Eutrophication

Source: Wikipedia. Scheme: Hans Hillewaert.

Eutrophication (literally: overnourishment) can happen if too many nutrients get into a lake's water. Algae will grow and as a consequence, so will the animal life, but more dying plants and creatures will use more oxygen, which results finally in the water's oxygen level sinking. Also, other species of algae might occupy the water, those might be toxic species. Also the amount of mud will increase, with harmful effects on ground-living creatures, such as mussels.