Why does as an excess of nutrients such as nitrates lead to animal death in bodies of freshwater?

This is an unexpected result as usually nutrients are considered beneficial to organisms. Some organisms do benefit enourmously from the nutrients, but this leads to the deaths of other. The high concentrations of nutrients allow high rates of algal growth (e.g. nitrates allow synthesis of protein), leading to an 'algal bloom'. The algal bloom prevents sunlight reaching plants on the lake bed, their rate of photosynthesis is greatly decreased and they die. Some of the algae also die. As a result there is a lot more dead biomatter in the lake, which causes a population explosion of decomposers, both bacterial and fungal. These do not photosynthesis but do respire. Since the rate photosynthesis has decreased but the rate of respiration increased the oxygen concentration starts to fall. This can prove lethal to animals such as fish which have a high oxygen demand due to their active lifestyle. The process is called eutrophication.BONUS THOUGHT (probably appropriate to discuss with an oxbridge candidate): In eutrophication, the populations of some organisms expand rapidly whilst others plunge to nothing. The stability of the ecosystem is decreased. In many ecological contexts, competition promotes stability, so perhaps one way of viewing this process is that the increased availability of nutrients leads to decreased competition and decreased stability. Stability is not the same as success however. For algae eutrophication is a huge success, despite there being a great loss of stability. What happens as the amount of nutrients in the water is decreased?