Distribution and near-bottom transport of larvae and other plankton at hydrothermal vents

Deep-Sea Research II 45 (1998) 423—440

Distribution and near-bottom transport of larvae and other plankton at hydrothermal vents Stacy L. Kim!,*, Lauren S. Mullineaux" ! Moss Landing Marine Laboratories, PO Box 450, Moss Landing, CA 95039, USA " Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 1 March 1997; received in revised form 29 July 1997; accepted 31 July 1997

Abstract Distributions of larvae of benthic invertebrates and other planktonic organisms (holoplankton) were determined near hydrothermal vents along the East Pacific Rise (9°50@N) and combined with current meter records to estimate the extent and direction of transport in near-bottom flows. Diurnal tidal currents were strong enough to transport larvae substantial distances (up to 2 km) across the ridge axis during a single 12-h excursion. Potential longerterm transport in mean flows, however, appeared to be relatively slow (typically less than 1 km d~1). The proportion of larvae dispersing in near-bottom flows, as opposed to becoming entrained into the buoyant plume (and transported up out of the near-bottom environment) was estimated for a range of vent community sizes and black-smoker buoyancy fluxes, using a buoyant-plume entrainment model. These estimates suggested that larvae were most often transported in near-bottom currents, but that plume-level dispersal dominated for short periods of the tidal cycle (0.5—3 h) when the currents were slower than 1—2 cm s~1. The plume exit temperature also affects entrainment rate, so the proportion of larvae in each transport pathway (near-bottom flows and buoyant plumes) should vary substantially among vent habitats surrounding different temperature vents. The presence of certain holoplankton groups in diffuse vent flows, and their elevated abundances within the axial ridge valley, raises the possibility that these groups may be specifically associated with vent habitats. ( 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction Hydrothermally active areas are temporally and spatially patchy habitats with unusual physical characteristics. Extreme temperatures, high pressure, and high * Corresponding author. E-mail: [email protected]. 0967-0645/98/$19.00 ( 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 7 ) 0 0 0 4 2 - 8

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concentrations of chemicals such as hydrogen sulfide stress the inhabitants. The strict requirements that individuals cope with such variables may limit the number of species that have succesfully invaded hydrothermal vent habitat, contributing to the low species diversity found in hydrothermal vent communities (Tunnicliffe, 1991). Most species found at vents are not found elsewhere in the deep sea. The proximal reason for the restriction of vent species to hydrothermal habitat is the dependence on the abundant food resource available at vents. The unusual hydrothermal chemical regime allows chemosynthetic bacteria to flourish near and in hydrothermal fluids, and these bacteria are the base of the food web for animals abundant at vents (Fisher et al., 1994; Van Dover and Fry, 1994). The large size and concomitant high food requirements of common vent invertebrates, and the often symbiotic associations with chemosynthetic bacteria preclude survival far away from a source of hydrothermal fluid. Despite these habitat limitations, vent organisms have a widespread, if patchy, distribution. The vestimentiferan tubeworm Riftia pachyptila is found from Guaymas Basin to the Galapagos spreading ridge, a distance of over 3000 km (Tunnicliffe, 1991). The distribution of vent habitat creates gaps of up to hundreds of kilometers in species distributions (Fornari and Embley, 1995). To maintain the species continuity of sessile organisms over such habitat breaks, a dispersive stage is required, such as planktonic larvae of vent invertebrates (Mullineaux and France, 1995). Planktonic larvae have minimal swimming capabilities relative to flow patterns near vents. Thus, flow may strongly influence larval distribution patterns. Larvae may be transported in near-bottom flows, entrained into rising buoyant plumes and transported several hundred meters above the seafloor (Mullineaux et al., 1995), or they may stay very close to parental populations by remaining demersal (Zal et al., 1995). The patchiness of vent habitat would a priori suggest that planktotrophic larvae, which can spend a long time in the water column and thus disperse further, would be advantageous over lecithotrophic or brooded larvae for vent species. However, vent species include many different developmental types; Lutz et al. (1984) have suggested that developmental style is phylogenetically constrained rather than distinctive to the habitat. Another parameter that will influence larval dispersal patterns is current regime. Near-bottom flows are characterized as slow (a few cm s~1) and topographically directed along the ridge crest (Cannon et al., 1991). This pattern of near-bottom flow near hydrothermal vents suggests that near-bottom transport of vent larvae, within their lifespan, would be constrained to a relatively limited area near vent communities. Entrainment into the buoyant plume raises larvae several hundred meters above the seafloor, where current speeds are higher and directions are less topographically steered. While plume-level dispersal may be important for initial colonization, and for maintenance of species continuity over large habitat gaps, recruitment from locally established populations is likely more important for growth of newly colonized populations and maintenance of existing ones. The relative numbers of larvae dispersing in near-bottom flows versus via other mechanisms, however, are unknown. Local dispersal pathways in near-bottom flows can be inferred from distributions of larvae of hydrothermal vent species in near-bottom waters. This approach is used in the

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present study to investigate a near-bottom larval dispersal pathway and compare it to the entrainment pathway described in Kim et al. (1994). Near-bottom plankton around vents has been sampled rarely (Smith, 1985; Berg and Van Dover, 1987), and only with large mesh nets (297 and 183 lm, respectively) that could not capture many of the small larvae of vent organisms. The collections showed a pattern of increased biomass very near vents, and an overall paucity of vent larvae (Berg and Van Dover, 1987). No distinct pattern in near-bottom distribution of vent larvae could be determined, though it was evident that lecithotrophic larvae are transported in the water column, as are planktotrophic larvae and juvenile stages. Transport of all larval types was supported by the results of Wiebe et al. (1988) and Mullineaux et al. (1995), who found larvae of vent invertebrates hundreds of meters above the seafloor. However, the details of horizontal and vertical gradients in plankton distributions near vents remain unknown. In addition to more detailed near-bottom sampling, new approaches, including species-level taxonomy, repeated sampling at one site, and ecological modeling, promise to yield new information on the distributions of vent larvae. The specific goals of this research were to: 1. Estimate the extent of transport of larvae of benthic vent species along and across the ridge axis, in near-bottom flows on short time scales (one tidal excursion to many days). 2. Estimate the relative numbers of larvae dispersing in near-bottom flows versus in the neutrally buoyant plume, using a standard buoyant plume model to predict entrainment of larvae. 3. Characterize the distribution of selected holoplanktonic (entire life cycle in the water column) groups near vents in order to determine whether a vent-associated plankton community may exist in addition to the much better known benthic community.

2. Materials and methods The goals of the present study required characterization of the vertical distributions of plankton and larvae and measurements of typical near-bottom flow velocities. Data were collected during research cruises aboard the R/» Atlantis II in December 1991, November 1994, and December 1995. The series of Alvin submersible dives were conducted in the Venture Hydrothermal Fields along the East Pacific Rise (Fig. 1). This is a fast spreading center located from 9°09@N to 9°54@N and 104°14@W to 104°18@W, which has been well mapped by Haymon et al. (1991). The area was volcanically active as evidenced by recent lava flows (Haymon et al., 1993). The ridge crest was at &2500 m depth, with a small axial graben (200 m wide and (100 m deep. Both black smokers and shimmering flows were common. Plankton samples were collected around the vents and in and around the hydrothermal plume, using a net system to quantify large-scale (hundreds of meters) distributions and a plankton pump to characterize smaller-scale patterns. The net

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Fig. 1. Positions of the plankton-tow samples (j), plankton-pump samples (v) and current-meter moorings (?) relative to the axial summit valley (shaded strip) near the study site on the East Pacific Rise (EPR). Labels correspond to notations used in Table 1 (tows), Table 2 (pumps), and Fig. 4 (moorings). Sampling positions near 9°30.0@N, 104°14.7@W (tow and pump samples 79!, 80!, 81!, and 82!) were not plotted due to poor navigation in that region. Inset shows field area on EPR axis between Clipperton (C) and Siqueros (S) transform faults. Seabeam data and inset map courtesy of D. Fornari.

system (Deep-Tow, designed by K. Wishner) had a mouth area of 0.25 m2 and nets of 64 lm mesh, fine enough to retain even small larvae. The system was attached to the side of the submersible Alvin, where the manipulator arm could be used to trigger the three nets sequentially. On completion of the tows, the nets were cinched shut to prevent backwashing or contamination during the remainder of the dive and recovery. Volumes filtered were determined from duration of the tows and navigational records showing distance traveled and submersible speed (approximately 26 cm s~1). The pump sampler had a moveable intake nozzle and five collection bins, also with 64 lm mesh. Unfortunately, difficulties with the various flow meter systems prevented

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any precise measurements of volumes pumped per unit time. The pump operated at a roughly constant rate, however, allowing samples to be compared by accounting for sampling duration. The objective of the net tows was to sample near the bottom (within 15 m) versus above the bottom (above 15 m), both within the axial valley and away from it, on a scale of tens to hundreds of meters. The pump system was directed at sampling plankton at increasing heights above the bottom both within and outside a hydrothermal plume. Pump samples provided limited volumes but could be taken in specific locations on a decimeter to meter scale. Time constraints limited the replication in pump sampling. On subsequent cruises, the pump was used to sample very close to the bottom. Recent information (authors’ unpublished data) suggests that this method of sampling may collect recently settled vent gastropods that are morphologically indistinguishable from planktonic larvae; hence, these latter pump samples were not used in defining near-bottom larval distributions. On return to the surface, samples were immediately rinsed from the collection containers with filtered seawater, and chilled. Deep-tows and pumps were quickly examined under a dissecting scope (at 50]) for larvae, and samples were preserved in buffered formalin within 4 h of return to the surface. Samples were later transferred to 70% ethanol, and sorted at 50] under a Wild M5A dissecting scope. Species considered capable of escaping the samplers (amphipods and copepods) were not counted, though they were among the dominant taxa in some samples. Larvae of benthic vent gastropods were distinguished on the basis of protoconch morphology; other specimens were identified to major taxa, but not assigned to vent and non-vent groups. Statistical analyses of vertical (near-bottom versus above 15 m) and lateral (in axial valley versus away) distributions of taxa in net tows were performed with separate two-sample t-tests (Systat 6.01) because the sampling design was unsuited for analysis of variance (ANOVA). Data were log-transformed to correct for correlations between mean and variance, and significance levels were adjusted for multiple tests using a Bonferroni correction. An Aanderaa RCM8 current meter was deployed near the sampling sites during the cruises. This is a rotor-and-vane current meter that records a minimum speed of 1.1 cm s~1; current speeds below this threshold were assumed to have an average value of 0.55 cm s~1. The current meter was positioned precisely within or outside the axial valley using Alvin. The lengths of deployments were constrained by the diving schedules of the cruises, and the sampling intervals by the memory capacity of the current meter. In 1991, the current meter was moored at two sites, one within (3 day deployment) and one outside (2 days) the axial valley, at a height of 10 m above bottom, with a 2 min recording interval. In 1994 the current meter was moored at a nearby site within the axial valley (11 days), and in 1995 it was moored at an additional site outside the axial valley (7 days). The latter two deployments were configured at a height of 5 m above bottom with a 5 min recording interval. The relative proportions of larvae that were transported in near-bottom flows versus entrained into a buoyant plume of hydrothermal fluid rising from a vent were estimated using a standard buoyant plume model (cf. Turner, 1973) and simple geometric arguments.

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3. Results 3.1. Plankton Larvae and adults of both benthic and planktonic invertebrates were captured in plankton samples. Holoplankton included siphonophores, pelagic gastropods, and larvaceans, in addition to the amphipods and copepods which were not quantified. The pelagic gastropods and some of the siphonophores were in larval stages, whereas the larvaceans were adults. The most common benthic group was vent gastropod larvae; larvae of bivalves and polychaetes also were found, but could not be assigned to specific vent taxa. Some of the taxa in the plankton, including adult stages of benthic foraminifera, ostracods, polychaetes and limpets, must have been resuspended from the seafloor. All taxa found in 1991 net tows tended to be more abundant near the bottom than 15 m above it (Table 1, Fig. 2). Although this pattern was quite distinct, especially for larvae of vent gastropods (roughly 9 times more individuals near bottom) and pelagic gastropods (roughly 12 times more individuals near bottom), no significant differences were found for any of the taxa, using a two-sample t-test on log-transformed data (P(0.05; with Bonferroni correction for multiple tests). The lack of significance, even

Fig. 2. Mean abundances of common taxa found in Deep-Tow net samples near hydrothermal vents in 1991. Positions of tows were outside the axial valley (Out) and inside the axial valley (In), within 15 m of the bottom (NB) or more than 15 m above bottom (AB). Error bars show standard errors; sample sizes given in Table 1.

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429

in taxa with very different mean abundances, was due largely to the high variability in abundances among replicate tows (including many zeros), and the uneven sampling effort (low number of tows in all positions except near-bottom, inside the axial valley). Horizontal patterns were less consistent than vertical ones; larvae of vent gastropods and pelagic gastropods tended to be more abundant inside the axial valley than outside it, whereas bivalve larvae and larvaceans were more abundant outside the valley (none of these differences was significant, using t-tests as above). No attempt was made to interpret the lack of occurrence of rare taxa (e.g. siphonophores, unknown cnidarian larvae, unknown nemertean larvae, and adult limpets) in the above-bottom and outside-valley tows because their absence could have been due to low sampling effort. In 1994 net tows, horizontal patterns of benthic foraminifera, bivalve larvae, larvaceans, and ostracods were similar to those in 1991. Patterns of larvae of vent gastropods and pelagic gastropods had changed; these taxa were no longer more abundant inside than outside the axial valley. Because replication was low in the pump samples and variability among samples at a given height above the bottom was high, detailed vertical patterns could not be distinguished reliably and statistical analyses were not attempted. The presence of vent gastropod larvae in the buoyant hydrothermal plume at heights of 10—45 m above bottom (mab), however, did suggest that they were being transported vertically in the plume. The relative rarity of vent gastropod larvae in net tows at heights greater than 15 mab indicated that comparable vertical transport did not occur outside the plume. Pump samples collected outside the plume were not necessarily outside the influence of hydrothermal vents; for example, sample 71-P (Table 2) was located in visible diffuse flow emanating from a vent. Amphipods swarmed in this immediate area (cf. Kaartvedt et al., 1994), and siphonophores and larvaceans were very abundant in this sample (Fig. 3). 3.2. Currents Near-bottom currents at the study site varied among the four current meter deployments, but shared a common feature of cross-axis reversals on time scales of less than a day (Fig. 4). The progressive vector diagrams in Fig. 4 were based on the assumption that currents everywhere in the region were the same as those measured by the current meter. Although the validity of this assumption is likely to decrease with distance from the point of measurement, these diagrams provide a useful method for displaying the temporal variation of current speed and direction at one location. Currents during the 1991 current meter deployments (Fig. 4a and Fig. 4b) were characterized by slow ((2 cm s~1) north—northwesterly mean flows oriented along the ridge axis, superimposed on what appeared to be diurnal tidal oscillations oriented across the axis. These records were too short to resolve tides using time-series analysis, but a longer record from December 1994 to April 1995 (authors’ unpublished data) confirmed the consistent occurrence of cross-axis, diurnal tidal currents at this site. Thus, we infer that the diurnal oscillations in the present study also were tidally driven. Mean cross-axis speeds during a single diurnal excursion ranged between roughly 2 and 5 cm s~1. In a laterally homogeneous flow, these speeds would

69-1 129

Net tow Volume filtered (cubic m)

Protozoa Benthic Foramnifera 0 Cnidaria Siphonophore 0 Unknown — larvae 0 Annelida Polychaete — adult 0 Polychaete — larvae 0 Nemertea Unknown — larvae 0 Mollusca Bivalve — larvae 0 Limpet — adult 0 Vent Gastropod — larvae 23 Pelagic Gastropod — larvae 70 Arthropoda Ostracod — adult 0 Bythograeid crab — megalopa 0 Chordata Larvacea — adult 0 Embryos 0

Dec. 1991

Cruise

0

0 0

0 5

0

5 0 32 19

0 0

0 0

37 0

0 19

0

0 0 46 19

19 0

0 0

70-3 216

9

70-2 108

In-NB

Position

45 0

0 0

0 0 45 22

0

0 0

11 0

45

71-1 89

45 0

0 0

0 0 54 45

0

0 0

0 0

9

71-2 111

2 0

0 0

0 0 4 4

0

4 2

0 0

0

72-1 454

0 0

0 0

0 0 10 0

0

0 0

0 0

0

72-2 103

0 0

0 0

0 0 0 47

0

0 0

0 0

7

75-3 150

0 0

0 0

4 0 20 24

0

4 0

4 0

20

76-1 252

0 0

0 0

0 0 13 3

0

0 0

0 0

0

76-3 315

0 0

0 0

0 0 6 16

0

0 0

0 0

10

79-1 313

0 0

0 0

3 3 43 43

0

0 3

0 0

9

79-3 349

0 0

5 0

0 0 18 36

5

0 9

0 5

45

80-1 221

0 0

0 0

0 0 0 17

0

0 0

0 0

17

80-2 59

0 5

5 0

0 0 22 55

0

0 0

0 0

33

80-3 182

0 0

0 0

0 0 0 23

0

0 0

0 0

23

81-1 128

0 40

0 0

0 0 320 560

0

0 0

0 0

80

81-3 25

0 4

0 0

0 0 0 20

0

0 0

8 0

0

82-3 246

Table 1 Abundance (no. 1000 m~3) of plankton in Deep-Tow net samples. Position is inside (In) or outside (Out) the axial summit valley, and within 15 m of the bottom (near bottom; NB) or greater than 15 m above the bottom (above bottom; AB). Values are rounded to integers to keep significant figures consistent with raw data

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58-1 151

Net tow Volume filtered (cubic m)

Protozoa Benthic Foramnifera 27 Cnidaria Siphonophore 0 Unknown — larvae 0 Annelida Polychaete — adult 7 Polychaete — larvae 0 Nemertea Unknown — larvae 0 Mollusca Bivalve — larvae 0 Limpet — adult 0 Vent Gastropod — larvae 0 Pelagic Gastropod — larvae 40 Arthropoda Ostracod — adult 73 Bythograeid crab — megalopa 0 Chordata Larvacea — adult 0 Embryos 0

Nov. 1994

Cruise

82

0 0

0 0

0

0 0 14 14

55 0

0 0

0 0

0 0

0

0 0 32 312

65 0

32 0

59-1 73

365

58-3 93

In-NB

Position

0 0

9 0

9 0 164 26

0

0 0

0 0

26

64-1 116

11 0

0 0

11 0 23 23

0

0 11

0 0

23

70-1 87

161 6

0 0

13 0 19 77

0

0 0

0 0

32

71-3 155

Dec. 1991

Out-NB

0 6

0 0

0 0 25 43

0

0 0

0 0

0

72-3 162

0 0

0 3

0 0 5 5

0

0 8

0 0

18

82-2 393

130 0

0 0

22 0 130 652

0

43 0

0 0

435

59-3 46

0 0

8 0

0 0 15 15

0

8 0

0 0

0

73-1 131

Nov. 1994

0 0

0 0

0 0 10 0

0

0 0

0 0

0

75-2 289

0 0

0 0

0 0 4 14

0

0 0

0 0

32

79-2 284

Dec. 1991

In-AB

0 5

0 0

0 0 0 0

0

0 0

0 0

0

82-1 216

S.L. Kim, L.S. Mullineaux / Deep-Sea Research II 45 (1998) 423—440 431

0

0

0 2

2

0

2

0 4

2

!Sample taken within a swarm of amphipods.

19

82-G 31

0

78-G 33

Pump sample Time filtered (min)

Protozoa Benthic Foramnifera Cnidaria Siphonophore Annelida Polychaete — larvae Mollusca Vent Gastropod — larvae Pelagic Gastropod — larvae Chordata Larvacea — adult

BP-1

Position

4

0 2

0

2

0

75-C 28

BP-10

2

4 0

12

0

0

77-G 34

39

4 18

0

0

0

77-B 17

BP-20

6

3 6

0

0

0

75-G 20

BP-45

24

0 0

0

108

0

71-P! 5

AC-1

2

2 15

0

0

0

78-C 31

0

2 2

4

0

0

78-R 30

1

0 3

0

0

1

80-C 42

1

0 0

0

1

0

82-R 45

6

0 6

0

0

0

75-R 19

AC-10

2

2 8

0

0

4

78-B 30

Table 2 Abundance (no. h~1 of sampling time) of plankton in pump samples. Position is within the buoyant hydrothermal plume (BP) or away from it in ambient currents (AC), at a height above the bottom in meters. Values are rounded to integers to keep significant figures consistent with raw data

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433

Fig. 3. Mean abundances of common taxa found in Deep-Tow net samples near hydrothermal vents in 1994. Positions of tows were outside the axial valley (Out) and inside the axial valley (In), within 15 m of the bottom. Error bars show standard errors; sample sizes given in Table 1.

correspond to a cross-axis displacement of water parcels of 0.8—2 km during a 12 h period. In the 1994 deployment, current records also revealed an along-axis mean flow, which reversed directions from south (at &1 cm s~1) to north (at &0.6 cm s~1) on day 8 of the 11-day record (Fig. 4c). Cross-axis flows were less prominent than in the 1991 records, but semidiurnal cross-axis currents, with speeds corresponding to 6 h excursions of up to 0.3 km, did occur during the interval. Mean currents during the 1995 deployment differed from previous intervals in their crossaxis orientation, but cross-axis diurnal fluctuations were still apparent (Fig. 4d and Fig. 5). Currents corresponding to each of these short-term patterns (i.e. relatively slow mean flows that persisted either along- or across-axis for periods of days to weeks, superimposed on cross-axis diurnal or semidiurnal oscillations) were observed in the longer-term (unpublished) current meter record recovered in April 1995 at this site.

434

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Fig. 4. Progressive vector diagrams of near-bottom currents around hydrothermal vents. This plotting technique assumes lateral homogeneity of flow (see Section 3.2). (a) 3-day deployment inside the axial valley, 10 mab, recording on 2 min intervals, 1312 (local time) 5 Dec. 1991 to 1216 8 Dec. 1991. (b) 2-day deployment outside the axial valley, 10 mab, recording on 2 min intervals, 1948 (local time) 8 Dec. 1991 to 0952 10 Dec. 1991. (c) 11-day deployment inside the axial valley, 5 mab, recording on 5 min intervals, 1212 (local time) 12 Nov. 1994 to 0927 23 Nov. 1994. (d) 7-day deployment outside the axial valley, 5 mab, recording on 5 min intervals, 0942 (local time) 11 Dec. 1995 to 1022 18 Dec. 1995.

4. Discussion 4.1. Near-bottom larval transport Analyses of larval transport were restricted to larvae of vent gastropods because they were the only abundant group that could confidently be assigned an origin in vent communities. In net tows within the axial valley, vent gastropod larvae were found in higher abundances near the bottom than above 15 mab. Abundances outside the axial valley (below 15 mab) were intermediate, suggesting that larvae were moved more efficiently horizontally than vertically. The simplest explanation for the observed larval distribution is advection in the predominantly horizontal (as opposed to

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435

Fig. 5. Example stick plot showing the diurnal oscillation in current velocity at 5 m above bottom starting at 0942 (local time) 17 December 1995. Data are at 5 min intervals.

vertical) currents. It is unlikely that predation or other factors were controlling larval distributions, as observed predator abundances (i.e. larvaceans), though greater outside the axial valley, were negligible above 15 mab where larval abundances were the lowest. The observation of substantial numbers of vent larvae outside the axial valley (away from the source vent communities) is not surprising given the potential for transport in cross-axis flows. The current meter records, although short, indicated that vent larvae may move back and forth across the ridge axis in diurnally oscillating currents, as they are advected slowly along the ridge axis. Alternatively, larvae may be transported directly off-axis, depending on their time of release. Because the excursions in cross-axis flows could be as great as 2 km (assuming laterally homogeneous flow), larvae are expected to occur in detectable abundances many hundreds of meters off-axis. 4.2. Entrainment in buoyant plumes Using the observed near-bottom current speeds and larval distributions, we can make some first-order estimates of the proportion of larvae of vent organisms that are dispersed in near-bottom flows. The two larval dispersal pathways considered here are transport in near-bottom flows versus entrainment into buoyant plumes emanating from smokers and transport into different flow patterns several hundred meters above the seafloor (as described in Kim et al., 1994). In a flow field, the proportion of entrained larvae depends largely on the initial buoyancy of the plume (B ; see Eq. (2) 0 below) and on the horizontal velocity u . Near-bottom water and larvae will be ) entrained in a region extending out to where the entrainment velocity into the plume equals the horizontal velocity away from the plume, the zero-flow surface (Fig. 6). Though the shape depicted in Fig. 6 is a simplification of the actual surface within which larvae will be entrained, this first order approximation allows estimation of the proportion of larvae transported in near-bottom versus plume-level flows.

436

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Fig. 6. Top view (a) and side view (b) of the proportion of larva-containing fluid over a hydrothermal vent community that will be entrained into a central buoyant plume emanating from the seafloor (filled circle denotes plume source). Heavy shading indicates the volume of larva-containing fluid (from heights of 0—20 m above bottom) that is entrained into the plume (v ); light shading indicates the larva-containing fluid % that is not entrained (v ). The radius of the benthic comunity is r . The entrainment radius (r ) is the distance " " % where the entrainment velocity into the plume (u ) is equivalent to the average near-bottom current speed % (u ). Calculations of the entrainment volume use d , which is calculated geometrically from r and r . See ) " " % text for equations.

By mass balance, 2r u "2nr u (1) % ) 1 % where r is the radius of entrainment, r is the radius of the plume, and u is the % 1 % entrainment velocity as calculated by a standard buoyant plume model (e.g. Turner, 1973, as used in Kim et al., 1994), given by

A B

B 1@3 0 u "cC % 1 z

(2)

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437

where c"0.07 and C "4.7 are empirical constants, B is the initial buoyancy of the 1 0 plume and z is the height above the bottom in meters. The plume radius, r , again from 1 the standard plume model, is given by r "bz (3) 1 where b"0.084 is an empirical constant. From these equations we can calculate r , and then the distance the community % extends (see Fig. 6b), d , can be calculated geometrically as " d "Jr2!r2 (4) " " % where r is the radius of the community surrounding a hydrothermal vent. From " descriptive studies of Hessler and Smithey (1983), we find that a typical hydrothermal community may extend up to 7 m around a central vent. The fluid volume that contains larvae, v , is " (5) v "nr2H " " where H is the height of the water column where larvae are present. A strong decrease in the abundance of vent gastropod larvae occurs between 10 and 15 mab (Table 1); extrapolating this decrease we will assume that negligible numbers of larvae are diffused or mixed by processes other then plume entrainment to higher than 20 mab, and that within this area, larvae are evenly distributed. This gives us H"20 m, and the volume that will be entrained into the plume, v , is % H nr2 r % #r d #r2 asin % dz v" (6) % " % " 2 r " 0 Then P, the proportion of available larvae entrained into the plume, is simply

PA

A BB

v (7) P" % v " and we can calculate the percentage of the larvae produced by a hydrothermal vent community that are transported in near-bottom flows, versus transported by buoyant plumes. Because v is a monotonically increasing function of r (Eq. (6)), and r is % % % directly proportional to u and inversely proportional to u (Eq. (1)), the proportion of % ) larvae entrained into the plume increases with higher entrainment speeds (i.e. more vigorous smokers) and decreases with higher current speeds. To answer the question of how prevalent plume-level transport might be in typical East Pacific Rise vent habitats, we wished to know how often currents were slow enough to allow most of the larvae to be entrained into the plume. We determined this by solving Eq. (6) numerically for small and medium-sized communities (r "2 and " 7 m), near moderate (B "3.61]10~5 m4 s~3, Kim et al., 1994) and vigorous 0 (B "10~4 m4 s~3, Little et al., 1987; Bemis et al., 1993) smokers, and calculating the 0 speeds (u ) at which larval entrainment dominated (i.e., proportion entrained *0.5) ) (Table 3). Time-series current measurements from the study site (the two longer

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Table 3 The proportion of the time near-bottom current speeds at the 9°50@N East Pacific Rise study site were slower than the threshold speed (u ) at which 50% of the larvae are entrained into a plume (i.e., v /v "0.5). ) % " Entrainment volume (v ) and volume containing larvae (v ) were calculated for small and medium-sized % " vent communities (community radius r "2 and 7 m, respectively) near moderate and vigorous black " smokers (buoyancy flux B "3.61]10~5 and 1]10~4 m4 s~3, respectively). Current meter records from 0 1994 and 1995 (Fig. 4c and Fig. 4d) were used to calculate the proportion of the time speeds were below the threshold u ) r " (m)

2 7

B 0 (m4 s~3)

1]10~4 3.61]10~5 1]10~4 3.61]10~5

u ) (cm s~1)

1.62 1.15 0.46 0.32

Proportion(u at 9°50@N ) 1994

1995

0.20 0.15 (0.15! (0.15!

0.26 0.15 (0.15! (0.15!

!Speeds(1.1 cm s~1 not recorded by Aanderaa current meter.

records; Fig. 4c and Fig. 4d) were then used to determine how often speeds were slower than this threshold u , i.e. how often larvae would be entrained into the plume. ) These calculations indicate that dispersal in the plume is not the prevalent larval transport pathway, as plume-level transport dominates less than 26% of the time, even in a small vent community near a vigorous smoker (Table 3). Furthermore, at a typical current speed of 4 cm s~1 (the mean of the 11-day record in Fig. 4c), only 3% of the larvae are entrained into a moderate smoker (B "3.61]10~5 m4 s~3) from 0 a medium-sized community (r "7 m). " Because the horizontal currents vary on tidal periods at our East Pacific Rise study site, the proportion of larvae entrained into the plume also will vary periodically. Plume-level transport appears to dominate only during the slowest current speeds of the tidal cycle. Other episodic events, however, such as the megaplumes described in Baker (1994), may entrain large numbers of larvae from an extensive area of the seafloor and transport them hundreds of meters into the water column. Megaplumes, resulting from the sudden release of a large volume of hydrothermal fluid, are not uncommon on the Juan de Fuca Ridge (four such plumes have been observed over a period of six years), and they may be responsible for occasional large pulses of larvae that disperse at the level of the neutrally buoyant plume. 4.3. Vent holoplankton All the common holoplanktonic groups quantified in the present study (siphonophores, pelagic gastropod larvae and larvaceans) were more abundant in net tows below 15 mab than higher off the seafloor. This observation is consistent with other reports of enhanced plankton abundances near the deep seafloor (Wishner, 1980; Smith, 1982); a pattern that is often attributed to a benthic or bentho-pelagic food source. The presence of these taxa within the axial valley, where they are exposed

S.L. Kim, L.S. Mullineaux / Deep-Sea Research II 45 (1998) 423—440

439

to hydrothermal fluids, however, raises the question of whether they are specifically associated with vents. In particular, the larvaceans and siphonophores, which were found in a pump sample taken in diffuse flow (in a swarm of amphipods), appear highly tolerant of the elevated temperature, reduced chemicals and heavy metals found in vent fluids. Although it is possible that these species occur throughout the deep sea, their prevalence in near-vent plankton samples suggest that they may be uniquely adapted to vent environments. Further taxonomic studies will be necessary to explore this possibility.

Acknowledgements We thank the captain and crew of the Atlantis II, and the crew of the DS» Alvin for their efforts in the field. We appreciate the helpful comments of Karl Helfrich and two anonymous reviewers on the manuscript. Aanderaa Instruments generously loaned a current meter for the initial deployments. This project was funded by NSF grants OCE-9019575 and OCE-9315554. WHOI contribution number 9392.

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