Eutrophication

The presence of high populations of photosynthetic protista, their DOM products, and the high numbers of Bacteria that can be supported by large amounts of organic matter lead to a condition known as eutrophication. Water is in a nutrient-rich state, biomass is high, and oxygen demand exceeds supply. Anoxic conditions rapidly develop, leading to the death of aerobic organisms.

The decomposition of dead material by prokaryotes leads to further demands for oxygen, and the entire environment becomes anaerobic, allowing for the growth of anaerobic Bacteria and Archaea with the production of methane, hydrogen sulfide, hydrogen, and many other products of anaerobic metabolism. Eutrophication is commonly seen around untreated sewage outfalls and dairy farm waste run-off, and in freshwater streams polluted by nitrate-rich agricultural field run-off.