| Summary: | 博士 === 國立中山大學 === 海洋科學系研究所 === 103 === Mesoscale eddies in the subtropical oligotrophic ocean are ubiquitous and play an important role in nutrient supply and oceanic primary production (PP). However, it is still unclear whether these mesoscale eddies can efficiently transfer CO2 from the atmosphere to deep waters via biological pump because of the sampling difficulty due to their transient nature. In May, 2007, particulate organic carbon fluxes (POCf), measured below the euphotic zone at the edge of warm eddy (WE) were 136–194 mg-C m−2 d−1 which was greatly elevated over that (POC flux = 26–35 mg-C m−2 d−1) determined in the nutrient-depleted oligotrophic waters in the Western North Pacific (WNP). In June and July, 2010, higher POCf (77–124 mg-C m−2 d−1) were also observed at the boundary of both WE and cold eddy (CE) in the WNP. The enhanced POCf at the edge of eddies was mainly attributed to both large phytoplankton (e.g. diatom) broken cells and zooplankton fecal pellets based on scanning electron microscopy (SEM) examination.
In April and September, 2013 and May, 2014, researches were conducted in the South China Sea (SCS) for POCf measuring to better understand the effect of carbon pump in eddies (i.e. CE and WE). The POCf was not only higher in the CE than WE (50–166 and 15–25 mg-C m−2 d−1, respectively) but also in cores than edges in those eddies (CE: 73–166 and 50–53; WE: 25 and 15 mg-C m−2 d−1). Raised POCf of the CE’s core in the Bashi Channel could be followed by more nutrients injection in the core rather than the edge (nitrogen, phosphate and silicate inventories within the euphotic zone were 1155、78、1445 and 395、28、536 mmol m−2, respectively). The elevated POCf found in the core of CE at the SouthEast Asian Time-series Study (SEATS) may be attributed to the core effect on the specific station and its nearby water for a longer period. As to the higher POCf in the core than the edge at the SEATS was probably due to the considerable lateral advection caused by anticyclonic mechanisms of the WE. Based on the result of preliminary particles enumeration and identification, it was that the elevated POCf in CEs was contributed by numerous larger phytoplankton (i.e. diatom: Chaetoceras, Thalassiosira, Nitzschia, Asteromphalus, Coscinodiscus and Pleurosigma) according to their significant and positive relationship between the POCf, the abundance and the carbon content. Additionally, we also deployed a deep sediment trap array (at the depth of 150, 1000, 2000 and 3200 m) at SEATS to understand particle transporting efficiency to deep water. The sinking flux of POC were 25、18、16 and 15 mg-C m−2 d−1 at SEATS suggesting that particles could be largely stored in deep water because the e ratio (POCf / I-PP) did not show significantly change. Therefore, the persistent POCf in the WE was possibly due to active microbial loop (e.g. bacteria) and kept more POC in the refractory condition based on the result of preliminary particles enumeration with more bacteria-covered particles in the surface water of the WE’s core site (WE’s core and others were 32 and 7%, respectively). Moreover, the application of POCf, I-Chl a and I-PP (integrated chlorophyll-a and PP within the euphotic zone) to estimate the quantification of biological pump in the subtropical oligotrophic ocean was reliable in accordance with a strong and positive correlation between ratios of POCf / I-Chl a and export ratio.
In conclusion, our result stand in the opposite side to the previous study that the POC flux of the eddy is not important and insignificant in the subtropical Pacific Ocean based on our field observations. The result suggests that mesoscale eddies in the oligotrophic waters, such as the subtropical WNP and SCS, can efficiently increase the oceanic carbon export flux. The edge and core of eddies in the WNP and SCS, respectively, act as a crucial conduit in carbon sequestration to deep waters.
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