(i) Eutrophication: It is the process in which there is a decrease in the concentration of dissolved oxygen in water because of excessive growth of algae or increase in the nutrient concentration in the water body. This occurs because of continuous sewage disposal in water body. It also occurs when fertilisers are washed into the water body from the nearby fields. Sewage and fertilisers provide a lot of nutrients to the phytoplanktons in the water body and result in excessive growth of algae, also known as algal bloom. When the algae subsequently die, the aerobic decomposers become active and rapidly consume the dissolved oxygen from the water body during the decomposition of algae. This drastic depletion of oxygen causes the death of the entire aquatic life in the water.
(ii) Biomagnification: It is the phenomenon of increase in the concentration of harmful non-biodegradable chemical substances in the body of living organisms at each trophic level of food chain. This occurs because of increase in the concentration of harmful chemicals such as pesticides (DDT, DDE, dieldrin, etc.) and heavy metals (mercury, cadmium, tin, lead, etc.) in the water bodies that cannot be degraded by the microorganisms. It leads to the poisoning of the predatory birds that eats fish such as kingfishers, ospreys and loons. For example, biomagnification of mercury in the fishes through the food chain results in Minamata disease in fish-eating human population.