Filtration and Excretion by Zebra Mussels: Implications for Water Quality Impacts in Lake Pepin, Upper Mississippi River

Because they are filter-feeders, zebra mussels can significantly reduce concentrations of seston (i.e., suspended particulate matter including algae and bacteria), resulting in increased water clarity (Reeders and de Vaate 1990; Holland 1993; Fanslow, Napela, and Lang 1995; Madenjian 1995; Klerks, Fraleigh, and Lawniczak 1996). Conversely, zebra mussels can impair water quality by excreting soluble nutrients and by increasing sediment biological oxygen demand (Andersson, Graneli, and Stenson 1988; Effler and Seigfried 1994; Gardner et al. 1995; Heath et al. 1995; Holland, Johengen, and Beeton 1995; Mellina, Rasmussen, and Mills 1995; Arnott and Vanni 1996; Effler et al. 1996; James, Barko, and Eakin 1997a; Effler et al. 1997). Since positive and negative effects on water quality can vary with differences in hydraulic residence time, thermal structure, availability of dissolved oxygen, seston loadings, and biotic variables, there is a need to determine the effects of zebra mussel filter-feeding activities on water quality conditions in aquatic systems. Relationships between seston filtration and soluble nutrient excretion were examined over a range of zebra mussel shell lengths (6 to 32 mm). This information was used in conjunction with zebra mussel shell length frequency distribution and areal estimates of zebra mussel population density to predict overall areal filtration and soluble nutrient excretion rates in Lake Pepin, Upper Mississippi River. Zebra mussels were first found in Lake Pepin in 1994 and have increased in population density considerably over the last 4 years.