No chips, no fish: ocean acidification

Oceans cover 70 per cent of the earths’ surface and constitute its largest carbon sink, absorbing about one third of human CO2 emissions over the last 200 years. However, in recent decades the level of those emissions has increased to the extent that their continued absorption by ocean water is changing and will further change its natural alkalinity with a pH of 8.2 units. Pure water has pH of 7.0 so strictly speaking, oceans are not becoming more acidic: they are becoming less alkaline.

Analysis of ice cores shows that alkalinity is now lower than it has been for 600,000 years and is continuing to fall. The average pH of ocean surface water has fallen by 0.1 of a unit since the industrial revolution and by the end of this century is predicted to have fallen by 0.35-0.5 units. The rate at which pH is now falling is estimated to be over 100 times greater than at any time during the past 100,000 year.

The problems are both the reduction of alkalinity (falling pH) and the speed with which it is happening. The former damages calcifying marine life, the latter gives it no time to adapt. The result: serious damage to the marine environment and adverse effects on the ability of crustacean and other fish to survive.

As CO2 entering the atmosphere continues to rise, its pressure on the ocean surface grows, causing increasing amounts of the gas to be dissolved by it. In colder waters CO2 is held in suspension while in warmer water at lower latitudes, more of it reacts with water to form carbonic acid. In both cases, rising quantities of CO2 pose serious and growing problems for calcifying marine animals such as algae, corals, plankton and molluscs, by reducing the concentration of calcites present in seawater.

Pteropods (Thecosomata), a small snail form of plankton are particularly vulnerable to this change since they make their shells from aragonite which is 50 per cent more soluble in water than other calcites. Reduced presence of this material makes it more difficult for juvenile pteropods to secrete and maintain robust protective shells. This greatly increases their vulnerability. So serious is this problem that marine scientists predict that pteropods are likely to become extinct within 40 years.

Pteropods are present in all the oceans. Such a wide variety of fish depend on them as their main source of nourishment and consume them in such vast numbers that they have been appropriately described by Dr Hoffmann (University of California) as “chips of the sea”.

Millions of humans depend on these fish, or other fish which prey on them, as their main - often their only - source of protein. At the very least, extinction of the pteropod will result in major depletion of commercial fish stocks and growing scarcity, even their total loss to those who depend on them most.

Seagrasses appear to thrive in the presence of elevated levels of CO2 and less alkalinity, with greater biomass being produced. Seagrass meadows provide important nursery areas for juvenile fish. On reaching maturity, many migrate to coral reefs, enhancing bio-diversity on or in close proximity to them. However, this is likely to become much more difficult because of the growing plight of coral reefs.

Coral reefs or more correctly the polyps, which protect their delicate bodies with calcites of many different shapes and colours, are confronted with similar problems to those of the pteropods. Reduction in the concentration of calcites in seawater makes it increasingly difficult for polyps to produce a strong and durable covering. This makes it more difficult for them to maintain the protection afforded by coral and increases their vulnerability to erosion, predators and warming temperatures.

Ultimately, the battle will be lost and polyps will die in their billions, resulting in the death of entire reefs, irrespective of their size. This is already occurring in the Caribbean and elsewhere.

Calcifying algae (Halimeda) are among the many animals which inhabit coral reefs. When they die their bodies accumulate, providing “cement” that strengthens and protects reefs from erosion and coral eating fish. However, the decreasing level of calcites confronts algae with the same problems experienced by coral polyps. Their numbers decrease as does the material they contribute to reef protection.

When a coral reef dies it is no longer able to provide habitat to the large number and variety of fish that use it as a nursery, a source of food, or for protection. When a reef dies, so do its inhabitants and the fish which rely on them for sustenance must look elsewhere. The latter include fish which humans need and use as a source of protein.