Periodic Reporting for period 2 - CAPTURE (Carbon pathways in the Southern Ocean)
Reporting period: 2023-01-01 to 2023-12-31
The Biological Carbon Pump (BCP) transfers atmospheric CO2, fixed by phytoplankton in the sun-lit upper ocean, as particulate organic carbon to the deep ocean. The BCP plays a key role in Earth’s climate by removing 10 Pg of carbon from surface waters each year, with the Southern Ocean (SO) pump representing 33% of the global BCP.
Carbon export from the BCP has long been solely attributed to the gravitational sinking of large particles following the spring phytoplankton bloom. Conspicuous imbalances in ocean carbon budgets have recently challenged this long-lived paradigm. Several lines of observational evidence have demonstrated the importance of additional export pathways that transfer all classes of particles to depth with different seasonality. Physical transport of organic matter by vertical mixing or active transport by zooplankton vertical migration should now be considered as major export pathways contributing to the BCP.
CAPTURE (CArbon PaThways in the soUtheRn ocEan) aimed at developing a mechanistic and quantitative understanding of the BCP export pathways using year-round and depth-resolved observations from heavily instrumented Biogeochemical Argo floats (BGC-Argo).
The project was organised around three work packages (WP):
• WP 1: Phytoplankton seasonality. How Environmental factors control phytoplankton seasonality in contrasting bio-regions of the SO?
• WP 2: BCP seasonality. How environmental factors and phytoplankton seasonality control the seasonal evolution of the BCP export pathways?
• WP 3: Zooplankton seasonality. How the coupling-decoupling between phytoplankton and zooplankton seasonality influence the functioning of the BCP?
The main conclusions are:
(1) Despite contrasting productivity regimes in the SO, environnemental forcing such as wind stress play an equally important role on phytoplankton seasonality.
(2) This seasonality in environmental forcing and phytoplankton biomass controls the seasonality of the BCP which is characterised by a succession of different C export pathways, resulting in a longer and sustained transfer of C to the ocean"s interior than previously recognised.
(3) Zooplankton play a central role in the functioning of the BCP, mostly by controlling the efficiency of the C transfer trough vertical migration, aggregation/fragmentation processes and predator-prey relationship.
Carbon export from the BCP has long been solely attributed to the gravitational sinking of large particles following the spring phytoplankton bloom. Conspicuous imbalances in ocean carbon budgets have recently challenged this long-lived paradigm. Several lines of observational evidence have demonstrated the importance of additional export pathways that transfer all classes of particles to depth with different seasonality. Physical transport of organic matter by vertical mixing or active transport by zooplankton vertical migration should now be considered as major export pathways contributing to the BCP.
CAPTURE (CArbon PaThways in the soUtheRn ocEan) aimed at developing a mechanistic and quantitative understanding of the BCP export pathways using year-round and depth-resolved observations from heavily instrumented Biogeochemical Argo floats (BGC-Argo).
The project was organised around three work packages (WP):
• WP 1: Phytoplankton seasonality. How Environmental factors control phytoplankton seasonality in contrasting bio-regions of the SO?
• WP 2: BCP seasonality. How environmental factors and phytoplankton seasonality control the seasonal evolution of the BCP export pathways?
• WP 3: Zooplankton seasonality. How the coupling-decoupling between phytoplankton and zooplankton seasonality influence the functioning of the BCP?
The main conclusions are:
(1) Despite contrasting productivity regimes in the SO, environnemental forcing such as wind stress play an equally important role on phytoplankton seasonality.
(2) This seasonality in environmental forcing and phytoplankton biomass controls the seasonality of the BCP which is characterised by a succession of different C export pathways, resulting in a longer and sustained transfer of C to the ocean"s interior than previously recognised.
(3) Zooplankton play a central role in the functioning of the BCP, mostly by controlling the efficiency of the C transfer trough vertical migration, aggregation/fragmentation processes and predator-prey relationship.
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.
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.
This project contributed to better understand the multi-faceted export pathways of the organic carbon in the Southern Ocean. I revealed for the first time the seasonal succession of these pathways that supports a long and sustained transfer of carbon to the ocean's interior. These results call for a more integrative approach to compute mesopelagic carbon budget, taking into consideration multiple export mechanisms over a complete annual cycle.
The first two BGC-Argo floats equipped with the newly-developed UVP6 were deployed in the Southern Ocean during the outgoing phase of my fellowship. The recent recovery of one of the float provides a unique opportunity to download the million of particle and zooplankton images taken by the UVP6 and stored in the float memory. This portfolio will provide unvaluable information on the morphology of particles and zooplankton, and thus on ecological processes at play in the system, leaving a CAPTURE legacy beyond the end of the project. More generally, results from this multidisciplinary project, combining physical oceanography, phytoplankton and zooplankton ecology and biogeochemistry, will help elucidate and predict the future development of key climate-related processes in the SO. Publications in high-impact peer-reviewed journals such as Nature Communications, Nature Geoscience and Geophysical Research Letters contribute to disseminate the results to a broad audience including scientists, general public and decision-makers.
The first two BGC-Argo floats equipped with the newly-developed UVP6 were deployed in the Southern Ocean during the outgoing phase of my fellowship. The recent recovery of one of the float provides a unique opportunity to download the million of particle and zooplankton images taken by the UVP6 and stored in the float memory. This portfolio will provide unvaluable information on the morphology of particles and zooplankton, and thus on ecological processes at play in the system, leaving a CAPTURE legacy beyond the end of the project. More generally, results from this multidisciplinary project, combining physical oceanography, phytoplankton and zooplankton ecology and biogeochemistry, will help elucidate and predict the future development of key climate-related processes in the SO. Publications in high-impact peer-reviewed journals such as Nature Communications, Nature Geoscience and Geophysical Research Letters contribute to disseminate the results to a broad audience including scientists, general public and decision-makers.