How does rising atmospheric CO2 affect marine organisms?

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The Growth Response of a Submersed Aquatic Plant to Elevated CO2 in the Air Above It
Idso, S.B. 1997. The Poor Man's Biosphere, including simple techniques for conducting CO2 enrichment and depletion experiments on aquatic and terrestrial plants. Environmental and Experimental Botany 38: 15-38.

What was done
For several different periods of several weeks each, the author grew specimens of the common freshwater aquarium plant corkscrew vallisneria (Vallisneria tortifolia) - which had been trimmed back to initially have only 3 cm of blade length and 3 cm of root length - in several 10- and 29-gallon aquariums filled with non-fertilized tap water and having a bottom cover of approximately 10 cm of common aquarium gravel, while the semi-sealed air spaces above these "Poor Man's Biospheres" were maintained at several different CO2 concentrations and their water temperatures (T) were maintained within 0.5C of either 18.2C (low T) or 24.5C (high T).

What was learned
Interestingly, the CO2-induced growth enhancement of the plants was determined to be linear, as opposed to the gradually declining growth enhancement at higher CO2 levels that is exhibited by most terrestrial plants; and it extended to the highest atmospheric CO2 concentration studied: 2100 ppm. This behavior, however, is not unique; for Idso reports that Titus et al. (1990), who studied the closely related Vallisneria americana, "observed that the biomass of their experimental plants also rose linearly with the CO2 content of the air above the water within which they grew, and that [it] did so from the value of the current global mean (365 ppm) to a concentration fully ten times larger [our italics]." As for the interactive effect of water temperature, the CO2-induced growth enhancement of the plants in the high-T treatment was 3.5 times greater than that of the plants in the low-T treatment.

What it means
The findings of Idso's study, together with those of Titus et al., suggest that the growth of submersed macrophytes may be more responsive to atmospheric CO2 enrichment than is the growth of most terrestrial plants. More effort should be thus directed to further examining this intriguing possibility.

Titus, J.E., Feldman, R.S. and Grise, D. 1990. Submersed macrophyte growth at low pH. I. CO2 enrichment effects with fertile sediment. Oecologia 84: 307-313.

Reviewed 4 October 2006