Phytoplankton assemblages in Onagawa Bay from 2012 to 2013 determined by DNA sequencing and pigment analysis

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.