Nutrients in freshwater
The problems caused by an excess of dissolved nutrients in freshwater systems
|Introduction||Aquatic plants (like any other plants) need two essential nutrients to grow: nitrogen (N) and phosphorus (P). In a healthy lake the nutrients occur in small amounts. But in large quantities, they can cause a major water pollution problem. Too many nutrients stimulate the rapid growth of plants and algae, clogging waterways and sometimes creating blooms of toxic blue-green algae. This process is called eutrophication. The result of this is that when the plants and algae die and decompose, they use up large amounts of oxygen (O2). So the amount of oxygen that is available for fish and other aquatic species will be reduced. In extreme cases it can lead to a completely oxygenl ess environment that can support nothing except a few species of anaerobic bacteria. It also can kill fish and other aquatic life and reduce the aesthetic and recreational value of the lake. |
The nutrients include nitrates found in sewage and fertilizers, and phosphates found in detergents and fertilizers. Human inputs of nutrients from the atmosphere and from nearby urban and agricultural areas can accelerate the natural eutrophication of lakes, a process called cultural eutrophication.
Nutrients from urban sources may be derived from domestic sewage, industrial wastes and storm drainage. The contribution of nitrogen and phosphorus per person per day averages 10.8 g N and 2.2 g P, though there is a considerable range. In the 1940s detergents were developed containing sodium tripolyphosphate, which softens water by neutralizing calcium and keeps dirt in suspension once it has washed off clothes.
These are the principal sources of nutrient overload causing cultural eutrophication in lakes. The amount of each source varies according to the types and amounts of human activities occurring in each airshed and watershed.