FISHERY BULLETIN: VOL. 69, NO, 3 



500 



400 



:300 



b200 



o 

 o 



IT 



Q. 



>- 100 



a: 



< 



or 



a 



JAN 



FEB 



MAR 



APR 



MAY 

 MONTH 



JUN 



JUL 



AUG 



SEP 



Figure 5. — Seasonal pattern of primary productivity ami chlorophyll o in the mid-Subarctic Pacific Region be- 

 tween the Aleutian Islands and lat 46° N. 



summer means. As might be expected, primary 

 productivity was limited by low available light 

 energy in the winter. The mean daily light in- 

 tensity was low (116 cal/cm-), and the avail- 

 ability of light to the cells was further limited 

 by their being distributed throughout the surface 

 mixed layer, which reached well below the eu- 

 photic zone to the halocline at about 100 m. 



In March, daily light intensities averaged 274 

 cal/cm- and thermal stratification had developed 

 sufficiently to decrease the mixed-layer depth — 

 thereby increasing exposure of the cells to light 

 at shallower depths and consequently stimulating 

 growth. The mean chlorophyll a concentration 

 (19.3 mg/m-) was clearly higher than for any 

 other season, and Pr and Pk (303 and 272 mg 

 C/m- per day) were more than three times as 

 high as in January and February (Figure 5, 

 Table 2). The Pr/C„ and Pk/Co ratios (16 

 and 14) were roughly twice those in midwinter, 

 indicating that productivity was no longer lim- 

 ited by low light intensities. Although measure- 

 ments in May 1968 were made at a considerable 

 distance east of the Adak Line and may not be 

 directly comparable, mean Pk values were re- 



markably similar in June 1966 and 1967 and 

 May 1968 (238, 241, and 246 mg C/m^ per day, 

 respectively) as were the mean chlorophyll val- 

 ues (14.4, 12.7, and 14.3 mg/m-, respectively). 



In August, the mean Pk and Pr values differed 

 significantly {Pk = 153 and Pr = 247 mg C/m^ 

 per day) . A large part of this difference may be 

 attributed to high productivity measured at lat 

 46° N in transition area T-2 (Pk- = 240 and 

 Pr = 664 mg C/m- per day) . These estimates 

 strongly influenced the means because the 

 weights assigned to each in the averaging pro- 

 cess were relatively large. The mean chlorojjhyll 

 a concentration (10.6 mg m') was slightly less 

 than in June, and the mean Pr/Cu (23) was 

 higher than in June. In September, mean Pb 

 and Pk were 201 and 2.50 mg Cm- per day, 

 and the mean chlorophyll o was 13.0 mg/m'. 

 These values were somewhat similar to the other 

 summer values, as were the ratios Pw/Ca (16) 

 and Pk/C„ (20). 



The general time-distributional pattern of pro- 

 ductivitj' and chloroi)hyll a between lat 46° and 

 51°40' N from January through September was 

 drawn from the above results (Figure .5). Pro- 



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