Pollution comes in many forms and has many impacts on the marine environment and marinelife that live there. Industrial and domestic pollution enters the marine environment mainly through rivers. But direct pollution from shipping is also significant.
Nutrients from agriculture, sewerage and industrial processes enter the sea in huge amounts. This can lead to algal blooms sometimes known as red tides, some of which can cause death in fish, marine mammals, birds etc.. Increased nutrient loads can also lead to reduction in dissolved oxygen. In many developed areas of the world, legislation is controlling the amount of eg untreated sewerage entering the sea.
Chemicals and heavy metals
These arise from land based industrial, atmospheric or ship sourced pollution and enter the food chain. The are concentrated in marine mammals and can cause reductions in fertility, reduced immunity from disease etc. Ironically, recently, Iceland whale meat exported to Japan was rejected because of excessive heavy metal content.
Plastics are abundant polluters of the marine environment. Plastic sheeting, bags etc are commonly seen in shipping lanes and are mistaken by cetaceans, turtles and seabirds for food, and ingestion is often fatal. The impact is most famously to be seen in Laysan Albatrosses living in the Pacific Ocean that feed in the area known as the Pacific Gyre where huge volumes of plastic debris are concentrated by currents. Thousands die each year from plastic ingestion. However, seabirds in all oceans are at risk. A study of dead Fulmars in the North Sea found plastics in 98% of stomachs. Balloons are also implicated.
Plastic pollution also occurs as very fine granules, either direct from shore-based industrial emissions or from breakdown of plastics in the ocean. Granules often mimic plankton and are therefore eaten by plankton feeding marinelife. The plastics eventually break down and then release chemicals into the marine environment.
Oil pollution from oil tanker wrecks, oil installation blow-outs and from illegal discharges at sea of fuel tanks is a well known and acute pollution source that kills a lot of the marinelife in the vacinity of the incident. The numbers of animals affected depends on many factors; proximity to breeding or wintering concentrations, size of spill, type of oil, sea currents and weather conditions. However the impact is probably longer and wider than immediately apparent as oil pollution will disperse, and break down and enter the foodchain.
Pollution incidents continue with little evidence of success in reducing their frequency or impact, even though pollution monitoring has increased, and many incidents are missed.
The rate at which species are being redistributed around the globe is increasing dramatically, often carried in seawater used as ballast by pan-global shipping - to balance the ship whilst transiting with little or no cargo. Taken on in one port and pump it out in another ocean. The seawater often includes fish, and crab larvae, algaes etc., some of which survive, and may thrive in their new home and can imbalance the ecosystem, either by predating on rare species or consuming their food source.
Most shipping uses low-grade bunker fuel oil, which has a sulphur content 2,000 times that of car diesel. The seas are a major sulphur sink (absorber), but the increased rates may lead to increasing acidification. Shipping emissions pollution is concentrated in the high shipping traffic areas, which include inshore European waters.
Shipping is also considered to contribute 3.5-4% of all climate change emissions. However, research suggests that the same pollution helps form clouds which reflect solar heat back to space, so helping cool the planet.
There are increasing moves to limit sulphur emissions from shipping, led by the UN.