Air-sea exchange of CO2 at a Northern California coastal site along the California Current upwelling system.

Uncertainty in the air-sea CO₂ exchange (CO₂ flux) in coastal upwelling zones is attributed to high temporal variability, which is caused by changes in ocean currents. Upwelling transports heterotrophic, CO₂ enriched water to the surface and releases CO₂ to the atmosphere, whereas the presence of nutrient-rich water at the surface supports high primary production and atmospheric CO₂ uptake. To quantify the effects of upwelling on CO₂ fluxes, we measured CO₂ 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 pCO₂ 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 CO₂ (~ 1.5±7 SD (standard deviation) gCm^-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 CO₂ flux (~ 0.2±3SDgCm^-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 CO₂ flux was highly related to salinity and SST during the pCO₂ measurement periods; high salinity and low SST corresponded to high pCO₂, and vice versa. We estimated that the coastal area off Bodega Bay was likely a source of CO₂ to the atmosphere based on the following conclusions: (1) the overall CO₂ flux estimated from both eddy covariance and pCO₂ measurements showed a source of CO₂; (2) although the relaxation period during the 2008 measurements were favorable to CO₂ uptake, CO₂ flux during this period was still a slight source, (3) salinity and SST were found to be good predictors of the CO₂ flux for both eddy covariance and pCO₂ 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 pCO₂ and SST and salinity, the average annual CO₂ flux between 1988 and 2011 was estimated to be ~ 35molCm^-2 yr^-1. The peak monthly CO₂ flux of ~ 7 mol Cm^-2 month^-1 accounted for about 30% of the dissolved inorganic carbon in the surface mixed-layer. [ABSTRACT FROM AUTHOR]