Effects of invasive zebra mussels on phytoplankton, turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.