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Although it is generally believed that cyanobacteria have high temperature optima for growth (> 20" C), mat-foming cyanobactm'a are dominant in many types of lakes, streams, and ponds in the Arctic and Antarctic. We studied the effect of temperature on growth (p) and relative pigment composition of 27 isolates of cyanobactm'a (matforming Oscillatoriaceae) from the Arctic, subarctic, and Antarctic to investigate whether they are a) adapted to the low temperature (i.e. psychrophilic) or b) tolerant of the low temperature of the polar regzons (i.e. psychrotrophic). We also derived a parabolic function that describes both the rise and the decline of cyanobactm'al growth rates with increasing temperature. The cyanobacteria were cultured at seuen different temperatures (5"-35" C at 5" C interuals), with continuous illumination of 225 pmol photons.m-2.s-1. The parabolic function jits the p-temperature data with 90 % conjidence for 75 % of the isolates. Among the 2 7 isolates of cyanobactm'a studied, the temperature optima (T,J for growth ranged from 15" to 35" C, with an average of 19.9" C. These results imply that most polar cyanobacteria are psychrotrophs, not psychrophiles. The cyanobactm'a grew over a wide temperature range (typically 20" C) but growth rates were low men at T& (average pmax of 0.23 2 0.069 d-I ) . extreme^ slow growth rates at low kmperature and the high temperature f i optimal growth imply that the cyanobact,ria are not adapted genetically to cold temperatures, which characterize their ambient environment. Other competitive advantages such as tolerance to desiccation, freeze-thaw cycles, and h g h t , continuous solar radiation may contribute to their dominance in polar aquatic ecosystems.

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Photosynthesis vs. irradiance relationships were determined for phytoplankton communities from seven lakes in the Canadian high Arctic, including ultraoligotrophic Char Lake, nutrient-enriched Meretta Lake, and two meromictic lakes. The derived photosynthetic parameters were low for all samples, with a mean (ϮSD) light-saturated photosynthetic rate (P ) of 0.46 (Ϯ0.28) g C g Ϫ1 chlorophyll a (Chl a) h Ϫ1 and a mean ␣ B (light-limitation parameter) parameters for phytoplankton in Arctic and Antarctic lakes are three-to sixfold lower than for marine algae, ice algae, and cultures over the same low-temperature range. This may be the result of more severe nutrient stress in high-latitude lakes relative to polar marine environments and to the persistence of nonactive pigments in cold freshwaters.

Dinoflagellates have substantially lower growth rates than other taxa of similar size. These low growth rates have been suggested to reflect the lower chlorophyll a to carbon ratio (Chl a:C) in dinoflagellates, but that speculation has never been widely tested. This study tests if the variations in growth rates among taxa are related to differences in Chl a:C using published data. I collected 92 data entries from the literature representing 31 species, mostly from two divisions (Chrysophyta and Pyrrophyta), and found a significant relation (r2= 0.39) between growth and Chl a:C. Since Chl a:C is almost independent of C content, I also developed a growth model using both C and Chl a:C. Together, the two variables explain 68% of variation in algal growth. However, a further 6.4% of the variance in growth can still be attributed to phyletic differences. Low Chl a:C is only a partial explanation for the low growth rates of the dinoflagellates.