Seasonal Inorganic Carbon and Nitrogen Transport by Phytoplankton in an Arctic Lake

Abstract

Euphotic zone profiles of dissolved inorganic carbon [Formula: see text] and nitrogen [Formula: see text] transport were taken from Toolik Lake, Alaska, at approximately 10-d intervals through 100-d growing seasons in 1980 and 1981. Rates of DIC transport ranged from <0.2 to 7.8 μmol∙L −1 ∙d −1 , with the maximum always at 0–1 m. Nitrate and ammonium transport rates ranged from <0.1 to 3 and 0.4 to 8 nmol∙L −1 ∙d −1 , with the vertical pattern variable. For the 1980 and 1981 investigative periods, DIC transport was 622 and 504 mmol∙m −2 (7.5 and 6.0 g C∙m −2 ), making Toolik one of the most oligotrophic lakes on record. DIN transport for the respective years was 34 and 41 mmol∙m −2 , giving molar C/N transport ratios of 18 and 12. For both years,[Formula: see text] was <20% of total DIN transport. Half-saturation constants for [Formula: see text] and [Formula: see text] transport were similar, averaging [Formula: see text] 0.11 ± 0.08 and 0.15 ± 0.13 μmol∙L −1 . During the ice-free period, mean turnover times were also comparable at 6.3 ± 8.2 [Formula: see text] and 2.6 ± 1.2 d [Formula: see text], while relative preference indices for both nutrients were often near 1.0, indicating transport equitable with availability. The total data show a population well adapted to utilize consistently low ambient concentrations of [Formula: see text] and [Formula: see text] (0.05–0.20 μmol∙L −1 ) in the ice-free season, but incapable of exploiting elevated (2–3 μmol∙L −1 ) under-ice levels of [Formula: see text].