Living at the bottom

While water is a three-dimensional environment, a large share of aquatic fauna lives at the bottom. The bottom is a safe place, but only for as long as good living conditions persist.

Benthic animals living at the bottom of a water body need stability in their lives. They must have enough food and space to live in and the bottom must not dry out at intervals, or be exposed to waves. The quality of the bottom, and also of the water, must remain favourable for the animals.

Strong regulation of the water level or loading from the catchment may put this stability at risk. When peatlands and acid sulphate soils are drained, the acidity level of water can spike. Sediments washed into lakes can cover gravel bottoms, or the bottoms may become muddy due to eutrophication. Eutrophication can also lead to oxygen depletion in the deep parts of a lake.

Some species tolerate changes in their living conditions better than others. Certain benthic animals can store oxygen in haemoglobin-like substances and, if the oxygen depletion is temporary, survive while other species die. The bottom does not become completely deserted, but the diversity of benthic species declines. This also happens when a lake becomes acidified or a stream bottom gets silted up. Benthic animals go down in numbers, and there are fewer species.

In a healthy lake or river, things are quite different: there are high numbers of benthic animals and a great diversity of species. Depending on the type of the water body, these species may include molluscs, segmented worms, true water bugs, flatworms, and plenty of larvae of flying insects. Some benthic animals dig themselves into the bottom sediment, while others crawl on the sediment surface or among vegetation.

Benthic animals are used as an indicator of the status of lakes and rivers. Samples are collected from rapids in rivers as well as in deep areas and from rocky shores of lakes. In these samples, both species composition and numbers of individuals are studied.