ABSTRACT

As a result of their high densities and ltration rates, dreissenid mussels are a major sink for particulate nutrients in lakes and rivers. These consumed nutrients are allocated to one of three fates: egestion as particulate feces and pseudofeces, assimilation into mussel biomass, and excretion in dissolved form. Of these three pathways, nutrient egestion and excretion are usually dominant, with the relative importance of egestion increasing when food supply is high and/or food quality is low. With the exception of several very high values, published mass-specic excretion rates generally range from 1 to 10 μmol gDW−1 h−1 for nitrogen and 0.1-2 μmol gDW−1 h−1 for phosphorus. Measured areal N excretion rates range from 155 to 624 μmol m−2 h−1, while those for P range from 4.3 to 213 μmol m−2 h−1. There is some evidence that P excretion rates for quagga mussels (Dreissena rostriformis bugensis) are lower than those

for zebra mussels (Dreissena polymorpha). Major factors that regulate nutrient excretion include temperature, mussel size, and food quantity/quality, with reproductive stage, dissolved oxygen concentration, and substratum type also playing apparent roles. At the ecosystem scale, the effect of nutrient recycling by dreissenids is modulated by hydrodynamic processes, including vertical and horizontal mixing, which inuence both the supply of phytoplankton to dreissenids and the distribution of excreted nutrients. Although the inuence of dreissenids on ecosystem-scale nutrient dynamics varies among systems, common ecosystem responses include a decrease in phytoplankton abundance, a decrease in total nutrient concentrations, an increase in the dissolved nutrient to phytoplankton biomass ratio, and an increase in the biomass of benthic macroalgae. In large lakes, the alteration of nutrient dynamics by dreissenids has led to a management conundrum in which maintenance of productive pelagic sh populations and reduction of

Harvey A. Bootsma and Qian Liao

CONTENTS

Abstract .................................................................................................................................................................................555 Introduction ...........................................................................................................................................................................556 Factors Regulating Nutrient Excretion by Dreissenids .........................................................................................................558