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 (δ13C, δ15N) 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 presence of submerged aquatic vegetation), 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, δ15N values of blue crabs were approximately 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 for supporting management within these hydrologically managed regions.
Highlights
IntroductionTidal mixing, and land-based freshwater runoff including river inflows drive fisheries production (Caddy & Bakun 1994)
Coastal upwelling, tidal mixing, and land-based freshwater runoff including river inflows drive fisheries production (Caddy & Bakun 1994)
In 2019, the Mississippi River exceeded flood stage water height for a record-breaking 5 months from early spring through late summer, with the region experiencing one of the wettest years in recorded history. This unprecedented freshwater coming down the Mississippi River provided a stark contrast in inflow between the active and inactive deltas compared in the present study
Summary
Tidal mixing, and land-based freshwater runoff including river inflows drive fisheries production (Caddy & Bakun 1994). Within the northern Gulf of Mexico, the Mississippi River has formed vast coastal wetlands within past and present deltas, supporting over 60% of fishery landings in this region since 1950 (Chesney et al 2000). This area is experiencing one of the fastest rates of wetland loss in the world due to a combination of anthropogenic and natural factors (Couvillion et al 2013). In Louisiana, only the Plaquemine-Balize Delta at the mouth of the Mississippi River and the Atchafalaya/Wax Lake Outlet at the mouth of the Atchafalaya River are active
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