Multitemporal comparison of wetland communities in diked and undiked wetlands in southern Georgian Bay

Abstract:

Hydrological connection between coastal wetlands and the Laurentian Great Lakes plays an important role in maintaining aquatic biodiversity in the wetlands by preventing monocultures of emergent vegetation from forming, by facilitating frequent exchange of chemical constituents between the wetlands and lakes, and by allowing daily and seasonal migration of fish in and out of the wetlands. We hypothesize that when a wetland becomes impounded or diked, the emergent vegetation will expand at the expense of aquatic habitat (open water and submersed aquatic vegetation), the water chemistry in the marsh will become altered, and the diversity of the fish community will become reduced. Conversely, there should be no long-term impact on avian diversity where water levels are not actively manipulated to maintain bird habitat. We tested these hypotheses by comparing changes in the wetland communities of Wye Marsh, a diked wetland in Georgian Bay, with that of a nearby undiked wetland, Matchedash Bay. We used available historic air photos (from 1930 to 2008) to quantify the amount of aquatic habitat in both wetlands. Consistent with our prediction, the amount of aquatic habitat decreased significantly through time in Wye Marsh, but not in Matchedash Bay; instead, area of open-water in the undiked wetland varied directly with mean water levels of Georgian Bay. Water chemistry in both wetlands reflected surrounding agricultural land-use and exhibited differences that could be predicted on the basis of the hydrological connection with Georgian Bay. Whereas diversity of the fish community in Wye Marsh was significantly lower than that in Matchedash Bay, the avian diversity showed no significant differences. We determined that diking wetlands is not a suitable solution to limit the loss of wetland habitat due to declining water levels in Georgian Bay.