Autotrophic flagellates and diatoms in the Northeast Water polynya, Greenland: summer 1993

Abstract

Abundance, biomass and distribution of autotrophic flagellates, autotrophic dinoflagellates and diatoms were measured in the Northeast Water (NEW) Polynya during summer, 1993, using epifluorescence microscopy (EFM) and high-pressure liquid chromotography (HPLC). Numerous discrepancies existed between results from EFM and HPLC. Phytoplankton communities determined microscopically at the depth of the chlorophyll maximum consisted primarily of flagellates in various size classes (average 1.3 × 107 cells 1−1, 8.5 μg Cl−1) and the small diatom Chaetoceros socialis (average 5×106 cells 1−1, 52 μg Cl−1), with subsurface maxima of both groups usually occurring close to the nitricline. A wide variety was observed in the composition of the phytoplankton community with respect to biomass of individual taxa. The biomass of autotrophic flagellates represented on average 23% of the total autotrophic biomass in all regions of the study area as defined by water mass type. Based on carbon units derived from pigment concentrations (HPLC), prasinophytes and haptophytes each contributed 24% to total flagellate (non-dinoflagellate) biomass and chrysophytes contributed 52%. In the northeast quadrant, influenced by the East Greenland Current, haptophyte contribution dropped to 9%. Most of the biomass of autotrophic flagellates was in cells < 8 μm in diameter; of diatoms, in cells < 10 μm in diameter. Highest picoplankton biomass (cells < 2 μm) occurred in ice-free water at moderate stability. The average contribution of autotrophic dinoflagellates to total autotrophic biomass was 6% (3 μg Cl−1, 1.7×105 cells 1−1). Ratios of microscopically measured autotrophic carbon to fluorometric chlorophyll a (i.e. community C:chl a) ranged from 11–265 (avg. 61), with maximum ratios attributable to regions dominated by C. socialis. Although differences between four areas of the NEW with respect to physical characteristics of the water column were observed, few corresponding biological differences could be verified. Similarly, phytoplankton community composition was weakly if at all correlated with physical or chemical characteristics of the water column. Future investigations should examine food web dynamics (i.e. grazing) and small-scale circulation to account for the mosaic pattern of phytoplankton features.