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Keywords:

  • Cold seep;
  • fauna;
  • Mosaic;
  • pockmark;
  • Regab;
  • temporal variation

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

The Regab pockmark is a large cold seep area located 10 km north of the Congo deep sea channel at about 3160 m water depth. The associated ecosystem hosts abundant fauna, dominated by chemosynthetic species such as the mussel Bathymodiolus aff. boomerang, vestimentiferan tubeworm Escarpia southwardae, and vesicomyid clams Laubiericoncha chuni and Christineconcha regab. The pockmark was visited during the West African Cold Seeps (WACS) cruise with RV Pourquoi Pas? in February 2011, and a 14,000-m2 high-resolution videomosaic was constructed to map the most populated area and to describe the distribution of the dominant megafauna (mussels, tubeworms and clams). The results are compared with previous published works, which also included a videomosaic in the same area of the pockmark, based on images of the BIOZAIRE cruise in 2001. The 10-year variation of the faunal distribution is described and reveals that the visible abundance and distribution of the dominant megafaunal populations at Regab have not changed significantly, suggesting that the overall methane and sulfide fluxes that reach the faunal communities have been stable. Nevertheless, small and localized distribution changes in the clam community indicate that it is exposed to more transient fluxes than the other communities. Observations suggest that the main megafaunal aggregations at Regab are distributed around focused zones of high flux of methane-enriched fluids likely related to distinct smaller pockmark structures that compose the larger Regab pockmark. Although most results are consistent with the existing successional models for seep communities, some observations in the distribution of the Regab mussel population do not entirely fit into these models. This is likely due to the high heterogeneity of this site formed by the coalescence of several pockmarks. We hypothesize that the mussel distribution at Regab could also be controlled by the occurrence of zones of both intense methane fluxes and reduced efficiency of the anaerobic oxidation of methane possibly limiting tubeworm colonization.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

Cold-seep ecosystems have been identified along active and passive margins worldwide, and are known to host rich and abundant chemosynthetic communities (Sibuet & Olu-Le Roy 2002). Many studies have described the distribution of the dominant faunal assemblages in relation to their environment in several cold seeps systems (Sibuet & Olu-Le Roy 2002; MacDonald et al. 2003; Olu-Le Roy et al. 2007a; Jerosch et al. 2007; Lessard-Pilon et al. 2010), and cold seeps are usually considered to provide more stable environments than hydrothermal vents. Although some decadal-scale stability of vent fauna was sometimes observed (Desbruyères 1998; Copley et al. 2007; Cuvelier et al. 2011), several studies about temporal variation of vent communities suggested that hydrothermal vents can be highly dynamic environments (Hessler et al. 1988; Shanks 1995; Shank et al. 1998; Mullineaux et al. 2000), especially when considering smaller spatial and temporal scales (Cuvelier et al. 2011). Existing observations of individual taxonomic groups at cold seeps revealed very slow growth rates and extremely long lifetimes, likely related to slow and steady fluxes of reduced compounds (Nix et al. 1995; Fisher et al. 1997; Smith et al. 2000; Bergquist et al. 2000). For instance, some tubeworm aggregations were estimated to be at least 250 years old (Fisher et al. 1997; Bergquist et al. 2000), and ages of several 100s of years have been assessed for aggregations of Bathymodiolus childressi (Smith et al. 2000).

Up to now, very few works have focused on the temporal variation of the faunal distribution (Lessard-Pilon et al. 2010) in a cold seep environment. Such information is not only important to increase our knowledge about community dynamics, it also allows a better understanding of the dynamics of venting activity. Indeed, chemosynthetic communities are highly dependent on their environment, primarily because distribution patterns of the dominant symbiont-bearing, habitat-creating taxa are linked to methane and sulfide levels and fluxes, and substrata (Sahling et al. 2002; MacDonald et al. 2003; Levin et al. 2003; Bergquist et al. 2005; Mau et al. 2006; Olu-Le Roy et al. 2007a). Distribution changes therefore could also reflect changes in venting activity.

Bergquist et al. (2003b) and Cordes et al. (2005b) suggested that community changes could be also time-related, and proposed a succession model for Gulf of Mexico seep communities, in which mussel beds become replaced by tubeworm communities as carbonate precipitates in the sediments. With time, tubeworm communities then contribute to reducing methane and sulfide availability at the sediment/water interface, leading to changes in the associated communities by allowing non-endemic species to enter and compete with chemosynthetic species.

Whatever the cause of flux change, mussel population mortality and movements are considered to reflect changes in seepage flow or chemistry (Roberts et al. 1990; Lessard-Pilon et al. 2010), whereas tubeworms tend to increase their dominance when fluid flow declines and can persist for years (Bergquist et al. 2003a,b; Cordes et al. 2005b). Finally, Lessard-Pilon et al. (2010) attributed a 15-year succession pattern between tubeworm and mussel populations to renewed or redirected active seepage.

During the West African Cold Seeps (WACS) cruise in February 2011, the Regab pockmark was intensively surveyed and a 14,000-m2 videomosaic was assembled to map the main populated area of the pockmark. A subset of this same area had already been described by Olu-Le Roy et al. (2007a), who provided a detailed description of the spatial patterns of the faunal assemblages, highlighting a high degree of spatial heterogeneity. This work was based on imagery data, and in particular on videomosaics, taken in 2001 during the BIOZAIRE cruise.

Using geo-referenced mosaics and geographic information systems (GIS), we provide a description of the current distribution of the dominant megafauna (mussels, tubeworms, clams) and its 10-year development in one of the most densely populated areas of the Regab pockmark. We identify vestimentiferan tubeworms, bathymodiolid mussels and vesicomyids that create the dominant habitats of the pockmark. To our knowledge this is the first study of the temporal variation of the distribution of chemosynthetic fauna at this scale and including such diverse habitats.

Site Description

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

The Regab pockmark is located on the passive Congo-Angola margin at 3160 m water depth, about 10 km to the north of the Congo deep-sea canyon. The pockmark is a circular-shaped depression on the sea floor that is less than 20 m deep and about 800 m wide (Charlou et al. 2004; Ondréas et al. 2005) (Fig. 1). Regab has been described as a ‘pockmark cluster’ as it is considered to be composed of several smaller pockmarks (Ondréas et al. 2005). These features are believed to result from sea-floor collapses following the release of over-pressurized interstitial fluids. This was suggested after seismic profiles showed the presence of a 300-m-deep subsurface pipe rooted in a palaeo-channel that acts as a reservoir for the accumulating fluids (Ondréas et al. 2005; Gay et al. 2006). Trapped fluids are mostly enriched in methane and are believed to be produced in deeper layers of sediment by microbial activity (Charlou et al. 2004). The presence of gas hydrates was observed both in hydrate outcrops at the sediment surface and in gravity cores down to a depth of 6 m (Charlou et al. 2004; Ondréas et al. 2005). Sulfide is produced from methane and seawater sulfate in the subsurface sediment by anaerobic methane oxidation, which has been identified in the different habitats (Cambon-Bonavita et al. 2009).

image

Figure 1. Location of the Regab pockmark; the insert map shows the approximate outline of the pockmark and the mosaic area.

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The most active area in terms of fluid escape is a 600-m-long and 200-m-wide 70 ºN-directed area located near the middle of the pockmark. This area corresponds to a zone of extensive carbonate crusts and it seems to host most of the fauna that have been identified at the pockmark (Ondréas et al. 2005). The faunal communities present at Regab are dominated by symbiont-bearing species including vestimentiferan tubeworms of the species Escarpia southwardae (Andersen et al. 2004), two species of Vesicomyidae bivalves, Laubiericoncha chuni and Christineconcha regab (von Cosel & Olu 2008, 2009; Krylova & von Cosel 2011), and one species of the mussel Bathymodiolus aff. boomerang (Olu-Le Roy et al. 2007b). These foundation species create habitats that support associated heterotrophic macro- and meio-faunal communities which vary in biomass and diversity among habitats (van Gaever et al. 2009; Menot et al. 2009; Olu et al. 2009).

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

WACS mosaic

Acquisition

Imagery used for the production of mosaics was acquired with a high-definition color video camera over two ROV dives during the WACS cruise with RV Pourquoi Pas?. The camera is mounted vertically on the ROV Victor 6000 and is dedicated to high-resolution mosaicking applications. The surveys were carried out in a structured way by performing parallel line surveys separated by 3-m intervals, from an average altitude of 3 m, so as to ensure overlap between the mosaic lines. The total surveyed area covers a rectangular surface of about 65 × 220 m2 (Fig. 2). The limits of this surface correspond to the limits of the ‘mosaic 2’ produced by Olu-Le Roy et al. (2007a) from images acquired during the BIOZAIRE cruise in 2001. The reason for this is to enable later comparison of the two mosaics. To minimize drift-induced positioning errors, the survey area was split into two equal subareas of 65 × 115 m2 each (Fig. 2). The survey required 21 lines per subarea to cover the entire surface. Each line was 115 m long to ensure overlap between the two subareas. Moreover, the ROV position was regularly reset onto markers at the beginning of lines to eliminate any drifting error before starting a new line. During the survey, the maximum observed drift error at the end of a line was about 3 m. The markers were also used to reset the ROV position when resuming the survey in the second mosaicking dive. Final navigation is therefore a hybrid navigation from USBL and dead-reckoning navigation reset with markers.

image

Figure 2. ROV navigation of the WACS mosaicking survey showing the distribution and the overlap of the two subareas (A and B), as well as which areas were surveyed during the two mosaicking dives (423 and 426). The navigation data was regularly reset onto known markers (black dots) to constrain global drifting error.

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Construction of the video-mosaic

The lines of mosaic were constructed using the Ifremer in-house MATISSE program (Vincent et al. 2003; Allais et al. 2004). The MATISSE program was first designed for online-videomosaicking, i.e. to build the mosaic while the survey is ongoing. However, due to compatibility issues between the program and the new camera and navigation systems of the ROV Victor 6000, building the mosaic involved numerous intermediate data manipulation steps and could not be performed in real-time. For instance, the HD-formatted video files (1920 × 1080 pixels) had to be converted into DVD-PAL format (720 × 576 pixels) before they could be read by MATISSE. This involved adding black bands on the video files to preserve the 16/9-ratio of HD frames. Conversion to DVD format was done with the CONVERTXTODVD commercial program. Navigation files also had to be rewritten according to an older standard to ensure compatibility with MATISSE. The navigation was then replayed with Ifremer TRIADE Software, a program that sends navigation entries to MATISSE at a real-time frequency in order to simulate an online mode. Mosaic lines were then constructed at a real-time pace.

Each line of mosaic was constructed separately instead of letting MATISSE run straight from the beginning to the end of the survey. The reason for this was to keep size of files small, and to allow more flexibility in the construction of the final areal mosaic.

GIS and spatial analyses

The separate lines were imported and geo-referenced into ARCGIS. Geo-referencing was done with the ROV navigation data, but care was taken to match corresponding features between overlapping segments on the same points.

For all mosaics, surficial features were manually delineated and polygons were created in ARCGIS to map the spatial distribution of each feature. Mapped features are similar to those used for the BIOZAIRE mosaic (Olu-Le Roy et al. 2007a). The main categories are: dense Mytilidae, sparse Mytilidae, dead Mytilidae, EscarpiaMytilidae co-occurrence, dense Escarpia southwardae, sparse E. southwardae, juvenile E. southwardae, recumbent E. southwardae, senescent E. southwardae, living Vesicomyidae, mixed (living and dead) Vesicomyidae, dead Vesicomyidae, carbonate concretions (Fig. 3). Areas of coverage were computed for each non-sparse category in ARCGIS, using the Mollweide equal-area projection.

image

Figure 3. Excerpts of the WACS mosaic illustrating the different faunal categories: (a) Mytilidae, dense; (b) Mytilidae, sparse; (c) Mytilidae, shells; (d) Escarpia southwardae, dense; (e) E. southwardae, sparse; (f) E. southwardae-Mytilidae co-occurrence; (g) E. southwardae, juvenile; (h) E. southwardae, recumbent; (i) E. southwardae, senescent; (j) Vesicomyidae, living (in the black sediments); (k) Vesicomyidae, living/dead (mixed); (l) Vesicomyidae, shells. Images taken by ROV Victor 6000

(© Ifremer, WACS 2011).

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The dense Mytilidae category refers to areas where the living mussel distribution is almost continuous and where the substratum is rarely visible. Conversely, sparse Mytilidae applies to areas where the substratum is clearly visible between the individuals. Such distinction was not made for the dead Mytilidae category. The dense E. southwardae category refers both to single large bushes of adult tubeworms, and to fields of bushes of adult tubeworms, whereas the sparse E. southwardae category corresponds to areas where bushes of adult tubeworms are not closely distributed and contain relatively few tubes (roughly 10 or less). The juvenile E. southwardae category refers to bushes where tubeworms are of strikingly small size in comparison with the adult community. The recumbent E. southwardae category designates bushes where tubes are disposed horizontally, and the senescent category refers to dead individuals and individuals in poor condition whose tubes lie on the sea floor. Patches of vesicomyid clams are categorized either as living, mixed (dead and living) or dead. Living clams are normally half buried and stand upright in the sediments, whereas dead clam shells are generally open and lying in the sediments. The ‘mixed’ category refers to patches that contain both living and dead clams. Carbonate crusts were mapped only where concretions could clearly be seen on the images, and the mapped areas often do not include the carbonated crusts that underlie the tubeworm population, the dense mussel beds, or thin sediment covers.

The delineation process was supported by the use of the full HD resolution video files, particularly for differentiating clams from mussels and living bivalves from dead bivalves. Vesicomyid clams comprehend two species, Laubiericoncha chuni and Christineconcha regab, which cannot be separated based on the images. However, both in 2001 and in 2011, C. regab was largely dominant in samples and on close-up views (von Cosel & Olu 2009; Decker et al. 2012).

BIOZAIRE mosaic

The BIOZAIRE mosaic corresponds to the ‘mosaic 2’ described in the literature (Olu-Le Roy et al. 2007a). Due to the absence of navigation data, the BIOZAIRE mosaic was never geo-referenced. But surfaces could be calculated anyway from the altitude of survey and the camera parameters. In this work we used the new WACS mosaic to geo-reference each individual segment (76 segments) of the BIOZAIRE mosaic, with an average root-mean-square (RMS) error of 0.03 m (SD = 0.1 m) and a maximum RMS error of 0.4 m. The geo-referencing was done in ARCGIS by registering features common to both mosaics, such as unchanged carbonate concretions, patches of dead shells, detritus and also bushes of tubeworms. The advantage of this technique is that it reduces the discrepancies between both mosaics, no matter how accurate the geo-referencing of the WACS mosaic is. In other words, the same polygon should have the same surface on both mosaics and patch sizes should be directly comparable, with a low relative error. However, differences in angles of perspective, in image quality, in visibility and in precision of delineation process also occur and cause some discrepancies in the computed areas. Digitized polygons for living and dead mussel patches are the most affected by such discrepancies.

To maintain consistency with the published work, BIOZAIRE polygons were not redrawn. Instead, the original polygons, drawn in PHOTOSHOP by Olu-Le Roy et al. (2007a), were reused. This meant exporting every polygon layer from PHOTOSHOP. Polygons were then imported as polygon features into ARCGIS and geo-referenced over the BIOZAIRE mosaic. The surface areas were recalculated according to the new geo-referencing data.

Additionally, qualitative direct visual comparison of the two mosaics allowed for identification of small-scale localized changes, which could not be observed from the digitized polygons.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

WACS mosaic (2011)

The surveyed zone almost fully covers a 14,000-m² rectangular area directed in a southwest-to-northeast direction (Fig. 1). Direct mapping of the main faunal assemblages and visible carbonate concretion areas is available for the entire study area (Fig. 4a). It shows that the substratum is composed of either soft sediments or harder carbonate concretions and that the faunal distribution is spatially non-uniform but instead is divided into areas of high and low fauna presence. Areas of high fauna presence can in turn be categorized based on the dominant type of fauna (Fig. 4b).

image

Figure 4. (a) Distribution of the main faunal categories and carbonate concretions based on the WACS mosaic. (b) Simplified areas of distribution of the main types of fauna according to the WACS mosaic; the remaining ‘blank’ part of the survey area corresponds to the ‘areas of low fauna presence’ (see text).

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Carbonate concretions were visible over a large portion of the survey area (Fig. 4b). The total measured extent exceeds 4400 m2. However, this is a minimum estimation since it does not include carbonate concretions that were not directly visible at the surface, i.e. concretions covered by sediments or underlying fields of tubeworms and mussels.

Areas of high fauna presence

Mussel distribution

The map of faunal distribution (Fig. 4a) shows that large mussel beds were round-shaped and always adjacent to the tubeworm fields. At the limit between the two aggregations, a transition zone with co-occurrence of mussels and tubeworms was often observed. In these transition zones, mussels were present on the substratum between the tubeworms but they were also attached to the tubeworms themselves. The mosaic and video material from ROV dives also indicated that areas of mussel occurrence tended to coincide with areas of hard substrata, i.e. of carbonate concretions, either bare or with thin sediment cover. Indeed, very few mussels were observed on soft sediment areas; however, because the substratum type cannot be reliably identified from the images under all mussel aggregates, the proportion of mussel aggregates that were located in soft sediments could not be quantified.

The dense mussel category within the study area covered a total area of 414 m2 (Table 1) and was mostly concentrated in two main (M2/M3, M1) and one minor (M4) areas (Fig. 4b).

Table 1. Areas of coverage of the different assemblages calculated from the WACS (2011) mosaic. Information about the distribution in soft sediments is given only for faunal categories, in which the substratum type could always be identified on the mosaic.
assemblagetotal area (m2)within soft sediments (m2)within soft sediments (%)
Escarpia, living2573
Escarpia/Bathymodiolus co-occurrence 560
Escarpia, juveniles97
Escarpia, recumbent70
Escarpia, senescent5756.499
Mytilidae, living414
Mytilidae, shells67
Vesicomyidae, living8886.298
Vesicomyidae, mixed534508.395
Vesicomyidae, shells633549.987

The largest mussel area, known from the BIOZAIRE mosaic as ‘M2/M3’, stretched out over 20 and 26 m in the SW–NE and NW–SE directions respectively; it had an approximate surface of 450 m2, of which at least 300 m2 were covered by dense mussels. Observations of video footages showed that a large part of the dense population in this area was located at the bottom of a depression between boulders of carbonate concretions (Fig. 4a). This mussel bed stretched out towards the north boundary of the mosaic and likely extended further.

The second main mussel area (‘M1’) was located at about a 100 m to the southwest of the first one. It was composed of two beds of dense living mussels, one of about 45 m2 and the other of about 30 m2. The population was almost entirely surrounded by dense bushes of tubeworms but image material shows that mussels were also present, although at a lower density. Patches of dead mussels seemed to be larger at M1, whereas the abundance of living mussels was visibly lower than at M2/M3.

Additionally a minor mussel patch was present at the northeastern limit of the mosaic. In this area, the densest mussel bed covered an area of <10 m2, but likely extended over the limit of the mapped area. This area is referred to as ‘M4’.

Tubeworm distribution

The majority of the tubeworm population within the area of study was concentrated in dense bushes. Bushes of tubeworms were isolated in some places but occurred more commonly in large and dense fields. In either case, living tubeworms seemed to occur only on carbonate concretions and mostly around the main mussel areas. However, the substratum under dense tubeworms was not always visible on the images, and it could not be ascertained whether all living tubeworms in the area of study occurred on carbonate concretions.

The largest field with high tubeworms density was up to 1400 m2 in area and was located near the middle of the study area, W–NW of M2/M3. This area was more elevated than in the rest of the study area due to the presence of blocks of hard concretions, which gave the relief a rugged surface. A relatively high visible abundance of mussels was observed within the transition zone between mussel and tubeworm populations. In this area the transition zone was up to 7 m wide.

The second largest field of tubeworms covered an area of about 600 m2 and surrounded M1 almost entirely. In this field, the zone of co-occurrence between tubeworms and mussels was very small and it was not observed along the mussel/tubeworm limit. The field stretched out farther towards the south-southwest and beyond the limits of the study area.

The next largest fields of dense tubeworms were located at the eastern and northeastern end of the mosaic. In this area, two fields of about 130 m2 each were separated by a zone of soft sediments and low fauna presence. A 55-m2 zone of co-occurrence between tubeworms and mussels could be observed in the vicinity of M4.

Juvenile tubeworms were mostly observed as isolated bushes or as small fields in the periphery of the large aggregations of dense tubeworms. Observed juveniles also seemed to occur consistently on carbonate concretions, but generally close to or at the limit between concretions and soft sediments (Fig. 5). They were never observed more than 3 m away from the limit of the concretions, and never on the most protruding, and likely thicker, concretions. Mussels were also observed within populations of juvenile tubeworms where those bordered the mussel beds.

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Figure 5. Excerpt from the WACS mosaic showing a limit between carbonate crusts (right) and soft sediments (left); juvenile vestimentiferans are visible on the right and fields of vesicomyid clams can be seen in the sediment on the left. A band of reduced sediments occurs along the border of carbonate crusts. Images taken by ROV Victor 6000

(© Ifremer, WACS 2011).

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Senescent and/or recumbent populations were rarely observed, and never within the main tubeworms aggregations. The main occurrences were located in the periphery of larger fields of tubeworms. Additionally, senescent tubeworms were often located over soft sediments (Table 1) and in the immediate vicinity of clam aggregations.

Vesicomyid clam distribution

Vesicomyids were observed in aggregates of very variable dimensions, and ranged from very small clusters of about 0.01 m2 to large fields of up to 400 m2, made up of living, dead or mixed (i.e. dead and living) individuals. However, most aggregates contained mixed individuals and it was hard to quantify the relative proportion of living and dead individuals from the images. In a few cases, small clusters of living clams could be observed and delineated within larger patches of mixed (dead and living) clams. Dimensions of individual aggregates of living clams in the survey area did not exceed 3 m2.

Clam communities seemed limited to the areas covered by soft sediments (Table 1). For instance, clusters of living vesicomyids were scattered across the mosaic but were almost consistently (273 of 276 clusters) observed in the areas covered with soft sediments (Table 1). Furthermore, 241 of 276 clusters of living vesicomyids were located in patches of dark reduced sediments, which correspond to 94% of the total area (88 m2) covered by clusters of living vesicomyids (Table 1). Generally, dead vesicomyids were more commonly observed on carbonate concretions (13%) compared with living (2%) and mixed (5%) vesicomyids.

The vesicomyid population was very patchy and heterogeneously distributed within soft sediment areas (Fig. 4a); in particular, it was concentrated mostly at the periphery of the main mussel/tubeworm aggregations (Fig. 4b). The largest field of vesicomyids occurred in the vicinity of M4; within this field, living and mixed (living and dead) vesicomyids covered 39 and 258 m2, respectively.

Areas of low fauna presence

Areas of low fauna presence exhibited strikingly low numbers of tubeworms, clams and mussels patches in comparison with the rest of the survey area (Fig. 4a). Apart from the highly mobile fauna such as the galatheids, most of the fauna in those areas was composed mainly of sparse patches of tubeworms (≤25 m2) or of living and mixed (dead and living) clams (≤20 m2). Dead clam shells were also frequently observed.

The least colonized zone was located to the south of the large mussel and tubeworm communities located in the middle of the survey area. The zone covered an area of about 1000 m2 and was mostly composed of soft, bioturbated sediments.

Comparison with BIOZAIRE mosaic (2001)

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

The BIOZAIRE mosaic (2001) does not provide contiguous areal coverage such as given by the WACS mosaic (2011), and large gaps occur between individual lines. Both mosaics overlap over a 4605-m2 area, which corresponds to a subset only of the WACS mosaic area. To compare the trend in faunal distribution, this overlapping area is shown for the BIOZAIRE and WACS videomosaics (Fig. 6). Overall, there were only small changes in the spatial location of the main faunal assemblages.

image

Figure 6. Distribution of the main faunal categories and carbonate concretions in the area of overlap between the BIOZAIRE (2001) and the WACS (2011) mosaics.

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The mussel distribution has remained mostly the same as it was during the BIOZAIRE cruise. Although it is hard to compare the size of the main aggregations due to the smaller coverage of the BIOZAIRE mosaic, there is evidence that M2/M3 contained a larger mussel population on the WACS mosaic than on the BIOZAIRE map, with fewer gaps between the different patches (Fig. 7a). Conversely, some small mussel beds at M1 seem to have disappeared and to have been replaced by dead mussel shells (Fig. 7b). M4 is not covered by the BIOZAIRE mosaic and cannot be compared.

image

Figure 7. Images taken from the Biozaire (left) and WACS (right) mosaics, representing almost the same areas of the sea floor: (a) at M2, some areas previously devoid of mussels are now fully colonized by mussels; (b) at M1, small beds of living mussels on the BIOZAIRE have been replaced by mussel shells on the WACS mosaic; (c) a recumbent tube of vestimentiferan showed no change in size and position between 2001 and 2011. Images taken by ROV Victor 6000

(© Ifremer, BIOZAIRE 2001 and WACS 2011).

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Tubeworm fields showed no change in distribution, the slight difference in polygon sizes being due more to the lower resolution of the BIOZAIRE imagery data than to actual distribution changes. From these results, the tubeworm community is believed to be the one that changed the least across the study area. Most bushes or even single tubeworms were found unchanged and in some cases in the exact same position as in the older mosaic (Figs 7c and 8). Tubeworms were indeed the most reliable features when geo-referencing the BIOZAIRE mosaic onto the WACS mosaic.

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Figure 8. Images taken from the BIOZAIRE (left) and WACS (right) mosaics, representing almost the same areas of the sea floor; (a, b) new patches of vesicomyids that did not exist at the time of the Biozaire cruise; (b) also shows that the patch of dead clams has been partly re-colonized; (c) a patch of mixed vesicomyids almost disappeared under sediment cover. Images taken by ROV Victor 6000

(© Ifremer, BIOZAIRE 2001 and WACS 2011).

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The vesicomyid clam population is the fauna that had changed the most since the BIOZAIRE cruise. Although the main clam fields have remained at the same locations, their sizes seem to have increased. In addition, at least 38 new patches of living clams were observed that did not exist during the BIOZAIRE cruise. Those new patches had a mean size of 0.5 m2 (SD = 0.6 m2) and covered a total area of 17 m2. They were often located in close vicinity to older patches (Fig. 8b) but were also in a few cases new settlements farther from previously existing patches (Fig. 8a). Conversely, 16 patches were identified on the BIOZAIRE mosaic which no longer existed in 2011, or at least had been buried under some sediments (Fig. 8c). They had a mean size of 0.3 m2 (SD = 0.2 m2), and covered a total area of 4.6 m2. About four of those 16 patches contained dead clams only.

Areas of coverage by dense mussels, tubeworms and clams were computed for both the BIOZAIRE and the WACS faunal distribution maps (Table 2). Areas of sparse mussel and sparse tubeworm occurrence are not shown due to too large errors in delineating sparse aggregations. In addition, to maintain consistency with previous work on the BIOZAIRE mosaic, areas with co-occurrence of living mussel and tubeworm are provided (Table 2). Given a total common area of 4605 m2 between the BIOZAIRE and the WACS mosaics, areas can be expressed in percentage of cover of the overlap area. According to these calculations, coverage changes were very low and remained below 2% of the total overlap area for every category. Patches of living tubeworms, tubeworms with mussels and mixed (dead and living) clams underwent the largest changes, with coverage increases of up to 1.5, 1.3 and 1.2% of the total overlap area, respectively (Table 2). The total areal extent of the other assemblages showed almost no change.

Table 2. Areas of coverage of the different assemblages in the overlap area between the Biozaire (2001) and WACS (2011) mosaics. The percentages are relative to the total area (4605 m2) covered by both mosaics.
assemblageBiozaire (m2)Biozaire (%)WACS (m2)WACS (%)trend (%)
Escarpia, living71615.578217.0+1.5
Escarpia/Bathymodiolus co-occurrence 2074.52685.8+1.3
Escarpia, juveniles250.5280.6+0.1
Escarpia, recumbent791.7601.3−0.4
Escarpia, senescent701.5952.1+0.6
Mytilidae, living1944.22074.5+0.3
Mytilidae, shells20.04140.3+0.3
Vesicomyidae, living230.5270.6+0.1
Vesicomyidae, mixed1413.11994.3+1.2
Vesicomyidae, shells1122.41252.7+0.3

Overall, the distribution of the carbonate concretions over the study area did not change between the BIOZAIRE and the WACS cruises. The higher resolution of the new mosaic allowed better definition of the limits of the concretions, especially in areas covered with tubeworms or mussels, and no major new area of occurrence was observed. On the contrary, in many places the carbonate concretions tended to slightly disappear under a thin sediment cover.

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

Faunal and carbonate distribution

The mosaic and the distribution map of the megafaunal communities give a very detailed view and full coverage of the entire study area. The results show that the megafauna at Regab is concentrated mainly in three distinct areas of high faunal presence, separated by areas of relatively lower presence. Such distribution indicates that the chemical fluxes that are required to sustain these chemosynthetic communities are heterogeneous over the study area. Indeed, the distribution of the main faunal assemblages showed a concentric spatial zonation pattern, starting from mussel beds in the middle to tubeworms and finally fields of vesicomyids towards the outside. In our study this spatial zonation pattern from mussels to vesicomyid clams was observed, to various extents, around the three main mussel areas (M1, M2/M3, M4).

A model presenting a concentric pattern has been proposed previously for the Regab pockmark (Gay et al. 2006) but it considered both mussels and tubeworms as methane-dependent species inhabiting the same carbonate-dominated facies (Olu-Le Roy 2006). Although Bathymodiolus aff. boomerang contains both methanotrophic and thiotrophic symbionts (Duperron et al. 2005) and, for the populations living in the Regab pockmark, is known to rely on methane as dominant energy source (Olu et al. 2009; Duperron et al. 2011), tubeworms are known to host sulfur-oxidizing symbionts (Dubilier et al. 2008) and to have very high demands in terms of sulfide supply (Cordes et al. 2003).

We postulate that the observed distribution is controlled by the strength of fluid advection and related methane fluxes, and propose a model in which the megafaunal distribution at Regab is structured by the presence of discrete zones of intense fluid advection and methane fluxes under the mussel beds (Fig. 9). The distribution of the other communities would therefore be related to decreasing advection rates with distance from the mussel beds. The existence of such localized pathways of high fluid advection rate is compatible with the current understanding that the center of the pockmark is composed of several smaller pockmarks (Ondréas et al. 2005). In this section we discuss the concepts of this model, and compare them with more detailed observations of the faunal distribution and of the presence/absence of carbonate crusts.

image

Figure 9. Summary schematic model (not to scale). The main aggregations are distributed in concentric patterns with the mussels in the middle, then the tubeworms on thick concretions, and finally the vesicomyid clams in the surrounding sediments. Mussels are present in an area of intense flux with significant release of methane to the water column. A transition zone is observed where mussels are present at the bottom and on the tubes of the vestimentiferans. Vestimentiferans are present on carbonate concretions but reach the sediments with their roots. Through sulfate release, they maintain the AOM and the sulfide production. Juvenile tubeworms are distributed near the limit of the crusts where the sulfide fluxes from the sediments are likely higher. The presence of dark, reduced sediments around the concretions indicate that part of the methane and sulfide fluxes are redirected from under the crusts towards more sulfate-rich zones where AOM occurs. Populations of vesicomyid clams occur in the surrounding sediments. Their patchy distribution suggests that this is controlled by discrete and transient fluxes from below.

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Our observations show that the main mussel beds occur in areas where carbonates form blocs of indurated sediments and concretions within slight depressions, and that the main tubeworm aggregations occur in areas with extensive, continuous and prominent carbonate crusts. The formation of authigenic carbonates is a byproduct of the anaerobic oxidation of methane (AOM) in the sediment (Boetius et al. 2000; Aloisi et al. 2002) and is an indicator of methane fluxes and microbial activity within the sediments. However, the formation of continuous carbonate crusts impacts the porosity and permeability of the sediments and hence reduces the possible pathways for methane- and sulfide-rich fluid escapes and for sulfate-rich seawater infiltration (Hovland 2002; Luff et al. 2004). Therefore, areas of mussel occurrence are likely to be characterized by higher fluxes between the sediments and the bottom water than the encrusted areas where tubeworms occur.

We propose that the three main mussel areas present in our study area are located on focused zones where the seepage activity is the strongest, and where fluid flow is intense enough for the methane fluxes to reach the sediment/water interface. Indeed, the distribution of mussels mainly in dense circular beds suggests the presence of localized areas of intense methane fluxes. This hypothesis was mentioned previously from the results of the BIOZAIRE mosaic (Ondréas et al. 2005; Olu-Le Roy et al. 2007a). It also is strongly supported by recent biogeochemical analyses in various Regab habitats, according to which extensive seepage of gaseous and dissolved methane was observed only under the mussel habitat (Pop Ristova et al. 2012). This is also in accordance with previous studies in other areas that showed that release of methane to the water column is indeed facilitated in areas of high flux (Boetius & Suess 2004; de Beer et al. 2006; Niemann et al. 2006).

Co-occurrence of mussels and tubeworms is commonly observed at the transition between the two populations. In such zones, numerous mussels are observed on the tubes of the vestimentiferan aggregations that directly border the mussel beds. This could be the result of space limitations within the mussel beds, which would constrain mussels to invade neighboring tubeworm aggregations. Indeed, the dual symbiosis of Bathymodiolus aff. boomerang allows this species to use both methane and sulfide, similarly to several other seep and vent mussels of the Bathymodiolus genus (Duperron et al. 2005). However, unlike in the gills of Bathymodiolus azoricus at Mid-Atlantic hydrothermal vents, symbionts in seep mussel gills at Regab and at Gulf of Mexico seeps are predominantly methanotrophic; the thiotrophic symbionts are likely limited by the absence or low level of sulfide in the seawater (Duperron et al. 2011). Moreover, the fact that mussel/tubeworm co-occurrence zones are mostly close to the main mussel beds may also support the hypothesis that methane fluxes are higher in those areas than in the tubeworm aggregations located farther from the mussel beds. This is in accordance with previous studies that indicate that mussel beds at Regab are located in areas with the highest concentrations of methane in the water (Charlou et al. 2004; Olu-Le Roy et al. 2007a). Such behavior was also observed for methanotrophic mussel populations in the Southern Barbados prism (Olu et al. 1996b) and along the Costa Rica margin (Mau et al. 2006). In the Gulf of Mexico, mussels associated with brine seeps with high methane concentration grow faster and are in better physiological condition than those from petroleum sites with low methane but high sulfide concentrations (Bergquist et al. 2004).

The distribution of the tubeworm aggregations around the mussel beds could reflect lower fluid advection rates than under mussel beds. Niemann et al. (2006) and de Beer et al. (2006) suggested that by preventing downward fluxes of sulfate-rich water into the sediments, intense fluid advection rates can hinder the efficiency of AOM. This is in accordance with results from Pop Ristova et al. (2012), who calculated the proportion of upward diffusing methane that is removed by AOM at Regab to be only 6–20% under a mussel habitat compared with 47–97% in lower flux areas such as under clam habitats. Such processes could result in different environmental conditions inside and outside the mussel beds, and partly control the relative distribution of tubeworms and mussels. Biotic interactions are known to occur between mussels and tubeworms at hydrothermal vents (Johnson et al. 1994; Desbruyères 1998; Lenihan et al. 2008). For instance, mussels of the species Bathymodiolus thermophilus are able to disperse fluids laterally (Johnson et al. 1994), which could make fluids unavailable for tubeworms, and to inhibit recruitment of other vent species (Lenihan et al. 2008). Such interactions could also apply in dense B. aff. boomerang aggregations. However, unlike vent tubeworms, which take up sulfide from the seawater (Arp et al. 1985), seep tubeworms such as Escarpia southwardae take up sulfide directly from the sediments through their roots (Julian et al. 1999; Andersen et al. 2004) and are unlikely to be affected by fluid dispersion by the mussels. Furthermore, sulfide is almost absent in the seawater overlying mussel beds at Regab (Olu-Le Roy et al. 2007a; Duperron et al. 2011). Therefore, the absence of tubeworms in mussel beds is more likely due to competition for space or reduced AOM efficiency rather than to biotic interactions.

Our observations show that large tubeworm aggregations around the main mussel beds correspond to areas where the carbonate crusts are most prominent, and likely the thickest. Carbonate precipitation likely reduced methane flux, thus enhancing AOM and sulfide production favoring tubeworm settlement and dominance over mussels. Moreover, as observed at Gulf of Mexico seeps, tubeworms of the species Lamellibrachia luymesi were shown to release sulfate through their roots into the sediments (Cordes et al. 2005a), preventing a potential sulfate-depletion of the sediments. This characteristic is believed to allow adult L. luymesi to fuel or even enhance the AOM (Cordes et al. 2005a; Dattagupta et al. 2008) to maintain their supply of sulfide. As a result, the vestimentiferan population contributes to the formation of carbonates, which is supported by our observations that tubeworms are present where concretions form continuous and prominent crusts.

Juvenile tubeworms consistently occur near or at the limit between carbonate crusts and bare sediments. Sulfide fluxes and concentrations are likely to be higher in such areas with unsealed sediment/water interface than in areas covered by thick crusts, and thus provide a suitable environment for the larvae to settle until they can self-maintain their supply of sulfide. According to Bergquist et al. (2002), the recruitment of new tubeworms of the species L. luymesi and Seepiophila jonesi is time-constrained and ceases in older aggregations, due to the presence of thick carbonate pavements and low concentrations of sulfide in the water (Bergquist et al. 2003a). Sulfide concentrations have indeed been reported to be higher around aggregations of juvenile rather than within older aggregations (Bergquist et al. 2003b).

This interpretation is further supported by the occurrence of bands of black reduced sediments along the limits of the vestimentiferan-hosting carbonate concretions (Fig. 5), indicating that AOM and sulfide release occur in those areas. These features also suggest that part of the methane fluxes trapped beneath the carbonates could be redirected toward the sides of the carbonate crusts.

The distribution of the vesicomyid population is likely related to even lower advection rates than the tubeworm aggregations. Vesicomyid clams require soft substrata to access sulfide through their foot and, thus, are generally excluded from encrusted areas. Results showed that the living vesicomyid clams are indeed mainly located in the soft sediment areas surrounding the tubeworms and mussel aggregations. However, according to our interpretation, those areas are also where methane and sulfide fluxes are lower. In particular, Olu-Le Roy et al. (2007a) observed that vesicomyid clams at Regab are located in areas with relatively low methane concentrations, in comparison with tubeworm and mussel habitats. This is consistent with the current understanding that methane/sulfide availability shapes the structure of the microbial and megafaunal communities (Olu et al. 1996a,b, 1997; Sahling et al. 2002; Sibuet & Olu-Le Roy 2002; Levin et al. 2003; Levin 2005; Ritt et al. 2011; Pop Ristova et al. 2012).

The vesicomyid clam environment is sometimes proposed as being a precursor to a tubeworm/carbonate environment (Sahling et al. 2008). This is partly supported by our observations of dead shell on some bare carbonate concretions. The presence of dead shells within the concretions has been reported previously (Pierre & Fouquet 2007). These observations suggest that areas with vesicomyids transfer to carbonated crust formations, either with or without vestimentiferans. However, we have also observed large patches of dead vesicomyid shells within the sediments, sometimes almost buried, which would suggest that seepage activity in these areas decreased or stopped and that living populations either died or moved away. Such areas might never turn into tubeworm/carbonate environments due to too low or too transient fluxes. Thus, vesicomyid populations might not be restricted to one particular successional stage of colonization, but be present in a range of areas representing different development stages of the seeping activity, and the observed patterns of distribution may reflect the spatial heterogeneity of fluid flux.

Bergquist et al. (2003b) and Cordes et al. (2005b) suggested that the relative distribution of mussels (Bathymodiolus childressi) and tubeworms (L. luymesi and S. jonesi) could be related to different stages of succession, and explain that mussel beds indicate an earlier stage of colonization that would later be replaced by tubeworms when the formation of carbonate concretions reduces the methane supply to the water column.

However, we propose that the situation at Regab is more complex, as the observed patterns of faunal distribution could also be related partly to spatial heterogeneity of the fluid advection regime. A particular feature of this site is the coalescence of several pockmarks within the Regab site, which may differ in their fluid flow regimes or their evolution stages (Ondréas et al. 2005; Gay et al. 2006). If the relative mussel/tubeworm distribution were solely related to different colonization stages, the mussel population would be expected to be observed mainly together with, or in the vicinity of, juvenile tubeworm aggregations. Although we observed juveniles around some small mussel clusters, the larger mussel beds present in the study area were predominantly bordered by large adult tubeworm aggregations. Considering the extremely slow growth rate of tubeworms (Fisher et al. 1997; Bergquist et al. 2000), this indicates that fluid advection in those areas has been going on for a relatively long time, but that recruitment of juvenile has not occurred or has been hindered.

Temporal comparison

The comparison of the maps of faunal distribution and of the computed areas reveals that the size of the areas of faunal occurrence has remained globally the same between the BIOZAIRE (2001) and the WACS (2011) cruises. We consider the discrepancies in computed values and mapped areas to be caused largely by uncertainties in the method. First, images for each mosaic have been taken with different camera and lighting setups, which results in different resolutions and visibilities between the BIOZAIRE and WACS mosaics. Furthermore, small perspective distortions can, in places, impact the precision of the relative geo-referencing of the mosaics, or make a feature look larger on one mosaic than on the other. Finally, the delineation process is a manual step that strongly depends on the interpretation and precision of the observer. For all these reasons, mapped features may look different and discrepancies in the computed areas may arise that are difficult to evaluate. Nevertheless, considering all these possible sources of uncertainty and the large size of the study area, the computed areas are remarkably consistent between the two mosaics. Indeed, quantitative results showed that the changes in coverage per category are lower than 2% of the overlap area, which suggest very little change between the two mosaics. However, based on qualitative observations described below, we consider that the calculated areas are impacted by errors of the method and cannot be used to analyze further such small variations in the areas of faunal cover.

For instance, our observations of the mosaics confirm that no change occurred in the population of tubeworms within the overlap area over the past 10 years. Indeed, an increase of the area covered by tubeworms would signify that recruitment occurred. However, we did not observe new juvenile aggregations, possibly because the observation period was too short and that juveniles are still too small to be seen, or because the recruitment in the area covered by the mosaics was somehow limited. Overall, the absence of changes in the vestimentiferan population is in accordance with the findings that some tubeworms (Lamellibrachia luymesi and Seepiophila jonesi) may be very slow-growing and long-living (Fisher et al. 1997; Bergquist et al. 2000). Nevertheless, Lessard-Pilon et al. (2010) observed evidence of tubeworm recruitment (Lamellibrachia spp. or Escarpia laminata) within discrete seep communities in the Gulf of Mexico over a 15-year period, confirming that changes within tubeworm communities could be observed on such time-scales. Their work was based on a comparatively small study area (23.4 m2) and observations were likely more detailed than in our study (4605 m2); indeed, they used photo datasets, which provided higher definition images and closer views of the sea floor. Hence, tubeworm recruitment may have occurred within our study area but it was not evident from our data.

Conversely, some changes in size of individual beds of living and dead mussels were observed which suggests that localized variations of methane fluxes or carbonate precipitation may have occurred. For instance, small-scale visual observations suggest that the dense mussel bed in M2/M3 might contain, in the overlap area, a greater abundance of mussels in 2011 than in 2001, and that some minor mussel beds at M1 might have disappeared. This could indicate that the intensity of fluxes increased in M2/M3 and decreased in M1. A decrease in activity in M1 would be consistent with the findings of Olu-Le Roy et al. (2007a), who also hypothesized that a decreasing methane flux occurs in this area, based on the lower density of the mussel beds. Alternatively, the mussel abundance might have been stable and the observed changes could reflect a rearrangement of the mussels. In either case, distribution changes are likely the result of local variations in environmental conditions.

However, the observed changes were localized and, overall, the mussel population showed very little variation. Despite the scale difference between the two studies, these findings are consistent with observations of Lessard-Pilon et al. (2010). Both studies agree that only little change was observed on the total area covered by foundation fauna. However, Lessard-Pilon et al. (2010) observed significant small-scale changes in the distribution of mussel populations (Bathymodiolus brooksi and Bathymodiolus heckerae), and reported that about 50% of the area originally covered by living mussels at one site had been, after a period of 15 years, either replaced by dead mussel shells or colonized by tubeworms (Lamellibrachia spp. or Escarpia laminata). Interestingly, as we observed at Regab, the hydrothermal vent mussel population of Bathymodiolus azoricus was described to be stable on a decadal scale in terms of overall percentage of colonization (Cuvelier et al. 2011), whereas small fluctuations occurred on shorter time scales and on smaller spatial scales for seep mussels of the Gulf of Mexico (Lessard-Pilon et al. 2010).

In our study, no change was observed in the tubeworm aggregations surrounding these areas where mussel distribution varied. Mussel populations are expected to be more dynamic and to respond faster to environmental changes than tubeworm populations (Lessard-Pilon et al. 2010), which may be insensitive to small variations of seepage activity. Alternatively, the increased occurrence of dead mussels in M1 could reflect a late stage of the successional model developed by Bergquist et al. (2003b), characterized by a decrease of mussel population due to a decrease of methane and maybe of sulfide in the water column. Indeed, mussels are associated with areas of vigorous seepage and high methane concentration (Nix et al. 1995; Bergquist et al. 2005). However, the model proposed by Bergquist et al. (2003b) is true for long time-scales and it is uncertain whether such a trend is detectable within a 10-year period.

Changes in the population of vesicomyid clams were more frequently observed than for the mussel population. Although we cannot conclude if the total living population did change globally, the location of the aggregates of living individuals shows relatively more differences in comparison to the other populations studied. Indeed, several patches of living individuals observed in the 2011 mosaic did not exist in 2001. Conversely some patches of vesicomyid clams that existed in 2001 did not exist anymore in 2011. Also, in some cases, old patches of dead clams were re-colonized by living clams. However, changes in the distribution of patches of living/dead clams are difficult to detect since the relative proportion of living and dead clams cannot be estimated from the images. Overall, the distribution of living vesicomyid clams is very patchy and is difficult to understand.

Sahling et al. (2008) proposed a model for pockmarks of the Congo fan (100 km north of Regab), in which the distribution of tubeworm (E. southwardae) and clam (of similar species than at Regab) assemblages is controlled by the depth of the gas hydrate deposits. The model considers that gas hydrates deposits act as ‘capacitors’ (Dickens 2003) that buffer the transient influxes of methane from below and that ensure a more stable diffusion of methane into the porewater above, thus sustaining long-living seep communities. We think that a similar control mechanism occurs at Regab. Indeed, the presence of gas hydrates at Regab is known both from direct observation of outcrops on the sediment surface and from sediment cores (Charlou et al. 2004; Ondréas et al. 2005).

However, although this model is supported by the presence of such large populations of long-living seep communities and by their spatial patterns of distribution, it does not fully explain the temporal changes observed within the clam populations. At Regab, most aggregates of living individuals are indeed located within patches of black sediments, indicating the occurrence of AOM; clams are mobile fauna, hence changes in areas of distribution must somehow reflect changes of sulfide availability. One possible explanation could be that gas hydrate deposits under clam communities are either absent or too thin to buffer the transient methane fluxes over such a time period. The observed changes in clam distribution would therefore be the response to the transient release of methane and subsequent transient sulfide production. Clearly, some monitoring of sulfide concentrations and some geological sampling under the clam aggregates would be required to further refine this interpretation. However, this is in accordance with other studies that suggest that vesicomyid clams are supported by diffuse or transient fluxes (Olu et al. 1996a,b).

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

In this study, mosaic-based mapping of the faunal distribution over an area of 14,000 m2 shows that the distribution of dominant megafaunal species (mussels, tubeworms, clams) at Regab is mostly concentrated within three main megafaunal aggregations. Within these three main aggregations, the faunal arrangement follows the same spatial pattern with the methanotrophic mussels in the middle, then the vestimentiferans and finally the vesicomyid clams on the outer zone. We interpret that each of these patterns is centered on a zone of high flux of methane-enriched fluids. Such zones of high fluid flow are responsible for the spatial variation of intensity of the fluxes reaching the upper sediments and, hence, structure the distribution of the chemosynthetic megafauna in the pockmark.

In addition, this study is the first to describe the 10-year variation of the megafauna distribution in a cold seep environment over a 4600-m2 large area. Quantitative comparison of the two mosaics revealed that the overall size of the dominant megafaunal populations of Regab did not change significantly (<2% of the comparison area), which indicates that the intensity of the methane and sulfide fluxes that reach the faunal communities has been globally stable at the scale of the comparison area. We interpret such continuity as possibly related to the presence in the sediments of gas hydrate deposits acting as ‘capacitors’ for the methane fluxes.

Nevertheless, this study also shows that small-scale and discrete distribution changes have occurred, as already observed at other seep and vent sites, but were too small to be reliably quantified with our methodology. Those changes occurred mainly within the living population of vesicomyid clams, suggesting that the clam community was exposed to more transient fluxes as compared with the mussel and tubeworm communities.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References

We would like to thank the captain and the crew of RV Pourquoi Pas?, and the ROV Victor 6000 team. Also, thanks to Olivier Soubigou, Christophe Bayle and Michael Aaron for their technical help and advices, and to James Collins for proofreading. I am grateful to the entire team of the LEP for receiving me into their team.

This work was supported by the Ifremer and by the European Commission under the EU Framework 7 funded Marie Curie Initial Training Network (ITN) SENSEnet (contract no. 237868).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Site Description
  5. Methods
  6. Results
  7. Comparison with BIOZAIRE mosaic (2001)
  8. Discussion
  9. Conclusion
  10. Acknowledgements
  11. References
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