Colonization patterns and densities of zebra mussel Dreissena in muddy offshore sediments of western Lake Erie, Canada.

Zebra mussels (Dreissena) have expanded rapidly throughout most of the Laurentian Great Lakes since their inadvertent release in 1986. These exotic molluscs now occur in great numbers on the bottom of western Lake Erie where they are found increasingly in deeper areas of the basin (average depth: 10 m), on soft, muddy substrates. This study is aimed at quantifying the density and the distribution patterns of mussel colonization in the basin as a first step in investigating the effect on sediment properties of such an abrupt change in benthic community structure. Underwater video imagery and diver-collected samples taken from representative offshore areas (seven sites) in western Lake Erie showed colonization levels of up to 20,000 live mussels per m(2) in soft sediments (adults with shells >10 mm comprised 47%). Digital side-scan sonar records confirmed that colonization patterns were not random but showed distinctive spatial signatures ranging from 30-m-long parallel stripes, to large ovate masses. Broad irregular mats were found in association with hard bottoms (bedrock, boulders, or wrecks and large debris). Mussel densities were averaged from the sites, assuming consistent relationships with substrate type and were combined with digitized percentage of areal coverage of major bottom types in western Lake Erie. This resulted in the first population figure of 10(13) in the basin. This figure includes molluscs of all sizes > 0.84 mm.