Lake

Lake formation (or birth) and evolution (or aging) are natural periods of lake existence as they are for all living things. Some lakes have a short lifespan of 100-1,000 years, although many lakes will exist for 10,000 years or longer, but there are lakes that only exist in damper seasons of the year. Because people who study lakes have considerably shorter life spans, the chemical, physical, and life-supporting properties of water are used to classify lake age. As water tends to support life, lakes are often assessed based on what life they can and do support: their fertility. The deposits in lake basins have strata, or layers, that reveal details about a lake's history. And a lake's present fertility is related to water stratification by regions of similar temperature and light penetration.

Fertility is governed by a number of biological and chemical factors. The photosynthetic plankton that grow on a lake's surface are eaten by zooplankton; these plankton make up the primary link in the lake's food chain. Photo plankton contribute to a lake's fertility as a food source and as an oxygen source through photosynthesis. Plankton are consumed by aquatic invertebrates which are, in turn, eaten by small and larger fish.

Minerals such as phosphorous and oxygen are also required for life to flourish. Phosphorous levels can vary over a range of parts per billion (ppb). Most fish require an oxygen concentration of at least 5 parts per million (ppm). Oxygen concentration is primarily due to photosynthesis in lake plants and surface wind agitation. Some oxygen can also come from tributary streams.

Lakes are classified as oligotrophic, mesotrophic, or eutrophic depending on age and whether they have little, some, or a lot of life, respectively. Oligotrophic lakes are the youngest and, usually, least fertile lakes; they tend to be deep with sparse aquatic vegetation and few fish. Mesotrophic lakes are middle-aged lakes that are less deep and more fertile than oligotrophic lakes. And eutrophic lakes (the oldest lakes) are most fertile and even more shallow than mesotrophic lakes. Eutrophic lakes eventually reach the point where demand for oxygen exceeds the oxygen supply. Eutrophic lakes have many aquatic life forms that eventually die and decompose; decomposition uses up oxygen that could have supported additional life. Decomposing material, detritus, collects on the lake's benthos (basin bottom), making the lake shallower. As oxygen becomes sparse, lakes approach senescence, full maturity to death.