ALBERT

Description: Air-sea exchange of CO 2 at a Northern California coastal site along the California Current upwelling system Biogeosciences Discussions, 9, 17707-17741, 2012 Author(s): H. Ikawa, I. Faloona, J. Kochendorfer, K. T. Paw U, and W. C. Oechel Uncertainty in the air-sea CO 2 exchange (CO 2 flux) in coastal upwelling zones is attributed to high temporal variability, which is caused by changes in ocean currents. Upwelling transports heterotrophic, CO 2 enriched water to the surface and releases CO 2 to the atmosphere, whereas the presence of nutrient-rich water at the surface supports high primary production and atmospheric CO 2 uptake. To quantify the effects of upwelling on CO 2 fluxes, we measured CO 2 flux at a coastal upwelling site off of Bodega Bay, California, during the summer of 2007 and the fall of 2008 using the eddy covariance technique and the bulk method with p CO 2 measurements from November 2010 to July 2011. Variations in sea surface temperatures (SST) and alongshore wind speeds suggest that the measurement period in 2007 coincided with a typical early-summer upwelling period and the measurement period in 2008 was during a typical fall relaxation period. A strong source of CO 2 (~1.5 ± 7 SD (standard deviation) g C m −2 day −1 ) from the ocean to the atmosphere during the upwelling period was concurrent with high salinity, low SST, and low chlorophyll density. In contrast, a weak source of CO 2 flux (~0.2 ± 3 SD g C m −2 day −1 ) was observed with low salinity, high SST and high chlorophyll density during the relaxation period. Similarly, the sink and source balance of CO 2 flux was highly related to salinity and SST during the p CO 2 measurement periods; high salinity and low SST corresponded to high p CO 2 , and vice versa. We estimated that the coastal area off Bodega Bay was likely a source of CO 2 to the atmosphere based on the following conclusions: (1) the overall CO 2 flux estimated from both eddy covariance and p CO 2 measurements showed a source of CO 2 ; (2) although the relaxation period during the 2008 measurements were favorable to CO 2 uptake, CO 2 flux during this period was still a slight source, (3) salinity and SST were found to be good predictors of the CO 2 flux for both eddy covariance and p CO 2 measurements, and historical data of daily averaged SST and salinity between 1988 to 2011 show that 99% of the data falls within the range of our observation in May–June 2007, August–September 2008 and November 2010–July 2011 indicating that our data set was representative of the annual variations in the sea state. Based on the developed relationship between p CO 2 and SST and salinity, the average annual CO 2 flux between 1988 and 2011 was estimated to be ~35 mol C m −2 yr −1 . The peak monthly CO 2 flux of ~7 mol C m −2 month −1 accounted for about 30% of the dissolved inorganic carbon in the surface mixed-layer.