Primary productivity and phytoplankton community structure in surface waters of the western subarctic Pacific and the Bering Sea during summer with reference to bloom stages

Abstract

The western subarctic Pacific and adjacent Bering Sea are recognized as one of the most productive regions supported by spring diatom bloom among the world’s oceans. Yet, phytoplankton growth and photosynthesis are often suppressed during summer. Little is known about factors controlling the summer primary productivity and phytoplankton community structure in the western subarctic Pacific and the Bering Sea, especially along the Russian coast extending from the Kuril Islands to the Gulf of Anadyr. A total of 24 stations belonging to two cruises conducted in June 2014 and August 2018 was categorized into three groups using hierarchical cluster analysis for physical properties such as seawater temperature, salinity, and mixed layer depth. Based on the biogeochemical characteristics of each cluster, we defined clusters 1, 2, and 3 as bloom, post-bloom, and pre-bloom conditions, respectively. Cluster 1, observed near the Kuril Islands and the Kamchatka Peninsula, was characterized by nutrient-rich cold water that supported the predominance of diatoms. Nutrient-depleted warm waters associated with shallow mixed-layer and small diatom populations were grouped into cluster 2, distributed in the Gulf of Anadyr and near the Kamchatka Peninsula. The deepest mixed layer depth was found in cluster 3, accompanied by the low proportions of diatoms. The spatial distribution of cluster 3 showed broad geographical coverages extending from the Kuril Islands to the Aleutian Basin. Thus, this study identified the relationships between summer primary productivity, phytoplankton community composition, and environmental variables in surface waters of the subarctic Pacific along the Russian coast. Overall, the present study addressed the shortcoming of in situ data in phytoplankton communities and environmental variables in the western subarctic Pacific and the Bering Sea.