Eutrophication of water bodies is mainly caused by -

Eutrophication or hyperophication, is when a body of water becomes overly enriched with minerals and nutrients which induce excessive growth of algae. This process may result in oxygen depletion of the water body. One example is an "algal bloom" or great increase of phytoplankton in a water body as a response to increased levels of nutrients. Eutrophication is often induced by the discharge of nitrate or phosphate-containing detergents, feilizers, or sewage into an aquatic system. Ref: https://en.m.wikipedia.org/wiki/Eutrophication