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 efficiencies ( 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.

Abundance and activity of picoplankton (here defined as cells passing a 1 pm Nucleporem screen) were studied in northern Foxe Basin, eastern Canadian Arctic. Substantial proportions (10 to 70 %) of the chlorophyll a content, ribulose-1,5-bisphosphate carboxylase activity (RuBPC, E.C. 4.1.1.39) and autofluorescent bodies present in whole seawater samples passed a 1 p screen in intact, photoautotrophic particles. A smaller fraction (10 to 25 %) of the light-dependent 14C fixation was found in this picoplankton fraction, the difference possibly explained by a selective effect of screening on photosynthetic activity rather than by heterotrophic uptake of algal exudates. About 10 % of the whole sample RuBPC was found to pass a 0.2 pm diameter screen, indicating the presence of autotrophy in marine ultramicrobacteria (ferntoplankton). A potential for growth in the ferntoplankton fraction was also indicated by substantial fixation of tritiated nucleic acid precursors into macromolecules.

Comparison of 2 water samples, one collected from 10 m, the other from the aphotic zone (1000 m) on the Costa Rica Dome in the eastern tropical Pacific Ocean, revealed the presence in both samples of pigmented cells of several diatoms, dinoflagellates and coccoid organisms. Measurements of carbon assimilation rates in temperature-controlled incubators across a light gradlent demonstrated that the assimilation number (mg C [mg Chl a]-' h-') of the 1000 m sample was about 0.8, slmilar to that of the 10 m sample. The ratios of RuBP carboxylase to other carboxylating enzymes were also similar between 10 m and the aphotic zone. However, the initial slope a and the inhlbltion parameter P were considerably higher for the deep sample than for the 10 m sample. Possible mechanisms by which these viable algae reached the aphotic zone are discussed.