WP 1:
This work package focused on two contrasted regions, the naturally iron-fertilized and productive waters on the Kerguelen Plateau and the iron-depleted low productive waters west of the plateau. A previous study demonstrated that wind stress controls the onset of the spring bloom in the productive waters of the plateau. Interestingly, observations from seven BGC-Argo floats deployed west of the plateau revealed that wind stress also plays a key role in low productive waters. Despite very different levels of productivity, the same mechanism controls the seasonal phytoplankton dynamics in these two contrasted regions. A carbon budget was computed in the region west of the plateau. This budget revealed a relatively high vertical carbon export for such a low productive region, suggesting a high export efficiency. A manuscript presenting these results is in preparation and will be submitted to Geophysical Research Letters. The main figures/results were presented during the French 2021 GMMC (“Groupe Mission Mercator Coriolis”) annual meeting.
WP 2:
Using year-round robotic observations and recent advances in optical signal analysis, the functioning of three BCP export pathways - the mixed layer pump, the eddy subduction pump and the biological gravitational pump was concurrently investigated. Physical forcing and phytoplankton phenology have been investigated during three phytoplankton bloom cycles in contrasting environments to demonstrate their influence on the magnitude and seasonality of these pumps, with implications for carbon sequestration efficiency. The main insight from this study is the seasonal succession of processes contributing to the export of organic carbon to the deep ocean, suggesting that the contribution of all these processes over a complete annual cycle should definitely be accounted for when computing regional mesopelagic carbon budgets. A clear overlap in the timing of these processes was identified which suggests that their joint contributions can boost the overall efficiency of the BCP. This work was published in Nature Communications and main results were presented at the SOCCOM workshop in 2021.
WP 3:
Two BGC-Argo floats were deployed during the SOLACE voyage which took place south of Tasmania in early 2021. These floats were equipped with the newly-developed Underwater Video Profiler (UVP6) sensor which takes images of zooplankton and marine snow to derive particle size spectrum. This sensor brought new insights into the characterisation of key processes involved in the BCP. Downward carbon export estimates, and sinking speeds of 14 particle size classes were inferred. More specifically, a sinking speed to particle size relationship and a depth-dependent bulk particle sinking speed relationship were computed, which are useful for the modelling community aiming at better representing the BCP in Earth System Models. In addition, diel zooplankton migration was characterised, especially the migration patterns, their timing, and their contribution to aggregation/fragmentation processes. This work demonstrates the potential of the UVP6 carried by autonomous platforms to remotely assess ecological processes involved in the BCP. Results have been presented at the Chinese AOMP workshop, the Ocean Science Conference 2022, the French Argo meeting 2022 and the Ocean Twilight Zone Symposium in 2023. A manuscript will be submitted in Nature Geoscience in March 2024.
