EVOLUTION OF THE GREAT LAKES ECOSYSTEMS THROUGH TIME: ICE, INNOVATION, INTRODUCTION, AND INVASION

The ecosystems of the Great Lakes have been remarkably dynamic since they formed approximately 5,000-7,000 years ago, after the retreat of the Laurentide ice sheet that covered the entire area of the modern lakes. Post-Pleistocene fossils deposited in the drainage basin after the retreat of the ice include rare specimens of walrus and whales. Fish and other aquatic species, including unionid bivalves, repopulated the deglaciated lake basins primarily from the Mississippi Basin. Relict Arctic fish species such as grayling persisted in the basin until habitat destruction and overfishing eliminated them. For similar reasons, native Atlantic salmon in Lake Ontario are now rare. More recent changes in Great Lakes fauna have been linked to intentional and unintentional introductions of non-native species. Sea lamprey invaded the upper lakes through the Welland Canal, which was built to bypass Niagara Falls and opened in 1929. These fish parasites virtually eliminated populations of the native lake trout. As a result, abundances of alewife, another non-native species that served as prey for the lake trout, increased dramatically. Salmonids were intentionally introduced to the Great Lakes and their tributaries to control alewife and support a sport fishery; some salmonid species continue to be actively stocked each year. Trans-Atlantic introductions of species from the Ponto-Caspian region have mostly come through ballast water discharge from ships, and have included dreissenid bivalves (zebra and quagga mussels), the round goby, and the spiny water flea. Species such as these have severely altered the lower food web and have been linked to rapid declines of important native invertebrates such as Diporeia. Many threats exist to the future of healthy Great Lakes ecosystems. Range expansion of harmful, non-native species and range contraction of native species due to climate change are likely. Continued nutrient pollution may increase harmful algal blooms. Potential introductions of species such as Asian carps, “killer shrimp”, or hydrilla further threaten the health of already-disturbed ecosystems. Recent efforts that may help to stabilize Great Lakes ecosystems include work funded under the U.S. Great Lakes Restoration Initiative and the development of new ballast water regulations by the U.S. Coast Guard.