LARRANCE: PRIMARY PRODUCTION IN MID-SUBARCTIC REGION 



. Pr/Co 



pk/Co 



SEPTEMBER 1966 



JANUARY 

 FEBRUARY 1967 



JUNE JULY 1967 



Figure 6. — Ratios of Pr/C„ and Py^/C^ in the Subarctic 

 Region along long 175°-17G° W, 1966-67. 



duction was low in the winter, increased sig- 

 nificantly in March, and was relatively steady 

 at intermediate levels throughout the summer. 

 Chlorophyll a also increased between February 

 and late March as a consequence of high pro- 

 duction but decreased significantly during the 

 spring and decreased slightly throughout the 

 summer. Some reasons for these changes can 

 be inferred from the nutrient and zooplankton 

 data and are discussed later. 



Although primary production apparently con- 

 tinued at a high rate through the spring, chloro- 

 phyll a concentrations were significantly less in 

 May and June than in March. Two probable 

 reasons for this decrease are a decrease in cell 

 chlorophyll content and an increased loss rate, 

 primarily due to grazing. It is impossible to 

 ascertain from the data which one of these 

 causes was most important, but some trial cal- 



culations can help explore the problem. The 

 standing stock (Si) at t days (time between 

 consecutive cruises) was predicted by the simple 

 growth equation 



S, = Soe""-"" 



where So is the initial standing stock and « and h 

 are growth and loss coefficients. Standing stock 

 was expressed in mg chlorophyll a/m^, and P/Ca 

 ratios were multiplied by \/F (ratio of cell 

 chlorophyll a to cell carbon) to give growth co- 

 efficients (o) in units of day^'. 



The growth rate (a) varied with time be- 

 cause P/Ca varied and \/F was assigned values 

 according to those reported elsewhere (Strick- 

 land, 1960; Eppley, 1968; and Strickland et al, 

 1969). Populations in nutrient-rich water 

 under suboptimum light intensities, conditions 

 extant in February and March, contain larger 

 amounts of chlorophyll per unit carbon than 

 those in nutrient-poor water under brighter 

 light. Eppley (1968) found F values of about 

 30 for deep nutrient-rich water and 90 for shal- 

 low water depleted of nutrients. Values of \/F, 

 therefore, were assumed to be 0.04 in February 

 and March and 0.01 in June. The June value 

 is probably too low because the water still con- 

 tained ample nutrients (although lower than in 

 March) for vigorous growth, but was selected 

 to maximize the decrement afl^orded to decreas- 

 ing cell chlorophyll content and therefore min- 

 imize St in June. 



Results of the calculations show (Table 3) that 

 changes in cell chlorophyll content could account 

 for only part of the decrease in chlorophyll con- 

 centration between March and June. The loss 

 coefficient ( b ) was computed for the period Feb- 

 ruary to March and assumed to remain constant 

 through June. The computed standing stock 

 {S,i) in June was 1096, about 80 times as high 

 as the observed value. (For comparison, stand- 

 ing stocks were also computed for X/F = 0.04 

 and \/F = 0.01, Table 3.) Since this (1096) 

 is the minimum that could be expected from a 

 loss of cell chlorophyll, grazing must have in- 

 creased during the period to further decrease 

 the chlorophyll concentration to its observed 

 level. A concomitant increase in zooplankton 

 corroborates this conclusion (discussed later). 



605 



