FISHERY BULLETIN: VOL. 69, NO. 3 



the upper 10 m at lat 43° N in September. The 

 deep chlorophyll layer at other stations could 

 easily have been missed because sampling below 

 the 1% light level was at standard depth. 



PHOSPHATE CHANGES AND THEIR 

 RELATION TO PRIMARY PRODUCTION 



An attempt was made to draw qualitative in- 

 ferences from phosjjhate data about the relative 

 levels of primary production between cruises to 

 obtain a somewhat more detailed picture of the 

 seasonal productivity pattern. The major fac- 

 tors generally affecting changes in dissolved 

 phosphate concentration in the oceans are: (1) 

 utilization by primary producers (P„), (2) re- 

 generation by zooplankton and bacteria (Pr) , 

 and (3) advective changes (P„). The relation 

 among these factors is given by the formula: 



Po 



P„ + Pr + Pa, 



where P,> is the net change of phosphate con- 

 centration with time, P„ is negative, Pr is posi- 

 tive, and Pa can be of either sign. Estimates 

 of Pu between cruises were calculated by apply- 

 ing a ratio of carbon to phosphorus (C:P) to 

 the carbon-14 data as discussed below, Pa was 

 estimated from calculations of vertical velocities 

 and phosphate concentrations measured at depth, 

 and Po was calculated from measurements of 

 phosphate concentrations; however, no adequate 

 estimate of Pr was possil)le. Two major assump- 

 tions were necessary to evaluate the above 

 parameters: 



1. The uptake ratio of C:P = 40 by weight 

 (Strickland and Parsons, 1965) . This value re- 

 lates the C and P content of cells, but not the 

 amounts assimilated. It is applied here, how- 

 ever, to carbon-assimilation rates measured by 

 the carbon-14 method, which measures rates be- 

 tween net and gross production. The resulting 

 estimate of phosphorus uptake, therefore, will 

 be larger than the actual amount of phosphorus 

 retained in new cell material. 



2. Phosphate concentrations and their in situ 

 changes were uniform within individual u])per- 

 zone domains. This assumjjtion permits us to 

 neglect the effect of horizontal advection. 



Since nutrients were generally abundant in 

 the Subarctic Region and changes were small 

 during the year, circulation and regeneration 

 must have supplied dissolved nutrients to the 

 upper zone at rates sufficient to keep pace with 

 their utilization, despite high assimilation rates 

 by the algae during spring and summer. The 

 amounts of phosphate supplied to the upper 50 m 

 by upwelling were estimated from calculations 

 of monthly mean vertical velocities (Wickett, 

 1966, 1968)' and observed phosphate concen- 

 trations at 50 m (Table 5). Wickett listed 

 meridional components of Ekman and total 

 transport for alternate points on a grid of 5- 

 degree units of latitude and longitude. To ob- 

 tain the phosphate estimates, the average of 

 monthly mean vertical velocities for grid points 

 at lat 45° N, long 175° W and at lat 50° N, long 

 180° W were used as single monthly estimates 

 applicable to the Adak line of stations north of lat 

 45° N. To obtain the net amount of phosphate ex- 

 changed through the 50-m surface, the net verti- 

 cal displacement of water during each month 

 was multiplied by mean phosphate concentra- 

 tions at 50 m. The computed vertical velocities 

 refer to the bottom of the Ekman layer which 

 extends to the halocline at 100 m in winter but 

 is limited by the thermocline in summer to as 

 shallow as 30 m. The error incun-ed, however, 

 by applying the velocities to 50 m rather than 

 any other level was probably within the range 

 of precision of the estimated velocities. 



Such estimates of vertical transport of phos- 

 phate must be considered minimal, because the 

 turbulent flux of properties across a surface 

 cannot be computed by using mean velocities. 

 That is, mean vertical velocities indicate net 

 upward flow, although water and its associated 

 jiroperties actually move up and down across 

 horizontal surfaces. When phosphate concen- 

 tration increases with depth (as it usually does) , 

 the shallower water loses less phosphate by 

 downward flux than it gains by equivalent up- 



' Wickett, W. P. 1966. Fofonoff transport com- 

 putations for the North Pacific Ocean, 1966. Fish. Res. 

 Board Can., Manuscr. Rep. Ser. (Oceanogr. Limnol.) 

 229, 92 p. (Processed.) 



1968. Transport computations for the North Pacific 

 Ocean, 1967. Fi.sh. Res. Boanl Can., Tech. Rep. 53, 92 p. 

 (Processed.) 



608 



