First in situ estimations of small phytoplankton carbon and nitrogen uptake rates in the Kara, Laptev, and East Siberian seas

Abstract. Carbon and nitrogen uptake rates by small phytoplankton (0.7–5 µ m) in the Kara, Laptev, and East Siberian seas in the Arctic Ocean were quantified using in situ isotope labeling experiments; this research, which was novel and part of the NABOS (Nansen and Amundsen Basins Observational System) program, took place from 21 August to 22 September 2013. The depth-integrated carbon (C), nitrate ( NO 3 - ), and ammonium ( NH 4 + ) uptake rates by small phytoplankton ranged from 0.54 to 15.96 mg C m −2 h −1 , 0.05 to 1.02 mg C m −2 h −1 , and 0.11 to 3.73 mg N m −2 h −1 , respectively. The contributions of small phytoplankton towards the total C, NO 3 - , and NH 4 + varied from 25 % to 89 %, 31 % to 89 %, and 28 % to 91 %, respectively. The turnover times for NO 3 - and NH 4 + by small phytoplankton found in the present study indicate the longer residence times (years) of the nutrients in the deeper waters, particularly for NO 3 - . Additionally, the relatively higher C and N uptake rates by small phytoplankton obtained in the present study from locations with less sea ice concentration indicate the possibility that small phytoplankton thrive under the retreat of sea ice as a result of warming conditions. The high contributions of small phytoplankton to the total C and N uptake rates suggest the capability of small autotrophs to withstand the adverse hydrographic conditions introduced by climate change.