Glacial melt water stratification driven nutrient limitation effects on Kongsfjorden post bloom phytoplankton composition, productivity, and photophysiology

Effects of nutrient limitation of post bloom phytoplankton composition, productivity and photophysiology were investigated at 3 locations (M: mid, J: jetty, G: glacier front) in a high Arctic fjord (Kongsfjorden, 79˚N) during June 2015. Glacial melt water intensified stratification during this period in all locations, whereas nitrate concentrations were low (average 0.2±μM), indicating post bloom conditions. Short term (60 h) nitrate addition experiments revealed increasing N-limitation of surface phytoplankton at central Kongsfjorden. This coincided with a pronounced change in the flagellate dominated phytoplankton community, replacing large (10 μm) chrysophytes (85% of chl-a) with smaller (5-2 μm) haptophytes (60% of chl-a) and prasinophytes (20% of chl-a). These changes were less pronounced at station G, where average surface nutrient concentrations were higher. Surface chl-a concentrations (5 m) were variable and showed no clear trends during the onset of nutrient limitation and changes in taxonomic composition. Furthermore, chl-a normalized maximum carbon fixation rates and water column productivity showed no significant trends. The initial rate of carbon fixation correlated to irradiance and surface salinity, in addition to chrysophyte chl-a. Active fluorescence (FRRf) derived chl-a normalized photosynthetic characteristic were more strongly associated to changes in taxonomic composition and chl-a than to environmental variables and N-limitation. FRRf based photosynthesis characteristics showed opposing trends compared to the carbon incorporation experiments, changing the electron requirement for carbon fixation from 7.7± in the beginning of June to 17± electrons at the end of June. We concluded that N-limitation caused pronounced changes in phytoplankton composition and size structure, with minor effects on chlorophyll-a and water column productivity.