Impact of Dredging on Diagenesis and Nutrient Release in a Restored Mediterranean Lagoon (Lake of Tunis, Tunisia)

Impact of Dredging on Diagenesis and Nutrient Release in a Restored Mediterranean Lagoon (Lake of Tunis, Tunisia) | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Impact of Dredging on Diagenesis and Nutrient Release in a Restored Mediterranean Lagoon (Lake of Tunis, Tunisia) Haifa Ben Mna, Walid Oueslati, Mohamed Amine Helali, Ayed Added This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4959425/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Three sediment cores from dredged and undredged areas of Northern lake of Tunis, a mediterranean lagoon in northern of Tunisia, were used to investigate behavior of nutrients related to diagenetic reactions in sediment and assess the release of reduced nitrogen and phosphorus from surface sediment to the water, 30 years after dredging. The results show diffusion from the sediment towards the water column. Expecting from the results that the degradation of organic matter and the resulting N and P fluxes would be greater in the dredged area due to the oxygenation of the environment, this process was more significant in the undredged area. A comparison with the pre-dredging sediments show that the influence of dredging is very remarkable over time. It has effectively reduced organic matter contents (TOC levels after dredging were 2 to 9,5 times lower) and consequently the fluxes of reduced nitrogen and phosphorus species across the sediment-water interface. Fluxes of ammonium decreased 153 times and those of phosphorus about 8 104 times. This explains the improvement in the quality of water and sediment in the northern lake of Tunis. Sediment dredging nutrients diffusives fluxes early diagenesis Mediterranean Lagoon Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Over the last decades, lake eutrophication has been a worldwide environmental problem. This phenomenon, caused by excessive inputs of nutrients, has led to serious problems for the aquatic ecosystem such as excessive harmful algal blooms and other aquatic plants, deterioration of water quality, fish and other aquatic organisms deaths, and loss of biodiversity (Yu et al., 2017 ). Previous studies have shown that because of internal phosphorus and nitrogen loading, eutrophication could last for many years even though the external sources of nutrients are reduced (Zhong et al., 2018 ; Sun et al., 2022). Various measures have been applied to manage lake eutrophication. Sediment dredging has been the most widely used technique worldwide to improve water quality and restore aquatic ecosystems by reducing nutrient release from sediment in many eutrophic lakes (Fan et al., 2004 ; Cao et al., 2007 ; Liu et al., 2016 ; Oldenborg and Steinman, 2019; Chen et al., 2019 ). Many previous studies revealed that dredging practices have reduced organic matter and internal nutrient loading in sediment (Kleeberg and Kohl, 1999 ; Zhong et al., 2008; Yu et al., 2017 ; Chen et al., 2018 ). Despite these encouraging results, some other studies have revealed that sediment dredging has not achieved the desired results, namely controlling nutrient release (Fan et al., 2004 ; Liu et al., 2016 ). The effectiveness of sediment dredging depends on several factors, such as the dredging technique, sediment characteristics, water body characteristics (depth, currents), external input, and dredging season (Kleeberg and Kohl, 1999 ; Liu et al., 2016 ; Zhong et al., 2018 ). The northern lake of Tunis was long used as a receptacle for wastewater and rainwater from the city of Tunis and neighboring towns (Ben Charada, 1997; Belkhir and Hadj Ali Salem, 1981). This pollution, which began in the early 20th century, led to a significant deterioration in the lake's water quality, resulting in eutrophication. This phenomenon resulted in massive mortality of organisms and the onset of dystrophic crises, leading to the release of foul-smelling odors during the summer season (Belkhir and Hadj Ali Salem, 1981; Zaouali, 1983 ; Turki and Hadj Ali, 1990 ). The increasingly alarming deterioration of the lake's condition prompted the Tunisian government to take urgent action, which resulted in a lake sanitation program. Restoration works took place between 1985 and 1988. It essentially consisted of dredging a large part of the lake, particularly in the north and the exchange channel with the sea. Approximately 10 million m3 of solid matter were dredged from the lake. Sediment dredging and restoration works at Tunis Lake have had a positive impact on this ecosystem. A clear reduction in nutrient levels and chlorophyll biomass was measured (Turki and Hadj Ali, 1990 ). The lake thus regained its equilibrium, with the disappearance of degenerative phenomena and the appearance of red water, as well as an increase in water transparency and a marked improvement in biological diversity with the return of marine animal and plant species. In the present study, we investigate nitrogen and phosphorus behavior related to diagenetic reactions in sediment collected from the dredged and undredged areas, while comparing the magnitude of nutrient release from the sediment-water interface to that measured before restoration and dredging carried out in the lagoon 30 years ago. 2. Experiments 2.1. Site description The Tunis Lake, located at the bottom of the Tunis Gulf to the east of Tunis City, is a Mediterranean lagoon complex separated into two parts, named North Lake and South Lake by the road leading from Tunis to La Goulette and the central navigation channel in a WSW-ENE direction (Fig. 1 ). The Tunis North Lake (36° 45'N – 36° 60'N and 10° 10'E – 10° 30'E), is the northern part of this lagoon complex covering an area of 24 km 2 (Chouba et al., 2010 ) and with an average water depth of 1.5m. It communicates with the Gulf of Tunis through Kheireddine Channel by means of a semi-diurnal tide (about 12 hours and 25 minutes) and the prevailing winds that allow the renewal of water and circulation inside the lake. In the past, before the restoration project, Tunis North Lake was regarded as one of the world’s most hypoxic coastal ecosystems, due to its location near Tunis city (Harbridge et al., 1976 ; Belkhir and Hadj Ali Salem, 1981; Pilkey et al., 1989 ; Chouba etal., 2010 ; Armi et al., 2010 ; Ben Charrada, 1992 ). The restoration works (carried out from 1986 to 1988) consisted essentially of the dredging and the deepening of a large part of the lake located at the north (Fig. 1 ). The depth of this dredged part became 2 to 4 m (Diawara et al., 2008 ). 2.2. Sediment sampling and porewater extraction Three sampling sites were selected in Tunis North Lake in October 2015: Sites LN1 and LN2 were chosen in the area of dredged sediment. The first site was located in the north of the dredging area, which is a highly urbanized area. The second site was near the stormwater outfalls, and site LN3 was selected in the southern part of the northern lake, an area that was not dredged. Sediment cores (70 cm in length and 75 mm in diameter) were collected manually in polyvinyl chloride tubes that were previously cleaned. The sediment cores were closed with rubber stoppers immediately after sampling to avoid sediment exposure to the atmosphere and were brought back to the laboratory. In the laboratory, all the cores were treated under a nitrogen atmosphere (N2) within four hours to avoid deterioration of the sediments. Sediment was laterally sliced into 2 cm sections within the top 20 cm, and 5 cm sections for the remaining sediment. Slicing was performed under an inert atmosphere (N2) using a glove box to avoid contact with oxygen. Each section, after measurement of pH and Eh (pH meter 82862 Weilheim WTW), was divided into two samples: one was immediately used for porewater extraction, and the other was dried at 50°C and preserved to analyze the TOC, carbonate, iron, and manganese contents of this sediment. Porewater was extracted under an inert atmosphere by centrifugation at 2500 rpm for 25 minutes. The centrifuged water samples and the overlying waters were filtered in an inert atmosphere through 0.45 µm filters. A water split was acidified with ultra-pure nitric acid to pH 2 and then stored at 4°C for dissolved iron and manganese analysis. The second water split was immediately used for nutrient and sulfate analysis. 2.3. Sediment analyses To determine the percentage of the fine fraction (particle diameter < 63µm), sediment recovered after centrifugation was separated into two size fractions: a fine fraction (particle diameter 63 µm). Separation was performed by wet sieving using a 63 µm nylon sieve and bi-distilled water. Organic Carbon content was determined by the ANNE method. This method allows the determination of the organic Carbon in the sediment by colorimetry after oxidation of the organic matter by potassium dichromate (K 2 Cr 2 O 7 -H 2 SO 4 ), in a sulfuric medium. Calcium carbonate content was measured using a Bernard calcimeter. Iron and manganese contents were measured by a flame spectrophotometer Thermo Scientific SOLAAR ICE 3300 series. 2.4. Porewater analyses Prior to iron and manganese analyzes, we used the iron co-precipitation technique developed by Welch et al. (1990) in order to precisely detrmine Fe and Mn concentrations by concentrating them and removing salts. Dissolved Fe and Mn were measured using flame spectrometer (Thermo Scientific ICE 3300 AA Spectrometer). Nutrients (phosphate, ammonium, nitrate and nitrites) in both pore and supernatant waters were analyzed using a Technicon AA3 AutoAnalyzer. Concentrations were determined according to methods described in Murphy and Riley ( 1962 ) for phosphate, Solorzano ( 1969 ) for ammonium, Bendschneider and Robinson (1952) for nitrite and Wood et al. ( 1967 ) for nitrate. Sulfates were measured with the Hoioarth ( 1978 ) method. The detection limits for analyzed species and the relative standard deviation (RSD) are summarized in Table 1 . Table 1 Detection limit and relative standard deviation of analyzed dissolved species. Dissolved species Detection limit RSD (%) NO 3 − (µmol l − 1 ) 0.05 0.17 NO 2 − (µmol l − 1 ) 0.01 0.08 NH 4 + (µmol l − 1 ) 0.01 0.37 PO 4 3− (µmol l − 1 ) 0.015 0.05 SO 4 2− (µmol l − 1 ) 0.02 2.2 Fe 2+ (µmol l − 1 ) 0.01 0.8 Mn 2+ (µmol l − 1 ) 0.002 1.4 2.5. Diffusive fluxes across the sediment-water interface (SWI) The diffusive flux (J) of a dissolved component was calculated from the porewater profile using Fick’s first law: $$\:\text{J}\:=\:-\:{\phi\:}\:.\:{\text{D}}_{\text{s}}\:.\left(\frac{\text{d}\text{C}}{\text{d}\text{z}}\right)$$ Where dC/dz represents the gradient at the sediment–water interface (Schulz and Zabel 2006). In applying Fick’s first law, the sediment was assumed to be in a steady state. The flux direction is opposite to the concentration gradient (when the concentration increased with depth, the flux was directed to the water column). Ф represents sediment porosity, D sed represents the molecular diffusion coefficient in the sediment, while D sed = D sw / θ² and θ² = 1 - ln (Ф²) which is related to the tortuosity ‘‘θ’’, temperature and chemical elements, according to Boudreau ( 1997 ). 3. Results 3.1. Sediment properties 3.1.1. The pH and Eh profiles Vertical variation of Eh throughout the cores shows a rapid decrease in the first 10 centimeters at all sampling sites and then values remain stable in the rest of the sediment columns. Values are positive in the first 2 centimeters and then conditions become anoxic in the rest of the sediment. In the deep layer, Eh reaches − 200 mV at LN1 site which is located in the dredged area. The variation of pH along the cores collected from the three sites is small (Fig. 2 ). Values range from 7,21 to 8 at LN1site, 7,08 to 8 at LN2 site, and 6,97 to 8 at LN3 site. Eh values are always lower than those measured in the water column (pH > 8). pH profiles at the three stations follow, overall, a constancy of values in the rest of the cores. 3.1.2. Main characteristics of the particulate fraction Total organic carbon (TOC) levels decrease slightly to very slightly in the first 10 centimeters of sediment in the three collected cores (Fig. 3 ). From a depth of 10 cm onwards the contents increase to reach maximum values at a depth of 15 cm at LN1 site (3.32%) and LN3 site (1.4%) and at a depth of 25 cm at LN2 site (2.1%). In the deeper sediments, TOC contents tend to decrease slightly. The highest TOC contents were measured throughout the core sampled at LN1 site. The results of the particle size analyses carried out on the sediments of cores lead to the same conclusions indicating a heterogeneous sediment in its texture (Fig. 3 ). Indeed, we noted fluctuations in the fine fraction (FF) profiles at three sampling sites that are related to the presence of large quantities of shell tests and plant filaments. The lithological aspect of sediment of the collected cores has been described previously in our study Oueslati et al. ( 2018 ). Indeed, the upper layer of LN1 and LN2 cores (taken from the dredging area) consists of a recent black mud rich in shell debris and plant filaments. Towards depth the hue of the old sediment becomes grey to light grey. LN3 core taken from the undredged area consists of black silt rich in shell fragments and plant filaments at the top and down to deeper levels. Oueslati et al ( 2018 ) showed that the presence of a section of sediment from 10 to 20 cm rich in test of the gastropod Hydrobia ventrosa at LN3 site and their absence in the sediments of LN1 and LN2 cores, was characteristic of a polluted sediment rich in organic matter before the restoration and dredging works. The carbonate content profiles did not show clear changes with depth (Fig. 3 ). Values are roughly constant along the collected cores reaching 60% at LN2 site and about 50% at LN1 and LN3 sites. The vertical distribution of particulate iron does not show significant variation along the collected cores (Fig. 3 ). Manganese distribution shows an overall increase towards the deeper layers of the sediment (Fig. 3 ). A slight decrease in concentrations was shown from the profiles of Fe and Mn contents in the surface layer of cores (0–4 cm). 3.2. Distribution of porewater major diagenetic counponds Nitrates (NO 3 −) profiles show some differences in concentrations among sites, but show the same trends (Fig. 4 ). Background concentrations are 12.54 µmol l − 1 at LN1 site, 24.57 µmol l − 1 at LN2 site, and 50.7 µmol l − 1 at LN3 site. Just below the sediment-water interface, levels increase with depth and peak in the first 6 centimeters at all three sites and then decrease rapidly with depth. The highest concentrations are measured at 2 cm depth at LN3 site located in the undredged zone (about 125 µmol l − 1 ). At LN1 site, the maximum levels reached at 2 cm depth are of the order of 101.3µmol l − 1 . At LN2 site, we notice on the nitrate profile the presence of three concentration peaks, the first is reached just below the SWI at 2 cm depth (50.8 µmol l − 1 ), the second which is the maximum is reached at 6 cm depth (95 µmol l − 1 ) and the third is reached at 12 cm depth (69.1 µmol l − 1 ). Nitrite (NO 2 − ) concentrations in the porewaters of three cores do not exceed 11.3 µmol l − 1 (Fig. 4 ). Their distribution with depth show a rapid increase in the first 6 cm of depth to reach peaks of 3µmol l − l (4 cm depth), 11.3 µmol l − 1 (6 cm depth), and 7.9 µmol l − 1 (2 cm depth) at LN1, LN2 and LN3 sites respectively. This increase is followed by a rapid decrease. Beyond 10 cm depth, new concentration peaks are noted at different levels of the cores: around 11 cm depth for LN2 and LN3 and around 30 cm depth for LN1 and LN3. This new increase is also followed by a rapid decrease. In the deep sediments, nitrite concentrations are low. Ammonium (NH 4 + ) concentrations increase rapidly below the SWI within 10–12 cm depth at all three study sites, reaching peaks of 126.1 µmol l − 1 at LN2 site and 190.45 µmol l − 1 at LN3 site at 10 cm depth (Fig. 4 ). At LN1 site, the maximum content (around 112.5 µmol l − 1 ) is reached at 20 cm depth. Beyond 10 cm there is a rapid decrease in concentrations at LN2 and LN3 sites. This is followed by a slight increase towards the deeper layers. In the first 10 cm, the highest NH 4 + concentrations are measured at LN3 site which is located in the undredged area. The vertical evolution of PO 4 3− is similar to that of NH 4 + at all three sites (Fig. 4 ). It mainly concerns the first 10–12 centimeters. It is characterized by an increase in concentrations below SWI until about 12 cm depth where concentrations decrease slightly towards the deep sediment layer. The highest concentrations are measured at LN2 site. Dissolved manganese (Mn 2+ ) is detected at the SWI at concentrations below 0.3 µmol l − 1 . As soon as the interface is crossed, we observe in the first few centimeters a significant production with a maximum that varies from site to site (Fig. 4 ). At LN1 site, the maximum concentration peak (1.24 µmol l − 1 ) is reached at 2 cm depth, while it is reached 6 cm below the SWI at LN2 site (of the order of 4.7 µmol l − 1 ) and at 8 cm depth at LN3 site (of the order of 4.35 µmol l − 1 ). This maximum production is quickly followed by a sharp decrease (concentrations reach low values). The vertical variation of dissolved iron (Fe 2+ ) is similar to that of Mn 2+ (Fig. 4 ). Indeed, we observe an increase in concentrations below the SWI to reach maximum values in the first 10 centimeters. The maximum concentration peak is reached at 4 cm depth at LN1site (3.59 µmol l − 1 ), at 6 cm at LN2 site (5.22 µmol l − 1 ) and at 8 cm depth at LN3 site (about 24.98 µmol l − 1 ). As for Mn 2+ , the maximum production of Fe 2+ is rapidly followed by a sharp decrease in concentrations. The maximum concentrations of dissolved Fe and Mn are measured at LN3 site located in the undredged area. Vertical variation of sulfates (SO 4 2− ) shows the same trends and very similar concentrations at three study sites (Fig. 4 ). In fact, for all the profiles, a decrease in content is noted from the SWI to the deep layer, with slight subordinate increases around 10 to 12 cm in depth. 4. Discussions 4.1. Basic sediment properties after the restoration works Considering the average rate of sedimentation in the northern lake of Tunis which is about 0,15 cm yr − 1 according to Added ( 2002 ), the surface layer of about 5 cm thickness at LN1 and LN2 sites located in the dredged area corresponds to a recent layer deposited after the restoration works. Beyond this depth, the sediment corresponds to older mud deposited before the dredging works. At the LN3 site located in the undredged area, the sediment column reflects the state of the lake before and after the restoration works. Indeed, considering the lithological aspect of the collected cores, Oueslati et al. ( 2018 ) were able to distinguish a sediment section located between 10 and 20 cm depth in the LN3 core rich in shells of the gastropod Hydrobia ventrosa, characteristic of the northern lake of Tunis before the restoration work. The slight depletion in TOC and the rapid decrease in pH and Eh in the first centimeters at the three sites suggest the presence of classical diagenetic processes in the surface layer. Indeed, a classical sedimentation model shows that organic matter undergoes bacterial degradation as soon as it is deposited on the sediment surface. This results in the anoxia of the sediment and the release of hydrogene ion in the porewater. According to this study, we note that the environment has evolved differently. Indeed, the degradation of organic matter is more significant in the undredged site than in the dredged area. In addition, sediment anoxia is higher in the dredged area than in the undredged area. This could be explained by the greater input of nutrients to the dredged area than to the undredged one. In fact, according to Turki and Hadj Ali ( 1990 ), nitrate and chlorophyll a levels were higher in the dredged area, despite the restoration and dredging works. Added ( 2002 ) studied diagenesis processes in cores taken from the northern lake of Tunis in 1986 just before dredging. TOC levels were on the order of 6,6% in the surface layers. When compared to the levels found in this study, they are 2 to 9,5 times higher. This was mainly due to the eutrophic state of the lake at that time. The high concentrations observed in the deep layer are probably the result of the accumulation of organic matter over the last few years (93 years is the time estimated by a sedimentation rate equal to 0.15 cm yr − 1 and a depth of 14 cm at the LN3 site). It is likely that during this time the depositional conditions were more favorable for the preservation of organic matter than at present. This phenomenon is also observed by Added ( 2002 ) in the cores taken from the northern lake of Tunis in 1986 just before the dredging works and explained it also by a preservation of organic matter 333 years before, considering the average rate of sedimentation in the lake. On the other hand, the increase with depth of the TOC contents could testify to a more important contribution of organic matter to the lake in the past which seems to have decreased after the restoration and dredging works carried out on the lake. When examining the lithological aspect of the LN2 core described by Oueslati et al. ( 2018 ), we notice that the maximum TOC contents are measured at 22–33 cm depth, in a layer constituted by greenish-gray mud rich in plant filaments. 4.2. Early diagenetic reactions 4.2.1. Nitrification, denitrification and ammonium release In undisturbed suboxic sediments, nitrate profiles showed generally a production below the SWI that is usually attributed to the bacterial nitrification, followed by a decrease in underlying levels resulting from denitrification processes induced by aerobic denitrifying bacteria (Froelich et al., 1978; Added, 1981 ; Added, 2002 ). At all three study sites, the nitrate profiles show levels evolving according to the theoretical model described above. In LN2 and LN3 sites, nitrate regeneration in the reducing sediments just below the thin oxic zone appears to be due to the oxidation of NH 4 + by oxygen or manganese oxide advected to depth by benthic macrofauna. Note that Oueslati et al. ( 2018 ) highlighted the presence of lamellibrach and gastropod shells and some annelids in the sediments. In addition, particulate Mn profiles show high concentrations in these sediment levels. This process has been observed by several authors such as Hyacinthe et al. ( 2001 ) in the sediments of Biscay Bay and Ben Mna et al. ( 2022 ) in the sediments of Bizerte Lagoon. Nitrites peak concentrations in the suboxic surface sediments and in the reducing sediments of the subjacent zone seems to be due to nitrate production related to the mineralization of organic matter by oxic respiration just below the sediment-water interface and by nitrification in the subjacent zone. Ammonium is the predominant dissolved nitrogen specie at three study sites. This is in agreement with the reducing conditions of the environment since the production of NH 4 + ions could only take place at Eh values lower than 260 mV (Fernex et al., 1989 ). In the first 10 to 12 cm of the sediment at the three sites, it was evident that there was a negative correlation between NH 4 + levels in the porewater and TOC levels in the sediment. This led us to conclude that the maximum production of ammonium ions in the top layer of the cores is clearly related to the anaerobic mineralization of organic nitrogen species contained in the organic matter. By examining the NH 4 + and TOC profiles, we arrived at unexpected results. Indeed, the maximum ammonification occurred at LN3 site (located in the undredged area) where the bacterial oxidation of the organic matter is more significant than at the other two sites. However, it was expected to see a more important bacterial oxidation of organic matter and consequently, a significant diagenesis of organic nitrogen (nitrification under the SWI and subsequent ammonification in the subjacent sediment) at the dredged sites (LN1 and LN2) taking into account the remediation works carried out and the current good environmental status of these sites. 4.2.2. Phosphate release The relatively high production of PO 4 3− ions in the first 10–12 cm is similar to that of NH 4 + and is certainly attributed to the bacterial mineralization of phosphorus compounds contained in organic matter. The homogenization of low concentrations at LN1 site could be attributed, on the one hand, to a significant bioturbation activity given that the sediment is rich in lamellibranch shells as reported by our previous work Oueslati et al. ( 2018 ), and on the other hand, to co-precipitation or adsorption on sedimentary mineral particles. Indeed, the high affinity of phosphorus for iron carbonates and oxy-hydroxides limits the concentration of PO 4 3− in porewaters (Added, 2002 ; Zaaboub et al., 2014 ; Ben Mna et al., 2022 ). Moreover, the study of the chemical fractionation of phosphorus conducted by our co-author Added ( 2002 ) on the sediments of the cores taken from the North Lake of Tunis just a few months before the restoration works, shows that mineral phosphorus accounted for 73% of the total phosphorus in the sediment and was the potential source of phosphates in the porewaters of the lake. It should be noted that the LN1 core sediments contain sufficient amounts of carbonate and relatively small amounts of particulate Fe. 4.2.3. Manganese and iron behavior We can conclude from the vertical distribution of dissolved manganese and iron that these species followed the typical behavior during the mineralization of organic matter in a sub-oxic sediment. We note in fact, a release of Mn 2+ and Fe 2+ ions in porwater of the surface layer followed by a rapid decrease. This behavior results, obviously, from the reduction of manganese and iron oxides and hydroxides following the anaerobic degradation of organic matter, then from the diffusion of dissolved Fe and Mn towards the thin surface oxic layer and their re-oxidation by oxygen and / or nitrates into iron and manganese oxides as was widely observed in several coastal environments (Hansen et al., 1994 ; Hyacinthe et al., 2001 ; Ben Mna et al., 2022 ), or towards the deep anoxic layers and their precipitation in the form of sulphides and / or carbonates ( Franklin and Morse, 1983 ;Huerta-Diaz and Morse, 1992 ; Caplat et al., 2005 ; Ben Mna et al., 2022 ). Note that the analysis of carbonate (Fig. 3 ) and sulfide levels in the sediment of the cores at the three sites (Oueslati et al., 2018 ) shows sufficient quantities of both metal substrates. The area of Mn 2+ production at LN2 and LN3 sites located in a slightly deep level compared to the LN1 site, was obviously related to the presence of bioirrigation phenomena in the upper sediment levels at LN2 and LN3 sites.According to the lithological description made by Oueslati et al. ( 2018 ), the upper layer at these sites is, already, rich in lamellibranch and gastropod shells. In addition, we measured high nitrate levels in this upper layer at the latter sites. 4.2.4. Sulfates The sulfate profiles indicate, overall, a typical behavior found in sediments. The decrease below the sediment-water interface is, in fact, attributed to sulfate reduction by sulfate-reducing bacteria during anaerobic degradation of organic matter. The low concentrations in the deep sediment layer reflect significant sulfate reduction at three study sites. This is indicated by the presence of sulfides in the sediment (Oueslati et al., 2018 ). 4.3. Effects of restoration works on some early diagenetic reactions We compared the results found in this study with those found by our co-author Added ( 2002 ) in 1986, just a few months before the dredging remediation work. The results of Added(2002) are shown in the Fig. 5 . Before the remediation and dredging works, maximal ammonium content measured was about 68 times higher compared with the current contents, indicating that organic nitrogen diagenesis in the Nordh Lake of Tunis was significant at the time. In addition, the vertical evolution of NH 4 + levels before the remediation work reflects the intense bacterial degradation of organic matter at the time. Similarly, for phosphate, the maximum content measured in the interstitial waters of core samples taken prior to lake restoration was of the order of 38 µmol l − 1 (Fig. 5 ). This is 8 times higher than that measured in this study, reflecting, as we have just concluded, the intense diagenesis of organic matter prior to lake restoration. Phosphate release at that time was essentially limited to the first ten centimetres, as is the case today. The decrease in PO 4 3− levels in deep sediments, either before or after lake restoration, probably corresponds to phosphate precipitation. Indeed, Added ( 2002 ) has shown that fluorapatite, hydroxyapatite and whitlockite were phosphate minerals likely to precipitate in the Nordh Lake of Tunis. Mn and Fe concentration are broadly comparable to those measured in 1984 and 1985 by Added ( 2002 ). These low concentrations before and after the restoration of the lake can be explained by the iron sulphides precipitation on the one hand, and the adsorption of Mn 2+ ions on these minerals on the other. Indeed, it has been shown that the Mn 2+ ion does not form sulphides easily, but is incorporated into pyrite in anoxic sediments with a high degree of pyritization (Morse and Luther, 1999 ). Knowing that the sediments of the northern lake of Tunis are favorable to pyritization (Oueslati et al., 2018 ). Compared to those measured by Added ( 2002 ) prior to remediation, SO 4 2− concentrations measured at the three study sites are slightly low. Concentrations prior to remediation remained quite high in the porewaters (Fig. 5 ), despite the significant sulfate-reduction signaled by the presence of sulfides (Added, 2002 ). The distribution of SO 4 2− at that time was globally dominated by the following processes (Added, 2002 ): a significant sulfate reduction in June, sulfide oxidation in November and March, bioirrigation in April, and wave-induced water mixing in October. The first process led to a significant decrease in sulfate contents near the interface, the second led to an increase in SO 4 2− contents, and the third and fourth processes allowed for a homogenization of sulfate contents between the supernatant water and the porewater. Thus, contrary to the current state in the lake, sulfate reduction was not total in the sediment before restoration. 4.3. Effects of dredging on the nitrogen species and phosphate fluxes across the SWI The estimated fluxes of nitogen species (nitrate, nitrite and ammonium) and phosphate show diffusion from the sediment towards the water column (Table 2 ). The diffusion of nitrates and nitrites towards the water column results from their release just below the sediment-water interface following bacterial nitrification. Positive fluxes of ammonium is related to release of these species into porewater via diagenetic reactions in the sediment. We note that the nitrate, nitrite and ammonium gradients are greater at LN3 station located in the undredged area. Compared to the ammonium fluxes calculated in the North Lake of Tunis before the remediation works (27800 µmol. m 2 .d − 1 ), those estimated in the present work are 153 times lower (Table 3 ). This important decrease could be explained by the nitrification of ammonium ions into nitrites and nitrates in the thin oxic surface layer which acts as a trap for these reducing species produced from the anaerobic mineralization of organic nitrogen in depth and diffused upwards near to the sediment-water interface. However, before the restoration and dredging, the conditions in the lake were eutrophic. The low diffusion fluxes of phosphorus ions could be explained by their accumulation in the surface sediments by adsorption and/ or co-precipitation with mineral substrates, particularly carbonates. The fluxes of phosphates in the North Lake of Tunis before the remediation works (60200 µmol.m 2 .d − 1 ) are strongly higher than the calculated fluxes at present (about 8 104 times higher). Table 2 Diffusive fluxes J(µmol m 2 d − 1 ) of nitrogen species (NO 3 − , NO 2 − and NH 4 + ) and phosphate (PO 4 3− ) across the SWI in the North Lake of Tunis. J(µmol m 2 d − 1 ) NO 3 − NO 2 − NH 4 + PO 4 3− LN1 191.57 2.73 55.33 0.16 LN2 84.90 6.40 14.92 0.76 LN3 240.87 14.64 181.70 0.31 Table 3 Fluxes (mmol m 2 d − 1 ) calculated by Added ( 2002 ) with the concentration gradients at the SWI in the North lake of Tunis before its restoration (during the year 1986). January April June August October The average fluxe NH 4 + 3.48 0.27 27.8 0 2.24 6.76 PO 4 3− 60.2 10.3 0 5 51.7 25.4 Conclusions The dredging carried out in the Northern Lake of Tunis (a formerly heavily polluted lake) thirty years before the date of this present sampling effectively improved the quality of the water and sediment of the lake. TOC concentrations and fluxes of reduced nitrogen and phosphates from sediment to water column have decreased signifantly after dredging. However comparig the dredged area with the undrdged area, we can conclude that it would be better to reduce the external loading of nutrient to the dredged area because the conditions are favorable to a return to the former eutropic state. Indeed, the degradation of organique matter in the undregded area is more significant than in the dredged area. The anoxia is higher in the dredgd site. Declarations Author Contribution Haifa Ben Mna wrote the main manuscript text. Walid Oueslati did the analysis and wrote a part of the text. Mohamed Amine Helali prepared figures and wrote a part of the text. Ayed Added wrote a part of the text. All authors reviewed the manuscript. References Added, A. (2002). Cycles biogéochimiques des sels nutritifs, du fer, du manganèse et du soufre dans les sédiments de deux systèmes côtiers du nord de la Tunisie: Lagune de Ghar El Melh et Lac Nord de Tunis. PhD thesis, Univ. Tunis II, Fac. Sci. Tunis, p. 266. Added, A. (1981). Etude géochimique et sédimentologique des sédiments marins du Delta du Rhône. PhD thesis. Univ. Pierre et Marie Curie Paris VI, p. 263. Armi, Z., Turki S., Trabelsi, E. & Ben Maïz, N. (2010). First recorded proliferation of Coolia monotis (Meunier, 1919) in the North Lake of Tunis (Tunisia). Environ. Monit.Assess. 164, 423–433. Belkhir, M. & Hadj Ali Salem, M. (1981). Contribution à l’étude des mécanismes l’eutrophisation dans le lac de Tunis : évolution des paramètres physico-chimique et biologiques. Bull. Inst. Natn. Scient. Tech Océanogr. Pêche Salammbo 8, 81–98. Ben Charrada, R. (1992). Le lac de Tunis après les aménagements. Paramètres physico-chimiques de l’eau et relation avec la croissance des macro-algues. Mar. Life 1, 29–44. Ben Charrada, R. (1997). Etude hydrodynamique et écologique du Golfe de Tunis. . PhD thesis, Univ. Tunis II, Fac. Sci. Tunis, p. 319. Bendschneider, K. & Rex J. R. (1952). A new spectrophotometric method for the determination of nitrate in sea water. J. Mar. Res. 11, 87-96. https://elischolar.library.yale.edu/cgi/viewcontent.cgi?article=1760&context=journal_of_marine_research https://elischolar.library.yale.edu/journal_of_marine_research/761 Ben Mna, H., Alsubih, M., Oueslati, W., Helali, M.A., Amri, S., Added, A. & Aleya, L. (2022). Diagenetic processes and nutrients diffusive fluxes at the sediment-water interface in the Bizerte Lagoon (North Tunisia). J. Afr. Earth Sci. 196, 104671. Boudreau, B. (1997). Diagenetic models and their implementation: modelling transport and reactions in aquatic sediments. Springer-Verlag, Berlin, 414 pp. Cao, X.Y., Song, C.L., Li, Q.M. & Zhou, Y.Y. (2007). Dredging effects on P status and phytoplankton density and composition during winter and spring in Lake Taihu, China. Hydrobiologia 581, 287–295. Caplat, C., Texier, H., Barillier, D. & Lelievre, C. (2005). Heavy metals mobility in harbour contaminated sediments: the case of Port-en-Bessin. Mar. Pollut. Bull. 50, 504–511. Chen, C., Kong, M. Wang, Y.Y. Shen, Q.S., Zhong, J.C. & Fan, C.X. (2019). Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China. Environ. Sci. Poll. Res. 27, 25861-25869. Chen, M., Cui, J., Lin, J., Ding, S., Gong, M., Ren, M. & Tsang, D.C. (2018). Successful control of internal phosphorus loading after sediment dredging for 6 years: a field assessment using high-resolution sampling techniques. Sci. Total Environ. 616, 927–936. Chouba, L., Chebil, L.A. & Herrey, S. (2010). Etude saisonnière de la contamination métallique des macroalgues de la lagune nord de Tunis. Bull. Inst. Natn. Scien. 37, 123–131. Diawara, M., Zouari-Tlig, S., Rabaoui, L. & Ben Hassine, O.K. (2008). Impact of management on the diversity of macrobenthic communities in Tunis north lagoon : systematics. Cah. Biol. Mar. 49, 1-16. Fan, C., Zhang, L.,Wang, J., Zheng, C., Gao, G. & Wang, S. (2004). Processes and mechanismof effects of sludge dredging on internal source release in lakes. Chin. Sci. Bull. 49, 1853–1859. Fernex, F., Baratie, R., Span, D. & Vandelei Fernandes, L. (1989). Variations of nitrogen nutrient concentrations in the sediment pore waters of the northwestern Mediterranean continental shelf. Continent. Shelf Res. 9, 767–794. Franklin, M.L. & Morse, J.W. (1983). The interaction of manganese(II) with the surface of calcite in dilute solutions and seawater. Mar. Chem. 12, 241–254. Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. & Maynard, V. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic : suboxic diagenesis. Geochim. Comochim. Acta 43, 1075–1090. Hansen, H.C.B., Borggaard, O.K. & Sorensen, J. (1994). Evaluation of the free energy of formation of Fe(II)-Fe(III)-hydroxyde-sulphate (green rust) and its reduction of nitrite. Geochim. Cosmochim. Acta 58, 2599–2608. Harbridge,W., Pilkey, O.H., Whaling, P. & Swetland, P. (1976). Sedimentation in the Lake of Tunis: A Lagoon Strongly Influenced by Man.Environmental Geology, 1 : 215-225. Hoioarth R. W. (1978). A rapid and precise method for determining sulfate seawater, estuarine waters, and sediment pore waters.Limnol. Oceanogr. 23, 1066-1069. https://doi.org/10.4319/lo.1978.23.5.1066 Huerta-Diaz, M.A. & Morse, J. (1992). Pyritization of trace metals in anoxic marine sediments. Geochim. Cosmochim. Acta 56, 2681–2702. Hyacinthe, C., Anschutz, P., Carbonel, P., Jouanneau, J.M. & Jorissen, F.J. (2001). Early diagenetic processes in the muddy sediments of the Bay of Biscay. Mar. Geol. 177, 111–128. Kleeberg, A. & Kohl, J.G. (1999). Assessment of the long-term effectiveness of sediment dredging to reduce benthic phosphorus release in shallow Lake Müggelsee (Germany). Hydrobiologia 394, 153–161. Liu, C.; Zhong, J.C.; Wang, J.J.; Zhang, L. & Fan, C.X. (2016). Fifteen-year study of environmental dredging effect on variation of nitrogen and phosphorus exchange across the sediment-water interface of an urban lake. Environ. Pollut. 219, 639–648. Morse, J.W. & Luther III G.W. (1999). Chemical influences on trace metal interactions in anoxic sediments. Geochim. Acta 63, 3373-3378. Murphy, J.& Riley, J.P. (1962). A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27, 31–36. Kimberly, A.O. & Alan D.S. (2019). Impact of sediment dredging on sediment phosphorus flux in a restored riparian wetland. Sci. Total Environ. 650, 1969–1979. Oueslati, W. Helali, M.A., Mensi, I. Bayaoui, M., Touati, H., Khadraoui, A., Zaabooub, N., Added, A. & Aleya, L. (2018). How useful are geochemical and mineralogical indicators in assessing trace metal contamination and bioavailability in a post-restoration Mediterranean lagoon? Environ. Sci. Poll. Res. 25, 25045–25059. Pilkey, O., Heron, D., Harbridge, W. Keer, F. & Thoronton, S. (1989). The sedimentology of three Tunisian lagoons. Marin Geology 88, 285-301. Schultz H.D. & Zabel M., (2006). Marine Geochemistry. Springer-Verlag, Berlin, Heidelberg, 2nd 869 870 edition. Solorzano, L. (1969). Determination of Ammonia in Natural Waters by the Phenolhypochlorite Method. Limnol. Oceanogr. 14, 799-801. Sun.C., Wang, S., Wang, H., Hu, X., Yang, F.,Tang, M., Zhang, M. & Zhong, J. (2022). Internal nitrogen and phosphorus loading in a seasonally stratified reservoir: Implications for eutrophication management of deep-water ecosystems. J. Environ. Manage. 319, 115681. Turki, S. & Hadj Ali, M. (1990). Evolution de l’état trophique dans le lac de tunis (partie Nord). Bull. Natn. Scient. Tech. Océanogr. Pèche Salammbo. 17, 61-74. Wood, E.D., Armstrong, F.A.J. & Richards, F.A. (1967). Determination of Nitrate in Seawater by Cadmium-Copper Reduction to Nitrite. Journal of the Mar. Biol. Assoc. U.K. 47, 23-31. http://sci-hub.tw/10.1017/S002531540003352X Yu, J.H.; Ding, S.M.; Zhong, J.C.; Fan, C.X.; Chen, Q.W.; Yin, H.B.; Zhang, L. & Zhang, Y.L. (2017). Evaluation of simulated dredging to control internal phosphorus release from sediments: Focused on phosphorus transfer and resupply across the sediment-water interface. Sci. Total Environ. 592, 662–673. Zaaboub, N., Ounis, A., Helali, M.A., Béjaoui, B., Lillebø, A.I., Ferreira da Silva, E. & Aleya, L. (2014). Phosphorus speciation in sediments and assessment of nutrient exchange at the water-sediment interface in a Mediterranean lagoon: implications for management and restoration. Ecol. Eng. 73, 115–125. Zaouali, J. (1983). Lac de Tunis: 3000 years of engieering and pol­lution. A bibliographical study with comments. UNESCO, Rapp. Mar. Sci. 26, 30-47. Zhong , J.C., Yu, J.H., Zheng, X.L.,Wen, S.L., Liu, D.H. & Fan, C.X. (2018). Effects of dredging season on sediment properties and nutrient fluxes across the sediment–water interface in Meiliang Bay of Lake Taihu, China. Water 10(1606), 1–16. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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. 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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-4959425","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":357927458,"identity":"6696ac11-afec-40f1-baac-e01d360f53f9","order_by":0,"name":"Haifa Ben Mna","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYBACAwYGNiAlx8AvAeZLyDAwHyBKizGD5AwGxgagFh4GtgQitRjcAGthIKzFXOwA24OfbQbyxrebjz+6UWMB1ML7AK8Wy9kJ7Ia9bQaG2+4cS2zOOQZyGLsBfofdTmCT4G37w7jtRo5hcw4bUIt8GwG/ALVI/m0zsN88A6TlH8gWNsJapHnbDBI3SAC15LYRocVydmKbtMw5g+QZN9ISZ+f2SfCwEdJiLp18TPJNmYFt/4zkA59zvtXJ8RPSwgCKDUZkRQQ1QMAf4pSNglEwCkbBCAUAjfE9CFhcDokAAAAASUVORK5CYII=","orcid":"","institution":"Université Tunis-El Manar","correspondingAuthor":true,"prefix":"","firstName":"Haifa","middleName":"Ben","lastName":"Mna","suffix":""},{"id":357927459,"identity":"d5ba5f81-23bc-48e1-96b9-0c71a7ee2be4","order_by":1,"name":"Walid Oueslati","email":"","orcid":"","institution":"Université Tunis-El Manar","correspondingAuthor":false,"prefix":"","firstName":"Walid","middleName":"","lastName":"Oueslati","suffix":""},{"id":357927460,"identity":"a6df98c4-e40b-43de-806a-f30a2ea1bcad","order_by":2,"name":"Mohamed Amine Helali","email":"","orcid":"","institution":"Université Tunis-El Manar","correspondingAuthor":false,"prefix":"","firstName":"Mohamed","middleName":"Amine","lastName":"Helali","suffix":""},{"id":357927461,"identity":"1c28c888-6da5-4e8e-81f3-628cec8b6da0","order_by":3,"name":"Ayed Added","email":"","orcid":"","institution":"Université Tunis-El Manar","correspondingAuthor":false,"prefix":"","firstName":"Ayed","middleName":"","lastName":"Added","suffix":""}],"badges":[],"createdAt":"2024-08-22 16:21:59","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4959425/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4959425/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":65459900,"identity":"c50b79b8-4572-4415-a624-8a7ec25d6f42","added_by":"auto","created_at":"2024-09-27 17:24:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":625535,"visible":true,"origin":"","legend":"\u003cp\u003eStudy area and location of sampling sites.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/68eb0d2ea83d7a01a907bb33.png"},{"id":65459141,"identity":"ea98bb8c-fb36-4e8c-9b2a-15906c673885","added_by":"auto","created_at":"2024-09-27 17:08:54","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":21984,"visible":true,"origin":"","legend":"\u003cp\u003eDepth profiles of Eh and pH at LN1, LN2 and LN3 sites.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/cc4c8c37bd181bef62d90df7.png"},{"id":65458532,"identity":"421bb4b4-ad19-4d3a-b9c3-fd7301b7eaac","added_by":"auto","created_at":"2024-09-27 17:00:54","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":42322,"visible":true,"origin":"","legend":"\u003cp\u003eDepth profiles of particulate organic carbon TOC, fine fraction (FF), carbonates, iron and manganese in sediment at LN1, LN2 and LN3 sites.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/f4b8965e15314a3f2a9a39e0.png"},{"id":65459518,"identity":"19e28610-736a-46b1-990a-e38387ec6fc3","added_by":"auto","created_at":"2024-09-27 17:16:54","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":78437,"visible":true,"origin":"","legend":"\u003cp\u003eDepth profiles of major diagenetic compounds.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/096b5c8e2b8c3161ebce20f8.png"},{"id":65459139,"identity":"a2d1845c-de85-4178-a19d-63cd420451a5","added_by":"auto","created_at":"2024-09-27 17:08:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":51364,"visible":true,"origin":"","legend":"\u003cp\u003eVertical distribution of ammonium, phosphate and dissolved iron and manganese before remdiation works according to Added (2002).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/dcb9b743d164330f0a54d518.png"},{"id":66786168,"identity":"0fd0581f-820a-4fe9-89fb-4d8f11b7bda7","added_by":"auto","created_at":"2024-10-16 12:53:52","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1532802,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4959425/v1/afb0c620-55c2-4142-a30c-8ab69ec5eb5b.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Impact of Dredging on Diagenesis and Nutrient Release in a Restored Mediterranean Lagoon (Lake of Tunis, Tunisia)","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOver the last decades, lake eutrophication has been a worldwide environmental problem. This phenomenon, caused by excessive inputs of nutrients, has led to serious problems for the aquatic ecosystem such as excessive harmful algal blooms and other aquatic plants, deterioration of water quality, fish and other aquatic organisms deaths, and loss of biodiversity (Yu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Previous studies have shown that because of internal phosphorus and nitrogen loading, eutrophication could last for many years even though the external sources of nutrients are reduced (Zhong et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Sun et al., 2022).\u003c/p\u003e \u003cp\u003eVarious measures have been applied to manage lake eutrophication. Sediment dredging has been the most widely used technique worldwide to improve water quality and restore aquatic ecosystems by reducing nutrient release from sediment in many eutrophic lakes (Fan et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Cao et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Oldenborg and Steinman, 2019; Chen et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Many previous studies revealed that dredging practices have reduced organic matter and internal nutrient loading in sediment (Kleeberg and Kohl, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Zhong et al., 2008; Yu et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Chen et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDespite these encouraging results, some other studies have revealed that sediment dredging has not achieved the desired results, namely controlling nutrient release (Fan et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). The effectiveness of sediment dredging depends on several factors, such as the dredging technique, sediment characteristics, water body characteristics (depth, currents), external input, and dredging season (Kleeberg and Kohl, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Liu et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Zhong et al., \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe northern lake of Tunis was long used as a receptacle for wastewater and rainwater from the city of Tunis and neighboring towns (Ben Charada, 1997; Belkhir and Hadj Ali Salem, 1981). This pollution, which began in the early 20th century, led to a significant deterioration in the lake's water quality, resulting in eutrophication. This phenomenon resulted in massive mortality of organisms and the onset of dystrophic crises, leading to the release of foul-smelling odors during the summer season (Belkhir and Hadj Ali Salem, 1981; Zaouali, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e1983\u003c/span\u003e; Turki and Hadj Ali, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). The increasingly alarming deterioration of the lake's condition prompted the Tunisian government to take urgent action, which resulted in a lake sanitation program. Restoration works took place between 1985 and 1988. It essentially consisted of dredging a large part of the lake, particularly in the north and the exchange channel with the sea. Approximately 10\u0026nbsp;million m3 of solid matter were dredged from the lake.\u003c/p\u003e \u003cp\u003eSediment dredging and restoration works at Tunis Lake have had a positive impact on this ecosystem. A clear reduction in nutrient levels and chlorophyll biomass was measured (Turki and Hadj Ali, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1990\u003c/span\u003e). The lake thus regained its equilibrium, with the disappearance of degenerative phenomena and the appearance of red water, as well as an increase in water transparency and a marked improvement in biological diversity with the return of marine animal and plant species.\u003c/p\u003e \u003cp\u003eIn the present study, we investigate nitrogen and phosphorus behavior related to diagenetic reactions in sediment collected from the dredged and undredged areas, while comparing the magnitude of nutrient release from the sediment-water interface to that measured before restoration and dredging carried out in the lagoon 30 years ago.\u003c/p\u003e"},{"header":"2. Experiments","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Site description\u003c/h2\u003e \u003cp\u003eThe Tunis Lake, located at the bottom of the Tunis Gulf to the east of Tunis City, is a Mediterranean lagoon complex separated into two parts, named North Lake and South Lake by the road leading from Tunis to La Goulette and the central navigation channel in a WSW-ENE direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The Tunis North Lake (36\u0026deg; 45'N \u0026ndash; 36\u0026deg; 60'N and 10\u0026deg; 10'E \u0026ndash; 10\u0026deg; 30'E), is the northern part of this lagoon complex covering an area of 24 km\u003csup\u003e2\u003c/sup\u003e (Chouba et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and with an average water depth of 1.5m. It communicates with the Gulf of Tunis through Kheireddine Channel by means of a semi-diurnal tide (about 12 hours and 25 minutes) and the prevailing winds that allow the renewal of water and circulation inside the lake.\u003c/p\u003e \u003cp\u003eIn the past, before the restoration project, Tunis North Lake was regarded as one of the world\u0026rsquo;s most hypoxic coastal ecosystems, due to its location near Tunis city (Harbridge et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e1976\u003c/span\u003e; Belkhir and Hadj Ali Salem, 1981; Pilkey et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e1989\u003c/span\u003e; Chouba etal., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Armi et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Ben Charrada, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1992\u003c/span\u003e). The restoration works (carried out from 1986 to 1988) consisted essentially of the dredging and the deepening of a large part of the lake located at the north (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The depth of this dredged part became 2 to 4 m (Diawara et al., \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2008\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Sediment sampling and porewater extraction\u003c/h2\u003e \u003cp\u003eThree sampling sites were selected in Tunis North Lake in October 2015: Sites LN1 and LN2 were chosen in the area of dredged sediment. The first site was located in the north of the dredging area, which is a highly urbanized area. The second site was near the stormwater outfalls, and site LN3 was selected in the southern part of the northern lake, an area that was not dredged. Sediment cores (70 cm in length and 75 mm in diameter) were collected manually in polyvinyl chloride tubes that were previously cleaned. The sediment cores were closed with rubber stoppers immediately after sampling to avoid sediment exposure to the atmosphere and were brought back to the laboratory. In the laboratory, all the cores were treated under a nitrogen atmosphere (N2) within four hours to avoid deterioration of the sediments. Sediment was laterally sliced into 2 cm sections within the top 20 cm, and 5 cm sections for the remaining sediment. Slicing was performed under an inert atmosphere (N2) using a glove box to avoid contact with oxygen. Each section, after measurement of pH and Eh (pH meter 82862 Weilheim WTW), was divided into two samples: one was immediately used for porewater extraction, and the other was dried at 50\u0026deg;C and preserved to analyze the TOC, carbonate, iron, and manganese contents of this sediment.\u003c/p\u003e \u003cp\u003ePorewater was extracted under an inert atmosphere by centrifugation at 2500 rpm for 25 minutes. The centrifuged water samples and the overlying waters were filtered in an inert atmosphere through 0.45 \u0026micro;m filters. A water split was acidified with ultra-pure nitric acid to pH 2 and then stored at 4\u0026deg;C for dissolved iron and manganese analysis. The second water split was immediately used for nutrient and sulfate analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Sediment analyses\u003c/h2\u003e \u003cp\u003eTo determine the percentage of the fine fraction (particle diameter\u0026thinsp;\u0026lt;\u0026thinsp;63\u0026micro;m), sediment recovered after centrifugation was separated into two size fractions: a fine fraction (particle diameter\u0026thinsp;\u0026lt;\u0026thinsp;63 \u0026micro;m) and a coarse fraction (particle diameter\u0026thinsp;\u0026gt;\u0026thinsp;63 \u0026micro;m). Separation was performed by wet sieving using a 63 \u0026micro;m nylon sieve and bi-distilled water. Organic Carbon content was determined by the ANNE method. This method allows the determination of the organic Carbon in the sediment by colorimetry after oxidation of the organic matter by potassium dichromate (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e-H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e), in a sulfuric medium. Calcium carbonate content was measured using a Bernard calcimeter. Iron and manganese contents were measured by a flame spectrophotometer Thermo Scientific SOLAAR ICE 3300 series.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Porewater analyses\u003c/h2\u003e \u003cp\u003ePrior to iron and manganese analyzes, we used the iron co-precipitation technique developed by Welch et al. (1990) in order to precisely detrmine Fe and Mn concentrations by concentrating them and removing salts. Dissolved Fe and Mn were measured using flame spectrometer (Thermo Scientific ICE 3300 AA Spectrometer).\u003c/p\u003e \u003cp\u003eNutrients (phosphate, ammonium, nitrate and nitrites) in both pore and supernatant waters were analyzed using a Technicon AA3 AutoAnalyzer. Concentrations were determined according to methods described in Murphy and Riley (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e1962\u003c/span\u003e) for phosphate, Solorzano (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e1969\u003c/span\u003e) for ammonium, Bendschneider and Robinson (1952) for nitrite and Wood et al. (\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e1967\u003c/span\u003e) for nitrate. Sulfates were measured with the Hoioarth (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e1978\u003c/span\u003e) method. The detection limits for analyzed species and the relative standard deviation (RSD) are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\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\u003eDetection limit and relative standard deviation of analyzed dissolved species.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDissolved species\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDetection limit\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRSD (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.17\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.08\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.015\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe\u003csup\u003e2+\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMn\u003csup\u003e2+\u003c/sup\u003e (\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5. Diffusive fluxes across the sediment-water interface (SWI)\u003c/h2\u003e \u003cp\u003eThe diffusive flux (J) of a dissolved component was calculated from the porewater profile using Fick\u0026rsquo;s first law:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\text{J}\\:=\\:-\\:{\\phi\\:}\\:.\\:{\\text{D}}_{\\text{s}}\\:.\\left(\\frac{\\text{d}\\text{C}}{\\text{d}\\text{z}}\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere dC/dz represents the gradient at the sediment\u0026ndash;water interface (Schulz and Zabel 2006). In applying Fick\u0026rsquo;s first law, the sediment was assumed to be in a steady state. The flux direction is opposite to the concentration gradient (when the concentration increased with depth, the flux was directed to the water column). Ф represents sediment porosity, D\u003csub\u003esed\u003c/sub\u003e represents the molecular diffusion coefficient in the sediment, while D\u003csub\u003esed\u003c/sub\u003e = D\u003csub\u003esw\u003c/sub\u003e / θ\u0026sup2; and θ\u0026sup2; = 1 - ln (Ф\u0026sup2;) which is related to the tortuosity \u0026lsquo;\u0026lsquo;θ\u0026rsquo;\u0026rsquo;, temperature and chemical elements, according to Boudreau (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Sediment properties\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1. The pH and Eh profiles\u003c/h2\u003e \u003cp\u003eVertical variation of Eh throughout the cores shows a rapid decrease in the first 10 centimeters at all sampling sites and then values remain stable in the rest of the sediment columns. Values are positive in the first 2 centimeters and then conditions become anoxic in the rest of the sediment. In the deep layer, Eh reaches \u0026minus;\u0026thinsp;200 mV at LN1 site which is located in the dredged area.\u003c/p\u003e \u003cp\u003eThe variation of pH along the cores collected from the three sites is small (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Values range from 7,21 to 8 at LN1site, 7,08 to 8 at LN2 site, and 6,97 to 8 at LN3 site. Eh values are always lower than those measured in the water column (pH\u0026thinsp;\u0026gt;\u0026thinsp;8). pH profiles at the three stations follow, overall, a constancy of values in the rest of the cores.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2. Main characteristics of the particulate fraction\u003c/h2\u003e \u003cp\u003eTotal organic carbon (TOC) levels decrease slightly to very slightly in the first 10 centimeters of sediment in the three collected cores (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). From a depth of 10 cm onwards the contents increase to reach maximum values at a depth of 15 cm at LN1 site (3.32%) and LN3 site (1.4%) and at a depth of 25 cm at LN2 site (2.1%). In the deeper sediments, TOC contents tend to decrease slightly. The highest TOC contents were measured throughout the core sampled at LN1 site.\u003c/p\u003e \u003cp\u003eThe results of the particle size analyses carried out on the sediments of cores lead to the same conclusions indicating a heterogeneous sediment in its texture (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Indeed, we noted fluctuations in the fine fraction (FF) profiles at three sampling sites that are related to the presence of large quantities of shell tests and plant filaments.\u003c/p\u003e \u003cp\u003eThe lithological aspect of sediment of the collected cores has been described previously in our study Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Indeed, the upper layer of LN1 and LN2 cores (taken from the dredging area) consists of a recent black mud rich in shell debris and plant filaments. Towards depth the hue of the old sediment becomes grey to light grey. LN3 core taken from the undredged area consists of black silt rich in shell fragments and plant filaments at the top and down to deeper levels. Oueslati et al (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) showed that the presence of a section of sediment from 10 to 20 cm rich in test of the gastropod Hydrobia ventrosa at LN3 site and their absence in the sediments of LN1 and LN2 cores, was characteristic of a polluted sediment rich in organic matter before the restoration and dredging works.\u003c/p\u003e \u003cp\u003eThe carbonate content profiles did not show clear changes with depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Values are roughly constant along the collected cores reaching 60% at LN2 site and about 50% at LN1 and LN3 sites.\u003c/p\u003e \u003cp\u003eThe vertical distribution of particulate iron does not show significant variation along the collected cores (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Manganese distribution shows an overall increase towards the deeper layers of the sediment (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). A slight decrease in concentrations was shown from the profiles of Fe and Mn contents in the surface layer of cores (0\u0026ndash;4 cm).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Distribution of porewater major diagenetic counponds\u003c/h2\u003e \u003cp\u003eNitrates (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;)\u003c/sup\u003e profiles show some differences in concentrations among sites, but show the same trends (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Background concentrations are 12.54 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at LN1 site, 24.57 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at LN2 site, and 50.7 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at LN3 site. Just below the sediment-water interface, levels increase with depth and peak in the first 6 centimeters at all three sites and then decrease rapidly with depth. The highest concentrations are measured at 2 cm depth at LN3 site located in the undredged zone (about 125 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). At LN1 site, the maximum levels reached at 2 cm depth are of the order of 101.3\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. At LN2 site, we notice on the nitrate profile the presence of three concentration peaks, the first is reached just below the SWI at 2 cm depth (50.8 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), the second which is the maximum is reached at 6 cm depth (95 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and the third is reached at 12 cm depth (69.1 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eNitrite (NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) concentrations in the porewaters of three cores do not exceed 11.3 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Their distribution with depth show a rapid increase in the first 6 cm of depth to reach peaks of 3\u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;l\u003c/sup\u003e (4 cm depth), 11.3 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (6 cm depth), and 7.9 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (2 cm depth) at LN1, LN2 and LN3 sites respectively. This increase is followed by a rapid decrease.\u003c/p\u003e \u003cp\u003eBeyond 10 cm depth, new concentration peaks are noted at different levels of the cores: around 11 cm depth for LN2 and LN3 and around 30 cm depth for LN1 and LN3. This new increase is also followed by a rapid decrease. In the deep sediments, nitrite concentrations are low.\u003c/p\u003e \u003cp\u003eAmmonium (NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) concentrations increase rapidly below the SWI within 10\u0026ndash;12 cm depth at all three study sites, reaching peaks of 126.1 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at LN2 site and 190.45 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at LN3 site at 10 cm depth (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). At LN1 site, the maximum content (around 112.5 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is reached at 20 cm depth. Beyond 10 cm there is a rapid decrease in concentrations at LN2 and LN3 sites. This is followed by a slight increase towards the deeper layers. In the first 10 cm, the highest NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e concentrations are measured at LN3 site which is located in the undredged area.\u003c/p\u003e \u003cp\u003eThe vertical evolution of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3\u0026minus;\u003c/sup\u003e is similar to that of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e at all three sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). It mainly concerns the first 10\u0026ndash;12 centimeters. It is characterized by an increase in concentrations below SWI until about 12 cm depth where concentrations decrease slightly towards the deep sediment layer. The highest concentrations are measured at LN2 site.\u003c/p\u003e \u003cp\u003eDissolved manganese (Mn\u003csup\u003e2+\u003c/sup\u003e) is detected at the SWI at concentrations below 0.3 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. As soon as the interface is crossed, we observe in the first few centimeters a significant production with a maximum that varies from site to site (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). At LN1 site, the maximum concentration peak (1.24 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is reached at 2 cm depth, while it is reached 6 cm below the SWI at LN2 site (of the order of 4.7 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and at 8 cm depth at LN3 site (of the order of 4.35 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). This maximum production is quickly followed by a sharp decrease (concentrations reach low values).\u003c/p\u003e \u003cp\u003eThe vertical variation of dissolved iron (Fe\u003csup\u003e2+\u003c/sup\u003e) is similar to that of Mn\u003csup\u003e2+\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Indeed, we observe an increase in concentrations below the SWI to reach maximum values in the first 10 centimeters. The maximum concentration peak is reached at 4 cm depth at LN1site (3.59 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), at 6 cm at LN2 site (5.22 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and at 8 cm depth at LN3 site (about 24.98 \u0026micro;mol l\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). As for Mn\u003csup\u003e2+\u003c/sup\u003e, the maximum production of Fe\u003csup\u003e2+\u003c/sup\u003e is rapidly followed by a sharp decrease in concentrations.\u003c/p\u003e \u003cp\u003eThe maximum concentrations of dissolved Fe and Mn are measured at LN3 site located in the undredged area.\u003c/p\u003e \u003cp\u003eVertical variation of sulfates (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) shows the same trends and very similar concentrations at three study sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In fact, for all the profiles, a decrease in content is noted from the SWI to the deep layer, with slight subordinate increases around 10 to 12 cm in depth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussions","content":"\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Basic sediment properties after the restoration works\u003c/h2\u003e \u003cp\u003eConsidering the average rate of sedimentation in the northern lake of Tunis which is about 0,15 cm yr\u003csup\u003e− 1\u003c/sup\u003e according to Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e), the surface layer of about 5 cm thickness at LN1 and LN2 sites located in the dredged area corresponds to a recent layer deposited after the restoration works. Beyond this depth, the sediment corresponds to older mud deposited before the dredging works. At the LN3 site located in the undredged area, the sediment column reflects the state of the lake before and after the restoration works. Indeed, considering the lithological aspect of the collected cores, Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) were able to distinguish a sediment section located between 10 and 20 cm depth in the LN3 core rich in shells of the gastropod Hydrobia ventrosa, characteristic of the northern lake of Tunis before the restoration work.\u003c/p\u003e \u003cp\u003eThe slight depletion in TOC and the rapid decrease in pH and Eh in the first centimeters at the three sites suggest the presence of classical diagenetic processes in the surface layer. Indeed, a classical sedimentation model shows that organic matter undergoes bacterial degradation as soon as it is deposited on the sediment surface. This results in the anoxia of the sediment and the release of hydrogene ion in the porewater.\u003c/p\u003e \u003cp\u003eAccording to this study, we note that the environment has evolved differently. Indeed, the degradation of organic matter is more significant in the undredged site than in the dredged area. In addition, sediment anoxia is higher in the dredged area than in the undredged area. This could be explained by the greater input of nutrients to the dredged area than to the undredged one. In fact, according to Turki and Hadj Ali (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e1990\u003c/span\u003e), nitrate and chlorophyll a levels were higher in the dredged area, despite the restoration and dredging works.\u003c/p\u003e \u003cp\u003eAdded (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) studied diagenesis processes in cores taken from the northern lake of Tunis in 1986 just before dredging. TOC levels were on the order of 6,6% in the surface layers. When compared to the levels found in this study, they are 2 to 9,5 times higher. This was mainly due to the eutrophic state of the lake at that time.\u003c/p\u003e \u003cp\u003eThe high concentrations observed in the deep layer are probably the result of the accumulation of organic matter over the last few years (93 years is the time estimated by a sedimentation rate equal to 0.15 cm yr\u003csup\u003e− 1\u003c/sup\u003e and a depth of 14 cm at the LN3 site). It is likely that during this time the depositional conditions were more favorable for the preservation of organic matter than at present. This phenomenon is also observed by Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) in the cores taken from the northern lake of Tunis in 1986 just before the dredging works and explained it also by a preservation of organic matter 333 years before, considering the average rate of sedimentation in the lake. On the other hand, the increase with depth of the TOC contents could testify to a more important contribution of organic matter to the lake in the past which seems to have decreased after the restoration and dredging works carried out on the lake. When examining the lithological aspect of the LN2 core described by Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), we notice that the maximum TOC contents are measured at 22–33 cm depth, in a layer constituted by greenish-gray mud rich in plant filaments.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Early diagenetic reactions\u003c/h2\u003e \u003cdiv id=\"Sec16\" class=\"Section3\"\u003e \u003ch2\u003e4.2.1. Nitrification, denitrification and ammonium release\u003c/h2\u003e \u003cp\u003eIn undisturbed suboxic sediments, nitrate profiles showed generally a production below the SWI that is usually attributed to the bacterial nitrification, followed by a decrease in underlying levels resulting from denitrification processes induced by aerobic denitrifying bacteria (Froelich et al., 1978; Added, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e1981\u003c/span\u003e; Added, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). At all three study sites, the nitrate profiles show levels evolving according to the theoretical model described above.\u003c/p\u003e \u003cp\u003eIn LN2 and LN3 sites, nitrate regeneration in the reducing sediments just below the thin oxic zone appears to be due to the oxidation of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e by oxygen or manganese oxide advected to depth by benthic macrofauna. Note that Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) highlighted the presence of lamellibrach and gastropod shells and some annelids in the sediments. In addition, particulate Mn profiles show high concentrations in these sediment levels. This process has been observed by several authors such as Hyacinthe et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2001\u003c/span\u003e) in the sediments of Biscay Bay and Ben Mna et al. (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) in the sediments of Bizerte Lagoon.\u003c/p\u003e \u003cp\u003eNitrites peak concentrations in the suboxic surface sediments and in the reducing sediments of the subjacent zone seems to be due to nitrate production related to the mineralization of organic matter by oxic respiration just below the sediment-water interface and by nitrification in the subjacent zone.\u003c/p\u003e \u003cp\u003eAmmonium is the predominant dissolved nitrogen specie at three study sites. This is in agreement with the reducing conditions of the environment since the production of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e ions could only take place at Eh values lower than 260 mV (Fernex et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e1989\u003c/span\u003e). In the first 10 to 12 cm of the sediment at the three sites, it was evident that there was a negative correlation between NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e levels in the porewater and TOC levels in the sediment. This led us to conclude that the maximum production of ammonium ions in the top layer of the cores is clearly related to the anaerobic mineralization of organic nitrogen species contained in the organic matter.\u003c/p\u003e \u003cp\u003eBy examining the NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and TOC profiles, we arrived at unexpected results. Indeed, the maximum ammonification occurred at LN3 site (located in the undredged area) where the bacterial oxidation of the organic matter is more significant than at the other two sites. However, it was expected to see a more important bacterial oxidation of organic matter and consequently, a significant diagenesis of organic nitrogen (nitrification under the SWI and subsequent ammonification in the subjacent sediment) at the dredged sites (LN1 and LN2) taking into account the remediation works carried out and the current good environmental status of these sites.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e4.2.2. Phosphate release\u003c/h2\u003e \u003cp\u003eThe relatively high production of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e ions in the first 10–12 cm is similar to that of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e and is certainly attributed to the bacterial mineralization of phosphorus compounds contained in organic matter.\u003c/p\u003e \u003cp\u003eThe homogenization of low concentrations at LN1 site could be attributed, on the one hand, to a significant bioturbation activity given that the sediment is rich in lamellibranch shells as reported by our previous work Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), and on the other hand, to co-precipitation or adsorption on sedimentary mineral particles. Indeed, the high affinity of phosphorus for iron carbonates and oxy-hydroxides limits the concentration of PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e in porewaters (Added, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Zaaboub et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Ben Mna et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Moreover, the study of the chemical fractionation of phosphorus conducted by our co-author Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) on the sediments of the cores taken from the North Lake of Tunis just a few months before the restoration works, shows that mineral phosphorus accounted for 73% of the total phosphorus in the sediment and was the potential source of phosphates in the porewaters of the lake. It should be noted that the LN1 core sediments contain sufficient amounts of carbonate and relatively small amounts of particulate Fe.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section3\"\u003e \u003ch2\u003e4.2.3. Manganese and iron behavior\u003c/h2\u003e \u003cp\u003eWe can conclude from the vertical distribution of dissolved manganese and iron that these species followed the typical behavior during the mineralization of organic matter in a sub-oxic sediment. We note in fact, a release of Mn\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e2+\u003c/sup\u003e ions in porwater of the surface layer followed by a rapid decrease. This behavior results, obviously, from the reduction of manganese and iron oxides and hydroxides following the anaerobic degradation of organic matter, then from the diffusion of dissolved Fe and Mn towards the thin surface oxic layer and their re-oxidation by oxygen and / or nitrates into iron and manganese oxides as was widely observed in several coastal environments (Hansen et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Hyacinthe et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Ben Mna et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), or towards the deep anoxic layers and their precipitation in the form of sulphides and / or carbonates ( Franklin and Morse, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e1983\u003c/span\u003e;Huerta-Diaz and Morse, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1992\u003c/span\u003e; Caplat et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2005\u003c/span\u003e ; Ben Mna et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Note that the analysis of carbonate (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and sulfide levels in the sediment of the cores at the three sites (Oueslati et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) shows sufficient quantities of both metal substrates.\u003c/p\u003e \u003cp\u003eThe area of Mn\u003csup\u003e2+\u003c/sup\u003e production at LN2 and LN3 sites located in a slightly deep level compared to the LN1 site, was obviously related to the presence of bioirrigation phenomena in the upper sediment levels at LN2 and LN3 sites.According to the lithological description made by Oueslati et al. (\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), the upper layer at these sites is, already, rich in lamellibranch and gastropod shells. In addition, we measured high nitrate levels in this upper layer at the latter sites.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e4.2.4. Sulfates\u003c/h2\u003e \u003cp\u003eThe sulfate profiles indicate, overall, a typical behavior found in sediments. The decrease below the sediment-water interface is, in fact, attributed to sulfate reduction by sulfate-reducing bacteria during anaerobic degradation of organic matter. The low concentrations in the deep sediment layer reflect significant sulfate reduction at three study sites. This is indicated by the presence of sulfides in the sediment (Oueslati et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Effects of restoration works on some early diagenetic reactions\u003c/h2\u003e \u003cp\u003eWe compared the results found in this study with those found by our co-author Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) in 1986, just a few months before the dredging remediation work. The results of Added(2002) are shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Before the remediation and dredging works, maximal ammonium content measured was about 68 times higher compared with the current contents, indicating that organic nitrogen diagenesis in the Nordh Lake of Tunis was significant at the time. In addition, the vertical evolution of NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e levels before the remediation work reflects the intense bacterial degradation of organic matter at the time.\u003c/p\u003e \u003cp\u003eSimilarly, for phosphate, the maximum content measured in the interstitial waters of core samples taken prior to lake restoration was of the order of 38 µmol l\u003csup\u003e− 1\u003c/sup\u003e(Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). This is 8 times higher than that measured in this study, reflecting, as we have just concluded, the intense diagenesis of organic matter prior to lake restoration. Phosphate release at that time was essentially limited to the first ten centimetres, as is the case today. The decrease in PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e levels in deep sediments, either before or after lake restoration, probably corresponds to phosphate precipitation. Indeed, Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) has shown that fluorapatite, hydroxyapatite and whitlockite were phosphate minerals likely to precipitate in the Nordh Lake of Tunis.\u003c/p\u003e \u003cp\u003eMn and Fe concentration are broadly comparable to those measured in 1984 and 1985 by Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). These low concentrations before and after the restoration of the lake can be explained by the iron sulphides precipitation on the one hand, and the adsorption of Mn\u003csup\u003e2+\u003c/sup\u003e ions on these minerals on the other. Indeed, it has been shown that the Mn\u003csup\u003e2+\u003c/sup\u003e ion does not form sulphides easily, but is incorporated into pyrite in anoxic sediments with a high degree of pyritization (Morse and Luther, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e1999\u003c/span\u003e). Knowing that the sediments of the northern lake of Tunis are favorable to pyritization (Oueslati et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eCompared to those measured by Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) prior to remediation, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e concentrations measured at the three study sites are slightly low. Concentrations prior to remediation remained quite high in the porewaters (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), despite the significant sulfate-reduction signaled by the presence of sulfides (Added, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e). The distribution of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e at that time was globally dominated by the following processes (Added, \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e): a significant sulfate reduction in June, sulfide oxidation in November and March, bioirrigation in April, and wave-induced water mixing in October. The first process led to a significant decrease in sulfate contents near the interface, the second led to an increase in SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2−\u003c/sup\u003e contents, and the third and fourth processes allowed for a homogenization of sulfate contents between the supernatant water and the porewater. Thus, contrary to the current state in the lake, sulfate reduction was not total in the sediment before restoration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003e4.3. Effects of dredging on the nitrogen species and phosphate fluxes across the SWI\u003c/h2\u003e \u003cp\u003eThe estimated fluxes of nitogen species (nitrate, nitrite and ammonium) and phosphate show diffusion from the sediment towards the water column (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The diffusion of nitrates and nitrites towards the water column results from their release just below the sediment-water interface following bacterial nitrification. Positive fluxes of ammonium is related to release of these species into porewater via diagenetic reactions in the sediment. We note that the nitrate, nitrite and ammonium gradients are greater at LN3 station located in the undredged area.\u003c/p\u003e \u003cp\u003eCompared to the ammonium fluxes calculated in the North Lake of Tunis before the remediation works (27800 µmol. m\u003csup\u003e2\u003c/sup\u003e.d\u003csup\u003e− 1\u003c/sup\u003e), those estimated in the present work are 153 times lower (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This important decrease could be explained by the nitrification of ammonium ions into nitrites and nitrates in the thin oxic surface layer which acts as a trap for these reducing species produced from the anaerobic mineralization of organic nitrogen in depth and diffused upwards near to the sediment-water interface. However, before the restoration and dredging, the conditions in the lake were eutrophic.\u003c/p\u003e \u003cp\u003eThe low diffusion fluxes of phosphorus ions could be explained by their accumulation in the surface sediments by adsorption and/ or co-precipitation with mineral substrates, particularly carbonates. The fluxes of phosphates in the North Lake of Tunis before the remediation works (60200 µmol.m\u003csup\u003e2\u003c/sup\u003e.d\u003csup\u003e− 1\u003c/sup\u003e) are strongly higher than the calculated fluxes at present (about 8 104 times higher).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\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\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\u003eDiffusive fluxes J(µmol m\u003csup\u003e2\u003c/sup\u003e d\u003csup\u003e− 1\u003c/sup\u003e) of nitrogen species (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e and NH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e) and phosphate (PO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e) across the SWI in the North Lake of Tunis.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\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\u003eJ(µmol m\u003csup\u003e2\u003c/sup\u003e d\u003csup\u003e− 1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLN1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e191.57\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.73\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e55.33\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLN2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e84.90\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e6.40\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.92\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.76\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLN3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e240.87\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e14.64\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e181.70\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.31\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" 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=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\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\u003eFluxes (mmol m\u003csup\u003e2\u003c/sup\u003e d\u003csup\u003e− 1\u003c/sup\u003e) calculated by Added (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2002\u003c/span\u003e) with the concentration gradients at the SWI in the North lake of Tunis before its restoration (during the year 1986).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eJanuary\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eApril\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eJune\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAugust\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eOctober\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eThe average fluxe\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNH\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e+\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.48\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.27\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e27.8\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.24\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e6.76\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e3−\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e60.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.3\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e51.7\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e25.4\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e \u003cp\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eThe dredging carried out in the Northern Lake of Tunis (a formerly heavily polluted lake) thirty years before the date of this present sampling effectively improved the quality of the water and sediment of the lake. TOC concentrations and fluxes of reduced nitrogen and phosphates from sediment to water column have decreased signifantly after dredging. However comparig the dredged area with the undrdged area, we can conclude that it would be better to reduce the external loading of nutrient to the dredged area because the conditions are favorable to a return to the former eutropic state. Indeed, the degradation of organique matter in the undregded area is more significant than in the dredged area. The anoxia is higher in the dredgd site.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eHaifa Ben Mna wrote the main manuscript text. Walid Oueslati did the analysis and wrote a part of the text. Mohamed Amine Helali prepared figures and wrote a part of the text. Ayed Added wrote a part of the text. All authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAdded, A. (2002). Cycles biog\u0026eacute;ochimiques des sels nutritifs, du fer, du mangan\u0026egrave;se et du soufre dans les s\u0026eacute;diments de deux syst\u0026egrave;mes c\u0026ocirc;tiers du nord de la Tunisie: Lagune de Ghar El Melh et Lac Nord de Tunis. PhD thesis, Univ. Tunis II, Fac. Sci. Tunis, p. 266.\u003c/li\u003e\n\u003cli\u003eAdded, A. (1981). Etude g\u0026eacute;ochimique et s\u0026eacute;dimentologique des s\u0026eacute;diments marins du Delta du Rh\u0026ocirc;ne. PhD thesis. Univ. Pierre et Marie Curie Paris VI, p. 263. \u003c/li\u003e\n\u003cli\u003eArmi, Z., Turki S., Trabelsi, E. \u0026amp; Ben Ma\u0026iuml;z, N. (2010). First recorded proliferation of Coolia monotis (Meunier, 1919) in the North Lake of Tunis (Tunisia). Environ. Monit.Assess. 164, 423\u0026ndash;433.\u003c/li\u003e\n\u003cli\u003eBelkhir, M. \u0026amp; Hadj Ali Salem, M. (1981). 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Res. 11, 87-96.\u003c/li\u003e\n\u003cli\u003ehttps://elischolar.library.yale.edu/cgi/viewcontent.cgi?article=1760\u0026amp;context=journal_of_marine_research\u003c/li\u003e\n\u003cli\u003ehttps://elischolar.library.yale.edu/journal_of_marine_research/761\u003c/li\u003e\n\u003cli\u003eBen Mna, H., Alsubih, M., Oueslati, W., Helali, M.A., Amri, S., Added, A. \u0026amp; Aleya, L. (2022). Diagenetic processes and nutrients diffusive fluxes at the sediment-water interface in the Bizerte Lagoon (North Tunisia). J. Afr. Earth Sci. 196, 104671.\u003c/li\u003e\n\u003cli\u003eBoudreau, B. (1997). Diagenetic models and their implementation: modelling transport and reactions in aquatic sediments. Springer-Verlag, Berlin, 414 pp.\u003c/li\u003e\n\u003cli\u003eCao, X.Y., Song, C.L., Li, Q.M. \u0026amp; Zhou, Y.Y. (2007). Dredging effects on P status and phytoplankton density and composition during winter and spring in Lake Taihu, China. 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The interaction of manganese(II) with the surface of calcite in dilute solutions and seawater. Mar. Chem. 12, 241\u0026ndash;254.\u003c/li\u003e\n\u003cli\u003eFroelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. \u0026amp; Maynard, V. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic : suboxic diagenesis. Geochim. Comochim. Acta 43, 1075\u0026ndash;1090.\u003c/li\u003e\n\u003cli\u003eHansen, H.C.B., Borggaard, O.K. \u0026amp; Sorensen, J. (1994). Evaluation of the free energy of formation of Fe(II)-Fe(III)-hydroxyde-sulphate (green rust) and its reduction of nitrite. Geochim. Cosmochim. Acta 58, 2599\u0026ndash;2608.\u003c/li\u003e\n\u003cli\u003eHarbridge,W., Pilkey, O.H., Whaling, P. \u0026amp; Swetland, P. (1976). Sedimentation in the Lake of Tunis: A Lagoon Strongly Influenced by Man.Environmental Geology, 1 : 215-225.\u003c/li\u003e\n\u003cli\u003eHoioarth R. 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Fifteen-year study of environmental dredging effect on variation of nitrogen and phosphorus exchange across the sediment-water interface of an urban lake. Environ. Pollut. 219, 639\u0026ndash;648.\u003c/li\u003e\n\u003cli\u003eMorse, J.W. \u0026amp; Luther III G.W. (1999). Chemical influences on trace metal interactions in anoxic sediments. Geochim. Acta 63, 3373-3378.\u003c/li\u003e\n\u003cli\u003eMurphy, J.\u0026amp; Riley, J.P. (1962). A modified single solution method for the determination of phosphate in natural waters. Anal. Chim. Acta 27, 31\u0026ndash;36.\u003c/li\u003e\n\u003cli\u003eKimberly, A.O. \u0026amp; Alan D.S. (2019). Impact of sediment dredging on sediment phosphorus flux in a restored riparian wetland. Sci. Total Environ. 650, 1969\u0026ndash;1979.\u003c/li\u003e\n\u003cli\u003eOueslati, W. Helali, M.A., Mensi, I. Bayaoui, M., Touati, H., Khadraoui, A., Zaabooub, N., Added, A. \u0026amp; Aleya, L. (2018). 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Total Environ. 592, 662\u0026ndash;673.\u003c/li\u003e\n\u003cli\u003eZaaboub, N., Ounis, A., Helali, M.A., B\u0026eacute;jaoui, B., Lilleb\u0026oslash;, A.I., Ferreira da Silva, E. \u0026amp; Aleya, L. (2014). Phosphorus speciation in sediments and assessment of nutrient exchange at the water-sediment interface in a Mediterranean lagoon: implications for management and restoration. Ecol. Eng. 73, 115\u0026ndash;125.\u003c/li\u003e\n\u003cli\u003eZaouali, J. (1983). Lac de Tunis: 3000 years of engieering and pol\u0026shy;lution. A bibliographical study with comments. UNESCO, Rapp. Mar. Sci. 26, 30-47.\u003c/li\u003e\n\u003cli\u003eZhong , J.C., Yu, J.H., Zheng, X.L.,Wen, S.L., Liu, D.H. \u0026amp; Fan, C.X. (2018). Effects of dredging season on sediment properties and nutrient fluxes across the sediment\u0026ndash;water interface in Meiliang Bay of Lake Taihu, China. Water 10(1606), 1\u0026ndash;16.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Sediment, dredging, nutrients, diffusives fluxes, early diagenesis, Mediterranean Lagoon","lastPublishedDoi":"10.21203/rs.3.rs-4959425/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4959425/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThree sediment cores from dredged and undredged areas of Northern lake of Tunis, a mediterranean lagoon in northern of Tunisia, were used to investigate behavior of nutrients related to diagenetic reactions in sediment and assess the release of reduced nitrogen and phosphorus from surface sediment to the water, 30 years after dredging. The results show diffusion from the sediment towards the water column. Expecting from the results that the degradation of organic matter and the resulting N and P fluxes would be greater in the dredged area due to the oxygenation of the environment, this process was more significant in the undredged area. A comparison with the pre-dredging sediments show that the influence of dredging is very remarkable over time. It has effectively reduced organic matter contents (TOC levels after dredging were 2 to 9,5 times lower) and consequently the fluxes of reduced nitrogen and phosphorus species across the sediment-water interface. Fluxes of ammonium decreased 153 times and those of phosphorus about 8 104 times. This explains the improvement in the quality of water and sediment in the northern lake of Tunis.\u003c/p\u003e","manuscriptTitle":"Impact of Dredging on Diagenesis and Nutrient Release in a Restored Mediterranean Lagoon (Lake of Tunis, Tunisia)","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-27 17:00:49","doi":"10.21203/rs.3.rs-4959425/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"63524dac-beff-42be-9939-0c7cbcc3ddd9","owner":[],"postedDate":"September 27th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-10-16T12:53:43+00:00","versionOfRecord":[],"versionCreatedAt":"2024-09-27 17:00:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4959425","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4959425","identity":"rs-4959425","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}