Temporal and spatial patterns of chemotaxonomic algal pigments in the subarctic Pacific and the Bering Sea during the early summer of 1999

The distribution and community structure of phytoplankton were investigated in the subarctic Pacific and the Bering Sea during summer 1999 using chemotaxonomic pigment markers analyzed by HPLC and the CHEMTAX program (Mar. Ecol. Prog. Ser. 144 (1996) 265). In addition, in vivo chlorophyll fluorescence was also applied to the estimation of phytoplankton abundance in the study area. During our survey, chlorophyll a concentrations were consistently low (<1 μg l−1) in the whole water column of the Western Subarctic Gyre (WSG) and the Alaskan Gyre (AG). In contrast, higher chlorophyll a concentrations (>1 μg l−1) were sometimes observed in the Bering Sea and a coastal domain near the Aleutian Islands. In the WSG stations, prasinoxanthin-containing prasinophytes consistently predominated (ca. 20–40% in terms of chlorophyll biomass), although prasinophytes have received little attention in previous studies of the subarctic Pacific. Diatoms were a secondary component in terms of chlorophyll biomass in the WSG. Phytoflagellates, such as prymnesiophytes, pelagophytes, and green algae (prasinophytes and chlorophytes), contributed 45–90% to the chlorophyll biomass in the AG stations. Diatoms were predominant in the phytoplankton community of the Bering Sea, but the dominant phytoplankton group was variable in the coastal domain of the Aleutian Islands. When we compared the WSG with the AG in terms of the abundance and community structure of phytoplankton during the study period, the two gyres appeared to be in a similar biogeochemical province: sufficient macronutrients, low-chlorophyll biomass (<1 μg l−1), and the dominance of small phytoflagellates. These results imply the dominance of a microbial food web, which is responsible for the recycling of biogenic carbon or nutrients in the euphotic zone of the two gyres.