Eutrophication is the excessive build up of nutrients in lakes, ponds, and other water bodies as a result of river and stream runoff containing pollution from fertilizer, sewage, and other waste products. While more nutrients in the water might at first sound like a good thing, it actually has harmful side effects, such as algae blooms that deplete water of oxygen and kill off aquatic species and other wildlife. Factory farms and private residents who fertilize their lawns are major culprits of this type of pollution. If you live by a fresh water source, such as a pond or stream, you can lessen the negative effects of runoff by creating a natural plant barrier between your lawn and the water. Among the plants that make ideal buffers are willow trees, birch, green ash, red maples, buttonbush, spice bush, some dogwood species, water oak and pin, sycamore, and smooth alder. These and other plants absorb the excess nutrients while also preventing erosion.