<P> Aquatic ecosystems perform many important environmental functions . For example, they recycle nutrients, purify water, attenuate floods, recharge ground water and provide habitats for wildlife . Aquatic ecosystems are also used for human recreation, and are very important to the tourism industry, especially in coastal regions . </P> <P> The health of an aquatic ecosystem is degraded when the ecosystem's ability to absorb a stress has been exceeded . A stress on an aquatic ecosystem can be a result of physical, chemical or biological alterations of the environment . Physical alterations include changes in water temperature, water flow and light availability . Chemical alterations include changes in the loading rates of biostimulatory nutrients, oxygen consuming materials, and toxins . Biological alterations include over-harvesting of commercial species and the introduction of exotic species . Human populations can impose excessive stresses on aquatic ecosystems . There are many examples of excessive stresses with negative consequences . Consider three . The environmental history of the Great Lakes of North America illustrates this problem, particularly how multiple stresses, such as water pollution, over-harvesting and invasive species can combine . The Norfolk Broadlands in England illustrate similar decline with pollution and invasive species . Lake Pontchartrain along the Gulf of Mexico illustrates the negative effects of different stresses including levee construction, logging of swamps, invasive species and salt water intrusion . </P> <P> An ecosystem is composed of biotic communities that are structured by biological interactions and abiotic environmental factors . Some of the important abiotic environmental factors of aquatic ecosystems include substrate type, water depth, nutrient levels, temperature, salinity, and flow . It is often difficult to determine the relative importance of these factors without rather large experiments . There may be complicated feedback loops . For example, sediment may determine the presence of aquatic plants, but aquatic plants may also trap sediment, and add to the sediment through peat . </P> <P> The amount of dissolved oxygen in a water body is frequently the key substance in determining the extent and kinds of organic life in the water body . Fish need dissolved oxygen to survive, although their tolerance to low oxygen varies among species; in extreme cases of low oxygen some fish even resort to air gulping . Plants often have to produce aerenchyma, while the shape and size of leaves may also be altered . Conversely, oxygen is fatal to many kinds of anaerobic bacteria . </P>

What are the threats to aquatic ecosystems in marathi