Chemical changes in the ocean are causing a whole host of ecological problems for marine species, and the effects are evident worldwide. The process is happening so quickly that many species are unable to adapt and evolve, putting them at risk of extinction. As all marine species are already having to adapt to increasing sea temperatures and other stressors, the addition of this other issue could be the tipping point in their survival, especially if the population size has already been reduced. Species with smaller population sizes are more at risk of than those with a healthy population size.
It is thought that coral reefs could be the first victims of ocean acidification, with one reef being destroyed every other day. The rate of disappearance of coral reefs is thought to be over twice as fast as that of rainforests. Coral reefs are built by many individual coral polyps, which secrete a hard calcium carbonate exoskeleton known as a ‘corallite’ to protect their vulnerable, soft body. To create these corallite exoskeletons, corals require aragonite and calcite, which are naturally occurring carbonate minerals present in the ocean. The occurrence of these essential minerals has greatly declined, and the rate of calcification in corals has reduced by 15 percent since 1990 and will continue to decline by up to 60 percent, which will be fatal for most reefs. Many coral reefs are already beginning to crumble, and when the calcium carbonate exoskeletons dissolve, more carbon dioxide is released, adding even more of this damaging gas to the ocean. The red and green algae that live in the corallite exoskeletons and make up around 90 percent of the total mass of coral reefs can also dissolve in acidic conditions.
Having a weakened exoskeleton increases the vulnerability of coral reefs to storms and heavy wave action. The frequency of extreme weather events is increasing due to climate change, and corals are becoming weaker and less able to cope with stormy sea conditions, which could lead to extensive damage and local extinctions during bad weather.
The Earth’s atmosphere currently has a carbon dioxide concentration of 380 parts per million, which is predicted to increase to 450 parts per million by 2030. When the atmospheric carbon dioxide concentration reaches this level, it is predicted that corals will be completely unable to form their exoskeletons due to a lack of calcite and aragonite in the ocean.
Ocean acidification is already slowing the growth of corals, and when their growth eventually stops, the reefs will gradually erode away. If nothing is done to combat ocean acidification, it is possible that all coral reefs may have disappeared by 2050. Around a quarter of all described marine species, which equates to roughly 1,000,000 species, rely on coral to survive and may become extinct if ocean acidification continues. As well as marine life, around 500 million people worldwide depend on coral reefs for their livelihoods and as a food source.
Molluscs, echinoderms and crustaceans
Mollusc, echinoderm and crustacean species such as mussels, sea urchins and lobsters all have a protective shell, which can be dissolved in acidic conditions, leaving the individual vulnerable to predation and disease. The decrease in availability of the molecules required for shell formation means that the individual spends a longer amount of time attempting to find them and less time finding food and reproducing, which has detrimental impacts on the overall population. This is especially damaging for species that have already experienced a decline in population size due to other stressors. Ocean acidification is responsible for some species creating deformed shells, which may also be detrimental to their survival.
Phytoplankton and plankton
Phytoplankton provides 50 percent of the atmospheric oxygen on Earth, removes a large amount of carbon dioxide from the atmosphere,and is the basis of the marine food chain. It is thought that its capacity to produce oxygen has reduced by six percent due to global warming and the effects of ocean acidification could reduce this further.
Non-photosynthesising planktonic organisms such aspteropods form protective shells and use the same materials as molluscs, crustaceans and echinoderms to do so. These materials are becoming less abundant as the ocean’s acidity increases. Both phytoplankton and plankton are at the base of the marine food chain, and their disappearance could have severely detrimental effects on the entire ecosystem.
Fish and marine mammals
The direct effects of ocean acidification will have repercussions on the entire marine food chain. The species which are directly affected are mostly at the bottom of the food chain, and their disappearance will reduce the amount of available prey items for other species, such as large fish and marine mammals, including the basking shark and humpback whale. It is likely that the lack of food will cause the population size of these species to decline, as well as many others. Humans will be negatively affected by fish population declines, as millions of people rely on the fishing industry for food and employment.