CLEARANCE RATES AND FILTERING ACTIVITY OF ZEBRA MUSSEL (DREISSENA-POLYMORPHA) - IMPLICATIONS FOR FRESH-WATER LAKES

Zebra mussel (Dreissena polymorpha) filter-feeding impacts on phytopla nkton in lakes depend on a number of factors including which phytoplan kton are grazed and the population filtration rate, which in turn depe nds on individual clearance rate (volume of water cleared of particles per unit time) and the percentage of mussels filtering. We used short -term fixed-volume suspension depletion experiments to compare clearan ce of different seston particles and to concurrently measure clearance rates and the percentage of mussels filtering for different sizes of zebra mussel both day and night. Zebra mussel readily cleared particle s less than or equal to 150 mu m, including cyanobacterial filaments, and large mussels collected particles as large as 1.2 mm. Clearance ra tes did not differ among six differently shaped phytoplankton taxa. Fi ltering activity (percentage of mussels actively filtering) was 6-9% h igher at night than during the day, but there were no diel changes in clearance rate. Clearance rate depended on mussel size, but filtering activity did not differ among shell length-classes. Filtering activity declined as zebra mussel depleted food concentration. Results suggest that filtering activity should be considered in population filtering impact assessments as a separate term from clearance rate because each may respond to different factors.