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

The applied nitrates may seep into water sources from the fields at times of rainfall. Here, they increase the nitrate concentration in the water, leading to an algal bloom. This layer of surface algae blocks light to bottom dwelling plants, thus limiting their ability to photosynthesise, so they eventually die. Saprobiotic bacteria decompose this organic material, while aerobically respiring to obtain their required energy. Because there is so much dead plant material, the bacteria continue to do this, using up so much oxygen that there is less oxygen avaliable for the fish. These aerobic organisms may eventually die.