Water: absolute requirement for normal growth and metabolism directly involved in all the chemical reactions of life — sometimes referred to as the forgotten nutrient. Any classification of "nutrients" is likely to be arbitrary given the status of nutrition as a developing science. Researchers keep becoming more aware of a wider range of nutrients esential for health.
An organism will metabolise any organic compound to use for its energy content, for structural purposes (growth or replacement of living structures), or for participation in chemical reactions necessary for life. Any particular substance can play more than one role in the body, though researchers lack a good understanding of these roles.
The discovery of the group of nutrients called phytonutrients reinforces the provisional nature of our knowledge. We know little about phytonutrients, organic compounds from plants which play an essential role in the normal functioning of a body and have complex hormonal effects on health or play an active role in the amelioration of disease. They are not fit readily into the scheme of the traditional nutrition categories.
Nutrients and the environment
While in essence true to the definition above, the term nutrients has a more limited meaning within the specialised fields of water quality and water pollution, referring specifically to plant fertilizers. In this context, 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 example) and over-stimulation of algae and microbes leads to an ecological process called eutrophication.
A surprisingly small number of elements provide interest or concern in this context: 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, though necessary for growth, classify as micronutrients due to the very small quantities required. Plants obtain carbon, hydrogen, and oxygen (elements most needed for growth) from the air and water, where all three elements occur in great abundance as water and as carbon dioxide. So nutrients the 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 always remain present in great abundance (C, H, O) or usually 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 become 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 gives cause for 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.
Nutrients are recycled at the ecosystem and biosphere system level through biogeochemical cycles:
