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Comprehensive investigations of the Canadian Arctic during late summer and early fall revealed the widespread occurrence of long-lived subsurface chlorophyll maxima (SCM) in seasonally ice-free waters. The vertical position of the SCM corresponded with the depth of the subsurface biomass maximum (SBM), at least in Baffin Bay, suggesting that SCM could be an important source of carbon for the food web. Most of these SCM were located well below the pycnocline in close association with the nitracline, implying that their vertical position was driven mainly by a shortage of inorganic nitrogen in the upper euphotic zone. The diversity of SCM configurations with respect to physical properties of the water column complicates the estimation of euphotic-zone chlorophyll and primary production from surface properties. High photosynthetic yields (F v /F m ) showed the phytoplankton to be photosynthetically competent and well acclimated to conditions of irradiance and nutrient supply near the surface and at the SCM. A well-defined primary nitrite maximum was associated with the SCM in the southwest Canadian Arctic, but not in the northeast where nitrite concentrations were highest much below the euphotic zone. This contrast is consistent with differences in vertical stratification, the light -dark cycle and, possibly, the physiological state and taxonomic composition of the phytoplankton community at the SCM. This study demonstrates that the SCM, once regarded as anecdotal due to under-sampling, are a dominant feature of the Arctic Ocean that should be considered in remote sensing studies and biogeochemical models.

Arctic marine unicellular eukaryotes are composed of microalgae and non-autotrophic protists. These eukaryotes comprise a well-diversified group of organisms that are either adapted to live in the upper water column of coastal and oceanic regions, here defined as phytoplankton/ pelagic communities, or in bottom horizons of sea ice and known as sympagic/sea-ice-associated communities. There are approximately 5,000 recognized legitimate marine phytoplankton species and an unknown number of sympagic eukaryotes. Although pelagic and sea-ice eukaryotes have been described since the exploration phase of the Arctic regions up to the early twentieth century, no synthesis regarding information from all Arctic seas have been undertaken, and no exhaustive current information provides the exact number and composition of species on a pan-Arctic scale. In a first attempt to assess the pan-Arctic diversity of pelagic and sea-ice eukaryotes, a wealth of data from various sources (e.g., scientific publications, unpublished reports, databases) were reviewed, while taxonomic data were confirmed with current nomenclature and classification. We report a total of 2,106 marine singlecelled eukaryote taxa with 1,874 phytoplankton and 1,027 sympagic taxa from four grouped pan-Arctic regions, namely Alaska, Canada, Scandinavia including Greenland and the Russian Federation. Both phytoplankton and sympagic taxa were present in four of the six supergroups of eukaryotes described by Adl et al. (J Eukaryot Microbiol 52:399-451, 2005), which are Archaeplastida (chlorophytes and prasinophytes), Chromalveolata (e.g., chrysophytes, cryptophytes, diatoms, dictyochophytes, dinoflagellates and prymnesiophytes), Excavata (euglenids) and Opisthokonta (choanoflagellates). The bulk of this marine biodiversity of Arctic microorganisms consists of large cells (>20 μm) mainly due to examination at low magnification under light microscopy. Future efforts should focus enhancing our knowledge of the biological diversity of small cells (<20 μm), which represent less than 20% of our actual biodiversity assessment of pan-Arctic regions.

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) 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...