What you are seeing on top of the water is the rapid growth of aquatic plants due to the presence of excess nutrients in the water. Although nutrients provide necessary food for these plants, excess nutrients can become toxic to other organisms by disrupting the balance of the aquatic ecosystem. Nutrients can come from failing septic systems, soil erosion created during and after construction and through inappropriate application of home lawn and garden fertilizers. It means degradation of your surface drinking water supply and suffocation of the fish that swim in your lakes and streams.
Fertilizers contain nutrients such as nitrogen, phosphorus and potash.
When it rains these substances run-off into streams and lakes, stimulating algae and weed growth. Pesticides contain compounds that can be toxic to aquatic life, along with polluting groundwater wells. You don't have to sacrifice a clean lake and safe drinking water for a green lawn. Leave your grass clippings on your lawn, or compost them with leaves for application as a natural fertilizer. This will give your lawn most of the nutrients it needs to stay healthy and green.