Primary Production and Nutrient Assimilation by Natural Phytoplankton Populations of the Eastern Canadian Arctic

Phytoplankton biomass (chlorophyll a) and primary production rates in Baffin Bay during summer 1978 were comparable to levels reported for other open water arctic and subarctic regions. Values were moderately high ([Formula: see text] mg Chl∙m −2 ; 227 mg C-fixed∙m −2 ∙d −1 ) considering the low mixed-layer nutrient (nitrogen) concentrations, low ambient temperatures ([Formula: see text] euphotic zone = −0.2 °C), and variable and moderately low daily solar radiation ([Formula: see text] MW∙m −2 ). Biomass maxima were consistently found at or near the bottom of the euphotic zone, and were 6 times higher than surface values on the average. Nitrate and ammonium were assimilated in approximately equal proportions despite the relatively greater abundance of nitrate in the euphotic zone, particularly below the mixed layer. Average C:N assimilation ratios were slightly lower (5:1) than the chemical composition ratio of the particulate matter (7:1). High phosphate assimilation rates reflected the abundance of this nutrient in the euphotic zone and resulted in low C:P (22:1) and N:P (6:1) assimilation ratios. Growth rates computed from carbon and nitrogen (NO 3 − + NH 4 + ) assimilation rates averaged 0.31 and 0.35 doublings∙d −1 , respectively, for the euphotic zone, and were half the maximum expected growth rates for prevailing water temperatures and optimal conditions of light and nutrients. Baffin Bay phytoplankton populations exhibited no obvious signs of severe nitrogen limitation despite low euphotic zone concentrations of the nutrient. Furthermore, the strong correspondence between: (1) normalized primary production rates (photosynthetic index) and incident solar radiation and (2) growth rates and incubation temperatures suggests that nutrients may play a relatively less important role in controlling arctic primary production than previously considered.Key words: phytoplankton, primary production, nutrients, arctic, light, and temperature