The Institute of Inland Waters in Greece has investigated the physical, chemical and biological characteristics of major Greek lakes, with regard to water use, quality and trophic status. Based on this and other limnological investigations, Greek lakes can be divided into three categories: warm monomictic deep lakes, warm monomictic shallow lakes, and dimictic shallow lakes. The water quality of some Greek lakes demonstrates the presence of high concentrations of nutrients and heavy metals, with reduced concentrations of diluted oxygen. High concentrations of ammonia, nitrate and phosphate are found in many lakes, while anaerobic hypolimnia are usually found in shallow lakes. Phosphorus is the main nutrient responsible for eutrophication because it is the limiting factor in most of the lakes that were investigated.