Phytoplankton productivity and its response to higher light levels in the Canada Basin
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Phytoplankton productivity in the Canada Basin was measured in the late summer season, from mid-September to mid-October 2009, using a 13C–15N dual tracer technique. To understand potential production changes associated with sea ice melting in the Arctic Ocean, we examined the effects of light enhancement and nitrate enrichment on the carbon productivity of phytoplankton from the chlorophyll a maximum layer. The daily carbon productivity in the Canada Basin in 2009 was very low, with a mean of 4.1 mg C m−2 (SD = 3.6 mg C m−2), compared with those reported in previous studies in the region. Among several explanations, the most plausible reason for the large difference in carbon productivity between this and the previous studies was strong seasonal variation in biomass and photosynthetic rate of the phytoplankton in the study region. Based on our results from light enhancement and nitrate enrichment experiments, we found that carbon productivity of phytoplankton in the chlorophyll a maximum layer could be stimulated by increased light condition rather than nitrate addition. Thus, potentially increasing light availability from current and ongoing decreases in the sea ice cover could increase the carbon production of the phytoplankton in the chlorophyll a maximum layer and produce a well-developed maximum layer at a deeper depth in the Canada Basin.