Bio-physical coupling at sub-mesoscale in the Southwest Indian Ocean: the RESILIENCE project
Oral presentation, WIOMSA 12th symposium, Thursday, October 13th 2022, room 6, 12:00 pm
Mesoscale eddies have been recognized as major contributors to global marine productivity. Cyclonic eddies, for instance, favor the vertical injection of essential nutrients into the euphotic zone, at vertical velocities of a few meters per day (eddy pumping). In the last decade, recent progress in our understanding of sub-mesoscale dynamics has pointed out that much greater vertical velocities, up to tens of meters per day, are found on the outskirts of mesoscale eddies, at the location of intensified density gradients and frontogenesis. Although these vertical velocities may favour increased productivity by bringing nutrient-rich waters into the upper mixed layer, they may also contribute to significant downward export of healthy living phytoplankton cells and sequestration of material into the deeper layer. The RESILIENCE cruise (fRonts, EddieS and marIne LIfe in the wEstern iNdian oCEan) aimed to investigate small-scale biophysical interactions (~1-10 km), in oceanic fronts surrounding mesoscale eddies (~100km) commonly found in the Mozambique Channel and along the east coast of South Africa. The global objective is to understand the role of these physical processes (vertical exchanges in particular) on ocean productivity and functioning of pelagic ecosystems.
Biological and physical parameters have been sampled simultaneously and continuously in the water column, as well as in the surface waters in the central part of the Mozambique Channel and on the east coast of South Africa. These two zones are known for their intense eddy activity and are in highly hydrodynamically contrasted systems: Large fronts in the Mozambique Channel; coastal-offshore interaction off the north coast of South Africa; semi-permanent eddy south of Durban forced between the coast and the Agulhas current). These eddies are by nature very dynamic and move horizontally. The sampling strategy have therefore been continuously adapted using remote sensing data (altimetry and ocean color) as well as the in situ data collected during the cruise.
Many different parameters have been sampled during the cruise. The fine scale measurement was done using an undulating body (Moving Vertical Profiler - MVP - between 0 and 300m), equipped with temperature, salinity and fluorescence sensors. These measurements were complemented with vertical profiles (CTD-O2-F) and discrete samples (biochemistry measurements). Plankton (phyto- and zooplankton) communities have also been investigated at different depths to determine the functional groups present as well as their metabolic rates.
Multicolor flying fish “Yellow Bandwing”. Flying fish (several species) have been observed mostly in the Mozambique Channel @Peter Ryan (UCT)
Phytoplankton investigations have been performed using continuous and high frequency acquisition equipment such as the CytoSense and Fluoroprobe instruments while an Underwater Vertical Profiler (UVP5) has been deployed at each station to measure the fine-scale vertical structure of the marine snow and zooplankton. Continuous acoustic measurements (echosounder EK80 with 5 frequencies) have been done underway as well as on station (AZFP and WBat instruments) to characterize the distribution of zooplankton and micronekton (intermediate trophic level). In parallel, some pelagic trawls have been deployed to collect and characterize the type of organisms measured by the acoustic instruments. Seabirds and marine mammals observers have also performed surveys in the sampled area.
Deployment of the MVP (Mooving Vessel Profiler), towed at the rear of the vessel and performing vertical profiles between the surface and 300m. The “fish” is equipped with several sensors (CTD, fluorometer) and allows high spatial resolution measurements @Clément Panelle (UF RESILIENCE)
The RESILIENCE cruise was achieved from 19 April to 23 May 2022, onboard the French R/V Marion Dufresne. A first data set, in particular those from continuous sub-surface measurements, has been available since the end of the cruise. Most data (those resulting from analysis in ground laboratories) will be available within the months following the cruise. A description of the operation done at sea, as well as a first overview of the data available shortly after the cruise, will be presented during the Symposium, with a focus on environmental data (hydrology, nutrients, chlorophyll, plankton).
In essence, RESILIENCE is a multidisciplinary (and international) cruise that has covered a wide variety of measurements at sea. From its conception to its realization, RESILIENCE relied on a strong collaboration between research institutions in France and South Africa. Training for students from South-Africa, Mozambique, Mauritius and France is also an important outcome of RESILIENCE. The cruise is part of the IIOE-2 programme.