How does rising atmospheric CO2 affect marine organisms?

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How Would U.S. Salt Marshes Respond to CO2-Induced Warming?
Kathilankal, J.C., Mozdzer, T.J., Fuentes, J.D., McGlathery, K.J., D'Odorico, P. and Zieman, J.C. 2011. Physiological responses of Spartina alterniflora to varying environmental conditions in Virginia marshes. Hydrobiologia 669: 167-181.

The authors write that "salt marshes are highly productive ecosystems that dominate the low-energy intertidal landscapes in mid- to high-latitude regions (Adam, 1990) and sequester large amounts of carbon (Odum and Fanning, 1973; Duarte and Cebrian, 1996)," and they note that in western Atlantic intertidal marshes, smooth cordgrass (Spartina alterniflora) is the dominant primary producer, citing Bruno and Bertness (2001).

What was done
Physiological measurements were made on smooth cordgrass, which is believed to possess C4 physiology, at two locations within the Virginia Coast Reserve's Long Term Ecological Research area -- Fowling Point Marsh (a lagoon salt marsh) and Oyster Marsh (a mainland fringing marsh) -- in order to investigate the dependence of the marsh plant's rate of photosynthesis on light, temperature and intercellular CO2 concentration, after which the functional relationships between these environmental variables and S. alterniflora's physiological responses were used to improve C4-leaf photosynthesis models that were ultimately employed to determine the net impact of potential increases in air temperature and atmospheric CO2 concentration on the productivity of smooth cordgrass in a future CO2-enriched and possibly warmer world.

What was learned
The six scientists discovered that rather than acting like a C4 plant, their modeling studies and field measurements indicated that "S. alterniflora exhibited physiological traits similar to C3-C4 intermediate plants," so that one could expect that "atmospheric warming in conjunction with an increase in atmospheric CO2 would enhance photosynthesis in S. alterniflora." In further support of this conclusion, they indicate that "McKee and Rooth (2008) reported a significant stimulation in S. alterniflora biomass in plants grown under elevated CO2," and they state that "warming experiments in New England (Charles and Dukes, 2009) demonstrated that S. alterniflora increased productivity with increasing temperature," adding that the plant's optimum temperature for photosynthesis will also "likely increase in response to enriched CO2 conditions," citing the study of Simon et al. (1984).

What it means
In the concluding paragraph of their report, Kathilankal et al. declare that "in a scenario of atmospheric warming and increased atmospheric CO2 levels, S. alterniflora will likely respond positively to both changes," and they suggest that these responses "will result in increased S. alterniflora productivity," which, we would add, should be good news for western Atlantic intertidal marshes and the many beneficial services that smooth cordgrass provides to those ecosystems.

Adam, P. 1990. Saltmarsh Ecology. Cambridge University Press, Cambridge, United Kingdom.

Bruno, J.F. and Bertness, M.D. 2001. Habitat modification and facilitation in benthic marine communities. In: Bertness, M., Gaines, S. and Hay, M. (Eds.). Marine Community Ecology. Sinauer Associates, Sunderland, Massachusetts, USA, pp. 201-218.

Charles, H. and Dukes, J.S. 2009. Effects of warming and altered precipitation on plant and nutrient dynamics of a New England salt marsh. Ecological Applications 19: 1758-1773.

Duarte, C.M. and Cebrian, J. 1996. The fate of marine autotrophic production. Limnology and Oceanography 41: 1758-1766.

McKee, K.L. and Rooth, J.E. 2008. Where temperate meets tropical: Multi-factorial effects of elevated CO2, nitrogen enrichment, and competition on a mangrove-salt marsh community. Global Change Biology 14: 971-984.

Odum, E.P. and Fanning, M.E. 1973. Comparison of fungi and bacterial biovolume in dead leaves of smooth chord grass (Spartina alterniflora). Estuaries 5: 246-260.

Simon, J.P., Potvin, C. and Strain, B.R. 1984. Effects of temperature and CO2 enrichment on kinetic properties of phosphor-enol-pyruvate carboxylase in two ecotypes of Echinochloa crusgalli (L.) Beauve., a C4 weed grass species. Oecologia 63: 145-152.

Reviewed 20 July 2011