DOI: 10.5061/dryad.jwstqjq6d
title: Biological productivity and community structure of phytoplankton in the oligotrophic Philippine Sea

description: 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.
description: 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 <i>a</i> concentration were filtered
 through Whatman GF/F filters (24 mm). The chl <i>a
 </i>concentrations were measured using a Turner Designs model 10-AU
 fluorometer (Turner Designs, USA), which was calibrated using commercially
 purified chl <i>a</i>. Using
 a<sup> 13</sup>C-<sup>15</sup>N dual tracer
 technique, <i>in situ</i> 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 <sup>13</sup>C and
 <sup>15</sup>N 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).