Élodie Martinez, a biogeochemist at the French National Research Institute for Sustainable Development (IRD), studies the variability of phytoplankton biomass. These micro-organisms, the first link in the aquatic food chain, produce a large proportion of the Earth’s oxygen and absorb a third of the carbon dioxide in the atmosphere.
“Women oceanographers”(7/12). They have made the ocean their object of study, sometimes even their main concern. Physicists, chemists, geologists or biologists, they all contribute to improving our knowledge of the marine environment. océans connectés sets out to meet them all over France.
By Marion Durand.
Cover photo © Marion Durand
Phytoplankton are some of our best allies. Microscopic and moving with the currents, this group of tiny organisms belonging to the plant kingdom provides us with many services. The oceanographer Élodie Martinez has made these microalgae her subject of study. “We talk a lot about the role of the Amazon rainforest, but phytoplankton are responsible for half of the oxygen in the atmosphere,” explains the biogeochemist. If we talk about the lungs of the planet, it’s partly thanks to phytoplankton.
Phytoplankton, made up of millions of microscopic plant species, absorb CO2 dissolved in the water through photosynthesis. When phytoplankton (or the organisms that consume them) die, they take this greenhouse gas with them, which is converted into organic matter made up of carbon. These small particles, known as “sea snow”, settle to the bottom of the oceans and sequester the carbon. “The role of phytoplankton in the biological pump and in the carbon cycle is extremely important,” says the oceanographer from the IRD.
Phytoplankton regulate the climate and are therefore essential to our survival. But these planktonic organisms are very fragile. Their abundance and availability vary from region to region and over time. Élodie Martinez analyses the physical and biogeochemical mechanisms behind phytoplankton variability. The oceanographer uses spatial remote sensing to analyse changes in biomass, i.e. the amount of matter available. The researcher has been working on “island effects” for about fifteen years. “Around islands, particularly in the South Pacific, we see large algal blooms, an increase in the concentration of one or more species of algae,” explains Élodie Martinez.
Efflorescence of diazotrophs (Trichodesmium) in the Coral Sea, image from the Landsat 8 satellite on 1 September 2019. ©️Joshua Stevens/NASA
These phenomena, also known as phytoplankton blooms, are indicators of the biomass of plant plankton and can be observed from space thanks to the colour of the water. The Brest oceanographer is trying to understand why these plant species are abundant around certain islands, as their presence attracts many species that feed on them. As the first link in the food chain, phytoplankton is eaten by zooplankton, which in turn are eaten by small pelagic fish and larger predators such as manta rays and whales.
“There are tens of thousands of islands in the Pacific. These blooms can have a significant impact on food webs, especially on large pelagic species such as tuna. So the presence of phytoplankton has an indirect effect on fisheries,” she explains.
Oases of nutrients close to the islands
In the waters of the tropical Pacific, islands are sources of nutrients for microscopic algae. But where do these food oases come from? This is the question posed by Élodie Martinez, who studies the Marquesas Islands in French Polynesia. Are the nutrients linked to upwelling, the movement of cold water from the depths to the surface? Do they come from oceanic eddies? Is it rain that washes the soil off the mountains and carries the nutrients to the ocean? “One of the processes involved could be the breaking of internal waves: there are waves, especially tidal waves, that propagate through the various deep layers of the ocean and when they reach the island, they break and inject the nutrient salt towards the surface, in this case iron in the case of the Marquesas Islands”.
To observe this phenomenon, the researcher relies on satellite observations, campaigns at sea and numerical modelling. “We have several approaches using different tools. We use physics, biology and marine ecology to get answers. The interdisciplinary aspect is what I really like about the research”.
At the Laboratoire of Ocean dynamics and et satellite remote (LOPS), a joint research unit of four French research and higher education institutions, Élodie Martinez is also studying the temporal variability of phytoplankton biomass on a decadal scale. Artificial intelligence, which is increasingly being used by scientists, can be used to improve calculations, refine forecasts or propose other hypotheses to understand the evolution of these micro-organisms over decades.
From pilot to oceanographer
Based in Brest, Brittany, for the last six years, Élodie Martinez has worked for a long time in Polynesia, within the UMR “Ecosystèmes Insulaires Océaniens”. She completed her first postgraduate internship in Tahiti and, in 2006, wrote a thesis on the seasonal and interannual variability of thermocline waters in French Polynesia.
Elodie Martinez in front of her office at the European Institute for marine studies (IUEM) ©️M.Durand
After a short stint in Hawaii, the engineer joined the marine optics team at the Villefranche-sur-Mer oceanography laboratory, specialising in spatial remote sensing and ocean colour. “These five years as a postdoc allowed me to refine my project and know what I wanted to develop as a research topic,” says the oceanographer, whose career was far from set in stone. Her father was a flight attendant, her mother a stewardess, and she dreamed of being in the air: “Ever since I was a little girl, I wanted to be an airline pilot, but a health problem prevented me from taking that path,” she recalls without regret. Life is a blessing, but I’m not sure I would have been as fulfilled”.
“I love being an oceanographer,” she adds with a big smile. “What I really enjoy is the research. It’s great to wake up one morning, read a scientific article and start a new research project to answer the questions that have been left unanswered! It’s never-ending, you’re never in your comfort zone, you can jump from one topic to another”.
She is also passionate about sharing her knowledge. Every year, she takes part in the “Elles bougent” initiative, which aims to encourage young girls in Year 9 to consider technical and scientific careers. “I was incredibly lucky to have parents who didn’t have a baccalauréat, but who always pushed my sister and me to do whatever we wanted, telling us that there were no limits. I want to pass that on to young girls!” The teenage girls she meets sometimes have a utopian vision of the oceanographer’s profession, imagining only their interlocutor on a voyage or campaign at sea. “What reassures the students is that there are no fixed career paths. You can start off in one direction and end up changing your specialism“.
In 2021, she and some fellow researchers created a card game for young Polynesian children to raise awareness of the importance of phytoplankton. “With this game of 7 families, we wanted to show children that phytoplankton are very small and invisible, but that their presence is essential” explains the 45-year-old oceanographer.
At a priority school in Tahiti, Élodie Martinez has also set up the “Adopt a float” programme, initiated by the Institut de la mer in Villefranche, which allows students to “adopt” an underwater robot and follow it as it moves around the ocean, raising their awareness of the importance of oceanographic observation. “It’s fantastic to see how interested young people are, and it’s not just the top of the class! Maybe it opens up new perspectives for them, but above all we want to raise their interest and awareness, because it’s their environment and they need to take care of it“.
TO READ IN THE SAME SERIES
Emina Mamaca, chemist and Ifremer’s spokesperson at the highest level of government
Sabrina Speich, oceanographer : ” I’m not afraid to fight for the climate”
Sophie Cravatte, a professional life dedicated to the Pacific Ocean
From Marseille to Nouméa, Sophie Bonnet unravels the mysteries of underwater deserts