Journal of Shellfish Research, Vol. 27, No. 1, 169–175, 2008.
HYDROTHERMAL VENT MUSSEL HABITAT CHEMISTRY, PRE- AND
POST-ERUPTION AT 9°50#NORTH ON THE EAST PACIFIC RISE
HEATHER A. NEES,1* TOMMY S. MOORE,1 KATHERINE M. MULLAUGH,1
REBECCA R. HOLYOKE,1 CHRISTOPHER P. JANZEN,2 SHUFEN MA,1
EDOUARD METZGER,1,3 TIM J. WAITE,1 MUSTAFA YÜCEL,1
RICHARD A. LUTZ,4 TIMOTHY M. SHANK,5 COSTANTINO VETRIANI,4
DONALD B. NUZZIO6 AND GEORGE W. LUTHER, III1*
1
College of Marine and Earth Studies, University of Delaware, Lewes, Delaware 19958; 2Department of
Chemistry, Susquehanna University, Selinsgrove, Pennsylvania 17870; 3Current institution: University of
Angers, Geologie-Laboratoire BIAF, Angers, France; 4Institute of Marine and Coastal Sciences, Rutgers
University, New Brunswick, New Jersey 08901; 5Biology Department, Woods Hole Oceanographic
Institution, Woods Hole, Massachusetts 02543; 6Analytical Instrument Systems, Inc., P.O. Box 458,
Flemington, New Jersey 08822
ABSTRACT Between October 2005 and March 2006, a seafloor volcanic eruption occurred at 9°50#N East Pacific Rise (EPR),
establishing a ‘‘time zero’’ for characterizing newly-formed hydrothermal vent habitats and comparing them to pre-eruption
habitats. Before the eruption, mussels (Bathymodiolus thermophilus) formed large aggregates between 9°49.6# and 9°50.3#N. After
the eruption, the few mussels remaining were in sparsely-distributed individuals and clumps, seemingly transported via lava flows
or from mass wasting of the walls of the axial trough. In situ voltammetry with solid state gold-amalgam microelectrodes was used
to characterize the chemistry of vent fluids in mussel habitats from 2004 to 2007, providing data sets for comparison of oxygen,
sulfide, and temperature. Posteruption fluids contained higher sulfide-to-temperature ratios (i.e., slopes of linear regressions)
(10.86 mM °C–1) compared with pre-eruption values in 2004 and 2005 (2.79 mM °C–1 and –0.063 mM °C–1, respectively). These
chemical differences can be attributed to the difference in geographic location in which mussels were living and physical factors
arising from posteruptive fluid emissions.
KEY WORDS: Bathymodiolus thermophilus, East Pacific Rise, hydrothermal vent, in situ electrochemistry, mussels, oxygen,
sulfide
INTRODUCTION
Hydrothermal vents harbor specialized organisms capable
of tolerating highly variable abiotic conditions (Fisher et al.
1988). These organisms thrive at the interface between diffuse
flow source waters and cooler ambient seawater. The source
waters are reducing fluids as they lack molecular oxygen and,
upon emission, are diluted by oxygenated seawater (Johnson
et al. 1986). Oxygen concentrations in diffuse flow fluids are
typically below detection limits until the temperature of the fluid
on mixing is less than 10°C to 12°C (Johnson et al. 1988a).
Temperature is considered to be a semiconservative tracer and
is often used to infer the extent of mixing between diffuse flow
and ambient seawater, which is then used to assume the chemical environment (Johnson et al. 1986, Johnson et al. 1988a).
Typically, high-temperature fluids are highly reduced and representative of vent fluid, whereas low temperature water is oxidized and more characteristic of ambient conditions (Johnson
et al. 1986, Le Bris et al. 2006).
Dynamic temperature and chemical habitat fluctuations
have dictated the physiological adaptations and capabilities of
vent fauna (Powell & Somero 1986, Johnson et al. 1988a). In
mixing with ambient seawater, large temporal temperature (tens
of degrees) and chemical variability (e.g., from 0.002–1 mM for
sulfide) occurs over seconds or days (Scheirer et al. 2006, Luther
et al. 2008). Temperature and chemistry can also vary as a
result of horizontal flow changes caused by semidiurnal and
*Corresponding authors. E-mail: [email protected], [email protected]
diurnal tidal periods (Johnson & Tunnicliffe 1985, Tivey et al.
2002, Scheirer et al. 2006). These temporal changes and variability contribute to the constantly changing, dynamic environment observed at 9°50#N on the East Pacific Rise.
Foundation or dominant species around which vent assemblages are formed on 9°50#N EPR include the tubeworms, Tevnia
jerichonana (Jones 1985) and Riftia pachyptila (Jones 1981), and
mussels, Bathymodiolus thermophilus (Kenk & Wilson 1985).
These organisms use chemosynthetic endosymbionts for their
nutrition, although mussels can also filter feed (Fisher et al.
1988, Page et al. 1991). Chemosynthesis occurs through aerobic
conditions with microbial oxidation of free sulfide (S free sulfide ¼
H2S + HS–) to produce sulfate and organic compounds (Luther
et al. 2001a).
Given that mussels have the ability to filter feed (Fisher et al.
1988, Page et al. 1991), they can depend less on the chemosynthesis of microbes when sulfide levels are low. Their endosymbionts are found within specialized cells in their gills (Powell &
Somero 1986, Belkin et al. 1986, Fisher 1995). To aid in feeding
and survival, mussels form large aggregates, reaching up to
hundreds of individuals, which can divert flow of vent fluid in
cracks from vertical to horizontal. This lateral diversion of vent
fluid may increase sulfide uptake (Le Bris et al. 2006, Johnson
et al. 1994, Johnson et al. 1988b). Chemistry is also different for
each individual organism depending on the location of a mussel
within an aggregate, resulting in microhabitat variation (Fisher
et al. 1988).
The 9°50#N EPR vent system was first discovered at a depth
of about 2,500 m in November 1989 from images recorded by
169
NEES ET AL.
170
sidescan sonar and photography on Argo-I (Fornari & Embley
1995). In April 1991, researchers returned to the area with the
DSV Alvin to further study the site and found a variety of posteruptive phenomena, including the seafloor covered with fresh
basalt, indicating that a recent (less than two weeks) volcanic
eruption had occurred (Haymon et al. 1993). The 1991 eruption
provided the opportunity to observe a vent system from very
early stages and study its development and faunal colonization.
Faunal succession after the 1991 eruption, as described by
Shank et al. (1998), identifies B. thermophilus as one of the later
species to colonize the vent system. Vents are first colonized by
thick, white microbial mats. Tubeworms then settle beginning
with the pioneer species T. jerichonana and followed by R.
pachyptila. Initial fluids of the new vent system after the
eruption were marked with high temperatures and high sulfide
concentrations (Von Damm 2000). The multiyear progression
of the vent system was characterized by decreasing temperature
and sulfide concentration levels (Shank et al. 1998).
Fifteen years after the 1991 eruption at 9°50#N EPR another
volcanic eruption was detected between 9°49#N and 9°51#N.
Based on data from seafloor seismometers, the eruption occurred between October 2005 and March 2006, with the greatest
seismicity determined to occur in late January 2006 (Tolstoy
et al. 2006). Large aggregates of B. thermophilus that dominated
many of the vent fields before the eruption were completely
overrun with lava. The few mussels that survived the eruption
were in sparsely-distributed groups, seemingly transported
down-slope via lava flows or from mass wasting of the walls
of the axial trough. The eruption also imparted posteruptive
chemical conditions to the vent fluids, returning the system to
high initial sulfide concentrations and temperatures (Shank
et al. 2006). The 2005 to 2006 eruption has provided the opportunity to study the chemical dynamics of B. thermophilus habitat
through comparisons between pre and posteruption environmental conditions. Through this comparison, we show that mussels can tolerate higher levels of sulfide after an eruption.
METHODS
In situ voltammetry measurements were conducted at
9°50#N EPR in April 2004 (Alvin Dives 3,996–4,012, April 8–
24), April-May 2005 (Alvin Dives 4,099–4,113, April 24 to May
10), June 2006 (Alvin Dives 4,201–4,207, June 25 to July 1), and
January 2007 (Alvin Dives 4,297–4,318 from January 13 to
February 3). There were a total of 13 dives, 5 dives, and 21 dives
collecting voltammetry data during 2004, 2005, and 2007. No
data for mussels in 2006 were available because of the limited
number of dives. Study sites (Fig. 1), where chemistry measurements on and near mussels were taken, included Marker 82,
Marker 89, Marker 119, Marker 141, East Wall, IO, and Tica
for 2004, Marker 82, Marker 119, Marker 141, East Wall,
Mussel Bed, and Tica for 2005, and Choo-Choo and Marker
7 for 2007. All data presented were measured directly over
mussels or within mussel aggregates and near the diffuse flow
source. Data presented include all electrochemical scans taken
for mussels, regardless of site, location, date, or time.
In situ voltammetry using solid state gold-amalgam (Au/Hg)
working microelectrodes were used from the DSV Alvin to
characterize the chemical environment of the vent system. The
electrodes were constructed from 100 mm gold wire housed in
polyethyl ether ketone (PEEK) tubing, and plated with mer-
Figure 1. Map of study sites from 2004, 2005, and 2007 where chemistry
measurements on and near mussels were taken. The figure on the left
displays all ten sites with a point representing each site. The figure on the
right displays an enlarged image of the nine northern site locations.
cury, as described by Brendel and Luther (1995). The electrodes
were controlled by the Analytical Instrument Systems, Inc.
(AIS) DLK-SUB analyzer (AIS ISEA-I) and operated from
within the DSV Alvin (Nuzzio et al. 2002). The analyzer
included inputs for a grounded Ag/AgCl reference electrode,
counter Pt reference electrode, four working electrodes, and a
temperature probe. Reference and counter electrodes were
attached to the side of the Alvin basket positioned in ambient
temperature (2°C). The Au/Hg working electrodes and temperature probe were mounted inside a Delrin or titanium wand with
the tips exposed at the end. Standard three electrode voltammetry experiments do not require reference and counter electrodes
to be located in close proximity to the working electrodes
(Luther et al. 1999, Luther et al. 2001a, Luther et al. 2001b).
Electrochemical scans were collected in situ and later analyzed. Cyclic voltammetry (scan rate 2000 mV s–1) was used to
measure the free sulfide (SH2S ¼ H2S + HS–, denoted also as
Sfree) and O2 concentrations. An electrode cleaning step with a
holding potential of –0.9 V or –1.0 V for 5 s, depending upon the
year of data collection, initiated the scan process. A conditioning step was then conducted, holding at the initial potential
(–0.05 V for 2007 and –0.1 V for all other years) for two seconds.
The measurement was taken by scanning from –0.05 to –1.8 V in
2007 and from –0.1 to –1.8 V in all other years. A program of up
to 10 individual electrochemical scans was performed over a
duration of 1.5 min to 2 min for each measurement in space.
These multianalyte electrodes detect O2 (detection limit, denoted as DL, <3 mM), H2S (DL < 0.2 mM), Fe(II) (DL < 10 mM),
Mn(II) (DL < 5 mM), and other S(-II) species such as polysulfides (Sx2–, DL < 0.2 mM) and thiosulfate (S2O32–, DL 30 mM)
(Brendel & Luther 1995, Luther et al. 2001a, Luther et al. 2001b,
Luther et al. 1999, Luther et al. 2008, Mullaugh et al. 2008).
During the times when no electrochemical information was
being collected, several cleaning scans were conducted in
ambient seawater to maintain the electrode surface integrity
and prevent electrode fouling.
CHEMICAL CHANGES AT 9°50#N EPR MUSSEL HABITATS
171
Calibration of each electrode was made before an Alvin dive
with a standard sulfide solution. The standard sulfide solution
was prepared with ACS reagent grade Na2S9H2O in degassed
deionized water and a calibration curve was constructed using
filtered seawater as the electrolyte. Electrodes were calibrated
for oxygen using filtered seawater saturated with dissolved
oxygen at room temperature. The electrochemical method used
in the calibrations was identical to that used during in situ data
collection. Peak heights within the electrochemical scans were
identified, measured, and converted to concentration, based on
the working electrode’s calibration. All electrochemical measurements were calibrated for temperature and flow (Luther
et al. 2001a, Luther et al. 2001b, Luther et al. 2008).
Statistical tests were conducted in SAS Version 9.1.2 for
Windows using Proc Reg and Proc Glm for simple linear
regression analyses and analysis of covariance (ANCOVA),
respectively. Simple linear regressions were used to identify if
linear relationships existed between Sfree and water temperature
(°C) and between O2 and water temperature (°C) for each sampling year in which mussels were observed (2004, 2005, 2007).
The absolute value (jxj) of the Studentized residual, Cook’s D,
RStudent, and DFFITS were used to identify when an individual datum influenced the linear relationship and R2 of a dataset.
Observations were identified as statistical outliers and removed
from the dataset only when the absolute value (jxj) of all four
tests was $2 and adequate justification was determined from
notations made during sample collection. The statistical significance of linear regressions was determined using critical values
of the F distribution (i.e., regression mean squares/residual
mean squares) for one-tailed hypotheses (Zar 1999, Appendix
B.4, Numerator DF ¼ 1; P # 0.05).
Analysis of covariance (ANCOVA) was used to determine if
the slopes of the regression lines from each sampling year for
Sfree and O2 versus temperature separately were equivalent. Fvalues for three regression functions (2004, 2005, 2007) were
calculated from pooled and common residual sum of squares
and pooled residual degrees of freedom. Calculated F-values
greater than critical values of the F distribution for one-tailed
hypotheses (Zar 1999, Appendix B.4, Numerator DF ¼ 2; P #
0.05) identified significant differences among slopes of the three
linear regressions.
RESULTS AND DISCUSSION
Most of the macrofauna present at 9°50#N EPR in 2004 and
2005 before the eruption occurred included B. thermophilus and
R. pachyptila. Bathymodiolus thermophilus were observed in
large numbers forming many aggregates, covering more than a
square meter on the ocean floor (direct observation). These
organisms were frequently found covering the tubes of living
R. pachyptila. However these aggregates were destroyed during
the eruption. This created a ‘‘time-zero’’ not only for the succession of biological communities but also with regard to chemical
conditions, returning the system to high initial temperatures and
sulfide concentrations (Von Damm 2000).
In contrast, most of the biology observed at 9°50#N EPR
after the eruption in 2006 and 2007 included microbial mats and
T. jerichonana. In 2007, few B. thermophilus mussels were
observed, with only one or a few individuals seen at a given
location. The sizes of the mussels seen were similar in length to
those observed before the eruption, suggesting that they were
Figure 2. Temperature (top), O2 (middle), and free sulfide (Sfree) (bottom)
ranges for waters in which mussels were observed. Open circles denote the
mean value, whereas filled circles represent the median value. A separate
Sfree range is seen in 2005, representing one set of electrochemical scans
collected at a location at East Wall in the presence of R. pachyptila and is
further discussed in the text. This set of data was determined to be a
statistical outlier and is included as a separate range.
NEES ET AL.
172
survivors of the eruption. In the case of the scattered mussels
located in the central Tica area, it is possible that these mussels
were either positioned in an area not affected by fresh lava flow,
temporarily transported into the water column, or higher on a
wall and then moved to the locations they were observed in
2007.
The ranges for O2, Sfree, and temperature in 2004, 2005, and
2007 measured among B. thermophilus differ between pre and
posteruption (Fig. 2, Table 1). A wide range of O2 values were
observed for B. thermophilus in 2004 and 2005, with a more
limited range observed in 2007. These wide ranges demonstrate
the amount of variability and fluctuations experienced by the
mussels in such a dynamic habitat (Johnson et al. 1988a) and
also by their position within an aggregation (Fisher et al. 1988).
The median values can be considered indicative of the conditions that the organisms typically experience. The median O2
concentrations were always greater than the median Sfree values
for all three years. This indicates that mussels prefer higher
concentrations of O2. Because they are not completely dependent upon endosymbionts for food (Fisher 1995) their need for
Sfree may not be as great.
In 2007 (post eruption), Sfree median and mean concentrations were higher than pre-eruption values but the temperature
mean and median were similar to 2004. A separate Sfree range is
given in 2005, representing one set of up to 10 electrochemical
scans collected at one of the diffuse flow sites in the East Wall
location. This specific diffuse flow location within East Wall
was not identified in 2004 and included an aggregation of
mussels surrounding five R. pachyptila tubeworms. The high
concentrations of Sfree measured at this particular location are
on the order typically observed in EPR tubeworm aggregations.
Within a meter of this particular measurement location, there
was an area with only mussels and the data from the mussel only
location resulted in concentrations found in the lower range
bracket for 2005. With the exception of this anomalous set of
scans mentioned, the temperature and Sfree concentration
ranges decrease, whereas the O2 concentration ranges increase
from 2004 to 2005.
Comparison of the oxygen-to-temperature data (Fig. 3)
indicates similar slopes (calculated from linear regressions) of
–3.67 mM °C–1, –5.33 mM °C–1, and –9.87 mM °C–1 for 2004,
2005, and 2007, respectively (F ¼ 0.70, P ¼ 0.5019, Fig. 3).
However, the median O2 concentration detected among mussels
in 2007 was greater than those recorded in 2004 and 2005 (Fig.
2). Because the source of oxygen is ambient bottom water and is
provided by the physical currents of the bottom water, no trends
were observed between years.
Comparison of the sulfide-to-temperature data for all years
illustrates large differences before and after the eruption (F ¼
4.73, P ¼ 0.0134, Fig. 3). Linear regressions of all data for each
year produce slopes of Sfree concentration versus temperature
that are commonly referred to as sulfide-to-temperature (S/T)
ratios. Two S/T ratios are displayed for 2005 because of the
anomalous set of scans mentioned previously (circled triangle,
Fig. 3). Statistically, this point is an outlier (jxj ¼ 2.8 – 25.1) and
has influence on the slope of the linear regression. When this
outlier is included in the linear regression, the S/T ratio is
determined to be 3.78 mM °C–1 which is not different from the S/
T ratio of 2.79 mM °C–1 determined for 2004. When the outlier is
excluded from calculations, a smaller S/T ratio of –0.063 mM
°C–1 is determined for 2005. Using this latter S/T ratio, 2004
and 2005 are potentially different.
After the eruption in 2007, vent fluids surrounding B.
thermophilus had a higher S/T ratio (10.86 mM °C–1) than those
observed pre-eruption. The difference in these ratios pre and
posteruption are mostly caused by water-rock interactions that
occur subsurface (Alt 1995, Von Damm et al. 1995, Von Damm
2000) but may also be attributed to the limited number of B.
thermophilus observed in 2007. In 2004 and 2005 mussels were
frequently located within or adjacent to diffuse flow sources, as
suggested by their placement in crevices where higher S/T ratios
existed. In 2007, however, mussels were positioned farther from
similar diffuse flow sources, because T. jerichonana were within
closer proximity to these waters.
These diffuse flow sources emitted higher sulfide concentrations (as high as 386.9 mM Sfree) than before the eruption.
Despite the tubeworm domination at the diffuse flow sources,
mussels were still surrounded by higher Sfree concentrations
than before the eruption. Without a large population, mussels
were unable to overcrowd tubeworm aggregates inhabiting
these richer diffuse flow sites in 2007. Recruitment cues,
currently unknown (Mullineaux et al. 1998), and highly variable
larval dispersion (Mullineaux et al. 2005), may explain why
tubeworms colonized before mussels and were able to increase
in population size. However, with greater populations before
the eruption in 2004 and 2005, tightly packed aggregates of
TABLE 1.
Temperature, O2, and free sulfide (Sfree) ranges for waters in which mussels were observed using in situ voltammetry.
Values of 3 and 0.2 mM for O2 and Sfree, respectively, indicate the detection limits for these species. Minimum and maximum
values were determined from the total number of electrochemical scans; whereas the number of mussel aggregates (i.e., sets of
individual scans) was used to calculate means and medians. Inserted in parantheses below the 2005 Sfree data are values for a single set
of scans determined to be an outlier point for the 2005 data. This outlier point had R. pachyptila among the mussel aggregation
at East Wall and indicated an anomalous set of data for diffuse flow, further discussed in the text. The data provided from the
scans for this outlier were not included in the overall 2005 data, but listed separately.
Observations
Temperature (°C)
O2 (mM)
Sfree (mM)
Scans
Aggreg.
Min.
Max.
Mean
Median
Min.
Max.
Mean
Median
Min.
Max.
Mean
Median
2004
2005
224
301
23
28
2.0
2.0
16.5
11.0
4.4
3.8
3.5
2.8
3
3
163.1
187.3
51.1
47.6
42.8
46.9
0.2
0.2
2007
46
5
2.0
9.5
4.6
3.5
56.7
125.7
86.6
79.6
0.2
69.1
32.5
(175.3)
87.2
7.7
2.9
(6.1)
27.4
3.2
0.7
(0.2)
14.1
CHEMICAL CHANGES AT 9°50#N EPR MUSSEL HABITATS
Figure 3. Oxygen-to-temperature (top) and sulfide-to-temperature (bottom) ratios for the waters in which mussels were observed. Each point
represents the mean value from a single set of electrochemical scans
consisting of up to 10 individual scans. Filled circles denote data collected
in 2004, open triangles represent data collected in 2005, and gray squares
denote data collected in 2007. Ratios are the slopes (m) of the linear
regression lines. Slope, R2, F value, and P values are provided in the plot.
Two ratios are displayed for 2005 due to a set of scans reflected by the
circled data point. This set of scans was collected at East Wall among
R. pachyptila and is further discussed in the text. Statistically, this point is
an outlier and has influence on the slope of the linear regression. Therefore,
a ratio is provided for 2005 which includes the outlier and a ratio is
provided for 2005 that excludes the outlier point.
B. thermophilus out-competed tubeworms as demonstrated by
exclusion cage experiments conducted at 9°50#N EPR by Lutz
et al. (2008). Although capable of dominating the diffuse
flow sources before the eruption, these Sfree concentrations were
still less than the concentrations experienced by the mussels in
2007.
173
Le Bris et al. (2006) conducted in situ flow injection analysis
to determine the total sulfide (S total sulfide ¼ H2S + HS– + FeS +
Sx2–, denoted as Stotal) concentrations surrounding faunal
habitats. Their study was conducted in 2002 at the sites Mussel
Bed and Biovent, where large aggregates of mussels were found.
Le Bris et al. (2006) report values for single aggregates and do
not combine data from other sources as do our data. The S/T
ratios of mussel aggregates range from 3.1 mM °C–1 to 16.6 mM
°C–1 at Mussel Bed and from 10.4 mM °C–1 to 10.9 mM °C–1 at
Biovent (Le Bris et al. 2006). In our study a single set of scans
were measured at Mussel Bed in 2005. The S/T ratio at Mussel
Bed in 2005 was 2.45 mM °C–1, which is less than the ratios
determined in Le Bris et al. (2006). Using the 2005 ratio, which
excludes the outlier, our data show a decrease from 2002 (when
the Le Bris et al. (2006) study was conducted) to 2004 and from
2004 to 2005. The decrease in ratios indicates that the diffuse
flow source was becoming less rich in sulfide and that the vent
system was in a state of overall decline in this area. It should be
noted that the Le Bris et al. (2006) study measured Stotal instead
of Sfree, which was determined in our study to account for such a
large difference. However, more in situ data at more sites and
locations were collected in this study compared with Le Bris
et al. (2006).
The trends of decreasing temperatures and Sfree concentrations in the ranges and the mean and median values from 2004
to 2005 also indicate that the vent system was declining. Further
indication from comparison of other studies shows that this
decline can be followed annually. Whereas Shank et al. (1998)
did not report S/T ratios, maximum temperatures and Stotal
concentrations of diffuse flow surrounding organisms were
provided. Shank et al. (1998) reported temperatures and Stotal
concentrations decreasing from 55°C and 1,900 mM in April
1991 (2 weeks after the 1991 eruption) to 24°C and 300 mM in
November 1995 (55 mo after the 1991 eruption). In the same
study, small mussels were observed approximately 42 mo after
the eruption (in October 1994) settling in cracks within basalt
(32°C, 800 mM Stotal). In November 1995, (55 mo after the eruption) aggregates of larger mussels were observed covering basalt
and even tubeworms (Shank et al. 1998).
Comparing these values to those in our study, the maximum
temperatures and Sfree concentrations were 16.5°C and 69.1 mM
in April 2004 and 11.0°C and 32.5 mM (not including the outlier
mentioned previously) in April-May 2005. These maximum
values show a decreasing trend in sulfide release throughout the
years. A continual decreasing trend in S/T ratios is observed
from data collected annually from 1991 to 1995 in Shank et al.
(1998), from data collected in 2002 in Le Bris et al. (2006), and
from data collected in 2004 and 2005 in our study. The increase
in mean and median values of temperatures and Sfree concentrations in 2007 confirm the occurrence of an eruption, providing a fresh start for the hydrothermal vent system. Further
evidence includes the highest temperature and Sfree concentration recorded for diffuse flow in 2007 were 31°C and 386.9 mM,
which occurred among T. jerichonana aggregates.
Polysulfides and thiosulfate, which have been observed at
Lau Basin (Mullaugh et al.; 2008, Waite et al. 2008), were not
detected in any of the electrochemical scans conducted near
mussels. Thiosulfate was also not detected at the Galápagos
Rift (Fisher et al. 1988). However, thiosulfate was detected at
9°50#N EPR by Gru et al. (1998) using discrete samples colleted
near R. pachyptila, and polysulfides have been noted near R.
NEES ET AL.
174
pachyptila by Luther et al. (2001b), who used the same in situ
electrochemical analyzer system used in this study. The presence
of thiosulfate at Lau Basin along with its absence (in situ DL #
30 mM) at 9°50#N EPR and the Galápagos Rift could be
because of differences in the hard substrates at these locations
on which the mussels reside. Porous substrates at Lau Basin
have a surface (or deeper) covering of iron and manganese
oxides (Fouquet et al. 1991a, Fouquet et al. 1991b). As vent
fluid passes through this substrate, sulfide adsorbs to the surface
and is then oxidized by oxidized iron and manganese resulting
in sulfide oxidation and detection of polysulfides and thiosulfate
(Mullaugh et al. 2008). These substrates are more permeable
than those found at 9°50#N EPR, which consists of basalt, a
glassy substrate having less noticeable oxidized iron and
manganese, which may account for the lack of thiosulfate
observed. In addition, Fe(II) and Mn(II) at 9°50#N EPR were
below detection limits.
system was declining before the eruption in 2006, as evidenced
by decreasing temperatures and Sfree concentrations, presented
not only from our study, but also from other studies conducted
since the 1991 eruption (Shank et al. 1998, Le Bris et al. 2006).
Large aggregates of mussels present before the eruption outcompeted tubeworms for colonization sites surrounding diffuse
flow sources, which were lower in Sfree concentration in 2004
and 2005. During the 2006 eruption, fresh lava flow destroyed
these mussel aggregates, leaving only a few individual survivors.
With posteruption high concentrations of sulfide, mussels were
apparently unable to colonize localized vent habitats in the
region. Instead, T. jerichonana quickly colonized the areas in
large aggregates. From insights gained during previous studies
(Shank et al. 1998), it is anticipated that mussels will once again
populate the local vent habitats as sulfide concentrations
decrease to levels observed before the eruption.
ACKNOWLEDGMENTS
CONCLUSION
Before the 2006 eruption, much of the hydrothermal vent
system at 9°50#N EPR was a mussel-dominated field, whereas
after the eruption, the vent system changed to a tubewormdominated field. This transformed biological community structure change was accompanied by increased vent fluid temperatures and Sfree concentrations. Hydrothermal flux in the vent
The authors thank C. Kraiya, J. Tsang, the DSV Alvin pilots,
and the crew and captain of R/V Atlantis for their help and
encouragement. This work was funded by NSF grants OCE0327353 (RAL and CV), OCE-0327261 and OCE-0451983
(TS), OCE-0326434 and OCE-0308398 (GWL). We dedicate
this paper to the life and memory of Dr. Mel Carriker, who was
a valued mentor, colleague and friend.
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