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1.
Håkon Mosby mud volcano (HMMV) is one of the most active and most studied seep sites in European waters. Many authors have described its thermal activity, dynamic of mud flows, and geochemical and microbial processes. It is characterised by a concentric zonation of successive biogenic habitats related to an activity and geochemical gradient from its centre to its periphery. Around the central area covered by mud flows, white and grey microbial mats occur among areas of bare sediment, whereas siboglinid tubeworm fields of Sclerolinum contortum and/or Oligobrachia haakonmosbiensis colonise the peripheral areas. The meiofaunal community is known to be structured among habitats, but the macrofauna has rarely been investigated and has never been sampled in situ. As part of the European project HERMES, using the ROVs Victor 6000 and Quest 4000, we sampled quantitatively the different habitats of the volcano for macrofauna sensus lato, retained on a 250‐ or 500‐μm sieve. We also sampled a newly discovered pockmark on Storegga slide (cne 5.6) and two pockmarks (G11, G12) in the Nyegga area. Macrofauna was identified and counted from phylum to family level. Our results on HMMV showed a gradient of increasing density and diversity from the volcano centre (1–3 taxa; 260 ind·m?2) to the peripheral siboglinid fields (8–14 taxa, 93,000 ind·m?2), with an intermediate situation for microbial mats. For macrofauna ≥500 μm, non‐siboglinid polychaetes dominated the communities of the central mud volcano area, white mats and S. contortum fields (83, 89 and 37% of the total, respectively), whereas gastropods dominated grey mats and O. haakonmosbiensis fields (89 and 44% of the total, respectively). Polychaete families followed the same pattern of diversity according to habitats within HMMV. Of 23 polychaete families identified, only one occurred in the centre, and three in the microbial mats. Capitellidae and Dorvilleidae (typical of organically and sulphide‐enriched areas) occurred at remarkably high densities in white microbial mats and in O. haakonmosbiensis fields. The S. contortum fields were the most diverse habitat with 12 polychaete families. The 250‐μm fraction showed similar taxa dominating the habitats, but taking meiofauna into account, nematodes became the major taxon in white mats and in S. contortum fields, where they were particularly large in size, whereas copepods dominated in other habitats. Meiofauna and macrofauna did not show the same patterns of density according to habitats. Using principal components analysis the habitats at HMMV were clearly distinct, and clustered according to dominant species of siboglinids and type of microbial mats. Pockmarks at Nyegga showed a similar concentric pattern of habitats around fluid sources as on the volcano, which seemed similarly to influence macrofauna composition, but at a much smaller scale. Total taxa and polychaete diversity are high in the S. contortum fields in these pockmarks as well. Regional‐scale comparisons including HMMV and Storegga suggested a higher influence of habitat‐type than seep‐site on the community structure.  相似文献   

2.
Methane seeps occur at depths extending to over 7000 m along the world's continental margins, but there is little information about the infaunal communities inhabiting sediments of seeps deeper than 3000 m. Biological sampling was carried out off Unimak Island (3200–3300 m) and Kodiak Island (4500 m) on the Aleutian margin, Pacific Ocean and along the Florida Escarpment (3300 m) in the Gulf of Mexico to investigate the community structure and nutrition of macrofauna at these sites. We addressed whether there are characteristic infaunal communities common to the deep‐water seeps or to the specific habitats (clam beds, pogonophoran fields, and microbial mats) studied here, and ask how these differ from background communities or from shallow‐seep settings sampled previously. We also investigated, using stable isotopic signatures, the utilization of chemosynthetically fixed and methane‐derived organic matter by macrofauna from different regions and habitats. Within seep sites, macrofaunal densities were the greatest in the Florida microbial mats (20,961 ± 11,618 ind·m−2), the lowest in the Florida pogonophoran fields (926 ± 132 ind·m−2), and intermediate in the Unimak and Kodiak seep habitats. Seep macrofaunal densities differed from those in nearby non‐seep sediments only in Florida mat habitats, where a single, abundant species of hesionid polychaete comprised 70% of the macrofauna. Annelids were the dominant taxon (>60%) at all sites and habitats except in Florida background sediments (33%) and Unimak pogonophoran fields (27%). Macrofaunal diversity (H′) was lower at the Florida than the Alaska seeps, with a trend toward reduced richness in clam bed relative to pogonophoran field or non‐seep sediments. Community composition differences between seep and non‐seep sediments were evident in each region except for the Unimak margin, but pogonophoran and clam bed macrofaunal communities did not differ from one another in Alaska. Seep δ13C and δ15N signatures were lighter for seep than non‐seep macrofauna in all regions, indicating use of chemosynthetically derived carbon. The lightest δ13C values (average of species’ means) were observed at the Florida escarpment (−42.8‰). We estimated that on average animal tissues had up to 55% methane‐derived carbon in Florida mats, 31–44% in Florida clam beds and Kodiak clam beds and pogonophoran fields, and 9–23% in Unimak seep habitats. However, some taxa such as hesionid and capitellid polychaetes exhibited tremendous intraspecific δ13C variation (>30‰) between patch types. Overall we found few characteristic communities or features common to the three deep‐water seeps (>3000 m), but common properties across habitats (mat, clam bed, pogonophorans), independent of location or water depth. In general, macrofaunal densities were lower (except at Florida microbial mats), community structure was similar, and reliance on chemosynthesis was greater than observed in shallower seeps off California and Oregon.  相似文献   

3.
There are thousands of seeps in the deep ocean worldwide; however, many questions remain about their contributions to global biodiversity and the surrounding deep‐sea environment. In addition to being globally distributed, seeps provide several benefits to humans such as unique habitats, organisms with novel genes, and carbon regulation. The purpose of this study is to determine whether there are unique seep macrobenthic assemblages, by comparing seep and nonseep environments, different seep habitats, and seeps at different depths and locations. Infaunal community composition, diversity, and abundance were examined between seep and nonseep background environments and among three seep habitats (i.e., microbial mats, tubeworms, and soft‐bottom seeps). Abundances were higher at seep sites compared to background areas. Abundance and diversity also differed among microbial mat, tubeworm, and soft‐bottom seep habitats. Although seeps contained different macrobenthic assemblages than nonseep areas, infaunal communities were also generally unique for each seep. Variability was 75% greater within communities near seeps compared to communities in background areas. Thus, high variability in community structure characterized seep communities rather than specific taxa. The lack of similarity among seep sites supports the idea that there are no specific infauna that can be used as indicators of seepage throughout the northern Gulf of Mexico, at least at higher taxonomic levels.  相似文献   

4.
Assessing the relative contribution of local diversity to regional biodiversity may be the key to understanding large-scale and even global patterns in species diversity. Here, the contribution of habitat heterogeneity of cold seeps at three spatial scales [micro-scale (ms), macro-scale (10 to 100s of ms), and mega-scale (10 to 100s of km)] to the total nematode biodiversity (genus level) along the Norwegian continental margin is evaluated. Due to the development of higher resolution bathymetry and increased bottom sampling in recent years, continental margins, once regarded as monotonous landscapes, are now acknowledged to have a high degree of habitat complexity and diversity. By calculating the additive partitioning of gamma diversity in alpha and beta fractions, we examined to what extent habitat diversity of seep sites significantly increases the nematode genus composition and diversity at different spatial scales. Siboglinidae patches and control sediments yielded comparably high levels of nematode genus richness. They exhibited low turnover rates within and across the different seep sites. In contrast, the bacterial mats at Håkon Mosby Mud Volcano (HMMV) and the reduced sediments at the Nyegga pockmarks harboured genus-poor nematode communities with an equally high dominance of one or two species, which were different for each seep. Different habitats, in particular at the HMMV, contributed significantly to the seep nematode richness. This study demonstrates that the presence of distinct habitat types within multiple seep sites contributes to the high diversity of nematode communities inhabiting the seeps in the Norwegian deep sea.  相似文献   

5.
The Håkon Mosby Mud Volcano (HMMV) is a methane seep that is densely populated by the bacteria Beggiatoa spp. as well as by tubeworms of the Family Siboglinidae. Analyses of samples from seven stations located in five different habitats (bare sediments, bacterial mats, siboglinid fields, sediments with small patches of siboglinids and areas of ‘normal’ sea floor) revealed differences in the density and species‐level diversity of nematode communities. Total densities increased from stations inside the mud volcano and on its rim towards the ‘normal’ areas outside the volcano. Nematode species diversity was similar in samples from the siboglinid fields and the bare HMMV sediments. However, the sediment with patches of siboglinids collected outside the volcano was characterised by a diverse nematode community comprising 47 species, whereas the control sediments without siboglinids yielded only 27 species. The nematode assemblage at the bacterial mat site included only two identified species, Halomonhystera disjuncta and Sabatieria ornata, with H. disjuncta being strongly dominant. Terschellingia distlamphida, S. ornata and Aponema ninae dominated nematode communities at the siboglinid fields and in bare HMMV sediments. Species dominating at stations inside the caldera were found in comparable low densities in the sediment with siboglinid patches collected outside the volcano, but were missing in the background samples, where species of Endeolophos, Acantholaimus and Desmoscolex were dominant. Species diversity generally decreased with sediment depth. A subsurface‐dwelling nematode community was observed in the siboglinid fields and the bare sediments. Background sediments showed a sharp decline with 18–20 species at 0–2 cm versus three species at 3–5 cm sediment depth. The trophic composition of the nematode fauna at the HMMV showed a prevalence of deposit feeders at almost all stations, whereas background sediments exhibited a uniform distribution of non‐selective and selective deposit feeders and epistrate feeders. The high percentage of deposit feeders inside the mud volcano could be related to the grain‐size distribution and the significantly higher bacterial biomass compared to the control sediments.  相似文献   

6.
Distributions and carbon isotopic compositions of microbial lipid biomarkers were investigated in sediment cores from the G11 and G12 pockmarks in the Nyegga sector of the Storegga Slide on the mid-Norwegian margin to explore differences in depth zonation, type and carbon assimilation mode of anaerobic methane-oxidizing archaea (ANMEs) and associated sulphate-reducing bacteria responsible for anaerobic oxidation of methane (AOM) in these cold seep environments. While the G11 site is characterised by black reduced sediments colonized by gastropods and Siboglinidae tubeworms, the G12 site has black reduced sediments devoid of fauna but surrounded by a peripheral occurrence of gastropods and white filamentous microbial mats. At both sites, bulk sediments contained abundant archaeal and bacterial lipid biomarkers substantially depleted in 13C, consisting mainly of isoprenoidal hydrocarbons and dialkyl glycerol diethers, fatty acids and non-isoprenoidal monoalkylglycerol ethers. At the G11 site, down-core profiles revealed that lipid biomarkers were in maximum abundance from 10 cm depth to the core bottom at 16 cm depth, associated with δ13C values of ?57 to ?136‰. At the G12 site, by contrast, lipid biomarkers were in high abundance in the upper 5 cm sediment layer, associated with δ13C values of ?43 to ?133‰. This suggests that, as expected from the benthic fauna characteristics of the sites, AOM takes place mainly at depth in the G11 pockmark but just below the seafloor in the G12 pockmark. These patterns can be explained largely by variable fluid flow rates. Furthermore, at both sites, a dominance of ANME-2 archaea accompanied by their bacterial partners is inferred based on lipid biomarker distributions and carbon isotope signatures, which is in agreement with recently published DNA analyses for the G11 pockmark. However, the present data reveal high discrepancies in the contents and δ13C values for both archaeal and bacterial lipid profiles, implying the possible involvement of at least two distinct AOM-related microbial consortia at the inferred AOM depth zonation of G11 and G12 pockmark sediments. In both sediment cores, the δ13C profiles for most archaeal lipids suggest a direct assimilation of dissolved inorganic carbon (DIC) in addition to methane by ANMEs (chemoautotrophy); constant and highly depleted δ13C profiles for PMI:3, an archaeal lipid biomarker presumably related to ANME-2, suggest a direct assimilation of 13C-depleted methane-derived carbon via AOM (methanotrophy). Evidently, the common approach of investigating lipid biomarker contents and δ13C signatures in cold seep sediments does not suffice to precisely discriminate between the carbon assimilation mode for each ANME archaeal group and associated bacteria. Rather, this needs to be combined with further specific labelling studies including different carbon sources (methane carbon, methane-derived organic intermediates and DIC) in order to unravel the metabolic pathways of each microbial consortium involved in AOM (ANME-1 vs. ANME-2 vs. ANME-3 archaeal group and associated bacteria).  相似文献   

7.
Cold seeps are among the most heterogeneous of all continental margin habitats. Abiotic sources of heterogeneity in these systems include local variability in fluid flow, geochemistry, and substrate type, which give rise to different sets of microbial communities, microbial symbiont-bearing foundation species, and associated heterotrophic species. Biogenic habitats created by microbial mats and the symbiotic species including vesicomyid clams, bathymodiolin mussels, and siboglinid tubeworms add an additional layer of complexity to seep habitats. These forms of habitat heterogeneity result in a variety of macrofaunal and meiofaunal communities that respond to changes in structural complexity, habitat geochemistry, nutrient sources, and interspecific interactions in different ways and at different scales. These responses are predicted by a set of theoretical metacommunity models, the most appropriate of which for seep systems appears to be the 'species sorting' concept, an extension of niche theory. This concept is demonstrated through predictable patterns of community assembly, succession, and beta-level diversity. These processes are described using a newly developed analytical technique examining the change in the slope of the species accumulation curve with the number of habitats examined. The diversity response to heterogeneity has a consistent form, but quantitatively changes at different seep sites around the world as the types of habitats present and the size-classes of fauna analyzed change. The increase in beta diversity across seep habitat types demonstrates that cold seeps and associated biogenic habitats are significant sources of heterogeneity on continental margins globally.  相似文献   

8.
Cold-seep environments and their associated symbiont-bearing megafaunal communities create islands of primary production for macro- and meiofauna in the otherwise monotonous and nutrient-poor deep-sea environment. To examine the spatial variation and distribution patterns of metazoan meiobenthos in different seepage-related habitats, samples were collected in two regions off Norway: several pockmarks associated with the Storegga Slide including the Nyegga pockmark area (730 m; 64°N), and the active, methane-venting Håkon Mosby Mud Volcano (HMMV) west of the Barents Sea (1280 m; 72°N). Based on sediment geochemistry and associated epifauna, three different habitat types were distinguished across the two regions: (1) reduced sediment with suboxic conditions, sometimes covered by bacterial mats, (2) sediment colonised by chemosynthetic, siboglinid tubeworms, and (3) sediment outside the influence of seepage and without a large chemosynthetic fauna. Meiofaunal communities varied strongly in terms of generic diversity and dominance among the different habitat types. Control sites and Siboglinidae polychaete fields both supported high nematode genus richness similar to normal deep-sea sediments, whereas the reduced sediments yielded a genus-poor nematode community dominated by one or two successful species. Meiofaunal densities in the different habitats were negatively correlated with macrobenthic densities. An extremely dense (>11,000 ind. 10 cm–2), mono-specific nematode population appeared to be restricted to the bacterial mats at HMMV. It consisted of a new cryptic species of the Halomonhystera disjuncta complex, which has been described from intertidal habitats in the North Sea. The reduced seep sediments at Nyegga did not yield H. disjuncta but were dominated by Terschellingia longicaudata, another cosmopolitan nematode species known to be abundant in organic-rich, oxygen-poor, shallow-water environments. These observations point to a past or recent connection between margins and shallow-water habitats.  相似文献   

9.
Abstract.  Infauna, including foraminifera and metazoans, were enumerated and identified from five types of seep habitats and two adjacent non-seep habitats. Collections were made with the deep submergence research vessel 'Alvin' from three areas of active seepage in the Gulf of Mexico (Alaminos Canyon [2220 m], Atwater Canyon [1930 m], and Green Canyon lease block 272 [700 m]) and on the Blake Ridge Diapir [2250 m], which is located off the southeastern coast of the United States. The seep habitats sampled included four types of microbial mats ( Beggiatoa , Thioploca , thin and thick Arcobacter ) and the periphery of a large mussel bed. Sediments under large rhizopod protists, xenophyophores, were sampled adjacent to the mussel bed periphery. A non-seep site, which was >1 km away from active seeps, was also sampled for comparison. Densities of most taxa were higher in the Gulf of Mexico seeps than in Blake Ridge samples, largely because densities in the thick microbial mats of Blake Ridge were significantly lower. Diversity was higher in the Thioploca mats compared to other microbial-mat types. Within an ocean basin ( i.e. , Atlantic, Gulf of Mexico) we did not observe significant differences in meiofaunal or macrofaunal composition in Beggiatoa versus Thioploca mats or thin versus thick Arcobacter mats. Foraminifera represented up to 16% of the seep community, a proportion that is comparable to their contribution at adjacent non-seep communities. In general, the observed densities and taxonomic composition of seep sites at the genus level was consistent with previous observations from seeps ( e.g. , the foraminifers Bolivina and Fursenkoina , the dorvilleid polychaete Ophryotrocha ).  相似文献   

10.
Macro- and meiofaunal communities were examined at four geomorphologically distinct sites with different gas- and oil-bearing fluid characteristics in the northern, central and southern basins of Lake Baikal. All sites had elevated concentrations of bicarbonate, nitrate, sulphate and chloride ions in pore fluids, with highest values at the Frolikha vent. Elevated levels of iron ions were found in pore waters of the St. Petersburg methane seep and the Gorevoy Utes oil seep. The chemical composition of pore waters at the Malenky mud volcano was similar to that reported in earlier work. Consistent with published data, the Frolikha vent (northern basin) and the St. Petersburg methane seep (central basin) were characterised by methane of mixed genesis (thermogenic + biogenic), whereas the methane source was mainly thermogenic at the Gorevoy Utes oil seep (central basin) and biogenic at the Malenky mud volcano (southern basin). In contrast to marine seep ecosystems, the macrofauna was dominated only by amphipods, giant planarians and oligochaetes, whereas bivalves were absent; the meiofauna was similar to its marine counterpart, being dominated by nematodes, cyclops, harpacticoids and ostracods. A statistically significant positive relationship was revealed between faunal abundance and the availability of bacterial mats on seep sediments. Moreover, ANOVA tests showed significant increases in both meiozoobenthic and macrozoobenthic densities at “hot spot” vent/seep sites relative to discharge-free reference sites. The isotopic composition of carbon and nitrogen at various trophic levels of these benthic vent/seep communities was found to differ markedly from that reported by earlier studies for the pelagic and other benthic food webs in Lake Baikal. As in marine seeps, the macrofauna had variable isotopic signatures. Light δ13C and δ15N values suggest the utilization of chemosynthetically fixed and/or methane-derived organic matter. By contrast, the heavy δ13C signatures of some mobile amphipods likely reflect consumption of photosynthetically derived carbon. These findings would at least partly explain why Lake Baikal is a notable outlier in global temperature–biodiversity patterns, exhibiting the highest biodiversity of any lake worldwide but at an extremely cold average temperature.  相似文献   

11.
Studies of the trophic structure in methane‐seep habitats provide insight into the ecological function of deep‐sea ecosystems. Methane seep biota on the Chilean margin likely represent a novel biogeographic province; however, little is known about the ecology of the seep fauna and particularly their trophic support. The present study, using natural abundance stable isotopes, reveals a complex trophic structure among heterotrophic consumers, with four trophic levels supported by a diversity of food sources at a methane seep area off Concepción, Chile (~36° S). Although methanotrophy, thiotrophy and phototrophy are all identified as carbon fixation mechanisms fueling the food web within this area, most of the analysed species (87.5%) incorporate carbon derived from photosynthesis and a smaller number (12%) use carbon derived from chemosynthesis. Methane‐derived carbon (MDC) incorporation was documented in 22 taxa, including sipunculids, gastropods, polychaetes and echinoderms. In addition, wide trophic niches were detected in suspension‐feeding and deposit‐feeding taxa, possibly associated with the use of organic matter in different stages of degradation (e.g. from fresh to refractory). Estimates of Bayesian standard ellipses area (SEAB) reveal different isotopic niche breadth in the predator fishes, the Patagonian toothfish Dissostichus eleginoides and the combtooth dogfish Centroscyllium nigrum, suggesting generalist versus specialist feeding behaviors, respectively. Top predators in the ecosystem were the Patagonian toothfish D. eleginoides and the dusky cat shark, Bythaelurus canescens. The blue hake Antimora rostrata also provides a trophic link between the benthic and pelagic systems, with a diet based primarily on pelagic‐derived carrion. These findings can inform accurate ecosystem models, which are critical for effective management and conservation of methane seep and adjacent deep‐sea habitats in the Southeastern Pacific.  相似文献   

12.
Mangroves are highly productive ecosystems that exhibit a diverse range of habitats, including tidal creeks and flats, forest gaps and interior forest with varying understory light intensity, tidal dynamics, geomorphological settings, and overall biological production. Within mangrove ecosystems, invertebrates and fish feed on heterogeneous food sources, the occurrence of which is unevenly distributed across the system. This provides a basis for testing models of carbon transfer across mangrove ecosystems. We hypothesized that the carbon transfer and assimilation by fish and invertebrates will vary across the different mangrove habitats and that such variations can be predicted by their stable isotope compositions. We analysed δ13C and δ15N signatures of consumers and their potential organic carbon sources across a tropical mangrove ecosystem in Vietnam. The δ13C values of crabs and snails significantly decreased from the tidal flat to interior forest, indicating that variations in carbon transfer and assimilation occurred at small scales <30 m. Reduced variation in δ13C of suspension‐feeding bivalves suggested that tidal water was a vector for large‐scale transport of carbon across the mangrove ecosystem. An analysis of co‐variance using habitat as a fixed factor and feeding habit and movement capacity of consumers as co‐variates indicated that habitat and feeding types were major features that affected the δ13C values of invertebrates and fish. The findings demonstrate that carbon transfer and assimilation across mangrove ecosystems occur as a diverse combination of small (<30 m) and large (>30 m) scale processes.  相似文献   

13.
We investigated the molecular composition (methane, ethane, and propane) and stable isotope composition (methane and ethane) of hydrate-bound gas in sediments of Lake Baikal. Hydrate-bearing sediment cores were retrieved from eight gas seep sites, located in the southern and central Baikal basins. Empirical classification of the methane stable isotopes (δ13C and δD) for all the seep sites indicated the dominant microbial origin of methane via methyl-type fermentation; however, a mixture of thermogenic and microbial gases resulted in relatively high methane δ13C signatures at two sites where ethane δ13C indicated a typical thermogenic origin. At one of the sites in the southern Baikal basin, we found gas hydrates of enclathrated microbial ethane in which 13C and deuterium were both highly depleted (mean δ13C and δD of –61.6‰ V-PDB and –285.4‰ V-SMOW, respectively). To the best of our knowledge, this is the first report of C2 δ13C–δD classification for hydrate-bound gas in either freshwater or marine environments.  相似文献   

14.
Relationships among seep community biomass, diversity, and physiographic controls such as underlying geology are not well understood. Previous efforts to constrain these relationships at the Blake Ridge Diapir were limited to observations from piloted deep-submergence vehicles. In August 2012, the autonomous underwater vehicle (AUV) Sentry collected geophysical and photographic data over a 0.131 km2 area at the Blake Ridge Diapir seeps. A nested survey approach was used that began with a regional or reconnaissance-style survey using sub-bottom mapping systems to locate and identify seeps and underlying conduits. This survey was followed by AUV-mounted sidescan sonar and multibeam echosounder systems mapping on a mesoscale to characterize the seabed physiography. At the most detailed survey level, digital photographic imaging was used to resolve sub-meter characteristics of the biology. Four pockmarks (25–70 m diameter) were documented, each supporting chemosynthetic communities. Concentric zonation of mussels and clams suggests the influence of chemical gradients on megafaunal distribution. Data collection and analytical techniques used here yield high-resolution habitat maps that can serve as baselines to constrain temporal evolution of seafloor seeps, and to inform ecological niche modeling and resource management.  相似文献   

15.
Cold seeps on the Hikurangi Margin off New Zealand exhibit various seabed morphologies producing different intensity patterns in sidescan backscatter images. Acoustic backscatter characteristics of 25 investigated seep sites fall into four distinct types characterised by variations in backscatter intensity, distribution and inferred structural heights. The types reflect different carbonate morphologies including up to 20-m-high structures (type 1), low-relief crusts (type 2), scattered blocks (type 3) and carbonate-free sites (type 4). Each seep corresponds to a single type; intermediates were not observed. This correlates well with published data on seep fauna at each site, with the four types representing three different faunal habitats of successive stages of seep development. Backscatter signatures in sidescan sonar images of cold seeps may therefore serve as a convenient proxy for variations in faunal habitats.  相似文献   

16.
To investigate a possible influence of submarine methane seepage on benthic foraminiferal communities, Rose Bengal stained (“live”) and empty tests of benthic foraminifera were studied from the sediment surface down to 15 cm sub-bottom depth of 12 sites at the Håkon Mosby mud volcano (HMMV). In addition, one reference site well away from the seep sites, but from similar water depths and the same general hydrographic setting was occupied for comparison. The HMMV is located at 1265 m water depth on the SW Barents Sea continental slope. Distinct living foraminiferal associations at the HMMV are linked to specific sedimentary, microbial, and macrofaunal habitats. In the center of the crater, and in crater areas completely covered by bacterial mats, Cassidulina reniforme is the only, albeit rare, living species. Below the top few millimeters, sediments are anoxic and devoid of living specimens. At the rim of the mud volcano, at sites densely populated by pogonophoran tube worms, three benthic foraminiferal associations are found; (i) a Fontbotia wuellerstorfi–Lobatula lobatula association living attached to the upper parts of pogonophoran tubes, which protrude into oxic water, (ii) a diverse Cassidulina neoteretis association populating dysoxic sediments of the surface centimeter, and (iii) a species-poor Bolivina pseudopunctata association colonizing the subsurface sediments down to four centimeters. Generally, we did not find endemic or seep indicative species or associations at the HMMV. However, the HMMV live faunas dominated by B. pseudopunctata are not found at the reference site nor are they described from comparable water depths and environments without gas seepages from the Norwegian-Greenland Seas.In the center and outer rim of the mud volcano, a C. neoteretis–Reophax guttifer dead association, similar to the one at the reference site, characterizes an assemblage of strongly corroded and partly displaced tests. At bacterial mat sites, a C. reniforme dead association corresponds to the living one. Thus both the living and the dead associations are indicative of a specific bacterial mat environment at the HMMV.  相似文献   

17.
We discovered and investigated several cold-seep sites in four depth zones of the Sea of Okhotsk off Northeast Sakhalin: outer shelf (160–250 m), upper slope (250–450 m), intermediate slope (450–800 m), and Derugin Basin (1450–1600 m). Active seepage of free methane or methane-rich fluids was detected in each zone. However, seabed photography and sampling revealed that the number of chemoautotrophic species decreases dramatically with decreasing water depth. At greatest depths in the Derugin Basin, the seeps were inhabited by bacterial mats and bivalves of the families Vesicomyidae (Calyptogena aff. pacifica, C. rectimargo, Archivesica sp.), Solemyidae (Acharax sp.) and Thyasiridae (Conchocele bisecta). In addition, pogonophoran tubeworms of the family Sclerolinidae were found in barite edifices. At the shallowest sites, on the shelf at 160 m, the seeps lack chemoautotrophic macrofauna; their locations were indicated only by the patchy occurrence of bacterial mats.Typical seep-endemic metazoans with chemosynthetic symbionts were confined to seep sites at depths below 370 m. A comparative analysis of the structure of seep and background communities suggests that differences in predation pressure may be an important determinant of this pattern. The abundance of predators such as carnivorous brachyurans and asteroids, which can invade seeps from adjacent habitats and efficiently prey on sessile seep bivalves, decreased very pronouncedly with depth. We conclude from the obvious correlation with the conspicuous pattern in the distribution of seep assemblages that, on the shelf and at the upper slope, predator pressure may be high enough to effectively impede any successful settlement of viable populations of seep-endemic metazoans. However, there was also evidence that other depth-related factors, such as bottom-water current, sedimentary regimes, oxygen concentrations and the supply of suitable settling substrates, may additionally regulate the distribution of seep fauna in the area.As a consequence of the pronounced pattern in the distribution of seep communities, their ecological significance as food sources of surrounding background fauna increased with water depth. Isotopic analyses suggest that in the Derugin Basin seep colonists feed on chemoautotrophic seep organisms, either directly or by preying on metazoans with chemosynthetic symbionts. In contrast, seep organisms apparently do not contribute to the nutrition of the adjacent background fauna on the shelf and at the slope. In this area, elevated epifaunal abundances at seep sites were caused primarily by the availability of suitable settling substrates rather than by an enrichment of food supply.  相似文献   

18.
Marine seep hunting surveys are a current focus of hydrocarbon exploration surveys due to recent advances in offshore geophysical surveying, geochemical sampling, and analytical technologies. Hydrocarbon seeps are ephemeral, small, discrete, and therefore difficult to sample on the deep seafloor. Multibeam echosounders are an efficient seafloor exploration tool to remotely locate and map seep features. Geophysical signatures from hydrocarbon seeps are acoustically-evident in bathymetric, seafloor backscatter, midwater backscatter datasets. Interpretation of these signatures in backscatter datasets is a fundamental component of commercial seep hunting campaigns. Degradation of backscatter datasets resulting from environmental, geometric, and system noise can interfere with the detection and delineation of seeps. We present a relative backscatter intensity normalization method and an oversampling acquisition technique that can improve the geological resolvability of hydrocarbon seeps. We use Green Canyon (GC) Block 600 in the Northern Gulf of Mexico as a seep calibration site for a Kongsberg EM302 30 kHz MBES prior to the start of the Gigante seep hunting program to analyze these techniques. At GC600, we evaluate the results of a backscatter intensity normalization, assess the effectiveness of 2X seafloor coverage in resolving seep-related features in backscatter data, and determine the off-nadir detection limits of bubble plumes using the EM302. Incorporating these techniques into seep hunting surveys can improve the detectability and sampling of seafloor seeps.  相似文献   

19.
Discovery of prolific natural methane seeps at Gullfaks, northern North Sea   总被引：1，自引：0，他引：1  
M. Hovland 《Geo-Marine Letters》2007,27(2-4):197-201
The Gullfaks and Kvitebjørn fields are located on the North Sea Plateau (135 m water depth), and on an ancient beach (135–190 m) deposited during the sea-level lowstand during the Last Glacial Maximum (LGM). There are several continuous seeps of mainly methane gas, where large patches of Beggiatoa bacterial mats occur. The ‘Heincke’ seep area, which is named after the German research vessel Heincke, has been targeted by scientists studying seep-associated processes and microbiology. The Gullfaks area has a long history of shallow gas and seepage. In 1980, well no. 34/10–10 had a blowout from a reservoir located 230 m below seafloor. The active Heincke seep location has no topographic expression, probably because the seabed consists of dense sand and gravel. Extensive bacterial mats (Beggiatoa sp.) are found on the seafloor at this seep site. Organisms such as hermit crabs were seen ingesting pieces of such mat, indicating ‘trophic bypass,’ where carbon derived directly from seeping methane is evidently feeding directly into higher trophic organisms. Ongoing and future research at this seep location in the North Sea can answer some important questions on the environmental impact of natural methane seeps on continental shelves.  相似文献   

20.
Previous studies at the Isla Vista oil seep have suggested that meiofauna, particularly nematodes, might be an important factor in explaining macrofaunal enrichment by making bacterial biomass available to the benthic food web. To explore this possibility, we analyzed meiofaunal abundance and microalgal pigments inside and just outside of bacterial mats at this natural oil seep.The bacterial mats occur where crude oil and natural gas are actively seeping out of the sediment; cores from within the mats contained a great deal of crude oil (up to 50 %). Meiofaunal abundances were the same in and out of the bacterial mats (averaging 1·-9 × 106 individuals m-2). However, dramatic changes in community structure were noticed. Harpacticoids made up 19 % of the fauna outside the mats but only 1 % inside. Pigment concentrations were also the same in both sites with phaeophytin dominating chlorophyll (120 compared to 29.8 mg m-2). The variance of both microalgal pigments and meiofauna was much greater inside than outside, suggesting that the bacterial mats are a more heterogeneous environment.Although the effect of crude oil toxicity is not clear, the high abundances of microbial and meiofaunal biomass support the hypothesis of benthic enrichment via microbes and meiofauna.  相似文献   

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