However, recent satellite observations have indicated the unexpected occurrence of points of unusually high concentrations of chlorophyll, the green pigment contained in phytoplankton. This would indicate the development of phytoplankton in these zones of otherwise low fertility. How can this occurrence be explained? IRD researchers and their co-workers studying the question (1) consider it as an accumulation of floating organic particles and not chlorophyll by-products from the biological activity of the ecosystem. These would initially have been spread over the water surface. They are thought to concentrate under the effect of converging movements induced by oceanic waves acting as a "hay rake". This original hypothesis, just published in Science, matches observations made in situ between Tahiti and New-Zealand. It brings new light on the survival strategy of species in the nutrient-poor parts of the ocean and should contribute to improved fisheries management in this area of the Pacific.

The oceans have their desert zones, in other words areas poor in nutrients and unfavourable for phytoplankton to develop. Half of the southern Pacific thus consists of great expanses of warm water with an average temperature of 28 C (a greater surface area than Europe), which receives no input of deep-source cold water, rich in nutrient salts.

However, in 2000 analyses of satellite observations on the colour of the ocean conducted by American scientists revealed unusually high concentrations of chlorophyll --the green pigment carried by phytoplankton-- in these unfertile areas. These accumulations were associated with the movement of Rossby waves and variations in ocean height they generate (2). An initial hypothesis proposed that Rossby waves induce an intermixing which prompts interm
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Contact: Bndicte Robert
presse@paris.ird.fr
33-1-48-03-75-19
Institut de Recherche Pour le Dveloppement
3-Dec-2003