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Autotrophs (primary producers) that manufacture their own food through photosynthesis -- plants, algae, or bacteria -- are dependent upon light streaming through the water. Autotrophs, in turn, are food for heterotrophs, like fish and insects. Types of organisms in any given stream may include invertebrates like mayflies, caddisflies, riffle beetles, aquatic worms, crustaceans, and mollusks. Algae will grow when light feeds it, as will aquatic bacteria and fungi. And of course, there are fish feeding off everything else.
Foliage Along the Bank
The foliage along the banks of a stream comprise its riparian zone. Plants and tree roots dipping into the stream provide nutrients to the stream's organisms. When those plants thin out enough to allow more light to reach the stream, the organisms requiring light to manufacture their own food (autotrophs) can grow larger. For instance, algae would form on rocks above and below the surface of the water, providing nutrients to other organisms.
Photosynthesis and pH
Daily sunlight provides the energy for photosynthesis, a process that uses up hydrogen molecules while enabling plants to manufacture their food. A stream's pH (measure of the balance between acidic and basic) varies by day and night. The daily photosynthesis diminishes hydrogen ions, making the water more basic. Without light, nighttime may find more free hydrogen ions, making water more acidic. In acidic waters, soluble heavy metals can make the water unsafe for aquatic life. Ideally the pH level should be neutral, providing a healthy environment for living organisms.
The Primary Ingredient: Fresh Water
The fresh water of a stream contains low levels of salinity, supporting a viable habitat to many organisms. Daylight provides the energy required for the stream water to evaporate, causing humidity, mists, and eventually recycling the water through the hydrologic cycle.