Hypoxia:
The 0₂ Blues

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.

The nutrients or chemicals that set the eutrophication process into motion come from many sources. There are natural sources of nutrients such as soil runoff. However, human or cultural activities tend to be major sources of nutrients that accelerate the eutrophication process. Cultural sources of nutrients include storm drain runoff, fertilizer, sewage, industry, and farm waste. You may recognize these sources as components of what is referred to as nonpoint source pollution.

Eutrophication may occur in a small pond, a large lake such as Lake Erie, or a coastal marine ecosystem such as the Gulf of Mexico. Eutrophication may play a major role in the process of a pond or small lake eventually filling itself up with organic material and becoming a bog or marsh. The bog or marsh may eventually become dry land. Recall that excessive amounts of organic materials are created by eutrophication with subsequent loss of oxygen. Without oxygen there is no decay. The organic material simply builds up over time and can fill up a small lake or pond.

These small aquatic ecosystems undergo a phenomenon known as overturn. Overturn is one complete cycle of top to bottom mixing of stratified or layered water masses. This phenomenon may occur in spring or fall, or after storms, and results in uniformity of chemical and physical properties of water at all depths. Pond overturn involves many of the same processes of eutrophication, such as nutrient enrichment and population blooms. In order to understand overturn we must incorporate physics principals to the chemical and biological understandings we have developed related to eutrophication. An important point here is that ecological systems and processes are the result of geological, chemical, physical, and biological interactions.

Overturn in a pond or lake generally occurs in the spring and in the fall. Let's follow the process by starting with winter conditions. In the winter there is a minimum of biological activity. The water temperature is cold from top to bottom. These conditions change with the onset of spring. Spring winds cause a stirring up of the organic and nutrient-rich bottom water which then mixes with the nutrient poor surface water. The warm conditions, along with the influx of nutrients, cause a phytoplankton bloom. The bloom stimulates the growth of the consumer populations, such as small crustaceans, that then graze down the phytoplankton population. This cycle does not go on forever. The phytoplankton uses up all the available nutrients (limiting factors) and are reduced in numbers. Here again we see nature's tendency to establish a balance or equilibrium in ecosystems. As summer approaches, biological activity is reduced because of reduced primary productivity. The nutrients that would be recycled through the death and decay process are locked into the bottom waters by the presence of a strong thermocline. A thermocline is a boundary between warm surface waters and cooler waters below. The thermocline acts as a density barrier preventing bottom and surface waters from mixing. The fall season brings about the breakdown of the thermocline, along with some mixing created by winds, which sets into motion another brief bloom, thus completing the cycle.