Phosphorus recycling by profunda quagga mussels (Dreissena rostriformis bugensis) in Lake Michigan

The effects of dreissenid mussels on plankton abundance and nutrient cycling in shallow, productive waters of the Great Lakes have been well-documented, but the effects of their more recent expansion into offshore regions have received much less attention. Understanding quagga mussel impact on Lake Michigan's phosphorus (P) fluxes is critical in assessing long-term implications for nutrient cycling and energy flow. In this study, P excretion and egestion rates were determined for mussels in the hypolimnion of Lake Michigan. Constant low temperatures and limited food supply contributed to a lower basal P excretion rate in profunda quagga mussels compared to the shallow phenotype. The P excretion:egestion ratio was approximately 3:2, highlighting the need to consider both of these pathways when assessing the effect of these filter feeders on nutrient dynamics. Total dissolved P (TDP) excretion rates ranged from 0.0002 to 0.0124 μmol mgDW− 1 d− 1, soluble reactive P (SRP) excretion rates ranged from 0.0002 to 0.0061 μmol mgDW− 1 d− 1, and particulate P (PP) egestion rates (feces + pseudofeces) ranged from 0.0007 to 0.0269 μmol mgDW− 1 d− 1. The ability of profunda mussels to alter P cycling dynamics is reflected in an increased hypolimnetic dissolved:particulate P ratio and the disappearance of the benthic nepheloid layer. On an areal basis, mussel P recycling rates are up to 11 times greater than P settling rates as determined by sediment traps, suggesting that mussel grazing has resulted in an increased delivery rate of P to the deep benthos and a shorter P residence time in the water column.