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The variability of particulate organic carbon (POC) sinking flux and its relationship to primary production were investigated in the upper twilight zone of the northwest Atlantic Ocean on 4 occasions during a Lagrangian study of the decline of a spring diatom bloom and its transition towards post-bloom conditions. POC sinking fluxes below the euphotic zone (50 to 75 m depth) decreased from (mean ± SD) 674 ± 105 to 281 ± 17 mg C m -2 d -1 throughout the senescence of the bloom and further decreased to 197 ± 2 mg C m -2 d -1 under post-bloom conditions. POC sinking fluxes below the euphotic zone were positively correlated to the production of large phytoplankton cells (≥5 µm) throughout the study period, highlighting the importance of the size structure of primary producers in shaping the export of POC from the euphotic zone. In contrast, POC sinking fluxes at 150 m depth showed little variation throughout the study period. Analysis of the vertical profiles of POC sinking flux revealed an increase in POC transfer efficiency from 50 to 150 m depth due to a decrease in POC recycling within the upper twilight zone throughout the decline of the bloom. More- over, POC recycling within the upper twilight zone was positively correlated with POC sinking fluxes below the euphotic zone throughout the study period, suggesting that recycling processes in the upper twilight zone respond rapidly and proportionally to the export of POC from the euphotic zone. It is hypothesized that the concomitant decreases in POC sinking fluxes below the euphotic zone and in POC recycling within the upper twilight zone compensated each other and could explain the fairly constant POC sinking fluxes at 150 m depth that were observed throughout the study period. Our results shed light on the importance of short-term variability in organic matter recycling within the upper twilight zone for the efficiency of POC export to depth.

Particulate organic carbon export in the upper twilight zone during the decline of the spring bloom