Describe how the presence of nitrates in a lake may lead to the death of fish.

Nitrates are present in artificial fertilisers and farmers add these to their crops to maximise growth. But, due to maybe heavy rainfall or excess fertiliser these nitrates can leach into lakes by surface run-off where the nitrates run-off the soil into a lake. The presence of nitrates leads to rapid growth of algae in the lake leading to an algal bloom, this is when a layer of algae forms on the surface of the lake, which blocks the sunlight from reaching any other plant. Because of this, plants found at the bottom of the lake die as they cannot photosynthesise, this leads to an increase in saprobiontic decomposers; which respire aerobically so use up oxygen in the lake, therefore, the fish do not have sufficient oxygen supply and so die.