Spatial and temporal variabilities of phytoplankton community structure in the northern North Pacific as determined by phytoplankton pigments

Particulate matter in surface waters of the northern North Pacific and the southern Bering Sea was analyzed for the spatial and temporal distribution of chlorophyll and carotenoid pigment compositions by high-performance liquid chromatography (HPLC). Samples were collected during 11 ship of opportunity cruises between Canada and Japan from April 1995 to June 1996. The cruises each took 13 days and were conducted every six weeks. Seasonal variability of the concentration of chlorophyll a showed different features among the three areas studied: the Alaskan Gyre (eastern part of the subarctic North Pacific), southern Bering Sea, and the area off the Kuril Islands (western part of the subarctic North Pacific). In the Alaskan Gyre, the concentrations of chlorophyll a were around 0.2–0.4 μg/l without any remarkable seasonal variation, consistent with previous studies. In the Bering Sea and the area off the Kuril Islands, a drastic change in chlorophyll a biomass was observed from spring to early summer, with extremely high levels, >2 μg/l, being found. The composition of the phytoplankton community was estimated by multiple regression analysis. A high concentration of chlorophyll a exceeding 1 μg/l was always accompanied by a high contribution of fucoxanthin-containing diatoms constituting up to 90% of the total chlorophyll a biomass. Chlorophyll b-containing green algae were one of the important algal groups in the northern North Pacific throughout the year as were fucoxanthin-containing diatoms and 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes. The results give new insights into the phytoplankton community structure of the subarctic North Pacific, namely, an ubiquitous basic structure made up of rather diverse algal groups, and a flourishing diatom population limited by area and season and superimposed on the basic structure. These two structures are thought to play their respective roles in the biogeochemical cycle in the surface of the northern North Pacific.