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. RMBEL.info
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 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.
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 (https://www.iisd.org/ela/about/who-we-are/). The now-famous eutrophication experiment proved that phosphorus, from anthropogenic sources, was the nutrient responsible for massive algal blooms in Lake Erie.
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.
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.
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 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.
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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