Productivity of sea ice algae: In situ vs. incubator methods

Abstract

Algae of annual sea ice in the high Arctic (Resolute Passage, N.W.T.) gave little evidence of serious photosynthetic impairment during the seasonal decline of the bottom ice bloom when measured in a light-gradient incubator using a popular technique. Measurements made in situ with a novel sampling and incubating device, however, yielded much lower production rates than expected from the incubator measurements. Maximum assimilation numbers averaged 0.223 mgC·mgChl a −1·h −1 in incubator experiments but only 0.018 in situ, while photosynthetic efficiency averaged 0.0137 and 0.0069 [mgC·mgChl a −1 ·h −1]·[ μmol photons·m −2·s −1] −1 in the incubator and in sutu, respectively. Problems of tracer diffusion, temperature differentials and extracellullar release of dissolved organic carbon can help explain the discrepancy, but differences in the spectral availability of light (PUR) between incubator and estimated in situ conditions would tend to worsen the disagreement. Bottom ice algae appear to remain viable and potentially productive during the decline of the bloom, but may achieve relatively little production in situ, depending on which technique for measuring production is considered most accurate.