FISHERY BULLETIN: VOL. 69, NO. 3 



PRIMARY PRODUCTIVITY ESTIMATES 



ADJUSTED FOR DIFFERENCES 



OF LIGHT INTENSITY 



Because productivity was measured in natural 

 light which differed (by as much as fivefold) 

 in total insolation from day to day, productivity 

 values were adjusted by two methods to permit 

 comparison of productivity estimates under 

 similar lig^ht conditions for purposes of detecting 

 possible differences in productivity among ocean- 

 ographic areas. One method applied the relation 

 given by Ryther (1956) and Ryther and Yentsch 

 (1957) of relative daily productivity beneath 

 a unit of sea surface to total daily surface ra- 

 diation. The measured daily light intensities 

 were averaged for each cruise and the corres- 

 ponding values of R (photosynthetic rate rela- 

 tive to photosynthesis at light saturation), 

 defined by R>i;her and Yentsch (1957), were de- 

 termined for the cruise mean of daily light and 

 for the light observed on the particular day in 

 question (K,,, and R,,,, respectively). Adjusted 

 productivity was then com])uted by Pr = P,„ y 

 Rav'Rm where Pr and P,„ are the adjusted and 

 observed productivities integrated through the 

 euphotic zone and have the units of mg C 'm- 

 per day. This ]irocedure amounts to using the 

 shape of Ryther's (1956) curve but not his ab- 

 solute values for estimating productivity. 



Since Ryther derived his curve from photosyn- 

 thesis measurements of phytoplankton from 

 Woods Hole Harbor, that relationship is likely 

 to differ from similar curves based on measure- 

 ments from other areas. An attempt was made, 

 thei'efore, to establish a simple emi)irical rela- 

 tion to estimate productivity in the mid-Sub- 

 arctic Pacific Region from chlorophyll and light 

 data obtained during the Kelez cruises. The 

 regression of measured daily productivity (P,,,) 

 per unit of chlorophyll a (C„) in the euphotic 

 zone 



(where Pm/Ca ^ 



, mg C as.similated/m- per day , 

 mg chloroi)hyll a m- 



on daily light intensity measured on the ship's 

 deck was computed (Figure 3) . Only data from 

 those stations where a full day's productivity and 

 light were measured were used for the relation. 



100 200 300 400 500 600 



SOLAf R4DI4TI0N (col/cm^ p«r do») 



Figure 3. — Relation between the ratio of daily primary 

 productivity to chlorophyll a (P,n/Ca) in the euphotic 

 zone and daily solar radiation above the sea surface in 

 the Subarctic Pacific Region, 1966-67. 



The intercept of the regression and the axes was 

 not significantly different from the origin. The 

 variability was generally large, as might be ex- 

 pected from data of this kind, and was especially 

 high for stations in the southern portion of the 

 study area (transitional and subtropical water). 

 This variability suggests that productivity re- 

 sponses to the environment in the Transitional 

 Domain and subtropical water were probably 

 different from tho.se north of about lat 46° N. 

 Furthermore, relatively few measui-ements were 

 taken south of lat 46° N — not enough for seasonal 

 comparisons. For these reasons, the regression 

 was computed for only those stations north of 

 lat 46° N. 



Productivities under average daily light in- 

 tensities for each cruise (Pk) were estimated by 

 multiplying the chlorojihyll a measured in the 

 eu|)hotic zone at a station by tlie estimate, from 

 the regression, of P Ca corresponding to the 



600 



