What is eutrophication of a watercourse?

Do you have the impression that the water of the lake at your cottage or of a stream that you have visited for a long time is becoming less and less clear? It is possible that this is not just an impression and that the watercourse in question becomes eutrophic. But what exactly is eutrophication?

Eutrophication is the phenomenon of aging of a watercourse which occurs during the accumulation of nutrients. This process takes place over several years. Like human beings, it is completely normal for a watercourse to age. What's less obvious is the speed at which it happens...

What causes eutrophication?

Theabundance of nutrients greatly accelerates eutrophication. These nutrients can come from various sources: fertilizer spreading, wastewater spills, runoff, soil erosion, etc. THE phosphorus is the main culprit of eutrophication, but other chemical elements (e.g. nitrogen) also have some impact. Thus, by increasing the nutrients in the water, the algae present in the water proliferate and grow abundantly.

These are mainly the low-flow bodies of water which are vulnerable to eutrophication, although high-flowing streams, such as rivers, can become so. Pollutants can accumulate more easily without the presence of a large quantity of moving water. It's a bit like unflushing a toilet. However, if contaminant discharges are very abundant, it is possible that, even with a relatively high flow rate, contaminants accumulate and begin the eutrophication process.

What about climatic changes? It turns out that increased temperature contributes to algae growth, while increased precipitation increases sediment input into waterways. Climate change can affect several other factors that alter the quality of waterways, including the physico-chemistry of environments, in particular oxygen.

Consequences of eutrophication

With the proliferation of algae a barrier is created on the surface of the water. This prevents the sun's rays from passing through, although they are useful for several other species (plants, fish, etc.) which are found at the bottom of the water. Little by little, these species begin to die and become a source of food for bacteria. During digestion, these bacteria consume a large part of the oxygen contained in the water and undigested residues of plants, algae and animals accumulate at the bottom of the watercourse. This accumulation of sediment, combined with the reduction in available oxygen, drastically changes living conditions in the aquatic environment.

Result: several organisms are not able to survive in this new habitat.

Stages of aging of a watercourse

Photo credit : ROBVQ

These changes can occur at widely varying speeds depending on the size of the water body, its depth and the intensity of nutrient inputs. Ultimately, a body of water at the end of its life will look more like a swamp than a lake or river.

The image shows the sequence of eutrophication from an oligotrophic lake (few nutrients) to a eutrophic lake (many nutrients), passing through the intermediate stage called mesotrophic. In Quebec, 31 % lakes are at a stage of eutrophication (oligomesotrophic), 16 % at an intermediate stage of eutrophication (mesotrophic), 8 % at an advanced intermediate stage (mesoeutrophic) and 2 % at an advanced stage of eutrophication. eutrophication (eutrophic and hypereutrophic).

Some studies have attempted to demonstrate this phenomenon of accelerated aging. The Experimental Lakes Region (ELR) Research Center's study on Lake 226 is recognized for its striking results. Since the lakes of the Kenora region of Ontario are protected from anthropogenic activities, they represent a perfect playground for scientists. In the 1970s, the RLE wanted to demonstrate the impact of contaminants such as carbon, nitrogen and phosphorus. The study has become the reference for eutrophication and phosphorus abundance.

What are the solutions to eutrophication?

If it is impossible for a human being to become younger, what about a stream? Well, it's possible: there are some restoration techniques to reverse the phenomenon of eutrophication of lakes.

In Quebec, the most effective, but expensive, method seems to be the use of Phoslock. Its use makes it possible to get rid of phosphorus in water by a principle of coagulation and flocculation. The chemical compound Phoslock attaches to the phosphorus and accumulates at the bottom of the watercourse. It was used on Lake Bromont and the results were quite conclusive: one year after its application, it made it possible to trap 70 % of the phosphorus contained in the watercourse. Moreover, two years after using Phoslock, we noticed a reduction in the presence of cyanobacteria.

There are other pilot projects attempted in Quebec such as the use of plant islands to capture phosphorus, the dredging experiment, the experiment with duckweed and pumping of unconsolidated sediments and their storage in a Geotube. However, no project has proven as promising as Phoslock.

Ultimately, Prevention is better than cure. Thus, it is crucial to limit our nutrient releases on an industrial and individual scale. Businesses, particularly the agricultural industry, are responsible for a large amount of phosphorus releases into nature associated with the use of pesticides and herbicides. For households, many cleaning products contain phosphorus. Theoretically, our sanitation systems should be able to remove phosphorus. However, not all wastewater treatment plants are actually capable of removing sufficient phosphorus. Also, riparian buffers represent a major tool to prevent nutrients from running off into watercourses.

In short, eutrophication is a growing phenomenon and it is essential to curb its proliferation!

Main photo: Unsplash