Ocean Acidification and How Little We Actually Know

Ocean acidification, sometimes referred to as “the other carbon dioxide (CO2) problem,” is a major concern for our future marine ecosystems.  The rate at which CO2 is taken up by the ocean increases as its concentration increases in our atmosphere, resulting in more acidic ocean water.  The resulting decrease in pH can cause a decrease in the saturation state of calcium carbonate (CaCO3) and raise the saturation horizon of CaCO3 (the natural horizontal boundary formed as a result of temperature, pressure, and depth, below which CaCO3 readily dissolves; lysocline).  The result of this process is expected to lead to decreased calcification in marine organisms. 

Decalcification of a Pteropod shell exposed over a two month period to
seawater with expected CO2 levels at century's end (Credit).

Doney et al. (2009) provides a good review on how ocean acidification can
 reduce calcification and growth rates of shell-forming marine organisms such as plankton, molluscs, echinoderms, and corals.

Understanding that a more acidic ocean would typically cause reduced calcification, the expected result would then be for otoliths (fish ear bones used to document fish age and growth, among other things) to decrease in size or have slower growth as a result of higher CO2 concentrations in the ocean.  As such, the authors of a recent article in Science hypothesized that otoliths in white seabass (Atractoscion nobilis) eggs and larvae reared in seawater with elevated CO2 would grow more slowly than they would in seawater with normal CO2 levels.
 

Picture of an otolith cross-section

However, the researchers found some unexpected results.  The otoliths actually significantly increased in size under the higher CO2 conditions.  The area of otoliths for 7-8 day old fish were 7-17% larger in higher CO2 conditions and estimated to have otolith masses 10-26% greater, despite fish being the same size.

Dorsal view of sagittal otoliths of 7-day-old white sea bass grown at (A) 430, (B) 1000, and (C) 2500 µatm p(CO2) seawater. Scale bars indicate 10 µm. (D) Ratio (treatment/control) of otolith area in relation to p(CO2)seawater. Mean ratios and their associated uncertainties (3) are plotted. The control level p(CO2) seawater was ~430 µatm [p(CO2) atmosphere ~ 380 µatm], for which otolith area ratio = 1. Checkley et al. 2009.

 
These results suggest that young fish can control the concentration of ions (H+ and Ca2+) in the endolymph surrounding the otoliths.  If there is constant pH (from the regulation of ions) and increased CO2 (from the water) in the endolymph this would result in an increase in carbonate and thus an increase in the saturation state of CaCO3, therefore accelerating otolith formation.  It is still unknown whether this increase in size could cause any ill effects in the fish or on our estimates of age and growth.


Further, other potential effects of ocean acidification include acidification of body fluids (hypercapnia), decreased growth and increased stress as organisms reallocate resources to maintain calcification, and cascading affects on the ecosystem as calcified organisms are adversely effected.  Global warming is complicated and can have unexpected affects.  Studies like Checkley et al. (2009) and others (i.e. Ries et al. 2009) are good reminders of how difficult it can be to predict the complex bio-physical relationships that exist in a dynamic ocean environment.

Dana Sackett


References:

Checkley Jr. DM, Dickson AG, Takahashi M, Radich JA, Eisenkolb N, Asch R. 2009. Elevated CO2 enhances otolith growth in young fish. Science 324:1683.

Doney SC, Fabry, VJ, Feely RA, Kleypas JA. 2009. Ocean acidification: the other CO2 problem. Annual Review of Marine Science 1:169-192.


Ries JB, Cohen AL, McCorkle DC.  2009.  Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification.  Geology 37:1131-1134.