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FLUCTUATIONS IN BIOLOGICAL PRODUCTION IN THE EQUATORIAL PACIFIC UNDERTHE EFFECT OF EL NINO

May 11, 1999

A region of the western equatorial Pacific, the Pacific warm pool, has exceptionally warm surface waters (an average 28.5°C) which have low salinity and are oligotrophic (nutrient-poor). At the Equator the Eastern edge of this pool comes into contact with cooler water (24°C on average) which is saline and nutrient rich (especially in nitrate) owing to the upsurge of mineral-salt laden waters from the depths (the equatorial upwelling). The contact zone marks a sharp salinity front which is also rich in zooplankton and fish (tuna especially).
During a warm episode (El Nino), under the effect of frequent Westerly winds in the equatorial Pacific and the currents these generate, the warm water mass extends and its eastern limit shifts along the Equator 2-3000 km eastwards in a few months. Conversely, during a cold phase (La Nina episode), the warm pool is pushed back westward of its mean position. This ocean dynamics, which is particular to the Pacific, has been described by a numerical oceanic circulation model (Ocean General Circulation Model) developed by LODYC (Laboratoire d'Océanographie Dynamique et de Climatologie/CNRS - IRD Paris VI), incorporating data collected from the Pacific basin by means of a specially deployed network of buoys and satellites.
What is the impact of these ocean dynamics on biological production in the equatorial Pacific? As data on the overall biogeochemical production for the Pacific are scarce, modelling has to be used if any answers are to be provided to this question. IRD research workers have therefore resorted to a biogeochemical model that simulates nitrate consumption by phytoplankton in the surface layers well exposed to sunlight. Nitrate was chosen as parameter because it is an important constituent of plant biomass and where it is absent primary production tends to cease. The overall biological production can be deduced from the available nitrate so determined. This nitrate model has been coupled with the OGCM elaborated by LODYC. Applied to an El Nino type event (that occurred in 1994), the coupled model gives a representation of the interactions between the ocean dynamics (made up of winds and current regimes) and biological fluxes.
The investigations indicate that, when the eastern edge of the oligotrophic warm pool is displaced several thousands of kilometres towards the east under El Nino's influence, and the cold water zone diminishes by the same extent, the area of highly productive nitrate-rich waters in the east of the basin also tends to decrease. Simulations of nitrate content fluctuations and biological fluxes resulting from this model have been affirmed by data recorded in the same part of the Pacific, during oceanographic cruises conducted by French (FLUPAC and OLIPAC) and American (EqPac) research teams between 1992 (La Nina phase) and 1994 (El Nino phase).
Using the variations in primary production thus determined, this modelling has led to figures for fluctuations in secondary production as a function of the ocean dynamics generated by ENSO. In particular it was shown that the zone rich in food for tuna (abundant in zooplankton and micronekton) is situated in the vicinity of the front between the warm pool and the equatorial upwelling cold water zone - and moves eastward in phase with El Nino events. This would explain why Pacific tuna (mainly skipjack, Katsuwonus pelamis ) shifts eastwards along with this front during the warm phases of the ENSO. These results, obtained using modelling, have been corroborated by tuna fisheries data which reveal that in years when El Nino is active, maximum catches are taken 2500 km further east than in the more usual climatic periods.
The IRD researchers are also developing this coupled model further by linking it with a third protocol which simulates the carbonate-carbon dioxide exchange system in the ocean, with the aim of better understanding of the processes involved in CO2 gas exchanges between the atmosphere and the sea could change in times of climate warming, examples of which are indeed provided by El Nino and its associated events.


For Further Information

CONTACT : Yves Dandonneau, IRD, Laboratoire d'océanographie dynamique et de climatologie (LODYC) IRD/CNRS/université Pierre et Marie curie,75252 Paris cedex 05
tel : 33 1 44 27 34 81, Fax : 33 1 44 27 38 05, e-mail : yd@lodyc.jussieu.fr

Bibliography
A.Stoens, C. Menkes, M.-H.Randenac, Y. Dandonneau, N. Griam, G.Eldin, L. Mémery, C. Navarette, J.-NM. André, T. Moutin et P. Rambault "The coupled physical-new production systeme in the equatorial Pacifique during 1992-1995 El Ni'±o", Journal of Geophysical Research, février 1999.
P. Lehodey, J.-M. André, M. Bretignac, J. Hampton, A. Stoens, C. Menkes, L. Mémery, N. Grima "Predicting skipjack tuna forage distibutions in the Equatorial pacific using a coupled dynamical bio-chemical model", Fisheries oceanography, 1998.

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