Benthic macroinvertebrate functional diversity regulates nutrient and algal dynamics in a shallow estuary

Proliferation of macroalgal blooms is regulated by grazing pressure and nutrient availability, which may be mediated directly by benthic macroinvertebrates or indirectly through feedback mechanisms. Using invertebrates common to a shallow estuary in Cape Cod, Massachusetts (USA), we determined effects of faunal diversity on benthic microalgae, net ecosystem metabolism, sediment nutrient fluxes, and macroalgal biomass and productivity. Laboratory microcosms contained sediments with single- and mixed-species invertebrate assemblages, in the presence of (1) no macroalgae, (2) a macroalgal monoculture, and (3) a realistic macroalgal polyculture. The deposit-feeding gastropod Ilyanassa obsoleta suppressed benthic microalgae, enhanced nitrate efflux from sediments, and maintained macroalgal standing stocks. Conversely, the burrowing, omnivorous polychaete Alitta (formerly Nereis) virens stimulated benthic microalgal growth, inhibited efflux of ammonium, and drastically reduced macroalgal biomass via grazing and translocation of thalli below the sediment surface. In the polyculture experiment, A. virens sequentially removed Gracilaria sp. (Rhodophyta), Ulva sp. (Chlorophyta), and finally Fucus vesiculosus (Phaeophyta). The bivalve Mya arenaria exhibited limited effects on benthic dynamics. In mixed-fauna assemblages, biomass and productivity of benthic microalgae and macroalgae were consistently lower than predicted, revealing non-additive effects of biodiversity. Communities dominated by I. obsoleta or other surficial grazers could indirectly promote macroalgal blooms via sustained release of sediment-derived nutrients and reduction of benthic microalgae. In contrast, omnivorous burrowers such as A. virens may buffer symptoms of eutrophication through inhibition of ammonium supply and direct grazing of bloom-forming macroalgae. Overall, our results highlight species-specific effects on key ecosystem functions, and demonstrate important feedbacks between top-down and bottom-up controls in shallow estuaries.