Sinking particle fluxes and biological carbon pump efficiency in the Labrador Sea during a Phaeocystis bloom decline

Abstract. The Labrador Sea is a key region for carbon dioxide uptake characterized by deep mixing during winter that supplies nutrients to the upper water column and fuels extensive phytoplankton blooms in spring. Yet, the efficiency by which organic carbon is exported from surface waters during these blooms, as well as their contribution to carbon sequestration, remain poorly constrained. Here, we present an unprecedented number of measurements of sinking export fluxes (particulate organic carbon, POC; and biogenic silica, bSi) collected in the central Labrador Sea during a 2-week-long process study that observed the decline of a historically large Phaeocystis bloom in spring 2022. This Phaeocystis bloom was unusually large and highly productive, extending over more than half of the Labrador Sea for 6 weeks. During the late stages of the bloom, we found that POC fluxes from the base of the euphotic zone to 500 m were variable but overall moderate to high (average of 8 ± 5 mmol C m^-2 d^-1). Nevertheless, evidence of shallow POC flux remineralization combined with the fact that POC fluxes in the bloom were not higher than in a region sampled outside of the bloom (average of 13 ± 3 mmol C m^-2 d^-1) suggested a limited role of Phaeocystis in carbon export. Large (>51 μm) particles collected using large volume pumps presented relatively low bSi/POC ratios and, therefore, diatoms did not appear to have an important ballasting role of Phaeocystis-derived material. Using in situ net primary production (NPP) rates, we determined that 2 weeks after the peak of the bloom, the total amount of NPP that reached 100 m below the euphotic zone was only 6 %. However, 3 weeks after the peak of the bloom, the value had increased to 29 %. The observed change was driven primarily by a decline in NPP over the sampling period, as POC fluxes remained relatively constant. Using satellite-derived NPP from the peak of the bloom until its end, we obtained an overall biological carbon pump (BCP) efficiency of 6 % placing this Phaeocystis bloom as a low BCP efficiency system. We stress the importance of long-term observations of both NPP and POC export for estimating meaningful BCP efficiencies. The results presented in this study provide a foundation for comparisons with other datasets collected during this ship-based process study and autonomous platforms present in the area during and beyond this study. These future efforts will provide the opportunity to increase the observational period and further elucidate the mechanisms leading to the low BCP efficiency found during the decline of this Phaeocystis bloom in the Labrador Sea.