Phytoplankton composition in the subarctic North Pacific during autumn 2005

Abstract

During a subarctic North Pacific cruise in autumn 2005, we investigated the similarities and differences between phytoplankton composition in the Alaskan Gyre (AG) and the Western Subarctic Gyre (WSG) using the chemotaxonomy program CHEMTAX, microscopy and flow cytometry. Chlorophyll a (Chl a) standing stock was almost constant (21.5‐31.4 mg m 22 ) throughout the two gyres. Prymnesiophytes were consistently the dominant group (21‐50% of Chl a standing stock) in both gyres. Diatoms were a minor component of Chl a standing stock (,10%); however, abundances of large diatoms (Thalassiosira spp., Fragilariopsis spp. and Neodenticula seminae) were higher in the AG than in the WSG. In the AG, green algae (chlorophytes and prasinophytes) were secondary components of Chl a standing stock (16‐35%). Pterosperma spp. (Prasinophyceae) partially contributed to the predominance of green algae in the AG. In the WSG, Synechococcus spp. accounted for 14‐27% of Chl a standing stock; however, the abundance of Synechococcus spp. reached 11 � 10 10 cells m 23 and was numerically much higher than that of eukaryotic plankton (,2 � 10 10 cells m 23 ). The similarities and differences between phytoplankton composition in the AG and WSG will influence the biogeochemical processes and food web dynamics in the subarctic North Pacific.