Ocean acidification is the lowering of ocean pH due to increasing levels of CO2 in the atmosphere (from fossil fuel burning, deforestation etc.). The absorption of CO2 has already acidified the surface layers of the ocean causing an overall decrease of 0.1 pH units since the pre-industrial period, which is equivalent to a 30% increase in acidity and a 16% decrease in carbonate ion concentrations. The surface ocean pH is projected to decrease by 0.3-0.4 pH units by 2100 (predicted to decline from approximately 8.2 in pre-industrial times to 7.8 by the end of this century). The changes in basic ocean chemistry due to ocean acidification are likely to have impacts on organisms that require calcium carbonate to build their shells or skeletons such as corals, and molluscs (oysters, mussels, pteropods, and abalone). There are three naturally occurring forms of calcium carbonate used by marine organisms to build shells, plates or skeletons: calcite (e.g. marine plankton coccolithophores), aragonite (e.g. corals, pteropods) and high magnesium calcite (e.g. starfish, sea urchins, brittle stars). The solubility & sensitivity to ocean acidification is higher with magnesium calcite and the least with calcite in the following order: magnesium calcite>aragonite> calcite.

Increasing ocean acidification can significantly reduce the ability of reef-building corals to produce their skeletons via reduced calcification. Successful fertilisation, larval settlement, recruitment, growth and survivorship of corals can be affected due to ocean acidification. A recent research shows that corals, echinoderms and molluscs are very sensitive to a decline in the pH value compared to crustaceans (Wittmann and Pörtner 2013). Many marine fish (about 25% of known marine fish) use coral reef as a habitat, shelter (refuge) and food. Coral reefs provide food and livelihood security for some 500 million people worldwide including 90% protein need of inhabitants of Pacific Island Developing Nations. Coral reefs are the primary economic driver in many tourist destinations and protect fragile coastlines from threats such as tsunamis and erosions.

Some experimental results showed that calcification is generally reduced in mussels under near-future CO2 levels. Projected future CO2 level (rise of ocean acidification) can impact on shell formation, larval development, and survival rate in abalone. A study on the early development of the oyster (Crassostrea gigas) found that shell calcification is reduced in juveniles and their body shape and size are also altered. Many mollusc species at the adult and juvenile stages have shown reduced growth and/or health under projected ocean acidification scenarios. Molluscs are food for commercial fish such as haddock, halibut, herring, flounder and cod. Clams, scallops, mussels, oysters, abalone and conchs provide direct protein sources for various island and coastal communities and are valuable commercial fisheries. Molluscs account for 8% of the global marine catch.

Though the effects of increased acidity on adult finfish seems to be minimal or supposed to be largely unaffected (since fish are able to control their acid-base balance by bicarbonate buffering, mainly across the gills and via the kidney), however, some recent experiments with tropical coral reef fish suggest that the sensory systems of fish can be affected by ocean acidification. For example, when clownfish (Amphiprion percula) were exposed to higher CO2 levels, they could not distinguish predator from non-predator and were found swimming toward predators, instead of away from them (Dixson et al. 2010). The loss of the senses of sight/smell/touch due to ocean acidification would thus reduce survival in commercially important fish species. Another experiment (Frommell et al. 2012) showed detrimental effects of ocean acidification on the developmental stages of Atlantic cod larvae (Gadus morhua). Exposure to elevated CO2 levels resulted in severe to lethal tissue damage in many internal organs in larval cod, with the degree of damage increasing with CO2 concentrations. As larval survival is the bottleneck to recruitment, ocean acidification has the potential to act as an additional source of natural mortality, which may affect populations of already exploited fish stocks. A small change in early life survival can generate large fluctuations in adult-fish abundance in the wild.

Antarctic krill (Euphausia spp.) is a key pelagic species in the southern region and represents the largest fishery resource. Many animals like whales, seals, penguins and fish are dependent on krill fishery. Marine ecosystems in particular krill populations could be vulnerable to ocean acidification. For example, when krill eggs were exposed to elevated seawater CO2 levels, hatch rates were found significantly lower, it also delayed embryonic development (Kawaguchi et al. 2013). The pteropod, or “sea butterfly” (with aragonite shells) are an important food source (for fish such as juvenile salmon, birds, tiny krill, and giant whales). They (pteropods) are also a good indicator of ecosystem health and play an important role in the oceanic carbon cycle. The shells of pteropods, Limacina helicina antarctica – living in the seas around Antarctica are being severely dissolved by ocean acidification according to a new study (Bednaršek et al. 2012). The main consequence of loss of shell due to ocean acidification will be increasing vulnerability of pteropods to predation and infection, which will in turn impact other parts of the food web.

Ocean acidification may cause an increase in jellyfish (Attrill et al. 2008). Jellyfish are key predators and can affect the abundance of zooplankton, fish larvae and eggs, which affects survival to the adult stage (or recruitment) of fish populations. As jellyfish are rarely the preferred food for other marine animals, any significant increase in their numbers could have major consequences for pelagic ecosystems and fisheries.

Nevertheless, rising CO2 may enhance productivity of non calcifying seagrasses, seaweeds as they require CO2 for photosynthesis and living, for example, photosynthetic organisms such as seagrasses showed higher growth rates, as much as five-fold or higher with acidification (Hendriks et al.2010 )

Question: Will ocean acidification be a threat to seafood security, commercial fishing and livelihoods? If so, how?