Analysis of Export and Transfer Efficiency around the PAP-Site Observatory: an update from the APERO project

The biological carbon pump (BCP) plays a key role in the regulation of atmospheric CO₂ levels. Export efficiency (Exp_eff), defined as the proportion of primary production (PP) that is exported as particulate organic carbon (POC) flux below the base of the euphotic zone (EZ), and transfer efficiency (T_eff), defined as the ratio of POC flux below the EZ and POC flux attenuated at a given depth in the twilight zone (TZ), are two of the main parameters used as metrics of BCP strength. The objective of this work is to investigate the factors that influence the variability of both parameters at different bloom stages. The APERO cruises aim to investigate the BCP, with emphasis on the TZ (200-1000 m), and were conducted at the PAP site oceanographic observatory during the decline of a spring bloom in June and July 2023. Water and particle profiles (0-1000 m) were collected at five stations and POC fluxes at the base of the EZ were obtained derived from ²¹⁰Po-²¹⁰Pb disequilibrium and high depth resolution sediment traps. In addition, data measured from 1989 to 2023 for POC fluxes, at the base of the EZ, derived from ²³⁸U-²³⁴Th and ²¹⁰Pb-²¹⁰Po disequilibrium and at 3000 m depth, derived from moored sediment traps were compiled. Exp_eff (Flux_Ez/satellite NPP time-integrated) and T_eff (Flux_3000m/Flux_Ez) were quantified, and both values were compared across different years and bloom stages. POC fluxes measured in APERO ranged from 3.1-17 mmol C m⁻² d⁻¹, which agrees well with the value measured in 2021 during the same bloom stage, 13 ± 3 mmol C m⁻² d⁻¹. Exp_eff  presents significant fluctuations and shows a strong intra-annual variability. It changed during the bloom development, from 5-20% in 1989 to 41% in 2012, from 16-42% in 1989 to 14% in 2021 during the bloom peak, and during the decline of the bloom, it decreased from 26 ± 4 % in 2021 to 2.2-11% during the APERO cruise. Finally, T_eff exhibits a strong intra-annual variability as the bloom progresses, changing from 6–23% at the beginning of the bloom in 1989, 16% in 2004, and 12% in 2012, to 5–14% at the peak in 1989, and 16% during the postbloom in 2009.