Pollution is a major threat to the Great Barrier Reef because there are so many different forms of pollution that are accessing the reef and putting it a risk. Urban and industrial waste, sewage, agricultural chemicals , and oil pollution are all forms of pollution which are poisoning reefs. These toxins are dumped directly into the ocean or carried by river systems from sources upstream into the ocean harming the marine species, coral and other marine organisms of the Great Barrier Reef. Some pollutants, such as sewage and runoff from farming, increase the level of nitrogen in seawater, causing an overgrowth of algae, which ‘smothers’ reefs by cutting off their sunlight. Litter is another form of pollution which can kill animals who think this litter is another creature.