Zebra Mussels in a Coastal Marsh: The Seasonal and Spatial Limits of Colonization

We investigated the seasonal cycles and spatial distribution of zebra mussels (Dreissena polymorpha) in a coastal marsh in Saginaw Bay, Lake Huron, from 1993 to 1995. This marsh was characterized by a sandy substrate and ice scour which removes emergent vegetation each winter. Due to this lack of permanent substrates, we expected that populations in the marsh would only be maintained during the ice free season, with recruitment from open waters responsible for recolonization each year. Instead, we found that substrate independent clusters over-wintered in the zone of emergent vegetation and were the source of recruitment to the marsh. Peak larval production occurred during July of both years with greatest densities between 300 and 350 m from shore. Settlement of these larvae onto Scirpus stems occurred by late August, accounting for the greatest numbers of mussels attached to stems. Mussel abundance was highest between 300 and 450 m from shore, but was very low inshore of 300 m. Losses of attached mussels from Scirpus stems reduced abundances by as much as 80% by the end of September 1994 despite the fact that the stems were still intact and healthy. The greatest losses occurred between 300 and 400 m from shore, while mussel abundance was increasing at the bayward edge of the emergent vegetation. The high abundance of larvae and attached juveniles near the center of the marsh, as well as the high loss of attached mussels in this area, may be related to a recently described form of horizontal stratification in coastal wetlands.