Deep-Sea Research II 92 (2013) 183–188

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Deep-Sea Research II

0967-06

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Cold-seep habitat mapping: High-resolution spatial characterizationof the Blake Ridge Diapir seep field

Jamie K.S. Wagner a,n, Molly H. McEntee a,1, Laura L. Brothers b, Christopher R. German c,Carl L. Kaiser c, Dana R. Yoerger c, Cindy Lee Van Dover a

a Division of Marine Science and Conservation, Nicholas School of the Environment, Duke University, Beaufort, NC 28516, USAb U.S. Geological Survey, Woods Hole, MA 02543, USAc Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

a r t i c l e i n f o

Available online 10 February 2013

Keywords:

Autonomous Underwater Vehicle (AUV)

Sentry

Deep-sea ecology

Vesicomya

Bathymodiolus

Chemosynthetic ecosystems

Pockmark

GIS

Sidescan sonar

45/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.dsr2.2013.02.008

esponding author. Tel.: þ1 919 818 1316.

ail address: jamie.ks.wagner@duke.edu (J.K.S.

esent address: Williams College, Williamstow

a b s t r a c t

Relationships among seep community biomass, diversity, and physiographic controls such as under-

lying 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.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Chemosynthetic communities associated with methane seepson continental margins occur in a variety of geographical andgeological contexts (Sibuet and Olu, 1998), including brine pools(MacDonald et al., 1990) and mounds (MacDonald et al., 2003) inthe Gulf of Mexico, fault lines at the Florida Escarpment (Paullet al., 1984), mud volcanoes in the Barbados Accretionary prism(Olu et al., 1997), and salt diapirs on Blake Ridge (Paull et al., 1996;Van Dover et al., 2003). Exploration of deep-sea cold seep systemscontinues to yield insight into the diversity and structure ofchemosynthetically-based biological communities, and habitatmapping of these communities in their geological context will bea critical tool to facilitate sustainable management of offshoreresources (Cochrane and Lafferty, 2002; Degraer et al., 2008).Cold seep communities are often associated with irregular bathy-metry (mounds, pockmarks, authigenic carbonate outcrops) relatedto seabed methane escape in an otherwise undifferentiated

ll rights reserved.

Wagner).

n, MA 01267, USA.

sedimentary environment (Hovland and Judd, 1988; MacDonaldet al., 1990; MacDonald et al., 2003). Mapping techniques thatresolve and ground-truth seafloor physiographic irregularities havepromise for regional-scale characterization of benthic habitats.

The Blake Ridge Diapir, located off the coast of South Carolina,eastern USA (Fig. 1A), includes several depressions or pockmarks(e.g., Paull et al., 1995), one of which [Ocean Drilling Program(ODP) Site 996] is known to support a cold seep community(Van Dover et al., 2003). Based on submersible observations,this seep community was reported to extend across an areameasuring�120 m x 240 m and to be dominated (biomass) bylarge mussels (Bathymodiolus heckerae; average adult shell length�200 mm) that host symbiotic methanotrophic and thiotrophicbacteria and form dense beds at the center of the seep field (VanDover et al., 2003). Vesicomyid (Vesicomya cf. venusta) clam bedsare also abundant elements of the faunal assemblage, thoughindividual clams are small (average shell length �20 mm).These clams host thiotrophic bacteria and are generally found atthe edges of the mussel beds and peripheral regions of the seepfield (Van Dover et al., 2003). This seep also hosts a number ofother abundant invertebrates, including several species of ophiuroids,holothuroids, asteroids, echinoids, octopus, squat lobster, and shrimp,along with several species of fish.

Fig. 1. Nested study areas: (A) Location of Blake Ridge Diapir study site, off the coast of South Carolina, eastern USA. (B) Detailed map of the study area (hillshaded

bathymetry with 5 m depth contours); dark grey box: area of sub-bottom survey; thick black line: sub-bottom profile line, shown in Fig. 3; light grey box: Blake Ridge

Diapir seep and area of AUV coverage, shown in Fig. 1C. (C) Sentry photosurvey tracklines, Blake Ridge Diapir seep. White circle indicates the location of ODP Site 996.

Fig. 2. Subsurface CHIRP profile beneath Pockmark A (see Fig. 1B).

J.K.S. Wagner et al. / Deep-Sea Research II 92 (2013) 183–188184

In addition to the multiple pockmarks seen in the area(Hornbach et al., 2007; Paull et al., 1995), branching migrationpathways beneath the Blake Ridge Diapir reported from CHIRPseismic reflection data (Hornbach et al., 2007) suggest that theremight be additional pockmarks in the area where methane-richfluids escape the seafloor and fuel chemosynthetic communities.

In this study, we examined the spatial distribution andpatterns of mussels and clams around ODP 996, as well as at3 newly discovered adjacent seep communities. In a previousinvestigation of the Blake Ridge Diapir seep, conducted withWoods Hole Oceanographic Institution’s (WHOI) deep-submergence vehicle Alvin, a relatively course-resolution map ofmegafauna distributions was generated from Alvin video (VanDover et al., 2003). In the current study, we used WHOI’sautonomous underwater vehicle (AUV) Sentry equipped withhigh-resolution mapping tools (sidescan sonar, multibeam echo-sounder, digital still camera) that, when employed close to theseabed (5 to 20 m above bottom), yield multi-scale datasets thatcan be integrated to create habitat maps of the seafloor. We haveused these tools to explore for seep communities and to char-acterize the relationship between biomass-dominant inverte-brates (Bathymodiolus heckerae and Vesicomya cf. venusta) andseafloor physiography.

2. Methods

The Blake Ridge Diapir (Fig. 1A) was explored using the NOAAShip Okeanos Explorer (research cruise EX1205L1) and WHOI’sAUV Sentry in July 2012. Operations focused on a 360-m�360-marea (Figs. 1B, 1C) that included ODP Site 996 (herein referred toas Pockmark A; 321 29.6230N, 761 11.4670W; 2155 m depth).The Okeanos Explorer collected 3.5 kHz subbottom data using aKnudsen 3260 CHIRP system with up to 40 m penetration (Fig. 2)

to image and identify the subsurface gas conduit underlying thestudy area.

In one pass of the AUV Sentry survey, sidescan sonar back-scatter data were collected using an Edgetech 2200-M 120 kHz/410 kHz sidescan sonar (5-m altitude, 70 tracklines, 360-mlength, 5-m spacing). In a separate pass, a Reson 7125 multibeamechosounder (400 kHz) was used to collect bathymetric data(20-m altitude, 14 tracklines, 360-m length, 25-m spacing), cover-ing an area of about 0.165 km2. Concurrent with the sidescansonar run, a photo survey (�7-s intervals; 12-bit, 1024 � 1024pixels, down-looking digital color camera) was conducted at anaverage speed of 0.73 m/s, covering an area of about 0.131 km2

(Fig. 1C).Each of the 5,568 georeferenced photos from the Sentry photo

survey was classified into one of six categories based on thedominant faunal characteristic (Fig. 3): (1) scattered clams; (2)

Fig. 3. Example photographs of classification: (i) scattered clams; (ii) dense clams; (iii) live mussels; (iv) dead mussels. Scale bar¼1 m.

J.K.S. Wagner et al. / Deep-Sea Research II 92 (2013) 183–188 185

dense clams; (3) dead mussels; (4) both live and dead mussels;(5) live mussels; or (6) no evidence of seep. After classification,Esri ArcGIS software was used to map the distribution of mega-fauna overlain on the geophysical data (processed sidescan sonarimagery mosaic and multibeam bathymetry). ArcGIS analysistools (e.g., 3D Analyst, Interpolate Line) were used to create depthprofiles of the main pockmarks from the final gridded bathymetrydata. Nearest neighbor analysis in ArcGIS was conducted on eachdominant faunal characteristic to determine the spatial statisticalsignificance of the distribution of each faunal type (Mitchell,2005). Specifically, the average Euclidian distance between eachgeoreferenced data point representing dominant faunal type wascompared to a hypothetical random distribution of such pointswithin the study area. This type of analysis assesses the likelihoodthat a given distribution pattern (clustered or dispersed) is aproduct of chance.

3. Results

Seismic reflection profiles across Pockmark A reveal an acous-tically transparent zone, which likely indicates the presence of gasin the pore space underneath the Blake Ridge Diapir seep (Fig. 2).The sidescan sonar imagery within the study area revealed fourdistinct patches of high backscatter (Fig. 4A), each coincident withone of four discrete bathymetric pockmarks (pockmarks A, B, Cand D; Fig. 4B); pockmark diameters ranged from 25 m to 70 m,and maximum relief ranged from 1.5 m to 3.5 m. Pockmarks A, Cand D showed evidence of mound formation within each pock-mark; pockmark B exhibited simple concavity. Several smallermounds and pockmarks (o15 m diameter) were also evidentwithin the high-resolution bathymetric data, surrounding themajor pockmarks. Apart from these pockmark/mound features,the bathymetry demonstrates relatively little local relief, and thesidescan sonar backscatter illustrates that the surrounding seabedis generally featureless in terms of seafloor texture (Figs. 4A, B).

Dense mussel beds in photographs (Fig. 3iii, 3iv) were coincidentwith higher backscatter areas in the sonar mosaic (Figs. 4A, C).Pockmarks A, B, C, and D all showed similar patterns of megafaunalcomposition and zonation, with dense bathymodiolin mussel beds

occurring either at the local low (Pockmark B) or central high(mounds within Pockmarks A, C and D) of all four pockmarks. Themost extensive live mussel beds tended to be surrounded by beds ofdead mussels, but minor live and live/dead mixed mussel beds werealso observed in association with smaller areas (o15 m) of irregularbathymetry (e.g., �35 m southeast of Pockmark D; Fig. 4D).Pockmark A supported the largest mussel beds, with live and deadmussels covering a combined area of �4700 m2. Mussel beds inPockmark D were only half as extensive (�2400 m2) and PockmarksB and C hosted still smaller mussel beds (�450 m2 and �600 m2,respectively). Dense patches of vesicomyid clams were peripheral tothe mussel beds and, with increasing distance from the musselcenter, became more scattered (Figs. 4D, 5). While mussels weremostly restricted to pockmarks and mounds within pockmarks,clams were found in relatively featureless terrain based on sidescansonar and covered a much greater areal extent than the mussels, upto 150 m beyond inferred localized sources of active fluid flow.

Nearest neighbor analysis of each faunal type revealed that,with one exception, all faunal types are non-randomly distributedthroughout the study area (Table 1), indicating that each group’sclustered distribution pattern is statistically significant and not aproduct of chance. Live mussels are the only faunal type with adistribution pattern that may be random, though live mussel datais limited by a much smaller sample size (n¼22, Table 1) than theother categories and is confounded by overlap of live mussels andthe live/dead mussel category.

4. Discussion

Three previously unreported seep communities were discov-ered in this study, all within 240 m of the ODP 996 seep site(Pockmark A) described by Paull et al. (1996), Van Dover et al.(2003), and Hornbach et al. (2007) and all hosting similardominant benthic invertebrates (i.e., clams and mussels).This finding – that three seep communities could be so close to anarea previously explored yet remained undiscovered – underscoresthe challenge of characterizing deep-sea ecosystems and the value ofnested, systematic surveys. Regional sub-bottom profiles (this study)indicated a large (4130 m diameter) acoustically transparent region

Fig. 4. Sidescan sonar, bathymetric, and faunal distribution maps from the Blake Ridge Diapir seeps site: (A) Sidescan sonar; areas of elevated backscatter are light while

areas of low backscatter are dark; arrows highlight prominent high backscatter features. Sidescan sonar has a 2 m resolution. (B) Bathymetric map showing four major

depressions (Pockmarks A, B, C & D). Multibeam data was gridded at 1 m resolution. (C) Seep megafauna distributions overlain on sidescan sonar map. (D) Seep megafauna

distributions overlain on bathymetric map. Black lines through each pockmark indicate the depth profiles, shown in Fig. 5.

J.K.S. Wagner et al. / Deep-Sea Research II 92 (2013) 183–188186

underlying the four pockmarks identified in the Sentry high-resolution multibeam survey (Fig. 2). The pockmarks presumablyoriginate from gas in pore spaces and sediment collapse (Hovlandand Judd, 1988). While sub-bottom profile data from this study didnot resolve individual conduits to each of the pockmarks, thepresence of four large pockmarks with associated chemosyntheticcommunities suggests that methane migrates to the seafloor in atleast four discrete locations.

Bathymodiolin mussels and vesicomyid clams rely on symbio-tic chemosynthetic bacteria for nutrition and are limited to areaswhere chemical flux is sufficient for endosymbiont nutrition(Sibuet and Olu, 1998). The roughly concentric patterns of bothmussel and clam bed distribution reported here suggest thatthese species differ in their habitat requirements or are competi-tively interactive. The mussel holobionts at Blake Ridge seeps areinferred to host methanotrophic and sulfide-oxidizing endosym-bionts (Van Dover et al., 2003). The ‘bulls-eye’ distribution ofmussels with respect to the seabed pockmarks indicates that themussels are centered on bathymetric irregularities generated bypoint-source methane seepage where fluid flux and methane andsulfide concentrations are greatest. Mussel beds tended to belive or dead, rather than mixed, suggesting that where mortality

occurs, it is locally catastrophic, with a common cause and timing.An explanation for the pattern of mussel mortality mapped in thisand other studies may be temporal variability in fluid flux andstarvation or toxicity (Heyl et al., 2007; Van Dover et al., 2003).An alternative or confounding interpretation could be deathcaused by disease or parasitism. Previously, a viral-like cellularinclusion in epithelial cells of digestive tubules in Blake Ridgemussels was deemed pathogenic, causing lysis of cells (Wardet al., 2004), and could be responsible for the mussel mortalityobserved. Parasitism was implicated as a factor in clam mortalityin a previous study at Blake Ridge (Mills et al., 2005), and couldalso play a role in the mortality of mussels, especially whereorganisms are nutritionally or physiologically stressed. Repeated,high-resolution mapping of mussel beds can be used to estimatepopulation dynamics; mapping in combination with measuresof porewater geochemistry (e.g., sulfide, sulfate, and methaneconcentrations) will help to resolve the question of whetherbiological or geochemical factors are the dominant cause ofmortality in this system.

In contrast to mussels, which host methanotrophs andthiotrophs, clam holobionts rely only on thiotrophic bacteria(Van Dover et al., 2003). We infer that relatively high sulfide

Fig. 5. Faunal distributions along pockmark depth profiles; profile lines shown in Fig. 4D. A, B, C, and D indicate the pockmark of origin for each depth profile, shown in

Fig. 4B. (A) i, ii, iii, and iv indicate where the example photos in Fig. 2 were taken.

Table 1Frequencies and nearest neighbor analysis of faunal distribution.

Dominant bio-type

character

Bio-type

frequency

Ratio of observed/expected average

distance of bio-types in the field

p-Value Pattern Likelihood the pattern is random (%)

Scattered clams 656 0.79 0.01 Clustered o1

Dense clams 465 0.73 0.01 Clustered o1

Dead mussels 100 0.68 0.01 Clustered o1

Both live and dead mussels 89 0.52 0.01 Clustered o1

Live mussels 22 0.83 0.15 Somewhat clustered The pattern may be due to random chance

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concentrations in porewater fluids exist over an areal extentmuch greater than that covered by mussel populations andhypothesize that this sulfide flux alone in the clam zone isinsufficient to support mussel holobionts. The alternative expla-nation that clams out-compete mussels in this region cannot beexcluded, but seems unlikely. The apparent absence of clams inthe core of the pockmarks is also striking; mussels are stackedone on top of another, in such dense array that there is nosediment visible, suggesting that competition for space could

occur and that mussels outcompete clams in these areas. Wecannot exclude the possibility, however, that fluid chemistry inthe pockmark sediments is not suitable to survival of the clams.Obviously, questions remain regarding what specific factors con-trol the striking zonation of clams and mussels.

Cold seep biological communities are typically described aslong-lived due to the stability of fluid flow at these sites relativeto hydrothermal vent systems (Fisher et al., 1997). However,lateral variation in the conduit system beneath seeps could cause

J.K.S. Wagner et al. / Deep-Sea Research II 92 (2013) 183–188188

flow intensity to fluctuate over time, consistent with bivalvemortality observed at the Blake Ridge Diapir. Analysis of carbo-nates from the Blake Ridge seep indicates that fluid flux isirregular and occurs over long periods of time (Paull et al.,1996). Fluid flux at each Blake Ridge Diapir pockmark may bevariable, both in time and in relation to neighboring pockmarks.Conceptually, such findings would be consistent with the idea of‘‘self-sealing’’ seeps in which individual outlets go through cyclesof active seepage and inactivity, due to factors such as carbonateprecipitates that can clog sub-surface conduits (Hovland, 2002).While transient flow may impact the longevity of individuals andpopulations associated with each pockmark, long-term biodiversityin the seep system will be maintained at other nearby points ofseepage. High-resolution seafloor geophysical data create a base-line for continued research of the spatial and temporal changes infaunal populations at the Blake Ridge Diapir. By tracking thetemporal dynamics of the underlying conduit structure (Brotherset al., in preparation) and combining this with time-series habitatmaps and fluid geochemical measures, we can develop a compre-hensive bio-physico-chemical model of seep dynamics.

Habitat mapping is an important tool for characterizing sea-floor ecosystems, and it informs policies related to the use andprotection of marine environments. Habitat mapping is a frequentpractice in near-shore marine environments (e.g., Brown andBlondel, 2009; Cochrane and Lafferty, 2002; Kostylev et al.,2001), but only recently is this practice being adapted for andapplied to mapping deep-sea habitats on multiple scales (severalhundred meters, as in Jerosch et al., 2007; Jones et al., 2010; Olu-Le Roy et al., 2007, or �15 m area, as in Lessard-Pilon et al., 2010).Backscatter data from sidescan sonar delineates seep-associatedhard substrata (e.g. shellbeds, carbonate) from muddy seafloorand provide a first-order assessment on presence/absence oflikely seafloor seeps, without requiring extensive photo surveys.Techniques used in this study improve our understanding ofoffshore ecosystems and can be applied to habitat mapping,studies of temporal evolution of seafloor seeps, ecological nichemodeling, and resource management.

Acknowledgements

We thank the crew of the NOAA Ship Okeanos Explorer, the shipand shore science teams, the NOAA onshore support team at theInner Space Center, University of Rhode Island, and M. Jones forassistance with ArcGIS. We thank S. Ackerman and two anon-ymous reviewers for helpful feedback. Paul Tyler has long been acolleague of the project PI (CLVD), first through his publications,then as a leader of the Chemosynthetic Ecosystems Project of theCensus of Marine Life, and finally as a shipmate on the R/VAtlantis. We are pleased to honor Paul’s outstanding contribu-tions to deep-sea research and celebrate his career as mentor tomany students of deep-sea science. This research was supportedby NSF award Bio Oce 1031050 and NOAA’s Office of OceanExploration and Research.

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