Eutrophication: It is an enrichment of water by nutrient salts that causes structural changes to the ecosystem.
Compounds that contain nitrogen and phosphorus (fertilizers) drive the eutrophication. These nutrients are generally not toxic.
Effects on aquatic life: They can have a large impact on the health of rivers and estuaries such as increased overgrowth of algae and aquatic plants, depletion of fish species, general deterioration of water quality.
Explanation:
To increase crop yield, fertilizers contain nitrates and phosphates.
When these get washed up into water bodies, the resulting increase of nitrate or phosphate in the water encourages algae growth, which forms a bloom over the water surface.
The algae leech the nutrient from the water body and kill aquatic life.
So, eutrophication in the lake is due to the presence of nitrogen and phosphorous.