Oceans acidifying at the highest rate since the last great mass extinction 65 million years ago, threaten marine life, coral reefs and nutrition for over a billion people according to a report (PDF)just released by the UN. Fossil fuel burning, cement production, deforestation and land use changes have increased atmospheric CO2 from 280 to 390 PPMs. Increasing atmospheric CO2 has dissolved in the oceans, increasing the acidity (decreasing the pH) by 30%. Increasing acidity is destabilizing shells and corals, threatening the web of life in the ocean.
Recent studies of a Mediterranean vent in shallow waters revealed key ecosystem changes in high CO2 areas compared to normal CO2 areas. Certain algae and seagrasses were found to grow more efficiently closer to the vents where there was also a large reduction in biodiversity, notably a loss of calcifying organisms adjacent to the vents where the pH was lowest.
Ocean acidification threatens global fisheries
Natural CO2 seeps acidify water causing calcium carbonate shells to dissolve.
Primary production forms the basis of the marine food web and is of great importance to maintaining fish stocks. Production is highest in areas rich in nutrients, such as upwelling zones and continental shelf areas. Globally approximately 80% of fish catch are from such areas.
Productivity ‘hotspots’ such as upwelling regions where cold water is rich in both nutrients and CO2, coastal seas, fronts, estuaries and sub-polar regions often supply the main protein source for coastal communities. However, many of these areas are also projected to be very vulnerable to ocean acidification this century.
As world populations rise alongside a predicted growth in coastal populations due to internal migration, the demand for ocean protein products is also likely to rise. Fish stocks, already declining in many areas due to over-fishing and habitat destruction now face the new threats posed by ocean acidification.
Aquaculture is threatened by ocean acidification
Marine invertebrates like molluscs and crustaceans, increasingly used in aquaculture, show negative responses to acidification at various life stages. An organism’s success depends on more than just the growth of its adult form. The life cycle of invertebrates from egg fertilization and development, to growth and timing of different juvenile larvae stages to their settlement, are critical to their own survival as well as that of both their prey and predators11. As most shellfish eggs and larvae are planktonic, these species are prone to the negative effects of ocean acidification. One study on the early development of the oyster (Crassostrea gigas) shows that shell calcification is reduced in juveniles; their body shape and size are altered, which suggests serious consequences for their survival into adulthood.
Ocean acidification threatens marine ecosystems
Ocean acidification is already destabilizing shells of key species at the bottom of the food chain in the Arctic Ocean. Most marine organisms that form shells build them from the mineral aragonite which built shells quickly before humans burned fossil fuels and cut down forests. However, aragonite is very sensitive to acidification of water. It rapidly destabilizes with increasing acidity.
Geochemists use the symbol Ω to describe the stability of minerals. Below Ω =1 a mineral dissolves. Minerals tend to precipitate more rapidly as the value of Ω increases above 1.
Ω less than 3 is unfavorable for the growth of many shelled marine organisms and coral reefs. As atmospheric CO2 levels rise above 450 parts per million (PPM) Ω drops below 3 for most of the earth's oceans. At the present level of 380 PPM CO2, Ω is already below 3 for the polar oceans.
credit: Ken Caldeira
Earth's coral reefs face extinction
Ocean acidification, combined with the effects increasing water temperatures will destroy most of the world's coral reefs if atmospheric CO2 levels continue to rise. Coral reef structures are already unstable in the Galapagos islands and the Pacific coast of Panama. As Ω for aragonite drops below 3 coral reef structures around the world will destabilize. Reefs, including Australia's Great Barrier reef, will die.
Great Barrier reef, Australia Photo: J.E.N. Vernon
A dire warning from Australian coral reef expert J.E.N. Vernon
You may well feel that dire predictions about anything almost always turn out to be exaggerations. You may think there may be something in it to worry about, but it won’t be as bad as doomsayers like me are predicting. This view is understandable given that only a few decades ago I, myself, would have thought it ridiculous to imagine that reefs might have a limited lifespan on Earth as a consequence of human actions. It would have seemed preposterous that, for example, the Great Barrier Reef — the biggest structure ever made by life on Earth — could be mortally threatened by any present or foreseeable environmental change.
Yet here I am today, humbled to have spent the most productive scientific years of my life around the rich wonders of the underwater world, and utterly convinced that they will not be there for our children’s children to enjoy unless we drastically change our priorities and the way we live.
...A decade or so ago, we thought that mass bleaching was the most serious threat to coral reefs. How wrong we were. It is clear now that there is a much more serious crisis on the horizon — that of ocean acidification. This will not only affect coral reefs (although reefs will be hit particularly hard), but will impact all marine ecosystems.
...All organisms that produce calcium carbonate skeletons (including shells, crabs, sea urchins, corals, coralline algae, calcareous phytoplankton, and many others) depend on their ability to deposit calcium carbonate, and this process is largely controlled by the prevailing water chemistry. As alkalinity decreases, precipitation of calcium carbonate becomes more and more difficult until eventually it is inhibited altogether.
The potential consequences of such acidification are nothing less than catastrophic.
Bleached coral, photo: J.E.N. Vernon
Humans are destroying the world's coastal zones, fisheries and coral reefs.