Picoplankton community structure in the subarctic Pacific Ocean and the Bering Sea during summer 1999

Abstract

We studied picoplankton community structures in the subarctic Pacific Ocean and the Bering Sea during summer 1999 using flow cytometric analysis. The picoplankton community in the studied area was comprised of Synechococcus spp., eukaryotic ultraplankton and heterotrophic bac- teria. Prochlorococcus spp. were not detected at any station. Abundances of Synechococcus and eukaryotic ultraplankton were at approximately the same level of 10 3 to 10 4 cells ml -1 within the upper euphotic layer in the subarctic gyres. An abundance of Synechococcus spp. higher than 5 · 10 4 cells ml -1 was found at the surface to 40 m depth in the northern Gulf of Alaska, whereas low Synechococcus spp. abundance (about 500 cells ml -1 ) was found in the upper euphotic layer in the Bering Sea. Abundances of heterotrophic bacteria were about 2 orders of magnitude higher than those of Synechococcus spp. and eukaryotic ultraplankton, with higher abundance generally occur- ring in the area of high autotrophic biomass. Although Synechococcus spp. and eukaryotic ultra- plankton occurred at comparable abundance, the latter contributed significantly more to photosyn- thetic carbon biomass, except in the northern Gulf of Alaska, where the biomass of Synechococcus spp. and eukaryotic ultraplankton were approximately equal. Cellular red fluorescence for Synecho- coccus spp. and eukaryotic ultraplankton increased by an average 4- and 2-fold, respectively, from the surface to the bottom of the euphotic layer, with the smallest increase occurring in the Bering Sea. Both the red fluorescence and forward light scatter (FSC, related mainly to cell size) per cell varied more than 2-fold spatially, with the highest value occurring in the Bering Sea. These variations were probably caused by differences in physiological conditions and species compositions. Overall, pico- phytoplankton was the dominant contributor to total autotrophic biomass in the subarctic North Pacific, but contributed only a small fraction to total autotrophic biomass in the Bering Sea. The West- ern Gyre (WG) and the Alaskan Gyre (AG) possess both similarities and differences in biogeochem- ical processes and microbial food-web dynamics. The slightly higher phytoplankton biomass, photo- synthetic efficiencies and growth rates in WG than AG suggests less severe iron limitation in the WG.