Dynamic of primary producers at a receding ice edge during early summer in the Canadian High Arctic

Sea ice strongly influences the dynamics of primary producers in the Arctic. In late spring-early summer, the snowmelt results in the formation of melt ponds on the surface of the sea ice that can be colonized by unicellular algae. Subsequently, the gradual melting of sea ice stabilizes the upper water column and increases the quantity of light reaching the surface, leading to the development of phytoplankton blooms. Due to global warming, these phenomena occur earlier in the season and on a larger spatial scale. To better quantify these effects, chlorophyll a (chl a) concentrations and primary production were measured in open water, at the ice edge and under the sea ice as well as in melt ponds in Lancaster Sound (Canadian Arctic) between 17 and 23 July 2014. The algal biomass and productivity were dominated by small cells (0.7-5 µm) in melt ponds and by large cells (>5 µm) in the water column under the ice and at the ice edge. In melt ponds, despite low algal biomass (<0.5 mg chl a m^-3), primary production rates were relatively high with values ranging from 1 to 21 mg C m^-3 d^-1. In the water column, primary production and chl a biomass ranged from 190 to 1409 mg C m^-2 d^-1 and from 20 to 82 mg m^-2, respectively. Maximum values were measured under the ice and at the ice edge on the western end of Lancaster Sound. These new measurements show the high diversity of primary production sources and their relative importance in Lancaster Sound.