When fertilisers leak into rivers or lakes, they can cause eutrophication. Basically, when the excess nitrates from the fertilisers leak into the water, they cause a huge boon period for plant life. Plant life grows so rapidly that the lake soon becomes over-crowded and the plants begin to die due to competition for light. The amount of microbes in the water then begins to increase rapidly as well, as they break down the dead plants. These bacteria use up all the oxygen in the water, starving fish of the oxygen. Other life in the lake begins to die off.

The moral of the story is that farmers need to take more care when spreading artificial fertilisers.