Nutrients and Eutrophication

To grow and survive, organisms need a variety of chemical elements and compounds. In aquatic ecosystems, nitrogen and phosphorus are the most important, as they are most often in short supply relative to the needs of plants, algae, and microbes. Other elements, like iron, manganese, and copper, are also needed in small amounts.

Forms of Nutrients

In aquatic ecosystems, nitrogen and phosphorus are found in particulate and dissolved phases, and in varying chemical forms:

• Organic particulate nutrients include living and dead organic matter such as bacterial, plant, and animal tissue
• Inorganic particulate nutrients include minerals and nutrients adsorbed (attracted to the surface) to suspended inorganic sediment particles
• Dissolved organic nutrients include numerous types of biological molecules, such as proteins, containing nitrogen and phosphorus
• Dissolved inorganic nutrients include forms of nitrogen and phosphorus. One of the most important forms of dissolved phosphorus is orthophosphate (PO^–³). Dissolved inorganic nitrogen is mainly present as nitrate (NO₃^–), and ammonium (NH₄⁺), but also as dissolved nitrogen gas (N₂), nitrite (NO₂^–), nitrous oxide (N₂O); and ammonia (NH₃).

The availability of nutrients for uptake and use by living organisms depends on the chemical form of the nutrient and the biochemical processes available to the organism. Some blue-green algae and bacteria can utilise nitrogen gas (N2), converting it to organic nitrogen—a process known as nitrogen fixation. Other bacteria, fungi, and plants use ammonium, and most can use nitrate. Phytoplankton and most plants assimilate orthophosphate, although some can access dissolved organic phosphorus using phosphatase enzymes. Particulate organic forms of phosphorus and nitrogen are slowly converted back into soluble forms.

Sources of Nutrients

Phosphorus can be released from minerals through weathering of rock, a process that is highly dependent on climate. Some inorganic materials in soils can bind phosphorus, preventing it from moving to the aquatic ecosystem. Artificial sources of nutrients include discharges of sewage, animal waste, and fertilizers. Direct discharge of wastewater or runoff can elevate concentrations of nutrients in aquatic ecosystems. Because nutrient concentrations often limit primary productivity and constrain biomass, inputs of nutrients can lead to large increases in the growth of algae or aquatic plants, just as fertilisation can increase growth and yield of crops.

Eutrophication

Eutrophication results from the presence of high concentrations of nutrients, which can lead to excessive biological growth, usually algae. While eutrophication is a natural process in the aging of some freshwater ecosystems, artificial (human-induced) eutrophication can degrade water quality and threaten aquatic species. High levels of algae can decrease the penetration of sunlight through the water column, preventing photosynthesis by submerged plants. Algae may release compounds with bad odour and tastes, or toxic effects. In addition, the death and decay of algae can reduce concentrations of dissolved oxygen. These changes can affect the diversity of plants, animals, and other aquatic organisms, and also interfere with human uses of water.

Nitrate Poisoning

Nitrate poisoning of cattle is a problem throughout the western portion of the basin. The WHO guideline for human health recommends that drinking water should not exceed levels of 100 mg/L and should ideally be below 50 mg/L. For more information on this subject, please refer to the section on Nitrates in groundwater.

Next: Chemical Water Parameters: Nitrates in Groundwater >