Environmental characteristics of Agulhas rings affect inter-ocean plankton transport.

© M.Ormestad-Kahikai/Tara Oceans

Tara Oceans

Villar, Farrant, Follows et al.

DOI: 10.1126/science.1261447

An oceanic “cold wash cycle” impacts plankton at a key choke point of global ocean circulation

Oceanic currents continuously transport vast quantities of seawater which distributes heat and salt around the world, thus regulating the Earth’s climate. These massive water displacements also influence marine life, especially plankton which drifts in currents against which they cannot swim. Indeed, such planetary wide stirring mixes plankton, favoring its diversity and ultimately the planet’s health (atmospheric carbon and oxygen, food, water quality).

One of the mightiest is the Agulhas current in the Southwestern Indian Ocean, roughly equivalent to 500 Amazon Rivers. At the tip of South Africa, this current occasionally buds off as monumental anticlockwise swirls (up to 300km across), called Agulhas rings, which slowly drift across the entire South Atlantic towards the coast of Brazil. Surprisingly steady, these marine Whirling Dervishes or ocean Black Holes can even be seen from space as sea water height anomalies that keep rotating for years. The fate of the entrapped Indian Ocean water they transport is the Atlantic and the Southern Ocean. Because of this so called Agulhas leakage, plankton communities across these well-connected systems was expected to be relatively similar.

An international interdisciplinary team of scientists (lead by CNRS and SZN) gathered around the Tara Oceans expedition to compare the physical and plankton properties of the three oceanic basins which converge at the Agulhas choke point. Using classical optical marine ecology tools, plankton looked very similar between the Indian and Atlantic Oceans, suggesting like other expeditions before it, that the Agulhas choke point was not a barrier to plankton dispersal. However, using high resolution genetic tools to zoom deeper into plankton cells, the Tara

Oceans team discovered that plankton diversity had in fact shifted between the two oceans. They also directly observed the inside of an Agulhas ring, finding that the harsh environmental conditions created by the Roaring Forties induced a strong unexpected modification of the plankton payload during its travel from the Indian Ocean to the Brazilian coast. Indeed, the metabolism of the ecosystem as a whole was found to be altered as a result of unusually high concentrations of nitrates, the ocean’s fertilizer. This phenomenon, supported by MIT computer simulations, is likely explained by the exceptional cooling and deep mixing that take place inside Agulhas rings, dubbed the plankton “cold wash cycle” by the study authors.

These results provide the first genomics-enabled comprehensive plankton description of Agulhas rings at a sensitive choke point central to predictive models necessary to anticipate global change.


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