Explain the consequences of using high levels of nitrogenous fertilisers

Eutrophication is the process of artificially increasing the mineral content of water as a result of nitrogen-containing fertilisers, especially nitrates, leaching from agricultural land. The nitrates and phosphates leach into waterways where they disrupt the mineral ion equilibrium in the water. This causes an algal bloom - the water becomes green and blocks sunlight. This means that deep plants cannot photosynthesise so die. The animals that feed on them then also die, and the short-lived algae soon die and are decomposed by saprobiontic fungi. (Remember saprotrophic nutrition is where the food is digested externally and then absorbed). These are aerobic organisms and use a lot of oxygen creating a biochemical oxygen demand. This causes an oxygen debt so fish and other organisms requiring oxygen die.