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Understanding phytoplanktonic plumes around the Marquesas Islands

Image of the Month - August 2021

Islands in the open ocean can generate their own nutrient-rich marine ecosystems downstream - like an oasis of life in the middle of the otherwise quite desertic ocean --, a phenomena called Island Mass Effect. Several mechanisms can explain this, such as coastal upwelling, sediments shed from the islands, winds, current-bathymetry interactions... 
The Marquesas archipelago (8°S-11°S/141°W-138°W), in the South Pacific, shows such an Island Mass Effect at a scale of hundreds of kilometers. This is especially visible in the satellite ocean color data, but the mechanism implied remains to be elucidated. 
 
Two episodes were noticed during and just after the first Moana-Maty cruise (Oct. and Nov. 2018), one south, one north of the archipelago. 
Mixing several remote sensing techniques with an in situ oceanographic campaign and a physical-biogeochemical model helped to better understand the cases. In particular, the Finite-size Lyaponov Exponents (FSLE) from altimetry highlight convergent structures ("high" values of FSLEs), transport barriers and small-scale fronts. Eddies are observed in the south, while the northern region is characterized by submesocale structures. Both dynamical activities impact the blooms. However, such blooms can only be reproduced with iron input coming from the islands, whose source is still to be identified with precision.
 
Different aspects of the phenomenon can be investigated using a set of data including remote sensing and in situ. With Swot, sub-mesoscale dynamics will be observed in sea surface height, and regardless of the weather. 

See also:

References

  • Cassianides, Angelina, Elodie Martinez, Christophe Maes, Xavier Carton, and Thomas Gorgues, 2020: Monitoring the Influence of the Mesoscale Ocean Dynamics on Phytoplanktonic Plumes around the Marquesas Islands Using Multi-Satellite Missions, Remote Sensing 12, no. 16: 2520. https://doi.org/10.3390/rs12162520
  • Rodier, Martine, 2018: MOANA-MATY 2018 cruise, RV Alis, https://doi.org/10.17600/18000580 
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