Why is the runoff from fertilized agricultural fields, even if free of pesticides, often harmful to the ecosystems of temperate lakes?

Answer The runoff causes a surface algal bloom, which reduces the penetration of light into the eater which reduces photosynthesis which reduces the amount of oxygen in the water.

Fertilizer acts in bodies of water (lakes and oceans) just as it acts on land. This fertilizer blocks the penetration of light into the water column which reduces the photosynthesis that occurs in the water, which (along with decomposition) reduces the amount of oxygen in the water which is detrimental of to oxygen-requiring life.

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