Abstract
Quantifying the temporal variability in phytoplankton productivity is essential for improving our understanding of carbon (C) and nitrogen (N) dynamics and energy flows in natural aquatic ecosystems. Samples were collected at three-day intervals from December 2018 to January 2019 from fixed station in Marian Cove, Antarctica to determine the C and N (NO 3 - and NH 4 + ) uptake by phytoplankton. Considerable fluctuations in the total C and N productivities were observed, which led to dynamic changes in the phytoplankton communities and a stronger coupling between the phytoplankton biomass. The increased rate of NO 3 - uptake coincided with an enhanced C uptake mainly by microphytoplankton (>20 µm), followed by an increase in NH 4 + uptake towards the end of sampling period. However, the <2 µm fraction (picophytoplankton) showed little variation in C and NO 3 - uptake, and the proportions of assimilated NH 4 + contributed to more than half of the total assimilated inorganic N. The increased NH 4 + did not increase the total phytoplankton biomass and C production. Interestingly, after January 9 (maximum chlorophyll a, C, and N uptake) there was a shift to a predominantly easterly wind (>6 m s -1 ), which rapidly decreased the total chl-a, C and N uptake rate to ~4% of the highest values (0.6 mg m -3 , 1.0 mg C m -3 h -1 , 0.1 mg N m -3 h -1 , respectively) on January 12. The phytoplankton community was also replaced by neritic and ice-related species. These findings suggest that strong temporal shifts in phytoplankton C and N assimilation are strongly influenced by external forces (wind stress).