Eutrophication is the process of nutrient enrichment in aquatic ecosystems. The term nutrient refers to any one of the chemical elements that are necessary for plant growth.
||For example, in
a garden or on farm crops, fertilizers are used to facilitate plant growth.
The most common chemicals in fertilizers are nitrogen (N), phosphorus
(P), and potassium (K). Examine the labels on fertilizers as well as
on containers of household plant food and you will see the percentages
of these chemicals used in each product.
When these chemicals, which are essentially fertilizers, enter an aquatic ecosystem they stimulate the growth and reproduction of algae and bacteria. These algae, bacteria, and other microscopic plant-like life are known as phytoplankton. Phytoplankon are responsible for what is called the primary production in an aquatic ecosystem. Primary production is the result of photosynthesis (see video lesson on the carbon cycle). In most aquatic ecosystems there is a balance or equilibrium between primary production, consumption by consumers, and decay processes. When excessive nutrients from natural or human sources enter an aquatic ecosystem phytoplankton production increases. The increase may be rapid and is called a phytoplankton bloom or an algal bloom.
A bloom or population explosion increases the numbers and total biomass of the phytoplankton population well beyond the capacity of predators or consumers to graze it down to the normal balanced level. The microscopic organisms that make up the phytoplankton have a short life span. After they die and decay bacteria consume them. These bacteria are consumers, technically called heterotrophs, and are organisms unable to make their own food. Organisms that make their own food are called autotrophs. An important part of the process is that heterotrophic bacteria consume oxygen. During a bloom, there is a large number, beyond the normal balance, of dead and decaying organisms, and thus, there is an increase in the population of heterotrophic bacteria. These bacteria can consume most of the available oxygen in the water, creating a low oxygen situation called hypoxia. When all the available oxygen is depleted it is called anoxia. Since most life needs oxygen, low oxygen conditions create considerable stress on organisms in the ecosystem, such as fish and invertebrates.