Nutrient pollution is caused when excess nutrients in the form of nitrogen and phosphorus are introduced into aquatic ecosystems, often due to leaching or surface water runoff. These excess nutrients can contribute to the degradation of downstream ecosystems. Nitrogen and/or phosphorus can drive harmful algal blooms, which block out the sun and hinder the growth of aquatic plants. Between decreased rates of photosynthesis and a ride in bacterial populations which feed on the algal mats and dying plants, oxygen levels can start to fall, killing off aquatic animals. This process is also known as eutrophication.