<P> Nutrient levels are important in controlling the abundance of many species of algae . The relative abundance of nitrogen and phosphorus can in effect determine which species of algae come to dominate . Algae are a very important source of food for aquatic life, but at the same time, if they become over-abundant, they can cause declines in fish when they decay . Similar over-abundance of algae in coastal environments such as the Gulf of Mexico produces, upon decay, a hypoxic region of water known as a dead zone . </P> <P> The salinity of the water body is also a determining factor in the kinds of species found in the water body . Organisms in marine ecosystems tolerate salinity, while many freshwater organisms are intolerant of salt . The degree of salinity in an estuary or delta is an important control upon the type of wetland (fresh, intermediate, or brackish), and the associated animal species . Dams built upstream may reduce spring flooding, and reduce sediment accretion, and may therefore lead to saltwater intrusion in coastal wetlands . </P> <P> Freshwater used for irrigation purposes often absorbs levels of salt that are harmful to freshwater organisms . </P> <P> The biotic characteristics are mainly determined by the organisms that occur . For example, wetland plants may produce dense canopies that cover large areas of sediment--or snails or geese may graze the vegetation leaving large mud flats . Aquatic environments have relatively low oxygen levels, forcing adaptation by the organisms found there . For example, many wetland plants must produce aerenchyma to carry oxygen to roots . Other biotic characteristics are more subtle and difficult to measure, such as the relative importance of competition, mutualism or predation . There are a growing number of cases where predation by coastal herbivores including snails, geese and mammals appears to be a dominant biotic factor . </P>

8. what type of environments make up a freshwater community