Role of saltmarsh production in subsidizing adjacent seagrass food webs: Implications for landscape-scale restoration

Abstract

Restoration is an increasingly employed method for rehabilitating and enhancing the ecosystem services of lost or degraded coastal habitats, such as supporting high faunal densities and biomass. In certain ecosystems, faunal densities along habitat ecotones are greater than the densities observed in either adjoining habitat, potentially due to the provision of spatial subsidies (e.g., increased food availability). We tested the hypothesis that seagrass-associated consumers immediately adjacent to a saltmarsh are the recipients of food subsidies sourced from saltmarsh primary production. The food web biomarkers we used to evaluate spatial flows of energy across estuarine habitat boundaries included the stable isotope ratios of carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S). We collected primary producers (N = 102) including macrophytic vegetation, benthic microalgae, and epiphytes, as well as 11 key consumers (N = 190) across seagrass meadows either immediately adjacent to (N = 8) or isolated from (N = 8) saltmarsh. We also estimated particulate organic matter (POM) from filter-feeding bivalves and used literature values as proxies for detritus. These food-web biomarkers reveal that seagrass-associated consumers derived 20–23% of basal primary production from the saltmarsh, regardless of saltmarsh proximity. Seagrass sources (epiphytes and vegetation) as well as POM, likely were the dominant contributors for these consumers. This finding indicates that other mechanisms (i.e., refuge provision) might be the primary driver resulting in increased nekton density within seagrass along seagrass-saltmarsh ecotones. While a synergistic restoration of saltmarsh and seagrass habitat may have a positive effect on faunal biomass, it appears there may not be a primary food subsidy benefit to seagrass-associated consumers.