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 mussel (Dreissena polymorpha) occur in deep-cold water and shallow-warm water, respectively, is inappropriate. We estimated dreissenid densities at sites affected by warmwater discharges into Lake Erie (Nanticoke, Ontario) to separate the effects of depth and temperature. Both species' densities increased with depth. After controlling for depth, quagga mussel were more abundant at thermally enriched sites than at sites unaffected by the discharge. This suggests that quagga mussel may occur in deep water not because such sites are cooler in summer, but because they are warmer over winter because of the inverse stratification in the range 0-4 degrees C. Zebra mussels were unaffected by the warmwater plume at a broad scale, but were almost entirely absent from a site al the mouth of the discharge canal. This is inconsistent with previous indications that zebra mussel can tolerate higher temperatures than 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 similar changes in other nearshore areas of the lower Great Lakes.