Abstract
The phytoplankton assemblages in Onagawa Bay were investigated by photosynthetic
pigment analysis and DNA sequencing over two consecutive years, from January 2012 to
December 2013. Chlorophyll a (Chl a) concentrations tended to be high from winter to spring,
and fucoxanthin, the source of which is mainly diatoms, was also high. Chlorophyll b concentration,
which is retained in picoprasinophytes, tended to be higher in June. Cyanobacteria tended
to appear in the summer, although, at less than 2%, in relatively small amounts. Since picoeukaryotes
and cyanobacteria are small, shellfish filters cannot trap them efficiently, so the amount of
nutrition obtained from them is relatively low and inefficient. Dinophysis norvegica (the causative
agent of diarrheal shellfish poisoning)was the dominant dinoflagellate species throughout
the study. Two species of dominant cryptophyte that were found are food sources of ciliate
Myrionecta rubra which is a food source for Dinophysis spp., so unfortunately, they render
Onagawa Bay susceptible to the growth of Dinophysis spp.. Phaeocystis spp. was the dominant
haptophyte. The combination of pigment analysis by HPLC and DNA Next Generation
Sequencing provided good data on seasonal phytoplankton variation, which is necessary to
understand the detailed feeding environment for shellfish raised in Onagawa Bay.