Lakes and rivers are extremely complex ecosystems. Nutrients are taken up by both algae and rooted weeds. The weeds act as a shelter for fish larvae and zooplankton, both of which eat algae and are, in turn, eaten by larger fish. Scientists have concluded that unpolluted lakes can absorb surprisingly large amounts of phosphates without uncertainty. When a fertilizer, such as a phosphate, is added more algae will grow, and consequently will the populations of zooplankton and fish.
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.