How does rising atmospheric CO2 affect marine organisms?

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Density-Dependent Growth Response of an Agricultural Weed to Elevated CO2
Wayne, P.M., Carnelli, A.L., Connolly, J. and Bazzaz, F.A.  1999.  The density dependence of plant responses to elevated CO2Journal of Ecology 87: 183-192.

What was done
An agricultural weed, field mustard (Brassica kaber), was sewn in pots at six densities, placed in atmospheric CO2 concentrations of 350 and 700 ppm, and sequentially harvested during the growing season to determine the effects of density on the CO2 growth response in this species.

What was learned
Early in stand development, elevated CO2 increased aboveground biomass in a density-dependent manner; with the greatest stimulation of 141% occurring at the lowest density (corresponding to 20 plants per square meter) and the smallest stimulation of 59% occurring at the highest density (corresponding to 652 plants per square meter).  However, as stands matured, the density-dependence of the CO2-induced growth response disappeared, and CO2-enriched plants exhibited an average aboveground biomass that was 34% greater than that of ambiently-grown plants across a broad range of densities.

What it means
As the CO2 content of the air rises, it is likely that plants will initially exhibit increased growth in a density-dependent manner.  However, as suggested by the results of this paper, the influence of density on growth will become less important as plants mature, as the CO2-induced increase in final biomass production was constant across a broad range of densities.  Moreover, the average final biomass of field mustard was similar to that reported for most herbaceous plants exposed to atmospheric CO2 enrichment (30 to 50% increase in biomass for a doubling of the air's CO2 content).  Thus, in regenerating forest ecosystems or old successional fields, it is likely that competition between densely growing plants of a given species will not reduce their CO2-induced growth responses to the extent that some have hypothesized.

Reviewed 15 October 1999