef\"\u003e2024\u003c/span\u003e)) for a full species list). All species of algae in this system were native, while the invertebrate assemblage was a mix of native and invasive species (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eExperiments\u003c/p\u003e\u003cp\u003e\u003cem\u003eAssembly.\u003c/em\u003e To test if macroalgae influence the formation of invertebrate assemblages during the establishment of natural communities, fouling communities were allowed to form on settlement plates in the presence or absence of algae. Polycarbonate settlement plates (15x15 cm) were attached to polycarbonate boards and submerged 1 m below the water surface. Four replicates of each treatment were deployed, and arranged in a Latin square. Treatments were: 1) natural community, where assemblages were allowed to form without any manipulation, and 2) algae removal, where algae were carefully removed with or forceps weekly in summer, bimonthly in fall and spring, and once a month in winter. The removal schedule was based on seasonal algae growth, which is fastest in summer and slowest in winter. I expected that algae would limit invertebrates in the natural community treatment and there would be a greater diversity and abundance of invertebrate species found in the removal treatment. This experiment was conducted over 18 months, from April 2012 to October 2013.\u003c/p\u003e\u003cp\u003eAt the end of the experiment settlement plates were collected, all algae and arborescent invertebrates trimmed to enable primary space occupiers to be identified, and communities photographed using an Olympus Stylus Tough 8010 camera. Digital photographs were later enlarged on a screen and percent cover and species richness of space occupying sessile invertebrates quantified using 200 random points overlaid on the central 12x12 cm area of a settlement plate to avoid edge effects.\u003c/p\u003e\u003cp\u003e\u003cem\u003eDisplacement\u003c/em\u003e. To test if macroalgae can displace established sessile invertebrates in fouling communities, the percent cover of macroalgae and invertebrates were manipulated using small tiles that were then assembled to form larger plates, depending upon the experimental treatment. Firstly, small 3x3 cm polycarbonate tiles were pre-seeded with either algae (tiles were placed on the sides of floating docks when algal recruitment was high) or invertebrates (tiles were placed underneath floating docks where low light levels prevented algal growth). Once seeded, 3x3 cm tiles, 25 in total, were then pieced together to create the larger 15x15 cm starting communities (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). For example, to create a starting community of approximately 50% algae and approximately 50% invertebrates, 12 (or 13) 3x3 cm tiles with pre-seeded with algae and 13 (or 12) 3x3 cm tiles pre-seeded with invertebrates were randomly assembled to create a larger 15x15 cm plate (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Table S2). This was achieved by gluing threaded polycarbonate rods into blind holes on the underside of the 3x3 cm tiles (i.e., the rod did not penetrate the upper settling surface) and then passing these rods through holes in a 17x17 cm, 0.5 cm thick, polycarbonate base plate and securing with polycarbonate nuts on the underside (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe four starting communities, or treatments, in this experiment were: 1) algae (50% algae tiles and 50% blank tiles) to test if invertebrate assemblages could form in the presence of established algae; 2) inverts (50% invertebrate tiles and 50% blank tiles) to test if incoming algae could still form and limit invertebrates; 3) inverts and algae (50% invertebrate tiles and 50% algae) to test if adult algae could displace adult invertebrates; and 4) inverts and algae removal (50% invertebrate tiles and 50% blank tiles with continual algal removal) as a control to determine if invertebrate mortality was influenced by colonization of macroalgae in treatment 2 (Table S2). I expected that established algae would limit invertebrate settlers and adults in treatments 1 and 3 respectively, depressing invertebrate abundance and diversity, and that incoming algae (treatment 2) would also have the same effect. Lastly, I expected that invertebrates would persist when algae were removed in treatment 4.\u003c/p\u003e\u003cp\u003eAlgae were removed from the control treatment in the same way and frequency as previously described for the \u003cem\u003eAssembly\u003c/em\u003e experiment. The duration of the experiment, number and arrangement of replicates, data collection, and response variables were also the same.\u003c/p\u003e\u003cp\u003e\u003cem\u003eRecruits.\u003c/em\u003e The presence or absence of algae was manipulated to test if invertebrate recruits, defined as newly settled individuals that have colonized plates, were affected by established algae. In this experiment, modifications were made to algae to identify potential mechanisms of exclusion. To eliminate whiplash, algal fronds were cut 1 cm above their bases to remove the distal thalli but keep other algal properties such as allelochemical composition and some micropredator habitat intact. Conversely to maintain algae structure and thus potential whiplash but to eliminate allelochemicals, a plastic structural mimic was used.\u003c/p\u003e\u003cp\u003eAs in the previous experiment, 3x3 cm tiles containing either normal algae, cut algae, plastic algal mimics, or no algae, were combined with 3x3 cm invertebrate recruit tiles to create starting communities. Twenty-five 3x3 cm tiles were assembled into larger 15x15 cm fouling communities depending on the experimental treatment. Starting communities, or treatments, were 1) algae normal (50% intact algae tiles and 50% invertebrate recruit tiles) to test if algae inhibit invertebrate recruits; 2) algae cut (50% modified algae tiles with distal thalli removed and 50% invertebrate recruit tiles) to test if invertebrate recruits can persist if some of the physical effects of algae, such as whiplash, are removed; 3) algae mimic (50% plastic algae mimic tiles and 50% invertebrate recruit tiles) to assess if the presence of the distal thalli (relative to treatment 2 where the distal thalli is removed) or the absence of allelochemicals might affect invertebrate recruits; and 4) no algae (50% blank tiles with algae absent and continually removed and 50% invertebrate recruit tiles) (control) (Table S2). I expected algae to inhibit recruits and that percent cover and diversity of invertebrates would be lowest in treatment 1 where normal algae is present, intermediate in treatments 2 and 3 where some of the characteristics of algae are eliminated, and highest in treatment 4 where no algae are present.\u003c/p\u003e\u003cp\u003eThe cutting and removal of algae in treatments 2 and 4 was performed twice a week. Distal thalli were cut 1 cm above the base using scissors and algae removal was as described in previous experiments. The algae present in the system at this time, \u003cem\u003eUlva intestinalis\u003c/em\u003e, was mimicked using green \u0026lsquo;Easter Grass\u0026rsquo; which was attached to tiles using Loctite Marine Epoxy. The experiment ran for five weeks, from June 26th\u003csup\u003eth\u003c/sup\u003e until July 31\u003csup\u003est\u003c/sup\u003e\u003csub\u003e,\u003c/sub\u003e 2012. This experiment was shorter than the previous two, as the aim was to tease out the influence of algae on early ontogenetic stages of invertebrates. The number and arrangement of replicates, data collection, and response variables were as above in \u003cem\u003eAssembly\u003c/em\u003e methods.\u003c/p\u003e\u003cp\u003e\u003cem\u003eSettlers.\u003c/em\u003e To test if and how the presence of algae influenced the settlement of invertebrate larvae, where settlers are defined as incoming larvae that settle on plates, I conducted an experiment similar to the \u003cem\u003eRecruits\u003c/em\u003e experiment described above, but the algae tiles (normal, cut, or mimicked) were arrayed with blank tiles to allow invertebrate settlement (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Starting communities, or treatments, were 1) algae normal (50% intact algae tiles and 50% blank tiles); 2) algae cut (50% modified algae tiles (distal thalli removed with scissors) and 50% blank tiles); 3) algae mimic (50% plastic algae mimic tiles and 50% blank tiles); and 4) no algae (all blank tiles, 100%, with algae absent and continually removed) (control). As in the \u003cem\u003eRecruits\u003c/em\u003e experiment, I expected algae to inhibit settlers and that percent cover and diversity of invertebrates would be lowest in treatment 1 where normal algae is present, intermediate in treatments 2 and 3 where some of the characteristics of algae are eliminated, and highest in treatment 4 where no algae are present.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe distal thalli were cut in the algae cut treatment, and algae removed from the no algae treatment as in the \u003cem\u003eRecruits\u003c/em\u003e experiment. Green polyethylene bags were cut into the shape of \u003cem\u003eUlva linza\u003c/em\u003e fronds (the algae present in the system at the time of the experiment) and attached to tiles using Loctite Marine Epoxy (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The experiment ran for 5 weeks, from August 8th to September 11th, 2012. The number and arrangement of replicates, data collection, and response variables were as above in the \u003cem\u003eAssembly\u003c/em\u003e experiment.\u003c/p\u003e\u003cp\u003eStatistical analyses\u003c/p\u003e\u003cp\u003eExperiments were analyzed as single factor MANOVAs. Response variables were percent cover of primary space occupying sessile invertebrates and diversity of space occupying sessile invertebrates. Percent cover data was transformed to logits, ln[p/(1-p)], to homogenize variances where p is the proportion of sessile inverts\u0026thinsp;+\u0026thinsp;0.025 (0.025 was added to avoid proportions of 0 or 1). Diversity was calculated using Shannon\u0026rsquo;s diversity index. MANOVAs were followed up with univariate ANOVAs to test for the contribution of each dependent variable using a Bonferroni-corrected α of 0.025. Post-hoc pairwise comparison tests, i.e., Tukey\u0026rsquo;s HSD, were carried out when ANOVA results revealed significant differences among treatments. Normality was tested using the Shapiro-Wilk test. Homogeneity of variance was tested using Levene\u0026rsquo;s median test. Compositional differences among treatments of primary space occupying sessile invertebrates were compared using PERMANOVA and non-metric multi-dimensional scaling (NMDS), both based on Bray-Curtis distances. Multivariate dispersion was tested using PERMDISP, which is a multivariate analog of Levene's test for homogeneity of variances. The contribution of individual species to the differences between groups was assessed using similarity percentage, or SIMPER. All analyses were carried out in R version 3.0.2 (R Core Team 2013) using the Vegan package (Oksanen et al. 2012).\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cem\u003eAssembly.\u003c/em\u003e In the \u003cem\u003eAssembly\u003c/em\u003e experiment, which tested if macroalgae influenced the formation of invertebrate assemblages when natural communities are allowed to establish, the presence of algae did not negatively impact invertebrates, contrary to what I expected. Neither percent cover nor diversity of space occupying sessile invertebrates differed between treatments (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and e, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) but treatments did influence community composition (Pseudo-F\u0026thinsp;=\u0026thinsp;2.15, p\u0026thinsp;=\u0026thinsp;0.03; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). In the natural community treatment, where both algae and invertebrate assemblages were allowed to form, there were more native \u003cem\u003eMytilus edulis, Crepidula plana\u003c/em\u003e, and algae/mud/tube complex, as well as the invasive \u003cem\u003eOstrea edulis\u003c/em\u003e (Table S3). In the algae removal treatment, there were more invasive colonial ascidians including \u003cem\u003eBotryllus schlosseri\u003c/em\u003e, \u003cem\u003eBotrylloides violaceus\u003c/em\u003e, and \u003cem\u003eDiplosoma listerianum.\u003c/em\u003e Thus, the algae removal treatment was dominated by invasive species whereas the natural community supported more native species. Across treatments native barnacles and invasive solitary ascidians\u0026mdash;\u003cem\u003eAscidiella aspersa\u003c/em\u003e, \u003cem\u003eCiona intestinalis\u003c/em\u003e, and \u003cem\u003eStyela clava\u003c/em\u003e\u0026mdash;did not vary (Table S3). These observations were supported by SIMPER. As species diversity did not differ between treatments and neither did species identities (Table S3), differing species abundances account for differences in community composition.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults of MANOVAs for \u003cem\u003eAssembly\u003c/em\u003e, \u003cem\u003eDisplacement\u003c/em\u003e, \u003cem\u003eRecruits\u003c/em\u003e, and \u003cem\u003eSettlers\u003c/em\u003e experiments\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSource\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003edf\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eWilks\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eApprox F\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003ePr(\u0026gt;\u0026thinsp;F)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eAssembly\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.904\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eDisplacement\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.522\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eRecruits\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.974\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eSettlers\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e69.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"5\"\u003e* Significant p-value\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults of univariate ANOVAs for percent cover and diversity of primary space occupying sessile invertebrates for the \u003cem\u003eAssembly\u003c/em\u003e, \u003cem\u003eDisplacement\u003c/em\u003e, \u003cem\u003eRecruits\u003c/em\u003e, and S\u003cem\u003eettlers\u003c/em\u003e experiments\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e\u003cp\u003ePercent cover\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e\u003cp\u003eDiversity\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSource\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u003cb\u003edf\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u003cb\u003eSS\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u003cb\u003eF\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003eP\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u003cb\u003edf\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u003cb\u003eSS\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u003cb\u003eF\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e\u003cb\u003eP\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eAssembly\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.96\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.904\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e1\u003c/p\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.005\u003c/p\u003e\u003cp\u003e0.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.809\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eDisplacement\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.65\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.522\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.12\u003c/p\u003e\u003cp\u003e0.38\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e1.23\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.342\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eRecruits\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.90\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.974\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.004\u003c/p\u003e\u003cp\u003e0.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e0.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.934\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eSettlers\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e726.60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026lt;\u0026thinsp;0.001*\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c7\"\u003e\u003cp\u003e0.82\u003c/p\u003e\u003cp\u003e0.50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c8\"\u003e\u003cp\u003e6.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c9\"\u003e\u003cp\u003e0.007*\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"9\"\u003e* Significant p-value\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eResults of PERMANOVA, using the Bray-Curtis distance metric, for community composition of primary space occupying sessile invertebrates for the \u003cem\u003eAssembly\u003c/em\u003e, \u003cem\u003eDisplacement\u003c/em\u003e, \u003cem\u003eRecruits\u003c/em\u003e, and \u003cem\u003eSettlers\u003c/em\u003e experiments\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSource\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003edf\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSS\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003ePseudo-F\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eP\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eAssembly\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1\u003c/p\u003e\u003cp\u003e6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.22\u003c/p\u003e\u003cp\u003e0.54\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e2.147\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e0.03\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eDisplacement\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.822\u003c/p\u003e\u003cp\u003e1.01\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e3.27\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u003cb\u003e0.001\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eRecruits\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e0.18\u003c/p\u003e\u003cp\u003e1.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e0.52\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.802\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003eSettlers\u003c/em\u003e\u003c/p\u003e\u003cp\u003eTreatment\u003c/p\u003e\u003cp\u003eResiduals\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3\u003c/p\u003e\u003cp\u003e12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.84\u003c/p\u003e\u003cp\u003e0.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e46.76\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e0.001*\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003ctfoot\u003e\u003ctr\u003e\u003ctd colspan=\"5\"\u003e* Significant p-value\u003c/td\u003e\u003c/tr\u003e\u003c/tfoot\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cem\u003eDisplacement.\u003c/em\u003e In the \u003cem\u003eDisplacement\u003c/em\u003e experiment, which tested if algae could displace established sessile invertebrates, percent cover and diversity did not differ between treatments (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb and f, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) but community composition did (Pseudo-F\u0026thinsp;=\u0026thinsp;3.27, p\u0026thinsp;=\u0026thinsp;0.001; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), similar to the findings from the \u003cem\u003eAssembly\u003c/em\u003e experiment. Composition in the algae removal treatment differed from the other three treatments where algae was allowed to be present in the system (algae, inverts, inverts and algae), all of which overlapped in the NMDS plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). As in the \u003cem\u003eAssembly\u003c/em\u003e experiment, communities with algae contained more native \u003cem\u003eM. edulis, C. plana\u003c/em\u003e, algae/mud/tube complex, and the invasive oyster \u003cem\u003eO. edulis\u003c/em\u003e. Higher abundances of invasive colonial ascidians were found in the algae removal treatment (Table S3). These findings are consistent with the results from the previous experiment. Across treatments, barnacles and solitary ascidians did not vary (Table S3). Again, all observations were supported by SIMPER. The native sponge, \u003cem\u003eHalichondria panacea\u003c/em\u003e, and the native encrusting bryozoan, \u003cem\u003eElectra pilosa\u003c/em\u003e, were also found in treatments with algae. The abundances of these, however, were not high enough to influence statistical results (Table S3) but does support the other findings in this and the previous experiment that algae appear to support native species.\u003c/p\u003e\u003cp\u003e\u003cem\u003eRecruits.\u003c/em\u003e Modified algae (\u003cem\u003eU. intestinalis\u003c/em\u003e)\u0026mdash;both plastic mimics and algae with the distal thalli removed\u0026mdash;used to test if algae negatively impacted invertebrate recruits as well as potential mechanisms for this, did not influence percent cover (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), diversity (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eg, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), or community composition (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Only invasive colonial ascidians \u003cem\u003eB. schlosseri\u003c/em\u003e and \u003cem\u003eB. violaceus\u003c/em\u003e occurred in these communities and their abundances did not vary across treatments (Table S3). Interestingly, the encrusting bryozoan, \u003cem\u003eElectra pilosa\u003c/em\u003e, a native invertebrate species, was again only found in communities with natural algae (treatments 1 and 2), albeit in very small numbers.\u003c/p\u003e\u003cp\u003e\u003cem\u003eSettlers.\u003c/em\u003e The \u003cem\u003eSettlers\u003c/em\u003e experiment tested if algae, \u003cem\u003eU. linza\u003c/em\u003e, negatively influenced invertebrate settlement and potential mechanisms for this. Treatments did alter percent cover (F\u0026thinsp;=\u0026thinsp;726.60, p\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and diversity (F\u0026thinsp;=\u0026thinsp;6.49, p\u0026thinsp;=\u0026thinsp;0.007; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh, Tables\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), but results were opposite to those expected i.e., algae did not negatively impact invertebrates but instead appeared to facilitate them. Diversity of invertebrates was higher in all treatments with algae, whether normal, cut or mimicked, and decreased slightly in the no algae treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eh), suggesting a role for the structure of algae in facilitating invertebrate settlement and colonization. Results were slightly different for percent cover of invertebrates. Here, percent cover was greatest in the algae normal and algae cut (distal thalli removed) treatments, intermediate in the algae mimic treatment, and least in the no algae treatment where algae were removed (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). This suggests that invertebrate larvae prefer to settle on some structure (Treatments 1, 2 and 3), but with a preference for natural algae (Treatments 1 and 2), regardless of whether the distal thalli have been removed or not. The differing results between experiments could be due to the various experimental time frames (18 months for \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments versus 5 weeks), with this experiment focusing on early stages of invertebrate assemblage formation, i.e., algae facilitating the settlement of invertebrate larvae, and the \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e representing climax communities.\u003c/p\u003e\u003cp\u003eCommunity composition also differed among treatments (Pseudo-F\u0026thinsp;=\u0026thinsp;46.76, p\u0026thinsp;=\u0026thinsp;0.001; Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed, Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and, as in the \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments, the treatment without algae was different from all of the treatments with algae, regardless of whether algae were normal, cut or mimicked (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). However, unlike in the \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments, all communities were dominated by invasive colonial ascidians, with their differing abundances accounting for differences in community composition (Table S3), observations that were supported by SIMPER. This difference could again be due to the differing time frames of the experiments, with native species not having enough time to colonize in this experiment or perhaps their larvae were not present in the water column during the short time period when this experiment was conducted. One native species, \u003cem\u003eC. plana\u003c/em\u003e, was found only in communities with natural algae (algae normal and algae cut) but did not occur in high enough abundances to influence results, but does suggest that algae support more native species.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eHere I showed that in a New England fouling community, the presence of algae affected community composition, specifically the balance between native and invasive species, and that, in the early stages of community assembly, some algae species (\u003cem\u003eU. linza\u003c/em\u003e) also appeared to facilitate the settlement of invertebrates. These findings add to our incomplete and lagging understanding of invasion resistance in the marine realm and may have implications for maintaining populations of autotrophs, informing management of both floating dock systems and the environments in which they are found.\u003c/p\u003e\u003cp\u003eAlgae facilitated invertebrate settlement\u003c/p\u003e\u003cp\u003eIn the \u003cem\u003eSettlers\u003c/em\u003e experiment, \u003cem\u003eU. linza\u003c/em\u003e appeared to facilitate invertebrate colonization by providing both structure and natural cues. Algae structure may facilitate invertebrate settlement by creating small eddies that increase the deposition of recruits (Sebens \u003cspan citationid=\"CR98\" class=\"CitationRef\"\u003e1983\u003c/span\u003e) and algae biofilms and chemical cues may further attract invertebrate larvae (Crisp \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e1974\u003c/span\u003e; Dobretsov \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Hadfield and Paul \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Pawlik \u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Tebben et al. \u003cspan citationid=\"CR118\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). There was some preference for natural over mimicked algae and this may reflect an overall preference of invertebrate larvae to settle into natural algae, higher rates of recruitment once settled, or greater colonization success once recruited. Some coral species have also been shown to settle preferentially into communities with natural rather than mimicked algae (Diaz-Pulido et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Nugues and Szmant \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIt was not expected that algae would facilitate invertebrates, instead it was expected that they would inhibit them. These unexpected results might be due to the types of algae present in fouling communities as opposed to other systems. The dominant species in this study were ephemeral green algae compared to large brown leathery macrophytes (see Steneck and Dethier \u003cspan citationid=\"CR109\" class=\"CitationRef\"\u003e1994\u003c/span\u003e) on rocky reefs, which are known to negatively impact invertebrates (Box and Mumby \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Connell \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; River and Edmunds \u003cspan citationid=\"CR93\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Witman \u003cspan citationid=\"CR135\" class=\"CitationRef\"\u003e1987\u003c/span\u003e). Furthermore, temperate algae species also do not produce as many defensive chemicals as their tropical counterparts (Norris and Fenical \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e1982\u003c/span\u003e; Bolser and Hay \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Hay \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) and studies examining the allelopathic effects of algae on invertebrates have focused on tropical reefs (Rasher and Hay \u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Rasher et al. \u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). In the Gulf of Maine there is some evidence that algal exudates negatively impact polychaete larvae (Warkus et al. 2011), but it is unclear whether this negative effect of algae on invertebrates is restricted to this taxon or this ontogenetic stage.\u003c/p\u003e\u003cp\u003eThe results from the short-term \u003cem\u003eSettlers\u003c/em\u003e experiment, that treatments with algae had a higher percent cover and diversity of invertebrates, contrasts the longer-term \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments, where the presence or absence of algae did not influence invertebrate cover or diversity. This could be due to the differing time frames of the experiments, with the \u003cem\u003eSettlers\u003c/em\u003e experiment teasing out what happens in the initial stages of invertebrate assemblage formation, i.e., that algae facilitate invertebrate colonization and, in general, this ecological process tends to be common in the early stages of community assembly (Clements \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1916\u003c/span\u003e; Connell and Slatyer \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1977\u003c/span\u003e; Dean \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Odum \u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e1969\u003c/span\u003e). It should be cautioned, however, that only one species of algae, \u003cem\u003eUlva linza\u003c/em\u003e, was used in this experiment, and thus any inferences about algae facilitating invertebrate settlement are restricted to this species.\u003c/p\u003e\u003cp\u003eAlgae affected community composition of invertebrates\u003c/p\u003e\u003cp\u003eIn the \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments, algae affected community composition, treatments with algae having more native invertebrate species than treatments without algae. Thus, algae might confer some invasion resistance by maintaining native invertebrate assemblages and subsequently limiting the abundance of invasive species. How might algae do this?\u003c/p\u003e\u003cp\u003eOne striking observation from the \u003cem\u003eAssembly\u003c/em\u003e and \u003cem\u003eDisplacement\u003c/em\u003e experiments is that communities with algae were associated with greater abundances of molluscan fauna i.e., the native blue mussel, \u003cem\u003eM. edulis\u003c/em\u003e, and the native slipper snail, \u003cem\u003eC. plana\u003c/em\u003e. Settlement of blue mussels into algae is well known (Bayne \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1964\u003c/span\u003e; \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Dobretsov and Wahl \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) and can be induced by waterborne algal compounds (Dobretsov and Wahl \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Similarly, \u003cem\u003eC. fornicata\u003c/em\u003e, a congener of \u003cem\u003eC. plana\u003c/em\u003e has also been shown to metamorphose in response to chemical cues from coralline algae (Taris et al. \u003cspan citationid=\"CR117\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). As a general rule, the greater the diversity of algae in an assemblage, the greater the diversity of molluscs (Azevedo 2008), and this relationship appears to be mediated by algal functional form, (Chemello and Milazzo \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Duarte et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), and functional identity of algae does affect meiofauna diversity in intertidal algae (Dijkstra et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Torres et al. \u003cspan citationid=\"CR122\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Veiga et al. \u003cspan citationid=\"CR124\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Regardless of the mechanism by which algae confer invasion resistance, understanding the importance of macroalgae as a microhabitat for small-sized species, which generally occupy the lowest trophic levels, can contribute to a better knowledge of the dynamics of these ecosystems and of the species that they harbor (Queiroz and Dias \u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eIf the strong biotic interactions between native algae and native invertebrates hinder the infiltration of invasive species (here that algae attract native invertebrates thus limiting invasive invertebrates), this is consistent with the biotic resistance hypothesis of species invasions (Beaury et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Maron and Vila 2001), where invasion success is negatively correlated with species richness (Levine and D\u0026rsquo;Antonio 1999; Shea and Chesson \u003cspan citationid=\"CR100\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). In algae communities, the functional diversity of algae has been shown to protect against invasion by other algae species (Britton-Simmons \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) and, in invertebrate communities, increased species richness significantly decreased invasion success, because species-rich communities more completely and efficiently used available space, the limiting resource in fouling communities (Stachowicz et al. \u003cspan citationid=\"CR107\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). However, no studies have investigated how algae and invertebrates together might resist invasion. Indeed, studies investigating the biotic resistance hypothesis, tend to focus on interactions between species in the same trophic level or that of consumer-resource and not between organisms of different kingdoms. Furthermore, facilitation is only incorporated into invasion biology when discussing how native species can facilitate non-native species (Cavieres \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) and not how native species can facilitate other native species and thus limit invasions via indirect effects. Thus, both facilitation (Bruno et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) and indirect effects (White et al. \u003cspan citationid=\"CR131\" class=\"CitationRef\"\u003e2006\u003c/span\u003e) need to be better incorporated into this field. Disturbance is also well-known to facilitate invasive marine invertebrates (Altman and Whitlatch \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), possibly due to their ability to colonize open space (Stachowicz et al. \u003cspan citationid=\"CR106\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Tyrrell and Byers \u003cspan citationid=\"CR123\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Kremer et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Janiak et al. 2013), and this may also explain why invertebrates were more abundant in removal treatments, that they were more successfully able settle into the open space provided when algae was removed than native species were.\u003c/p\u003e\u003cp\u003eManagement of floating docks for macroalgae\u003c/p\u003e\u003cp\u003eIf algae do facilitate and maintain native species assemblages in floating dock systems, then ensuring the persistent of these taxa is important. Unfortunately, manmade structures introduce shade to the marine environment, reducing plant growth, productivity, and survival (Burdick and Short \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Dyson and Yocom \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). In eelgrass beds, one management strategy has been to construct docks out of materials that will allow sufficient light penetration for photosynthesis, such as fiberglass grating in single-family docks in Florida (Landry et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and glass prisms in ferry terminals in Washington State (Blanton et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). As macroalgae in rocky habitats also show a decrease in biomass and percent cover with shade from manmade structures (Pardal-Souza et al. \u003cspan citationid=\"CR86\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), macroalgae might also benefit from docks made of similar materials (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Other suggestions include orientating the sides of docks towards the sun (Burdick and Short, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1999\u003c/span\u003e), using rugose textured materials on the side of docks to allow increased surface area for algae to grow (Dyson and Yocom \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Logan et al. \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), transplanting algae on to manmade structures (Firth et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Marzinelli et al. \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Susini et al. \u003cspan citationid=\"CR112\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), and the addition of structures such as seawall stairs, vegetation baskets, and vertical gardens (Dyson and Yocom \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). While it is becoming increasingly popular to design manmade structures with the natural environment in mind (Dafforn et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), the impacts of floating docks on macroalgae are still largely understudied (Lambert et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), and ecological designs for docks, piers, and overwater structures must address the effects of shading on vegetation (Dyson and Yocom \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Marine urban sprawl in general needs to be managed more (Dafforn et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Firth et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), and invasive species along in coastal and estuarine environments should also be managed to the same extent as overexploitation, pollution, and climate change (Williams and Grosholz \u003cspan citationid=\"CR132\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs fouling communities predominantly occur in harbors and marinas, these locations should also be managed to promote healthy algae populations. These systems tend to be characterized by increased suspended sediments (Iannuzzi et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1996\u003c/span\u003e) that reduce light penetration and thus photosynthesis by algae (Airoldi 2003), and are highly polluted with associated reductions in biodiversity (Ferrario et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kenworthy et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), including many species of brown and red algae (Fowles et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Marinas also have reduced water flow, trapping sediments inside, as well as increased turbidity, temperature and pH (Rivero et al. \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). These marked environmental changes have ecological consequences and should be a primary consideration during the planning process (Rivero et al. \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eWorldwide, macroalgae are under threat from sedimentation, loss of habitat, invasive species, pollution, hypoxia, and acidification (Brodie et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Mineur et al. \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Schiel \u003cspan citationid=\"CR97\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Steneck et al. \u003cspan citationid=\"CR110\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Walker and Kendrick \u003cspan citationid=\"CR128\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). Any factor that reduces algal cover thus has the potential to shift floating dock invertebrate communities towards those dominated by invasive species, which in turn increases the risk of invasion to nearby native habitats (Glasby and Connell \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Lambert and Lambert \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Ruiz et al. \u003cspan citationid=\"CR95\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Simkanin et al. \u003cspan citationid=\"CR101\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Managing floating docks (Blanton et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Landry et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Dyson and Yocom \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and marinas (Rivero et al. \u003cspan citationid=\"CR94\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) to enhance macroalgae growth might be one way to reduce fouling community dominance by invasive species. As well as hosting a diversity of native fauna, algae highly productive and provide multiple ecosystem services and need to be managed accordingly (Duffy et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Harley et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Mineur et al. \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Smale et al. \u003cspan citationid=\"CR102\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Strong et al. \u003cspan citationid=\"CR111\" class=\"CitationRef\"\u003e2015\u003c/span\u003e).\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003eFunding was provided by the Doctoral Dissertation Research Grant Program from University of Massachusetts Boston and the Lerner-Gray Fund for Marine Research from the American Museum of Natural History.\u003c/p\u003e\n\u003ch2\u003eCompeting Interests\u003c/h2\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003ch2\u003eAuthor Contributions\u003c/h2\u003e\n\u003cp\u003eMW designed the study, conducted the experiments, analyzed the results, and wrote the manuscript.\u003c/p\u003e\n\u003ch2\u003eAcknowledgements\u003c/h2\u003e\n\u003cp\u003eThank you to the two anonymous reviewers who helped shape and refine this manuscript as well as to R. Etter who assisted with many aspects of the work. Field assistance was provided by E. Franck, D. Katzmark, and S. Morello and Dorchester Yacht Club kindly let me conduct my experiments there.\u003c/p\u003e\n\u003ch2\u003eData availability\u003c/h2\u003e\n\u003cp\u003eData (Wagstaff, \u003cspan class=\"CitationRef\"\u003e2024\u003c/span\u003e) is available from Data Dryad: \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.5061/dryad.ghx3ffbxv\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAiroldi L, Cinelli F (1997) Effects of sedimentation on subtidal macroalgal assemblages: an experimental study from a Mediterranean rocky shore. J Exp Mar Biol Ecol\u003cem\u003e \u003c/em\u003e215:269\u0026ndash;288.\u003c/li\u003e\n\u003cli\u003eAldred N, Clare AS (2014) Mini-review: impact and dynamics of surface fouling by solitary and compound ascidians. 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Mar Biol 81:61\u0026ndash;68. \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"biological-invasions","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"binv","sideBox":"Learn more about [Biological Invasions](https://www.springer.com/journal/10530)","snPcode":"10530","submissionUrl":"https://submission.nature.com/new-submission/10530/3","title":"Biological Invasions","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Ascidians, competition, floating docks, fouling communities, invertebrates, macroalgae, invasive species","lastPublishedDoi":"10.21203/rs.3.rs-7706424/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7706424/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eFouling communities are tractable ecological systems that form on manmade structures in the marine environment where hypotheses about community assembly, disturbance, and species invasions can be tested. While much of this work has focused on sessile invertebrates, on hard substrates in the marine environment, invertebrates and macroalgae, organisms from two different kingdoms, can compete for the same limiting resource, space. In rocky subtidal habitats, research suggests that algae may exclude invertebrates or impact invertebrate post-settlement mortality and, to investigate this in fouling communities, I conducted manipulative experiments on the sides of floating docks. In three out for four experiments, macroalgae did not exclude invertebrates but did alter invertebrate community composition, communities with algae having more native species, mainly molluscs, whereas communities without algae were dominated by invasive species, specifically colonial ascidians. In one experiment, macroalgae also appeared to facilitate invertebrate settlement in the early stages of community assembly, mediated by both algae structure and natural chemical cues. If macroalgae presence in fouling communities can shift the balance of invertebrate assemblages towards those containing native species, this suggests a possible role for macroalgae in resisting invertebrate invasions. Both floating docks and marinas could thus be managed to enhance autotroph persistence and more studies in invasion biology could investigate facilitation, indirect effects, and interactions between organisms from different taxonomic groups.\u003c/p\u003e","manuscriptTitle":"The role of macroalgae in structuring a New England fouling community and the implications for floating dock management to ameliorate invasive species","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-27 14:03:55","doi":"10.21203/rs.3.rs-7706424/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-10-13T12:53:08+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-13T08:50:56+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Biological Invasions","date":"2025-10-01T14:55:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-25T11:43:20+00:00","index":"","fulltext":""},{"type":"submitted","content":"Biological Invasions","date":"2025-09-24T14:47:28+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"biological-invasions","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"binv","sideBox":"Learn more about [Biological Invasions](https://www.springer.com/journal/10530)","snPcode":"10530","submissionUrl":"https://submission.nature.com/new-submission/10530/3","title":"Biological Invasions","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"972fc855-abdf-496f-a738-b5bd55a83140","owner":[],"postedDate":"October 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-04-27T16:12:35+00:00","versionOfRecord":{"articleIdentity":"rs-7706424","link":"https://doi.org/10.1007/s10530-026-03810-w","journal":{"identity":"biological-invasions","isVorOnly":false,"title":"Biological Invasions"},"publishedOn":"2026-04-20 15:58:46","publishedOnDateReadable":"April 20th, 2026"},"versionCreatedAt":"2025-10-27 14:03:55","video":"","vorDoi":"10.1007/s10530-026-03810-w","vorDoiUrl":"https://doi.org/10.1007/s10530-026-03810-w","workflowStages":[]},"version":"v1","identity":"rs-7706424","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7706424","identity":"rs-7706424","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}