Algal colonization of annual sea ice in the hlgh Arctic approximates plate culture. presenting a model system for physiological studies of natural populations of marine microalgae. Time series of observations were made in the Northwest Passage during the latter half of the spring bloom. Although in situ temperature, salinity and irradiance were nearly constant, the photosynthetic performance of ice algae as indicated by maximum assimilation rates (pmB, mg C mg chl-' h-') and photosynthetic

more »

... iencies ( a , nlg C mg chl-' h-' (p E m-' s-')-') displayed large, low-frequency fluctuations. In contrast, the photoadaptive index, Ik (LIE m-* S-'), varied little up until the last few days of our study when snow cover melted and transmitted light increased rapidly. When compared to cells from other snow covers or light histories, algal populations from a snow-free area exhibited higher asslmdation rates and photoadaptive indices but had similar photosynthetic efficiencies and lower standing stocks. Nutrient fluxes in the 'surface mixed layer' also varied by about an order of magnitude over the fortnightly tidal cycle. Tidally dominated vertical mixing results in a pulsed nutrient regime which is apparently reflected in a modulation of algal photosynthesis and growth.