PHOTOSYNTHESIS‐IRRADIANCE RELATIONSHIPS IN SEA ICE MICROALGAE FROM McMURDO SOUND, ANTARCTICA¹

ABSTRACT Sea ice microalgae in McMurdo Sound, Antarctica were examined for photosynthesis‐irradiance relationships and for the extent and time course of their photoadaptation to a reduction in in situ irradiance. Algae were collected from the bottom centimeter of coarse‐grained congelation ice in an area free of natural snow cover. Photosynthetic rate was determined in short term (1 h) incubations at −2° C over a range of irradiance from 0 to 286 μE·m −2 ·s −1 . Assimilation numbers were consistently below 0.1 mg C·mg chl a −1 ·h −1 . The I k 's 3 averaged only 7 μE·m −2 ·s −1 , and photosynthesis was inhibited at irradiances above 25 μE·m −2 ·s −1 . Photosynthetic parameters of the ice algal community were examined over a nine day period following the addition of 4 cm of surface snow while a control area remained snow‐free. A reduction of 40% in P m B relative to the control occurred after two days of snow cover; α, β, I k , and I m were not significantly altered. Low assimilation numbers and constant standing crop size, however, suggested that the algal bloom may have already reached stationary growth phase, possibly minimizing their photoadaptive response.