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

The effect of under-ice irradiance on in situ growth and producbon of sea ice microalgae was investigated at McMurdo Sound, Antarctica in 1982. Five 100 m2 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 )rE m-2 S-'. Standing crop, growth rate and photosynthetic rate were greatest in snowfree ice (Q-0) where chlorophyll a concentration increased from 0.1 to 76 mg m-2 in the platelet layer (p = 0.41 d-') and from 0.05 to 9 nlg m-2 in bottom congelation ice (p = 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 bottonl ice approximates cumulative production. Peak algal standing crop at Q-0 was estimated to be 3.2 g C m-', based on a carbon to chlorophyll ratio of 38. However, net pnmary production based on in situ measurements of photosynthetic rate was 10-fold higher, at 41 g C m-' Thls f~nding suggests that prevlous estimates of sea ice production must be revised sharply upward.