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Nekton community dynamics within active and inactive deltas in a major river estuary: potential implications for altered hydrology regimes

Caleb B. Taylor, John Andrew Nyman*, Megan K. La Peyre

*Corresponding author:

ABSTRACT: High fisheries production within estuaries is associated with coastal upwelling, tidal mixing, and land-based runoff facing increasing impacts from climate and human activities. Active river deltas receive large riverine inflows compared to inactive river deltas providing contrasting estuaries to compare impacts of river inflow on estuarine nekton. We quantified nekton assemblages and stable isotopes (δ 13C, δ 15N) of commercially important blue crab, Callinectes sapidus Rathbun, 1896, within an active and inactive delta in coastal Louisiana to explore the impacts of differing riverine inflow. Crustaceans dominated estuarine assemblages differing only by season and not delta type, with summer and fall supporting highest densities. Fish density and assemblages differed by the interaction of season and delta due to differences during the 2019 record high spring river inflow. During this period, the active delta supported reduced fish densities and richness compared to the inactive delta. Nekton densities across deltas and seasons reflect a combination of species’ life history characteristics and habitat conditions. The high spring river discharge in 2019 impacted habitat availability (reduced SAV presence), water conditions (decreased temperature and salinity), and potentially displaced nekton to unsampled habitat areas (i.e., interior marsh surface) within the active delta. While differences in nekton density and assemblages were only evident during the high spring river discharge, δ 15N values of blue crabs were ~1.5 times higher in the active delta potentially indicating more terrestrial influence. Understanding how altered inflow impacts environmental variables supporting estuarine nekton production remains critical to support management within these hydrologically managed regions.