What is Eutrophication in ecology explain briefly?

Freshwater ecosystems demonstrate the close relationship between aquatic and terrestrial ecosystems. Until recently, most people did not appreciate how much of an effect land ecosystems have on freshwater ecosystems. Aquatic ecosystems go through a natural aging process once they are formed. This aging process is called eutrophication, from the Greek word eutrophus, meaning well fed, or well nourished. In ecological terms, this means that nutrients are slowly added to a pond or lake over time by natural processes.

In addition to the nutrients that come from the aquatic plants and animals themselves, other nutrients are continually added to an aquatic system from surrounding land ecosystems. These sources include soil erosion and runoff, and organic material, such as plant and animal matter that finds its way into the ecosystem. For instance, on land, plant leaves and branches die and decompose; animals defecate and die. Decomposers convert their bodies and waste into particulate organic matter, which gets incorporated into the ground. However, these materials can also be washed into the body of water by rain and runoff. The aquatic ecosystem gradually becomes able to support more and more living components, or biomass. Over time, what started off as a nutrient-poor aquatic habitat receives nutrients (the process is called nutrient loading), and becomes rich in organic matter.

An example of the process would be a deep pocket carved out of the bedrock by glaciers about 14,000 years ago. Initially the lake would be deep, and filled with relatively clear, cold water able to hold large amounts of oxygen. The bottom of such a lake consists mostly of inorganic sediment. Although there would be many different species in the phytoplankton making up the flora in the water column, the numbers of individuals of each species would be low, reflecting the low amounts of nutrients. Trout and salmon are the types of fish best suited to these early, oligotrophic conditions.

As time went on, the nutrient supply would become more abundant in terms of nitrogen and especially phosphorous. These nutrients act as fertilizers for phytoplankton, which would increased in number. Increased phytoplankton supports a larger food web, and the overall plant and animal components would increase. As the bottom sediments gradually receive larger and larger amounts of organic material, the built-up shoreline would be able to support larger, vascular plants. The total amount of nutrients being cycled would grow over time. Growth of organisms in the water would make the water more turbid (less clear). The oxygen levels in the water would also decrease due to the increased biological activity. As the lake became shallower, the temperature of the water would increase, decreasing the capacity for absorbing oxygen. The types of fish and plants are able to tolerate these conditions are probably catfish, sunfish, and weeds. Bass are the major species of fish inhabiting intermediate, or mesotrophic conditions.

The late stages of aging, or eutrophic conditions, are what we would call "polluted water." The process of eutrophication can be accelerated by human activities. Rain water runoff from lawns, farms (plant and animal), domestic sewage treatment facilities and industrial wastes can all fertilize an aquatic ecosystem in "cultural eutrophication." To summarize the aging process, the water depth decreases, the water becomes more cloudy and turbid, the temperature rises, the oxygen level decreases, the nutrient level increases and the species diversity declines. Ultimately, the lake might become entirely filled in with sediment and organic material and produce a terrestrial ecosystem.