I study Lake Saint-Pierre because it’s a great place to observe high biodiversity; there is a great variety of organisms in the water and on the lake bottom especially in Lake Saint-Pierre because there are many different types of habitats. When you go from the littoral areas to the centre of Lake Saint-Pierre, the water quality changes; the habitat and vegetation change. When you observe these different habitats, you see all kinds of different species and different adaptations. The water quality is very different because in the St. Lawrence, in Lake Saint-Louis, on one side we have water from the Ottawa River, which joins the St. Lawrence after passing through the Lake of Two Mountains, and on the other side we have water from the Great Lakes that arrives from Lake Saint-François. There are two distinct water masses, one on the north side and one on the south side of the river. In the centre, these waters mix. In Lake Saint-Pierre, the northern and southern water bodies are even more clearly separated because the ship channel is twelve metresdeep and creates a veritable barrier between the north and south shores. The water masses therefore present different characteristics. The aquatic plants that grow in them are also rather different and are associated with highly divergent communities of invertebrates, both zooplankton and benthic organisms. The littoral zone is also a good place for zooplankton because this area provides them with some shelter from predators, which include all the small juvenile fish that can be found in the littoral zone during the spring spawning season. So there is lot of variability and a great diversity of organisms, which are still very poorly known.
Usually, when people study lakes, they take samples from the centre, where there are no plants. They barely look at the edges of the lake, where there is vegetation. The advantage of research in Lake Saint-Pierre is that we can take a cross-section, following the littoral zone into the pelagic zone, and note what changes can be observed in the communities.
Zooplankton is an extremely important part of all aquatic food chains. Zooplanktonic organisms feed on algae, and all juvenile fish feed on zooplankton, at least as fry or as very young fish. When they get larger, they can seek out invertebrates on the lake bottom, known as benthos. Once they grow even bigger, they may also eat other fish. Zooplankton is therefore essential in transferring energy and matter. In-depth scientific studies have shown that zooplankton is a very important factor in explaining how contaminants bioaccumulate and are transferred from water to fish via algae.
I should start by saying that the water level of Lake Saint-Pierre normally varies between the spring, when there are many floods, and the low water periods in the summer, when the water level usually falls. We predict that climate change with global warming will increase evaporation. Less water will flow into the St. Lawrence River from the Great Lakes. We believe that over the next 50 years the water level of Lake Saint-Pierre may fall by around one metre. A decrease of one metre in depth could mean that four metres or more of the lakebed will be exposed, even up to several dozen metres in certain areas. Plant populations in Lake Saint-Pierre will therefore change greatly depending on water levels. Researchers from the St. Lawrence Centre have found that if the water level falls by one metre, emergent plants will spread. These are plants whose roots are underwater but whose leaves, fruits or flowers rise above the water. Emergent plants will tend to replace submerged plants, which grow entirely underwater. Since submerged plants come in a greater variety of shapes than emergent plants, they shelter more invertebrates, which provide food for fish. Therefore, food biomass will decrease if emergent plants become predominant throughout Lake Saint-Pierre. From an aesthetic point of view, the lake will become a big swamp, instead of the beautiful fluvial lake we see today. This is a serious issue. Next year, I will organize a scientific symposium on large rivers called "Les grands fleuves vont à la mer," involving France and Quebec as part of the "Entretiens Jacques Cartier," where we will discuss similar problems affecting large rivers in China, European rivers – in France and in other countries – and rivers in Latin America. We will try to bring researchers together to talk about this issue. Researchers who study rivers are currently developing collaborative networks.
