20 FEBRUARY 2021 : What on Earth is going on under our feet?

After more than a month at sea, the Marion Dufresne has travelled 10159 km and navigated on changing sea floor, ranging from 100 m to 5 km deep. Under our feet are organisms of various sizes: from microscopic to the largest whales in the world! At the “centre” of this complex food chain are the so-called micronektonic organisms, which range in size from 1 to 20 cm. This group consists mainly of fish, crustaceans, molluscs and gelatinous organisms; which are a delicacy for the populations of top predators (birds and mammals) that are very abundant in the Southern Ocean!

To follow these organisms’ dynamics, since we left Reunion Island, day and night, whether it is windy (it is often the case) or raining, we monitor our acoustic data recorded by Marion Dufresne’s EK80 multi-frequency echo sounder. Every 3.5 seconds, the latter emits 5 sound signals in the water column and records the signals reflected by the organisms it encounters in its beam: it allows us to observe the densities of organisms between the surface and more than 1000 m deep. Our objective? Study the 3-D distribution and variability of zooplankton and micronekton in relation to physical and climatic dynamics.

Cédric and Lloyd interpret a signal measured at a few dozen metres deep. @ Sara Sergi (LOCEAN)

Between 200 and 1,000 metres deep, at the interface between the surface and abyssal ecosystems, are the most important and least exploited fish stocks on our planet. However, their distribution, their structuring by the environment and their biodiversity remains little known to oceanographers. To overcome this lack of knowledge, the THEMISTO programme (Toward Hydroacoustics and Ecology of Mid-trophic levels in Indian and SouThern Ocean, PI: Cédric Cotté) aims to understand and quantify the processes by which environmental variability structures the pelagic ecosystems of the Indian zone of the Southern Ocean. In these depths also takes place the daily migration of the most important organisms on the planet! Every night, organisms migrate vertically up to 1000 m towards the surface, to feed all night long and return to the depths at dawn. Studying their distribution is therefore crucial because it has repercussions on the stability and composition of marine ecosystems, but also on climate change through the transport of carbon in the deep layers of the oceans, on a daily basis!

Left: First diel vertical migration observed on board the Marion Dufresne during the SWINGS campaign, off Reunion Island (frequency 38 kHz). The colour represents the measured organism density (in dB m-1) as a function of depth and latitude: the warmer the colour, the greater the organism acoustic density. Right: Micronekton sample collected by trawling during the campaign dedicated to the MyctO-3D-MAP project, in this case a myctophid fish (centre) surrounded by a large number of Hyperic amphipods Themisto gaudichaudii and some euphausiids Euphausia triacantha.  @Nolwenn Béhagle.

On a large scale, from the warm subtropical waters to the cold waters south of the Polar Front, we observed heterogeneous and contrasted distributions of organism densities. These initial observations are not surprising because they are linked to the passage of marked transitions in salinity, temperature, oxygen (and other oceanic tracers), called oceanic “fronts”. By collaborating with specialised research teams, the results obtained and combined should lead to a better understanding of the dynamics of mesopelagic ecosystems and their spatial and temporal relationship with ocean dynamics in the Indian part of the Southern Ocean.

The remaining 5300 km or so will allow us to observe these organisms’ dynamics on our way back: there is still plenty work to be done!

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