Numerous nonnative species have invaded New York State’s Oneida Lake in recent decades and have had a lasting impact on the ecosystem structure. Among the most recent and successfully established invasives are two dreissenid mussels, zebra mussels (Dreissena polymorpha) and quagga mussels (Dreissena rostriformis bugensis) These mussels are important ecosystem engineers that significantly modify their environment by improving water clarity, transferring resources to the benthic sediment, and reducing water column chlorophyll a concentrations. The round goby (Neogobius melanostomus), another potentially high-impact invader, recently established in Oneida Lake and feeds heavily on these mussels. In lakes where the two dreissenid species co-occur, quagga mussels are known to displace zebra mussels, although the reasons for this are not well understood. The purpose of this study was to investigate the mechanisms allowing quagga mussels to outcompete zebra mussels by comparing the growth and mortality rates in the presence of round gobies. To determine growth and mortality rates, we placed quagga and zebra mussels in open and closed minnow traps for two months and recorded their measurements at the start and the end of a two-month period. We expected quagga mussels to have a higher growth rate and a lower mortality rate than zebras.
Results/Conclusions:
The zebra mussel mortality rate was 20% higher than quagga mortality rate. Smaller mussels (<12mm) were consumed more than large mussels (>12mm), suggesting that round gobies select for zebra mussels because they are typically smaller. Open traps that were exposed to predation had a higher mortality rate than closed traps for zebra mussels, but not quaggas. Contrary to our hypothesis, zebra mussel growth rate was higher than that of quagga mussels. The size-specific predation of gobies on small zebras over larger quaggas suggest a shift in population size structure towards larger mussels. If round gobies have a stronger impact on zebra mussels than quagga mussels, we expect that the invasion of round gobies will hasten the zebra mussel decline, allowing quagga mussels to dominate. In the past two years, we have observed a decline in total mussel biomass in Oneida Lake which may be due in part to the rapid growth of the goby population. If these trends continue, we expect lake-wide ecological changes such as decreased water clarity in the near future. Thus, continued monitoring of dreissenid mussels and round goby populations will be important to anticipating the trajectory of ecosystem changes in Oneida Lake.