What is considered a toxic dose to an infant may be harmless to an adult. Concentration and effect Oxygen is a good example of how varying the concentration of a substance changes the environmental effects. There are times, depending on an individuals health, when more oxygen is needed, but in general, if we were to inhale pure oxygen for any length of time, we would suffer toxic effects. Conversely, if we were to receive too low a concentration, we would be unable to breathe. The same concept applies to nutrients and aquatic organisms of various types.
Nutrients are important elements in streams, for they serve as sources of food.
If no nutrients are present in waterways, nothing grows, but when the proper balance of nutrients and chemical trace elements are present, the various species of algae eaten by many insects, invertebrates, and fish grow successfully. They, in turn, are utilized as food. Besides blocking the sunlight essential to the health of submerged plants and other organisms, these bothersome green and blue-green alga and other plants thrive at the expense of the desirable food species. The overabundance of plant life affects the water system's Dissolved Oxygen as well. When the undesirable species begin to die, they sink to the bottom of the water column, where bacteria and other microorganisms cause them to decompose.