FISHERY BULLETIN: VOL. 84, NO. 3 



plankton (Voituriez and Herbland 1984). We can 

 then suggest that new nitrates have been assim- 

 ilated during El Nino in the western Pacific at lat. 

 6-9°N. The 2 /imoles-L -1 drop in nitrate concentra- 

 tion is observed in the interval 17°-22°C, corre- 

 sponding to a 35 m thick water layer (Anonymous 

 1972-84), so that the amount of new nitrates used 

 by photosynthesis is 70 fimoles-m" 2 , or 980 

 mg-m- 2 . If CIN = 9.01 and C/Chl = 114 in surface 

 waters of the oligotrophic central North Pacific 

 (Sharp et al. 1980), this amount of nitrogen corre- 

 sponds to 77 mg Chi a-m -2 . It represents an im- 

 portant supply in an ecosystem where the chloro- 

 phyll concentration is usually low. 



Figure 3 shows the variations of integrated 

 chlorophyll (0-200 m) between lat. 6°N and 9°N at 

 long. 137°E, obtained from the Ryofu Maru data 

 (Anonymous 1972-84). Values during the 1982-83 El 

 Nino are similar to those since July 1981, i.e., below 

 50 mg-m -2 . SSCC from the same data set also 

 shows low values during the 1982-83 El Nino, con- 

 flicting with the results mapped on Figure 1. Re- 

 cent El Nino events in 1972 and 1976 resulted in 

 a drop of the sea level in the western Pacific (Meyers 

 1982). Low sea level was also recorded during an 

 El Nino like event in the western Pacific in 1979-80 

 (Donguy and Dessier 1983). These low sea level 

 episodes during which the thermocline is shallow 

 (Wyrtki 1978), yet do not correspond to high SSCC 



or high integrated chlorophyll values in the Ryofu 

 Maru results (Fig. 3). It seems however that the 

 nutricline depth is shallower during these four epi- 

 sodes (Fig. 3). All of them are moreover charac- 

 terized by a shift in the nutrient-temperature rela- 

 tionship (Fig. 2) indicating a consumption of new 

 nutrients. We are dealing with an SSCC enrichment 

 in the northwestern tropical Pacific which persists 

 for several months (October 1982-March 1983) and 

 is consistent with an input of new nutrients from 

 below, but which does not appear in the chlorophyll 

 concentrations measured every 6 mo on the Ryofu 

 Maru. Both data sources have weaknesses. The 

 SSCC monitoring does not measure what occurs 

 below the surface. A significant correlation exists 

 between SSCC and integrated chlorophyll on broad 

 data sets (Lorenzen 1970; Piatt and Herman 1983), 

 but oligotrophic ecosystems often show no relation- 

 ship or, sometimes, inverse relationships (Hayward 

 and Venrick 1982). The Ryofu Maru data at 137 °E 

 between 6°N and 9°N allow a look at this problem 

 (Fig. 4): the correlation between SSCC and in- 

 tegrated chlorophyll is significant at the 1% level. 

 The value r = 0.52 obtained with individual stations 

 increases to r = 0.70 when enlarging the spatial 

 scale (i.e., taking mean values between 6°N and 9°N 

 instead of individual stations); a further improve- 

 ment would probably be obtained by enlarging the 

 time scale, but appropriate time series do not exist 



SSCC(mg m-3) 



A- 

 .2- 







— i »— ^ 1 1^ r- — -t ^ 



Integrated Chlor. (0-200m , mg.m-2) 



100-. 

 50- 



0- 



—I r-^ 1 1 1 —  



Depth of nutricline ( m ) 



100- 



50- 

 0- 



70 



75 



80 



Figure 3.— Long-term evolution of lat. 6°N-9°N averaged parameters related 

 to the primary production (data from the RV Ryofu Maru cruises at long. 

 137°E, Anonymous 1972 to 1984). Upper and middle panels: the chlorophyll 

 concentrations primarily expressed in active chlorophyll a and pheophytin have 

 been converted into chlorophyll a equivalents (Dandonneau 1979). Lower panel: 

 the continuous line joins the depth of P0 4 = 0.35 /^moleL" 1 ; open circles 

 represent the depths of N0 3 = 1 ^mole-L" 1 . Thickened marks on the horizon- 

 tal axis indicate the low sea level episodes in the western tropical Pacific. 



692 



