Until 1992, zebra mussels (Dreissena polymorpha) were an important food source for diving ducks in Lake Markermeer (The Netherlands). After 1993, the mussel biomass sharply declined, and the current population is in poor condition (maximum shell length <15 mm) compared to populations from adjacent Lake IJsselmeer. Lake Markermeer is characterized by high concentrations of total suspended matter (TSM), including 70% inorganic suspended sediments (ISS) affected by wind-driven resuspension, which may influence the food uptake by the mussels. We assessed whether the poor condition of the zebra mussels in Lake Markermeer could be attributed to the high TSM concentrations, and especially the fraction of ISS. In situ measurements of near-bottom TSM concentrations were linked to wind-driven wave action, and experiments were carried out using increasing natural sediment (NS) concentrations combined with afixed concentration of a green alga (Scenedesmus obliquus) to determine the clearance rates (CR) of adult and juvenilezebra mussels. CR on NS particles decreased with increasing NS concentrations. The CR on S. obliquus, however, increased with an increasing NS concentration up to 0.75 g L-1 for adult mussels, indicating that zebra mussels can tolerate relatively high TSM concentrations and selectively graze on phytoplankton. Zebra mussels eventually reduce their CR at high TSM concentrations; the CR of juvenile mussels on S. obliquus collapsed at NS concentrations >0.5 g L-1. The NS concentrations used in this study are high, but analyses of the near-bottom TSM concentrationsshow that these values occur frequently and may be partly responsible for the poor mussel status in Lake Markermeer. |
