Regional Studies in Marine Science 2 (2015) 45–53

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Regional Studies in Marine Sciencejournal homepage: www.elsevier.com/locate/rsma

Geochemistry of two shallow CO2 seeps in Shikine Island (Japan) andtheir potential for ocean acidification research
Sylvain Agostini a,∗ , Shigeki Wada a , Koetsu Kon a , Akihito Omori b , Hisanori Kohtsuka b ,Hiroyuki Fujimura c , Yasutaka Tsuchiya a , Toshihiko Sato a , Hideo Shinagawa a ,Yutaro Yamada a , Kazuo Inaba aaShimoda Marine Research Center, University of Tsukuba, Japanb

Misaki Marine Biological Station, University of Tokyo, Japanc

Department of Chemistry, Biology and Marine Science, University of the Ryukyus, 1 Senbaru, Nishihara-cho, Okinawa 903-0213, Japanhighlights• Report on the discovery of two CO2 seeps in the temperate Pacific Ocean, Japan.
• Geochemistry survey showed the suitability for ocean acidification research.
• Confounding factors and their mitigation for the use of CO2 seeps in ocean acidification research is highlighted.
• Potential and past research of CO2 seeps in Japan is discussed.

articleinfo

Article history:
Received 24 March 2015
Received in revised form
11 July 2015
Accepted 11 July 2015
Available online 14 July 2015
Keywords:
CO2 seep
Temperate Pacific Ocean
Japan
Ocean acidification
Geochemistry

abstract
Shallow CO2 seeps, where CO2 gas is venting underwater, offer great potential for studies into the effectsof ocean acidification at the ecosystem level. To our knowledge, only two tropical system and two temperate systems of such seeps have been described worldwide. Here we describe two new temperate systems:
the Mikama Bay and Ashitsuke sites, located on Shikine Island, Japan. The Mikama Bay site is located in ashallow bay. Investigation of the gas and water chemistry showed that the gas contained 98% CO2 and up
to 90 ppm H2 S. Total alkalinity was constant in time and space with an average of 2265 ± 10 µ mol kg−1 .
Mapping of Eh and pH showed that the low pH zones were the largest when currents were moderate.
Under moderate currents, Eh values were globally higher and total sulfides concentration lower, supporting that a longer residence time of the bay water allow the oxidation of the sulfides to sulfates. Zones
suitable for acidification studies: with a pH lower than 8.0, low saturation state of calcite and aragonite,
and non-detectable sulfide concentration, can be defined a few meters from the main venting zone. The
second site, Ashitsuke, is located in the inter-tidal zone on a shore composed of boulders. Several areas
showed reduced pH sometimes restricted to a few meters and up to 20 m long along the shoreline. Temperature was higher in some of the reduced pH zones suggesting the presence of hot springs in addition
to vents. This paper also highlights the need for discovering additional CO2 seeps, which by their natureoften lack comparable replicates and can be confounded by factors other than CO2 . In this regard, Japan
offers great potential as it is home to numerous active volcanoes, representing potential venting sites inclimates ranging from tropical to sub-polar.
© 2015 Elsevier B.V. All rights reserved.

1. Introduction
Increased atmospheric carbon dioxide (CO2 ) has led to a decrease in the pH of the oceans by 0.1 in the last 100 years. Ocean pH∗ Correspondence to: 5-10-1 Shimoda, 422-0015 Shizuoka, Japan. Tel.: +81 558
22 6697.
E-mail address: agostini.sylvain@shimoda.tsukuba.ac.jp (S. Agostini).
http://dx.doi.org/10.1016/j.rsma.2015.07.004
2352-4855/© 2015 Elsevier B.V. All rights reserved.

levels are predicted to further decrease by 0.5 unit of pH in the next
100 years, a phenomenon known as ocean acidification (Doneyet al., 2009). This unbuffered decrease in pH has been shown toaffect a wide range of organisms in the oceans. Calcifiers, organisms that build skeletons or external shells from calcium carbonate,such as corals, foraminifera, and gastropods, are particularly sensitive to an increase in seawater pCO2 , as it lowers calcium carbonate saturation state (Cohen and Holcomb, 2009). In contrast, otherorganisms may benefit from increased water pCO2 , such as the

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S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

Fig. 1. Map of terrestrial volcanoes (red triangles) and submarine volcanoes (blue triangles) along the Japanese coastline, and at the two sites surveyed in this study.
Insets show the Izu peninsula and the Izu Islands (top right), and Shikine Island with the Mikama Bay and Ashitsuke sites (bottom right). Shallow venting sites highlighted
in Table 1 are shown (green diamonds). Location data for volcanoes are provided by the Hydrographic and Oceanographic Department of the Japan Coast Guard(http://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/list-2.htm, in Japanese). (For interpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)coccolithophore Emiliania huxleyi, which can use the additional available CO2 for photosynthesis under certain conditions(Iglesias-Rodriguez et al., 2008), diatoms or seagrass which wereshown to increase under high pCO2 (Johnson et al., 2011). Suchvariability in the response of organisms to ocean acidification
makes it difficult to predict the overall effects of ocean acidification at the ecosystem level. An experimental approach would require that an important number of species are studied just to havean idea of the effect of acidification on one ecosystem, and thisapproach would still not provide an understanding of the effects
of inter-species interactions. Therefore, alternative approaches are
required. Dr. Hall-Spencer et al. (2008) reported the effects of natural CO2 vents on a coastal ecosystem near the island of Ischia,
Italy, providing acidification information at the ecosystem level forthe first time, and revealing the potential of CO2 vents as a new
approach to research on ocean acidification (Hall-Spencer et al.,2008).
Natural CO2 seeps (submarine CO2 vents) occur in various
places around the world, and are generally associated with volcanic activity. After the first report by Hall-Spencer et al. (2008),
other studies of such systems have been published. For example, Fabricius et al. (2011) reported the effects of increased CO2
on a coral reef in Papua-New-Guinea, and Inoue et al. (2013) focused on the impact on a coral reef in the Ryukyus Archipelago inJapan. More recently, the ecosystems around other CO2 seeps in
New Zealand have been studied (Brinkman and Smith, 2014), andother temperate CO2 seep ecosystems have been reported from theMediterranean Sea (Boatta et al., 2013), near the Ischia site previously described (Hall-Spencer et al., 2008). In addition to these CO2
seeps, other naturally acidified coral reef ecosystems have been described from Puerto Morelos, México where springs discharge low
pH waters into the reefs (Crook et al., 2013, 2011). Therefore, to ourknowledge, the natural acidification of only two temperate ecosystems and three tropical ecosystems, when the Puerto Morelos is
included, have been studied thus far, which comprise a very small
fraction of the diverse marine ecosystems on Earth.

Japan lies at the junction of four tectonic plates, and as a result
is home to numerous volcanic systems, many of which are on the
coastline or comprise submarine volcanoes (Fig. 1). The Japanesearchipelago extends across a wide range of latitudes, 24–46°N,
from tropical to sub polar zones. As such, there is a high probabilityof discovering marine CO2 seeps in different climates in theseregions around Japan. Deep hydrothermal vents with high CO2
contents have been actively studied (i.e.: Ohta and Kim, 2001) but
the identification and description of shallow seeps are still scarce.
One critical exception to this is the shallow CO2 seep discovered on
the volcanic island of Iwotorishima in the Ryukyu arc (Inoue et al.,2013), which marked the beginning of ocean acidification researchusing natural vents in Japan. The Izu–Bonin–Mariana arc system isa good candidate for hosting CO2 seeps. One of the advantages ofthis arc system is that it ranges from temperate to tropical zones
and therefore may provide the opportunity to study the effects ofocean acidification on diverse marine ecosystems.
In this paper, we report the discovery of two CO2 seeps in the
Izu arc and present results from studies on geochemistry at thesesites. We also discuss the drawbacks in using natural vents in ocean
acidification research and based on our own experiences, propose
ways in which these drawbacks can be mitigated. Finally, we
discuss the overall potential of the Japanese arc systems for using
CO2 seeps to study the effects of ocean acidification on marine
ecosystems.
2. Materials and methods
2.1. Study sites
Two different sites were investigated: Mikama Bay andAshitsuke. Both sites are situated on the south coast of Shikine
Island, off the coast of the Izu peninsula (Fig. 1). Shikine Island
is located at 34°N, and it is in a temperate zone, under the
influence of the Kuroshio warm current. As a result, numerous

S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

47

Table 1
Chemistry data: pH, Eh, and total sulfides in the waters of Mikama Bay. Sample locations are shown in Fig. 2.
Datelocation
20141030
Mikama20150422
MikamaSampleID

LatitudeLongitude

Depth
(m)

Temp
(°C)

Salinity

Eh (mV)

Total sulfide(nmol l−1 )PH

Alkalinity(µmol kg−1 )pCO2(µatm)

Ω ca

Ω ar

1
2
3
4
5
6
7
8

34.32022
34.32016
34.32045
34.32038
34.31986
34.31984
34.31894
34.32063

139.20304
139.20308
139.20277
139.20288
139.20294
139.20303
139.20652
139.20300

5.30
5.05
3.15
3.75
6.10
9.25
6.15
2.05

24.80
25.80
26.80
27.80
28.80
29.80
30.80
31.80

34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2

−1
−64
−47
−47

<40
>410

−45

240
280
<40
<40
<40
>410

8.03
7.84
7.55
7.80
8.06
8.06
8.14
6.86

2251
2238
2264
2239
2251
2261
2255
2251

596
976
2 040
1 081
549
552
442
10 458

3.74
2.55
1.40
2.35
3.96
3.97
4.60
0.30

2.46
1.68
0.92
1.55
2.60
2.61
3.03
0.20

9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

34.32000
34.32021
34.32008
34.31947
34.31955
34.32061
34.32146
34.32150
34.32114
34.32003
34.32006
34.31973
34.32016
34.32069
34.32027
34.31974
34.31945
34.32007
34.31965

139.20340
139.20300
139.20360
139.20370
139.20450
139.20510
139.20460
139.20440
139.20500
139.20660
139.20680
139.20650
139.20560
139.20560
139.20360
139.20310
139.20280
139.20290
139.20330

5.70
4.40
6.70
8.70
8.25
8.95
3.15
2.20
7.50
3.35
45.00
3.60
8.70
8.55
5.55
4.75
3.85
2.45
6.75

15.65
16.05
15.70
15.53
15.48
15.55
16.20
16.05
15.80
15.60
15.20
15.40
15.40
15.40
15.70
15.58
15.48
15.92
15.50

34.5
34.5
34.5
34.5
34.5
34.6
34.5
34.5
34.5
34.5
34.6
34.6
34.6
34.6
34.6
34.6
34.6
34.6
34.6

69
38
64
112
134
142
96
94
69
149
133
112
155
166
89
99
125
87
105

<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40
<40

7.99
7.78
7.98
8.12
8.13
8.06
7.46
7.58
7.80
7.96
7.70
7.97
8.12
8.13
7.92
8.04
8.09
7.85
8.08

2272
2267
2272
2275
2274
2268
2271
2267
2271
2269
2275
2267
2266
2269
2275
2267
2273
2278
2279

626
1 066
642
445
433
519
2 333
1 742
1 014
675
1 287
655
442
431
749
547
481
898
495

2.69
1.76
2.64
3.47
3.54
3.09
0.89
1.15
1.82
2.53
1.45
2.56
3.45
3.52
2.34
2.97
3.28
2.05
3.22

1.73
1.13
1.7
2.23
2.27
1.98
0.57
0.74
1.17
1.62
0.93
1.65
2.22
2.26
1.51
1.91
2.11
1.32
2.07

0m
2m
4m
6m
25m
30m
50m

–
–
–
–
–
–
–

–
–
–
–
–
–
–

–
–
–
–
–
–
–

22.9
22.3
21.4
21.2
21.7
22.1
20.6

24.0
34.0
34.1
34.1
33.9
33.5
34.0

–
–
–
–
–
–
–

–
–
–
–
–
–
–

7.248
7.552
7.999
8.026
7.879
7.502
8.222

2225
2225
2225
2222
2203
2203
2219

4 463
1 949
626
582
851
2 210
340

0.54
1.28
3.16
3.31
2.47
1.13
4.68

0.34
0.84
2.06
2.15
1.61
0.73
3.04

3

−8
73

20141030
Ashitsuke

scleractinian corals and other tropical organisms can be observedin the marine ecosystem surrounding the island (Supplementary
Video, Appendix A). Initial geochemical surveys were carried out
to evaluate the potential of the sites before further investigations
were made into water and gas chemistries.
2.2. Gas chemistry
Gas was sampled at four different locations across the venting
areas in 22nd June 2014. The gas was collected in 500 ml plastic bottles by inverting a funnel above the vents (Supplementary
Video 1, Appendix A). It was then transferred into a 50 ml syringe
while still under water, closed with a cork, and brought onto thevessel where CO2 and H2 S were immediately measured using 2HT(range 10%–100%) and 4LT (0.2–2 ppm) test tubes (Gastec corporation, Japan), respectively. Three samples of gas were collected inMikama Bay and one sample was collected at the main venting siteof Ashitsuke.
2.3. Water geochemistry
In Mikama Bay, the area of interest was surveyed with a multisensor (U-5000G, Horiba Ltd, Kyoto Japan) coupled with a GPS. Bottom water temperature, pH calibrated on the NBS scale, Eh, depth,and salinity were measured. Bottom water samples were collectedusing a Van Dorn water sampler at different locations around thebay (Fig. 2). The samples were then immediately transferred inglass DO bottles. Total sulfides were measured using a Gastec test

tube for hydrogen sulfide (H2 S) (Gastec 4LT, Japan) mounted on an
impinger; this occurred immediately after sampling on board theresearch vessel Tsukuba II. Twenty milliliters of the water samples
were transferred into the impinger, 2 ml of 16 N H2 SO4 were added,
and the resulting H2 S produced was then pushed through the test
tube for measurement. The remaining water sample was filtered at
0.45 µm with cellulose acetate filters (Dismic 045, Advantec Japan)
and stored for total alkalinity measurements.
To assess the variability of the geochemistry in Mikama Bay thechemistry mapping and hydrology surveys was carried twice: on
the 30th of October 2014 and on the 22nd of April 2015. In addition,
monitoring of the variation of pH over time at a fix location was
conducted by fixing a TOA DKK multisensor at a depth of 5 m
close to the venting area (Fig. 2: star mark) on the 1st of May
2015. During both mapping surveys the depth, current velocity
was measured at 2 and 5 m in October 2014 and April 2015
respectively using a Doppler current profiler (CI-68, FURUNO),
and wind velocity were recorded using the instruments (KV-5026,
KOSHIN DENKI KOGYO) aboard the research vessels Tsukuba II.
At the Ashitsuke site, pH, temperature, dissolved oxygen,salinity and Eh were measured along a 50 m transect (line 0)
on the shore on the 30th October 2014 and on the 22nd April
2015 on two transect of 100 m (line 1) and 60 m (line 2)(Supplementary Fig. 1, Appendix A), using two Orion 4-Star pHDO sensors (Thermo Scientific) equipped with an RDO probe, apH electrode (8156BNUWP, Thermo Scientific, calibrated on the
NBS scale), a conductivity electrode (Orion 013010MD, Thermo
Scientific) and an ORP electrode (918BNMD, Thermo Scientific).
The 0 m points are always the closest point from the same venting

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S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

Fig. 2. pH (A: 20141030; B: 20150422) and Eh (C: 20141030; D: 20150422) in Mikama Bay. Diamonds show the location of water sampling for total sulfide measurement
and total alkalinity measurements. The red starts show the location for time variation measurement of pH Contour lines show the depth in meters. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)zone. Water samples for total alkalinity were taken every 5 m alongline 0, on the 30th October 2014.
All water samples for total alkalinity were filtered at 0.45 µm
with cellulose acetate filters (Dismic 045, Advantec Japan). Total
alkalinity was measured via titration with HCl at 0.1 mol l−1 witha Metrohm titrator (785 DMP titrino) and total alkalinity wascalculated by Gran plot from titration point with a pH between
4.0 and 3.0. Carbonate chemistry parameters: partial CO2 pressure(pCO2 ), Aragonite and calcite saturation state (Ωaragonite , Ωcalcite ),and dissolved inorganic carbon (DIC) were calculated using the
CO2SYS software package (Pierrot et al., 2006) using pH (NBS scale)and total alkalinity with the constants from Roy et al. (1993).
Chemistry and hydrography data were plotted on maps using
the free opensource software QGIS (QGIS, 2009). For vector
plots (wind and currents) the plugin ‘‘Vector field renderer’’(ccrook/QGIS, 0000) was used. Depth contour lines were produced
in QGIS from recorded depth at random points, interpolated using
the interpolation plugin (Inverse distance weighting algorithms)
and corrected by hand for anomalies.
3. Results
3.1. Mikama Bay
3.1.1. Preliminary survey
The venting zone is located in a bay surrounding by cliffs, on
the south coast of the Shikine Island. During our first survey inJune 2014, abundant gas discharge was found in the north westof the bay, and a hot spring also discharges water nearby. The
venting zone was characterized by a low pH in the water (<7), andtemperature (22 °C) and salinity (34) was constant throughout theBay. The smell of hydrogen sulfide could be detected at the surface
close to the main venting site. The Bay has a steep slope towards

the ocean, with depth reaching over 25 m at the entrance of the
Bay (contour line in Figs. 2 and 3).
3.1.2. Gas chemistry
Gas release is very abundant at the main vents, and the vent
fluxes are mostly constant (Supplementary Materials: Video 1,
Appendix A). A sufficient amount of gas (500 ml) could be collected
in a few seconds at three random locations in the venting zone.
The gas contained 98 ± 3% CO2 (n = 3). H2 S was also present,with concentrations varying from 24 to 90 ppm, with an averageof 60 ± 33 ppm (n = 3).
3.1.3. Geochemistry
Assessment of the water chemistry in the bay showed steep pH
and Eh gradients. During the survey in October 2014, the lowest pH
recorded, directly above the vents, was 6.86, which corresponded
to an Eh value of −70 mV (Fig. 2(A), (C)). In April 2015, threezones with reduced pH were observed (Fig. 2(B), (D)). At the firstzone, corresponding to the zone found in 2014, the minimum pH
observed was 7.34 with an Eh of 32 mV. At the second zone, in asmall bay in the center of Mikama Bay, the minimum pH observed
was 7.27 with an Eh of 67 mV. Finally, at the third venting zone
on the west of Mikama Bay the minimum observed pH was 7.33
with an Eh of 85 mV. Zones with a pH of approximately 7.9 and aslightly positive Eh were observed to the east of the venting site
in October 2014. In April 2015 survey, all Eh values were positive.
Salinity and temperature were similar between the venting area
and the remainder of the bay, with averages of 35.2 ± 0.2 and25.1 ± 0.2 °C in October 2014, and 34.6 ± 0.1 and 15.6 ± 0.3 °C
in April 2015. Total sulfide concentrations in the water varied with
Eh levels. In locations and time of sampling, where Eh values werepositive or close to zero, total sulfides were below the detection
limit (<40 nmol l−1 ), as it was the case for all samples in April 2015,

S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

49

A

B

Fig. 4. Variation in pH and depth at a fix location in Mikama Bay on the 1st of May
2015.

3.2.2. Gas chemistry
The volume of gas being vented is not as important as in Mikama
Bay and is mostly intermittent. Therefore, only one sample wascollected. The gas contained 56% CO2 and 3.3 ppm H2 S.

Fig. 3. Currents (blue arrows) and wind (gray arrows) recorded in Mikama
Bay during the surveys on the 30th of October 2014 and 22nd April 2015. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

and where Eh values were negative, concentrations varied from240nmol l−1 to more than 410 nmol l−1 (Table 1). Total Alkalinity
was relatively constant in time and space with an average value of2265 ± 10 µmol kg−1 . Calculated carbonate parameters of water
samples taken, showed maximum observed pCO2 of 10,458 µatm
in October 2014 and 2333 µatm in April 2015 corresponding toundersaturated value with regard to calcite and aragonite (Table 1).
Moderate turbulent winds, ranging 0.6–11.5 m s−1 , average
4.5 m s−1 , were observed in October 2014, which were associatedwith current velocities at 5 m in the surface water ranging from0 to 1.6 knot, average 0.4 knot, during the surveys (Fig. 3(A)).
Moderate North–North–East winds, ranging 1.5–8.6 m s−1 ,average 5.1 m s−1 , were recorded in April 2015, and associatedwith low current velocities, ranging from 0 to 0.2 knot, average
0.04 knot (Fig. 3(B)).
Logging of pH which was conducted every minute during 3 h in
May 2015 (Fig. 4) showed an average pH of 7.75 ± 0.05, ranging
7.59–7.88. During the 3 h, the depth increased from 4.7 to 5.7 m,
but the variation in pH were not associated with the change of
depth on the observed period.
3.2. Ashitsuke
3.2.1. Preliminary survey
The site is located on the shore near to the Ashitsuke harbor(Supplementary Fig. 1, Appendix A). The shore is composed of rocks
varying in size, with the largest more than 2 m in diameter. At low
tide, gas seeps can be seen in tidal pools, and hot spring wateris also discharged into several pools. At high tide, the seeps arebeneath the water.

3.2.3. Geochemistry
Water chemistry was measured at high tide when the seeps
are covered by water (Figs. 5 and 6). During the survey in 2014,a very steep gradient in pH was observed between 0 m (closest
from the seep) and 5 m along the transect, with the lowest value of
7.25 at 0 m. Another drop in pH was observed at approximately
25 m, which appears to be caused by another spring. At 0 m, a
decreased salinity (24), slightly increased temperature (22.9 °C)
and increased Eh (121.9 mV) were recorded, but these values
returned to a more typical level, either 1 m from the seep, fora salinity of 34 and Eh of 53.3 mV, or 5 m from the seep fora temperature of 21.2 °C. A similar trend, but with a smaller
amplitude, was also observed at approximately 25 m. The alkalinity
was mainly constant along the transect with a value of 2220 ±7 µmol kg−1 . The corresponding carbonate parameters: pCO2 ,Ωcalcite and Ωaragonite ranged from 340 µatm (50 m) to 4463 µatm
(0 m), 0.54–4.68 and 0.34–3.04, respectively (Table 1).
During the 2015 surveys, several low pH zones were observedin both north (Fig. 7A, Supplementary Fig. 1: line 1) and southtransects (Fig. 7B, Supplementary Fig. 1: line 2) with minimum
observed pH of 6.65 and 7.60 respectively. The decrease of pH wasassociated with an increase of temperature reaching up to 22 °C at
23 m of the north transect.
4. Discussion
The results presented here from the Mikama Bay and Ashitsukesites highlight the potential of these sites for the study ofacidification at the ecosystem level with zones naturally enrichedin CO2 . The presence of potentially confounding factors is
acknowledged, but if study methodologies are well planned andthese factors are mitigated, then these sites can provide valuable
information on the effects of acidification at ecosystem levels. The
confounding factors found in this study were the presence of H2 S in
the venting gas at Mikama Bay, with concentrations up to 192 ppm,
and the limited spatial range of the pH gradient and increased
temperature in Ashitsuke. These factors are not unique to thesesites, and are often present in other natural CO2 seep systems,
potentially also having an effect on the ecosystems around the

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S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

A

B

Fig. 5. Chemistry of seawater at high tide along a 50 m transect (line 0) in Ashitsukein October 2014. Temperature, salinity, pH and Eh were measured every 1 m.

sites (Riebesell et al., 2010; Barry et al., 2010). In order to allow
for and mitigate the effects of these factors, different approaches
are proposed and discussed below.
The first potentially confounding factor often associated withvolcanic vents is the release of toxic gases such as hydrogen sulfide(H2 S). In seawater, H2 S dissociates into the hydrosulfide ion (HS− )and the bisulfide ion (S2− ). These three species are toxic and arereferred to as the total sulfides. In water containing a sufficientamount of oxygen, and with an appropriate pH, temperature andEh, sulfides will be oxidized to diverse forms including, at theusual oxygen concentrations of surface water, non-toxic sulfate
ions (Bagarinao, 1992). The venting gas at Mikama Bay contained ameasurable amount of H2 S, the Eh values close to the venting sitewere low in one of the survey, and a relatively high concentrationof total sulfides was recorded here, showing that a non-negligible
portion of the H2 S released remained in the form of sulfides in
the water in that region. However, at greater distances from theventing sites, Eh values became positive and no total sulfides
could be detected, suggesting that most of the released H2 S had
been oxidized to sulfate. During the 2015 surveys, Eh values wereglobally higher than during the 2014 surveys and the zones where
pH was affected tended to be larger. This difference could be
attributed to slowlier current at the time of the 2015 surveys(Fig. 3) and therefore a longer turn over of the water in the Bay.
The oxidation of sulfide is rapid, with a half time ranging from 0.4 h
to 65 h in air-saturated seawater (Bagarinao, 1992; Millero, 1986),

Fig. 6. Temperature and pH measured along the north and south transect (line 1
and 2 respectively). Temperature and pH were measured every 1 m.

and a high residence time of the water will allow the complete
oxidation of sulfides to sulfate on a short distance range around thevent. The toxicity of sulfides to marine organisms is still not wellunderstood, and toxic concentrations are not well constrained.
However, sulfides in water solution were found to be toxic tomammals at a concentration of 2 µmol l−1 , while negative effects
on fish physiology can be seen at concentrations greater than
0.45 µmol l−1 H2 S, and concentrations of 280 µmol l−1 were foundto cause hepatic malfunctions in the freshwater common carp.
Some exceptions to this general pattern exist, however, with somefish species tolerant to concentrations as high as 300 µmol l−1(Tobler et al., 2006). The effect of sulfides on algae is very variable,with some thriving in high concentrations of 250–500 µmol l−1
and other inhibited by sulfide concentrations of 30–60 µmol l−1(Bagarinao, 1992). The seagrass Thalassia testudinum was shown
to be tolerant to underground exposure of sulfide at mmol l−1
levels (Koch and Erskine, 2001). Only limited literature is available
on the toxicity of sulfides for marine invertebrates. Toxic sulfide
levels are suggested to be 200 µmol l−1 and higher (Völkel andGrieshaber, 1995), but often only meiofauna and deep-sea thermalvent organisms have been considered (Thiermann et al., 2000), andother benthic and planktonic invertebrates that are not typically
exposed to high sulfide concentrations may have a lower tolerance.
The concentrations of H2 S and total sulfides found in Mikama Bay
are comparable to those of some other natural CO2 seeps thathave been used for acidification studies. Venting gas at the Ischia

S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53

system of Hall-Spencer et al. (2008), the system on IwotorishimaIsland (Inoue et al., 2013) and two of the sites in Papua New
Guinea (Fabricius et al., 2011) are reported to contain undetectableamounts of H2 S. In contrast, a site on Vulcano Island in Sicily,
and the Dobu site in Papua New Guinea contained measurable
amounts of H2 S. At Dobu, 163 ppm H2 S was measured in theventing gas (Fabricius et al., 2011). The chemistry at the Vulcano
Island system in Levante Bay is well described, and data from 1984
are available (Boatta et al., 2013), showing that H2 S concentrationsin the venting gas are highly variable in both space and time,with concentrations ranging from 21,600 ppm to less than 50 ppm
in nearby sites. Eh and total sulfides were also measured at theLevante system, and showed a similar trend to that in Mikama Bay,with the most extreme values above the venting sites. Further fromthe vents, Eh and pH levels were higher, reaching levels at which
sulfates will dominate (Boatta et al., 2013). The concentrations
found in Mikama Bay, are well below the toxic concentrations
reported in the literature, except for directly above the vents where
the total sulfide concentrations were higher than 410 nmol l−1 . At
positive Eh values most of the sulfide will be oxidized, and total
sulfide concentrations of less than 40 nmol l−1 were found at Ehlevels higher than −8 mV. This relationship provides us with amethod of ensuring that the potential effects of H2 S in venting
gas are mitigated. With a simple mapping of Eh and pH levels, it
is possible to select zones where a decrease in pH is still present
because of the high concentration in CO2 in the venting gas, butwhere most of the H2 S has been oxidized to non-toxic sulfates, as
indicated by positive Eh values. To confirm the suitability of thestudy zone, long-term measurements of pH and Eh are required, as
the chemistry around venting sites was shown to be highly variable
and will be strongly dependent on the current velocity and water
residence time.
The spatial and temporal variability of the water chemistryaround venting sites is not a good representation of future changes
expected because of an increase in atmospheric CO2 (Kerrison et al.,2011). However it has been suggested that acidification due toincreased atmospheric CO2 will also be accompanied by increase
pH variation both at the organism scale (Agostini et al., 2013)
and ecosystem level (Anthony et al., 2011; Schulz and Riebesell,2012) and therefore the study of effects of rapid pH variation as
seen in CO2 seeps, on the organisms physiology may also be useful(Dufault et al., 2012). The variability of pH at venting systems is
mainly determined by two different factors: the amount of gas
released, and the water circulation. Water circulation will not only
determine the amount of gas that will dissolve in the water masspassing the venting zone, but also the oxidation of H2 S and therelease of CO2 into the atmosphere. All the venting systems thathave been studied have shown significant variation in pH overtime, with a variation of up to 1 unit of pH (Hall-Spencer et al.,2008; Fabricius et al., 2011; Kerrison et al., 2011). Mikama Bay is
not different and variations of 0.5 units were observed during a 3 hmonitoring of pH. Variations are often coupled with the tidal cycle(Inoue et al., 2013) however this could not be observed in our study
as it would require longer monitoring. The effects of the tidal cycle
can be seen on two levels. Firstly, it has been shown that changing
water pressure as a result of a change in depth directly affects
the amount of gas released (Furushima et al., 2009). Secondly,
the tidal cycle will also affect the residence time of the water,
particularly in coral reefs where water exchange with the open
ocean is highly reduced at low tides. The residency of water andthe currents in the venting zone will also determine the steepnessof the gradient. In Mikama Bay, a sharp gradient was observed overless than 100 m on the west of the Bay in October 2014 and larger(over around 200 m) in April 2015, which is more distinct than atother venting systems where the pH gradient and affected zones
span several hundreds of meters (Boatta et al., 2013). The steepness

51of the gradient could be caused by the bay structure, including
its depth which rapidly increased going south and its opening tothe ocean waters. In Ashitsuke the gradient is even steeper thanin Mikama bay, with pH returning to values greater than 8 over
only a few meters. At this site, the seeps occur within the intertidal zone, and therefore, at high tide they are covered by highly
active water with a high pH, limiting the zone in which pH canbe affected. The pH at Ashitsuke is also in consequence expected
to be highly variable. Long term monitoring of the pH in the intertidal zone will be required for further investigation. The study ofnatural venting sites has been criticized owing to the fact that thecommunity inhabiting the venting sites may have come from far
away and therefore, may not reflect an adapted community thatwould form under the predicted future acidification caused by anincrease in atmospheric CO2 over the next hundred years (Barryet al., 2010). To mitigate the effects of potentially ‘‘temporary’’
residents of the venting sites, future studies could focus on nonmotile organisms. Moreover, special focus should be given to the‘‘winners’’, the dominant species in the low pH zones, as they likely
show a natural adaptation to the low pH values, and therefore may
represent the future dominant species. Finally, observations madeduring field studies, especially of physiological effects, should be
confirmed through experiments in a controlled environment whenpossible (i.e. Inoue et al., 2013). Addressing fluctuations in pH
over time is more problematic. Fluctuations observed at venting
sites does not clearly represent long-term changes as a resultof atmospheric change. Careful mapping of the water chemistry
and data collection over an extended period of time can highlightareas where these fluctuations have a smaller amplitude, possibly
further from the venting sites (Kerrison et al., 2011) Laboratory
experiments, where fluctuations in pH can be controlled andlimited, should be used to confirm the effects observed in the field.
The reduction in pH in Ashitsuke was often associated with ahigher temperature. Several hot springs are present close to theseeping zones. These hot springs are mainly hot seawater and nochanges in alkalinity was observed. A hot springs is also present inMikama Bay (north west shore) but no increased in temperature
was detected probably due to the large volume of water in thebay and the distance of the studied zone from the hot spring. The
temperature in Ashitsuke were lower than in Mikama Bay during
the October 2014 survey. This difference could be attributed tothe cooling by air. The local increase in temperature in Ashitsuke
will limit its use for pure ocean acidification studies but could be
used to study the simultaneous effect of increased temperature andreduced pH. Sites along the south transect showed temperature
increased by less than 1 °C and reduced pH that correspond to thepredicted value for 2100.
Finally, because of the nature of their formation, it is often
difficult to find comparable replicates of venting sites. In manycases, nearby venting sites can be identified, and often, two(Hall-Spencer et al., 2008; Brinkman and Smith, 2014) or threeventing sites (Fabricius et al., 2011) have been combined in CO2
seep studies. In other cases only one venting site of comparable
characteristics could be found in an area (Inoue et al., 2013). In
Mikama Bay at least two reduced pH zones were observed andcould be used as replicates. However due to their proximity, their
independence can be discussed and they do not represent theoverall diversity of ecosystem and geomorphology that can beobserved in Shikine Island. Due to the lack of replication and thecomplexity inherent in interpreting the data given the various
potential confounding factors, it is difficult to consider ecosystems
close to CO2 seeps as a window into future ocean acidification
effects on the diverse marine ecosystems. To mitigate the effectsof this lack of replication, two different approaches are possible.
Firstly, the statistical analysis carried out on any obtained data
should be rigorously selected. Havenhand et al. (2010) suggested

52

S. Agostini et al. / Regional Studies in Marine Science 2 (2015) 45–53
Table 2
Japanese seeps reported in literature for which the gas composition is known. The seeps are potentially suitable for the study of the effects of acidification
at the ecosystem level.
Venting site

Gas composition

Funka-asane

CO2

95.1%

H2 S
Others

4.18%
0.7%

Lat

Long

Depth (m)25°27′ 20′′ N

141°14′ 20′′ E

20

Hydrographic Department,
Maritime Safety Agency (1994)

Taketomia

CO2
CH4
N2
O2
H2 S
He

0.45–1.9%
68.2–69.5%
30.4–34.1%
0.7–1.3%
0.008–0.015%<0.0006%

24°17′ 12′′ N

123°52′ 54′′ E

20

Noguchi et al. (2006)

Backarc Basin

CO2
H2 S
Others

86%–92%
3–5.5%
3.5–1127°16′ 54′′ N

127°4′ 48′′ E

1300–1550

Sakai (1994)

Ogasawara arc

CO2
NH4
H2 S
Others

3.92–149.8 mM/kg
0–5.59 mM/kg
1.2–9.66 mM/kg
0.5–11.0 mM/kg

29°47′ 39′′ N

140°20′ 31′′ E

692–2300

Sakai (1994)

Wakamiko
CalderaCO2

50.7–92.6%

31°39′ 2′′ N

130°45′ 09′′ E

75–200Shitashima (2009)

30°47′ 35′′ N

130°18′ 19′′ E

Satsuma Iwojima

H2 S
H2
N2
CH4
O2
Ar

<1.37%<0.72%<10.8%<26.5%<3.22%<0.157%

CO2>98%

Sato et al. (2014)a

Low CO2 content in the Taketomi site is not suitable for acidification research but has provided valuable information on the dynamics and physics ofventing.

a statistical methodology suitable for the study of CO2 seeps at theecosystem level. They proposed that the response of the ecosystem
obtained at the venting sites, should be compared with the average
response observed at various nearby control sites. An alternative
approach is to confirm the ecological effects observed at a ventingsite by comparing them with the ecological effects observed
at different seeps located at similar latitudes and in similar
environments. Such ‘‘pseudo-replication’’ adds extra weight to theneed of the findings of CO2 seep studies. However, the number ofseeps that have been described is still too low, with only two intemperate zones and two in tropical zones, or three if the Mexican
site is included. Here, we report one more temperate shallow siteat Mikama Bay, and one intertidal site at Ashitsuke. The likelihoodof finding other CO2 seeps in Japan is high, considering the numberof active volcanoes along the Japanese coastline (Fig. 1).
Japan is already well known for its studies of deep hydrothermalvent (Sakai et al., 1990; Sakai, 1994; Shitashima, 2009) but a simple review of the literature points to more shallow and epipelagic
venting systems (Shitashima et al., 2008; Kikawada et al., 2005;
Noguchi et al., 2006; Hydrographic Department, Maritime Safety
Agency, 1994) that could be used for studying ocean acidification.
Some of these potential locations and their gas chemistry are summarized in Table 2. Among these, the ‘‘Wakamiko Caldera’’ system
has already been used to assess the effect of CO2 sequestration inmarine sediments (Shitashima et al., 2008). Japan extends acrossa wide range of latitudes, and the Izu–Bonin–Mariana arc in particular, which hosts the sites reported in this study, has a significant number of volcanoes that are possibly associated with venting
sites, in climates ranging from temperate to tropical. Combining
the wide latitude range of Japan and the abundance of volcanoes,
Japan has the potential to host a wide varieties of CO2 seep systems and therefore could afford researchers great opportunities for

ocean acidification research using naturally CO2 enriched systems.
Our future studies will continue to focus on investigating suitablesites along the Izu–Bonin arc.
Acknowledgment
This work was supported by JAMBIO, Japanese Association for
Marine Biology, as a program of Joint Usage/Research Center by the
Ministry of Education, Culture, Sports, Science and Technology.
Appendix A. Supplementary materials
Supplementary material related to this article can be foundonline at http://dx.doi.org/10.1016/j.rsma.2015.07.004.
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