Trophic state index
aquatic plants and algae. Usually the water body will be dominated either by aquatic plants or algae. When there are a lot of aquatic plants the water tends to be clear. When there is a lot of algae the water tends to be darker. The process of  photosynthesis in the algae supplies oxygen to the fish and plants which live these waters. Sometimes an excessive algae bloom will occur. This can kill fish by using all the oxygen in the water. The process of eutrophication can occur naturally and by human impact on the environment.