Glacial melt water stratification promotes nutrient limitation in Kongsfjorden, Spitsbergen, thereby altering phytoplankton composition but not productivity.

Effects of nutrient limitation on post bloom phytoplankton composition and productivity were investigated along a 3 station transect in Kongsfjorden (Spitsbergen) in June 2015. Increasing glacial melt water input intensified stratification at all locations. Short term (60 h) nitrate addition experiments revealed increasing N-limitation of surface phytoplankton over time in the middle of the fjord. This coincided with a pronounced change in the flagellated phytoplankton community, replacing large (10 μm) chrysophytes (85% of chl-a) with smaller haptophytes (60% of chl-a) and prasinophytes (20% of chl-a). Changes were less pronounced near the glacier, where average surface nutrient concentrations were higher. Surface chl-a concentrations (5 m), chl-a normalized maximum carbon fixation rates and water column productivity were variable and showed no clear trends over time. The initial rate of carbon fixation correlated inversely to irradiance and surface salinity, in addition to a positive correlation to chrysophyte chl-a. We conclude that N-limitation causes pronounced changes in Kongsfjorden phytoplankton composition and size structure, with minor effects on chlorophyll-a and water column productivity.