MONITORING THE SEA SURFACE CHLOROPHYLL CONCENTRATION 



IN THE TROPICAL PACIFIC: 

 CONSEQUENCES OF THE 1982-83 EL NINO 



Yves Dandonneau 1 



ABSTRACT 



The sea surface chlorophyll concentration (SSCC) is routinely measured in the tropical Pacific using filtra- 

 tions made aboard merchant ships that sail from New Caledonia to Japan, North America, Panama, New 

 Zealand, and Australia. About 4,000 measurements are collected every year, allowing a tentative monitor- 

 ing of SSCC in the Pacific. Heavy smoothing made it possible to map quarterly charts of SSCC which 

 cover the 1982-83 El Nino episode. The usually enriched belt which corresponds to the equatorial upwell- 

 ing vanished after September 1982, except for a reduced zone east of long. 120° W, where a moderate 

 enrichment persisted throughout the warm event. It recovered after July 1983, spreading westwards 

 to long. 170°E. During the mature phase of El Nino (October 1982-June 1983), an enriched zone ap- 

 peared in the western Pacific, centered at about lat. 7°N, consistent with a rise of the thermocline in 

 this region. An examination of oceanographic data collected in this region since 1970 shows that nutrients 

 from below the thermocline are consumed by the phytoplankton during each El Nino. This western Pacific 

 enrichment was weakened with time, and the period from April to June 1983 was characterized by low 

 SSCC values over most of the tropical Pacific. Unusually high SSCC values are reported in subtropical 

 zones, during the austral winters of 1982 and 1983 in the southwestern Pacific and during the 1982 autumn 

 in the northeastern Pacific, which may be due to advection of rich water from higher latitudes and to 

 intensified vertical mixing by strong westerly winds, respectively. 



El Nino was first observed and experienced in Peru, 

 where it was given its name and became a familiar 

 part of Peruvian life. Although the southern oscilla- 

 tion was identified more than 60 yr ago (Walker 

 1924), the relation between the El Nino phenomenon 

 and ocean-scale features was only established after 

 the 1957-58 event by Bjerknes (1966). It is now well 

 established that El Nino is simply the most obvious 

 consequence of important oceanographic and 

 meteorological changes in the Pacific Ocean 

 (Donguy and Henin 1976; Quinn et al. 1978; Cane 

 1983). One would expect biological changes at the 

 same scale. These, however, have only been studied 

 in the eastern Pacific (Walsh 1981; Chelton et al. 

 1982; Barber and Chavez 1983) where a pronounced 

 decrease in phytoplankton biomass and primary pro- 

 duction is observed. Farther west in the equatorial 

 zone, the decrease in primary productivity has been 

 shown only by indirect observations on marine birds 

 (Schreiber and Schreiber 1984) and on abnormal 

 distributions of some fishing grounds in relation to 

 changes of water mass (Donguy et al. 1978; Yama- 

 naka 1984). The difficult problem of monitoring the 

 intensity of primary production on a large scale is 



troupe SURTROPAC, Centre ORSTOM, B.P. A5, Noumea, 

 New Caledonia. 



4S7 -US' 



Manuscript accepted January 1986. 

 FISHERY BULLETIN: VOL. 84, NO. 3, 1986. 



usually reserved for satellite-borne sensors. A 

 modest attempt, however, is in progress, as a part 

 of the SURTROPAC program (ORSTOM, Noumea) 

 based upon chlorophyll samples taken by voluntary 

 observers on ships of opportunity. Each year about 

 4,000 sea surface chlorophyll concentrations (SSCC) 

 are collected in this way, distributed along maritime 

 lanes from the Tasman Sea to Panama, North 

 America, or Japan. These data cover the tropical 

 Pacific from lat. 30 °S to 30 °N, and from long. 

 140°E to 80°W. There are large gaps both in space, 

 between the main lanes, and in time, between con- 

 secutive crossings. But, on a quarterly basis, the 

 SSCC data are numerous enough to allow a crude 

 view of the whole tropical Pacific Ocean, with the 

 advantage of using a single methodology. The con- 

 sequences of the 1982-83 El Nino can thus be ex- 

 amined, and most of the attention will be directed 

 towards the central and western Pacific, where pres- 

 ent knowledge is very incomplete. 



METHODS 



Chlorophyll Measurements 



SSCC measurements are made according to a non- 

 extractive method (Dandonneau 1982). Twenty milli- 



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