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Life within oceanic eddies reflects their history

While the physics and the ecology of oceanic eddies are increasingly understood, their biological productivity cannot always be explained by the processes observed at a given point in time. In the Mozambique Channel, models show that it also depends on their own life history.

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The biological primary production of the oceans derives from photosynthesis by micro-algae. It uses nutrients (nitrates, phosphates, silicates, etc.) which are not fully recycled in the surface layers, because a part of the organic matter is exported to deeper waters through sedimentation. However this depletion is compensated by the input of nutrient-rich waters transported laterally or vertically by various hydrological processes, among which mesoscale (50 to 500 km) eddies. It is commonly assumed that the biological activity within an eddy is related to the upward or downward movement of water in its core, according to its cyclonic or anticyclonic rotation and leading respectively to enrichment or impoverishment. But the reality is far more complex and the biogeochemistry within an eddy depends on its history and its interaction with other mesoscale structures or with the continental shelf.

 The aim of this paper is to analyze the life history of eddies presenting contrasting surface biological properties (nutrients, chlorophyll): is their planktonic production stimulated by vertical injection of nutrients in their core, or by the offshore transport and retention of productive coastal waters?

 The studied area is the Mozambique Channel, where the energy of eddies is among the highest in the world; eddies appear at a frequency of about 4–5 per year, have a diameter of up to 300 km and can extend to the bottom. The importance of their ecological role was demonstrated at all levels of the ecosystem, from phytoplankton to tunas including zooplankton or seabirds and their prey.

The study combined a circulation model (eddy formation and movement) and a biogeochemistry model (trophic levels and nutrient cycles). These models were initialized from a set of monthly data deriving from observations and simulations. Before starting a simulation over the five-year period of study, models were run over a first five-year period to allow for their stabilization. The comparison with satellite data shows that, in spite of a few differences, the simulation correctly reproduces the large-scale patterns of regional circulation in the Mozambique Channel and the associated biochemical responses. Several eddies appearing after stabilization were selected, and the results of the models made possible to track them back in time since their formation.

 Very different situations were encountered, involving cyclonic (C1 to C3) and anticyclonic (A1 to A4) eddies. 

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Satellite image (left) and model output (right) showing eddies A1 and C2. Chlorophyll concentration is shown by colors and sea-level anomaly by black lines (level higher in A1, lower in C2)

- C1 formed south of Madagascar and moved west-south-west; nitrate and chlorophyll were initially high but decreased progressively by lack of a significant external input.

- During its formation, A1 entrained coastal nutrient-enriched waters from the north of Madagascar. This initial nutrient input was taken up by phytoplankton but, as the eddy propagated and passed close to the coast, it was twice slightly enriched by nutrients from coastal waters.

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Eddy A1 trajectory and nitrate and chlorophyll concentrations

- C2 was generated in the extremely unproductive waters of central Mozambique Channel and moved westward without any nutrient input from enrichment processes.

- Born at the same place as C1, A2 was initially depleted in nitrate and was even more so after having merged with another anticyclonic eddy. A change of trajectory occurred during its last month of existence and resulted in its enrichment through lateral injection of nutrients.

 

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Eddy A2 trajectory and nitrate and chlorophyll concentrations

- A3 travelled across almost the entire Mozambique Channel, from North to South; planktonic production was fuelled by nitrates before being limited by exhaustion of the iron available for the algae.

- Two eddies interacting with the continental shelf of Mozambique were also studied. A4 was characterized by reduced chlorophyll concentrations at its core and increasing levels towards its boundary; as it moved southward, its southern outer zone merged with eddy C3 which was rich in chlorophyll.

The analysis of these situations showed that the properties of an eddy at a given point in time can be misleading as they result from the conditions of its formation and from its life history over several months.

The three anticyclonic eddies (A1, A2 and A3) had nitrate and chlorophyll concentrations varying by two orders of magnitude. Both A1 and A3 exhibited their highest nitrate concentrations at the location of their origin, relatively close to the shelf. No additional nutrients were injected into their surface layers during their three-month passage through the Mozambique Channel. Enrichment of A2 occurred at the end of its trajectory, when it moved closer to the shelf. The interaction of eddies with the coastal domain is a process which enriches them, but also transports coastal nutrients offshore.

Cyclonic eddies C1 and C2 also showed contrasting and different characteristics that could be explained by their origins. C2 was formed in low productivity waters and did not undergo any significant evolution along its course. By contrast, C2 was born in a region where deep nutrient-rich waters are brought to the surface (coastal upwelling); it retained water properties from where it originated and displayed the largest surface concentration of nitrate and the highest levels of new production of all the eddies that were investigated.

These examples illustrate the complexity of the ecosystem and the importance of models to study the mechanisms responsible for primary production within eddies. The next step is to implement a study including the higher trophic levels (predators).

 

 

The paper

José Y.S., Aumont O., Machu E., Penven P., Moloney C.L., Maury O., 2014. Influence of mesoscale eddies on biological production in the Mozambique Channel: several contrasted examples from a coupled ocean-biogeochemistry model. Deep-Sea Research II, 100: 79-93.

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The authors

This work is a result of the collaboration between researchers of laboratories from France ( Laboratoire de physique des océans of IUEM in Brest and UMR "Ecosystèmes Marins Exploités" in Sète) and South Africa (Marine Research Institute of the University of Cape Town).


The journal

was founded in 1953 and split in 1993 through the creation of a new journal (Deep-Sea Research II: Topical Studies in Oceanography) which does not publish isolated papers but special issues from international and interdisciplinary projects and scientific conferences. Volume 100 was published in February 2014 and is a collection of 17 papers (117 pages) on mesoscale dynamics and ecosystem responses in the Mozambique Channel.

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