Nutrient pollution is one of the most widespread types of water pollution. It is caused by nutrients like nitrogen and phosphates being introduced into the water from waste, agriculture or the use of fossil fuels. Nutrient pollution causes unusual diseases in fish and stimulates algae production, some forms of which are toxic. Zoo-plankton eat the toxic algae and pass the toxicity up the food chain, all the way to birds, mammals and humans. Non-toxic algae and seaweed growth is also stimulated, which causes the water to become murky, making it harder for sea animals to find food and for sunlight to reach sea plants. When the algae dies, it uses oxygen in it's decomposition process, lowering the oxygen levels, eventually killing the sea life.
Chemicals can get into the water a number of ways. Pesticides used on land leach into the water. Chemicals can be intentionally or unintentionally dumped into the waterways or there can be an oil spill. Any introduction of chemicals into the waters have ill effects on sea life. Fish kills can result. Any sea creature can ingest chemicals like mercury and pass them up the food chain, infecting all who have consumed the chemical. Some of the possible effects on animals that have ingested chemicals include reduced reproduction, stunted growth, abnormal behavior and even death.
Marine debris, simply put, applies to all of the trash that makes its way into the waterways. Sea animals accidentally ingest some of the marine debris, thinking it is food, causing health problems or death. Fish, dolphins and other types of sea life can get wrapped up in trash like plastic rings or discarded nets, causing drowning or possible amputation. The presence of marine debris in the seas and oceans can degrade habitats such as coral reefs and beds of sea grass.
Thermal pollution is the introduction of heated water into a body of water. It can shock and kill sea life accustomed to it's habitat being a certain temperature as well as deplete the oxygen stores in the water. The extra heat from thermal pollution may disrupt spawning, reducing the population of some sea life.
Noise pollution, caused by sonar used on boats and ships and by engine noise, disrupts the ability of marine mammals, such as dolphins and whales, from communicating with each other and finding food. Noise pollution has also been attributed as a cause of whale beaching and the sounds can damage marine mammals interior physiology, causing internal bleeding and resulting in death.