Land-based sources (such as agricultural run-off, discharge of nutrients and pesticides and untreated sewage including plastics) account for approximately 80% of marine pollution, globally. Marine habitats worldwide are contaminated with man-made debris. Oil spills remain a concern, though actual spills have decreased steadily for several decades.
Excessive nutrients from sewage outfalls and agricultural runoff have contributed to the increasing incidence of low oxygen (hypoxic) areas known as dead zones, where most marine life cannot survive, resulting in the collapse of some ecosystems. There are now close to 500 dead zones with a total global surface area of over 245,000 km², roughly equivalent to that of the United Kingdom. The excess nitrogen can also stimulate the proliferation of seaweeds and microorganisms and cause algal blooms. Such blooms can be harmful (HABs), causing massive fish kills, contaminating seafood with toxins and altering ecosystems.
Litter can accumulate in huge floating garbage patches or wash up on the coasts. Light, resistant plastics float in the Ocean, releasing contaminants as they break down into micro-particles that animals mistake for food. Fish and birds can choke on these particles, get sick as they accumulate toxins in their stomachs, or become entangled in larger debris.
As the world saw in 2010, the Gulf of Mexico deep-water oil spill had a devastating effect on the entire marine ecosystem, as well as the populations that depend on the marine areas for their livelihoods. Smaller oil spills happen every day, due to drilling incidents or leaking motors, negatively impacting birds, marine mammals, algae, fish and shellfish.