An imbalance in the natural nutrient content of water. Bodies of water naturally contain nutrients such as phosphorus and nitrogen which are essential to plant growth. Fertilizers contain excess nutrients and when these enter the water supply, perhaps due to water running off a field into a river, the nutrients cause an imbalance in the make up of the water. As nutrients are important to plant growth on land, the same applies to plants in the water. Therefore, too many nutrients in the water encourage the growth of weeds and algae. This can make the water highly polluted and result in oxygen depletion.