The ocean is populated by microscopic planktonic organisms that regulate global biogeochemical cycles and allow life to thrive in the marine ecosystem. Every spring, the Patagonian shelf and shelf-break experience massive phytoplankton blooms.
On board Tara, the scientific team studied the bloom of a very special variety of coccolithophores: Emiliania huxleyi. This is a calcifying microalga with a high biogeochemical impact, and its annual bloom around December on the margins of the Argentinean Sea represents one of the largest events of this type on a global scale.
If this phenomenon takes place every year in December, it is thanks to the combination of various factors that allow the micro-organisms to develop: sunshine, temperature, nutrients present in the water. From one year to the next the blooms will be different. Sometimes they are very localised and concentrated, other times they are scattered and diffuse.
Build an international collaboration: The Gayoso expedition.
The name of the expedition was chosen to pay tribute to the first person that observed this coccolithophores species: Ana Maria Gayoso.
The Gayoso expedition involved two oceanographic vessels. The Tara, a French vessel of the Tara Océan Foundation, partner of AtlantECO, and the Houssay, an Argentinean vessel. The expedition was carried out in two stages. The Houssay set off first from Ushuaïa to Buenos Aires. The scientific team sampled the Argentinian coastline upstream of the bloom to be able to compare the data between the bloom period and the non-bloom period. One month later, the schooner Tara went the opposite way and started hunting for coccolithophores.
The specific questions that lead the work of the scientists crew were :
What are the ecosystem dynamics of a coccolithophore bloom and demise?
What are the contributions of different mortality agents (allelopathy, viruses, bacteria, grazers) in the bloom's demise and its metabolic footprint?
What is the role of infochemicals as mediators (notably through vesicles) of microbial interactions?
What are the large-scale biogeochemical consequences of microbial interactions?
How does the Patagonian bloom relate to global coccolithophore biology?
Hunting the coccolithophores bloom from space to the ocean
The ecosystem in which the bloom occurs is very dynamic, so to answer all these questions and to study the bloom as exhaustively as possible, the scientific team has multiplied the observation and research tools.
The scientific team used satellite imagery. They studied images from previous years to determine the areas where the probability of finding the bloom was highest. Satellite images from the previous days were also analysed in order to locate the areas where the bloom was beginning.
In addition, drifting buoys were deployed from ships to identify moving water masses and track them to understand the temporal evolution of the bloom.
Finally, as mentioned above, the expedition has relied on collaboration between two vessels to extend the possibilities of a comprehensive survey in time and space, to augment the measurements with additional instruments and to facilitate the transfer, handling and recovery of the buoys.
Report from Flora Vincent (Weizmann Institute), chief scientist onboard Tara from Buenos Aires to Ushuaia
We conducted 12 stations in total covering 3000 km from North to South. In this region, there is a very strong current from South to North. A major challenge was to detect coccolithophore patches from space, using a combination of different satellite images: modeled chlorophyll, reflectance etc. An important phase in the expedition was to conduct a Lagrangian drift a.k.a to locate, and follow a patch of coccolithophores as if “we were plankton”. We used buoys to keep track of the water mass, and we were able to follow it. We started south, and drifted North for 7 days in a row.
A very strong memory relates to the night called the “cocco chase” on December 14th. Few days within the lagrangian, our coccolithophore counts decreased heavily. We faced a strong dilemna: to stay and continue our drift here, or to go check out a new patch for higher cocco counts ? At 8pm on the 14/12 we decided to go prospect to the East of our position, for 25nm until the next jet stream. Around 2am on 15/12, several of us woke up to analyze the data of the inline system: a continuous sampling of surface waters to check for coccolithophores. Counts were increasing. We managed to relocate in a patch of higher cocco concentration, released drifters around 4am thanks to the sailors, and sampled our first station at 6am. That was intense.
Our leg involved a lot of scientific discussion. The sailors have been extremely helpful in deciding the trajectory, and there was a constant back in forth between scientific objectives and navigation constraints. We were lucky to do a lot of sailing.
We also had a tradition of making “bets” in the morning. Each time we would sample the water, I had to count cocco concentrations using a flow cytometer on board. Before measuring, I would go around people that were awake and ask for their estimate. Everyone played; some of us were always wrong (like me, always too high), some of us were quite often right (like Guillaume the deck engineer and Matthieu the deck officer), but the winner and the looser always got a Kinder delivered by Conny, our chemist on board. One of the first questions people would ask in the morning was “so, how much today ?”. The sailors were in the quest of cocco with us, understood and shared our excitement when counts were high, and our concern when counts were low. It’s important to show that science can also be a game that’s accessible to all.
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