Effects of high CO2 on the fixed nitrogen inventory of the Western English Channel

Abstract

<p>In this study, we investigated the impact of CO<sub>2</sub> induced seawater acidification on the concentration of inorganic nutrients, nitrate, nitrite, ammonium, soluble reactive phosphorous (SRP) and silicate at a coastal site in the Western English Channel (WEC). Samples were collected and filtered weekly during the spring to summer transition between 17th March and 21st July 2008. Ambient pH varied by 0.2 units over the study period, while purging with CO<sub>2</sub> (380, 500, 760 and 1000 ppmv) resulted in a maximum pH decrease of 0.4 units. Surface nitrate was depleted during the spring phytoplankton bloom and nitrogen limitation was prevalent thereafter. Acidification did not change the concentrations of nitrate, nitrite, SRP or silicate. An average increase in total ammonium of 0.17 μmol L<sup>-1</sup> (20) was observed in acidified seawater during the pre-bloom period. We suggest that this increase was not a biological interaction but due to the influx of gaseous ammonia from the atmosphere; CO<sub>2</sub> induced acidification shifted the seawater dissolved ammonium-ammonia equilibrium towards the former and thereby decreased the sea-air concentration gradient of ammonia. Using the results from this study, a simple zero-dimensional model for the WEC was constructed which shows that this region will become a net sink for 300 mol m<sup>-2</sup> a<sup>-1</sup> for NH3 if atmospheric CO<sub>2</sub> concentrations increase to 717 ppm. The increase in the total ammonium inventory of the WEC surface waters may increase phytoplankton biomass in the future.</p>