Fertilizer runoff can cause an excess of nutrients in the water. The excess of nutrients can cause blooms of algae and phytoplankton. The growth of algae and phytoplankton is limited by the availability of the nutrients in the water, and with the influx of fertilizer they bloom uncontrollably. The bacteria that degrade these algae and phytoplankton can use all of the oxygen in the water while they degrade the dead organisms and create dead zones in the oceans.
