Microzooplankton grazing on the picophytoplankton and bacteria in the Chukchi Sea and East Siberian Sea, Arctic Ocean

In the summer of 2012, we measured microzooplankton grazing rate on picophytoplankton and bacterioplankton in the Chukchi Sea and East Siberian Sea using the icebreaker R/V Araon as part of the Korean Arctic Research Program. A variety of environmental conditions and trophic condition were encountered, from low chlorophyll-a (< 0.1 ug L^-1) in the Chukchi borderland to diatom bloom (maximum 17.1 ug L^-1) in the northern part of the East Siberian Sea which is characterized by high phytoplankton biomass driven by the influx of more productive waters from the river. Of the microzooplankton, naked ciliates dominated in low chlorophyll-a concentration area and small-HDF dominated in high chlorophyll-a concentration sites. Picophytoplankton biomass accounted for 11 to 83% of total phytoplankton and for a greater percentage in the Chukchi borderland (average 70%). Microzooplankton grazing rate varied by the assemblage composition of picoplankton and microzooplankton. Microzooplankton exerted higher grazing pressure on picophytoplankton compared to bacterioplankton. Picophytoplankton growth rate and mortality rate ranged from undetectable (i.e. not significant) to 2.0 d^-1 and undetectable to 2.4 d^-1, respectively. Microzooplankton removed >100% daily picophytoplankton production, and grazing rate was highest in the Chukchi borderland. Bacterioplankton growth rate and mortality rate ranged from undetectable (i.e. not significant) to 1.74 d^-1 and undetectable to 1.07 d^-1, respectively. Microzooplankton often removed average 89% of daily bacterioplankton production. Our study suggests the importance of microbial loop in the planktonic ecosytstems of the Arctic Ocean. Therefore, microzooplankton were the major consumers of picoplankton production, and that their grazing is one of the most important losses affecting the piocophytoplankton and bacterioplankton biomass during summer in the Arctic Ocean.