This is very easy, since the lakes are near a crop field, runoff fertilizers have obviously contaminated the water, and to prevent it : Careful contouring of the land can help prevent this by slowing and spreading the flow of water, giving it time to soak in before it runs downhill. Terraces, swales and buffer zones can be used to keep fertilizer from entering ponds and other waterways. That was from google for a more detailed response.
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