Patterns in the distribution and abundance of zebra mussels (Dreissena polymorpha) in the St. Lawrence River in relation to substrate and other physico-chemical factors

Using SCUBA and an in situ method of quantifying substrate characteristics, I describe patterns of zebra mussel (Dreissena polymorpha) distribution along the St. Lawrence and Hudson Rivers and in Oneida Lake, New York, and develop empirical models for their abundance. Calcium-poor waters originating from rivers draining the Canadian Shield resulted in a lack of zebra mussel along the north shore of the St. Lawrence River east of Montreal until Portneuf despite an abundance of suitable substrate. Calcium concentrations of 15 mg/L or less were found to limit the distribution of zebra mussel. The entire south shore from Cornwall, Ontario to Ile d'Orleans, Quebec was colonized by zebra mussel wherever suitable substrate was found. In the Hudson River, along the south shore of the St. Lawrence River and in Oneida Lake variability in density was primarily related to substrate type which explained between 38% and 91% of the variance. Other factors such as Secchi depth, calcium concentration of the water, the presence of crayfish, native unionid abundance and the maximum width of the river at the site increased the amount of explained variance across the different systems. The influence of substrate type on zebra mussel density and the predictions of the model were also tested using data from the literature, where substrate type explained 75% of the variability in density. The scatter of the literature data above the predictions of the empirical model suggests that North American zebra mussel populations may continue to grow before reaching equilibrium levels. While water chemistry parameters may be useful predictors of the presence or absence of zebra mussel in a given water body, physical factors play a far greater role in determining local abundance