How does rising atmospheric CO2 affect marine organisms?

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CO2 Effects on Tropical Marine Fish Embryos and Larvae
Munday, P.L., Donelson, J.M., Dixson, D.L. and Endo, G.G.K. 2009. Effects of ocean acidification on the early life history of a tropical marine fish. Proceedings of the Royal Society B 276: 3275-3283.

The authors say "there is concern that continued increases in atmospheric CO2 over the next century could have significant impacts on a wide range of marine species, not just those with calcified skeletons." In the case of fish, however, which "control their tissue pH by bicarbonate buffering and the exchange of ions, mostly across the gills," they say that "small changes in internal or external pH can readily be compensated (Heisler, 1989; Claiborne et al., 2002)." Fish embryos and young larvae, on the other hand, are said by them to be "more sensitive to pH changes than are juveniles and adults," citing the work of Brown and Sadler (1989). Therefore, they indicate that "significant effects of ocean acidification are most likely to be detected in these early life stages," which possibility they thus go on to explore.

What was done
Working with a 70,000-liter recirculating sea water system at James Cook University's experimental marine aquarium facility, Munday et al. grew wild-caught pairs of the orange clownfish (Amphiprion percula) in 70-liter tanks containing sea water simulating a range of ocean acidification scenarios for the next 50-100 years -- 390 (current day), 550, 750 and 1030 ppm atmospheric CO2 -- while documenting various aspects of egg, embryo and larval development.

What was learned
The four researchers, all from the School of Marine and Tropical Biology of Australia's James Cook University, determined that "CO2 acidification had no detectable effect on embryonic duration, egg survival and size at hatching." In fact, they say that it actually "tended to increase [italics added] the growth rate of larvae." Eleven days after hatching, for example, they observed that "larvae from some parental pairs were 15 to 18 per cent longer and 47 to 52 per cent heavier in acidified water compared to controls," further noting there was a "positive [italics added] relationship between length and swimming speed," and that "large size is usually considered to be advantageous for larvae and newly settled juveniles."

What it means
In discussing current concerns over potential effects of the ongoing rise in the air's CO2 content on marine fish, Munday et al. state that "the most common prediction is that ocean acidification could [negatively] affect individual performance (e.g. development, growth, survival, swimming ability)," especially during the early life history of such fish. However, they indicate that "contrary to expectations," their findings indicated that "CO2-induced acidification up to the maximum values likely to be experienced over the next 100 years had no noticeable effect on embryonic duration, egg survivorship and size at hatching for A. percula, and tended to have a positive effect on the length and weight of larvae."

As for adult fish, the Australian scientists note that "most shallow-water fish tested to date appear to compensate fully their acid-base balance within several days of exposure to mild hypercapnia [a deleterious condition produced by having more than the normal level of carbon dioxide in the blood due to exposure to elevated CO2 concentrations]," citing the work of Michaelidis et al. (2007) and Ishimatsu et al. (2008) in this regard. Hence, it would appear that climate alarmism over this issue is significantly overblown.

Brown, D.J.A. and Sadler, K. 1989. Fish survival in acid waters. In: Morris, R., Taylor, E.W., Brown, D.J.A. and Brown, J.A. (Eds.). Acid Toxicity and Aquatic Animals. Cambridge University Press, Cambridge, United Kingdom, pp. 31-44.

Claiborne, J.B., Edwards, S.L. and Morrison-Shetlar, A.I. 2002. Acid-base regulation in fishes: cellular and molecular mechanisms. Journal of Experimental Zoology 293: 302-319.

Heisler, N. 1989. Acid-base regulation in fishes. I. Mechanisms. In: Morris, R., Taylor, E.W., Brown, D.J.A. and Brown, J.A. (Eds.). Acid Toxicity and Aquatic Animals. Cambridge University Press, Cambridge, United Kingdom, pp. 85-96.

Ishimatsu, A., Hayashi, M. and Kikkawa, T. 2008. Fishes in high CO2, acidified oceans. Marine Ecology Progress Series 373: 295-302.

Michaelidis, B., Spring, A. and Portner, H.O. 2007. Effects of long-term acclimation to environmental hypercapnia on extracellular acid-base status and metabolic capacity in Mediterranean fish Sparus aurata. Marine Biology 150: 1417-1429.

Reviewed 9 December 2009