Revisiting the CO2"source" problem in upwelling areas – a comparative study on eddy upwellings in the South China Sea

[摘要] The causes for a productive upwelling region to be a source of CO₂ areusually referred to the excess CO₂ supplied viaupwelling of high dissolved inorganic carbon (DIC) from deep water. Furthermore, we hypothesize that microbialactivity plays a significant role on top of that. To test this hypothesis, multiplebiogeochemical parameters were investigated at two cyclonic-eddy-inducedupwelling sites, CE1 and CE2, in the western South China Sea. The data showedthat upwelling can exert significant influences on biological activities inthe euphotic zone and can also impact on particulate organiccarbon (POC) export flux depending on upwelling conditions, such as themagnitude, timing, and duration of nutrient input and consequent microbialactivities. At CE2, the increase of phytoplankton biomass caused by theupwelled nutrients resulted in increase of POC export flux compared tonon-eddy reference sites, while at CE1 the microbial respiration of organiccarbon stimulated by the upwelled nutrients significantly contributed to theattenuation of POC export flux. These results suggest that on top ofupwelled DIC, microbial activities stimulated byupwelled nutrients andlabile organic carbon produced by phytoplankton can play acritical role for an upwelling area to be outgassing or uptaking CO₂.We point out that even though an upwelling region is outgassing CO₂, carbonsequestration still takes place through the POC-based biological pump aswell as the refractory dissolved organic carbon (RDOC)-based microbialcarbon pump.

[学科分类] 地球化学与岩石