Sedimentary facies, geomorphic features and habitat distribution at the Hudson Canyon head from AUV

multibeam data

.

Martina Pierdomenicoa a, Vincent G. Guidab b, Leonardo Macellonic c, Francesco L. Chioccia a, Peter A. Ronad d,

Mary I. Scrantone e, Vernon Asperc c and Arne Diercksc c

a) a Department of Earth Sciences, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy.

b) b NOAA NE Fisheries Science Center, 74 Magruder Road, Highlands, NJ 07732, United States.

c) c National Institute for Undersea Science and Technology, University of Mississippi, 310 Lester Hall,

University (MS) 38677, United States

d) d Deceased, formerly Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New

Brunswick, NJ 08901-8521, United States

e) e School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794, United

States

Corresponding author: Martina Pierdomenico. martina.pierdomenico@uniroma1.it. +39 06 4991 4935

ABSTRACTbstract

Mapping of physical benthic habitats at the head of Hudson Canyon was performed by means of integrated

analysis of acoustic data, video surveys and seafloor sampling. Acoustic mapping, performed using AUV-

mounted multibeam sonar, provided ultra-high resolution bathymetric and backscatter imagery for the

identification of geomorphological features and the characterization of surficial sediments. Habitat

characterization in terms of seafloor texture and identification of benthic and demersal communities was

accomplished by visual analysis of still photographs from underwater vehicles. Habitat classes were defined

on the basis of the seafloor texture observed on photos and then compared with the geophysical data in order

to associate habitats to acoustic classes and/or geomorphological features. This enabled us to infer habitat

distribution on the basis of morpho-acoustic classes and extrapolate results over larger areas. Results from

bottom trawling were used to determine the overall biodiversity within the identified habitats. Our analysis

revealed a variety of topographic and sedimentological structures that provide a wide range of physical

habitats. A variety of sandy and muddy substrates, gravel patches and mudstone outcrops host rich and

varied faunal assemblages, including cold-water corals and sponge communities. Pockmark fields below 300

m depth suggest that methane-based chemosynthetic carbonate deposition may contributes to creation of

specific benthic habitats. Hummocky terrain hasave been delineated along the canyon rims and associated

with extensive, long-term burrowing activity by golden tilefish (Lopholatilus chamaeleonticeps). These

results show the relationships of physical features to benthic habitat variationariability, support the notion of

the area as a biodiversity hotspot and define essential habitats for planning of sustainable regional fisheries.

Keywords:

Hudson submarine Canyon, seafloor mapping, benthic habitat, backscatter imagery.

Introduction

1. Introduction

The Iincreasing human impacts on the marine environment raise the need for to promote management

approaches capable of sustaining the health of ecosystems through the preservationpreservation of their

structure, functioning and key processes (De Young et al., 2008). On the other hand, any sustainable

approach requires a good knowledge of the spatial distribution and ecological functioning of marine

ecosystems, over a range of different scales (Cogan et al., 2009; Tallis and& Polasky, 2009; Cogan et al.,

2009; Salomidi et al., 2012). This is particularly true for the deep sea, whose ecosystems are still poorly

known (Wilson et al., 2007; Harris and& Baker, 2011; Heyman and& Wright, 2011).

In this context, the mapping of benthic habitats has become a major tool in the assessment and

monitoring of marine ecosystems and planning of Marine Protected Areas (Pickrill and Todd, 2003; Harris et

al., 2008; Muñoz et al., 2009; Howell et al., 2010; Copeland et al., 2013). The process of producing seafloor

habitat maps requires the integration of marine biology, geology, oceanography and geophysics, in order to

produce simplified representations of the seafloor related to the distribution of biological communities at

different spatial scales. The termnotion of ‘habitat,’ traditionally related to the place where an organism

ordinarily can be found, has evolved in the direction of a spatially recognizable area characterized by

physical and environmental conditions that support a particular biological community, together with the

community itself (Valentine et al., 2005; Coggan et al., 2007; Valentine et al., 2005). The rapid development

in recent years of high-resolution seafloor mapping techniques, such as Side Scan Sonar (SSS) and

Multibeam Echosounders (MBES), significantly significantly increased theboosted our capability to image

and map the seafloor over large areas (Brown and& Blondel, 2009; Brown et al., 2011a). Full coverage

bathymetry and the derived information of the seafloor integrated with ground truth data enableallow the

recognition of different habitats and provide an interpretative tool for predicting their distribution and those

of the marine resources that they support (Brown et al., 2011b and references therein). The recent use of

cutting-edge technologies such as Remotely Operated Vehicles (ROV) and Autonomous Underwater

Vehicles (AUV) as platforms for acoustic systems led to a significant improvement in the scale to which

deep- sea habitats can be identified and in the description of the associated biota (Grasmueck et al., 2006;

Dolan et al., 2008; Vertino et al., 2010; Huvenne et al., 2011; Macelloni et al., 2013).

Substratum type, topographic relief, sediment composition, and geomorphology of the seabed

arehave been identified as important descriptors of biological patterns (Buhl-Mortensen et al., 2009; Harris

and& Baker, 2011). Seafloor complexity and habitat heterogeneity are recognized to play a key role in

enhancing the faunal biomass and biodiversity in deep- sea environments (Buhl-Mortensen et al., 2010;

Vanreusel et al., 2010; De Leo et al., 2014). Characterization of the seabed in terms of terrain parameters

such as slope, aspect or curvature and geomorphic features, along with the segmentation of MBES and SSS

data into acoustic facies (i.e., regions showing similar acoustic properties or features), commonly referred to

as Acoustic Seabed Classification (ASC, Anderson et al., 2008), thus offers a valuable tool for delineating

regions of the seafloor that may support specific communities and thus provide distinct habitats (Wilson et

al., 2007; Buhl-Mortensen et al., 2009; Savini and& Corselli, 2010).

We present a case study of benthic habitat mapping in the upper reach of Hudson Canyon based on

the integrated analysis of acoustic and groundtruth data.

Hudson Canyon, located about 180 km SE of New York City, is under evaluation for the assignment

of HAPC (Habitat Area of Particular Concern) status and represents a fisheries and biodiversity hot spot

(Stevenson et al., 2004; Mayo et al., 2009). It is also the focus of a collaboration between the NOAA

Northeast Fisheries Science Center, the Mississippi Mineral Research Institute (MMRI), the National

Institute for Undersea Science and Technology (NIUST), Stony Brook and Rutgers Universities. This

collaboration aims at creating an integrated database that includes existing and newly collected data, such as

acoustic mapping, visual ground-truthing, hydrographic, sedimentological and trawl data collections, as a

basis for the study of benthic habitats and for the development of habitat suitability models for fisheries

species.

The aims of this study are therefore:

(1) To produce the first detailed map (tens of meters) of the benthic habitats at the Hudson Canyon

head using very high-resolution acoustic data from an AUV-mounted MBES;

(2) To derive a quantitative relationship between acoustic parameters and ground truth results to classify the

study area into categories related to different seafloor characteristics;, and

(3) To qualitatively outline the biodiversity of the area by using information from photos and video imagery

and trawl samples.s.

2. Hudson Canyon

Hudson Canyon (Fig. 1) is the largest submarine canyon on the eastern U.S. Atlantic margin and one

of the largest inof the world (Ericson et al., 1951; Pratt, 1967). It extends for over 400 km, from the outer

shelf down to the upper continental rise at about 3500 m depth (Heezen et al., 1959; Pratt, 1965). The canyon

head deeply incises the continental shelf starting at 80 m water depth, about 40 km shoreward of the shelf

margin, and is composed of two branches, NW-SE and N-S oriented, that merge at a depth of about 120 m

into a segment oriented parallel to the main canyon course (Stanley and& Freeland, 1978; Butman et al.,

2006).

From the head to the base of the continental slope (about 2200 m depth), the canyon is up to 12 km

wide and up to an 1100 m incised depth with respect to the surroundings (Butman et al., 2006). The canyon

displays a flat thalweg, 500 to 900 m wide. The canyon walls are characterized by multiple ridges

perpendicular to the thalweg axis, separated by a dense network of gullies (Twichell and Roberts, 1982;

Butman et al., 2006).

The Hudson Shelf Valley, a shallow trough extending across the continental shelf, connected the

Hudson River to the canyon head during glacial low stands (Knebel et al., 1979). Holocene gravel and coarse

sand-deposits of fluvial origin are present at the canyon head and show evidence of reworking by currents

and bioturbation (Schlee and& Pratt, 1970). These coarse relict sediments occur down to a water depth of

130 to 175 m, where a sharp boundary exists with the present muddy cover. This appears to record a long-

term separation of different energy zones, i.e. below the boundary current speeds necessary to erode fine

sediments decrease significantly in frequency and intensity (Stanley and Freeland, 1978).

The general pattern of circulation in the area involves shelf waters and the warmer and more saline

slope-waters, separated by the abrupt gradient called the shelf-slope front (Wright, 1983; Chapman, 1986).

On average, shelf waters move towards the southwest parallel to bathymetric contours at speeds of 5-10 cm/s

at the surface and 2 cm/s or less at the bottom (Burrage and& Garvine, 1988; Stevenson et al., 2004).

Stratification of the water column occurs over the shelf and within the top layer of slope water during the

spring-summer and persists until autumn, while a permanent thermocline exists in slope waters from 200 to

600 m depth (Aikman, 1984). Moreover, the “cold pool,”, i.e. a bottom shelf water characterized by a

minimum temperature of 1.1 to 4.7°C (Amstrong, 1998), is present on the continental shelf bottom at depths

ranging between 40 and 100 m, from the spring to early fall. The upper reach of the canyon is affected by

complex oceanographic dynamics including internal waves and bottom currents flowing parallel to the

canyon axis (Keller et al., 1973; Hotchkiss and& Wunsch, 1982). Measurements of bottom currents

velocities within the canyon revealed a semidiurnal reversal of flow (Keller et al., 1973), suggesting active

resuspension and transport of fine material through the canyon to the outer continental rise (Keller and

Shepard, 1978; Shepard et al., 1979).

At the head of the canyon, intense mix due to breaking and dissipation of internal waves was

observed by Hotchkiss and Wunsch (1982). Moreover, Church et al. (1984) suggested that the interaction of

the canyon with the Mid-Atlantic Bight (MAB) ‘cold pool’ water may promote enhanced nutrient exchange

and biological production. Climatological CZCS satellite observations of surface chlorophyll indicate

enhanced surface primary productivity at the regions near Hudson Canyon and other shelf break canyons

(Ryan et al., 1999).

As observed for other canyons of the northwestern Atlantic margin (Hecker et al., 1983), Hudson

Canyon is suitable to host a rich and more varied fauna compared to the surrounding shelf and slope areas.

Rowe et al. (1982) found that macrofaunal composition inside the canyon did not differ substantially from

the adjacent slope (except for high densities within the canyon’s head). Nevertheless, commercial and

recreational catches in the shelf areas surrounding Hudson Canyon indicate the occurrence of a great variety

of demersal fishes and invertebrates (Jacobson et al., 2009; Mayo et al., 2009; Jacobson et al., 2009).

Stevenson et al. (2004) reported intense bottom otter trawl activity for the period between 1995 and 2001 in

the shelf areas around the canyon. Limited observations also suggest that the canyon increased

concentrations of krill that attract larger numbers of marine mammals in the Hudson Canyon area (Greene et

al., 1988). Hecker and Blechschmidt (1980) reported eEvidence of cold-water corals within Hudson Canyon.

are reported by Hecker and Blechschmidt (1980), who They found abundant populations of the soft coral,

Eunephthya fruticosa, in the deeper portion of the canyon. Solitary stony cold-water corals were observed on

the shelf around Hudson Canyon and in the head of the Canyon (Packer et al., 2007).

Moreover, Wwide shelf areas around the head of the canyon display a unique rough topography with

relief of 1-10 m. This irregular hummocky topography is attributed to seafloor erosion and burrowing

activity of golden tilefish (Lopholatilus chamaeleonticeps) and associated species of crustaceans (Able et al.,

1982; Twichell et al., 1985).

.

3. Data and Methods

To produce the benthic habitat map of the upper reach of Hudson Canyon we used a large

varietyvariety of data collected between 2004 and 2011 in the framework of different projects. The primary

dataset consists of bathymetric and backscatter data from a Kongsberg EM2000 (200 KHz) multibeam sonar,

mounted on a NIUST “Eagle Ray” Autonomous Underwater Vehicle (AUV). Groundtruth data include still

photos and grab samples collected by the United States Geological Survey (USGS) Sea Bottom Observation

and Sampling System (SEABOSS) towed video vehicle for areas shallower than 200 m, and photos acquired

by the NIUST “Mola Mola” AUV at greater depths (Fig. 2). In addition, demersal fishes and benthic

megafaunal catches from trawls provided individuals for the taxonomical identification of the benthic fauna

observed on photos and were used, along with the still photos, to define the overall biodiversity within the

different identified habitats. All data used in this study are shown on the map in Fig. 2.

3.1. Visual and sediment groundtruth data

To characterize and classify the different types of habitat across the study area, we consideredtook

into consideration photos and seafloor sediment samples collected during two cruises in 2004 and 2011. The

2004 dataset was collected aboard NOAA Ship Delaware II using the USGS SEABOSS towed video

vehicle. SEABOSS has two video cameras (forward and downward looking), a downward looking 35 mm

camera, and a modified Van Veen grab sampler. Quartz halogen lights provide illumination for the video,

and an electronic flash unit provides lighting for still photography. Dual lasers provide accurate photographic

range and scale information. The system is tethered and essentially “flown" over the seafloor by a shipboard

operator while the support vessel is drifting. Images from both video cameras were recorded on tape, but

were also viewed in real time, allowing collection of representative still photographic images. Each

deployment of SEABOSS consisted of drift transects of continuous video of variable duration, but averaging

26 minutes apiece. Still photos of features of biological and geological interest were taken at irregular

intervals as they appeared on video, with an average rate of one photo per minute. As SEABOSS hangs

nearly vertically from the ship during deployment, ship-mounted GPS provided georeferencing for these

photos. Stations for this cruise have been chosen on the basis of previous low-resolution acoustic data in

order to include canyon margins and walls, the thalweg and the adjacent shelf. In this study we only analyzed

the still photos acquired by SEABOSS. A total of 727 photos from 16 video-transects within the study area

(Fig. 2) were analyzed. Sediment samples were taken by the SEABOSS Van Veen grab at the end of each

transect and used for grain size analyseis, which. Grain size analyses for Seaboss samples were performed at

the USGS Woods Hole Science Center according to standard methodologies practiced by that laboratory at

the time of their collection (Poppe and& Poloni, 2000).

The 2011 dataset was collected aboard NOAA Ship Henry B. Bigelow by using the NIUST “Mola

Mola” AUV. Two long baseline (LBL) and one ultra-short baseline (USBL) bottom moorings were placed

on the canyon bottom to insure positional precision. Mola Mola was programmed to travel at an altitude of

3.0 ± 0.2 m above the bottom and to take a still photo every 4 seconds. Four deployments of Mola Mola

within the study area (Fig. 2) collected a total of 1638 photos. Stations were planned to include some

physical habitats occurring in the deepest part of the study area that were not surveyed during previous

cruises.

Habitat characterization was accomplished through the analyses of the still photos, that included

enumeration of demersal fishes and macrofaunal invertebrates and description of seafloor texture. In this

study the seafloor texture is the main factor taken into account to classify habitats, following the approach

suggested by Green et al. (1999) for meso- and macro-habitats (i.e., tens to few hundreds of meters). We thus

defined seven classes of benthic habitats, corresponding to as manyuch seafloor types identified on the

photos: Gravel and cobbles;, Mudstone outcrops;, Sand with gravel;, Sand;, Muddy sand/Mud;, Sub-

outcropping and outcropping rock; and Hummocky terrain (i.e., seafloor areas characterized by a rough

micro-topography produced by heavily burrowed semilithified clay outcrops, overlainied by a veneer of fine

sediment). To confirm and detail the nature of the seafloor corresponding to the different habitat classes, we

used the sediment samples taken by the SEABOSS Van Veen grab at the end of each transect. An additional

habitat class was defined for the photo-transects collected in correspondence of pockmark areas, where high

concentrations of dissolved methane were measured in near bottom-water samples (Rona et al., 2009).

Habitat classes were mapped along each transect and then compared with bathymetric and backscatter data.

.

3.2. Multibeam and backscattering data

Acoustic mapping of Hudson Canyon was accomplished during three cruises in 2007, 2008 and

2009, aboard the NOAA vessels Ronald H. Brown and Henry B. Bigelow. Ten deployments of “Eagle Ray”

lastingof about 18 hours each allowed the mapping of an area of about 140 km2 between 85 and 700 m depth,

corresponding to the first 25 km of the upper reach of the canyon (Fig. 2). The vehicle traveled at a constant

altitude of about 60 m above the seafloor following 150-m spaced parallel track lines aligned along the

canyon axis. A single bottom mooring, kept approximately in the middle of survey area with disposable

weights, provided a fixed georeference. A hydrophone array was used for communication between the ship

and the AUV.

Raw acoustic data were processed with the Caris HIPS& SIPS® 7.0 software, allowing the correction

of bathymetric data by accounting for sound velocity variations, tides, out-of-sequence beams and spikes.

The filtered data were used to produce a digital elevation model (DEM) of the study area with a 3 m

horizontal resolution (Fig. 3A). A slope map (Fig. 3B) was extracted from the bathymetric data and used to

identify gradients in elevation that are indicative of specific topographic features (e.g., outcrops, pockmarks).

Backscatter data were processed using Geocoder®, a software tool developed by Fonseca and Calder (2005).

Raw backscatter data were radiometrically corrected to remove variable acquisition gains, power levels,

insonification area and grazing angles; geometric corrections were applied to compensate for the navigation

and transducer attitude and a feathering algorithm was used to reduce the seam artifact between overlapping

lines during the mosaicking of the data. Final product was a “normalized” backscatter grid with a 1 m

horizontal resolution (Fig. 3C).

3.3. Acoustic seabed classification and mapping of benthic habitats

In order to estimate the distribution of the habitats observed on the photos transects for the entire

mapped area, we statistically investigated the relationships between photo-derived habitat classes and the

backscatter intensity. Mean intensity of backscatter was extracted from a 10 m-radius circular area centered

on each classified still-photo; photos located in the nadir area are eliminated from the analysis. Comparison

between the backscatter values associated withto different habitat classes (Fig. 4 and Fig. 5) enabledallowed

us to identify five distinct acoustic classes (i.e. areas characterized by different backscatter intensities)

corresponding to different substrates (Fig. 6). One-way permutational multivariate analysis of variance

(PERMANOVA) was used to test the significance of differences in backscatter intensity pertaining to each

habitat class (Fig. 6 C). The first and third quartile of mean backscatter intensity corresponding to each

habitat class (Fig.5) were used to choose the limits for acoustic classes (Fig. 6B), and semi-automated

mapping of areas of similar backscatter intensity was accomplished with the software Global Mapper®.

However, the relationship between acoustic classes and habitat classes was not always

consistentunivocal, and the distribution of the habitats obtained from the photo transect analysis did not

perfectly match the acoustic classification of the seafloor (i.e., different habitat classes were included in the

same acoustic class or did not show defined correspondence with backscatter intensity, Fig. 7). Furthermore,

the hummocky terrain wasere observed on a SEABOSS transect outside the multibeam coverage (Fig. 4),

and comparison with backscatter was not possible for this habitat class. We thus created new morpho-

acoustic classes baseding on geomorphological interpretation from the bathymetric data (Fig. 8), that better

match the habitat distribution as seen on the photos. In these cases habitat distribution was inferred from the

association with specific geomorphological features or topographic characteristics of the seafloor, and

polygons of habitats were constructed manually. From the analysis of high-resolution bathymetric data,

backscatter imagery and integration with groundtruth data, we identified and mapped 8 habitat classes,

related to sediment characteristics and/or specific geomorphological features. The resulting map of benthic

habitats of the upper reach of Hudson Canyon is shown in Figure 9.

3.4. Trawl catch data

Trawl catch data used in this study were selected from a large dataset acquired during annual cruises

conducted in and around Hudson Canyon and the adjacent outer shelf between years 2004 and 2011. Fifteen

stations, sampled over the years within the study area during winter, autumn and summer, were chosen, for a

total of 58 trawls (Fig. 2). At almost stations two different trawl gear were used: a 2 m beam trawl (13x6 mm

mesh size) and a 36’ Yankee otter trawl (38 mm and 27 mm mesh size for the trawl net and cod end,

respectively). Otter trawls were towed for 30 min. at approximately 3.8 kt, and beam trawls for 15 min. at

approximately 1.5 kt. Catches were sorted to the lowest practicable taxon (LPT), and all fishes and mega-

invertebrates were counted and weighed by LPT. Large samples were enumerated by extrapolation from

subsamples. The list of the species collected in trawls and their frequency of occurrence isare shown in

Appendix A and B.

4. Results

4.1. Sediment distribution and backscatter intensity

The distribution of backscatter intensity (Fig. 3C and Fig. 4) displays a clear pattern, with low-

backscatter facies along the thalweg, indicating the occurrence of fine-grained sediments, in contrast with the

high-backscatter facies distributed on the shelf and along the canyon walls. The highest backscatter values

are present along the northeastern rim of the canyon and part of the western rim (not entirely covered by

acoustic mapping). A general decrease of backscatter intensities with increasing depth is also visible along

the canyon walls, particularly evident along the more gently sloping western flank. A hHistogram of

frequency of backscatter values, characterized by a marked bimodal distribution of intensities (Fig. 6B),

reflects the distribution of the two main types of sediment present along the shelf and within the thalweg,

corresponding respectively to sandy and silty sediment. The very high backscatter facies along the

northeastern and the western rim of the canyon are produced by a mixed substrate of gravel, cobbles and

boulders on sand. We distinguished two acoustic classes within the high backscatter facies:; areas

characterized by backscatter intensities > -7 db coincidinge with dense beds of gravel and cobbles and

mudstone outcrops on sandy seafloor (Acoustic class 1 – Habitat A and B), that mostly occur along the upper

portion of canyon flanks;, andwhile widespread areas at canyon rims and along the eastern portion of the

adjacent shelf are characterized by a minor presence of gravel or shell debris on sand corresponding to

slightly lower BS values, included between -8 db and -10 db (Acoustic class 2 – Habitat C).

Seafloor samples taken within the medium-backscatter facies of the shelf and canyon walls

correspond to medium sands, with a minor component of gravel and/or silt fractions. All the sandy samples

are containedcomprised in the interval of backscatter intensity ranging from -10 to -14 db (Acoustic class 3 –

Habitat D).

The low backscatter facies along the thalweg (BS intensity ranging from -14 db to -30 db, the lowest

intensities occurring in the area) corresponds to fine-grained sediment, ranging from silty sands to silt/ mud

(Acoustic class 4 – Habitat E). Acoustic class 5, even if not calibrated with photos or seafloor samples, likely

consists of very fine sediment (silt/mud) which. This sediment is mainlyostly present at the base of the flanks

and is absent in the central part of the thalweg. Very low backscatter values (that are likely related to an

increase of the fine fraction of sediment) were observed in the deepest part of the thalweg, below 600 m

depth.

Sub-outcropping rock (Habitat F), observed on the photos in correspondence of narrow ridges at the

base of the eastern wall of the canyons, isare characterized by medium-to-high backscatter facies and were

associated with BS intensities encompassing acoustic classes 1, 2 and 3.

4.2. Benthic habitats of the upper reach of Hudson Canyon

The characteristics of the identified habitats are summarized in Fig.ure 10 and discussed in the bullet

list below: Habitat A– Mudstone outcrops; Habitat B – Gravel and cobbles; Habitat C – Sand with gravel;

Habitat D – Sand; Habitat E – Muddy Sand/Mud; Habitat F –Outcropping/Sub-outcropping rock; Habitat G

– Pockmark fields; and Habitat H – Hummocky Terrain.

4.2.1. Habitat A, Habitat B and Habitat C (Gravelly substrates and mudstone outcrops)

A belt of very high BS intensity was present along the entire eastern rim of the canyon, between 90

m and 150 m depth, and on part of the western rim (Fig. 4). High backscatter intensities were also recorded

on the shelf surrounding the canyon head, specifically in the eastern sector, whereas only a few patches of

high backscatter were present on the western side (Fig.4). Video surveys in high BS zones depicted

mudstone outcrops, beds of gravel, cobble and occasionally large boulders. Three main habitat classes were

recognized in the high backscatter zone, based on visual estimates of the percentage of the coarse sediment

fraction and the presence of mudstone outcrops: Habitat A - Mudstone outcrops; Habitat - B Gravel and

cobbles; and Habitat C - Sand with gravel (Fig. 10). Mudstone outcrops, forming seafloor relief of 1-2 m,

were observed only along the eastern rim, at depths ranging from 100 to 160 m. This kind of substrate

creates very specific conditions that allow the colonization of rich and varied faunal assemblages. Gravel

beds and mudstone outcrops supported abundant epifauna, including sponges and zoanthids. American

lobsters (Homarus americanus), galatheid crabs (Munida iris iris), and chain dogfish (Scyliorhinus retifer)

were common in trawls and in seafloor videos and photos within theseis habitats.

4.2.2. Habitat D (Sandy seafloor)

Video observations and sediment samples depicted an overall sandy seafloor in a widespread area of

medium-high backscatter that was present on the shelf and along the canyon walls down to ~200 m depth;

this was defined as Habitat D - Sand, where sand predominates, with rare occurrence of shell debris or sparse

pebbles (Fig. 10). Thisis bottom type was colonized by the typical fauna of the Mid- Atlantic Bight (Wigley

and Theroux, 1981; Mahon et al., 1998). Specifically, several types of small tubes were observed as well as

occasionally burrowing ophiuroid arms protruding from the seafloor. Sessile fauna was mainly represented

by tubicolous polychaetes and burrowing anemones (Edwardsia sp.), and mobile epifauna included cancrid

crabs, hermit crabs and margined sea stars (Astropecten americanus), the latter being the most abundant

species observed. Hakes (Urophycis chuss, Urophycis regius) and gulfstream flounders (Citharichthys

arctifrons) were also frequently detected in videos and photos.

4.2.3. Habitat E (Muddy substrates)

The entire thalweg and the lower portion of the western flank, in the southern sector of the mapped

area, were characterized by low backscatter intensity (Fig. 4). Video observations within the thalweg showed

a muddy seafloor that we referred to as Habitat E - Muddy sand/Mud (Fig. 10), with tufted burrows and

numerous brittle stars. Epifauna were almost entirely represented by white sea pens (Stylatula elegans); some

snake eels (Ophichthidae) were also present.

4.2.4. Habitat F (Outcropping/Sub-outcropping rock)

Narrow and steep ridges with rough morphology were present on the northeast flank of the canyon

near the base of the walls (Fig. 6 and Fig. 11C). Such steep terrains were first interpreted as outcrop of hard

substratum, because of their high backscatter facies. However, despite it, rock surfaces could not be

distinguished on most of video imagery, possibly due to the occurrence of draping sediments. High densities

of large anemones (tentatively identified as Bolocera tuaediae) dominated the bottom of these steep areas,

along with a large number of decapod shrimp (possibly Atlantopandalus propinquus).

4.2.5. Habitat G (Pockmark fields)

Several circular depressions interpreted as pockmarks were observed near the base of the canyon walls at

depths ranging from about 300 to 500 m (Fig. 6 and Fig. 11D). These features, likely associated with past

and present fluid circulation, displayed a sub-circular shape with diameters ranging from 50 to 400 m and a

rim-to-floor relief up to 25 m deep (clearly increasing with diameter). The slope of their walls ranged

frombetween 15° toand 25°. Backscatter imagery showed higher intensities on the floor of the depressions as

compared with the surrounding areas (Fig. 11D), suggesting possible present-day activity with fluid emission

from the seafloor, and potential authigenic carbonate crusts generating a stronger reflectivity within the floor

of the depression. A recent multibeam survey revealed the presence of gas bubble plumes rising in the water

column from the seafloor inside the Hudson Canyon (Skarke et al., 2014). Furthermore, chemical analysis

from bottom water samples collected within the canyon axis at depths ranging from 450 and 520 m revealed

high methane anomalies (to 100 nM, an order of magnitude above background) in near bottom water in the

proximity of these features (Rona et al., 2009). Smaller pockmarks, 5 to 100 m wide, were also observed

along the canyon thalweg at depths ranging from 500 to 570 m. These depressions, elongated or irregular-

shaped, had a maximum rim-to-floor relief of about 2 m and again were characterized by high backscatter

values on their bottoms.

Photos within pockmark fields depicted high densities of Bolocera tuediae (Fig. 10). Other species

frequently observed include deep sea red crab (Chaceon quinquedens), witch flounder (Glyptocephalus

cynoglossus), and longfin hake (Urophycis chesteri). Heavy marine snow concentrations in the near-bottom

water were observed.

4.2.6. Habitat H (Hummocky terrain)

An area of about 6 km² characterized by a rough topography was observed at both canyon rims,

starting at about 130 m depth (Habitat - H, Fig. 10 and Fig. 11B). This rugged morphology corresponds to

the hummocky terrain described by Able et al. (1982) and Twitchell et al. (1985), associated with extensive,

long-term burrowing activity by golden tilefish (Lopholatilus chamaeleonticeps). Along the eastern flank,

the hummocky terrain isare present only on a flat area of the canyon margin and isare absent below the rim,

whereas on the western side itthey extends downslope toward the canyon axis, forming finger-like features

along several of the ridge crests, down to 250 m depth. Backscatter data show heterogeneous intensities due

to the hummocky terrain of the two areas, with higher values on the eastern margin. Bathymetric data show

that this rough topography is produced by uneven ridges and depressions few tens to few hundreds of meters

wide and up to 5 m high, on which is superimposed a smaller-scale roughness of the seafloor (some

decimeters high). Seafloor photo images taken by the SEABOSS vehicle within 1 km of the southern corner

of the mapped area visually confirm the uneven microtopography, with the occurrence of semilithified clay

substrate and tilefish burrows (Fig. 11B).

5. Discussion

5.1. Distribution and mapping of physical habitats

Analysis and interpretation of high-resolution acoustic and groundtruth data showed that the upper

reach of Hudson Canyon is characterized by complex topographic features and sedimentary structures,

giving rise to a wide range of physical habitats in a relatively small area.

Comparison of acoustic data with ground truth images showed a good matching between seafloor

zoning defined on backscatter segmentation and the visual census of habitat types.; PERMANOVA pairwise

post hoc test results (Fig. 7C) assess the strength of the relationships between seafloor type and backscatter

intensity, indicatingshowing significant differences in relative backscatter intensities among the classified

seafloor types and allowing us to use this acoustic parameter to map habitats over the entire study area even

in the absence of direct observation. However, backscatter intensity alone was not able to distinguish all

benthic habitats.

Specifically, we used backscatter as proxy for the distribution of Habitat B – Gravel and, Habitat C -

Sand with gravel, Habitat D – Sand, and Habitat E – Muddy Sand/Mud, that are associated with narrow and

distinct backscatter intensity ranges (Fig.7). Backscatter intensity of Habitat A - Mudstone outcrops is

indistinguishable from the adjacent gravel beds (p value from PERMANOVA pairwise post hoc test >0.01);

however, seafloor relief produced by the outcrops enabledallowed the mapping of this habitat by using

information from bathymetric data. Similarly, distribution of Habitat F - Outcropping/Sub-outcropping rock,

corresponding to the same backscatter values as gravel beds and sands with gravel, was inferred from

association with the very rough and steep ridges along the base of the eastern wall of the canyon in the

southern sector of the mapped area (Fig. 6, Fig. 11A). Outcrops create a rugged morphology with slopes

commonly exceeding 25°, and reaching 40° in places. Habitat G - Pockmark fields, distributed along the

deepest portion of the thalweg was characterized by muddy seafloor corresponding to the same acoustic class

of Habitat - E. The evidence of increased concentration of methane in the proximity of the pockmarks (Rona

et al., 2009) led us to distinguish this habitat from adjacent muddy seafloor of the thalweg.

Distribution and acoustic signature of the Habitat H - Hummocky terrain, for which no groundtruth

data within the mapped area were available, are different along the two margins of the canyon. This habitat

corresponds to high acoustic backscatter along the eastern side of the canyon, where it is restricted to the

canyon margin and is mainly characterized by a patchy distribution of bigger depressions and reliefs on a

smooth seafloor (Fig. 11B). On the contrary, on the western rim the hummocky terrain isare associated with

a small- scale rugosity and does notdon’t yield high backscatter values, possibly because the underlying

substrate is masked by a draping fine sediment veneer over much of its extent. However, the distribution of

this habitat was resolved by the very high-resolution bathymetry, which allowed a fine mapping of the

features produced by the long- term burrowing activity on semilithified clays by the tilefish. The distinctive

morphology produced by tilefish burrowing activity and associated species of crustacean was already

observed by Able et al. (1982) and Twichell et al. (1985), who mapped an area of rough topography about

800 km² located on the outer shelf around the head of Hudson Canyon, between 150 and 500 m depth. In this

study, high resolution bathymetry from an AUV allowed us to refine the boundaries of the hummocky area

and map for the first time with high detail its extent in correspondence of the canyon rims.

Concerning the thalweg area (corresponding to Habitat E - Muddy Sand/Mud), the decrease of

backscatter from the base of the canyon walls toward the thalweg likely reflects a gradual decrease in the

mean grain size of the surficial sediment with increasing amount of muddy component over sand. However,

video transects across the thalweg showed a general homogeneity and dominance of the muddy habitat

below a specific backscatter threshold of backscatter. This observation suggests that backscatter variation

over muddy sediments does not relate with substantial changes in the distribution of macro-epibenthic

communities. This is particularly true at least for the shallowest part of the thalweg.

5.2 Observation on benthic communities

Photos analysis and trawl catch data analysis provided important information about the overall

biodiversity within the different habitats observed, supporting the evidence of distinctive habitats provided

by the presence of the gravel beds and mudstone outcrops in the upper reach of Hudson Canyon.

Unfortunately, trawls were not planned to groundtruth the habitat distribution, so that the considerations

arising from their analysis are generic and not specific.

The presence of gravel deposits at the canyon head, early documented by Stanley and Freeland

(1978), together with mudstone outcrops observed at the eastern margin of the canyon, creates favorable

conditions for the development of unique habitats hosting rich faunal assemblages. These particular types of

seafloor, corresponding to the Habitats A, B and C, support a substantial epifauna of sponges and zoanthids,

and provide habitats for benthic and demersal species commonly associated with structurally complex

habitats.

Together with megabenthic and demersal species that are widespread along sandy bottoms of the

continental shelf of Mid Atlantic Bight (i.e., margined sea stars Astropecten americanus; hakes Urophycis

chuss and Urophycis regius; gulfstream flounders Citharichthys arctifrons; spiny dogfishes Squalus

acanthias), trawl catches included Aamerican lobsters (Homarus americanus), galatheid crabs (Munida iris

iris), black bellied rosefish (Helicolenus dactylopterus), a large number of egg cases of chain dogfish

(Scyliorhinus retifer) and a rare deep-water keyhole limpet, Diodora tanneri. Juveniles of black sea bass

(Centropristis striata) and other seasonal migrants from shallow water, like northern and striped sea robins

(Prionotus carolinus, P. evolans), were also caught on winter trawls at the canyon margins. Black sea bass

are well-known to seek structured habitats such as rocks and wrecks during their seasonal inshore residence

(Drohan et al., 2007). The preferred habitat of this fish during their offshore (winter) period is less clear.

Catches of juveniles of this species in winter trawl samples, frequently correspond to the gravelly bottoms of

the eastern rim of the canyon, suggesting that these hard bottom habitats, relatively rare on the outer shelf,

may play a role as overwintering habitat for this migrant species. Dense patches of cold-water cup corals (the

solitary stony coral Dasmosmilia lymani) were observed on photos and videos corresponding to the gravelly

habitats at two sites along the rims of the canyon (Fig. 12). Trawl catch data confirm the presence of this

species and also indicate that D. lymani continued to persist at these two stations for at least 7 years. These

observations suggest that this kind of canyon rim habitat may be of importance for juveniles of

overwintering seasonal migrants, as a nursery for chain dogfish, and as a repository of rare deep-water

species, as already observed for other canyons along the northwestern Atlantic margin (Hecker, 2001). Direct

observations of the deepest part of the thalweg were restricted to specific geomorphological features

supposed to represent distinct habitats (i.e., pockmark fields and rocky outcrops). Trawl catches in the area

(around 700 m depth) indicated the occurrence of species typical of the upper slope environment. The catch

was dominated by deep- sea red crabs (Chaceon quinquedens) and witch flounder (Glyptocephalus

cynoglossus) with typically associated species (marlin spike grenadier Nezumia bairdi, offshore hake

Merluccius albidus, thorny skate Amblyraja radiata). It also included a large number of unidentified attached

anemones, gooseneck barnacles, and sea spiders, interpreted as hard-bottom associates.

Hudson Canyon is an highly productive area that is supposed to contains essential habitats for a large

number of finfish species (summer flounder, silver and red hake, black sea bass, butterfish, tilefish), long fin

squid, and shellfish (lobster, deep- sea red crabs). Catch data from the Northeast Fisheries Science Center

(NEFSC) suggest a strong and persistent role of the canyon in enhancing fisheries on the surrounding shelf

(NEFMC, 2014). For its importance as commercial and recreational fishing “hot spot,”, Hudson Canyon has

been often cited as a priority area for conservation (Hecker, 2001); however, it has not been extensively

explored as other submarine canyons along the US continental margin, and there is’s still need to improve

our knowledge of this system to address management concerns.

5.3 Informing HAPC conservation planning

While the information above is of general interest for the development of an ecosystem approach to

fisheries management, particularly on an area basis, the specific management issue currently under

consideration is the establishment of Habitat Areas of Particular Concern (HAPCs) with regard to deep- sea

corals (MAFMC and& NMFS, 2014). Two major options are under consideration for this fisheries

management plan amendment: (1) management of activities within broad zones defined by depth; and (2)

versus management of discrete zones centering on submarine canyons. Hudson Canyon would be involved in

either case. Although the historic records of corals in Hudson Canyon are limited, recent habitat suitability

modeling reported in the documentation for these proposals suggests much of this canyon has a high

suitability for various types of deep- sea corals as well as a high value for its fisheries resources (MAFMC

and& NMFS, 2014). Herein lies an issue for management consideration; how best can deep- sea coral habitat

conservation be balanced with the exploitation of seafood resources? Making informed management

decisions in this regard require good, detailed information regarding the actual distribution of deep- sea

corals, fisheries resources, and the habitat factors that influence their distributions. Actual coral distribution

has not been well-documented, so that much of the large-scale distribution of corals has been inferred from

habitat suitability modeling.

Proponents of the recent effort to model deep- sea coral habitat suitability along the northeast coast

of the United States admit that the use of models based on correlation of limited records of coral presence

(but not absence) with broad spatial scale information on physical/geological attributes can be prone to

errors. Furthermore, a high suitability in this case does not mean corals are actually present, nor does it

predict density or species composition. Many areas are not adequately surveyed for deep sea corals and the

exact locations of coral “hot spots” may depend on physical features on spatial scales finer than those

modeled (MAFMC, 2014). Use of such models was necessary in this case because of the sheer size of area to

be assessed (the entire northeastern U.S. coast) and the short time schedule for completion. Our data,

although far more limited in extent and requiring more time to acquire, provides a much more detailed

account of habitat features on much finer spatial scales and includes both presence and absence of species of

deep- sea corals and biological as well as geological habitat features. Further, although not reported here, our

data includes quantitative estimates of deep- sea coral and sponge densities that can be useful for developing

more quantitative habitat descriptions and models. For this reason we believe that our results can be of value

in more precisely defining deep- sea coral habitat value of Hudson Canyon for management purposes, but

also in providing better raw material for more sophisticated habitat suitability models that could include both

positive and negative distributional factors.

6. Conclusions

A first benthic habitat map of the upper reach of Hudson Canyon was produced by the analysis and

integration of geophysical and groundtruthing datasets.

The AUV-mounted multibeam sonar provided ultra-high resolution bathymetric and backscatter

imagery allowing the identification of geomorphological features and the estimation of surficial sediment

distribution at a high level of detail (tens of meters). Identification of benthic and demersal communities

showed rich and varied faunal assemblages, including species commonly associated with structurally

complex habitats and deep- sea ecosystems. Data analysis revealed that the upper reach of Hudson Canyon is

characterized by a great variability of sedimentary and topographic features; a variety of sandy and muddy

substrates, gravel patches, mudstone and rocky outcrops, hummocky terrain and pockmark fields provide

different benthic habitats in a relatively limited area. Gravelly bottoms and mudstone outcrops at the head of

the canyon create the conditions for hosting a rich fauna including sponges, zoanthids and deep water corals.

Trawl catch data suggest that these habitats are of importance for juveniles of overwintering seasonal

migrants, as a nursery for chain dogfish, and as a repository of rare deep water species. Pockmark fields,

occurring below 300 m depth, associated with active methane venting suggest that methane-based

chemosynthetic carbonate deposition may contribute to creation of specific habitats. Previously described

hummocky terrain associated with extensive, long-term burrowing activity by golden tilefish (Lopholatilus

chamaeleonticeps) wasere clearly delineated along the canyon rims.

The upper reach of Hudson is characterized by a great seafloor heterogeneity that, along with the

interacting hydrographic regime and methane discharge, may contribute to create favorable conditions for

the development of fertile habitats, enhancing the local productivity and promoting biodiversity. These

results support the relevance of the area as a biodiversity hotspot and contribute to the development of a plan

for sustainable regional fisheries.

Acknowledgements

This paper is dedicated to the memory of Dr. Peter Rona (8-17-1934/2-20-2014). The authors wish

to acknowledge Dr. Page Valentine and Dann Blackwood of USGS and the captains and crews of the

NOAA vessels Ronald H. Brown, Delaware II, and Henry B. Bigelow for their critical assistance in the

collection of data, and the Directors of the NOAA Northeast Fisheries Science Center and National Institute

of Undersea Science and Technology for their support of this work. The authors wish to acknowledge two

anonymous reviewers whose critical suggestions greatly improved the manuscript. We are grateful to Dr.

Donna Johnson for the English revision.

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Figure Caption s

Figure 1. A.) Bathymetric map of northwestern Atlantic margin with the study area denoted by the red

polygon: Inset - Northwestern Atlantic Ocean. HSV- Hudson Shelf Valley; HC-Hudson Canyon. 1B).

Bathymetric map of Hudson Canyon head; contour lines every 50 m. Bathymetry data: NOAA National

Geophysical Data Center (http://www.ngdc.noaa.gov).

Figure 2. Groundtruth stations along the upper reach of Hudson Canyon. The area surveyed by the AUV-

mounted EM2000 MBES corresponds to the orange polygon. Trawls stations used for the description of the

biotic assemblages are also included.

Figure 3. Digital elevation model (A), slope (B) and backscatter map (C) obtained from the processing of

multibeam data collected with Eagle Ray AUV.

Figure 4. The map shows the backscatter mosaic draped on the shaded relief of MBES data. The lines

overlying the backscatter map represent the photo-transects, with the different colors referring to the habitat

classes defined from the photo analysis. Numbers indicate the location of the seafloor samples used to

confirm the nature of seafloor observed on the photos, and cake diagrams show gravel, sand and mud weight

proportion from grab samples.

Figure 5. Seafloor photo images, backscatter facies and main backscatter statistical parameters pertaining to

each habitat class derived from the photo analysis.

Figure 6. A.) Acoustic classes map obtained from supervised segmentation of backscatter mosaic. 6B.)

Supervised segmentation of the backscatter mosaic into acoustic classes; backscatter intensity corresponding

to grab samples (numbered as in Fig. 4) is shown above the histogram of frequency of backscatter values;

box plots showing backscatter intensity ranges pertaining to each classified habitat class (named as in Fig. 5)

are also included. See Fig. 5 for description of habitat classes. Statistics parameters represented by the box

plots are shown in the inset on the bottom right. 6C.) Table showing PERMANOVA pairwise post hoc test

results (unshaded) and average distance between habitat classes based on Euclidean distance of

untransformed backscatter intensity values (shaded). * p> 0.01, **p<0.01.

Figure 7. Box-plot of the distribution of backscatter intensity related to the different habitats identified from

the photo images. Statistics parameters represented by the box plots are shown in the inset in Fig.6.

Figure 8. Map of the geomorphological features of the upper reach of Hudson Canyon.

Figure 9. Benthic Habitat Map of the upper reach of Hudson Canyon.

Figure 10. Description of the seafloor characteristic, benthic faunal assemblages and distribution of the

habitat classes derived from the analysis of photos.

Figure 11. Geomorphological features that contribute to the creation of specific habitats.

Appendix A. List of species collected using the 2 m beam trawl gear and their frequency of occurrence. Total

of taxa collected is also included.

Appendix B. List of species collected using the 36’ Yankee otter gear and their frequency of occurrence.

Total of taxa collected is also included.