Immersion on board an oceanographic research vessel
In recent weeks, researchers from the EPOC laboratory were on a mission aboard a research vessel studying the West Gironde Mud Patch. A scientific expedition presented in pictures.
Between mid-October and mid-November, MAGMA-2, a scientific mission took place on board the oceanographic research vessel Côtes de la Manche. What was its objective? To study the West Gironde Mud Patch, a sediment patch located in the Bay of Biscay, about 25 km off mouth of the Gironde Estuary. Divided into three parts, known as legs, this 35-day sea expedition focused on the ecological status of the benthic habitat, i.e. the organisms living near the bottom of the sea.
The first leg, lasting two weeks, brought together scientists from the Oceanic and Continental Environments and Paleo-Environments laboratory (EPOC - CNRS, EPHE and the University of Bordeaux) and the Emerging Chronic Risks team (CHROME - University of Nîmes). The scientists came from disciplines ranging from biology to biogeochemistry, and their goal was to study and characterize the ecological and biogeochemical functioning of the benthic habitat. To do so, they focused on ten sites, called stations, where they conducted operations using five instruments to collect data and take samples at depths ranging from 30 to 75 m.
"The particularity of the West Gironde Mud Patch is that it receives particles coming from the Estuary", explains Bruno Deflandre, a teacher-lecturer at the University of Bordeaux conducting his research at the EPOC laboratory and scientific manager of the MAGMA-2 legs. "It is also subject to storms, the frequency and intensity of which are increasing with global change. It is also an area of intense fishing for common sole and Norway lobster, which makes it a particularly interesting object of study."
During oceanographic expeditions, the scientists and some of the sailors spend a large part of the working day on the deck, at the back of the ship, as this is where the instruments are launched from. These devices allow the scientists to study the water column, but also the sediments of the sea floor.
The wheelhouse, also known as bridge, is the place where navigation is carried out and from where the captain directs maneuvers, as well as the launching of scientific instruments.
Among these instruments, the Sediment Profile Imager (SPI) produces pictures of the sedimentary structure. Immersed 15 times per station, it penetrates the mud and takes high resolution pictures showing the water-sediment interface and the first ten centimeters of mud in detail: sediment color, burrows, living organisms, etc.
One objective of the mission was to sample the benthic macrofauna present in the sediment, notably with the Hamon grab, which collects the equivalent of a large bin of sediment. This was followed by long hours of sifting. Leaning over a 1 mm mesh sieve, the scientists collected the organisms from the silt with tweezers, an exercise that requires good eyesight!
The silt was also sampled with a corer. Equipped with tubes measuring 60 cm long and 10 cm in diameter, this tool simultaneously collects six sediment cores while perfectly preserving the water-sediment interface. Like the other instruments, its launching is a delicate moment which requires good coordination between the sailors and scientists on the deck, and the captain in the wheelhouse.
The cores were then cut and prepared for analysis: organic matter, radioelements, microbiology, foraminifera... Many differents experiments will be caried out in the laboratory once back on land.
The working days on board research vessels are often long. In the laboratory to finish processing the samples, and on the deck where operations sometimes start before sunrise to make the most of any favorable weather. The weather is a major constraint during oceanographic missions: too much swell or too much wind may damage the equipment.
In addition, some instruments need to be immersed for many hours, like this autonomous profiler. Equipped with very sharp and fragile glass microelectrodes, it was deployed for five to eight hours on the sea floor and measured the vertical distribution of pH, O2, and resistivity in the sediment.
Including the crew of sailors and scientists, fifteen people were on board the 25-meter-long oceanographic vessel day and night. They met in the "mess" at 11am and 7pm precisely to enjoy a meal concocted by the cook.
With some suffering from sea sickness, others from the cold, days at sea are not always easy. However, some sunsets at sea are stunning...
The MAGMA-2 mission on board the R/V Côtes de la Manche
The R/V Côtes de la Manche is a vessel belonging to the French Oceanographic Fleet operated by Ifremer. Following the first leg, the other two parts of the MAGMA-2 mission, also conducted by researchers from the EPOC laboratory but also from Ifremer, specifically looked at the characterization of the transfer of particles and associated contaminants between the Estuary and the Mud Patch, but also the fate of contaminants within the sediment and organisms of this benthic habitat.
Teacher-researcher of the University of Bordeaux at the EPOC laboratory