Lake Saint-Pierre is truly ideal for this kind of work. The first time I saw it, I was amazed. I fell in love with the place. Not just because it is beautiful, but also because there are so many different kinds of aquatic plants. The greatest concentration of benthos can be found among aquatic plants. Lake Saint-Pierre also has environments with very different chemical and physical characteristics. There are distinct water masses that flow into the lake from its tributaries and from the Great Lakes. So there’s really...It’sa bit like a natural laboratory because within the same lake we can observe very different environments and see how organisms respond to these different conditions.
We have found a very close correlation between the types of plants and filamentous algae we observe and the type of benthos that can be found. The north shore is influenced by tributaries from the Laurentians. This water mass is less rich and has a different type of vegetation. There are no benthic cyanobacteria, which can instead be found in the southern part of the river, and therefore the invertebrates are different. In the southern part of the river, tributaries like the Saint-François and the Yamaska deliver high nutrient loads. There are thick growths of aquatic plants just downstream of these tributaries, as well as large amounts of filamentous green algae, which can sometimes be seen floating on the surface. These are very rich environments and are great for invertebrates because there is a lot to eat, a lot of food, and a lot of diversity. Many benthic organisms travel within clumps of filamentous algae. This environment displays great diversity. Water eventually flows through these large plant beds and many nutrients are taken up by the plants – especially nitrogen, not so much phosphorus. A little further downstream, we enter an area where there is still a lot of phosphorus but not very much nitrogen. These conditions are very favourable for cyanobacteria. Cyanobacteria can absorb atmospheric nitrogen and therefore have an edge over other vegetation, which requires nitrogen in the water. In this environment, we can find abundant growths of benthic cyanobacteria (Lyngbya) on the bottom and far fewer aquatic plants, less Vallisneria and less of all other kinds of aquatic plants. There are also changes in the benthos. With fewer aquatic plants, there is less substrate for benthos to live on. In growths of the cyanobacteria Lyngbya we mainly find lots of gammarids.
It is important to study benthos because it provides food for fish. We are investigating whether the lower density of invertebrates per square metre in the upstream part of the lake could eventually affect fish.
