The most direct is the toxic effect of many insecticides on fish. Although product labels warn of fish toxicity, we usually don t pay much attention if we don t have a fish pond. Some studies suggest that diazinon seldom migrates below the top inch of soil, but other studies have found it in wells in California and tap water in Ottawa and Japan. Studies of disulfoton claim that it strongly bonds to soil, and most groundwater samples have not found it, although it has been found in groundwater studies in California. The second impact of insecticides is by killing or reducing populations of insects that would normally be food for the hatchlings.
Herbicides used on lawn weeds also kill aquatic plants.
Pesticide toxicity studies have included fish toxicity, but little study has focused on non-target water plants and insects. I suspect we may see more study in the future. Phosphorous, in particular, fertilizes an algae that is toxic to fish and mammals, and when it grows in huge quantities, the dying and decaying algae rob the water of oxygen, asphyxiating the fish. If you live along a stream, the current push is to restore habitats with native plants. Although most recommendations seem to focus on our usual Puget Sound brush, I suspect we can include some locally native rose species such as rosa nutkana and rosa muriculata .