Seasonal variability of the protist community and production in the southern Okhotsk Sea revealed by weekly monitoring

Abstract To evaluate seasonal changes in a protist community (diatoms, dinoflagellates and ciliates) and their production in the southern Okhotsk Sea, water sampling was conducted approximately once a week from July 2016 to July 2017 at Mombetsu Port. Diatoms were the predominant category of the three taxa throughout the study period. The abundances of all taxa were high from March to September and were low from October to February. Four groups (A-D) were identified by cluster analysis according to abundance. Group A, of which Thalassiosira spp. were dominant, occurred in March. Group B showed the highest abundance (7.7 × 104cells L−1), with Chaetoceros spp. and Pseudo-nitzschia spp. being dominant. In group C, Odontella spp. and pennate diatoms were dominant and occurred from November to February. Group D was mainly observed in late October, and pennate diatoms were dominant. Seasonal changes in the predominant species of diatoms were explicitly observed, and these changes were associated with water mass exchanges. For dinoflagellates, abundance was highest in July 2016, and biomass and production were highest in September. Large fluctuations in the ciliate biomass in July 2016 might have been induced by the high growth rate, which reflects the high chlorophyll a concentration and water temperature and a decrease in predation pressure. The estimated primary production was lower than that reported in previous studies, which could reflect differences in the depth of the research areas and the methods used for estimating production. In the coastal area of the southern Okhotsk Sea, where production is high, not only the species composition but also the protist community varied clearly with season because of variations in the water mass.