Deep carbon export peaks are driven by different biological pathways during the extended Scotia Sea (Southern Ocean) bloom

Estimating the amount of organic carbon leaving the upper water column and becoming sequestered in the deep ocean is a major challenge in our understanding the oceanic C cycle. This study investigate deep sediment trap material collected at a 5 day resolution over a 4 month period covering the bloom in the northern Scotia Sea. This region is characterised by extensive and long-lived phytoplankton blooms that collectively take up the greatest amount of atmospheric carbon dioxide yet measured in the Southern Ocean. In particular, the study resolved multiple peaks of POC flux during the northern Scotia Sea bloom resulting from distinctly different export processes. This work also examine the contribution of diatoms and calcifying species to the POC flux as well as determining FP (faecal pellet) characteristics, including biogeochemical composition and sinking velocity. Results showed that POC flux during the bloom was concentrated in three major POC peak events. The first two POC peaks were large, of similar intensity (24.5 mg C m−2 d−1 in early November and 22.5 mg C m−2 d−1 in early December), and were dominated by faecal pellets (FPs, 60–66%). FP biogeochemical composition during these two POC peaks was characterised by a dominance of carbonate (>48%), as well as a higher sinking speed (up to 347 m d−1) compared with other FP in the rest of the samples. Results suggest that the large presence of calcium carbonate in these FPs contributed to their relatively high sinking velocity which, in turn, promoted their higher level of POC export to bathypelagic depths. Intact single cell diatoms were relatively important in the first POC peak (mainly characterised by Fragilariopsis kerguelensis and Thalassionema nitzschioides), while detritus including semi-grazed phytodetritus were more abundant during the second POC peak, suggesting a change in the upper ocean C cycle between the two POC peaks. A third, smaller, POC peak (12.7 mg C m−2 d−1 in January) was dominated by diatom resting spores, mainly Chaetoceros Hyalochaete sp. (>70% of total diatoms contribution), which were, almost exclusively, fully intact cells. Our high resolution sampling allowed insights into short time-scale processes that influence the ultimate magnitude of the substantial annual POC flux in this region. Overall our findings highlights that the temporal sequence of biological events in the surface layers during the bloom has a strong influence on both the magnitude and the composition of the fluxes.

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