A nutrient is any element or compound that is necessary for or contributes to an organism's metabolism, growth, or other functioning. Nutrients can be divided into many categories, including, but not limited to:
While in essence true to the definition above, the term nutrients has a more limited meaning within the subjects of water quality and water pollution, referring specifically to plant fertilizers. In this sense, certain mineral compounds can have an adverse impact on water quality because of their ability to promote plant and algae growth. An excessive growth of aquatic plants can clog waterways (see Giant salvinia for an example) and over-stimulation of algae and microbes leads to an ecological process called eutrophication.
The number of elements of interest or concern in this context is surprisingly small: really just nitrogen and phosphorus in most aquatic systems. Mineral compounds involved are ammonia, nitrites, nitrates, and orthophosphates. Organic compounds also may contribute, in as much as they also contain nitrogen and phosphorus. The reason only a few chemicals are of concern has to do with the fact that plants are made up mostly of compounds of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and phosphorus (P), and lesser amounts of sulfur (S), potassium (K), magnesium (Mg), and calcium (Ca). These elements constitute the macronutrients. Many other elements are necessary for growth, yet because these others are needed in very small amounts they are classified as micronutrients. Plants obtain carbon, hydrogen, and oxygen (elements most needed for growth) from the air and water, where all three elements are very abundant as water and carbon dioxide. So nutrients having greatest potential to influence plant growth in aquatic environments would be those elements necessary for plant growth, but likely to be limiting — that is, once used up, plant growth stops. Of the nine macronutrients, nitrogen and phosphorus are most likely to become limiting. The others are always present in great abundance (C, H, O) or usually present in amounts that exceed the requirements of aquatic plants or algae.
Farmers apply fertilizer nutrients in the form of nitrogen, phosphorus, and potassium (N, P, and K with perhaps micronutrients) to prevent these elements from becoming limiting in the soil. These elements are concentrated in wastewaters from animal pens and septic or sewage systems. And these elements (especially N and P) in runoff or wastewater discharges reaching streams, lakes, or seas will promote aquatic plant growth. Abundant plant growth itself is a concern in assessing water quality. The most abundant "plants" in most aquatic environments are algae. When essential nutrients are plentiful, algae multiply. If these algae are microscopic phytoplankton, their growth increases the turbidity of the water. The water then becomes cloudy and colored a shade of green, yellow, or brown (sometimes red). An abundance in an aquatic system of any algae, or of higher plants, can signal excessive inputs of nutrients.