Uptake rates of dissolved inorganic carbon and nitrogen by under-ice phytoplankton in the Canada Basin in summer 2005

The Ocean Exploration cruise in June-July 2005 allowed detailed description of the in situ under-ice primary productivity of phytoplankton in the Canada Basin. On the basis of a C-13-N-15 dual isotope tracer technique, the estimated daily carbon production rate of under-ice phytoplankton ranged from 20.4 to 178.3 mg C m(-2) day(-1), whereas the daily nitrogen production rate ranged from 5.5 to 50.9 mg N m(-2) day(-1). These ranges are much higher than those recorded in previous studies, probably due to high seasonal and regional differences in the basin. Reduced nitrogen limitation for phytoplankton and the relative light attenuation through different sea-ice conditions may also be factors influencing the higher carbon and nitrogen uptake rates in 2005. Based on this study, new carbon production by phytoplankton growing beneath sea ice is estimated to range from 0.66 to 6.75 g C m(-2) year(-1), indicating that 25.2-66.4% of total annual primary production might potentially be exported from the euphotic zone in the deep Canada Basin.