3.3. Stream Production

Habitat biologists use the word production when they talk about streams because they think about the habitat in a stream as a kind of factory where fish can be produced. So when they talk about the biological production they generally mean how many fish can be found or "produced" in streams.

How well a stream "produces" is a combination of many factors. When we try to improve or enhance habitat, as in the Adopt-a-Stream program we are also trying to improve stream production.

The vegetation growing alongside the stream has a strong influence on production because the habitat provided in a stream is tightly linked to its surroundings. Small streams in forested regions receive much of their materials from the surrounding forest ecosystem. Needle and leaf debris, twigs and branches are the main sources of nutrients to feed plants and animals in the stream.

There are two important food chains related to streams. The first (sometimes called the Internal Pathway or autochthonous) is based on what is produced in the stream through algae on the rocks and wood in the stream via photosynthesis. The required inputs to make this work are the "correct" mix of chemicals, nutrients and sunlight. It is this layer of algae on the rocks that often makes the stream slippery for wading anglers. However, in most shaded, woodland streams the amount of sunlight reaching the stream is small and limits algal production. Typically, the most stable streams have the best development of algae, since the suspended sediments in freshets scour the stream bottoms and remove algae growths in less stable watercourses. Algal growth is used in either its live or decomposed state by invertebrates (usually immature insects) which in turn are preyed upon by other species. These insects are the basic food for fish. Because of their low organic retention and their greater light interception, larger streams depend primarily on in stream production of this algae as a food source.

The second important food chain (sometimes called the External Pathway or allochthonous) is based on what is produced in the forests and is dependent on plant material from streamside vegetation entering the stream. There is actually a steady drop of plant material from land into the stream throughout the year, but the vast majority falls in the autumn when large amounts of leaves, small twigs, etc., become incorporated in the stream bottom. There the material begins the relatively slow process of decomposition through the action of fungi and bacteria. This material is colonized by microbes and is then consumed by stream invertebrates (mainly insects), which are in turn prey for carnivorous invertebrates and fish.

This illustration is one example of some of the different aspects of stream life