U.S. Department of Commerce


Date of this Version



Published in J. Great Lakes Res. 21(4):411-416.


After the zebra mussel, Dreissena polymorpha, was first discovered in Lake St. Clair in 1988 and rapidly spread throughout the Great Lakes, broad ecological changes became apparent in regions where this species was most abundant. Previous investigations documented dramatic increases in water clarity (Hebert et al. 1991, Marsden et al. 1993, Leach 1993), declines in chlorophyll and phytoplankton abundances (Leach 1993, Nicholls and Hopkins 1993, Holland 1993), and changes in benthic invertebrate communities (Griffiths 1993, Dermott et al. 1993, Stewart and Haynes 1994). These studies were conducted in various regions and defined impacts on a specific ecosystem component. Taken together, results indicate a major shift in energy flow patterns from the pelagic to the benthic region. Such a shift is so fundamental that every component of the food web from bacteria to fish would likely be affected either directly or indirectly. Thus, to truly understand the significance of shifts in energy flow patterns, impact assessments should be as holistic as possible within a given ecosystem. Despite the magnitude of observed impacts, however, establishing cause-effect relationships at the ecosystem-level is not an easy task. Not all changes occur at the same rate, and natural variation, along with man-induced influences such as nutrient abatement programs, confound interpretations of any mussel-induced impacts. For this reason, ecosystem- 411 level impacts can best be examined, interpreted, and certainly modeled, by integrating studies of both ecosystem structure and function within a specific, well-defined region.