Abstract
Abstract. The causes for a productive upwelling region to be a source of CO2 are usually referred to the excess CO2 supplied via upwelling of high dissolved inorganic carbon (DIC) from deep water. Furthermore, we hypothesize that microbial activity plays a significant role on top of that. To test this hypothesis, multiple biogeochemical parameters were investigated at two cyclonic-eddy-induced upwelling sites, CE1 and CE2, in the western South China Sea. The data showed that upwelling can exert significant influences on biological activities in the euphotic zone and can also impact on particulate organic carbon (POC) export flux depending on upwelling conditions, such as the magnitude, timing, and duration of nutrient input and consequent microbial activities. At CE2, the increase of phytoplankton biomass caused by the upwelled nutrients resulted in increase of POC export flux compared to non-eddy reference sites, while at CE1 the microbial respiration of organic carbon stimulated by the upwelled nutrients significantly contributed to the attenuation of POC export flux. These results suggest that on top of upwelled DIC, microbial activities stimulated by upwelled nutrients and labile organic carbon produced by phytoplankton can play a critical role for an upwelling area to be outgassing or uptaking CO2. We point out that even though an upwelling region is outgassing CO2, carbon sequestration still takes place through the POC-based biological pump as well as the refractory dissolved organic carbon (RDOC)-based microbial carbon pump.
Highlights
Upwelling is an oceanographic phenomenon that can bring nutrient- and dissolved inorganic carbon (DIC)-rich sub-thermocline water upwards and can lead to ecosystem biogeochemical responses (Mann and Lazier, 2006)
The results from the present study indicate that when nutrient-rich deep water with low abundance of prokaryotes is upwelled to the upper ocean, a corresponding rapid growth of phytoplankton and an initial reduction in total bacterial respiration might result in an increase of particulate organic carbon (POC) export flux, as demonstrated in the case of CE2, whose age was younger and whose upwelling was stronger than that of CE1
In the case that phytoplankton blooming does not occur or picophytoplankton dominates the community, as in the case of CE1 during the later intensification period, the injected nutrients and phytoplankton originated labile organic carbon could stimulate microbial respiration, which exacerbates the attenuation of POC flux
Summary
Upwelling is an oceanographic phenomenon that can bring nutrient- and dissolved inorganic carbon (DIC)-rich sub-thermocline water upwards and can lead to ecosystem biogeochemical responses (Mann and Lazier, 2006). Cyclonic eddies elevate isopycnals in the upper ocean to induce upwelling-like motion to varying degrees (Sweeney et al, 2003), and provide diverse scenarios for studies on subsequent biogeochemical responses. Other studies suggest that eddies make a minor contribution to regional biogeochemical budgets (Oschlies and Garçon, 1998; Oschlies, 2002), and upwelling areas can be CO2 outgassing (Rosón et al, 1999; Chen et al, 2007). The recognized fact is that upwelled DIC-rich deep water through isopycnals elevation could release CO2 and result in outgassing (Burt et al, 2013). Microbial contributions to such outgassing have not yet been fully elucidated
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