Physical control of spring–summer phytoplankton dynamics in the North Water, April–July 1998

Abstract

Abstract A 4-month multidisciplinary expedition, beginning at the end of winter to track the spring phytoplankton bloom to its termination in summer, was conducted from April to July 1998. The aim of the expedition was to investigate possible mechanisms responsible for the high biological productivity of the North Water, the most productive and largest polynya in the Northern Hemisphere. The aims of the present study were to investigate: (1) the effects of the physical forces, driving the formation of the polynya, on the dynamics of the phytoplankton stock in the polynya over the spring–summer period; and (2) the factors that limit the maximum biomass of phytoplankton. Contrasting physical characteristics, including ice concentration, surface mixed-layer depth (MLD), salinity and temperature in the surface mixed layer (SML), were observed between the east and west sides of the polynya. The Greenland (eastern) side of the polynya was characterized by a shallow SML, warm temperature, and high salinity relative to the Ellesmere Island, Canada (western) side. Chlorophyll a (Chl)>1 mg m −3 was observed in late April on the eastern side, and in late May on the western side. The peak phytoplankton bloom occurred in the southeastern part of the polynya, with average Chl of 15 mg m −3 (240–300 mg m −2 ) in the euphotic zone during the end of May and beginning of June. The increased phytoplankton biomass was associated with higher salinity and warmer temperature on the eastern side of the polynya. Low temperature in April and May decoupled the increase of Chl biomass from the shallowed SML, as predicted by Sverdrup's model. As Chl in the euphotic zone increased to 5 mg m −3 , the proportion of light absorption by phytoplankton could not increase further with Chl biomass, which might have limited the increase of primary production in the water column. Although the initial nutrient inventories largely determined the maximum biomass of phytoplankton, self-shading occurred in the build-up phytoplankton biomass to ∼5 mg m −3 , which retarded the timing of the peak bloom. Both sensible heat due to deep warm water entrainment into the SML and the biological heating effect via phytoplankton light absorption appear to contribute to the pattern of phytoplankton distribution in the North Water.