Lake ecosystem: Lake zones

Have you ever wondered why there are so many plants on the shore of a lake, but hardly any in the middle? Or why you can’t ever see any big fish close to the shore, but there are always so many insects and frogs there? Depth, temperature, the amount of light, oxygen and food, vary in different parts of a lake. And different plants and animals like certain conditions better than others. Looking at conditions and organisms in the lake, we can divide a lake into four different zones.

Let’s look at the shore first. Around the edge of the lake, the land slopes into shallow water. This area is usually rich in minerals and nutrients dissolved in water. Sunlight reaches all the way to the bottom through the shallow water. This is the nearshore or littoral zone.

Plants and algae need nutrients and sunlight to grow, so the conditions in the littoral zone are perfect for them. This is where you can find different types of reeds, water lilies, or pondweed. These plants make great hiding places and shelters for small animals such as snails, insects, some crustaceans and small fish. These plants and small animals attract in turn many bigger animals that eat them, such as frogs, ducks, or swans. Let’s move to the area of open water in the middle of the lake.

The layer of water close to the surface gets most sunlight. This is the limnetic zone. The water on the surface mixes with oxygen from the air. That’s one reason why there is so much oxygen in the limnetic zone. A lot of oxygen is also produced by tiny, free-floating algae called phytoplankton.

Apart from producing oxygen, phytoplankton is a source of food for many animals living in the lake, such as insects, fish, or crustaceans. The limnetic zone ends at the depth where sunlight can no longer reach. Below the limnetic zone, there is the dark and cold deep water - the profundal zone. There aren’t any plants living in deep water, so there isn’t much food! There isn’t much oxygen either, because the water in the profundal zone often doesn’t mix with the oxygen-rich water on the surface.

Most organisms can’t survive without enough oxygen, except for a few types of worms and crabs, or an occasional fish. But some types of fungi and bacteria can easily survive with very little oxygen! They can even live on the lake floor, in the benthic zone. The lake floor is often covered with a layer of organic matter - animal waste, dead algae or dead animals, which have sunk to the bottom. Bacteria and fungi feed on the organic matter and break it down into simpler substances.

By doing so, they produce carbon dioxide and nutrients, which can be used by plants and algae to grow! So, the four zones of a lake are: the nearshore or littoral zone, open water or limnetic zone, deep water or profundal zone, the benthic zone or lake floor. The different conditions, such as the amount of light, food, and oxygen in each of the lake zones, affect what kind of organisms live there. The organisms in turn affect their environment too - they can use or produce oxygen, eat, or become food for others. Living and nonliving things all depend on each other.

Together they create the ecosystem of a lake.