The effects of invasive dreissenid mussels on the offshore food web of Lake Simcoe, Ontario

Abstract

Dreissenid mussels have successfully invaded North America, including the Great Lakes and their surrounding waterbodies. With their establishment, there have been many notable impacts including the rerouting of energy flow from offshore to nearshore energy sources. This study aimed to determine if such a shift had occurred in Lake Simcoe, Ontario following the establishment of zebra mussels (Dreissena polymorpha) and, more recently, quagga mussels (Dreissena rostriformis bugensis) with an emphasis on the fishes in the offshore region. With the relatively long time period of 13 years between establishment dates of zebra mussels and quagga mussels, Lake Simcoe provided a unique opportunity to differentiate the possible effects of zebra mussels independently from quagga mussels. All offshore fish investigated in this study appeared to decrease in trophic position (TP), closely following dreissenid invasion, except for lake trout, which increased. Interestingly, for smelt, herring, and lake trout this result may not be due directly to mussel establishment, but rather other factors impacting Lake Simcoe such as changes to the zooplankton community, as well as increased natural recruitment and ongoing stocking practices of lake trout and whitefish. Whitefish trophic position appeared to be consistent with increased dreissenid mussel abundance, as observations of whitefish stomachs in recent years revealed heavy reliance on dreissenids. In addition, and contrary to expectations, the percentage of littoral carbon incorporated in cold water fish decreased over time, especially in the most recent years. A shift towards nearshore resources was observed following zebra mussel invasion in 1996 among all offshore fish species, however, this quickly shifted towards more offshore resources in the following years. Therefore, it appears that the invasion of dreissenid mussels to Lake Simcoe has produced a shift in energy pathways to favour more nearshore benthic-littoral energy production, in a similar manner to observed changes in the Great Lakes following dreissenid establishment. However, shifts in energy pathways in Lake Simcoe were observed to correct back to more offshore pelagic sources in years following initial dreissenid establishment. Ongoing monitoring of the Lake Simcoe offshore fish community is important to understand the effects of dreissenid mussels into the future.