Georgian Bay Wetlands

Status and long-term trends in ecosystem health of coastal wetlands of eastern Georgian Bay

Of the key ecosystem services provided by wetlands, one of the most important is provision of habitat for numerous fish, birds and wildlife species. Nowhere is this more important than in the thousands of pristine coastal marshes lining the shoreline of eastern Georgian Bay. Since wetlands occur at the interface between water and land, both aquatic species (fish, water lilies, pondweeds) and terrestrial species (wet meadow plants, insects), as well as semi-aquatic species such as turtles and amphibians use coastal marshes as habitat. A key component that maintains high biodiversity is the large inter-annual fluctuations in water levels that have varied by up to 2 meters between annual high and low water years over the past century. Since most wetland plants grow at an optimal water depth (for submerged or floating species) or soil wetness (for meadow/terrestrial species), as water levels go up or down, the plant communities must also shift with the shoreline accordingly. Repeated water-level cycles of 5 to 7 years prevent any one group of plants from dominating the landscape. A healthy aquatic plant community consists of a diverse mix of submersed aquatic vegetation (pondweed and waterweed), floating species (lilies and water shield) and emergent plants (bulrush and cattails) that create a three-dimensional structure allowing small fish to hide from predators, and large fish to ambush their prey.

Observations During Recent Low and High Water Levels

In 1999, water levels dropped below the long-term mean, and remained below historic norms for 14 years. During this time period, the McMaster team sampled wetlands to determine their ecosystem health using a suite of ecological indices. During this low-water period (2003 to 2009), submergent and emergent plants migrated lakeward, while meadow species migrated towards the shoreline and colonized the previously submerged land. Within 10 years, the meadow and upland species like pine trees had become firmly established, and in some cases, where rock sills prevented water of Georgian Bay from flooding the wetland, aquatic habitats simply dried up. Just as suddenly, however, water levels rose above the long-term mean beginning in 2014 and have remained high through 2019. For the first few years, a  majority of the 50 wetlands visited were still in very good health, as indicated by the Water Quality Index (WQI) scores, that consider the relative amount of naturally occurring primary nutrients, water turbidity and planktonic algae in water samples, and the Wetland Macrophyte Index (WMI) scores, that consider the presence of pollution intolerant plant species. Although the mean Wetland Fish Index (WFI) scores were still indicating “Very Good” status for wetlands, they had significantly declined whereas the WQI scores had significantly increased. The McMaster team attributes this discrepancy to the effect of increased water level on the catch efficiency of fykenets.

In 2019 it appears that the currently excessive high water is negatively impacting fish nursery and habitat in certain wetlands because of the negative impact on suitable emergent wetland growth. Further information will be shared as the McMaster team proceeds with this research, which is benefiting from logistical and financial assistance from Georgian Bay Great Lakes Foundation.

Non-native Invasive Species

Despite the relatively good health of Georgian Bay wetlands, they have not been immune to the spread of non-native invasive species. When the McMaster team sampled wetlands 10 or more years earlier, most of the wetlands were free of round gobies, whereas now they are present in nearly all sites although not necessarily in abundance. They only saw them in abundance in their nets for the first time in 2005 in Severn Sound. Notable invasive plants include eurasian milfoil (Myriophyllum spicatum) and the common reed (Phragmites australis). Eurasion milfoil is an aquatic species that can form large, dense patches in the water column of wetlands, and limit the foraging ability of resident fish species. Phragmites is a relatively recent invader that has spread throughout the province into coastal wetlands, and along highway and roadway corridors. An aggressive competitor, Phragmites is an emergent plant with showy seed heads that can spread by seeds or via underground stems. Its tendency to form tall dense stands makes them particularly devastating for migratory semi-aquatic species such as the Blanding’s turtles. Research in a Lake Erie wetland showed that Phragmites accounted for up to 170 ha (13%) of lost habitat to Blanding’s turtles. Given that many wetlands in Georgian Bay are small (< 2 ha), it is frightening to imagine what the establishment of this common reed could do within the Biosphere Reserve. All these threats are compounded by the effects of a changing climate. Shifts in seasonality, timing and duration of seasons, precipitation, and water levels all have the potential to directly and indirectly affect the functioning and quality of wetland habitats. The shift away from historic, normal conditions may affect the resilience of Georgian Bay’s wetlands and open the door for future invaders or facilitate the spread of those already present.

Use of wetland habitat in Georgian Bay for muskellunge spawning and nursery

The McMaster team’s ongoing research (2012 to present) on muskellunge highlights how the prized fishery has also been affected by human activities and climate change. While trophy-caliber adults continue to be captured in Georgian Bay, they  have been unable to find any muskellunge young-of-the-year (YOY) at historically confirmed muskellunge nursery areas in the Severn Sound area.

Suddenly in 2019, the Ministry of Natural Resources and Forestry (MNRF) reported finding several muskeelunge YOY in this area. The McMaster team is working in collaboration with the Ministry on this development.

The Focus of Further Research

Further research should focus on accurate forecasts of water-level scenarios, understanding how wetland ecosystems may respond to new climate regimes, and developing adaptive strategies to manage these wetlands.