Ocean mixing and food supply

Strong mixing in cold nutrient-rich waters along the eastern boundaries of the world's oceans appears to reduce rather than stimulate growth of phytoplankton (microscopic plant-like organisms). In the nutrient-poor open ocean, on the other hand, mixing has a positive effect on phytoplankton growth. This is the surprising conclusion of a recent article by French oceanographer Vincent Rossi and colleagues in EGU's open access journal Nonlinear Processes in Geophysics (NPG). Since phytoplankton are the base of the ocean food chain (they are eaten by zooplankton, which are then eaten by small fish) this finding provides new understanding of some of the most important regions for human food production.

Driven by alongshore winds, cold and nutrient-rich bottom water upwells along the eastern margins of the Atlantic and Pacific Oceans. Although they represent less than 1% of the total surface of the world’s oceans, these Eastern Boundary Upwelling Systems (EBUS) are the most productive regions of the world. Their important coastal biological productivities support the largest commercial fisheries and identify them as key regions for human food supply. They include the Canary and the Benguela (South-African/Namibian coast) upwelling systems in the Atlantic Ocean and the Humboldt (Peruvian/Chilean coast) and Californian upwelling systems in the Pacific Ocean. In this multidisciplinary work, we focused on the small scale (1-100 km) processes studying the influence between eddy induced mixing and phytoplankton distributions.

Based on satellite data, we consider chlorophyll concentrations (as a proxy of phytoplankton) from ocean color sensor and we estimate the horizontal mixing of the surface ocean using the combined product from altimetry and scatterometry. The biological and physical characteristics of these four areas reveal similarities as well as differences. Using a comparative approach among EBUS, we documented the variations in space and time of their mixing activity.
The most important finding is the global negative correlation between surface horizontal mixing and chlorophyll contents over the four areas. This negative effect is opposite to the one deduced from previous modelling studies. A positive effect of small-scale turbulence on phytoplankton development was shown in the open ocean. However, in upwelling areas, a high eddy-induced mixing seems to modify the three dimensional flow and in turn diminish the uplift of nutrient rich waters.

To bring together these two theories, we finally suggest the possibility of a changing response of the phytoplankton to small-scale turbulence, from a negative effect in the very productive coastal areas, to a positive one in the open ocean. This study provides new insights for the understanding of the primary productivity in the ocean. Being the base of world marine ecosystems, these results about phytoplankton also contribute to an improved understanding of global marine element cycles and fish stocks variability.

Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems, V. Rossi, C. López, E. Hernández-García, J. Sudre, V. Garçon, and Y. Morel. Nonlinear Processes in Geophysics, 16, 557-568, 2009.
Abstract: http://www.nonlin-processes-geophys.net/16/557/2009/npg-16-557-2009.html

Information for editors
Author contact
Dr Vincent Rossi
Laboratoire d'Études en Géophysique et Océanographie Spatiale
CNRS, Observatoire Midi-Pyrénées
18 avenue Edouard Belin, Toulouse
31401 Cedex 9, France
Phone: +33(0)5.61.33.30.07
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