Eutrophication of a lake occurs when the algae that reside there bloom- they reproduce at a much faster pace than normal. This is due to the input of excessive nitrates and phosphates.
When algae over grow and later die off, oxygen in the lake is depleted as bacteria use the oxygen to consume the algae. The lake may become a low oxygen environment which can devastate organisms that reside there. As fish die off, the quality of the lake further deteriorates leading to more oxygen depletion as bacteria decompose the dead organisms. The sunlight may be prevented from entering the lake due to the algae bloom which further disrupts the food chains operating in this ecosystem.
Nutrient pollution is the main reason for eutrophication due to human activities. This is due to chemical fertilizers applied to lawns and to farms and from discharge from septic and sewer systems. Runoff from farmlands, golf-courses, private homes and areas being developed can eventually find its way into a body of water and cause it to become polluted. That is because wastes and chemical fertilizers contain nitrates and phosphates.
Humans can avoid eutrophication by limiting the use of chemical fertilizers and by applying cover crops to prevent erosion of soil into nearby lakes and streams. Vegetative buffer zones can be planted or maintained near a body of water to prevent the entry of pollutants. These are some ways to avoid eutrophication of a lake.