It is impossible to overstate the importance of phosphorus in the aquatic food web, as it forms the foundation of life in these waters. Everything from the tiniest phytoplankton all the way up to top predators like lake trout and loons rely on phosphorus.
You may have heard the terms “nutrient-rich” and “nutrient-poor”. In the context of water quality, these terms refer to the level of nutrients like phosphorus and nitrogen in the water, here we talk specifically about phosphorus. Water that is considered nutrient-rich has sufficient levels of phosphorus to nourish the microscopic algae that support larger organisms such as crustaceans, small fish, and even top predators.
Offshore vs Nearshore Water
Phosphorus levels in offshore and nearshore waters of eastern Georgian Bay are so different that researchers need to monitor both areas to get a true sense of the region’s water chemistry. (Remember our 2018 article titled Nutrients Nearshore vs Offshore: What’s the Difference?).
Eastern Georgian Bay’s deep offshore waters are cold, colourless, and ultra-oligotrophic, meaning very low in phosphorus. The nearshore waters, in contrast, are shallower, warmer, and tend to be richer in the type and levels of phosphorus necessary to support a robust food web. Exactly how much phosphorus, and therefore how productive these nearshore waters of eastern Georgian Bay are, depends on several factors.
- Proximity to rivers draining water from the phosphorus-rich, forested watersheds of the Canadian Shield. Rivers carry nutrients from their watersheds which ultimately end up in the Bay.
- How much mixing occurs between nearshore and offshore waters. The amount of water exchange between the nearshore and offshore is determined by the connections between embayments and the open Bay. More water exchange, or mixing, means more potential for the dilution of nutrients by the ultra-oligotrophic (nturient-poor) offshore waters.
- Distance from developed areas of shoreline and intensive agriculture. Rivers, drains, and shorelines also carry pollutants and excess nutrients washing off of city streets and farm fields.
It’s not only phosphorus levels that vary between offshore and nearshore waters. Notice the considerable differences in these other indicators as well.
|Nearshore (Big Sound)
|River Discharge (Seguin River)
|Dissolved organic carbon (mg/L)
So the question is this: can eastern Georgian Bay embayments sustain their robust food webs in such close proximity to the nutrient-poor, offshore waters of Georgian Bay and Lake Huron?
The answer is a resounding “yes.” The embayments around Parry Sound are a comparatively nutrient-rich, ecological refuge from the ultra-oligotrophic waters of the open Bay, and the Ontario Ministry of Environment, Conservation and Parks (MECP) has the water quality data to prove it.
Water Quality in the Big Sound and Connected Embayments
In 2016, MECP assessed the waters in the Big Sound and connected nearshore embayments, as well as the region’s deep offshore waters.
The 2016 data confirm that both watershed and Georgian Bay waters shape the chemistry of these embayments, but not all embayments are the same. Take a look at the Big Sound and Deep Bay on the map. The Big Sound has a direct connection with the open Bay, meaning that a lot of water can exchange fairly easily between these two areas. Deep Bay, on the other hand, has a very limited connection with the open Bay, in fact, its connection to the open Bay is through the Big Sound.
Another excellent example of differences between embayments can be seen by looking at the Inner Harbour and Five Mile Bay. The Seguin River, with a watershed of roughly 1,001 km2, drains into the Inner Harbour. On the other hand, only smaller streams and creeks drain directly into Five Mile Bay. Therefore, water chemistry in the Inner Harbour is influenced more by watershed inputs than Five Mile Bay.
These differences result in ‘gradients of nutrients’ within and across embayments, meaning that nutrient levels change as you move away from river discharge or as water exchange with the open Bay is limited.
The data show that, even accounting for nutrient gradients, the embayments are a nutrient-rich, healthy refuge to the diverse range of organisms that can thrive in these nearshore waters. Total phosphorus levels ranged from 5-8 micrograms per litre in all the embayments surveyed. By way of comparison, the deep offshore waters of Georgian Bay had total phosphorus levels of only 1-3 micrograms per litre. These differences might not sound like much, but they matter a lot to the small organisms in the water.
One of the embayments studied–Deep Bay–is more nutrient-rich than the others. A couple explanations are possible. First, as was already mentioned, Deep Bay’s connection with the open Bay is comparatively narrow and shallow, which shelters Deep Bay from Georgian Bay’s nutrient-poor waters. Second, it is the only embayment of the seven studied where the bottom waters can become so low in oxygen that phosphorus that was once held in the lake sediments is released into the water.
Move Over Mussels!
The study also found that the water chemistry of these embayments is no longer favourable for invasive zebra and quagga mussels (read more about these invasive species here). Zebra and quagga mussels were beginning to make inroads in the region in 1990. However, changes in water chemistry since the 1990s, specifically changes in calcium concentrations, have created an aquatic environment that does not appear to support mussel growth. These mussel invaders require calcium concentrations of roughly 15 mg/L or higher. The waters around Parry Sound are at or below this calcium threshold. This means that the mussels are unlikely to be abundant in these embayments and therefore do not directly affect water quality in the immediate area.
If you do happen to see invasive mussels in Georgian Bay, please report your sighting online at https://www.eddmaps.org/ontario/. EDDMapS, or the Early Detection and Distribution Mapping System, can be used to report sightings of invasive species, either through a web browser at home, or in the moment via their app.
This blog is based on research by Todd Howell, Ontario Ministry of the Environment, Conservation and Parks, Environmental Monitoring and Reporting Branch. Mr. Howell worked with the ministry’s Great Lakes field group and was supported by Brian Thorburn, Ryan Mototsune, John Thibeau, and Lakshika Girihagama.