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Abstract
Global estimates of salt marsh degradation and loss are between 20-50% by the end of the century especially in Southern Louisiana where this loss is currently one of the highest in the world. Restoration efforts in this area have the goal of creating marsh land that is as similar in function, habitat, and ecosystem services as surrounding reference marshes. Our research takes the next step of quantifying those goals by evaluating the biodiversity and community structure of newly created sites relative to reference sites along a freshwater siphon-induced salinity gradient. We sampled on-marsh nekton species abundance and composition from two created and four reference sites varying distances from the siphon from 2018 to 2022 using wire mesh traps deployed at marsh subhabitats (ponds, creeks, and edges). We also sampled macroinvertebrate communities through use of nylon mesh bags for two months (May through July) in 2018, 2019, and 2021. Sampling occurred when the siphon was off in 2018 and 2019 and on in 2021 and 2022 causing a salinity gradient from 0 to ~12. The aims of this study were to 1) determine whether there is an associate between environmental factors and the abundance and distribution of on-marsh nekton and macroinvertebrate communities within restored and reference sites, and 2) determine whether population demographics of select on-marsh nekton species difference between restored and reference sites and/or across a salinity gradient. We found that nekton abundance slightly increased between siphon off and on years with biodiversity minimally declining closest to the siphon. In regards to subhabitats, pond community composition was more stable over time, due to the presence of euryhaline, Cyprinodontiforme fishes. We saw a change in creek and edge communities due to a replacement of shrimp species from brown and grass shrimp to Ohio shrimp in the presence of freshwater input closest to the siphon. When comparing on-marsh nekton and macroinvertebrates, there was only a correlation in Shannon diversity, but not in abundance. Finally, length and biomass population demographics were examined and, although variable between species, minimal differences were found between restored and reference sites and freshwater input did not seem to have a strong effect on these species demographics either. Interestingly, we did find a large amount of differences within the site mid-distance from the siphon through. Information gained here can help determine how combining newly-created sites with reduced salinity sustains species diversity as our results indicate that restored and reference marshes have high similarity less than five years post completion, and that siphon operation mostly affects the sites closest to it.