How does rising atmospheric CO2 affect marine organisms?

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CO2 Effects on Forest Understory Plants and Arthropods
Sanders, N.J., Belote, R.T. and Weltzin, J.F.  2004.  Multitriphic effects of elevated atmospheric CO2 on understory plant and arthropod communities.  Environmental Entomology 33: 1609-1616.

What was done
The authors studied effects of atmospheric CO2 enrichment on the plant and arthropod communities of the understory of a closed-canopy sweetgum plantation (which reduces the light available to the understory between 70 and 95% during the growing season) in a FACE study where the air's CO2 content was increased by approximately 48%.

What was learned
There were large adjustments in the relative productivities of the five dominant species that account for more than 90% of the biomass and annual production of the understory vegetation.  Overall, however, the total understory productivities of the two CO2 treatments were not significantly different from each other.  Also, Sanders et al. report that "C:N ratios for four of the five dominant plant taxa did not differ between ambient and elevated CO2," and that "there were no overall treatment or species x treatment effects" with respect to this parameter.

The three scientists further report they "found no effect of elevated CO2 on herbivory," and that "even for the one species that showed an effect of CO2 on C:N ratio, herbivores did not compensate by foraging more."  In addition, total arthropod abundance did not differ between ambient and elevated CO2 plots, nor did the abundances of detritivores, omnivores or parasitoids.  In summary, as they thus concluded (for this experiment, at least), "changes in plant community composition did not translate into differences in arthropod communities."

What it means
Sanders et al. state that "idiosyncratic, species-specific responses to elevated CO2 may buffer one another: the abundances of some species increase while others decrease," and, therefore, "to understand the potential effects of global [environmental] change on the complexity of multitrophic interactions that structure most communities, field experiments on entire communities are necessary."  Even then, however, it may still be a daunting task to correctly predict responses to projected future increases in atmospheric CO2 concentration and related or unrelated changes (or non-changes) in various climatic parameters.

Reviewed 18 May 2005