Simulated changes in extent of Georgian Bay low-marsh habitat under multiple lake levels

The extent of coastal wetlands in Georgian Bay is controlled primarily by the water level of Lake Huron, which directly affects the amount of critical habitat available for fish and wildlife communities. Lake-levels have historically fluctuated by nearly 2 m and that range could increase in the future. This prompted us to investigate how quantity and quality of wetland habitat in Georgian Bay may be affected by different lake-level scenarios. The extent of low-marsh habitat was modeled with a generalized linear model that used hydrogeomorphic features (i.e. depth, slope, and exposure) as predictors. We simulated lake levels between 175.5 m and 177.5 m at 0.5 m-increments, and found that the total area of low marsh peaked at 176.0 m (7113 ha) and declined sharply as lake levels increased or decreased. In contrast, low-marsh volume was highest at 176.5 m (3.84 × 107 m3) but remained relatively stable across all modeled lake levels. We derived an average elevation profile for low-marsh habitat across the study area that showed a shallow “step” between 175.5 and 176.0 m, flanked by steeper upslope and downslope sections. At historically low lake levels low-marsh habitat would have been dominated by shallow water (< 0.5 m), whereas at higher lake levels it would have been dominated by deeper (0.5–2.0 m) water. The geomorphology at low lake levels (i.e. 176.0 m) appears to favour large areas of shallow habitat at the expense of deeper habitats that could have supported more structurally complex, submersed aquatic vegetation.