Filamentous calcareous alga provides a substrate for coral-competitive macroalgae in the degraded lagoon of Dongsha Atoll, Taiwan

Background The chemically-rich seaweed Galaxaura is not only highly competitive with corals, but also provides substrate for other macroalgae. Its ecology and associated epiphytes remain largely unexplored. To fill this knowledge gap, we herein undertook an ecological assessment to explore the spatial variation, temporal dynamics, and epiphytic macroalgae of G. divaricata on patch reefs in the lagoon of Dongsha Atoll, a shallow coral reef ecosystem in the northern South China Sea, repeatedly impacted by mass coral bleaching events. Methods Twelve spatially independent patch reefs in the Dongsha lagoon were first surveyed to assess the benthic composition in April 2016, and then revisited to determine G. divaricata percent cover in September 2017, with one additional Galaxaura -dominated reef (site 9). Four surveys over a period of 17 months were carried out on a degraded patch reef (site 7) to assess the temporal variation in G. divaricata cover. Epiphytic macroalgae associated with G. divaricata were quantified and identified through the aid of DNA barcoding. Results Patch reefs in the Dongsha lagoon were degraded, exhibiting relatively low live coral cover (5-43%), but high proportions of macroalgae (13-58%) and other substrates (rubble and dead corals; 23-69%). The distribution of G. divaricata was heterogeneous across the lagoon, with highest abundance (16-41%) in the southeast area. Temporal surveys from site 7 and photo-evidence from site 9 suggested that an overgrowth by G. divaricata was still present to a similar extend after 17 months and 3.5 years. Yet, G. divaricata provides a suitable substrate some allelopathic macroalgae (e.g., Lobophora sp.). Conclusions Our study demonstrates that an allelopathic seaweed, such as G. divaricata , can overgrow degraded coral reefs for extended periods of time. By providing habitat for harmful macroalgae, a prolonged Galaxaura overgrowth could strengthen negative feedback loops on degraded coral reefs, further decreasing their recovery potential.