ABUNDANCE OF DREISSENA-POLYMORPHA AND DREISSENA-BUGENSIS IN A WARMWATER PLUME - EFFECTS OF DEPTH AND TEMPERATURE

The generalization that quagga mussel (Dreissena bugensis) and zebra m ussel (Dreissena polymorpha) occur in deep-cold water and shallow-warm water, respectively, is inappropriate. We estimated dreissenid densit ies at sites affected by warmwater discharges into Lake Erie (Nanticok e, Ontario) to separate the effects of depth and temperature. Both spe cies' densities increased with depth. After controlling for depth, qua gga mussel were more abundant at thermally enriched sites than at site s unaffected by the discharge. This suggests that quagga mussel may oc cur in deep water not because such sites are cooler in summer, but bec ause they are warmer over winter because of the inverse stratification in the range 0-4 degrees C. Zebra mussels were unaffected by the warm water plume at a broad scale, but were almost entirely absent from a s ite al the mouth of the discharge canal. This is inconsistent with pre vious indications that zebra mussel can tolerate higher temperatures t han quagga mussel. Even in shallow water unaffected by the discharge, quagga mussel were more abundant than zebra mussel. Because Nanticoke was among the first places in North America to be colonized by quagga mussel, their numerical dominance at Nanticoke may be followed by simi lar changes in other nearshore areas of the lower Great Lakes.