Status of Georgian Bay wetlands

Chow-Fraser, P. and Croft, M.
Unpublished technical report
2015 March


There are more than 3700 coastal wetlands along the eastern coast of Georgian Bay. They are unique among other coastal wetlands in the basin of the Laurentian Great Lakes, being some of the most pristine (Chow-Fraser 2006; Cvetkovic and Chow-Fraser 2011), and receive minimal impacts from agriculture, industry or urbanization. The largest current threat to coastal wetlands comes from increased cottage development and recent sustained low water levels (Midwood and Chow-Fraser 2012; Fracz and Chow-Fraser 2013; Leblanc et al. 2014). Secondly, the geomorphology of these wetlands is very different compared to those in Lakes Erie and Ontario, which are often densely vegetated with large expanses of cattails (Typha spp.) and are nutrient rich. These large marshes of the lower Great Lakes have underlying sedimentary bedrock and shallow slopes, often occurring at
rivermouths, behind barrier beaches and in protected embayments. By contrast, the wetlands in
eastern Georgian Bay tend to be small (< 2 ha), and form wetland complexes that function as
large wetland units (Midwood and Chow-Fraser 2015).

These wetlands in eastern Georgian Bay have pre-Cambrian granitic rock that shapes the morphology of the wetlands. There are, however, some wetlands in the southeast portion of Georgian Bay that have sedimentary limestone bedrock, similar to that in the Bruce Peninsula and Manitoulin Island (Midwood et al 2012). The third aspect that makes eastern Georgian Bay unique is the archipelago, rumoured to have 30,000+ islands, that dot the coast between Severn Sound and the French River. Often, there are small protected wetlands in the lee of these islands that provide excellent habitat for turtles and fish.

Similar to other wetlands of Lake Huron, those in Georgian Bay are naturally oligotrophic and have bulrushes (Schoenplectus spp.) as the dominant emergent vegetation. Lakes Huron-Michigan have experienced sustained low water levels since the late 1990s. The lack of inter-annual water-level fluctuation has negatively impacted both the species richness and diversity of plant and fish communities in the wetlands (Midwood and Chow-Fraser 2012). This is because changing water levels prevent the establishment of plant monocultures, allowing species to occupy different niches as the water level changes, and diverse aquatic plant communities tend to attract a diverse community of fish (Cvetkovic et al. 2010). The low water levels have been attributed to factors including long-term climate variability and the more recent effects of global climate change, glacial isostatic rebound, as well as past dredging of the St. Clair River.

This report summarizes the ecosystem status of coastal wetlands in eastern and northern Georgian Bay and the North Channel according to quaternary watersheds. Wetlands had been sampled with the same protocols for water quality, macrophyte and fish communities between 1998 and 2014.