Everything changes with time, including lakes, ponds, rivers and streams. The aging process of these bodies of water is called eutrophication, and it's a process which slowly moves the water from supporting primarily animal life to supporting primarily plant life. In the process, the depth and borders of the water may change, giving value to terrestrial life near its banks and leaving behind a rich, fertile plain. However, not all eutrophication is a good thing, and it can have disastrous effects if the process speeds up through artificial means. It is important to differentiate between natural and cultural eutrophication, and understand the implications of each to the local ecosystem.
Overview of eutrophication
Eutrophication basically means the rising level of nutrients in any given body of water. This includes such substances as nitrates and phosphorus, which encourage plant growth. The plants consume the nutrients, and then they age and die, releasing the nutrients back into the water. Over time, the cumulative nutrients allow for almost unchecked plant growth, while choking out fish, invertebrates and other such aquatic life.
Eventually, only plants and the sediment from dead organisms remain in the water. This sediment layer gets thicker as more plants grow and die, insects drown in the water and other organic debris accumulates. Over time, this sediment pushes up the floor of the body of water, causing the water itself to spill over its old banks and redistribute over the surrounding ground. It may take decades, but eventually the body will disappear altogether, leaving behind first a marshy area and then a firm, fertile plain.
Cultural versus natural eutrophication
The process of natural eutrophication introduces nutrient sources from the natural environment around the body of water. This might include dead aquatic and nearby terrestrial plants, dead fish, waste from all the living organisms, or runoff when it rains.
Cultural eutrophication is, essentially, any eutrophication that occurs due to human activity. Some definitions put it as caused by excessive human activity, though in this context it can be difficult to determine what level of activity is considered excessive. This might include industrial waste, sewage dumping (especially in urban areas), and increased runoff in areas with poor erosion control due to human development.
The impact of natural versus cultural eutrophication
Natural eutrophication can take hundreds of years to completely stagnate and redistribute a body of water. Very large natural bodies of water, such as the Great Lakes, do not experience eutrophication on a measurable level due to their exceptionally low bio-load compared to the volume of water. The ocean also does not appear to undergo natural eutrophication. These natural redistributions over time help keep the maximum amount of land mass healthy and productive as possible.
The impact of cultural eutrophication, depending on the degree to which excess nutrients are dumped into the water supply, can be catastrophic. These nutrients can reach toxic levels. High levels of nutrients may cause a massive algae bloom, which places a huge bio-load on the area in a relatively short amount of time. The algae block a large percentage of the sunlight that naturally filters down through the water, which in turn kills many plants on the bottom. Die-off from this domino effect produces even more nutrients, pushing some species of flora into overdrive. By the time it's done, the entire body of water could be sterilized or contain only one or two species of plant life. The process happens relatively quickly, and with only a fraction of the sediment buildup needed for the natural cycle or redistribution.