Sea ice microbial communities. VI. Growth and primary production in bottom ice under graded snow cover

Abstract

The effect of under-ice irradiance on in situ growth and production of sea ice microalgae was investigated at McMurdo Sound, Antarctica in 1982. Five 100 m 2 quadrats on annual ice were delimited in early October with 0, 5, 10, 25 and 100 cm snow cover; under-ice irradiances ranged from <0.02 to 100 μE m- 2 s-1. Standing crop, growth rate and photosynthetic rate were greatest in snow-free ice (Q-0) where chlorophyll a concentration increased from 0.1 to 76 mg m -2 in the platelet layer (μ = 0.41 d -1 ) and from 0.05 to 9 mg m -2 in bottom congelation ice (μ = 0.29 d -1 ) over 5 wk. Blooms occurred later in quadrats with 5, 10, and 25 cm snow cover,- however, growth rates were less than half that in Q-0. We tested the hypothesis that microalgal standing crop in bottom ice approximates cumulative production. Peak algal standing crop at Q-0 was estimated to be 3.2 g C ixT 2 , based on a carbon to chlorophyll ratio of 38. However, net primary production based on in situ measurements of photosynthetic rate was 10-fold higher, at 41 g Cm -2 This finding suggests that previous estimates of sea ice production must be revised sharply upward.