Top-down Control by Ribbed Mussels (Geukensia demissa) on Plankton Communities in Southeastern U.S. Salt Marshes

Abstract Benthic grazers and suspension feeders, such as oysters and mussels, may play a disproportionate role in controlling algal biomass and plankton community composition in estuarine systems. Nonetheless, their effects on plankton communities remain poorly understood across environmental contexts, in part from a paucity of in situ measurements. Here, we conducted in situ manipulative field experiments to evaluate the effects of the Atlantic ribbed mussel, Geukensia demissa , on bulk chlorophyll a (µg L − 1 ); absolute cell abundance (individuals mL − 1 ) of plankton groups > 10 μm in diameter (i.e., pennate and centric diatoms, dinoflagellates, and heterotrophic nano- and microzooplankton); and relative cell abundance (%) of plankton trophic levels (i.e., auto-, mixo-, and heterotrophic) across three salt marsh sites in the southeastern U.S. (Georgia and Florida). Mussel grazing reduced both chlorophyll a concentrations and total absolute cell abundances by 120–150% compared to cordgrass only plots and the ambient water column. Mussel effects on plankton groups were consistently strong and negative, ranging from − 1.12 to − 2.51. At two of the three sites, mussels reduced pennate and centric diatoms to a greater extent (> 155%) than dinoflagellates (~ 100%). This selective grazing likely shifted plankton community composition toward a more mixotrophic structure, whereby the relative contribution of autotrophic diatoms declined from roughly 80% and 65% in ambient waters to 70% and 45% in mussel plots, respectively, while the relative abundance of dinoflagellates increased. Given the essential role of the plankton community in mediating the ecological and geochemical functioning of estuaries, our results highlight that, within natural salt marshes, mussels play a significant role each tidal cycle in modifying such functions through their prolific and selective grazing on this community.