Describe an aquatic ecosystem, and explain how ph, co2, n, p, and salinity affect the ecosystem

An aquatic ecosystem is basically a biological community of interacting organisms and their physical environment, but confined to within an aquatic environment. Water quality variables such as those mentioned affect aquatic ecosystems because the animals living in water have evolved to tolerate certain ranges of concentrations of these variables. Animals in general tolerate a pH of around neutral, i.e. close to seven, and can generally tolerate a narrow range around seven. Nutrients such as nitrogen and phosphorus can cause aquatic ecosystems to become eutrophic. This generally indicates high algal growth in aquatic environments as nutrients allow exponential plant growth. This effectively changes the trophic state of a water body. The death of algae once the nutrients run out results in anoxic conditions, which in turn kills all the other animals within the ecosystem. Carbon dioxide is not really a pollutant within aquatic ecosystems. 

If a species is threatened with extinction, economists would predict that greedy or mean people are again ruining the environment, taking advantage of something they need to survive, or directly predation on the specie. According to the experts of the International Union for the Conservation of Nature (IUCN), a species is considered critically endangered when it is facing an extremely high danger of extinction in the wild. The statistics only count the species that live freely.