Types of Salt Marsh Edge and Export of Trophic Energy from Marshes to Deeper Habitats

Abstract

We quantified nekton and estimated trophic export at salt marshes with both erosional and depositional edges at the Goodwin Islands (York River, Virginia, USA). At depositional-edge marshes, we examined trophic flows through quantitative sampling with 1.75 m2 drop rings, and through gut content analyses of captured nekton. Six habitats were sampled on a transect from the marsh surface to the unvegetated subtidal. Consumption of animal prey by nekton, and export of trophic energy via transient nekton, was estimated for each habitat and for the entire marsh transect. The marsh edge was the habitat where we estimated the greatest contribution to export per m2. An estimated 28.0 g dry weight of animal tissue was removed as prey per m2 of depositional marsh edge into the open estuary over 150 days, primarily by blue crabs Callinectes sapidus. When the entire marsh was examined, however, marsh interior areas provided most of the trophic support for resident and transient species due to the greater area of the interior. When we considered the entire tidal cycle, the unvegetated intertidal area was also very productive, and contributed substantially to these trophic pathways. The blue crab Callinectes sapidus was the biomass dominant and probably the most important predator in all habitats. In a separate study, erosional-edge marshes facing open bays were examined with an 80 m2 enclosure net that sampled the marsh edge and the adjacent unvegetated area. For these marshes we report a high biomass of larger transient species. Blue crabs were the biomass dominant at every sampling date (mean from June to September 1996, 0.31 inds m-2 and 2.81 g dry weight m-2. A high biomass of transient fish species was also seen (mean from June - September, 1.90 inds m-2 and 1.25 g dry weight m-2). We suggest this high biomass implies that these species receive an appreciable benefit from this habitat. The high biomass of predators also suggests the potential for export from erosional-edge marsh areas. Although the gear used to examine erosional and depositional marsh edges was clearly different, both types of edge saw considerable use by transient species. We therefore conclude that marshes with both erosional and depositional edges can export significant biomass to deeper water ecosystems as the consumption of marsh secondary production by transient nekton.