Nutrient-poor oceans generate their food ’hot spots’

Half of the southern Pacific Ocean is an immense reservoir of warm subtropical water, yet poor in nutrients and little conducive for the development of living organisms.

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 intermingling between the layers of warm water at the surface and the deep cold nutrient-rich water levels. This mixing wouls generate surface influx of nitrates, favourable for phytoplankton development. This hypothesis cannot explain, however, why the chlorophyll concentration peaks are always observed at the warmest spots where the water accumulates under the effect of the passing waves.

The IRD oceanographers and their co-workers investigating these effects (1) consider rather that the Rossby waves act like a rake over the ocean surface, in this way concentrating all floating particles or debris in these places where warmer water accumulates owing to greater sun exposure. This excludes the possibility of nutrients ascending from the deep cold waters by mixing. In the convergence zones produced by wave movements, there would not be any new production of phytoplankton as had been suggested, but rather an accumulation of floating organic particles of a different origin. This floating material’s optical properties are similar to those of chlorophyll, so it gives the same effect as captured by satellite observation of ocean colour, in a way misleading the calculation systems which use these satellite colour data to estimate the chlorophyll concentration.

The researchers have devised a model for testing this original hypothesis and attempting to identify the origin of these floating particles. Such material would be organic by-products from the biological activity, however low-key, at work in the ocean’s surface layer. Instead of plunging down into the deeper layers, part of this organic debris could come back to the surface, maybe thanks to gas bubbles produced by bacteria during fermentation processes, or riding on lipids (lighter than water), for example. The simulations performed led to validation of this hypothesis. It was also confirmed by way of measurements of chlorophyll concentration, determined in situ in the surface layer, during quarterly campaigns on “Geochemistry, Phytoplankton and Ocean Colour” in the South Pacific, between Tahiti and New-Zealand (3). These observations suggest that the satellite detection system as designed cannot distinguish between chlorophyll and the organic particles, and that the chlorophyll concentration calculated from images of the convergence zones is overestimated.

This study sheds new light on how marine ecosystem processes work in association with the overall physical dynamics of the ocean. In the oligotrophic oceanic environments in question, the water movements generated by the passage of equatorial waves gathers and accumulates in restricted locations what little organic matter there is. What is initially scattered wide over the ocean surface is concentrated into oases of nutrients for fish. The results provide possible clues to the question of survival of marine species in nutrient-poor habitats. They could have significant applications in fishing and in particular for tuna stock management. However, the exact nature of these floating particles remains, however, to be identified. Research investigations are already planned, notably as part of the MATI and Biosope projects of the national programme Proof.

(1) IRD scientists from the Laboratory of Dynamic Oceanography and Climatology of the Institut Pierre-Simon Laplace (Paris) and the Laboratory of geophysical studies and spatial oceanography (Toulouse), researchers from the MREN (Maison de la recherche en environnement naturel) – UMR 8013 CNRS /Université du Littoral.

(2) Equatorial waves are generated by wind variations, atmospheric pressure, etc. They have been found in all latitudes, but they play a prime role at the Equator which acts as a wave-guide. In the tropics two main types of wave can be distinguished: Kelvin waves, which propagate from West to East along the Equator, and Rossby waves, which slowly cross from East to West in tropical latitudes. Kelvin waves arrive at the American coasts where they are reflected to set off back towards the West, on the North side and and the South side of the Equator, in the form of Rossby waves.

(3) There were 12 of these “GeP&CO” campaigns, conducted from 1999 to 2002 as part of a French national programme Proof (Processus océaniques et flux). The objective was to study the variability of phytoplankton populations and its influence on the geochemistry of the oceans.