An in situ incubation technique used successfully to measure the photosynthetic carbon assimilation of internal algal assemblages within thick multiyear Arctic sea ice was developed and improved to measure the photosynthetic carbon assimilation within young sea ice only 50 cm thick (Eastern Weddell Sea, Antarctica). The new device enabled some of the first precise measurements of in situ photosynthetic carbon assimilation in newly formed Antarctic sea ice.