Phosphorus Recycling By Profunda Quagga Mussels in Lake Michigan

Abstract

Quagga mussels (Dreissena rostiformis bugensis) act as ecosystem engineers in the southern basin of Lake Michigan, altering physical habitats and biogeochemical processes. Adapted to cold and oligotrophic conditions, profunda quagga mussels thrive on the soft substrate of deeper depths. At a 55 m site (10,000 mussels m-2) offshore from Milwaukee, WI, profunda mussel biomass (g m-2) was 1/3 of biomass (g m-2) measured at a 10 m comparison site (5,000 mussels m-2). Higher densities but less biomass is due to profunda mussels having less tissue for a given length and the population per m2 comprising of mostly small mussels ( Internal recycling is a critical component of Lake Michigan's P cycle. In the pre-dreissenid period, P was utilized in the hypolimnion by phytoplankton, settled passively to the lake bottom, and largely returned to the system via resuspension. Profunda quagga mussel grazing has altered P fluxes by reducing sediment resuspension and increasing nutrient cycling in the benthos. Mussel grazing rates are higher than passive settling rates due to vertical mixing replenishing the food supply above mussel beds. Dreissenids act as both nutrient recyclers and sinks, but in the deeper depths of Lake Michigan, profunda quagga mussels filtering year-round appear to have altered P residence time in the benthos.