How does rising atmospheric CO2 affect marine organisms?

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Progressive Nitrogen Limitation in a Brackish Tidal Marsh
Reference
Keller, J.K., Wolf, A.A., Weisenhorn, P.B., Drake, B.G. and Megonigal, J.P. 2009. Elevated CO2 affects porewater chemistry in a brackish marsh. Biogeochemistry 96: 101-117.

Background
The authors write that in the historic long-term (15 years and counting) study of the effects of an extra 340 ppm of CO2 on emergent aquatic plants growing in a brackish marsh on the Rhode River subestuary of the Chesapeake Bay (USA), the increase in vegetative growth induced by the extra CO2 "has the potential to influence soil nutrient dynamics," since "storage of nitrogen as biomass under elevated CO2 conditions may induce progressive nitrogen limitation." However, they report that the CO2-induced stimulation of biomass production by Schoenoplectus americanus (a C3 sedge) "was sustained from 1987 through 2004, suggesting that nitrogen has not progressively limited plant production, possibly due to the 'open' nitrogen cycling (e.g., through tidal exchange) in this marsh (Erickson et al., 2007)."

What was done
At the 15-year point of the study (January 2002), the authors initiated measurements of the concentrations of numerous substances found in the porewater of the wetland soil at depths of 10, 30 and 75 cm below the soil surface at approximately monthly intervals that continued up until December 2006.

What was learned
Keller et al. report that "porewater concentrations of nitrogen (as ammonium) and phosphorus did not decrease despite increased plant biomass in the C3-dominated community, suggesting nutrients do not strongly limit the sustained vegetation response to elevated CO2."

What it means
The five U.S. researchers write that "accumulating evidence from this and other studies suggests that progressive nutrient limitation will not decrease the elevated-CO2 response of C3-dominated communities through time in systems with 'open' nutrient cycles," which conclusion would likely apply to all tidal wetlands.

Reference
Erickson, J.E., Megonigal, J.P., Peresta, G. et al. 2007. Salinity and sea level mediate elevated CO2 effects on C3-C4 plant interactions and tissue nitrogen concentration in a Chesapeake Bay tidal wetland. Global Change Biology 13: 202-215.

Reviewed 11 November 2009