Phytoplankton growth and microzooplankton grazing along a sub-Arctic fjord (Godthåbsfjord, West Greenland)

We evaluated the role of microzooplankton (sensu latto, grazers <500 µm) in determining the fate of phytoplankton production (PP) along a glacier-to-open sea transect in the Greenland subarctic fjord, Godthåbfjord. Based on the distribution of size fractionated chlorophyll a (chl a) concentrations we established four zones: i) Fyllas Bank, characterized by deep chl a maxima (ca. 30-40 m) consisting of large cells; ii) the mouth and main branch of the fjord, where phytoplankton was relatively homogeneously distributed in the upper 30 m layer; iii) inner waters influenced by glacial melt water and upwelling, with high chl a concentrations (up to 12 µg L-1) in the > 10 µm fraction within a narrow (2 m) subsurface layer; and iv) the Kapisigdlit branch of the fjord, ice-free, and characterized with a thick and deep chl a maximum layer. Overall, microzooplankton grazing impact on primary production was variable and seldom significant in the Fyllas Bank and mouth of the fjord, quite intensive (up to > 100% potential PP consumed daily) in the middle part of the main branch of the fjord and Kapisigdlit branch, and rather low and unable to control the fast growing phytoplankton population inhabiting the nutrient rich waters in the upwelling area in the vicinity of the glacier. Most of the grazing impact was on the <10 µm phytoplankton fraction, and the major grazers of the system seem to be > 20 µm microzooplankton, as deducted from additional dilution experiments removing > 20 µm. Overall, little or no export of phytoplankton out of the fjord to the Fyllas Bank can be deducted from our data