LARRANCE: PRIMARY PRODUCTION IN MID-SUBARCTIC REGION 



'o 100 200 300 400 500 600 700 



ZOOPLANKTON (ml/IOOOm^) 



Figure 8. — Relation of chlorophyll a to net zooplankton 

 in the upper 150 m of the Subarctic Pacific Region, 

 1966-68. Dashed lines represent 95% confidence limits. 



The zooplankton value at the only nearshore 

 station (Adak Bay) in March was the lowest 

 observed. These data suggest that the zooplank- 

 ton increase occurred later in the spring in 

 coastal areas than farther offshore, a condition 

 which could have permitted the phytoplankton 

 bloom to reach much higher levels near shore 

 before being controlled by grazing. 



ANNUAL PRIMARY PRODUCTION 



The annual primary production between lat 

 46° and 51°40' N was estimated from values of 

 Pk, Pr, and Pm. The data from all cruises were 

 combined into a single composite year for ap- 

 portioning productivity to time periods. The 

 lowest estimate was for Pk (72 g C/m- per year) ; 

 Pm and Pr were somewhat higher (82 and 85 g 

 C/m- per year). If the estimate of primary 

 productivity from phosphate changes is correct, 



annual productivity could be as high as 100 g 

 C/m^. These estimates are considerably higher 

 than those reported by McAllister (1969), mod- 

 erately higher than those of Koblents-Mishke 

 (1965), and lower than those of Anderson (in 

 press). 



No reason is apparent from the quantitative 

 data for the difference between annual produc- 

 tivity at station "P" (48 g C/m^ per day; Mc- 

 Allister, 1969) and that in the mid-Subarctic 

 Region. If zooplankton is the main factor lim- 

 iting productivity, more zooplankton and less 

 chlorophyll should be expected at station "P" 

 than south of the Aleutians; however, zooplank- 

 ton density was lower and chlorophyll a concen- 

 trations were about the same or slightly higher 

 at station "P." More detailed seasonal obser- 

 vations from the mid-Subarctic, as well as a 

 comparison of plankton communities in the two 

 locations, will probably be necessary to explain 

 the observed differences in annual productivity. 



CONCLUSION 



The main conclusions drawn from the primary 

 productivity and related data are listed below: 



1. No significant differences in primary pro- 

 ductivity were found consistently among the var- 

 ious oceanographic domains or other waters of 

 the Subarctic Pacific Region except that Adak 

 Bay and the Coastal Domain south of Adak 

 Island were generally more productive than 

 areas farther south. 



2. The annual cycle of productivity is typical 

 for temperate oceans: productivity is low in win- 

 ter, increases in the spring until more or less of 

 a bloom develops, and declines by summer to rel- 

 atively steady intermediate levels. Productivity 

 in autumn and early winter was not studied. 



3. Light limits productivity during winter; 

 less light penetrates the surface than during 

 other seasons, and the greater thickness of the 

 mixed layer in the upper zone further reduces 

 the light available to the algae. 



4. During spring and summer, nutrients and 

 light are plentiful, but zooplankton appears to 

 graze the standing stock of phytoplankton to 

 relatively low levels — thereby limiting produc- 

 tivity of phytoplankton. 



611 



