Thick ice cover is a feature of cold-temperate, polar, and alpine lakes and rivers throughout much of the year. Our observations from Canadian lakes and rivers across the latitudinal gradient 46-80ЊN show that their overlying ice contains low concentrations of dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) relative to the underlying waters. The CDOM exclusion factor (water/ice) ranged from 1.4 to 114 and was typically greater than twice the exclusion factor for inorganic solutes. Application of synchronous fluorescence analysis to lake ice samples and experimentally frozen lakewater indicated that only less complex, lower molecular weight molecules were retained within the ice. Consistent with this analysis, DOC-specific absorption showed that the DOC in the ice was generally less colored than that in the underlying waters. The reduced CDOM absorption within the ice allowed relatively high ultraviolet (UV) transmission despite the elevated scattering within the ice and resulted in UV diffuse attenuation coefficients up to eight times lower in the ice than in the underlying waters. This relatively low attenuation by the ice would cause organisms trapped near the surface by inverse stratification to experience high UV exposure prior to ice breakup. The ice exclusion effect gives rise to a concentrated zone of CDOM and DOC that is likely to favor heterotrophic and mixotrophic processes and influence biogeochemical interactions.

) were observed in brackish waters of the Beaufort Sea. These results confirm that picophytoplankton can dominate not only in warm oligotrophic waters, but also in a perennially cold ocean during late summer. KEY WORDS:Abundance · Photosynthetic eukaryotes · Picophytoplankton · Biomass · Nanophytoplankton · Microphytoplankton · Canadian Arctic Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 54: [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70] 2009 1988). However, Richardson & Jackson (2007) challenged this view by showing that the share of picophytoplankton in carbon export can match their relative contribution to total net primary production due to the inclusion of small cells into large aggregates that sink rapidly or are grazed by mesozooplankton. Considering the findings of Richardson & Jackson (2007), the conventional view that picophytoplankton contribute little to carbon export should be revisited. Hence, both large and small phytoplankton play a crucial role in the marine biogeochemical cycle.Large phytoplankton cells, including diatoms, prymnesiophytes and dinoflagellates, produce seasonal blooms under specific hydrographic conditions . For instance, the production of large phytoplankton is governed by variations in the vertical stability of the water column, through its effects on nutrient replenishment and the residence time of algal cells in the euphotic zone (e.g. Tremblay et al. 1997). In addition, the duration of the production period is sensitive to the seasonal melt dynamics of sea ice (Fortier et al. 2002). In northern Baffin Bay (BB), an intense diatom bloom characterized by cells > 5 µm begins as early as the end of April when the North Water polynya opens up . In the Canadian Archipelago, particularly in Barrow Strait, the phytoplankton bloom typically develops in July and August, corresponding to the timing of the ice break-up for this region (Michel et al. 2006). In the Chukchi and Beaufort seas, high chlorophyll concentrations are observed in regions along the ice edge and are associated with an overwhelming predominance of diatoms and prymnesiophytes (Hill et al. 2005). In the Barents Sea, largecelled phytoplankton dominate during blooms at the marginal ice zone and are of particular importance for the production of organic matter and the vertical export of carbon .Several studies have shown that small phytoplankton cells (< 5 µm) can also play an important role in carbon fixation in the Arctic Ocean and adjacent seas (Legendre et al. 1993, Gosselin et al. 1997. Picophytoplankton contribute most of the production and biomass in warm and nutrient-poor waters (Agawin et al. 2000). Recent studies have shown that picophytoplankton are often well represented numerically in cold Arctic seawaters. Indeed, eukaryotic cells < 2 µm often dominate the phytoplankton assemblage, reaching up to 28 000 cells ml -1 during the initial spring bloom in the central Arctic Ocean but usually ranging between 1000 and 10 000 cells ml -1 during t...

Ultraviolet-B (UVB, 280-320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (Ͻ240 m) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (Ͻ1 m), heterotrophic flagellates (2-10 m), small phytoplankton (Ͻ5 m), large phytoplankton (5-20 m), and ciliates (15-35 m) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB conditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of heterotrophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.The pelagic planktonic community functions through a web of energy and nutrient exchanges mediated by a diverse array of producers and consumers, which ultimately depend on the energy supplied by sunlight. Following the discovery of stratospheric ozone depletion (Farman et al. 1985) and the resulting increase in intensity of biologically harmful UVB radiation (280-320 nm) reaching Antarctic waters, the majority of UVB studies have focused on phytoplankton be- AcknowledgmentsWe thank F. Rassoulzadegan, F. Azam, and T. Sime-Ngando for comments; C. Lovejoy and L. Bérard for help with the identification of some planktonic species; and D. Bourget and N. Lafontaine for nutrient analyses.This work was supported by NSERC of Canada, Fonds FCAR of Québec, and FODAR (University of Québec). International collaboration was made possible by NATO collaborative research grant (CRG 95139) to S.D. and P.M. This investigation is a contribution to the research programs of the Groupe de Recherche en Environnement Côtier.