FILTRATION-RATES OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA) ON NATURAL SESTON FROM SAGINAW BAY, LAKE HURON

Filtration rates of the zebra mussel (Dreissena polymorpha) on natural seston from two different regions in Saginaw Bay were determined on a monthly basis from April to October in 1992 and 1993. The two regions represent contrasting trophic conditions, with the inner bay more eut rophic than the outer bay. Mean filtration rate was 16.2 mL/mg/h (rang e 4.0 to 40.7 mL/mg/h) over the entire 2-year period. Filtration rates on seston from the inner bay were significantly lower than rates on s eston from the outer bay in 1992, but no differences were apparent in 1993. Lower rates were attributed to higher concentrations of seston ( chlorophyll, particulate organic carbon, and total suspended solids) f ound in the inner bay in 1992. in 1992, overall filtration rates were related to seston concentrations as described by a negative exponentia l function. In 1993, seston concentrations were uniformly low, and a r elationship between filtration rates and concentrations was not observ ed Further, filtration rates were not related to seston composition, a s determined by the ratio of POC:TSS and chl:TSS. Maximum filtration r ates were apparently related to temperature, with highest maximum rate s occurring at 10-20 degrees C. Based on measured filtration rates and overall standing stocks, the Dreissena population in the inner bay wa s capable of filtering the volume of the inner bay 1.3 times per day i n 1992 and 0.2 times per day in 1993.