Hydrothermal vent mussel habitat chemistry, pre- and post-eruption at 9[degrees]50' north on the East Pacific Rise.
Article Type: Report
Subject: Hydrothermal vent ecology (Research)
Habitat (Ecology) (Research)
Electrochemistry (Research)
Mussels (Environmental aspects)
Mussels (Chemical properties)
Mussels (Distribution)
Authors: Nees, Heather A.
Moore, Tommy S.
Mullaugh, Katherine M.
Holyoke, Rebecca R.
Janzen, Christopher P.
Ma, Shufen
Metzger, Edouard
Waite, Tim J.
Yucel, Mustafa
Lutz, Richard A.
Shank, Timothy M.
Vetriani, Costantino
Nuzzio, Donald B.
Luther, George W., III
Pub Date: 03/01/2008
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: March, 2008 Source Volume: 27 Source Issue: 1
Topic: Event Code: 310 Science & research; 690 Goods & services distribution Advertising Code: 59 Channels of Distribution Computer Subject: Company distribution practices
Geographic: Geographic Scope: United States Geographic Name: East Pacific Rise Geographic Code: 1USA United States
Accession Number: 178358794
Full Text: ABSTRACT Between October 2005 and March 2006, a seafloor volcanic eruption occurred at 9[degrees]50'N East Pacific Rise (EPR), establishing a "time zero" for characterizing newly-formed hydrothermal vent habitats and comparing them to pre-emption habitats. Before the eruption, mussels (Bathymodiolus thermophilus) formed large aggregates between 9[degrees]49.6' and 9[degrees]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 [micro]M [degrees][C.sup.-1]) compared with pre-emption values in 2004 and 2005 (2.79 [micro]M [degrees][C.sup.-1] and 0.063 [micro]M [degrees][C.sup.-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


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[degrees]C to 12[degrees]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 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[degrees]50'N on the East Pacific Rise.

Foundation or dominant species around which vent assemblages are formed on 9[degrees]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 ([summation] free sulfide = [H.sub.2]S + H[S.sup.-]) to produce sulfate and organic compounds (Luther et al. 2001 a).

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[degrees]50'N EPR vent system was first discovered at a depth of about 2,500 m in November 1989 from images recorded by 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 post-eruptive 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[degrees]50'N EPR another volcanic eruption was detected between 9[degrees]49'N and 9[degrees]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.


In situ voltammetry measurements were conducted at 9[degrees]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 [micro]m gold wire housed in polyethyl ether ketone (PEEK) tubing, and plated with mercury, 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[degrees]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.sup.-1]) was used to measure the free sulfide ([summation][H.sub.2]S = [H.sub.2]S + H[S.sup.-], denoted also as [S.sub.free]) and [O.sub.2] 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 (-.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 [O.sub.2] (detection limit, denoted as DL, <3 [micro]M), [H.sub.2]S (DL < 0.2 [micro]M), Fe(II) (DL < 10 [micro].M), Mn(II) (DL < 5 [micro]M), and other S(-II) species such as polysulfides ([S.sub.x.sup.2-], DL < 0.2 [micro]M) and thiosulfate ([S.sub.2][O.sub.3.sup.2-], DL 30 [micro]M) (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.

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 [Na.sub.2]S x 9[H.sub.2]O 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 [S.sub.free] and water temperature ([degrees]C) and between [O.sub.2] and water temperature ([degrees]C) for each sampling year in which mussels were observed (2004, 2005, 2007). The absolute value ([absolute value of x]) of the Studentized residual, Cook's D, RStudent, and DFFITS were used to identify when an individual datum influenced the linear relationship and [R.sup.2] of a dataset. Observations were identified as statistical outliers and removed from the dataset only when the absolute value ([absolute value of x]) of all four tests was [greater than or equal to] 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 = 1; P [less than or equal to] 0.05).

Analysis of covariance (ANCOVA) was used to determine if the slopes of the regression lines from each sampling year for [S.sub.free] and [O.sub.2] versus temperature separately were equivalent. F-values 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 = 2; P [less than or equal to] 0.05) identified significant differences among slopes of the three linear regressions.


Most of the macrofauna present at 9[degrees]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[degrees]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 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 [O.sub.2], [S.sub.free], and temperature in 2004, 2005, and 2007 measured among B. thermophilus differ between pre and posteruption (Fig. 2, Table 1). A wide range of [O.sub.2] 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 [O.sub.2] concentrations were always greater than the median [S.sub.free] values for all three years. This indicates that mussels prefer higher concentrations of [O.sub.2]. Because they are not completely dependent upon endosymbionts for food (Fisher 1995) their need for [S.sub.free] may not be as great.

In 2007 (post eruption), [S.sub.free] median and mean concentrations were higher than pre-eruption values but the temperature mean and median were similar to 2004. A separate [S.sub.free] 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 [S.sub.free] 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 [S.sub.free] concentration ranges decrease, whereas the [O.sub.2] 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 [micro]M [degrees][C.sup.-1], -5.33 [micro]M [degrees][C.sup.-1], and -9.87 [micro]M [degrees][C.sup.-1] for 2004, 2005, and 2007, respectively (F = 0.70, P = 0.5019, Fig. 3). However, the median [O.sub.2] 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 = 4.73, P = 0.0134, Fig. 3). Linear regressions of all data for each year produce slopes of [S.sub.free] 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 ([absolute value of x] = 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 [micro]M [degrees][C.sup.-1] which is not different from the S/T ratio of 2.79 [micro]M [degrees][C.sup.-1] determined for 2004. When the outlier is excluded from calculations, a smaller S/T ratio of 4).063 [micro]M [degrees][C.sup.-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 [micro]M [degrees][C.sup.-1]) than those observed pre-emption. 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 [micro]M [S.sub.free]) than before the eruption. Despite the tubeworm domination at the diffuse flow sources, mussels were still surrounded by higher [S.sub.free] 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 B. thermophilus out-competed tubeworms as demonstrated by exclusion cage experiments conducted at 9[degrees]50'N EPR by Lutz et al. (2008). Although capable of dominating the diffuse flow sources before the eruption, these [S.sub.free] concentrations were still less than the concentrations experienced by the mussels in 2007.


Le Bris et al. (2006) conducted in situ flow injection analysis to determine the total sulfide ([summation] total sulfide = [H.sub.2]S + H[S.sup.-] + FeS + [S.sub.x.sup.2-], denoted as [S.sub.total]) 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 [micro]M [degrees][C.sup.-1] to 16.6 [micro]M [degrees][C.sup.-1] at Mussel Bed and from 10.4 [micro]M [degrees][C.sup.-1] to 10.9 [micro]M [degrees][C.sup.-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 [micro]M [degrees][C.sup.-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 [S.sub.total] instead of [S.sub.free], 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 [S.sub.free] 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 [S.sub.total] concentrations of diffuse flow surrounding organisms were provided. Shank et al. (1998) reported temperatures and [S.sub.total] concentrations decreasing from 55[degrees]C and 1,900 [micro]M in April 1991 (2 weeks after the 1991 eruption) to 24[degrees]C and 300 [micro]M 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[degrees]C, 800 [micro]M [S.sub.total]). 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 [S.sub.free] concentrations were 16.5[degrees]C and 69.1 [micro]M in April 2004 and 11.0[degrees]C and 32.5 [micro]M (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 [S.sub.free] 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 [S.sub.free] concentration recorded for diffuse flow in 2007 were 31[degrees]C and 386.9 [micro]M, 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 Galapagos Rift (Fisher et al. 1988). However, thiosulfate was detected at 9[degrees]50'N EPR by Gru et al. (1998) using discrete samples colleted near R. pachyptila, and polysulfides have been noted near R. 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 [less than or equal to] 30 [micro]M) at 9[degrees]50'N EPR and the Galapagos 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[degrees]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[degrees]50'N EPR were below detection limits.


Before the 2006 eruption, much of the hydrothermal vent system at 9[degrees]50'N EPR was a mussel-dominated field, whereas after the eruption, the vent system changed to a tubeworm-dominated field. This transformed biological community structure change was accompanied by increased vent fluid temperatures and [S.sub.free] concentrations. Hydrothermal flux in the vent system was declining before the eruption in 2006, as evidenced by decreasing temperatures and [S.sub.free] 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 out-competed tubeworms for colonization sites surrounding diffuse flow sources, which were lower in [S.sub.free] 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.


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 OCE-0327353 (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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(1) College of Marine and Earth Studies, University of Delaware, Lewes, Delaware 19958; (2) Department of Chemistry, Susquehanna University, Selinsgrove, Pennsylvania 17870; (3) Current institution: University of Angers, Geologie-Laboratoire BIAF, Angers, France; (4) Institute of Marine and Coastal Sciences, Rutgers University, New Brunswick, New Jersey 08901; (5) Biology Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543; (6) Analytical Instrument Systems, Inc., P.O. Box 458, Flemington, New Jersey 08822

* Corresponding authors. E-mail:hnees@udel.edu, luther@udel.edu
Temperature, [O.sub.2], and free sulfide ([S.sub.free]) ranges for
waters in which mussels were observed using in situ voltammetry.
Values of 3 and 0.2 [micro]M for [O.sub.2] and [S.sub.free],
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 [S.sub.free] 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 ([degrees]C)

       Scans   Aggreg.   Min.   Max.   Mean   Median

2004    224      23      2.0    16.5   4.4     3.5
2005    301      28      2.0    11.0   3.8     2.8
2007     46       5      2.0     9.5   4.6     3.5

           [O.sub.2] ([micro]M)           [S.sub.free] ([micro]M)

       Min.   Max.    Mean   Median   Min.    Max.     Mean    Median

2004    3     163.1   51.1    42.8    0.2      69.1     7.7      3.2
2005    3     187.3   47.6    46.9    0.2      32.5     2.9      0.7
                                             (175.3)   (6.1)    (0.2)
2007   56.7   125.7   86.6    79.6    0.2      87.2    27.4     14.1
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