High-marsh invertebrates are susceptible to eutrophication

In salt marshes, high-marsh habitats are infrequently flooded (typically only during spring tides). Organisms in these habitats, however, may still be susceptible to the effects of increased nutrients delivered by tidal water (i.e. eutrophication). In a Massachusetts salt marsh, I examined the responses of the epibenthic invertebrates in the Spartina patens-dominated high marsh to long-term (7 yr) and landscape-level (4-5 ha) nutrient enrichment. In this ecosystem-level experiment, nutrients (N and P; similar to 15x reference conditions) were added to the flooding waters of tidal creeks-which flooded the high marsh during spring tides-to mimic cultural eutrophication. Three detritivores: Melampus bidentatus (gastropod), Philoscia vittata (isopod), and Orchestia grillus (amphipod) numerically dominated the benthic invertebrate community (97% by abundance). These species had higher densities (47 to 199% increase) in enriched versus reference creeks. Melampus size structure shifted to larger individuals with enrichment. End-of-season aboveground biomass and detritus stocks of S. patens did not differ between treatments; thus, increased litter quality and/or alternative food-source increases (e. g. microbes) led to increased detritivore density/biomass. Predator densities (spiders and Tabanus larvae) increased 125 to 160% with enrichment, likely due to increased prey densities (including Orchestia and Philoscia). Analysis of similarities (ANOSIM) revealed that communities were dissimilar between treatments; differences were driven primarily by changes in detritivore abundance. These results suggest that despite being infrequently flooded and thus infrequently exposed to elevated nutrients, high-marsh invertebrates are susceptible to eutrophication. Hence, the high marsh should be integrated into our understanding of how eutrophication impacts saltmarsh functioning.