Nitrates and phosphates from fertiliser applied to crops may enter ponds and lakes. Explain how nitrate may cause the death of fish in fresh water.

Plants need three main nutrients to grow, which can be helpfully abbreviated as NPK: that's nitrates, phosphates and potassium. Many gardeners and farmers will apply these fertilisers to crops in order to increase productivity. However, one common problem with traditional nitrate or phosphate fertiliser application is that of eutrophication - when nitrate runs off the soil (e.g. after a rainfall event) and into nearby water sources such as rivers, ponds or lakes, resulting in enrichment of the water with these minerals and ultimately leading to algal blooms. The excessive growth of algae covers the surface of the water, blocking out light to the photosynthesising plants below the surface and leading to their death. The plants are then decomposed by saprobiotic organisms (e.g. bacteria) which respire aerobically and so use up much of the oxygen in these bodies of water as they go about the decomposition. The resulting depletion of oxygen in the water leads to large fish kills by asphyxiation. Because of this risk of eutrophication and fish death following excessive fertiliser application, many farmers are being encouraged to reduce their fertiliser use, and some new technologies use the colour of plants as an indicator to show how much fertiliser is needed, and so allowing for a more precise and targeted application.