Ocean Acidification

Anthropogenic actions, such as the burning of fossil fuels and the clearing of forests, are increasing the flux of carbon dioxide into the atmosphere. As the diagram below demonstrates, atmospheric carbon dioxide concentrations have been increasing over the past hundred years and are now at about 393ppm, compared to a pre-Industrial Revolution level of 280ppm.

http://www.pmel.noaa.gov/co2/story/Ocean+Acidification

Increasing atmospheric carbon dioxide concentrations impact not only terrestrial, but also marine ecosystems. At the atmosphere-ocean interface carbon dioxide dissolves into the water. This is a natural phenomenon. What is unnatural is how much carbon dioxide is currently being dissolved into the oceans now. More carbon dioxide in the atmosphere means more is entering marine waters. When carbon dioxide dissolves with water it forms carbonic acid (see diagram below). This compound dissociates into hydrogen ions, which decreases the ocean's pH. A "normal" ocean pH is around 8.1-8.2. Currently the oceans are experiencing a pH of 7.7 (Welch, 2011). The pH scale is logarithmic, which means a pH of 7 is ten times more acidic than a pH of 8. Therefore this observed decline of about 0.4 in pH results in about a 40% increase in hydrogen ion concentration levels. This drop in pH is dramatic and has the potential to drastically alter marine ecosystems.

http://snowcrablove.blogspot.com/2012/03/whats-ocean-acidification.html

Many studies have analyzed the impact of increasing ocean acidity on shellfish. Results demonstrate that lower pH can increase physiological stress for many marine calcifiers. Negative impacts include a decrease in calcification rates due to less carbonate ions, reduction in sensory abilities, slower development, and greater susceptibility to disease (Friedman, 2011). Overall it is still largely unknown how calcifiers, including geoducks, will adapt to large-scale changes in seawater pH. Research thus far seem to indicate that species' response to lower pH is not uniform. While some calcifiers, such as coccolithophores, seem to be the "losers" (Kleypas et. al,2006), geoducks appear to be the "winners".

It is projected that some calcifiers, such as coccolithophores (image on the right (http://www.co2.ulg.ac.be/peace/intro.htm)), will be worse off with more acidic oceans because  these organisms depend on coccoliths for building their calcified shells and the shell formation process is dependent on saturation of carbonate ions. Acidifying oceans are resulting in less saturated levels of carbonate ions, which decreases the rate of calcification for species, such as coccolithophores. Geoducks, however, do not rely on coccoliths for their shells.



It is still not clear exactly why geoducks are doing so well in acidifying oceans when other shellfish are more physiologically stressed. One possible explanation for this observed difference in performance levels between species could be due to the lack of multivariate studies. The synergistic impacts of increasing carbon dioxide levels, rising temperatures, changing light density, and nutrient concentrations have yet to be determined for marine species but could be vital towards understanding future shellfish population trends. Geoducks may be doing better than other marine species because they can adapt quicker to these changing variables. A probable explanation for the biological success of geoduck populations with climate change could be that they have greater plasticity (potential of individuals to respond over the short term) potential. Geoducks' evolutionary response to changing climatic factors may not be constrained by antagonistic genetic correlations as was found with some terrestrial species, such as prairie plants (Etterson, 2001).

Geoducks have shells composed of aragonite, a calcium carbonate compound. This compound reacts with carbonic acid, which increases as the ocean acidifies (Brickey et. al, 2012). Some scientists project that the observed rise in ocean acidity will have large effects on aragonite-shelled organisms, such as geoducks. To the contrary, geoduck larvae seem to have higher survival rates in lower pH (Bascom, 2011). This may be a result of their preference for deeper waters, which are more acidic.

The table above lists several interesting research findings on impacts of changing seawater chemistry on marine calcification. (Figure- Kleypas et. al., 2006).

Ocean acidification is not the only phenomenon that has the potential to affect geoduck fitness and population size. Rising  temperatures are also contributing to this species' performance. Click on the next page to find out more about the biological impacts of a warming ocean.