Environmental forcing of phytoplankton community structure and function in the Canadian High Arctic: contrasting oligotrophic and eutrophic regions

We assessed phytoplankton dynamics and its environmental control across the Canadian High Arctic (CHA). Environmental (hydrographic, atmospheric, sea ice conditions) and biological variables (phytoplankton production, biomass, composition) were measured along 3500 km transects across the Beaufort Sea, the Canadian Arctic Archipelago, and Baffin Bay during late summer 2005, early fall 2006 and fall 2007. Phytoplankton production and chlorophyll a (chl a) biomass were measured at 7 optical depths, including the depth of subsurface chl a maximum (Z(SCM)). Phytoplankton taxonomy, abundance, and size structure were determined at the ZSCM. Redundancy analyses and non-metric multidimensional scaling were used to assess relationships between phytoplankton composition in relation to biological and environmental variables. In late summer/fall, the CHA was characterized by (1) an oligotrophic flagellate-based system extending over the eastern Beaufort Sea, the peripheral Amundsen Gulf, and the central region of the Canadian Arctic Archipelago; and (2) a eutrophic diatom-based system located in Baffin Bay, Lancaster Sound, and in a hotspot in the central Amundsen Gulf. The oligotrophic regions were characterized by low production and biomass of large phytoplankton cells (> 5 mu m) and relatively high abundance of eukaryotic picophytoplankton (< 2 mu m) and unidentified nanoflagellates (2-20 mu m). The eutrophic regions were characterized by high production and biomass of large cells and relatively high abundance of centric diatoms, mainly Chaetoceros spp. The distinction between the 2 regimes was explained by differences in stratification (density gradient) and nitrate concentrations at the Z(SCM). This study demonstrates the key role of vertical mixing and nutrient input in shaping the structure and function of phytoplankton communities in the CHA, showing how ongoing environmental changes have the capacity to alter the diversity of biogeographic regions in the CHA.