Stream Ecosystems

The Qualities of a Stream

Streams begin to flow from ponds, lakes, springs or drainage areas. A stream's water level can vary widely depending on precipitation levels (which are the most variable in different seasons of the year).

Streams have two complementary and interacting zones, called riffles and pools, which differ in chemistry, intensity of current, and depth. Riffles are the turbulent, fast-flowing areas which are the most productive parts of the stream. The pools are the quiet, still waters which are the sites of decomposition of organic material. Since nutrients are always carried farther down the stream, upstream areas depend even more on in-stream objects for cover and obstacles, such as logs, snags and boulders, to form pools. This is something we are replacing in the streams where necessary, to provide habitat for all the different organisms in the stream.

How much life is in a stream depends partly on the nature of the stream bed, but also on the width of the stream. Streams on bed rock are less productive than streams with gravel or rubble bottoms since organisms have more places to hide in or attach to. Limestone bottom streams like those on Manitoulin are productive because limestone contributes nutrients to the ecosystem.

When the streams width is considered, it has been found that narrower streams are more productive than wider streams, due to the extra shade and vegetative debris they receive. For example, streams which are two metres wide or less are four times as rich in bottom organisms as those 6 or 7 metres wide. Streams that are too shallow become too hot and don't provide enough space and cover for fish to live or spawn. This is why we are rebuilding banks to optimal widths where they have eroded away.

The Sources of Food for Stream Inhabitants

The beginning of the food web in a stream ecosystem is organic material from outside the stream. Referring to the diagram below, about half the nutrients in a stream come from leaves and woody debris dropped from streamside vegetation (which is why this vegetation is so important and we are replacing it) (1), while the other half leaks in from the surrounding earth (2) and is carried by rain running off the banks (3). Up to half of a leaf can dissolve within two weeks, while bacteria and fungi grow on the leaf and invertebrates (microscopic animals) finish breaking it down (4).

So What Does Live in the Stream?

There is a web of relationships which make up the food chain of "what eats what" in any ecosystem. In every ecosystem the green plants are producers, which means they make wood, leaves, fruit, etc. out of carbon dioxide, nutrients and water with the aid of sunlight energy. This group is actually more productive in streams than in standing water (such as lakes) because of the constant flow of nutrients carried along by the water.

The animals are the consumers and feed either on the plants or other animals. Different streams around the world have different species living in them, depending on the climate and altitude where they are, how much water they hold and how fast it flows, and finally what rocks, soils and vegetation surrounds them.

Invertebrates are named for the many different roles they plan in the ecosystem, such as shredders, collectors, scrapers, piercers and gougers. They are also the major source for the stream's predators (most notably the fish). Examples of these predators include stream salamanders and fish such as trout.