TEMPORAL PATTERNS OF PRIMARY PRODUCTION IN A LARGE ULTRA-OLIGOTROPHICANTARCTIC FRESH-WATER LAKE

A large ultra-oligotrophic Antarctic freshwater lake, Crooked Lake, wa s investigated between January 1993 and November 1993. The water colum n supported a small phytoplankton community limited by temperature, nu trient availability and, seasonally, by low photosynthetically active radiation. Chlorophyll a concentrations were consistently low (< 1 mu g l(-1)) and showed no obvious seasonal patterns. Production rates wer e low, ranging from non-detectable to 0.56 mu g C l(-1) h(-1) with hig hest rates generally occurring towards the end of the austral winter a nd in spring. The pattern of carbon fixation indicated that the phytop lankton was adapted to low light levels. Chlorophyll a specific photos ynthetic rates (assimilation numbers) ranged fi-om non-detectable to 1 .27 mu gC (mu g chlorophyll a)(-1)h(-1). Partitioning of photosyntheti c products revealed carbon incorporation principally into storage prod ucts such as lipids at high light fluxes with increasing protein synth esis at depth. With little allochthonous input the data suggest that l ake dynamics in this Antarctic system are driven by phytoplankton acti vity.