Filtration and excretion by zebra mussels: Implications for water quality impacts in Lake Pepin, upper Mississippi River

Filtration and soluble nutrient excretion were examined over a range of zebra mussel (Dreissena polymorpha) shell lengths (range = 6 to 32 mm) in experimental laboratory systems and combined with in situ shell length frequency distribution and areal estimates of zebra mussel population density to make predictions of overall areal filtration and soluble nutrient excretion rates in Lake Pepin, upper Mississippi River, USA. Zebra mussels removed seston and excreted ammonia and soluble phosphorus in laboratory systems. When normalized with respect to ash-free dry mass (i.e., mug g(-1) AFD mass d(-1)), smaller zebra mussels filtered seston and excreted soluble nutrients at a higher rate than larger zebra mussels. Although overall zebra mussel density is currently very low in Lake Pepin (similar to 150 ind. m(-2)), lakewide chlorophyll filtration rates of similar to7 mg m(-2) d(-1) were equivalent to chlorophyll loading into Lake Pepin via external sources and represented a turnover of 11 days for chlorophyll standing crop in the lake. Lakewide estimates of excretion of soluble phosphorus of similar to3 mg m(-2) d(-1) by zebra mussels in Lake Pepin were comparable to internal phosphorus loading from anoxic sediments in other eutrophic freshwater aquatic systems. Our results suggest that zebra mussels are currently having a modest impact on phytoplankton dynamics and P and N recycling in this system.