In the Philippine Sea, mesoscale eddies have been frequently observed, but little is known about their contributions to the biological productivity and community structure of phytoplankton in the region. In situ carbon and nitrogen uptake rates and phytoplankton community structure were investigated in the Philippine Sea based on the 13C-15N dual-tracer technique and photosynthetic pigment analysis from late August to September 2018. Based on the comparison of the phytoplankton community structure among the cold eddy, warm eddy, and outer area, we found that the phytoplankton composition was different in the warm eddy with relatively lower contributions of large phytoplankton. However, a distinct enhancement in phytoplankton biomass over the euphotic zone was not detected even in eddy occurrence. In terms of community, the picoplankton Prochlorococcus was persistently dominant, which was attributed to a low supply and chemical composition of nutrients. The average integrated primary productions (IPP) were 80.4, 75.9, and 76.3 mg C m−2 d−1 for the cold eddy, warm eddy, and outer area, respectively. The IPP under the eddy condition was not largely different from that in reference sites, even though the vertical distribution of productivity maximum was found to be different depending on eddy condition. These weak biological responses to mesoscale eddies might be attributed to the sustaining picoplankton-dominant community structure and no substantial upward inputs of nutrients to the euphotic zone. Our findings represent an important contribution to understanding the biological response of mesoscale eddies in the oligotrophic Philippine Sea.

Seawater samples were collected at six depths corresponding to light levels of 100, 50, 30, 12, 5 and 1% of surface photosynthetically active radiation (PAR, 400 to 700 nm) using a CTD-mounted rosette sampler with 24 10 L Niskin bottles. Duplicate samples for dissolved inorganic nutrient concentrations (nitrite+nitrate, ammonium, phosphate, and silicate) were analyzed onboard immediately after collection using an automated nutrient analyzer (SEAL, QuAAtro, UK) following the QuAAtro multitest methods. The water samples (500 mL) used for measuring total chl a concentration were filtered through Whatman GF/F filters (24 mm). The chl a concentrations were measured using a Turner Designs model 10-AU fluorometer (Turner Designs, USA), which was calibrated using commercially purified chl a. Using a ¹³C-¹⁵N dual tracer technique, in situ primary production of phytoplankton was measured from six photic depths. The samples were analyzed at the Alaska Stable Isotope Laboratory of the University of Alaska, Fairbanks, USA. The abundances of ¹³C and ¹⁵N were measured by a Thermo Finnigan Delta+XL mass spectrometer. Phytoplankton pigments were measured from four photic depths (100, 30, 12, and 1% of PAR). The pigment concentrations were detected using a high-performance liquid chromatography system (HPLC-Agilent Infinite 120), and pigment separations were performed using a Zobrax Eclipse XDB C8 column (250 × 4.6 mm, 5 μm).