Briefly describe the environmental effects of nitrogen fertilisers in a body of water

The nitrogen fertiliser would leach into the water. This would cause algae to grow exponentially on the surface of the water, therefore would reduce the amount of light available to plants below the surface. Light is the limiting factor.

The plants would die (as cannot photosynthesise) and saprobiotic organisms would feed off the dead plants, as well as use up the oxygen in the water. Oxygen is the limiting factor.  Reduction in oxygen causes aerobic organisms in water to die. The water is now putrid.