Lake Connectivity: Impacts on Lake Superior

This is the second in a series of articles about eutrophication that have been contributed by Nathan Wilson, a PhD candidate of Lakehead University. The previous article can be found here: Classifying Lakes: Eutrophication in the Boreal Forest Ecozone. It goes over basic terminology for classifying lakes, which is used in the following article that examines how changes in the Lake Superior watershed are impacting Lake Superior itself.

We like to believe that Lake Superior is unaffected by human impacts, especially in comparison to other Great Lakes. Unfortunately, this may not be so true. In fact, Lake Superior has a long history of human impacts from legacy issues like the industrial revolution and the collapse of commercial fisheries, to more recent issues of phytoplankton blooms, high water levels, and reduced ice cover. Some historical, yet often overlooked, human impacts on Lake Superior include the Long Lac and Ogoki diversions and the damming of Lake Nipigon and the Nipigon River to provide more water to the Great Lakes. The subsequent sea lamprey introduction, which contributed to the near-loss of the lake trout fishery in Lake Superior, is a good example of how alterations in one area can have resulting impacts in connected systems. Humans altering the connectivity within Lake Superior, as well as the Great Lakes as a whole, has had significant and long-lasting impacts.

Lake Superior Ability to Buffer Against Impacts Overestimated

In the 1980’s, degraded environmental conditions in Lake Superior led to the establishment of four areas of concern (AOC) – Thunder Bay, Nipigon Bay, Jackfish Bay and Peninsula Harbour, as well as the St. Marys River at Sault Ste. Marie. Today, the historic impacts of direct industrial related degradation to Lake Superior have for the most part been addressed; however, it is no longer possible to restore the ecosystem to its former function in areas like Thunder Bay where ecosystems, such as coastal wetlands within the harbour, were filled in to allow for industrial development. This is a permanent loss of ecosystem function within Lake Superior that was justified due to the assumption that Lake Superior’s vast size gave it the inherent ability to buffer change.  A more recent indication that human impacts continue to afflict Lake Superior would be last year’s (2018) extensive cyanobacteria (also known as blue-green algae) bloom at the Duluth Harbour on the southwest shore of Lake Superior. Because Lake Superior is an oligotrophic lake, with minimal nutrients and productivity, the observation of cyanobacteria, which is normally associated with higher nutrient contributions, was unexpected.  

Lake Superior is the largest freshwater lake by surface area at 82,000 km2, with an average depth of 147 m and a maximum depth of 406 m, and holds around 12,000km3 of freshwater. It takes approximately 191 years for a drop of water entering Lake Superior today to exit into Lake Huron via the St. Mary’s River at Sault Ste. Marie. Lake Superior’s catchment area, or watershed, is 127,000km2this includes the two diversions, Ogoki and Long Lac, on the north shore. The Lake receives water from about 200 rivers around the basin, the largest of which include the Nipigon River, St. Louis River, White River, Pic River, and Kaministiquia River. Lake Superior is classified as a dimictic oligotrophic lake. It is home to more than 80 different fish species, although it has fewer dissolved nutrients and is less productive when compared to the other Great Lakes. 

We assume the massive size and volume of water in Lake Superior and it’s oligotrophic characteristics buffer the lake from small changes resulting in major impacts, but there is an emerging shift in our understanding of Lake Superior’s sensitivity to change. The scientific community has shown that Lake Superior is warming faster than any other great lake. This is very important as the temperature has major impacts on a number of other ecological and environmental factors such as fish growth and reproduction. Dr. Jay Austin of the Large Lake Observatory in Duluth has shown that Lake Superior is actually very sensitive to small changes. Dr. Austin’s work focuses on the long-term effects of climate change on lakes. Much of his recent work has specifically looked at the variation of ice cover on Lake Superior and seasonal temperature changes. 

Understanding Nutrient Contribution in Lake Superior Important

If Lake Superior is, in fact, more sensitive than previously thought, it is important to re-examine the state of lakes and rivers that feed into Lake Superior. There are a large number of headwater lakes in Lake Superior’s watershed that are connected via rivers to the lake proper. If there is a contamination issue within a headwater lake, eventually that water will be transported to Lake Superior. Although it is a massive lake, we are already seeing signs and symptoms that inland problems are potentially causing problems for Lake Superior. Because of the connection of aquatic systems and the fact that Lake Superior receives and provides water for such a massive area, it is essential that we pay attention and take responsibility for the small inland lakes and water systems that may be at risk.  

This past summer, Ontario’s Ministry of Environment Conservation and Parks (OMOECP) reported several cyanobacteria blooms, in lakes within Lake Superior’s watershed. At least one of these blooms was confirmed to be producing cyanotoxins. In 2012 and 2018, on the south shore of Lake Superior, the U.S. waters around Duluth experienced cyanobacteria blooms, something few thought possible. The blooms were the result of significant rain events in the area, described as 500-year rain events by the U.S. National Weather Service. Infosuperior also reported a cyanobacteria bloom in the northern waters of Lake Superior after a rainy fall season this year. As previously discussed in Classifying Lakes: Eutrophication in the Boreal Forest Ecozone, increased precipitation events result in more nutrients from the surrounding catchment area being picked up by surface water and eventually deposited into Lake Superior.

Unfortunately, because cyanobacteria blooms in Lake Superior were not anticipated, there was no equipment in place to record the exact conditions leading up to and during the blooms in Lake Superior’s south shore. Now researchers such as Dr. Robert Sterner from the Large Lakes Observatory along with his Ph.D. student Kaitlyn Reinl are working to understand the land-lake connection and where the bloom cells originate. There have been other reports of nuisance blooms around Lake Superior specifically of didymosphenia geminata (AKA Didymo or rock snot), a diatom that can have significant ecological impacts as it forms dense mats on the bottom of rivers. Didymo blooms can get so bad that they displace fish from the river. 

There are differences around Lake Superior when it comes to the ecological inputs and potential contaminants that are transported from the watershed to the lake. If you have travelled around Lake Superior you may have noticed a significant difference in the abundance of human development between the northern and southern catchment areas. There is very little agricultural development along the northern region of the lake; however, around Duluth and the St. Louis River in the south, the human development, both urban and agricultural, increases. As previously mentioned, human developments are significant sources of anthropogenic eutrophication driving nutrients. It should be noted that although reports of cyanobacteria and other algae blooms appear to be increasing, it is important to acknowledge there is not enough data to verify the cause of the increase. Northwestern Ontario is an understudied region with respect to eutrophication and cyanobacteria blooms and as people become more educated and conscientious about ecological health there is an increasing likelihood that reporting from the public will increase.

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Classifying Lakes: Eutrophication in the Boreal Forest Ecozone

Many policies and programs aimed at maintaining healthy lakes depend on measuring their level of eutrophication. Lake Superior is oligotrophic: relatively deep, clear and nutrient poor. – Image from

What is Eutrophication?

Lakes are classified based on trophic state, which is a measure of the nutrient level, clarity, and abundance of living organisms in a body of water. The trophic state for lakes ranges from oligotrophic lakes (deep, cold, clear water lakes with limited fish, plant and algal growth)  to eutrophic lakes (shallow, warm water lakes with an abundance of small fast growing fish, plants and algae). Mesotrophic is the middle state between these two. Eutrophication of a lake is the enrichment of nutrients (phosphorus, nitrogen, carbon and many others) that allows for increased growth of aquatic plant life (phytoplankton), which results in the depletion of the lake’s dissolved oxygen. Depleted oxygen or anoxia in lakes, can result in the lake becoming uninhabitable for fish. Anoxic conditions in lakes are also linked to internal loading (nutrients released from lake sediments) which further contributes to eutrophication and algal blooms.  

Eutrophication is a natural process. It normally takes centuries for lakes to transition from oligotrophic to eutrophic as precipitation (snow, and rain) erodes rocks and soil, picking up and transporting nutrients and particulates into lakes, which gradually become shallower and more nutrient-rich. Nutrients support primary production (plant and algal growth), which provides an abundant food source for zooplankton and small fish, which in turn support larger fish. As nutrient and sediment inputs continue over decades, the system will begin to have such an abundance of primary production that the oxygen within the system will not support fish and the lake becomes eutrophic.

The three trophic states of lakes from left to right, Oligotrophic, Mesotrophic, and Eutrophic. Note the increasing amount of sediment and vegetation in and around the lake as a result of increasing nutrients within the water. – Image credit to RMB Environmental Laboratories.

Eutrophic lakes are normally associated with warm geographic regions where lakes reach higher temperatures and where numerous sunny days provide the necessary light intensity to support large phytoplankton communities. The concept that natural climate conditions control a lake’s ability to produce large phytoplankton blooms has lead to the assumption that boreal shield lakes are generally oligotrophic.

Anthropogenic Eutrophication

Anthropogenic eutrophication is eutrophication expedited by human impacts, such as agriculture and urban development, which increase the nutrient load on nearby lakes. In the 1960s, research into anthropogenic eutrophication took off in response to the rapid eutrophication of Lake Erie. The scientific community was divided on the topic: large soap companies pushing carbon as the limiting nutrient responsible for Lake Erie’s issues, while others tried to argue that it was phosphorus.    

In 1969, David Schindler and colleagues established the Experimental Lakes Area in Northwestern Ontario where they could do whole ecosystem experiments on real lakes, in large part, to understand what causes anthropogenic eutrophication. Their eutrophication study divided Lake 226 with a plastic curtain into the North and South basin. The Northside had phosphorus, nitrogen, and carbon added, while in the Southside, only nitrogen and carbon were added. The North basin developed sickly pea green algal blooms while the south remained clear. This was a piece of crucial evidence that paved the way for worldwide policy changes.  

Aerial image of Experimental Lake Area (ELA) Lake 226 showing the curtain dividing the North and South basins and show the classic green algae bloom. – Image is taken from

Both nitrogen and phosphorus were added to a separate lake, Lake 227, until 1990. From 1990 to present only phosphorus has been added with continued algal blooms ( The now-famous eutrophication experiment proved that phosphorus, from anthropogenic sources, was the nutrient responsible for massive algal blooms in Lake Erie.  

Aerial image of Experimetnal Lake Area (ELA) Lake 227 showing lakewide algal bloom. – Image is taken from

How is Eutrophication Measured?

The level of Eutrophication in a lake can be approximated by measuring the clarity of its water, the concentration of chlorophyll in the lake, and the total phosphorus content of the lake. Clearer water means less ‘stuff’ floating around in the water and is associated with more oligotrophic lakes. Less light penetration usually means more ‘stuff’ (i.e. algae) in the water and is associated with more eutrophic lakes.

The clarity of water determines how far light can penetrate into the water and light is a driving factor for photosynthesis and chlorophyll concentration. Light penetration is measured using a secchi disc, a flat disc of about 4 inches in diameter that is checker pattern, black and white, in quarters.

A secchi disk is used to measure the clarity of a body of water. – Image form:

The disc is lowered into the water until you can no longer distinguish the pattern, then brought back up so you can just barely see it. This is the light penetration or secchi disc depth. Chlorophyll ‘a’ is the pigment associated with plants and a higher concentration of chlorophyll is associated with higher concentrations of algae in the water. Additionally, total phosphorus (TP) provides an indication of the available nutrients used by the algae in the water. Higher TP is again associated with higher algae, which will mean higher chlorophyll, which means limited light penetration.

Research by Vollenwieder (1969) and by Dillion and Rigler (1974) determined that collecting total phosphorus concentrations during the spring freshet, when ice and snow are melting and rushing into rivers and lakes, was ideal because the water within the lake is mixing. A sample of water at this time could provide a representative sample of the whole. Specifically, the TP at this time could be used to predict the concentration of phytoplankton seen on the lake later in the summer, removing the need for the additional sampling of lakes throughout the open water season.

Image depicting the phosphorus cycle for terrestrial and aquatic ecosystems. – Image is from

Currently, the Ontario Ministry Of Environment, Conservation, and Parks collects samples and monitors surface water conditions following this line of thought. Technicians go to lakes throughout Ontario during the spring (April and May) when lakes are mixed to collect water samples for analysis. This work provides data on the current abundance of TP in and entering lakes. The Ontario guideline stipulates that 20ug (micrograms) per litre of water is the assigned threshold in which a water sample is deemed micrograms too high and associated with a high probability of nuisance algal blooms.

Is Eutrophication a Problem in the Boreal Forest Ecozone?

Boreal Shield: “The Boreal Shield is the largest (about 20 percent of Canada, or over 1.9 million km2) of all the ecozone units. It is broadly U-shaped and extends from northern Saskatchewan east to Newfoundland. From northern Saskatchewan, it passes north of Lake Winnipeg, the Great Lakes and the St Lawrence River to Newfoundland. The rocky hills, coniferous forests and abundance of lakes are recurring parts of this well-known landscape”. (

Boreal Shield Lakes

The studies previously mentioned in this article, which advanced our understanding of phosphorus’s role in aquatic ecosystems,  were conducted at the Experimental Lakes Area (ELA), a large section of land with 58 lakes located just East of Kenora and within the Boreal Forest Ecozone. The lakes at the ELA are oligotrophic Canadian Shield lakes and are a small portion of the thousands of lakes in Northern Ontario that fall into the Boreal Forest Ecozone.

Lakes within the Boreal Forest Ecozone are generally assumed to be oligotrophic lakes: cold, clear, relatively deep, and generally pristine. Because of limited nutrients, lakes in this region are not associated with having algal problems. Although the ELA contains lakes that are representative of many oligotrophic lakes in the Boreal Forest Ecozone, there is a wide variety of lakes with varying trophic states throughout this region. This is clear to those who have had the opportunity to visit the lakes. Furthermore, oligotrophic lakes are not immune to eutrophication. Even the region’s deepest and most well-known oligotrophic lake, Lake Superior, experienced a cyanobacterial bloom last summer.

Sediment flowing into Lake Superior observed after major flooding in June, 2018. Flooding in Summer 2018, which brought nutrients and sediment into Lake Superior, was closely tied to algae blooms. – Image from NASA earth observatory

Anthropogenic Eutrophication of Boreal Shield Lakes

As previously stated, anthropogenic eutrophication is often caused by phosphorus loading associated with agriculture and urban development, but other anthropogenic nutrient sources exist. In Northern Ontario, there is very little agriculture, with forestry and mining being the major industries.

Although comparatively minimal nutrient contribution is associated with mining and forestry, there is still a possible increase of contaminants entering the aquatic systems. These contaminants are not exclusively nutrients but can also be harmful chemicals. When forested areas are cleared for logging or mining it can cause increases in surface-water runoff. Rainwater running over the ground, no longer absorbed by the vegetation, picks up nutrients and transports these nutrients into the watershed where they are available for use by phytoplankton.

Northern Ontario watersheds are also home to camps (lakeside cabins) that provide great fishing and leisure activities. The development of these properties can also contribute to nutrient enrichment of the lakes where they are built resulting in loss of the riparian buffer along the shore that would help keep surface runoff from entering directly into the lake. If you have the opportunity to walk the trails around Thunder Bay you will notice the many sections along McVicar or the MacIntyre where people’s lawns come right down to the river’s edge. Any grass or plant fertilizer that happens to be on the lawn during a rain event can easily end up in the river and ultimately Lake Superior.

Author: Nathan Wilson is a PhD candidate at Lakehead University. His focus is on examining lakes within Northwestern Ontario to better understand nutrients and cyanobacteria.

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