The genus
The Kuroshio is a western boundary current in the North Pacific Ocean that originates in the North Equatorial Current and bifurcates to the east of the Philippines. The main stream of the Kuroshio enters the East China Sea (ECS) northeast of Taiwan, flows out through the Tokara Strait, and runs along the Japanese islands of Shikoku and Honshu. While the Kuroshio and its adjacent waters are characterized by highly oligotrophic conditions, phytoplankton and zooplankton communities in the Kuroshio are distinct compared to those from adjacent waters (McGowan, 1971). McGowan (1971) suggested that some plankton species are delivered by the Kuroshio to the north from the equatorial region.
The abundance of the cyanobacterial genus
Nitrogen fixation by
In the present study, we simultaneously determined
Algal blooms in an oligotrophic region may indicate a nitrogen fixation
hotspot (Wilson and Qiu, 2008; Shiozaki et al., 2014c). To identify the
locations of intensive algal blooms, we used a data set of chlorophyll (chl)
To examine the current field, geoelectrokinetograph and ship-mounted
acoustic Doppler current profiler (ADCP) data from the uppermost layer for
the summers between 1953 and 2008 were obtained from the Japan Oceanographic
Data Center (
Experiments were conducted during summer on-board the R/V
Water samples for all of the experiments, with the exception of
determination of the dissolved iron concentration, were collected using an
acid-cleaned bucket and Niskin-X bottles. The depth profile of light
intensity was determined immediately before the water sampling using a light
sensor (during the KT-06-22, KT-07-21, KT-09-17, and KT-10-19 cruises) or an
empirical equation (during the
Water was sampled to estimate the dissolved iron concentration from 0.5 m
depth during the KT-06-21 and KT-07-22 cruises and from 10 m depth during
the KT-09-17 cruise using an acid-cleaned Teflon bellows pump (AstiPure
PFD2; Saint-Gobain) with Teflon tubing (inner diameter
The dissolved iron concentration was determined using an automatic Fe(III)
flow injection analytical system (Kimoto Electric Co., Ltd.) using a
chelating resin pre-concentration and chemiluminescence detection method
(Obata et al., 1993). A buffer solution of 10 M formic acid and 2.4 M
ammonium formate was added to the samples. The sample pH was adjusted to 3.0
with 20 % ammonium hydroxide (NH
Samples for the incubation experiments were collected vertically at all of
the stations, except at stations T0621, GN-3, and T0905, where samples were only
collected from the surface. All samples were collected in duplicate in
acid-cleaned 4.5 L polycarbonate bottles. During the
A recent study demonstrated that commercial
Water samples were collected for microscopic analysis at all light depths
during the
We used non-metric multi-dimensional scaling (nMDS) to investigate the
spatial differences in the environmental variables that could influence
Numerical particle-tracking experiments were conducted to investigate the
transport of water masses at the surface from areas around the Miyako
Islands in the summer season from 2003 to 2009. Surface velocity data were
derived from the FRA-JCOPE2 reanalysis product (Miyazawa et al., 2009),
which is an eddy-resolving (1/12
To examine differences in the output depending on the start time within the
same year, we also performed experiments starting on 1, 11, and 21 June and
1 July in 2009. The ratio of particles that reached areas downstream of the
Tokara Strait (hereafter Area K) (Fig. 7), including the particles'
entrainment to the Kuroshio, to total particles released from the Miyako
Islands was computed in all experiments. It should be noted that these
experiments contained the following two uncertainties. First, the
distribution of
The average surface current field indicated that the main stream of the
Kuroshio flowed along the continental shelf in the ECS, and then passed to
the south of the Kyushu Islands and Shikoku (Fig. 1b). In addition, the
Kuroshio branch bifurcated northward at 25 and 30
Distribution of
The sea surface temperature (SST) ranged from 25.1 to 30.5
Vertical profiles of phosphate and nitrogen fixation in the East
China Sea
Summary of
Surface
The abundance of
The surface abundance of
The SST was lower to the northwest of the Miyako Islands than in adjacent
waters, and chl
nMDS ordination of sampling stations with environmental variables.
Station T0904 was located near the upwelling water; its SST of 29.0
During the same cruise, we encountered a
Relationships
As the Kuroshio generally flows along the continental slope north of the
Miyako Islands (Fig. 1b), particles around the Miyako Islands were not
transported along the typical path of the Kuroshio to the northeast,
especially at their initial stages (Fig. 7a). Some particles migrated around
the Miyako Islands, or turned south after they passed the Tokara Strait.
Nevertheless, the particles delivered to Area K east of the Tokara Strait
increased as time elapsed, and the ratio of particles delivered to Area K to
the total released particles ranged from 13 to 56 % (30
Phosphate concentrations were consistently low within the MLD in all of the
studied areas, and the maximum abundance of
The surface distribution of the dissolved iron concentration demonstrated no
significant variation among the areas. The dissolved iron concentration
(0.19–0.89 nM) was higher than that in the western North Pacific
subtropical region (0.15–0.4 nM) (Brown et al., 2005). Obata et al. (1997)
demonstrated that the vertical distribution of the dissolved iron
concentration in the ECS showed two peaks (at the surface and in the deep
water), suggesting that aerial dust significantly contributes to the high
dissolved iron concentration at the surface in all of our study areas. In
accordance with our results, previous modeling studies estimated the amount
of dust deposition to be similar in all four areas (Jickells et al., 2005;
Mahowald et al., 2009). Therefore, iron availability for
Satellite data analysis indicated that there was a “pipeline” of material
transport from the Miyako Islands to the Kuroshio, and this was supported by
numerical simulations. According to the hypothesis of Marumo and Asaoka (1974),
the growth of
Although there was no statistically significant difference in
Johnson et al. (1999) reported that the iron supply increased around the
continental shelf because re-suspension from the bottom to the euphotic zone
becomes significant. However, in the continental shelf of the ECS, the
abundance of
In our study, we found a
The numerical simulation demonstrated that released particles from the
Miyako Islands were generally transported to the northeast and flowed along
the Kuroshio during summer between 2003 and 2009. Thus, if
Studies on nitrogen fixation around islands in the study region are fairly
limited (Liu et al., 2013), and the present study is the first report of a
We hypothesize that the high abundance of
T. Shiozaki, S. Takeda, S. Itoh, and K. Furuya designed the experiment; T. Shiozaki, S. Takeda, T. Kodama,
X. Liu, F. Hashihama, and K. Furuya collected the samples at sea. T. Shiozaki determined nitrogen
fixation and abundance of
We thank J. Ishizaka, the captains, crew members, and participants on board
the T/V