Effects of zebra mussel invasion on primary production and unionid mussel survival in Douglas Lake, Michigan.

Abstract

Douglas Lake, a northern Michigan kettle lake, was invaded by zebra mussels (Dreissena polymorpha) in 2001. We studied the effects of this invasion on primary production and native mussels (Unionidae). We analyzed the chlorophyll a content of benthic algal matrix removed from rocks in areas with many and few zebra mussels, and observed a decrease in primary production with increased zebra mussel pumping. We sampled zebra mussel volume on macrophytes from two depth transects, and found a relationship between zebra mussel volume and macrophyte dry mass (y=3.34x+0.17, p<0.0001, r=0.9932), but not percent organic mass (y=56.44x+0.07313, p=0.44998, r=0.34). We speculate that zebra mussels colonize macrophytes differentially by plant morphology. We sampled unionid mussels and compared their ratios of alive to dead with the zebra mussel biovolume encrusting them in deep vs. shallow water. Analysis of Variance revealed a significant difference in the ratio of live unionids to dead between shallow and deep sites (F=6.46383, df=1, p=0.018203) and among species (F=8.13166, df=2, p=0.002133), and a Least Squares Difference test for difference in zebra mussel volume between shallow and deep water was also significant (df=10, p=0.028716), which we interpret as evidence of a refuge for unionids from zebra mussel colonization.