Starting in the 1930s nutrient levels in the Great Lakes began to increase. By the 1970s, nutrient pollution and harmful algal blooms had become a major concern in the Great Lakes and St. Lawrence River. In response, the Canadian and United States governments signed the Great Lakes Water Quality Agreement in 1972. This agreement led to laws that decreased the amount of phosphorus in detergents and improved municipal and industrial wastewater treatment. As a result, the river had become visibly clearer by the 1980s, shifting from mucky water, with high nutrient levels, to clearer water, with low nutrient levels.

If you’ve spent time on the river, you may have memories of cutting your feet on mussel shells. These sharp shells come from zebra and quagga mussels, invasive species that are both native to Eurasia. Zebra mussels were first found in the St. Lawrence River in 1989 and were shortly followed by their cousin, the quagga mussel. Although they are small, they can have major impacts on ecosystems—a single zebra mussel can filter up to 3 litres of water each day!

Through filter feeding, zebra and quagga mussels absorb large amounts of phosphorus from the water. So, as their populations grew, they absorbed more phosphorus and made the water clearer. These changes have also been noted through observations of water clarity by community members like local diver, Ron Macdonald.

It is important for us to pay attention to this continued decline of phosphorus because levels are now lower than they were before industrial development. While clear water may seem like healthy water, too little nutrients and algae can also be a problem for ecosystems. Phosphorus is an important food for plants and algae, which in turn feed fish and other animals. If nutrient levels drop too low, it can impact other species in the river, as we found in the case of Yellow Perch.

There are two main nutrients that are important for feeding all life in the river: phosphorus and nitrogen. Unlike phosphorus, nitrogen was not a focus in the 1972 Great Lakes Water Quality Agreement. While phosphorus levels went down, we found that nitrogen levels went up until about 2000 and then started to decline in the mid 2000s.

This was a surprising finding that hasn’t been previously documented in the river. Researchers are still looking into different reasons why this is happening.

The decline in nitrogen follows zebra and quagga mussel invasion. Nitrogen is an important ingredient in making mussel shells. As the invasive mussel population grew, more shells were made and left behind in the water. Instead of the nitrogen being recycled and available in the water, it is now stored in the shells of mussels. More research is needed to determine if this could explain the decline in nitrogen levels. 

While nutrients in the main channel (deeper water) of the river are declining, there is a different story for the river’s nearshore areas and tributaries (small rivers and creeks). The nearshore area runs along the shoreline and provides habitat for birds, fish, and other wildlife. The nearshore area is also the part of the river that is directly impacted by human activity on the land.

Although nutrient levels in the main channel of the river are currently very low, they are much higher in the nearshore area. One reason for this difference is that nutrients from runoff and local tributaries tend to concentrate in nearshore areas.

Another reason for this difference, which might come as a surprise, is the introduction of invasive mussels. The large invasive mussel populations are concentrated along the edges of rivers and lakes. Through their filter feeding the mussels capture nutrients and trap them in this nearshore zone.

Climate change is a major threat to aquatic ecosystems globally and we are already seeing impacts on the St. Lawrence River. Climate change varies regionally and is reshaping local weather and ecosystems, with impacts expected to intensify in the coming decades. 

Climate projections suggest that spring may bring warmer-than-average temperatures along with intense rainfall and strong storms, leading to high water surges and increased surface runoff. These conditions can increas nutrient levels in the St. Lawrence River, particularly nitrogen, as rain can wash fertilizers from agricultural lands into the water.

Projections indicate that summers could be much warmer—by as much as 2 to 4 degrees Celsius by 2040—with years of extreme drought. Increased temperatures can encourage algal blooms and trigger the release of phosphorus stored in the sediments of lakes and rivers.