Benthic macroinvertebrate diversity in a shallow estuary: controls on nutrient and algal dynamics

Nutrient loading has produced radical shifts in the structure and function of shallow estuaries, with substantial impacts upon the community composition of invertebrates and autotrophs. The progression of eutrophication is regulated by both bottom-up and top-down factors, which may be mediated by fauna via trophic interactions and nutrient release. We conducted field surveys of benthic invertebrates in West Falmouth Harbor (WFH), MA, an estuary subject to localized wastewater enrichment, across a 1 km gradient of suspected degradation. Dominant taxa included gammarid amphipods and nereidid, orbiniid, and capitellid polychaetes, with lower diversity and opportunistic species characterizing the inner basin nearest the sewage plume. Using a series of microcosm experiments with WFH sediments, we investigated the effects of macroinvertebrate diversity on the biomass and productivity of macroalgae in monoculture and polyculture, and upon benthic microalgae and nutrient fluxes in the absence of macroalgae. Microcosms included single- and mixed-species treatments comprised of common fauna with different functional traits, as related to feeding behaviors, mobility, and modes of bioturbation. The burrowing, omnivorous polychaete Alitta (formerly Nereis) virens stimulated benthic microalgal growth, inhibited ammonium fluxes from sediments to the water column, and decreased macroalgal standing stocks. In polyculture, A. virens preferentially removed annual, bloom-forming taxa, and reduced the tissue nitrogen content of the remaining perennial macroalga. Conversely, the surficial, deposit-feeding gastropod Ilyanassa obsoleta reduced benthic microalgae, enhanced nutrient release, and maintained macroalgal biomass and productivity. Within mixed-fauna assemblages, A. virens disproportionately diminished macroalgal biomass. Benthic communities dominated by I. obsoleta or functionally comparable species could thereby accelerate feedbacks with eutrophication, through bottom-up supply and indirect promotion of macroalgal blooms. In contrast, activities of A. virens or similar fauna may buffer symptoms of nutrient loading. These findings have important implications for linking biodiversity, on multiple trophic scales, to ecosystem functioning in shallow estuaries