Ecosystem-Level Effects of Zebra Mussels (Dreissena polymorpha): An Enclosure Experiment in Saginaw Bay,Lake Huron

We examined the short-term effects of zebra mussels (Dreissena polymorpha) on ecosystem processes in late August 1991 in Saginaw Bay, Lake Huron. Four 1,600-L enclosures, made of Fabreen with a diameter of 1 m, a depth of 2 m, and closed at the bottom, were used to enclose natural plankton communities. These communities were dominated by diatoms with some chlorophytes, chrysophytes, and cyanophytes. Phytoplankton growth was limited by P-availability. Two enclosures were held as controls, and zebra mussels encrusting unionid shells were suspended in two of the enclosures: one enclosure (HZ) contained approximately four-fold greater numbers of mussels than the other (LZ). The concentration of suspended particles, chlorophyll, and algal biomass in HZ and LZ declined over a 6-day interval. Diatom numbers declined more than other taxa. Phytoplankton growth rates in HZ and LZ increased to near μ_max; there was no apparent change in photosynthetic parameters a or P_max scaled for chlorophyll. Soluble reactive P (SRP) increased significantly (p < 0.05) in HZ but not LZ. Dissolved organic P (DOP) and ammonium ion were elevated; dissolved organic carbon (DOC) was unchanged in HZ and LZ. The rate of phosphate uptake by bacteria and algae declined to less than 2% of controls; this rate decrease could not be explained simply by grazing losses or isotope dilution. The rate of ammonium regeneration by the plankton and the potential rate of ammonium uptake by the plankton did not differ significantly in HZ or LZfrom the control enclosures. Our findings indicate that the zebra mussel can have significant short-term effects on phytoplankton abundance, water transparency, water chemistry and phosphorus dynamics. We propose a model of zebra mussel effects that suggests high densities of zebra mussels may indirectly alter and control those processes that are rate-limited or concentration-limited by nutrient availability.