Spatially limited influence of a Mississippi River freshwater diversion on estuarine nekton communities in coastal Louisiana ( U.S.A. )

Introduction Freshwater diversions and marsh creation are prominent methods in coastal restoration, including in Louisiana (U.S.A.) where land loss rates rank among the highest in the world. However, concerns over impacts from altered salinity, including impacts on estuarine ecological structure and local fisheries, create conflicts over potential tradeoffs of freshwater additions for restoration. Objectives We assessed impacts of freshwater addition on estuarine nekton occupying reference and created marshes near the West Pointe à la Hache freshwater diversion by quantifying changes in estuarine nekton community structure to identify the most affected species and differences in responses between marsh types. Methods We sampled estuarine nekton community structure at created and reference marsh sites ranging from 0.6 to 11.2 km from the freshwater diversion's input using trawls while the diversion was paused (2018 and 2019) and after it resumed (2021 and 2022) to assess ecological impacts of the diversion on the Barataria Bay estuary. Results Freshwater addition influenced nekton communities nearest the diversion's input, with decreases in abundance of estuarine species and increases in freshwater species while the diversion operated. Influence was less detectable at sites over 5 km away. Created and reference marshes supported similar nekton communities regardless of diversion operation. Conclusions Our study indicated spatially limited direct influence of the freshwater diversion on nekton communities, including fisheries species, and provides insights into future management and design of coastal restoration.