Large seasonal variation in phytoplankton production in the Amundsen Sea

To better estimate annual primary production in the Amundsen Sea, which is one of the highest productivity regions in the Southern Ocean, the seasonal variations in carbon and nitrogen uptake rates of phytoplankton were investigated in this study. Based on 13C–15N dual isotope tracer techniques, the average daily productivities for the Amundsen polynya (AP), Pine Island polynya (PIP) and non-polynya regions were 0.25, 0.16 and 0.12 g C m−2 day−1, respectively. The average daily uptake rates of total nitrogen were 60.2, 53.5 and 34.8 mg N m−2 day−1 for the AP, PIP and non-polynya stations, respectively. In spite of the high concentration of nitrate in the Amundsen Sea, daily nitrate uptake rates (mean ± SD = 0.02 ± 0.01 g N m−2 day−1) were lower than ammonium uptakes for all productivity stations in this study, which resulted in a significantly lower f-ratio (mean ± SD = 0.44 ± 0.24) than that (mean ± SD = 0.71 ± 0.15) of the previous year. The substantially lower uptake rates of carbon and nitrogen and the f-ratio, especially in the AP, are due to a large seasonal variation in the uptake rates mainly caused by the shorter daytime duration and partly due to lower light availability induced by deeper mixed conditions in the present study compared with the previous study in 2010/2011. The large seasonal variation in daily phytoplankton production should be considered to better estimate annual production as a basic food source for higher trophic levels in the Amundsen Sea.