Plenty more fish in the sea? - Part Two

Figure 1 - The Carbon Cycle (source: WBGU 2006)

As mentioned last week, ocean acidification is another problem faced by many of the world's fisheries. Central to understanding this phenomenon is the carbon cycle, represented here in the image. The numbers in bold represent carbon reservoirs in gigatonnes (Gt), while the other numbers represent carbon fluxes in gigatonnes per year. We can see that together, the intermediate/deep ocean and the surface layer lock up about 38,000 Gt and that the largest annual influx to the oceanic store comes from the atmosphere. The flux dynamic between the surface and ocean is driven by different partial pressures of CO2 between the atmosphere and seawater, leading to an annual uptake of 2 Gt of carbon by the ocean each year - a figure that amounts to about 30% of annual anthropogenic emissions.

When it has been exchanged and passed into the ocean, CO2 is able to react with other chemicals. The ocean establishes a reversible equilibrium reaction involving CO2:

Figure 2 - Carbon in the ocean

 Carbon dioxide reacts with carbonate ions and water in the ocean to form acidic bicarbonate ions. With a continuous influx of CO2 into the equation due to anthropogenic carbon emissions, the partial pressure of CO2 increases and the equation equilibrium shifts towards bicarbonate, producing more of the acidic ions

Now while slight changes in ocean pH would not have direct impacts on fish, it is important to consider the implications elsewhere in the ecosystem. Increased acidity is predicted to disrupt planktonic crustacean communities, which would have a knock-on effect on the juvenile fish and fish larvae that rely on them for food. Likewise, bivalve molluscs and echinoderms are forecast to decline in the face of changing acidity. They provide a source of food for a variety of adult fish species. With both juvenile and adult populations at risk, it's clear that ocean acidification puts significant pressure on fish populations. It is believed that to date acidity has gone from 8.2 to 8.1 - an increase of 30%.

Additionally, ocean acidification leads to problems with coral reefs too. The process of calcification, which is instrumental in the construction of coral skeletons, is impaired by the increased CO2 concentrations. This happens because higher CO2 concentrations shift the equilibrium in the above equation towards the right, in favour of bicarbonate and at the expense of carbonate. Consequently, coral reefs are less able to expand, and we know from the last post how important reefs are in marine ecology.

So far, we've looked primarily at the implications of climate change for marine fisheries. Next time we'll consider the impacts on fluvial fish stocks - that's salmon to look forward to.