Trophodynamics of Nekton Assemblages and Relationships with Estuarine Habitat Structure Across a Subtropical Estuary

Abstract

Understanding the spatiotemporal dynamics of trophic structures, or trophodynamics, is important for assessing the overall condition and function of estuarine ecosystems. We examined spatial and seasonal trophodynamics of nekton assemblages across the seascape of a subtropical estuary (Charlotte Harbor, FL, USA) using historical nekton data from an ongoing monitoring program. We also examined relationships between trophodynamics and the spatial distribution of vegetated habitats, such as salt marshes, mangroves, and seagrass beds, as well as their absence along hardened shorelines (seawalls or riprap) using seascape metrics (area and edge density of habitats within several search radii of sample locations). We found that nekton assemblages in shallow waters (from the shoreline to a depth of ≤ 1.5 m) transitioned from mainly planktivores in the lower Peace and Myakka Rivers to mainly benthivores (those feeding on benthic infauna, epifauna, or seagrass) in the middle and lower portions of Charlotte Harbor. Overall nekton assemblages throughout the estuary appeared to be largely sustained through benthic pathways in areas with abundant seagrass and nearby mangrove habitat, particularly during the cool-dry winter when planktivore densities were seasonally low. The dramatic seasonal decline in planktivore densities we found in the lower rivers may be related to low freshwater inflow and increased predation resulting from seasonal recruitment pulses of several omnivorous species. Trophodynamics linked with seascape metrics of estuarine habitat structure described here may be used as a spatially explicit baseline of the estuarine seascape to monitor ecosystem health as well as parameterize spatial ecosystem models used in fisheries management in Charlotte Harbor and similar subtropical estuaries.

The influence of freshwater inflow and seascape context on occurrence of juvenile spotted seatrout Cynoscion nebulosus across a temperate estuary

“…Larger, continuous patches of seagrass may allow the juveniles to feed over a larger habitat area on benthic resources which are typically relatively abundant in seagrass while remaining protected from predators thereby reducing the need to move into or through unvegetated areas [ 67 ]. Relatively large areas of seagrass habitat measured at broad spatial scales has recently been related to relatively high densities of benthic feeders within nekton communities in other estuaries [ 64 , 67 ], suggesting that increased benthic resources within large areas of seagrass habitat may be responsible for observed habitat use patterns in juvenile seatrout as they grow and transition from plankton to benthic food sources [ 20 ].…”

“…Although juveniles of many estuarine-dependent species are associated with estuarine biogenic habitats, species-habitat relationships have largely been examined by characterizing the presence or absence of habitat types observed within sample sites [6]. The influence of habitat proximity on the taxonomic composition of estuarine nekton assemblages is well established [58][59][60][61][62][63][64][65]. However, the present study is one of the first to examine relationships between juvenile seatrout occurrence and seascape context (but see [15]).…”