Difficulties only arise when the lake is already impure. Zooplankton are sensitive to their environment and many substances are toxic to them. If any of these substances, including phosphates, are present the zooplankton population cannot increase. Adding phosphates to this polluted system will case algae growth. The floating masses cut off the light supply.
Weeds die and decompose using up dissolved oxygen, and causing sulfurous smells and plagues.
Deprived of shelter and food, the fish larvae starve. The lake is well on the way to catastrophe. Without wetlands there would be a minimal amount of fresh drinking water due to the fact that wetlands filter the waters of our lakes, rivers and streams, sequentially reducing contamination of water. The plant growth in wetlands removes phosphates and other plant nutrients washed in from the surrounding soil, consequently restricting the growth of algae and aquatic weeds. This growth is a serious problem in some of Canada's major waterways, where dead and decaying algae deprive the deeper waters of their oxygen.