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

[摘要] It is not well understood whether coastal upwelling is a net CO₂ sourceto the atmosphere or a net CO₂ sink to the ocean due to high temporalvariability of air–sea CO₂ exchange (CO₂ flux) in coastal upwellingzones. Upwelling transports heterotrophic, CO₂ enriched water to thesurface and releases CO₂ to the atmosphere, whereas the presence ofnutrient-rich water at the surface supports high primary production andatmospheric CO₂ uptake. To quantify the effects of upwelling on CO₂flux, we measured CO₂ flux at a coastal upwelling site off of BodegaBay, California, with the eddy covariance technique during the summer of 2007and the fall of 2008, and the bulk method with partial pressure of CO₂of surface water (pCO₂) data from November 2010 to July 2011.Variations in sea surface temperatures (SST) and alongshore wind velocitysuggest that the measurement period in 2007 coincided with a typical earlysummer upwelling period and the measurement period in 2008 was during atypical fall relaxation period. A strong source of CO₂(~ 1.5 ± 7 SD (standard deviation) g C m⁻² day⁻¹)from the ocean to the atmosphere during the upwelling period was concurrentwith high salinity, low SST, and low chlorophyll density. In contrast, a weaksource of CO₂ flux (~ 0.2 ± 3 SD g C m⁻² day⁻¹)was observed with low salinity, high SST and high chlorophyll density duringthe relaxation period. Similarly, the sink and source balance of CO₂flux was highly related to salinity and SST during the pCO₂measurement periods; high salinity and low SST corresponded to highpCO₂, and vice versa. We estimated that the coastal area off BodegaBay was likely an overall source of CO₂ to the atmosphere based on thefollowing conclusions: (1) the overall CO₂ flux estimated from both eddycovariance and pCO₂ measurements showed a source of CO₂; (2)although the relaxation period during the 2008 measurements were favorable toCO₂ uptake, CO₂ flux during this period was still a slight source;(3) salinity and SST were found to be good predictors of the CO₂ fluxfor both eddy covariance and pCO₂ measurements, and 99% of thehistorical SST and salinity data available between 1988 and 2011 fell withinthe range of our observations in May–June 2007, August–September 2008 andNovember 2010–July~2011, which indicates that our data set wasrepresentative of the annual variations in the sea state. Based on thedeveloped relationship between pCO₂, SST and salinity, the study areabetween 1988 and 2011 was estimated to be an annual source of CO₂ of~ 35 mol C m⁻² yr⁻¹. The peak monthly CO₂ flux of~ 7 mol C m⁻² month⁻¹ accounted for almost 30% of thedissolved inorganic carbon in the surface mixed layer.

[学科分类] 地球化学与岩石