Eddies, food trucks of the ocean

Ocean eddies fed by deepwater upwellings contain essential fats and oils for marine food webs, say German researchers

Eddies form in the ocean, when nutrient-rich water welling up from the depths, collides with coastlines — similar to those roiling, swirling currents of water seen in rivers, downstream of rocks. Same idea, bigger scale.

Typically, 10-100 km in diameter, ocean eddies are vital to marine ecosystems, as they carry upwelling deep water and its abundant organic carbon and nutrients offshore, boosting the productivity of the relatively nutrient-poor open ocean. 

“These eddies are basically the food trucks of the ocean,” explains geochemist Dr Kevin Becker of the Helmholtz Centre for Ocean Research in Kiel, Germany. “They transport nutrients from the highly productive coastal upwelling regions to the open ocean, where these nutrients are released and are likely to influence biological productivity.”

‘Biological productivity’ starts with those nutrients fuelling the growth of phytoplankton  —  microscopic marine algae, explains Dr Nick Hardman-Mountford, formally a biogeochemical oceanographer at CSIRO. Phytoplankton in the ocean’s surface layers are responsible for about half the world’s photosynthesis.

Not surprisingly, marine food webs, including those which humans exploit for fish, rely on phytoplankton. And when these algae die and sink to the bottom of the ocean, they sequester carbon — dead phytoplankton get buried in deep-ocean sediments.  

But, while it’s well known that oceanic eddies transport carbon and nutrients, most of the reporting has been at coarse scales.  The exact composition and nutritional quality — important for zooplankton and fish — of eddy-borne nutrients and carbon has not really been unexplored.

Until now.

Lead author, Becker and colleagues from the Helmholtz Centre and the University of Bremen in Germany, have dived into the detail, analysing samples collected from a ‘mesoscale’ — middle-sized — oceanic eddy off Mauritania, in north-west Africa, during an oceanographic expedition for their REEBUS project (Role of Eddies in the Carbon Pump of Eastern Boundary Upwelling Systems).

The same-sized eddies happen in south-east Australia, with the southern movement of the East Australian Current (EAC) where upwellings feed nutrients into the nitrogen-limited Tasman Sea, says Dr Giselle Firme, formally of the University of Technology, Sydney. 

Almost a thousand different lipids — fats and oils used by animals for energy — were found by Becker and his team in the Mauritanian eddy. They compared water samples taken inside the eddy with those taken outside, using high-resolution mass spectrometry (HRMS) to break the material into individual atoms, make them into ions, then weighed them by sending the ion stream through a magnetic field — lighter ions move faster.

And because different organisms use different lipids, the researchers could work out which groups they had in the samples.

“Lipids also contain chemotaxonomic information that allows us to determine the composition of microbial communities,” adds Becker. “Based on their chemical signatures, we can distinguish, for example, between lipids from phytoplankton, bacteria, and archaea species.” 

There was a distinct difference between the lipid ‘signature’ in the eddy and that of the surrounding ocean water, which meant that the two sets of microbial communities were distinctly different. Inside the eddy the lipids were richer in energy and essential fatty acids, critical for zooplankton and fish that cannot make their own, says Becker. The eddy also contained almost three times the amount of lipids found in the surrounding open ocean and coastal waters, he adds.

Becker concluded that coastal eddies in the Mauritanian upwelling region transport about 10,000 tonnes of organic carbon out into the open ocean each year. 

“Our study highlights the central role of mesoscale eddies in the local carbon cycle and provides a basis for future investigations of their importance on a global scale,” concludes coauthor, Prof Anja Engel, of the Helmholtz Centre. 

The paper was published in Nature Communications

Eddies in the Gulf Stream