Nekton (fishes and decapod crustaceans) use of brackish submerged aquatic vegetation (SAV) habitat has been largely overlooked in comparison to fresh and marine SAV habitats. This study was designed to examine nekton use of brackish marsh SAV habitat and non-vegetated bottom. Specifically, we examined the effects of habitat location (defined by SAV distance from marsh edge) and complexity (defined by SAV biomass) on nekton community density, biomass, and diversity in shallow water brackish marsh ponds, which contained monospecific beds of Ruppia maritima L. Three habitat types were investigated: (1) inner-pond SAV (SAV habitat > 1 m from marsh edge), (2) near marsh-edge SAV (SAV habitat < 1 m from marsh edge), and (3) non-vegetated bottom. We tested the null hypotheses that nekton density, biomass, and diversity were unrelated to habitat type or habitat characteristics. Ninety-six quantitative samples were taken with a 1 m 2 throw trap between September 2001 and July 2002. Using multivariate analysis of variance (MANOVA), a significant habitat type by season interaction was found (Wilks' λ = 0.32, F 24, 259 = 4.21, p < 0.0001) for nekton density, biomass, and diversity. Simple linear regression showed a positive relationship between SAV and nekton biomass (p < 0.0001); however, distance of the vegetated habitat from the marsh edge did not significantly influence SAV or nekton biomass. SAV biomass, but not location, appears to be a dominant factor that influences nekton use within these small brackish marsh ponds. This study highlights some of the intricacies associated with the identification and use of broad scale habitat classification for management purposes. Although brackish marsh SAV habitat types support higher densities of nekton compared to non-vegetated habitat, variation within this gross classification indicates a wide range of habitat 'value' that needs to be considered for management purposes.

Spatial and temporal variability characterize submerged aquatic vegetation (SAV) assemblages, but understanding the complex interactions of environmental drivers of SAV assemblages remains elusive. We documented SAV composition and biomass across a salinity gradient in a coastal estuary over 12 mo. Ten macrophyte species were identified. The dominant species, Ceratophyllum demersum and Myriophyllum spicatum, accounted for over 40% of total biomass. Only Ruppia maritima occurred across the salinity gradient. Salinity, water depth and clarity delineated 3 assemblages: a saline assemblage, and 2 groups of fresher-water species, one associated with deeper water and lower water clarity and the other associated with shallow water and higher water clarity. These assemblages exhibited intra-annual variation, with at least 5 times more biomass in late spring/mid-summer compared to early winter. This pattern was consistent across the estuary, although the difference between peak and low biomass varied by habitat type; brackish exhibited the greatest magnitude. This variation is likely due to higher variation in salinity and the species composition of this habitat. As climate change and coastal restoration impact timing and range of salinity, water depth and clarity in this region, these data can be used to help inform predictive models and management decisions.

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.