Spatio-temporal variation of Prochlorococcus and phytoplankton community between Shimizu coast and Suruga bay, Northwest Pacific Ocean

Abstract A biweekly study was conducted in the surface waters of the Shimizu coast and Suruga Bay in the Northwest Pacific Ocean during January 2005 to February 2006. The diagnostic pigment fucoxanthin (Fuco), representing diatoms, and divinyl-chl a (div-chl a), representing Prochlorococcus (picophytoplankton), were highest in the near shore (type-1) water. The contribution of diatoms to the phytoplankton biomass; i.e. chlorophyll a (Chl a), was highest ( ∼ 76%) in the type-I water and lowest ( ∼ 64%) in the offshore (type-III) water. The Prochlorococcus contribution was ∼ 13% and ∼ 5% in the type-I and III water, respectively. The div-chl a concentration reached a maximum of 2.92 mg m−3 in July (summer) followed by 0.9 mg m−3 smaller peak in September, before decreasing to 0.01 mg m−3 in January (winter). Zeaxanthin, a pigment which is a biomarker for Synechococcus (picophytoplankton), peaked in May (autumn), while chlorophyll b (Chl b) and lutein, representing chlorophytes, were elevated in August (summer). In contrast, the Fuco was highest (16.3 mg m−3) in June (spring). The Prochlorococcus concentrations peaked in July and September, when the water temperature was above 20 °C. However the lowest concentrations were observed between 1 °C and 15 °C, suggesting this community experienced temperature adaptation and was highest in type-I and type-II (i.e. mid-shore) water. Results revealed that Prochlorococcus had a greater contribution of temperature adaptations than diatoms to the ecosystem in comparison of the surface area of individual to volume ratio and played a major role in the biogeochemical cycle of the region.