J Oceanogr (2014) 70:435–445
DOI 10.1007/s10872-014-0244-6

ORIGINAL ARTICLE

Biogeochemical responses associated with the passage of a cycloniceddy based on shipboard observations in the western North PacificChiho Sukigara • Toshio Suga • Katsuya Toyama •Eitarou Oka

Received: 18 March 2014 / Revised: 25 August 2014 / Accepted: 31 August 2014 / Published online: 20 September 2014
Ó The Oceanographic Society of Japan and Springer Japan 2014

Abstract
A shipboard high-resolution hydrographic
survey in the subtropical region of the western North
Pacific conducted from October to November 2008 detected part of a cyclonic eddy around 30°N, 145°E. This eddy
had propagated westward in the region south of the Kuroshio extension for at least 6 months as a wavelike disturbance. Within this eddy, isopycnals shallowed between
a depth of 600 m and just below the surface mixed layer. In
addition, maximal dissolved oxygen concentrations wereobserved in the subsurface layer between depths of 50 and
100 m. Nitrate was depleted within this subsurface maximal oxygen layer. These results suggest that nutrients inthe deeper layers were supplied into the euphotic layer as aresult of the uplift of isopycnals in the eddy, fueling thephotosynthesis of phytoplankton in the subsurface andemitting an excess of oxygen due to new production.
Compared with the outside of the eddy, the enhancement of

C. Sukigara (&)
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japane-mail: suki@hyarc.nagoya-u.ac.jp
T. Suga
Department of Geophysics, Graduate School of Science, Tohoku
University, Sendai, Japan
T. Suga
Research Institute for Global Change, Japan Agency of MarineEarth Science and Technology, Yokosuka, Japan
K. Toyama
The Program in Atmospheric and Oceanic Sciences, Princeton
University, Princeton, NJ, USA
E. OkaAtmosphere and Ocean Research Institute, The University ofTokyo, Chiba, Japan

oxygen and the decrease of nitrate in the center of theeddy were estimated to be 2.7 mol O2 m-2 and0.22 mol N m-2, respectively. The primary productivity
calculated using the eddy transition speed of 5.1 km day-1
was 548 mg C m-2 day-1 at the center of the eddy. The
enhanced primary productivity due to the passage of theeddy is likely to have an important role in the ecosystemand on material cycling in the subtropical region.
Keywords Mesoscale process  Physical–biologicalprocess  Cyclonic eddy  Primary productivity  Nutrient
supply1 Introduction
Mesoscale perturbations are ubiquitous in the ocean, andenrich the spatial and temporal variability of the oceanic
structure. They also influence the distribution of materials,
biomass, and biogeochemical processes through variationsin the physical conditions. McGillicuddy and Robinson(1997) described the mesoscale dynamic processes that
operated during nutrient cycling in the Sargasso Sea. Theirconceptual model explained that cyclonic eddies andmode-water eddies raise the density surface toward thesurface, while anticyclonic eddies push the density surface
down. When deep water rich in nutrients is injected into theeuphotic zone by the upwelling created by eddies, phytoplankton can use the nutrients, and the biomass and theprimary production is enhanced. On this basis, nitrate
fluxes in the Sargasso Sea associated with eddies were
estimated to be 0.35 ± 0.10 mol N m-2 year-1 using a
regional, eddy-resolving, coupled physical–biologicalmodel. Furthermore, the nitrate flux in the Sargasso Sea
was estimated to be 0.19 ± 0.10 mol N m-2 year-1 using

123

436

a statistical model based on satellite-derived sea surface
height (SSH) anomalies and the mean relationship between
density and nitrate concentration (McGillicuddy et al.
1998). These estimated values of the nitrate flux by eddies
were larger than those of nitrogen supply by winter convection (0.09 ± 0.04 to 0.17 ± 0.05 mol N m-2 year-1;
Michaels et al. 1994), diapycnic diffusion (0.05 ± 0.01
mol N m-2 year-1; Lewis et al. 1986), nitrogen fixation(0.07 mol N m-2 year-1; Gruber and Sarmiento 1997), or
atmospheric deposition (0.03 mol N m-2 year-1; Knapet al. 1986). However, Oschlies (2002) reported that typicalvalues of the simulated eddy-induced nitrate supply near theSargasso Sea were only 0.05 mol N m-2 year-1, based onan eddy-resolving coupled ecosystem–circulation model for
the North Atlantic. Martin and Pondaven (2003) suggested
that the vertical nitrate fluxes due to eddy pumping based on
altimetry were likely to be overestimated, due to the spatial
bias of eddy location and the low efficiency of nutrient
uptake by phytoplankton in response to eddy pumping.
Although model estimates of the nutrient fluxes associatedwith eddies vary widely, these results strongly suggested that
cyclonic eddies and mode-water eddies might play a substantial role in nutrient supply to the oligotrophic subtropicalsurface layer. Therefore, field observations are required toverify the impact of eddies on biological processes.
Interdisciplinary field studies in subtropical regions(e.g., E-Flux programs in the North Pacific (Benitez-Nelson et al. 2007) or the EDDIES program in the North
Atlantic (McGillicuddy et al. 2007) have been conducted toinvestigate eddy-induced perturbations of the physical andbiological environment, the response of the plankton
community to eddies, and the resulting impacts on biogeochemical cycling and export (Benitez-Nelson and
McGillicuddy 2008). These programs have revealed that
biological and biogeochemical responses within eddies
differ due to variations in the eddy magnitude, and thetiming and duration of nutrient inputs caused by differences in the eddy formation process, intensity, age andmovement. For example, Cyclone Opal observed in the
E-Flux program, which was a strong eddy formed by a
combination of strong winds and topography in the lee ofthe Hawaiian Islands, involved an eddy-induced nitrate
injection (Mahaffey et al. 2008) and an extraordinary diatom bloom (Bibby et al. 2008; Brown et al. 2008). Thesestrong biological signals were suggested to have resulted
from large isopycnal displacements of over 100 m at theeddy core that lasted for about one month (Dickey et al.
2008; Nencioli et al. 2008). Furthermore, this eddy wassuggested to have been continuously entraining nutrientsfrom the deeper layer to the euphotic zone within themoving trail of the eddy (Nencioli et al. 2008). In anothercase, a mode water eddy in the Sargasso Sea was observedwith an enhanced nutrient injection and large diatom

123

C. Sukigara et al.

Fig. 1 Location of CTDO2 stations with water sampling (closed
stars), CTDO2 stations without water sampling (open stars), XCTD
stations (closed circles), and XBT stations (open circles) for the KH08-3 cruise leg 2 from 22 October to 4 November 2008

biomass (Bibby et al. 2008; Ewart et al. 2008), despite theadvanced age of the eddy. This was suggested to be theresult of a continuous upwelling of nutrients by eddy–windinteractions with strong mixing at the core of the eddy
(Greenan 2008). The nutrient injection processes and theassociated biological responses intricately vary dependingon the eddy features, the surrounding circumstances, andthe ecosystem in the eddy (Martin and Pondaven 2003;
Siegel et al. 2011). High-resolution and three-dimensional
observations are required to investigate the impacts ofeddies on the biogeochemical processes.
In this study, we focused on an eddy in the subtropicalregion several hundred kilometers south of the Kuroshio
extension (KE) in the western North Pacific. This area is in
the path of westward propagating cyclonic and/or anticyclonic eddies (Ebuchi and Hanawa 2000, 2001). Few
studies have reported on the physical structure of eddies
and particularly their influence on biological processes inthis region. Our aim was to analyze the relationships
among the physical structures, eddy movement, and biogeochemical responses.

2 Observation
A high-resolution hydrographic survey was conductedfrom 9 October to 7 November 2008 in the western North
Pacific southeast of Japan during the KH-08-3 cruise leg 2

Biogeochemical responses associated with the passage of a cyclonic eddyof R/V Hakuho-Maru, targeting the mesoscale distributionof the North Pacific subtropical mode water (Oka et al.
2011). Two meridional lines at 143°E and 146°E and three
shorter zonal lines at 27.5°N, 30°N, and 34°N were surveyed using a conductivity–temperature–depth–oxygen
profiler (CTDO2, Model9-plus; Sea-Bird Electronics Inc.,
Bellevue, WA, USA) at intervals of 300 or 1° in latitude/
longitude, and an expendable conductivity–temperature–depth profiler (XCTD; Tsurumi Seiki Co. Ltd., Yokohama,Japan) or an expendable bathythermograph (XBT, Tsurumi
Seiki Co. Ltd.) at 100 intervals (Fig. 1). A salinity profile ateach XBT station was estimated using the observed temperature and temperature–salinity relations at nearby
CTDO2 stations (Oka et al. 2011). Water samples forsalinity, nutrient, dissolved oxygen (DO), and chlorophylla (Chl a) measurements were collected at 1° intervals at
CTDO2 stations.
Salinity and oxygen sensors in the CTDO2 system were
calibrated using water samples. Salinity and DO in sampled
water were measured using an onboard Portasal 8410
salinometer (Guideline Instruments, Smiths Falls, ON,
Canada) and titration unit (MPT Titrino 798; Metrohm
Shibata, Tokyo, Japan), respectively. The saturation concentration of DO was computed for each layer from theobserved potential temperature and salinity (Weiss 1981).
The DO saturation ratio (DO%) was calculated by dividing
the observed DO by the saturation concentration of DO.
Nutrient (nitrate: NO3-, nitrite: NO2-, phosphate: PO4and dissolved silicate: SiO2) concentration was measured
with a TRACCS 2000 autoanalyzer (Bran?Luebbe, Norderstedt, Germany) after the cruise. The nitrate was
reduced to nitrite using a cadmium–copper column and theconcentration was determined by absorption spectrometryof an azo dye coupled with N-(1-naphthyl)-ethylenediamine dihydrochloride. The concentration of nitrite wasdetermined by absorption spectrometry of an azo dye(Armstrong et al. 1967). The concentrations of phosphateand dissolved silicate were determined by absorption
spectrometry of heteropoly-molybdenum blue (Murphy
and Riley 1961; Koroleff 1983). Nutrient samples were
kept in a freezer for measurement. The vertical resolutionof nutrients was 10 m in the top 200-m layer and 100 m inthe 200–600-m layer. Chl a samples were measured
onboard using a fluorometer (10-AU; Turner Designs,
Sunnyvale, CA, USA). The vertical resolution of chlorophyll was 10 m in the top 160-m layer and 20 m in the
160–200-m layer. The light environment of the watercolumn was obtained using a photosynthetically active
radiation (PAR) sensor (QSP-2200; Biospherical Instruments Inc., San Diego, CA, USA). Current velocity anddirection during the entire cruise were measured by a shipmounted acoustic Doppler current profiler (ADCP, Ocean
Surveyor 38 kHz; Teledyne RD Instruments Inc., Poway,

437

CA, USA). The profile depths ranged from 32 to 1296 m,
with a vertical resolution of 16 m.
The spatial distribution of SSH and its anomalies in theobservation period was based on delayed-time maps ofabsolute dynamic topography and sea level anomalies
produced by merging the data from two satellites, Jason-1
and Envisat, at the AVISO (http://www.aviso.altimetry.fr/
en/home.html; Ducet et al. 2000). The spatial and temporal
resolutions were 0.25° in latitude and longitude and every
7 days.

3 Results
In this study, we focused on the subtropical area south of
the KE, whose southern flank was located at 32.5°N on the
146°E line and 34°N on the 143°E line. Vertical sections of
the hydrographic and biogeochemical data for each transect
are shown in Fig. 2. On the 146°E line, isopleths ofpotential density (isopycnals) below 200 m depth shallowed sharply from 32°N to 33°N in association with KE(Fig. 2a).
A distinct uplift of the isopycnals from a depth of 600 m(26.6rh) to a seasonal pycnocline (up to 50 m and 24.0rh)
appeared around 30°N on the 146°E line. The largest uplifton the 146°E line was observed at 30.3°N, and the 25.2rh
isopycnal reached 130 m. This was substantially shallower
than in the vicinity of the same transect; the depth of the
25.2rh isopycnal was 200 m at 29°N and 250 m at 32°N.
On the 30°N line, isopycnals denser than 25.0rh tended toshallow toward the west. The depth of the 25.2rh isopycnal
reached a depth of 100 m at 145°E and was the shallowestof all stations (Figs. 2a, 3a). An obvious heaving of the
25.2rh isopycnal also appeared at 29.2°N on the 143°E
line. The magnitude and width of uplift was smaller than at
the 146°E line. The spatial distribution of SSH in theobservation period showed a moderate depression centeredat 29°N and 145°E (Fig. 4). The position of the depression
in the SSH roughly coincided with the uplift of the isopycnals at 146° and 143°E.
The downward decrease in DO below a depth ofapproximately 200 m associated with the increase of
potential density was caused by oxygen consumption during respiration by organisms and remineralization by bacteria. Exceptionally high DO and DO% layers appeared
from 150 to 400 m at 143°E and from 400 to 600 m at
145°E on the 27.5°N transect (Figs. 2b, c, 3b, c). The
former resulted from subtropical mode water, which is
depicted in the vertical distribution of potential density andpotential vorticity (Oka et al. 2011). The latter was associated with North Pacific central mode water contained in a
subsurface mesoscale eddy (Oka et al. 2009). Supersaturations of DO% were found in the layer from 50 to 100 m

123

438

C. Sukigara et al.

(a)
0
50

Depth [m]

100
150
200
200

400

600
34N

33N

32N

31N

30N

29N

28N

143E

(b)

143E 144E 145E 146E

28N

29N

30N

31N

32N

33N

146E

27.5N

145E 146E

147E

30N

0
50

Depth [m]

100
150
200
200

400

600

(c)

0
50

Depth [m]

100
150
200
200

400

600
34N

33N

32N

31N

30N

29N

28N

(d)

0

Depth [m]

143E

50

143E 144E 145E

28N

29N

27.5N

30N

31N

32N

33N

146E

145E 146E 147E
30N

100
150

(e)

0

Depth [m]

200

50
100
150
200
34N

33N

32N

31N
143E

123

30N

29N

28N

144E 145E
27.5N

28N

29N

30N

31N

146E

32N

33N

145E 146E 147E
30N

Biogeochemical responses associated with the passage of a cyclonic eddyb Fig. 2 Spatial distributions of a potential density (rh) (kg m-3),

b dissolved oxygen (DO) concentration (lmol kg-1), c DO saturation
ratio (%), d nitrate concentration (lmol L-1), and e chlorophylla (lg L-1) in the hydrographic sections of the KH-08-3 cruise leg 2.
Closed stars, open stars, closed circles, and open circles on the
figures indicate CTDO2 stations with water sampling, CTDO2 stations
without water sampling, XCTD stations, and XBT stations, respectively. Colored stars indicate the CTDO2 stations with water
sampling in the eddyin the southern stations (Figs. 2c, 3c). Significant supersaturated subsurface oxygen maximum (SOM) layers wereobserved at five stations, C031 (31°N, 146°E), C032 (30°N,
146°E), C033 (30°N, 147°E), C034 (30°N, 145°E), andC042 (29°N, 143°E), where isopycnals were lifted as
described above (Fig. 2c). SOM layers are often observed
and extend from the bottom of the surface mixed layer tothe bottom of the euphotic zone, where light intensity falls
to 1 % of that at the surface (Shulenberger and Reid 1981;
Hayward 1994).
The depth of the euphotic layer observed using the PAR
sensor in this cruise was estimated to be between 50 and
70 m from the surface. When the mixed layer depth was
defined as the depth at which the difference in temperaturefrom the 5 m depth was 0.1 °C, the thickness of the mixed
layer on the south of 32°N was at a \ 50 m depth from thesurface. The nitrate concentration within the euphotic
layer, whose depth was deeper than the depth of the mixed
layer, in the observed subtropical region was low(\0.1 lmol L-1) because of its utilization by phytoplankton in photosynthesis and the poor supply from deeper
layers. Therefore, it was considered reasonable that thedepth of the top of the nitracline almost corresponded to thedepth of the bottom of euphotic layer (Figs. 2d, 3d).
However, the nitracline deepened to a depth of around
100 m at the five stations (C031, C032, C033, C034, andC042) where the isopycnals were uplifted. The low nitrateconcentration in these stations was found not only in theeuphotic zone, but also below the euphotic zone
(80–100 m) and increased significantly from 80 to 130 m.
The low concentration of nitrate and the relatively highconcentration of DO below the euphotic zone in thesestations suggested the following scenario: The water withrich nitrate and low DO below the euphotic zone was
uplifted into the euphotic zone, nitrate in the water was
utilized for new production by phytoplankton, throughwhich excess oxygen emitted, and then the water with low
nitrate and high DO sunk below the euphotic zone. The
spatial variations of phosphate and silicate concentrations
were similar to that of nitrate. The low concentrations ofsilicate at the five stations (C031, C032, C033, C034, andC042) suggest the increase of diatoms, as observed in themode-water eddy in North Atlantic and the cyclonic eddyin North Pacific (Benitez-Nelson et al. 2007; McGillicuddy
et al. 2007; Bibby et al. 2008; Brown et al. 2008).
Vertical peaks of Chl a often appeared in the subsurface
in the oligotrophic subtropical region, which is referred to
DO concentration [μmol kg-1]

Potential density [kg m-3]
22
0

23

24

25

26

(b)

DO saturation [%]

(c)

50

Depth [m]50

Depth [m]170 190 210 230 250 270
0

100

80
0

100

150

150

200

200

200

Nitrate concentration [μmol L-1]

Chlorophyll a concentration [μg L-1]

0

0

2

4

6

8

(e)

110

120

0

0.2

0.4

0.6

0.8

1.0

50

Depth [m]

50

100

0

100

100

150

(d)

90

50

Depth [m](a)Depth [m]Fig. 3 Vertical profiles ofa potential density (kg m-3),
b dissolved oxygen (DO)concentration (lmol kg-1),
c DO saturation (%), d nitrateconcentration (lmol L-1), ande chlorophyll a concentration(lg L-1). Colored and graylines are the profiles for stations
inside and outside the eddy,
respectively. Broken lines at the
70 m depth indicate the bottomof the euphotic zone

439

100

C031 (31oN, 146oE)
C032 (30oN, 147oE)

150

150

C033 (30oN, 146oE)
C034 (30oN, 145oE)

200

200

C042 (29oN, 143oE)

123

440

C. Sukigara et al.

Fig. 4 The altimetric sea surface height map southeast of Japan on 22
October 2008. Thick black lines indicate the hydrographic sections of
the KH-08-3 cruise leg 2

as the deep Chl a maximum (DCM; Cullen 1982). HighChl a concentrations ([0.5 lg L-1) were observed arounda depth of 60 m at 29° and 33°N on the 146°E line, and
31°N on the 143°E line (Fig. 2e). However, the DCM at thestations with uplifted isopycnals (C031, C032, C033, C034,
and C042) was deeper (&100 m) than at the other stations(Figs. 2e, 3e). The Chl a concentrations in the deeper DCM
layer were about 0.3 lg L-1 and lower than those in otherstations.

4 Discussion
4.1 A mesoscale cyclonic eddy and biochemical
responses to the passage of the eddy
Uplifts of isopycnals around 30°N (Fig. 2a) appeared
inside the low SSH region (Fig. 4). Therefore, a cycloniceddy was suggested to have had an influence on the
structure of the water mass and the biogeochemical processes in this area. The ADCP velocity at 50 m depth
obtained along the observation lines (Fig. 5) revealed avelocity field dominated by the presence of the cyclonicflow of the eddy. The direction of the current was westward
between 32°N and 30°N, northeastward between 30°N and
29°N along the 146°E line, and northward between 146°E
and 147°E along the 30°N line. These directions showedthat the region from 29°N to 32°N on the 146°E line andfrom 145°E to 147°E on the 30°N line was located within
the eastern half of the cyclonic eddy. Station C034 at 30°N,
145°E was probably closest to the core of the eddy among
the CTD stations along the 146°E line and the 30°N line.
Current directions around station C032 at 30°N, 147°E
indicated that the eddy had already passed this station.

123

Fig. 5 ADCP velocity (arrows) at 50-m depth measured during theKH-08-3 cruise leg 2. The closed (open) circles indicate the locations
where CTDO2 (XCTD and XBT) observations were made. Thin
contours denote altimetric sea surface height on 22 October 2008.
Thick contour represents a geopotential anomaly contour of 1.85
dynamic meters relative to 794 dbar, the deepest common level of thehydrographic data

Along the 143°E line, the direction of the current was
southwestward between 29°N and 30°N, and eastward
between 28°N and 29°N. Current speeds reached
50 cm s-1 for the southeastward and westward currents,
but were as low as 10 cm s-1 at station C042 (29°N,
143°E). The region from 28°N to 30°N on the 143°E line
was thought to be occupied by the western half of the eddy.
We also drew contours of geopotential anomaly computed from the hydrographic data with the ADCP velocity,following Martin and Richards (2001) (Fig. 5). The contour of 1.85 m at the sea surface relative to 794 dbar (thedeepest common level of the hydrographic data) includes
the four CTD stations (C031, C033, C034, and C042) withthe biogeochemical response, and neighbors the remaining
station (C032) to its immediate east. Therefore, this contour is expected to outline the cyclonic eddy.
The shape of the eddy, as suggested by the distributionof current velocity and SSH during our observation period,
was an irregular circle with a minor axis 300 km in lengththat extended in the northwest and southeast directions, anda major axis 500 km in length that extended in the northeast and southwest directions (Fig. 4). From February to
April 2008, the eddy approached the meandering KE ataround 29°N, 154°E and its intensity increased, as illustrated by the depression of SSH (Fig. 6). After late April
2008, the eddy departed from the KE, propagated westward
with the same intensity, reached the area around 29°N,

Biogeochemical responses associated with the passage of a cyclonic eddy

AVISO MSLA 28-30oN
JUL 2009

APR 2009

JAN 2009

OCT 2008

JUL 2008

APR 2008

JAN 2008
130E

140E

150E

160E

Fig. 6 Time series of sea surface height anomalies along the zonal
band of 28–30°N. Vertical and horizontal broken lines show theposition and the time of the observation, respectively

145°E (Oka et al. 2011) in October 2008, and was finallyabsorbed by the Kuroshio at around 29°N, 133°E in May
2009. The speed of propagation of the eddy from April
2008 to May 2009 was estimated as 5.8 km day-1(6.7 cm s-1). The transit speed of the center of the eddy
from 8 October to 5 November 2008, including the shipobservation period, was calculated 5.1 km day-1(5.9 cm s-1) based on the satellite altimetry-derived eddy
trajectory data set (http://cioss.coas.oregonstate.edu/eddies/)
produced by Chelton et al. (2011). The data set indicatedthat the origin of this eddy could be dated back to midApril 2007 at around 27.2°N, 167.6°E. The age of the eddy
since generation at the time of observation was 18 months,
and 6 months had passed since the eddy had separated
from the KE. Ebuchi and Hanawa (2000) reported typicalshapes, behaviors, and spatial distribution of mesoscale
cyclonic and anticyclonic eddies in the Kuroshio recirculation region. They reported that typical eddies in thisregion are circular in shape with a diameter of about
500 km, and the surface velocity, SSH anomaly, and phasespeed of westward propagation were 15–20 cm s-1,
15 cm, and 7 cm s-1, respectively. The eddies were considered to be successive wavelike disturbances rather thansolitary eddies.
This study focused on five CTD stations (C031, C032,
C033, C034, and C042) that were located in the cycloniceddy. These stations had common characteristics, such as
uplifted isopycnals in a shallow layer, large SOM, a deepand steep nitracline, and a deep DCM compared withlocations outside the eddy (Figs. 3, 7). Because the temperature–salinity relationship inside the eddy was similar tothat outside the eddy (Fig. 8), the water in the eddy wasconsidered to have originated from south of the KE. These441

observations suggested that the uplift of isopycnals by theeddy injected nutrients from the deeper layer to a nutrientdepleted euphotic zone, enhanced the primary productivityof phytoplankton through nutrients from the disphotic
zone, and emitted surplus oxygen due to photosynthesis in
the area where the eddy passed (McGillicuddy et al. 1998;
Nencioli et al. 2008).
Figures 3 and 7 show that the five stations each had
different features in their vertical structure and biologicalparameters. The potential density below 50 m depth atstation C034 was higher than at the other stations, whichsuggests that station C034 was the closest station to theeddy center. Because the depth of the euphotic zone was
70 m, the density at the bottom of the euphotic zone was
24.8rh at station C034. Compared with the potential density profiles at the stations outside the eddy, this isopycnal
surface was presumably uplifted 30 m from a deeper layer.
Of all the stations in the eddy, station C034 also had thehighest level of DO in the SOM layer. The nitracline wasshallower than at other stations and was located just below
the euphotic zone. The high new productivity would haveoccurred at station C034.
Station C033 had the second highest potential density atthe bottom of the euphotic zone and the second largest
SOM layer. Of particular note was that the level of DO
from 100 to 130 m depth at this station was relatively high.
Nitrate was depleted down to a depth of 100 m and thenitracline was the deepest among the five stations. The
distance from the eddy core to station C033 was larger thanthat at station C034. These features, including the large
SOM layer and deep nitracline, suggest that the isopleths at
station C033 sank to a deeper layer due to the biologicalresponses, after they were uplifted into the euphotic zone,
as occurred at station C034. Furthermore, these biologicalresponses below the euphotic zone at station C033 suggested that the sinking of the water mass happened within a
few weeks, which was estimated by the average transition
speed of the eddy (5.1 km day-1) and the distance between
stations C034 and C033 (about 100 km). If a long time had
passed after the sinking of the isopleths, DO and nitrateanomalies should have been dissipated by diffusion andadvection or offset by respiration and remineralization.
Vertical profiles of potential density, DO, and nutrients at
stations C031, C032, and C042, which were located at theedge of the eddy, were similar to each other.
4.2 Quantitative estimation of the biochemical
responses to the passage of the eddy
The estimation of a nutrient flux, which was uplifted fromthe deeper layer to the euphotic zone by the passage of thecyclonic eddy, is important for understanding the biological productivity and material cycling in this region. In

123

442

(a) 22(b) 22
Potential Density [kg m-3]

23

Potential Density [kg m-3]

Fig. 7 Vertical potential
density profiles of a dissolved
oxygen (DO) concentration(lmol kg-1), b DO saturation(%), c nitrate concentration(lmol L-1), and d chlorophyll
a concentration (lg L-1).
Colored and gray lines are theprofiles for stations inside andoutside the eddy, respectively

C. Sukigara et al.

24
25
26
27

23

24

25

28
0

50

100

150

200

250

26

300

60

70

(c) 22(d) 22

Potential Density [kg m-3]

Potential Density [kg m-3]

23

24

25

26

90

100

110

120

23

24

25

26
0

1

2

3

4

5

6

7

8

0

0.2

C032 (30oN, 147oE)

C031 (31oN, 146oE)ooC034 (30 N, 145 E)

theory, the level of new primary production should bereflected by increases in organic carbon, DO, and thedecrease of nutrients. However, these signals dissipate withtime due to physical processes such as advection and diffusion, and biological processes such as grazing, particle
sinking, and decomposition. Therefore, sparse and traditional snapshot observations from research vessels cannot
collect sufficient data for estimating a nutrient flux and thesubsequent primary production influenced by the eddy.
Here, we attempted to make a quantitative estimation of thebiological impact of the passage of the eddy by comparing
biogeochemical parameters inside and outside the eddy
using the high-resolution hydrographic survey data set.
To define the vertical variation of DO at a station outside the eddy, averaged DO concentrations for every 0.1rh
were estimated at six stations (C030, C035, C036, C040,
C041, and C044) located outside the eddy. The increased
DO associated with the eddy passage was estimated fromthe difference between each value in the eddy and theaveraged values outside of the eddy at the same density.

0.4

0.6

0.8

1.0

Chlorophyll a [µg L-1]

Nitrate [µmol L-1]

123

80

Oxygen saturation [%]Dissolved oxygen [µmol kg-1]

C033 (30oN, 146oE)
oC042 (29 N, 143oE)

This estimation was based on the hypothesis that the eddy
did not transport water, but only propagated the uplift of
isopycnals westward. Table 1 shows the integrated differences in the DO from the seasonal thermocline(23.2–23.7rh) to 25.2rh, where large SOM layers existed(Figs. 3, 7). A large oxygen emission was seen at station
C034 (2522.6 mmol O2 m-2) close to the eddy center. If
this DO emission at station C034 occurred due to thepassage of the eddy, an average emission rate of
64.7 mmol O2 m-2 day-1 was obtained using the transit
time (39 days) of the eddy, which was estimated by theradius of the eddy (200 km) and the transit speed (about
5.1 km day-1). This value corresponds approximately tothenetcommunity
productivity(NCP)of
45.7 mmol C m-2 day-1 (548.4 mg C m-2 day-1) using
the typical elemental ratio of the photosynthesis andremineralization processes (C:N:P:–O2 = 106:16:1:150;
Anderson 1995). The largest oxygen emission in this study
was observed at station C033 (Table 1), which was passedby three-quarters of the eddy.

Biogeochemical responses associated with the passage of a cyclonic eddy

Anderson 1995). The –O2N ratio in C034 was notably high(20.2). The –O2N ratios (3.5–5.4) in the layers below the
100 m depth at four stations except C034 were smaller thanthose (14.1–21.0) in the layers above the 100 m depth. Low–O2N ratios in the deeper layers resulted from low ratios of
O2 increase to NO3 decrease. The rate of oxygen consumption by respiration of organisms in the deeper layer
would surpass the rate of NO3 production from organic
matter by decomposition and nitrification. The high –O2N
ratio in C034 was derived from the calculation, which only
contains the data above the 98 m depth.

30

20

15

10

5 Conclusion
Ocean Data View

Potential Temperature θ [degC]

25

5

0
33.5

443

34

34.5

35

Salinity [psu]C031 (31oN, 146oE)

C033 (30oN, 146oE)

C042 (29oN, 143oE)

C032 (30oN, 147oE)

C034 (30oN, 145oE)

C058 (37oN, 143oE)

Fig. 8 Temperature–salinity curve for the CTDO2 station. Coloredand black closed dots and black lines indicate the profiles at stations
inside and outside the eddy, respectively. Open dots and black lines
indicate stations in the subarctic region

The concentration of nitrate at each density level insideand outside the eddy was estimated in the same way as forDO, but the values for some density levels were linearlyinterpolated because of the coarser sampling than that usedfor DO. The largest nitrate supply (216.5 mmol N m-2)
was recorded at station C033, which also had the largest
oxygen emission (Table 1). The estimated net nitrate supplied by the passage of the eddy was comparable to thatreported in a previous study regarding a cyclonic eddy
(cyclone Opal) in the North Pacific (about
200 mmol N m-2; Mahaffey et al. 2008).
The ratios of O2 emission to NO3 supply (–O2N ratio)
varied from 10.5 to 20.2 (Table 1). The –O2N ratio waslarger than the empirical stoichiometric ratio (8.8–10.1;

Table 1 Estimated oxygen
emission, nitrate supply, and –O2N ratio

Our shipboard high-resolution survey in the subtropicalregion of the western North Pacific revealed the detailed
water structure and biological activities in and around a
cyclonic eddy. This eddy originated from the meander ofthe KE at 29°N, 154°E, and propagated westward. A
remarkable uplift of the isopycnals from a depth of about
600 m to the seasonal thermocline was observed in theeddy. The inside of the eddy was characterized by high DO
and low nitrate concentration in the subsurface layercompared with locations outside the eddy. High DO values
in thick SOM layers and deep nitraclines in the eddysuggested that the uplift of isopycnals by the cyclonic eddy
induced the injection of nutrients from the disphotic zone,
and increased the nutrient uptake and emission of oxygenby phytoplankton. The reduced nitrate in the eddy compared with locations outside was believed to be due to a
supply from the deeper layer, which was utilized by phytoplankton as new production in the water column. The
average nitrate reduction for five stations in the eddy was
182.0 mmol N m-2. This value was comparable to thenitrate supplied by winter convection in the area where
North Pacific subtropical mode water was formed (Sukigara et al. 2011).
The area of formation and the trajectory of the observed
cyclonic eddy are important features in the Kuroshio
recirculation region (Ebuchi and Hanawa 2001). If thisobserved eddy supplied nitrate into the euphotic zone overStation(location)

Range of depth(range of density)

Layer
thickness(m)

O2 emission(mmol m-2)

NO3 supply(mmol m-2)

–O2N
ratio

C031 (31N, 146E)

38–158 (23.6–25.2)

121

1994.5

190.1

10.5

C032 (30N, 147E)

46–147 (23.7–25.2)

102

1955.9

169.8

11.5

C033 (30N, 146E)

41–131 (23.3–25.2)

912659.3

216.5

12.3

C034 (30N,145E)

40–98 (23.2–25.2)

592522.6

124.9

20.2

C042 (29N, 143E)

32–151 (23.2–25.2)

120

2251.0

208.9

10.8

123

444

the area where the eddy passed, nitrate at a level of
2.0 9 1011 mol N would be induced into the biogeochemical process in this region. This level was estimatedusing the nitrate supplied per unit square (about
0.2 mol N m-2) and the area over which the eddy passed[1.0 9 1012 m2 = 5.0 9 105 m (as the eddy diameter) 9 2.0 9 106 m (as the distance that the eddy moved)].
Sasai et al. (2010) reported that during 2002–2006, on
average, two cyclonic eddies per year were generated andpassed through this area. The process of nutrient supply
due to the passage of the cyclonic eddy passage had an
important role in the high-stratification season in the oligotrophic subtropical region. To further investigate thedetailed impacts of the biogeochemical processes following the passage of an eddy, more interdisciplinary studies,including broad and high-resolution three-dimensional
observations around an eddy and a diversified range ofsampling parameters, are needed. More meaningful data
sets can be obtained by adding even more shipboard
observations, intensive observations by autonomous profiling floats with biological (DO, nitrate, and Chl a) sensors, long time-series observations using a moored systemwith a mounted sediment trap and/or biogeochemical sensors, and high-resolution three-dimensional physical–biological models.
Acknowledgments This work was partly supported by Grants-inAid for Exploratory Research (no. 17651002) Scientific Research inPriority Areas, ‘‘Western Pacific Air-Sea Interaction Study (WPASS)’’ from the Ministry of Education, Culture, Sports, Science andTechnology; by the Grant-in-Aid for Scientific Research (B) (no.
25287118) from the Japan Society for Promotion of Science (KAKENHI); by the Agriculture, Forestry and Fisheries Research Council(AFFRC) for the study of ‘‘Population Outbreak of Marine Life’’; byGrants-in-Aid for Creative Scientific Research (no. 17GS0203) from
the Ministry of Education, Culture, Sports, Science and Technology
and for Scientific Research in Priority Areas ‘‘Comprehensive studies
of global greenhouse gas cycles in the atmosphere, terrestrial biosphere and oceans’’. The authors thank the captain, crews, and scientists of the R/V Hakuho-Maru of the Japan Agency for MarineEarth Science and Technology (JAMSTEC). Thanks are also given tomembers of the Physical Oceanography Group at Tohoku University
and members of the laboratory of Satellite Biological Oceanography
in Hydrospheric Atmospheric Research Center at Nagoya University
for their helpful discussions throughout this study.

References
Anderson LA (1995) On the hydrogen and oxygen content of marine
phytoplankton. Deep Sea Res I 42:1675–1680
Armstrong FAJ, Stearns CR, Stickland JDH (1967) The measurementof upwelling and subsequent biological processes by means of
the TechniconTM AutoanalyzerTM and associated equipment.
Deep Sea Res 14:381–389
Benitez-Nelson CR, Bidigare RR, Dickey TD, Landry MR, Leonard
CL, Brown SL, Nencioli F, Rii YM, Maiti K, Becker JW, Bibby
TS, Black W, Cai WJ, Carlson CA, Chen F, Kuwahara VS,
Mahaffey C, McAndrew PM, Quay PD, Rappé MS, Selph KE,

123

C. Sukigara et al.
Simmons MP, Yang EJ (2007) Mesoscale eddies drive increased
silica export in the subtropical Pacific Ocean. Science 316:1017.
doi:10.1126/science.1136221
Benitez-Nelson CR, McGillicuddy DJ (2008) Mesoscale physical–
biogeochemical linkages in the open ocean: an introduction to
the results of the E-Flux and EDDIES programs. Deep Sea Res II
55:1133–1138
Bibby TS, Gorbunov MY, Wyman KW, Falkowski PG (2008)
Photosynthetic community responses to upwelling in mesoscaleeddies in the subtropical North Atlantic and Pacific Oceans.
Deep Sea Res II 55:1310–1320
Brown SL, Landry MR, Selph KE, Yang EJ, Rii YM, Bidigare RR(2008) Diatoms in the desert: plankton community response to amesoscale eddy in the subtropical North Pacific. Deep Sea Res II
55:1321–1333
Chelton DB, Schlax MG, Samelson RM (2011) Global observationsof nonlinear mesoscale eddies. Prog Oceanogr 91:167–216
Cullen JJ (1982) The deep chlorophyll maximum: comparing verticalprofiles of chlorophyll a. Can J Fish Aquat Sci 39:791–803
Dickey TD, Nencioli F, Kuwahara VS, Leonard C, Black W, Rii YM,
Bidigare RR, Zhang Q (2008) Physical and bio-optical observations of oceanic cyclones west of the island of Hawai’i. Deep
Sea Res II 55:1195–1217
Ducet N, Le Traon PY, Reverdin G (2000) Global high-resolution
mapping of ocean circulation from TOPEX/Poseidon and ERS-1
and -2. J Geophys Res Oceans 105(C8):19477–19498. doi:10.
1029/2000JC900063
Ebuchi N, Hanawa K (2000) Mesoscale eddies observed by TOLEXADCP and TOPEX/POSEIDON altimeter in the Kuroshio
recirculation region south of Japan. J Oceanogr 56:43–57
Ebuchi N, Hanawa K (2001) Trajectory of mesoscale eddies in the
Kuroshio recirculation region. J Oceanogr 57:471–480
Ewart CS, Meyers MK, Wallner ER, McGillicuddy DJ, Carlson CA(2008) Microbial dynamics in cyclonic and anticyclonic modewater eddies in the northwestern Sargasso Sea. Deep Sea Res II
55:1334–1347
Greenan BJW (2008) Shear and Richardson number in a mode-water
eddy. Deep Sea Res II 55:1161–1178
Gruber N, Sarmiento JL (1997) Global patterns of marine nitrogen
fixation and denitrification. Glob Biogeochem Cycles
11:235–266
Hayward TM (1994) The shallow oxygen maximum layer andprimary production. Deep Sea Res I 41:559–574
Knap A, Jickells T, Pszenny P, Galloway J (1986) Significance of
atmospheric-derived fixed nitrogen on productivity of theSargasso Sea. Nature 320:158–160
Koroleff F (1983) Determination of silicon. In: Glasshoff K, Ehrhardt
M, Kremling K (eds) Methods of seawater analysis, 2nd edn.
Verlag Chemie, Weinheim, pp 174–183
Lewis MR, Harrison WG, Oakley NS, Hebert D, Platt T (1986)
Vertical nitrate flux in the oligotrophic ocean. Science
234:870–873
Mahaffey C, Benitez-Nelson CR, Bidigare RR RR, Rii YM, Karl DM(2008) Nitrogen dynamics within a wind-driven eddy. Deep Sea
Res II 55:1398–1411
Martin AP, Pondaven P (2003) On estimates for the vertical nitrate
flux due to eddy pumping. J Geophys Res 108(C11). doi:10.
1029/2003JC001841
Martin AP, Richards KJ (2001) Mechanisms for vertical nutrienttransport within a North Atlantic mesoscale eddy. Deep Sea Res
II 48:757–773
McGillicuddy DJ, Robinson AR (1997) Eddy-induced nutrient supply
and new production in the Sargasso Sea. Deep Sea Res I
44:1427–1450
McGillicuddy DJ Jr, Robinson AR, Siegel DA, Jannasch HW,Johnson R, Dickey TD, McNeil J, Michaels AF, Knap AH

Biogeochemical responses associated with the passage of a cyclonic eddy
(1998) Influence of mesoscale eddies on new production in the
Sargasso Sea. Nature 394:263–266
McGillicuddy DJ, Anderson LA, Bates NR, Biddy T, Buesseler KO,
Carlson CA, Davis CS, Ewart C, Falkowski PG, Goldthwait SA,
Hansell DA, Jenkins WJ, Johnson R, Kosnyrev VK, Ledwell JR,
Li QP, Siegel DA, Steinberg DK (2007) Eddy/wind interaction
stimulate extraordinary mid-ocean plankton blooms. Science
316. doi:10.1126/science.1136256
Michaels AF, Knap AH, Dow RL (1994) Seasonal patterns of ocean
biogeochemistry at the U.S. JGOFS Bermuda Atlantic timeseries study site. Deep Sea Res I 41:1013–1038
Murphy J, Riley JP (1961) Modified single solution method for thedetermination of phosphate in natural water. Anal Chem Acta
27:31–36
Nencioli F, Kuwahara VS, Dickey TD, Rii YM, Bidigare RR (2008)
Physical dynamics and biological implications of a mesoscale
eddy in the lee of Hawai’i: cyclonic Opal observations during
E-Flux III. Deep Sea Res II 55:1252–1274
Oka E, Toyama K, Suga T (2009) Subduction of North Pacific centralmode water associated with subsurface mesoscale eddy. Geophys Res Lett 36:L08607. doi:10.1029/2009GL037540
Oka E, Suga T, Sukigara C, Toyama K, Shimada K, Yoshida J (2011)‘‘Eddy resolving’’ observation of the North Pacific subtropicalmode water. J Phys Oceanogr 41. doi:10.1175/2011JPO4501.1

445Oschlies A (2002) Can eddies make ocean deserts bloom? Glob
Biogeochem Cycles 16. doi:10.1029/2001GB001830
Sasai Y, Kelvin JR, Ishida A, Sasaki H (2010) Effects of cyclonicmesoscale eddies on the marine ecosystem in the Kuroshio
extension region using an eddy-resolving coupled physical–
biological model. Ocean Dyn 60:693–704
Shulenberger E, Reid JL (1981) The Pacific shallow oxygen
maximum, deep chlorophyll maximum, and primary productivity, reconsidered. Deep Sea Res A 28:901–919
Siegel DA, Peterson P, McGillicuddy DJ Jr, Maritorena S, Nelson NB(2011) Bio-optical footprints created by mesoscale eddies in the
Sargasso Sea. Geophys Res Lett 38:L13608. doi:10.1029/
2011GL047660
Sukigara C, Suga T, Saino T, Toyama K, Yanagimoto D, Hanawa K,
Shikama N (2011) Biogeochemical evidence of large diapycnal
diffusivity associated with the subtropical mode water of theNorth Pacific. J Oceanogr 67:77–85
Weiss RF (1981) Oxygen solubility in seawater. In: UNESCO
Technical Papers in Marine Science, No. 36:22

123