Deep-sea hydrothermal vents host intriguing communities that are fuelled by chemosynthesis and occur on active seafloor geological features such as mid-ocean ridges, back-arc basins, and seamounts. The habitat in these settings is patchy and subject to catastrophic eruptive disturbance, so the persistence of vent species depends on their ability, predominantly as larvae, to disperse and colonise other vents. In order tounderstandthe resilience of this system, it is important to quantify how quickly vent communities recover from disturbance,what processes influence their ability to recover, and how processes at an individual vent field affect community dynamics, persistence, and species diversity on the regional scale.
The ability of a species to disperse successfully from one vent patch to another depends on factors including the spatial separation of the vents, larval dispersal capabilities, and habitat requirements of new colonists. Vents occur in patterns aligned with locations of magma intrusion into the ocean crust at plate boundaries, often along topographic features such asmid-ocean ridges, volcanic arcs, and backarc spreading centres. In regions with relatively high magma supply rate, such as the East Pacific Rise, vent fields are separated by a few to tens of kilometres, whereas in areas with slower spreading rates, such as the Mid-Atlantic Ridge and some back-arc spreading systems, vent fields may be separated by hundreds of kilometres. Larvae are transported in ocean currents, and must locate suitable physical and chemical habitat within a vent field to colonise successfully. Vent fluid chemistry varies across individual vent orifices and over time. Species differ in their vent habitat requirements, so colonisation of any one species depends not only on its ability to reach a new vent but on the habitat characteristics it encounters there. Even if larvae colonize successfully, a population may fail to persist as the local conditions change, or as it interacts with other species during succession.
Disturbances at vent-field scale that eradicate the local community occur mainly through volcanic eruptions or landslides. The frequency of disturbance may range from almost continuous at vents on submarine arc volcanoes with multiyear eruptions (e.g. NW Rota-1, a seamount in the Mariana Islands, northwest of Rota Island) to very infrequent at vents on slow-spreading ridges. At spreading ridges, the frequency of volcanic eruptions (like the spacing of vents) is related to magma supply: at the fast-spreading East Pacific Rise, vent communities are eradicated by seafloor eruptions on decadal timescales, whereas at the slow-spreading Mid-Atlantic Ridge an eruption might not occur for 10 000 years. These natural eruptive disturbances appear to be a fundamental regulating feature of vent populations and communities at volcanically active areas, as both the disturbance frequency and vent spacing are expected to influence connectivity among vents.
A fundamental question in vent ecology is the resilience of communities to disturbance, including the recovery time in an individual vent field, and the regional persistence of groups of communities (metacommunities) connected across fields by dispersal. Efforts to investigate recovery of vent communities, by monitoring colonisation and species assembly, have taken advantage of local extinctions driven by seafloor volcanic eruptions at vents on the East Pacific Rise and Juan de Fuca Ridge. These field studies, all conducted in frequently disturbed systems, have been interpreted to suggest that typical vent community recovery times may be as short as 2–4 years, and are characteristic of a resilient fauna adapted to disturbance events. The caveats on those estimates, and assumptions underlying the interpretations, need to be examined critically. Identifying a pre-disturbance baseline can be difficult, and the time to re-establish a disturbed state may be much shorter than to establish a climax community, especially if the regional diversity is high. Furthermore, recovery of vent communities in other regions exposed to much lower frequencies of catastrophic disturbance, such as the Mid-Atlantic Ridge or some regions of the western Pacific, may be slower. Communities in those regions appear to be relatively stable over periods of years to decades, in the absence of disturbance on a vent-field scale. Understanding the processes contributing to recovery of both community structure and function is essential for interpreting these field results and using them to predict resilience more generally across a spectrum of disturbance regimes.
Renewed interest in extracting resources from deep-sea mineral deposits has led to an urgent need for a better understanding of vent community recovery and metacommunity resilience. The prospect of seafloor mining of sulphide deposits puts vent communities at risk in some of the areas where connectivity and resilience are least well known. The first demonstration of sulphide mining at a deep-sea vent occurred in the Okinawa Trough in 2017. As of 2019, eight nations had polymetalic sulphide mining exploration contracts under the International Seabed Authority, and another eight had allowed mining research or exploration at vents in waters of their national jurisdiction.
In a paper published in the journal Proceedings of the Royal Society Series B: Biological Sciences on 23 December 2020, Lauren Mullineaux, and Susan Mills of the Biology Department at the Woods Hole Oceanographic Institution, Nadine Le Bris of the Benthic Ecogeochemistry Laboratory at Sorbonne Université and Stace Beaulieu, Stefan Sievert, and Lauren Dykman, also of the Biology Department at the Woods Hole Oceanographic Institution, present the results of a study which aimed to investigate the rate and nature of ecological succession after a catastrophic disturbance.
Mullineaux et al. took advantage of a massive seafloor eruption in 2006 on the East Pacific Rise where we were able to initiate post-eruption observations within months and continue them uninterrupted for more than a decade, providing an extensive colonisation time series that is unprecedented for the deep sea. These data enable a critical evaluation of recovery time in a frequently disturbed vent system, and its relevance to a broader spectrum of disturbance regimes. The vent system represents an intriguing case for succession, where both connectivity and disturbance are closely tied to earth dynamics, and informs broader efforts to understand and predict changes in community structure in response to natural and human disturbance.
Experiments and habitat characterization for this study were conducted on the East Pacific Rise, on a series of cruises starting five months after a catastrophic eruption in January 2006 that eradicated the communities in a vent field located along the ridge between 9°48' and 9°52'. Vent communities at 9°47' North and further south persisted. A prior eruption in 1991 had also eradicated communities in this part of this ridge segment. Mullineaux et al. had been monitoring larval supply and colonization at these vents in the years between the two eruptions, and participated in a rapid-response cruise directly after the 2006 event. In the 11 years since, Mullineaux et al. have accumulated a near-continuous record of colonisation at sites at 9°50' North and neighbouring vents, in some cases with associated measurements of larval supply and/or vent fluid composition.
Colonisation monitoring was focused at a diffuse-flow vent site near P-vent (9.838° North, 104.291°West, 2509 m depth) where venting continued through the duration of the study. Colonisation surfaces were deployed and recovered using the Human-occupied submersibles Alvin and Nautile, and the remotely operated vehicle Jason, with recoveries occurring at time points of 9, 11, 22, 33, 96, 106, and 135 months after the eruption. A few early (nine months) samples were included from a transient vent near the Ty/Io vent, 200 m south of P-vent, but that site was abandoned when it became clear that venting had waned precipitously after the first cruise.
Colonists (larvae that had settled) were collected on experimental surfaces, termed ’sandwiches’, comprising six layered 10 × 10 cm Lexan plastic plates, each 0.7 cm thick and separated from each other by 1 cm to provide roughly 12 000 cm² surface area. For each deployment, replicate sandwiches were placed in each of three distinct habitat types, characterised by temperature as hot (7–30°C), warm (3–7°C), or cool (2–3°C). These habitat types correspond to the faunal zones listed by Fiorenza Micheli, Charles Peterson, Lauren Mullineaux, Charles Fisher, Susan Mills, Gorka Sancho, Galen Johnson, and Hunter Lenihan, in a paper published in 2002, as Vestimentiferan (Tubeworm), Bivalve, and suspension-feeder, respectively. Habitat temperature was measured at the base of each sandwich, on deployment and recovery, with a temperature probe for 1–2 min until a clear maximum value was observed. The maximum value of the recovery temperature was used to characterize the thermal environment of the surface, for consistency with prior studies. These temperature measurements are useful for characterising the general thermal habitat of a sandwich, even though the specific temperature experienced by an individual colonist could vary over the extent of the surface and over tidal and shorter timescales. Due to logistical limitations, not all habitats were sampled for each interval. In many cases, the thermal habitat changed over the course of a deployment, as temperatures overall tended to decrease with time. To account for this variation, both the deployment zone and the recovery temperature were taken into account when interpreting colonisation results.
On recovery, each sandwich was placed in an individual collection compartment for transport back to the ship and subsequent analysis of colonists. On board, sandwiches and their attached colonists were preserved in 80% ethanol, as were any individuals that had become detached in the compartment and were retained on a 63 μm sieve. In the laboratory, each surface was examined under a dissecting microscope and all metazoan colonists (including detached individuals greater than 1 mm) were enumerated and identified to species or the lowest taxonomic group possible. Meiofauna that were too small or mobile to be retained reliably in the samples, or outside our taxonomic expertise, were not included in the analyses.
Species or morphogroup abundance was assessed for each recovery date on three replicate sandwiches from each available habitat. Abundance of a subset of species, selected based on their demonstrated role in prior succession studies and/or prominence in the current samples, was compared across time with a non-parametric Kruskal–Wallace test (a non-parametric method for testing whether samples originate from the same distribution). Analyses of the full community species composition were conducted with nonmetric multidimensional scaling to visualize the change in composition over time. Relative abundance (compared to total number of individuals in the sample) was used in the nonmetric multidimensional scaling to compensate for different deployment intervals or surface types, and log transformed to avoid dominance of pattern by abundant species. Numbers of colonists varied widely between samples, so diversity was assessed by a rarefaction analysis, with total individuals pooled over replicates. Single-celled Protozoans were not included in nonmetric multidimensional scaling or rarefaction analyses, as their occasional outlier abundances obscured patterns displayed by the Metazoans.
Colonisation data from prior to the eruption were available from several vents located within a few hundred metres of the P-vent site, and were selected from the East Wall vent collections to represent a range of thermal habitats similar to P-vent, an extended deployment time (37 months), and the most mature successional stage available (85 months after the previous eruption in 1991). These pre-eruption colonisation experiments had been conducted on basalt blocks with similar collecting characteristics but smaller surface area than the sandwiches, so comparisons with post-eruption results were made on relative abundance.
Free sulphide (i.e. sulphide and hydrogen sulphide ions and dihydrogen sulphide molecules), pH, and temperature were measured in close vicinity to colonisers at 11, 24, 33, and 96 months after the 2006 eruption as in. Mullineaux et al. used a silver sulphide electrode (0.8 mm diameter) associated with a miniaturised glass electrode (1.5 mm) connected to individual potentiometric loggers at 11, 22, and 33 months. A 0.1 mm disc mercury amalgam electrode connected to an underwater potentiostat was used to measure free sulphide by cyclic voltammetry at month 96. The electrodes were combined with a temperature probe (3 mm diameter). Precise positioning and stabilisation of the tip using the submersible arm allowed continuous data records of 1 to 3 min on each location (at the frequency of 4–12 measurements per minute for potentiometry and one triplicate measurement per minute for voltammetry). Average values defined for each measurement sequence integrate fluctuations and allow characterising chemical gradients as a function of temperature with a spatial resolution much finer than the sandwich size.
Abiotic conditions around sandwiches at P-vent after the eruption show a typical increase of free sulphide and decrease of pH with temperature consistent with mixing of local diffuse vent fluids with ambient seawater being the primary driver of habitat abiotic ranges. When compared with the first time point (12 months post-eruption), the narrower ranges of sulphide, pH, and temperature at 24, 33, and 96 months indicate habitat conditions with lower vent fluid contribution, depleted in sulphide and less acidic. Temperatures recorded at 24 and 33 months were typically low because vent fluid flux had decreased at the location of sandwich deployments. Temperatures at 96 months were higher because monitoring had moved to areas with more vigorous venting.
Colonisation of prominent species varied across the series of deployments, showing a distinct progression over time. The Limpet Ctenopelta porifera appeared only in the first 22 months, and primarily in hot habitat. This species may have failed to persist because its preferred high-temperature habitat (typically greater than 15°C; electronic supplementary material) lasted only 2 to 3 years after the eruption. Its arrival is of particular interest because it was a pioneer from afar and had not been reported within 350 km of this site prior to the 2006 eruption. Siboglinid (Vestimentiferan) Tubeworms, including individuals identified as Riftia pachyptila and Tevnia jerichonana, and other individuals too small to identify to species, arrived predominantly in the first 33 months post-eruption, mostly in hot or warm habitat. They were represented by a few individuals in the 96, 106, and 135 month samples, including some in cool habitat. The Limpet Lepetodrilus tevnianus also colonised mostly in hot and warm habitat and declined after 33 months. The Vent Mussel Bathymodiolus thermophilus occurred in all habitat types, became abundant at 22 and 33 months after the eruption, and persisted through 135 months. The Limpet Gorgoleptis spiralis showed a similar temporal pattern, but was found in hot habitat only at 106 months. The small, tube-building Polychaete species Laminatubus alvini arrived at 22 months, persisted through 135 months, and occurred in all habitats. The Protozoans Abyssotherma pacifica (a chambered Foraminiferan), and Metafolliculina sp. (a tube-dwelling Ciliate) also occurred at 22 months and beyond in all habitats.
A distinct, directional trend in community composition was evident over the months following the 2006 eruption. In each of the hot, warm, and cool habitats, composition of the post-eruption samples became progressively more similar to pre-eruption samples during the first 96 months. Later in the series, however, composition bypassed or deviated from the pre-eruption state. Although the pre-eruption samples were intended to represent a ’baseline’ community, it is important to remember that they had been collected only 85 months after the previous eruption in 1991. The observed changes in post-eruption composition were directional along one dimension (horizontal axis), but not the other. In some cases, the differences between replicate samples at one recovery date were equal to, or greater than, differences between dates. Both of these patterns indicate that factors other than time influence composition, and microhabitat variation is a likely candidate, as recovery temperatures of the sandwiches varied across replicates.
Species richness of post-eruption colonists, as measured by rarefaction, generally increased over time For many recovery dates, sample sizes were too low to reach an accurate assessment of full richness (as displayed by lack of asymptote in the curve), but the species number at a given sample size allows for comparison across dates. In the hot habitat, post-eruption richness increased from 9 to 33 months after the eruption, and at 106 months exceeded the pre-eruption level. By contrast, in the warm and cool habitat, post-eruption richness reached or exceeded the pre-eruption level by 22 months.
A compilation of vent species reported from the area near 9° North East Pacific Rise can be added to Mullineaux et al.'s observations to identify the potential species pool for recolonisation of disturbed vents. Filtering this list so it includes only taxa that could be sampled with sandwiches results in a total of 91 taxa. Of these, 67 were observed in the post-eruption samples overall, but only 29 were in the 135-month samples, representing 74% and 32% of the regional species pool, respectively.
More than a decade after an eruption destroyed vent communities on the East Pacific Rise, the species composition of new colonists appeared to still be changing and the successional stage appeared to be different than in the prior, pre-eruption, community. These results suggest that important successional processes continue well beyond the 3–4 years that have previously been monitored after a seafloor disturbance event, and that prior estimates of recovery time need to be re-evaluated.
Temporal changes in environmental conditions clearly influenced succession in this system, especially early in the sequence. In the months directly following the eruption, rapid changes occurred in vent fluid flow distribution, resulting in closure of some newly opened vents, and rapid changes in thermal and chemical habitat, as observed at P-vent. Mullineaux et al. suspect that the rapid decrease of the Gastropod Ctenopelta porifera and other species during this stage may have been driven by loss of the unique habitat created by the unusually vigorous venting of sulphide-rich fluids directly from basalt orifices. Within 20 months past the eruption, maximal temperatures in the hot zone had dropped from 30°C to below 15°C, and sulphide concentration had declined substantially compared to the post-eruption maximum. However, spatial variation in environmental conditions across the hot, warm, and cool zones persisted, and supported different faunas across those zones. This environmental variation across the vent landscape creates a mosaic of habitat types, with variable chemical resource supply and abiotic constraints, which needs to be considered when characterising succession and quantifying recovery.
After two years into the successional sequence, new species continued to arrive (10 total), even though the temperatures had stabilized to a range of 2–10°C across the habitats, andto 2.0–4.2°C directly at the sandwiches.Other species disappeared as colonists (11 total), although they may have persisted in the surrounding community as adults. In these later years, it is likely that differences in species’ life-history traits, such as reproduction and larval dispersal, and species interactions become more important in driving patterns such as the gradual increase in species richness. These types of biological processes can be detected only through long-term monitoring and sampling across environmental zones.
Evaluating community recovery typically involves a comparison of species diversity or composition to a predisturbance, or undisturbed, ’baseline’. If Mullineaux et al. adopt this approach, using the pre-eruption species richness as baseline, then they would conclude from rarefaction curves that P-vent was approaching recovery as early as 2 years after the eruption in the warm and cool habitats. In our observations, the diversity even surpassed the pre-eruption state after roughly 3 years. If Mullineaux et al. use species composition instead (occurrence and relative abundance in nonmetric multidimensional scaling plots), then they conclude that the community was approaching recovery to baselinewithin 8–9 years after the eruption. However, the species composition was still changing at 11 years, and was dissimilar to the pre-eruption state, indicating that the communities may have bypassed the preeruption communities along the successional sequence, and may continue to change. The pre-eruption observations likely did not represent an equilibrium or climax community, as they were from samples collected 7 years after the previous eruption in 1991. This comparison illustrates the challenge in selecting and evaluating a pre-disturbance baseline community. An alternative approach to quantify recovery is to compare community composition during succession to regional species pools. Using the species list developed by
