Transformation of Freshwater Ecosystems by Bivalves - A case study of zebra mussels in the Hudson River

19 B ivalves (clams and mussels) are among the most familiar of aquatic organisms. Many have been used by humans for centuries as important sources of food and ornament, and some species are economically important pests, fouling water intakes and other structures. It is only recently, however, that ecologists have begun to understand that bivalves also play many important roles in ecosystems (e.g., Dame 1996). The functional importance of bivalves, especially in fresh water, is still not fully appreciated. For example, recent fresh water ecology textbooks (Wetzel 1983, Horne and Goldman 1994, Allan 1995, Petts and Calow 1996) scarcely mention the ecological roles of bivalves (the words “bivalve,” “clam,” and “mussel” do not even appear in the index of any of these books). By contrast, pelagic grazers, such as Daphnia, are widely recognized as having strong effects on ecosystems (e.g., Leibold 1989, Carpenter and Kitchell 1993, Jürgens 1994, Pace et al. 1998b). In this article, we show that bivalves are in fact dominant filterfeeders in many shallow-water ecosystems. Moreover, human activities often radically alter the density and composition of bivalve communities, in so doing inadvertently transforming ecosystem structure and function. We illustrate these points with a detailed case study of the effects of the zebra mussel (Dreissena polymorpha) invasion on the Hudson River ecosystem.