The structure of the phytoplankton community in the subsurface chlorophyll maxima in the western North Pacific Ocean

The phytoplankton taxonomic composition of the subsurface chlorophyllmaximum layer (SCM) was compared with that of the surface layerin July, September and November 1979 in the western North PacificOcean. Besides the use of chemical fixatives for preservablephytoplankton, serial dilution culture method was employed fornon-preservable flagellates and monads, i.e., fragile forms.The SCM was characterized by a high species diversity of preservablephytoplankton and by the numerical dominance of fragile forms.Among the fragile forms, Micromonas and Ochromonas were dominant.This dominance is consistent with their viability at low lightintensity. Cluster analysis revealed that, for blue-green algae,coccolithophorids, silicoflagellates and diatoms, a marked differenceexisted in species assemblages between the SCM and the surfacebut there was no distinct difference in dinoflagellate assemblages.The difference between both layers is discussed, as relatedto the mechanisms of formation of the SCM. ¹Present address: Nodai Research Institute, Tokyo Universityof Agriculture, 1-1-1, Sakuragaoka, Setagaya-ku, Tokyo 156,Japan.