FISHERY BULLETIN: VOL. 84, NO. 3 



R = would give an infinite weight to an obser- 

 vation k available at x k = Xj and y k = jjj. We would 

 then obtain tj = t k regardless of the other observa- 

 tions. This is acceptable only if the instrumental and 

 sampling errors on t k were null, which is not the 

 case. Thus, R accounts for the errors on the obser- 

 vations. We choose R 2 = 25, which, together with 

 a = 2 and p tj > (25 + 160)" \ performed an effi- 

 cient smoothing and preserved the large-scale infor- 

 mation. 



RESULTS 



The sequence of quarterly mean SSCC for 1982 

 and 1983 is presented in Figure 1, together with the 

 positions of the data. The western part, north of lat. 

 20°N, is poorly sampled. The data range between 

 0.05 and 0.20 mg-m -3 . The highest values are 

 found during the northern spring of 1982, and the 

 northern winter of 1983. The 1982 winter, and the 

 spring and fall of 1983 exhibit a few values >0.10 

 mg-m" 3 . The 1982 winter and fall show low SSCC, 

 like the summer of both years, below 0.10 mg-m -3 . 



The eastern part, north of lat. 10°N, has gener- 

 ally low SSCC values, often below 0.05 mg-m -3 . 

 Exceptions are the spring of 1982 at the extreme 

 north, and, mainly, the fall of 1982 during which the 

 mean values exceeded 0.20 mg-m -3 off California. 



Low SSCC values are observed in the western 

 part between the Equator and lat. 20°N until the 

 summer of 1982. They are abruptly replaced at the 

 end of 1982 by high values which persist until March 

 1983. Later, low values, generally below 0.05 

 mg-m" 3 , dominate again between lat. 5°N and 

 20°N, while SSCC >0.10 mg-m" 3 shift back south- 

 ward to the Equator. 



The equatorial zone shows high SSCC in January- 

 March 1982, between America and long. 160° E. 

 Values higher than 0.10 mg-m -3 spread from lat. 

 10°N and 10°S in the central Pacific, and to 15°S 

 at 120°W. From April to June 1982, the enriched 

 zone shifts eastwards and southwards. The east- 

 wards shift continues between July and September 

 and is accompanied by a decrease of SSCC in the 

 eastern Pacific, with mean values <0.15 mg- nr 3 . 

 From October 1982 to June 1983, a narrow band 

 with SSCC between 0.10 and 0.15 mg-m" 3 in the 

 eastern Pacific is the only remnant of the equator- 

 ial enrichment. A normal situation returned after 

 the El Nino, in July-September 1983, with SSCC 

 values >0.15 mg-m -3 spreading westwards to 

 long. 170°E. In October-December 1983, SSCC 

 >0.10 mg-m -3 are seen all along the Equator. 



South of lat. 20° S, an SSCC increase is observed 



during the austral winter. The increase started in 

 April-June in 1982, the maximum was reached in 

 July-September, with SSCC >0.20 mg-m -3 spread- 

 ing northward to 22 °S, and low values were seen 

 again in October-December. The increase during the 

 austral winter of 1983 was of a lesser extent, being 

 well developed only during July-September, with 

 SSCC >0.20 mg-m -3 limited to the south of 28°S. 



The intermediate zone, from lat. 10°S to 20°S, 

 between the equatorial upwelling and higher lati- 

 tudes where a winter increase is observed, gener- 

 ally has low chlorophyll concentrations, below 0.10 

 mg-m -3 . The lowest concentrations are seen in 

 austral summer, from October 1982 to June 1983, 

 and in October-December 1983. The highest concen- 

 trations are associated with a strengthening of the 

 equatorial upwelling (around long. 140 °W in April- 

 June 1982; westwards spreading of richer waters 

 from the eastern Pacific in July-September 1982 and 

 1983). 



When looking at the whole series of maps, the 

 most striking feature is the reduction of the equa- 

 torial upwelling enriched area after the onset of El 

 Nino. The most pronounced stage was in April-June 

 1983, with poor waters over most of the tropical 

 Pacific. On the contrary, a zone centered at lat. 

 10°N, west of the dateline, which is usually occupied 

 by chlorophyll-poor waters, had higher SSCC dur- 

 ing the 1982-83 El Nino. 



DISCUSSION 



Equatorial Upwelling 



The collapse of the equatorial upwelling after the 

 onset of El Nino, when westerlies have replaced the 

 trade winds at the Equator, consistently results in 

 a decrease in SSCC. This decrease has already been 

 documented for the eastern Pacific in the Galapagos 

 Islands region by Feldman et al. (1984) using sea 

 color satellite images. It corresponds to a decrease 

 in primary production of the whole photic layer 

 (Barber and Chavez 1983). The data presented here 

 show that the equatorial zone was impoverished 

 westwards to nearly 180°. This is in agreement with 

 the reproductive failure and disappearance of sea- 

 bird communities at Christmas Atoll (lat. 2°N, long. 

 157°W) in November 1982; Schreiber and Schrei- 

 ber (1984) attributed these events to the establish- 

 ment of an oligotrophic oceanic ecosystem instead 

 of a productive one. Successful reproduction started 

 again for some birds species in June 1983, and hatch- 

 ing occurred in July-September 1983, when SSCC 



690 



