Light and nutrient effects on microbial communities collected during spring and summer in the Beaufort Sea

Abstract

Between autumn 2003 and summer 2004, the icebreaker CCGS ‘Amundsen’ occupied a station in Franklin Bay, Beaufort Sea. Two microcosm experiments were carried out in spring, while the water column was covered by sea ice, to test whether phytoplankton assemblages sampled from ice-covered (spring) surface waters would be able to bloom when exposed to increased light and to what extent this bloom would be limited by light or nutrients. Two additional experiments were carried out during a sea-ice free period in summer. For the spring experiments (April 2004), surface water collected through an ice hole was distributed into 10 l Nalgene bottles and subjected to different photosynthetically available radiation (PAR; 5 to 60 μmol photons m–2 s–1) and nutrient addition treatments. A similar setup was repeated in July and August 2004. In the spring experiments, chlorophyll a (chl a) concentration showed an initial decline, probably reflecting a combination of autotrophic cell losses and photoacclimation effects. After about 2 wk, chl a started to increase exponentially at PAR above 10 μmol photons m–2 s–1, due to the proliferation of autotrophic nanoflagellates. In summer, chl a increased immediately after enclosure, mainly due to diatom (Thalassiosira/ Porosira) growth. In spring, nutrient addition had no effect on the net rate of chl a accumulation, while phytoplankton increased faster and reached higher biomass in the nutrient-amended bottles in summer. The relationships between nutrient consumption and production of chl a and particulate organic matter in our experiments suggested an important utilization of dissolved organic nutrients.