Anthropogenic Changes in the Ocean

The oceans are affected by human activity in several ways.Ocean acidification: the oceans absorb about a quarter of the CO₂ humans emit. Dissolved CO₂ reacts with water to form carbonic acid, which releases hydrogen ions:CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻Surface ocean pH has fallen from about 8.2 to about 8.1 since the Industrial Revolution. Because pH is a log scale, this is about a 26% increase in H⁺ concentration. The extra H⁺ ions combine with carbonate ions, so less carbonate is available for corals, molluscs and plankton to build their calcium carbonate shells and skeletons, which may even start to dissolve.Warming: warmer water holds less dissolved oxygen, and corals under heat stress expel the photosynthetic algae living in their tissues. This coral bleaching leaves the coral without its main food source, and if it lasts the coral dies. Coral reefs support about a quarter of marine species.Eutrophication: fertiliser and sewage washed into coastal waters cause algal blooms. When the algae die, aerobic bacteria decomposing them use up the oxygen, creating dead zones where fish cannot survive.Plastic pollution: plastics break into microplastics, which are eaten by animals from plankton to whales and can build up along food chains. Larger items entangle turtles, seabirds and seals.Together with overfishing, these changes threaten marine food webs and the people who depend on them.