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A Largely Unappreciated Effect of Elevated Atmospheric CO2 on Coastal Seawater Nitrogen Content
Reference
Wyatt, N.J., Kitidis, V., Woodward, E.M.S., Rees, A.P., Widdicombe, S. and Lohan, M. 2010. Effects of high CO2 on the fixed nitrogen inventory of the Western English Channel. Journal of Plankton Research 32: 631-641.

Background
The authors write that "the assimilation of inorganic nutrients fuels phytoplankton growth," and, therefore, that "any alteration in the bioavailability of these nutrients is likely to impact productivity and, by extension, climate regulation through the uptake of CO2 by marine algae." In this regard they note that "the reduction of surface ocean pH anticipated for the next century will alter the equilibrium coefficient between dissolved ammonia (NH3(aq)) and ammonium (NH4+) shifting the equilibrium towards NH4+ (Zeebe and Wolf-Gladrow, 2001; Bell et al., 2007, 2008)," such that the future decease in ocean pH due to the ongoing rise in the air's CO2 content could result in the transfer of more gaseous NH3 from the overlying atmosphere to the ocean, as has been noted by Jacobson (2005).

What was done
To further explore this scenario, Wyatt et al. collected surface seawater samples from a coastal monitoring site in the Western English Channel (WEC) from 17 March to 21 July 2008, which included two distinct phases of the annual spring phytoplankton bloom (a pre-bloom period of five weeks and the bloom proper of eleven weeks). In addition, they measured ambient pH for carbonate system estimates, as well as dissolved inorganic nutrients; and they equilibrated the samples with CO2-in-air mixtures that resulted in CO2 concentrations of 380, 500, 760 and 1000 ppm that led to pH values of 8.05, 8.01, 7.87 and 7.76, respectively, which are to be compared with the mean ambient value of 8.18.

What was learned
The six scientists report that the phytoplankton community "was predominantly limited by the availability of inorganic nitrogen," and that "during early and mid-summer, NHX became the primary source of inorganic nitrogen." Interestingly, they also found that "an overall increase in NHX concentrations by 20% was observed between the present day CO2 treatment (380 ppm) and 1000 ppm."

What it means
Wyatt et al. write that "as excess CO2 dissociates in the oceans, the increased hydrogen ion concentration ionizes NH3(aq) and decreases the ratio of NH3(aq):NH4+," and that this reduction in NH3(aq) "would lead to an imbalance in the equilibrium between NH3(aq) in the surface water and gaseous NH3 in the overlying atmosphere resulting in the drawdown of atmospheric NH3 to the surface ocean." Based on this finding, they further calculate that, whereas the surface waters of the WEC "are a net source of 150 Ámol/m2/year of NH3 to the atmosphere at present (2009)," it is likely that "the WEC will become a net sink of 300 Ámol/m2/year for atmospheric NH3 as atmospheric CO2 rises to 717 ppm and the surface pH decreases to 7.83," due to the increase in phytoplanktonic productivity driven by the increased transfer of gaseous NH3 from the air to the surface waters of the WEC. And this phenomenon would (1) boost the productivity of higher oceanic trophic levels, (2) help sequester more carbon at the bottom of the sea, and thereby (3) reduce the rate of increase in radiative forcing that is speculated to fuel global warming, which many believe is provided by the ongoing rise in the air's CO2 content.

References
Bell, T.G., Johnson, M.T., Jickells, T.D. and Liss, P.S. 2007. Ammonia/ammonium dissociation coefficient in seawater: a significant numerical correction. Environmental Chemistry 4: 183-186.

Bell, T.G., Johnson, M.T., Jickells, T.D. and Liss, P.S. 2008. Ammonia/ammonium dissociation coefficient in seawater: a significant numerical correction (vol. 4 pg 183, 2007). Environmental Chemistry 5: 258 U8.

Jacobson, M.Z. 2005. Studying ocean acidification with conservative, stable numerical schemes for non-equilibrium air-ocean exchange and ocean equilibrium chemistry. Journal of Geophysical Research 110: 10.1029/2004JD005220.

Zeebe, R.E. and Wolf-Gladrow, D.A. 2001. CO2 in Seawater: Equilibrium, Kinetics, Isotopes. Elsevier Oceanographic Book Series, Amsterdam, 346 p.

Reviewed 28 July 2010