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Deep-sea benthic megafaunal communities on the New England and Corner Rise Seamounts, Northwest Atlantic Ocean
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Deep-sea benthic megafaunal communities on the New England and Corner Rise Seamounts, Northwest Atlantic Ocean Abby Lapointe1, Les Watling1 and Allen M. Gontz2 1 2
Department of Biology, University of Hawaii at M¯anoa, Honolulu, HI, United States Department of Geological Sciences, San Diego State University, San Diego, CA, United States
Abstract Combined, the New England and Corner Rise seamount chains in the Northwest Atlantic extend approximately 1700 km from the continental slope southeast of George’s Bank to the vicinity of the Mid-Atlantic Ridge. Three-dimensional reconstructions of the seamounts were compiled from multibeam bathymetry data. Analysis of the benthic communities was based on data compiled from collected specimens, in situ images, and video footage. A total of 17 peaks were analyzed, sampled at depths ranging from 713 to 3000 m. The major factors influencing the structure of the megabenthic communities are depth, position along the chain, and substrate. Overall the New England Seamounts are more diverse than Corner Rise, and the highest taxon diversity was observed between 1700 and 2200 m. Below 2300 m only four species of corals were observed that were not found at shallower depths, and below 2700 m no corals were found that were not recorded in shallower water. These results suggest that there is a distinct difference in the structure of the benthic communities on the deepest (,2700 m) and shallowest ( . 1000 m) seamounts and peaks in the bathyal Northwest Atlantic. At intermediate depths the community structure varies depending on location along the chain (longitude), depth (water temperature), and substrate.
Keywords: Seamount; benthic communities; corals; Northwest Atlantic; New England Seamounts; Corner Rise Seamounts
Introduction Seamounts, submerged mountains in the sea, comprise a significant component of the hard substrate areas at bathyal depths and have been found to be hotspots of biomass and biodiversity in the deep ocean (Pitcher et al., 2007). Although estimates on the exact number of seamounts vary significantly depending on the methodology and thresholds used, there are, most likely, at least 14,000 seamounts globally (Kitchingman and Lai, 2004). Yet
Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00057-9 © 2020 Elsevier Inc. All rights reserved.
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Figure 57.1 Bathymetric map of the New England (1 12) and Corner Rise (13 15) Seamount groups. Inset lower left indicates position in the Northwest Atlantic, NES, New England Seamounts, CR, Corner Rise Seamounts.
despite their abundance and importance to marine biodiversity, relatively little is known about their benthic communities (Clark et al., 2010; Koslow et al., 2015). The New England and Corner Rise seamount chains in the Northwest Atlantic combined extend approximately 1700 km from the continental slope southeast of George’s Bank to the vicinity of the Mid-Atlantic Ridge (Fig. 57.1). The New England Seamount chain extends 1200 km from Bear Seamount in the west to Nashville Seamount in the east (Uchupi et al., 1970). The Corner Rise Seamounts are located about 300 km east of the New England Seamounts near the boundary of the abyssal plain and Mid-Atlantic Ridge (McGregor et al., 1973). The New England Seamounts are estimated to be of Mesozoic origin, ranging in age from about 103 to 82 million years (Duncan, 1984). The Corner Rise Seamounts are younger, estimated at approximately 75 million years in the western portion and 38 million years in the east (Epp and Smoot, 1989). Both seamount groups were formed by volcanic activity over a mantle plume hotspot that created the Monteregian Hills southeast of Montreal, Canada, about 140 mya and, due to seafloor spreading and movement of the North Atlantic plate, now resides southwest of Great Meteor Seamount east of the Mid-Atlantic Ridge (Sleep, 1990). The plume was strong during the formation of Bear Seamount. However, as the plates shifted and the direction of motion changed, the strength of the plume weakened during the formation of the Corner Rise Seamounts (Sleep, 1990). Additional seamounts may have existed between the New England Seamounts and Corner Rise but have been buried by sediments from the Sohm abyssal plain (Epp and Smoot, 1989), although this seems unlikely unless those seamounts were quite small.
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Figure 57.2 Contours of water temperature over the New England and Corner Rise Seamount chain. Shaded bars depict the depth ranges sampled on the seamounts/peaks, arranged by longitude of their location. See Table for abbreviations. Source: Data from NOAA World Ocean Atlas, 2013, ver. 2: www. nodc.noaa.gov/OC5/woa13/.
The rough bathymetry of the New England and Corner Rise seamounts was described in 1959, based on single-beam echo soundings using a precision depth recorder (Heezen et al., 1959). McGregor et al. (1973) outlined the bathymetry of the Corner Rise Seamounts in more detail and a series of submersible dives on the New England Seamounts in 1974 provided the first visual observations and details about their structure and bathymetry (Houghton et al., 1977). With the advent of multibeam sonar technology, fast and accurate surveys of the ocean floor are now possible and much of the New England and Corner Rise Seamounts have been mapped. The water column throughout the Northwest Atlantic is oxygen-rich, with oxygen levels of approximately 6.0 mL L21 at the surface to 3.0 mL L21 at 3000 m depth (NOAA World Ocean Atlas, 2013, ver. 2: www.nodc.noaa.gov/OC5/woa13/). The temperature contours across the length of the seamount chains, as measured during 41 dives on 17 seamounts at depths from 700 to 3000 m, along with the range of depths of remotely operated vehicle (ROV) dives on each seamount, are shown in Fig. 57.2. Details of the cruises are described below. The temperature on the majority of the New England Seamounts at depths where images or samples were taken was fairly constant between 3 C and 4 C, with only Manning and Rehoboth having dive sites in the 4 C 2 5 C range. Proceeding west to east from
920 Chapter 57 Manning in the New England Seamounts to Milne-Edwards Peak in Corner Rise, the surface water is warmer and extends deeper into the water column due to the influence of the Gulf Stream. However, the only area sampled with water warmer than 6 C was Ku¨kenthal Peak on Corner Seamount, and the only area sampled colder than 3 C was on Mytilus Seamount. Most of the dive sites on the Corner Rise Seamounts were in water approximately 1 C warmer than all the New England Seamounts other than Manning and Rehoboth. The dominant water mass covering the New England Seamounts is Labrador Sea Water which forms a Deep Western Boundary Current at the western end, but this water mass gives way to Upper North Atlantic Deep Water and then to North Atlantic Deep Water to the east and in deeper water, respectively (Talley et al., 2011).
Naturalness The majority of the seamounts in these two seamount groups are physically undisturbed. There have been surveys of the manganese crusts on a few seamounts, but no mining has occurred (Manheim and Lane-Bostwick, 1989). The New England Seamounts are, with the exception of Bear Seamount, generally too deep to fish economically, but the US National Marine Fisheries Service conducted 56 exploratory benthic trawls on Bear Seamount during the years 2000 14 (Moore et al., 2003, Shea et al., 2017). Several peaks on the Corner Rise Seamounts were most likely fished over the period 1976 96 (specific seamount data have not been released; Vinnichenko, 1997) and evidence of bottom-trawling was seen on two peaks during our ROV dives (Waller et al., 2007, Watling et al., 2007). These shallower peaks also showed evidence of human activity in the form of discarded bottles and bricks, and a discarded piece of trawl net was seen at 2100 m on Nashville Seamount. No further evidence of fishing on Nashville was observed, but the dives were generally below the depth where fishing would occur.
Mapping and exploration cruises 2003 14 With funding from the National Oceanic and Atmospheric Administration (NOAA), National Undersea Research Program (NURP), and Office of Ocean Exploration Research (OER), a series of cruises to the New England and Corner Rise seamounts were undertaken. In 2003 bathymetric mapping using the R/V Atlantis and dives with the submersible Alvin were completed on Bear, Kelvin, and Manning seamounts under the program Mountains in the Sea I; in 2004, the NOAA ship Ronald H. Brown was used for mapping and support of sampling dives with the ROV Hercules during the Mountains in the Sea II cruise with samples obtained from Balanus, Bear, Kelvin, Manning, and Retriever seamounts. In 2005 the Deep Atlantic Stepping Stones expedition used the same ship and ROV system, mapping and exploring Yakutat, Caloosahatchee, and Corner seamounts in the Corner Rise group, and Nashville, Manning, and Rehoboth seamounts in the New England chain. Later in 2005 mapping with the
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R/V Atlantis and one dive with Alvin focused on Picket Seamount during the CanyBal expedition. In all, 35 ROV or submersible dives were completed on 13 seamounts in those programs. In 2013 and 2014 NOAA’s Office of Ocean Exploration Research (OER) program conducted two cruises, Okeanos New England Seamount Exploration, and Our Deepwater Backyard, Exploring Atlantic Canyons and Seamounts, respectively, resulting in a series of ROV dives on several seamounts using the ship Okeanos Explorer and ROV Deep Discoverer. Seamounts explored in those programs included Mytilus, Physalia, Atlantis II, Gosnold, and Kelvin, adding six ROV dives and four additional seamounts.
Geomorphic features and habitats Three-dimensional reconstructions of the seamounts were compiled from multibeam bathymetry data collected during the cruises from 2003 to 2005, along with data from the US Law of the Sea cruise from the slope of the northeastern United States (Gardner, 2004) and ETOPO2 bathymetric data as baseline, are presented in Figs. 57.1 and 57.3. Seamount reconstructions were made using the software Fledermaus ver. 6 (www.qps.nl). Because of their extreme ages, few of the seamounts have retained their conical shape and most show features of slumping and resultant debris flows to the abyssal plain (Mitchell, 2001), for example, Fig. 57.3C and D. In addition, many of the seamounts are large structures with multiple peaks. This is especially true for Kelvin, Manning, and Nashville seamounts in the New England chain, and Corner and Caloosahatchee seamounts in the Corner Rise group (Fig. 57.3F and G). The Shipboard Scientific Party (1979) for Leg 43 of the Deep-Sea Drilling Project suggested that the elongate shape of Nashville Seamount could have resulted from a chain of closely spaced volcanos, or from a series of fissure eruptions. Both explanations may apply to most of the New England and Corner Rise seamount groups. Also, as noted by Taylor et al. (1975) for Gilliss Seamount, none of the seamounts in these two groups exhibit a caldera at the summit or moat at the base. Some of the seamounts show carbonate caps, most likely of Eocene age, and a few could have been emergent at some time in their history (Uchupi et al., 1970). Auster et al. (2005) proposed a hierarchical classification scheme for characterizing seamount landscapes as fish habitat. The two major habitat classes on these seamounts are basalt and fine-grained sediment, which can each then be divided into habitat subclasses and microhabitats. The substrate on the New England and Corner Rise seamounts is mainly basalt, covered by varying levels of biogenic sand. Several of the seamounts contain numerous habitat subclasses and microhabitats, including ridges, walls, boulders, and ledges in some areas and fine-grained sediment in others. A few seamounts may also contain clastic debris, compressed ash, or carbonate crust (Vogt and Tucholke, 1979). Houghton et al. (1977) noted that all surfaces were encrusted with manganese oxides and we saw varying levels of encrustation on subfossil and fossil corals. The average rate of deposition
Figure 57.3 Three-dimensional images of representative seamounts from the New England and Corner Rise Seamount chains: (A) from back to front, Bear, Physalia, and Mytilus Seamounts; (B) from left to right, Retriever, Picket, and Balanus Seamounts; (C) Manning Seamount viewed from the south; (D) Kelvin Seamount viewed from southeast; (E) Nashville Seamount viewed from the south; (F) Corner Seamount, from the south, showing “Kukenthal Peak” (left) and “Goode Peak” (right) and with Rockaway Seamount in the left background; other seamounts unnamed; (G) Caloosahatchee Seamount, facing north, with “Milne-Edwards Peak” (left) and “Verrill Peak” (right); other seamounts unnamed; (H) Yakutat Seamount, facing north and unnamed seamount in the background. Scale for elevation included in (D).
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Figure 57.4 Bray Curtis, group average cluster dendrogram of seamount relationships using presence absence data. Dashed lines in clusters indicate no significant difference among included seamounts.
of the manganese was estimated to be about 3 mm million years21 (Houghton et al., 1977) indicating that some of the dead coral we observed could be quite old.
Biological communities Analysis of the benthic communities of the seamounts was based on data compiled from greater than 200 collected specimens, 38,433 in situ images, and video footage obtained during the NOAA sponsored cruises detailed above. Images were analyzed for a total of 33 dives on 17 peaks, sampled at depths ranging from 713 to 3000 m. There were two short duration dives conducted at 3900 m on Kelvin and Retriever seamounts, but the area surveyed was very limited and the fauna was sparse, and so those dives are not included in this paper. The major factors influencing the structure of the communities on the New England and Corner Rise seamounts are depth, position along the chain (coded as “longitude”), and substrate composition. It is likely that slope is also an important factor but we have not yet analyzed that variable. Cluster analysis of all seamounts using species presence/absence data, not accounting for depth, produced three main groups (Fig. 57.4): (1) the three deepest sampled seamounts—Mytilus, Physalia, and Atlantis—all sampled below 2400 m; (2) the New England Seamounts—Retriever, Balanus, Kelvin, Gosnold, and Rehoboth; and (3) the Corner Rise Seamounts—Ku¨kenthal, Milne-Edwards, Verrill Peaks, and Yakutat Seamount.
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Figure 57.5 Total taxa found on each seamount/peak in the New England and Corner Rise Seamount groups.
Five seamounts or peaks did not fall into any group—Bear, Goode Peak, Picket, Nashville, and Manning. Overall, the New England Seamounts east from Retriever to Nashville are more diverse than those at Corner Rise (Fig. 57.5). The lowest diversity was on the three seamounts located closest to the continental shelf—Bear, Mytilus, and Physalia. Yakutat, located farthest east, is the most diverse seamount in the Corner Rise chain. Although the two seamount chains share several taxa, Bamboo C1, Paragorgia sp. cf. coralloides, and Corallium bathyrubrum are widespread throughout the New England Seamounts but were not recorded at Corner Rise. Anthomastus sp. and Iridogorgia magnispiralis are also common and widespread on the New England Seamounts, but only one and two specimens, respectively, were recorded at Corner Rise. On Ku¨kenthal Peak, 11 colonies of a purple plexaurid were recorded, as well as one colony on Yakutat Seamount, but this species was not found on the New England Seamounts. The most widespread coral taxa in this portion of the Northwest Atlantic, observed frequently on the New England Seamounts and Corner Rise, are Bathypathes sp. and Metallogorgia melanotrichos. Bear is the oldest of the New England Seamounts, located at the edge of the continental shelf southeast of Georges Bank. Uchupi et al. (1970) described Bear Seamount as being covered by Globigerina sand and ooze, with basalt pebbles dispersed throughout. In our survey from 1428 to 1780 m, the most abundant coral species are Bamboo C1 and M. melanotrichos, with patchy distributions of Anthomastus sp., Chrysogorgia tricaulis,
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Desmophyllum dianthus, Keratoisis grayi, Calyptrophora antilla, Bamboo D1a, and Leiopathes sp. The dominant sponge on the seamount is Euplectella. Goode Peak, located on Corner Seamount, was sampled from 1858 to 2128 m. The substrate on Goode is basalt rock protruding from varying depths of biogenic sand. The most abundant species is a sponge in the family Rossellidae (24 colonies). The bamboo coral Bamboo D1c occurs below 1900 m, and was most abundant around 2100 m. Candidella imbricata and Parantipathes larix were found between 1800 and 2000 m, but were not observed below 2000 m. Conversely, M. melanotrichos was recorded below 2000 m, but was not observed at shallower depths. Additional corals recorded include Bathypathes sp., Paramuricea sp., C. tricaulis, Corallium niobe, and an unidentifiable solitary coral. Picket Seamount is dominated by an undescribed bramble bamboo coral, Bamboo D2f for which 60 1 colonies were recorded. It is possible that this coral is found on other seamounts throughout the Northwest Atlantic; however, genetic analysis was only conducted on specimens collected from Picket Seamount and image data alone was not sufficient to determine which of the several bramble bamboo species are occurring on other seamounts. Additional abundant species on Picket include M. melanotrichos, Acanthogorgia armata, Corallium bayeri, and sponges in the genus Hertwigia. On Nashville Seamount, the farthest east seamount of the New England chain, the substrate is mainly basalt rock ledge with biogenic sand sediment (Fig. 57.6). The benthic community is abundant and diverse around 2100 m, and the landscape is densely populated with colonies of the corals M. melanotrichos, I. magnispiralis, Paramuricea sp., Bamboo J3a, Calyptrophora microdentata, and sponges in the genera Farrea and Hertwigia. With increasing depth, biomass and diversity decreases, and below 2500 m C. bathyrubrum and C. bayeri dominate the habitat with patchy distributions of other species in low abundances. The southwest peak on Manning Seamount between 1400 and 1500 m hosts a unique benthic community not observed on central Manning or on any of the other seamounts in the region. The substrate is biogenic sand with small stones, basalt rock, and compressed ash ledges, as well as botryoidal surfaces (manganese crust) and fossil coral debris consisting of Lophelia, Enallopsammia, Desmophyllum, and bamboo coral skeletons. The benthic community is densely populated with the primnoid whip coral Calyptrophora clinata, as well as a large fan-shaped Calyptrophora sp. which could not be identified from images and no specimens were collected. The most diverse of all of the seamounts sampled was Kelvin, located in the New England Seamount chain (Fig. 57.6). Kelvin was sampled from 1715 to 2606 m and throughout those depths the water temperature was in the 3 C 2 4 C range (Fig. 57.2). The majority of the substrate on Kelvin is basalt rock with varying levels of biogenic sand. However,
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Figure 57.6 Some representative seamount communities. (A) Manning Smt., SW peak, 1820 m, basalt ledge with unbranched bamboo corals, and the scleractinian corals Enallopsammia (yellow) and Lophelia pertusa (pink). (B) Manning Smt., SW Peak, 1850 m, basalt outcrop with abundant sponges, octocorals, and brittle stars, surrounded by a large death assemblage of Desmophyllum dianthus. (C) Balanus Smt., 1900 m, small basalt outcrop covered with octocorals Corallium, on the right, and Metallogorgia melanotrichos on the left, in a sea of small pebbles and biogenic sand. (D) Nashville Smt., 2230 m, colonies of Iridogorgia magnispiralis on a nearly vertical basalt wall with biogenic sediment drape. (E) Kelvin Smt., summit area, 1750 m, current swept biogenic sand with abundant xenophyophores. (F) Nashville Smt., 2200 m, sea urchin on heavily burrowed biogenic sand. Source: Courtesy NOAA Ocean Exploration and are publicly available.
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Figure 57.7 MDS of taxa analyzed by depth of occurrence. Similarity values from Bray Curtis cluster analysis.
a region of the seamount around 1950 m was characterized by a thick cover of fossil coral skeletons, mainly from the genus Desmophyllum. The benthic communities show distinct variation with depth, both within and among seamounts (Fig. 57.7). Multidimensional Scaling (MDS) and cluster analysis of the benthic communities arranged by depth produced six groups at 40% similarity (Bray Curtis, presence absence): (1) 700 2 999 m; (2) 1100 1199 m; (3) 1200 2 1399 m; (4) 1400 2 2199 m; (5) 2200 2 2699; and (6) 2700 2 2999 m and three groups at 20%: (1) 700 1199 m; (2) 1200 2699 m; and (3) 2700 2999 m. The major fauna breaks where the communities noticeably change structure are around 1200 m, and then again around 2600 m. MDS and cluster analysis of the communities arranged by water temperature produced four distinct groups (Fig. 57.8): (1) ,3 C; (2) 3 C 2 5 C: (3) 5 C 2 6 C; and (4) 9 C 2 13 C. The highest taxon diversity of the megabenthic fauna was observed between 1700 and 2100 m (Fig. 57.9). The temperature at these depths is mainly between 3 C and 4 C (Fig. 57.2). Diversity was lowest at the depths sampled shallower than 1200 m, as well as at depths below 2700 m (Fig. 57.9). However, it should be noted that sampling effort was minimal in the shallowest and deepest depth ranges and species accumulation curves suggest that the deepwater community could possess as many as 80 megafaunal species. Only one location, Ku¨kenthal Peak on Corner Seamount, was sampled at depths shallower
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Figure 57.8 MDS of taxa analyzed by occurrence in water of various temperatures at 1 C increments.
than 1000 m, and this was an area that had been previously trawled by commercial fisheries (Vinnichenko, 1997, Waller et al., 2007, Watling et al., 2007). The only data from the 1100 m depth range was based on less than 5 minutes at the end of a dive on Verrill Peak, Caloosahatchee Seamount. Sampling below 2700 m occurred at Mytilus and Atlantis Seamounts in the New England Seamount chain. Below 2700 m, the temperature decreases to below 3 C. All of the New England Seamounts were sampled at depths below 1300 m. At Corner Rise, Ku¨kenthal, Milne-Edwards, and Verrill peaks were all sampled above 1300 m, and the deepest sample site was on Yakutat Seamount at 2400 m. The substrate on Ku¨kenthal Peak between 700 and 1000 m is a carbonate crust, possibly of Eocene age, with isolated areas of exposed basaltic bedrock. There is a steep temperature gradient between around 9 C to over 12 C at the shallowest depth sampled. The summit area had been heavily trawled, most likely in the 1970s to 1990s (Vinnichenko, 1997), and the basal structures of a variety of corals and sponges were found but most were dead, although there were several young colonies of P. larix and a few colonies of the plexaurid gorgonian coral, Placogorgia sp., were observed. The latter species was not found at deeper depths of the New England or Corner Rise seamounts. Other corals in this community include the scleractinian Lophelia pertusa, Acanella arbuscula, and Muriceides sp. However, above 1000 m, the community was dominated by sponges. The 1100 m community on Verrill Seamount was dominated by several colonies of A. arbuscula, with P. larix and Placogorgia sp. also observed on a basalt rock.
Figure 57.9 Left panel, total species, cumulative hours spent, and average species recorded per hour spent in each 100 m depth interval across all seamounts. Right panels, observed species accumulation curves (Sobs) and Chao-1 estimates of species likely to be found with increasing sample sites (seamount/peak) at 1400 and 1900 m, and for the deep interval, 2400 3000 m.
930 Chapter 57 From 1200 to 2299 m the communities are much more diverse (Fig. 57.9), even accounting for the increased time spent at those depths. The corals A. arbuscula and P. larix are still present at these depths, but were not found below 2300 and 2400 m, respectively. Forty additional species of corals appear below 1200 m. On all of the seamounts sampled at depths above 2300 m, coral taxa dominated the overall benthic megafaunal community diversity, with sponges and other invertebrates, mainly anemones, contributing to varying degrees (Fig. 57.5). However, on the deepest seamounts sampled at depths below 2400 m, sponges were the most diverse on Mytilus Seamount while other invertebrates were most diverse on Physalia, and on Atlantis the diversity was similar between corals and sponges (14 and 13 taxa, respectively). Below 2300 m, there were only four species of corals observed that were not found at shallower depths—Chrysogorgia averta, Chrysogorgia agassizii, and two species of undescribed bamboo corals. Below 2700 m, the temperature decreases to less than 3 C and there were no corals found that were not recorded in shallower water. Certain species, including the black corals Telopathes magna and Bathypathes sp., as well as Paragorgia johnsoni, were found at these deep depths and also as shallow as 1200 m. These findings indicate that the corals at the deeper depths ( . 2700 m) are extending their range from shallower habitats, rather than being sourced from the abyss. Total bottom time varied at the different depths sampled (Fig. 57.9). Although a pattern can be seen between bottom time and the total number of taxa recorded, the diversity observed is not simply an artifact of sampling effort. At 1900 m, diversity was highest, although the most sampling effort was in the 2000 m depth range (38 hours). The bottom time at 1300 and 1900 m was the same (36.5 hours), although many fewer taxa were recorded in the 1300 m depth range. At 1200 and 1300 m, the hours spent on the bottom varied substantially (3 and 36.5 hours, respectively) although the number of taxa recorded were similar. Conversely, the bottom time at 2400 and 2500 m was similar (11 and 10.5 hours), but more taxa were observed at 2500 m. Species accumulation curves at intermediate and deep depths approach an asymptote, indicating a saturation point where more sampling will not produce more taxa. The saturation point is reached by adding more sampling sites rather than total bottom time. Sampling effort was lowest, in terms of both bottom time and sample sites, at depths shallower than 1300 m. Future research at these depths on the New England and Corner Rise seamounts will help to further delineate the benthic community structure. We observed a distinct difference in the community structure of the benthic communities on the deepest (,2700 m) and shallowest ( . 1000 m) seamounts and peaks in the bathyal Northwest Atlantic. At intermediate depths, the community structure varies depending on location along the chain (longitude), depth (temperature), and substrate. We hypothesize that the benthic communities are structured largely due to the water masses in which they
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reside, and the precise nature of these relationships is currently being explored in more detail and will be the subject of a future publication.
Acknowledgments We gratefully acknowledge the NOAA NURP, and Office of OER for the many years of support for cruises to the New England and Corner Rise Seamounts. Each cruise involved detailed mapping of the seamounts, as well as dives with submersibles and ROVs, requiring the expertise of a host of crew and technicians on the R/Vs Atlantis and Ronald H. Brown, as well as pilots and technicians for the HOV Alvin and ROV Hercules. We sincerely appreciate the time and effort of all involved. The ROV Hercules was made available by the Institute for Exploration, University of Rhode Island, but we would also like to express our appreciation to Robert Ballard for his invention of the “telepresence” concept, which we tested on our 2004 and 2005 cruises, and which is now standard operating procedure for Okeanos cruises. We are especially indebted to D. Scheirer of the US Geological Survey, Menlo Park for compiling the grids on which the 3D reconstructions are based and to K. Scanlon, US Geological Survey, Woods Hole, for early help with the Geographic Information System (GIS) analysis of National Oceanographic Data Center. (NODC) data. We also greatly appreciate the help and camaraderie of our many collaborators on these cruises and for help with species identifications, especially S. France, P. Auster, R. Waller, C. Kelley, and T. Shank.
Abbreviations ATL BAL BEA GIS GOO GOS KEL KUK MAN NODC PHY MIL MYT NAS MDS PIC REH RET VER YAK
Atlantis II Seamount, New England Seamounts Balanus Seamount, New England Seamounts Bear Seamount, New England Seamounts Geographic Information System Goode Peak, Corner Seamount, Corner Rise Gosnold Seamount, New England Seamounts Kelvin Seamount, New England Seamounts Kukenthal Peak, Corner Seamount, Corner Rise Manning Seamount, New England Seamounts National Oceanographic Data Center Physalia Seamount, New England Seamounts Milne-Edwards Peak, Caloosahatchee Seamount, Corner Rise Mytilus Seamount, New England Seamounts Nashville Seamount, New England Seamounts Multidimensional Scaling Picket Seamount, New England Seamounts Rehoboth Seamount, New England Seamounts Retriever Seamount, New England Seamounts Verrill Peak, Caloosahatchee, Corner Rise Yakutat Seamount, Corner Rise
References Auster, P.J., Moore, J., Heinonen, K.B., Watling, L., 2005. A habitat classification scheme for seamount landscapes: assessing the functional role of deep-water corals as fish habitat. In: Freiwald, A., Roberts, J.M. (Eds.), Cold-water Corals and Ecosystems. Springer-Verlag, Berlin, pp. 761 769.
932 Chapter 57 Clark, M.R., Rowden, A.A., Schlacher, T., Consalvey, M., Stocks, K.I., Rogers, A.D., et al., 2010. The ecology of seamounts: structure, function, and human impacts. Ann. Rev. Mar. Sci. 2, 253 278. Duncan, R.A., 1984. Age progressive volcanism in the New England Seamounts and the opening of the Central Atlantic Ocean. J. Geophys. Res. 89, 9980 9990. Epp, D., Smoot, C., 1989. Distributions of seamounts in the North Atlantic. Nature 337, 254 257. Gardner, J.V., 2004. U.S. Law of the Sea Cruise to Map the Foot of the Slope and 2500-m Isobath of the Northeast US Atlantic Continental Margin. Center for Coastal and Ocean Mapping/Joint Hydrographic Center, University of New Hampshire, Durham, NH. Heezen, B.C., Tharp, M., Ewing, M., 1959. The floors of the ocean: I. The North Atlantic. Geol. Soc. Am. 65, 1 122. Houghton, R.L., Heirtzler, J.R., Ballard, R.D., Taylor, P.T., 1977. Submersible observations of the New England seamounts. Naturwissenschaften 64 (7), 348 355. Kitchingman, A., Lai, S., 2004. Inferences on potential seamount locations from mid-resolution bathymetric data. Seamounts Biodivers. Fisher. 12 (5), 7 12. Fisheries Centre Research Report, University of British Columbia. Koslow, J.A., Auster, P., Bergstad, O.A., Roberts, J.M., Rogers, A., Vecchione, M., et al., 2015. Biological communities on seamounts and other submarine features potentially threatened by disturbance. United Nations World Ocean Assessment. L. Inniss, A. Simcock, et al. (Eds.) ,http://www.un.org/Depts/los/ global_reporting/WOA_RegProcess.htm.. Manheim, F.T., Lane-Bostwick, C.M., 1989. Chemical composition of ferromanganese crusts in the World Ocean: a review and comprehensive database. US Geological Survey Open-File Rept. 89 020. McGregor, B.A., Betzer, P.R., Krause, D.C., 1973. Sediments in the Atlantic Corner Seamounts: control by topography, paleo-winds, and geochemically-detected modern bottom currents. Mar. Geol. 14, 179 190. Mitchell, N.C., 2001. Transition from circular to stellate forms of submarine volcanoes. J. Geophys. Res. 106 (B2), 1987 2003. Moore, J.A., Vecchione, M., Collette, B.B., Gibbons, R., Hartel, K.E., Galbraith, J.K., et al., 2003. Biodiversity of Bear Seamount, New England seamount chain: results of exploratory trawling. J. Northwest Atlant. Fisher. Sci. 31, 363 372. Pitcher, T.J., Morato, T., Hart, P.J.B., Clark, M.R., Haggan, N., Santos, R.S. (Eds.), 2007. Seamounts: Ecology, Fisheries & Conservation. John Wiley & Sons. Shea, E.K., Judkins, H., Staudinger, M.D., Dimkovikj, V.H., Lindgren, A., Vecchione, M., 2017. Cephalopod biodiversity in the vicinity of Bear Seamount, western North Atlantic, based on exploratory trawling from 2000 to 2014. Mar. Biodivers. 47, 699 722. Shipboard Science Party, 1979. Site 382: Nashville Seamount—Volcanism along the eastern New England Seamount chain. Initial Rep. Deep Sea Drill. Proj. 43, 31 93. Sleep, N.H., 1990. Monteregian Hotspot Track: a long-lived mantle plume. J. Geophys. Res. 95 (B13), 983 990. Talley, L.D., Pickard, G.E., Emery, W.J., Swift, J.H., 2011. Descriptive Physical Oceanography: An Introduction, sixth ed. Elsevier, Burlingham, MA. Taylor, P.T., Stanley, D.J., Simkin, T., Jahn, W., 1975. Gilliss Seamount: detailed bathymetry and modification by bottom currents. Mar. Geol. 19, 139 157. Uchupi, E., Phillips, J.D., Prada, K.E., 1970. Origin and structure of the New England seamount chain. Deep Sea Res. Oceanograph. Abstr. 17 (3), 483 494. Vinnichenko, V.I., 1997. Russian investigations and deep water fishery on the Corner Rising Seamount in Subarea 6. NAFO Sci. Council Stud. 30, 41 49. Vogt, P.R., Tucholke, B.E., 1979. Ch. 42. The New England Seamounts: Testing Origins. Initial Reports of the Deep Sea Drilling Project 43, 847 856. Waller, R., Watling, L., Auster, P.J., Shank, T., 2007. Anthropogenic impacts on the Corner Rise seamounts, north-west Atlantic Ocean. J. Mar. Biol. Assoc. UK 87, 1975 1976. Watling, L., Waller, R., Auster, P.J., 2007. Corner Rise Seamounts: the impact of deep-sea fisheries. ICES Insight 44, 10 14.
Deep-sea benthic megafaunal communities on the New England and Corner Rise Seamounts, Northwest Atlantic Ocean Abby Lapointe1, Les Watling1 and Allen M. Gontz2 1 2
Department of Biology, University of Hawaii at M¯anoa, Honolulu, HI, United States Department of Geological Sciences, San Diego State University, San Diego, CA, United States
Abstract Combined, the New England and Corner Rise seamount chains in the Northwest Atlantic extend approximately 1700 km from the continental slope southeast of George’s Bank to the vicinity of the Mid-Atlantic Ridge. Three-dimensional reconstructions of the seamounts were compiled from multibeam bathymetry data. Analysis of the benthic communities was based on data compiled from collected specimens, in situ images, and video footage. A total of 17 peaks were analyzed, sampled at depths ranging from 713 to 3000 m. The major factors influencing the structure of the megabenthic communities are depth, position along the chain, and substrate. Overall the New England Seamounts are more diverse than Corner Rise, and the highest taxon diversity was observed between 1700 and 2200 m. Below 2300 m only four species of corals were observed that were not found at shallower depths, and below 2700 m no corals were found that were not recorded in shallower water. These results suggest that there is a distinct difference in the structure of the benthic communities on the deepest (,2700 m) and shallowest ( . 1000 m) seamounts and peaks in the bathyal Northwest Atlantic. At intermediate depths the community structure varies depending on location along the chain (longitude), depth (water temperature), and substrate.
Keywords: Seamount; benthic communities; corals; Northwest Atlantic; New England Seamounts; Corner Rise Seamounts
Introduction Seamounts, submerged mountains in the sea, comprise a significant component of the hard substrate areas at bathyal depths and have been found to be hotspots of biomass and biodiversity in the deep ocean (Pitcher et al., 2007). Although estimates on the exact number of seamounts vary significantly depending on the methodology and thresholds used, there are, most likely, at least 14,000 seamounts globally (Kitchingman and Lai, 2004). Yet
Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00057-9 © 2020 Elsevier Inc. All rights reserved.
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Figure 57.1 Bathymetric map of the New England (1 12) and Corner Rise (13 15) Seamount groups. Inset lower left indicates position in the Northwest Atlantic, NES, New England Seamounts, CR, Corner Rise Seamounts.
despite their abundance and importance to marine biodiversity, relatively little is known about their benthic communities (Clark et al., 2010; Koslow et al., 2015). The New England and Corner Rise seamount chains in the Northwest Atlantic combined extend approximately 1700 km from the continental slope southeast of George’s Bank to the vicinity of the Mid-Atlantic Ridge (Fig. 57.1). The New England Seamount chain extends 1200 km from Bear Seamount in the west to Nashville Seamount in the east (Uchupi et al., 1970). The Corner Rise Seamounts are located about 300 km east of the New England Seamounts near the boundary of the abyssal plain and Mid-Atlantic Ridge (McGregor et al., 1973). The New England Seamounts are estimated to be of Mesozoic origin, ranging in age from about 103 to 82 million years (Duncan, 1984). The Corner Rise Seamounts are younger, estimated at approximately 75 million years in the western portion and 38 million years in the east (Epp and Smoot, 1989). Both seamount groups were formed by volcanic activity over a mantle plume hotspot that created the Monteregian Hills southeast of Montreal, Canada, about 140 mya and, due to seafloor spreading and movement of the North Atlantic plate, now resides southwest of Great Meteor Seamount east of the Mid-Atlantic Ridge (Sleep, 1990). The plume was strong during the formation of Bear Seamount. However, as the plates shifted and the direction of motion changed, the strength of the plume weakened during the formation of the Corner Rise Seamounts (Sleep, 1990). Additional seamounts may have existed between the New England Seamounts and Corner Rise but have been buried by sediments from the Sohm abyssal plain (Epp and Smoot, 1989), although this seems unlikely unless those seamounts were quite small.
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Figure 57.2 Contours of water temperature over the New England and Corner Rise Seamount chain. Shaded bars depict the depth ranges sampled on the seamounts/peaks, arranged by longitude of their location. See Table for abbreviations. Source: Data from NOAA World Ocean Atlas, 2013, ver. 2: www. nodc.noaa.gov/OC5/woa13/.
The rough bathymetry of the New England and Corner Rise seamounts was described in 1959, based on single-beam echo soundings using a precision depth recorder (Heezen et al., 1959). McGregor et al. (1973) outlined the bathymetry of the Corner Rise Seamounts in more detail and a series of submersible dives on the New England Seamounts in 1974 provided the first visual observations and details about their structure and bathymetry (Houghton et al., 1977). With the advent of multibeam sonar technology, fast and accurate surveys of the ocean floor are now possible and much of the New England and Corner Rise Seamounts have been mapped. The water column throughout the Northwest Atlantic is oxygen-rich, with oxygen levels of approximately 6.0 mL L21 at the surface to 3.0 mL L21 at 3000 m depth (NOAA World Ocean Atlas, 2013, ver. 2: www.nodc.noaa.gov/OC5/woa13/). The temperature contours across the length of the seamount chains, as measured during 41 dives on 17 seamounts at depths from 700 to 3000 m, along with the range of depths of remotely operated vehicle (ROV) dives on each seamount, are shown in Fig. 57.2. Details of the cruises are described below. The temperature on the majority of the New England Seamounts at depths where images or samples were taken was fairly constant between 3 C and 4 C, with only Manning and Rehoboth having dive sites in the 4 C 2 5 C range. Proceeding west to east from
920 Chapter 57 Manning in the New England Seamounts to Milne-Edwards Peak in Corner Rise, the surface water is warmer and extends deeper into the water column due to the influence of the Gulf Stream. However, the only area sampled with water warmer than 6 C was Ku¨kenthal Peak on Corner Seamount, and the only area sampled colder than 3 C was on Mytilus Seamount. Most of the dive sites on the Corner Rise Seamounts were in water approximately 1 C warmer than all the New England Seamounts other than Manning and Rehoboth. The dominant water mass covering the New England Seamounts is Labrador Sea Water which forms a Deep Western Boundary Current at the western end, but this water mass gives way to Upper North Atlantic Deep Water and then to North Atlantic Deep Water to the east and in deeper water, respectively (Talley et al., 2011).
Naturalness The majority of the seamounts in these two seamount groups are physically undisturbed. There have been surveys of the manganese crusts on a few seamounts, but no mining has occurred (Manheim and Lane-Bostwick, 1989). The New England Seamounts are, with the exception of Bear Seamount, generally too deep to fish economically, but the US National Marine Fisheries Service conducted 56 exploratory benthic trawls on Bear Seamount during the years 2000 14 (Moore et al., 2003, Shea et al., 2017). Several peaks on the Corner Rise Seamounts were most likely fished over the period 1976 96 (specific seamount data have not been released; Vinnichenko, 1997) and evidence of bottom-trawling was seen on two peaks during our ROV dives (Waller et al., 2007, Watling et al., 2007). These shallower peaks also showed evidence of human activity in the form of discarded bottles and bricks, and a discarded piece of trawl net was seen at 2100 m on Nashville Seamount. No further evidence of fishing on Nashville was observed, but the dives were generally below the depth where fishing would occur.
Mapping and exploration cruises 2003 14 With funding from the National Oceanic and Atmospheric Administration (NOAA), National Undersea Research Program (NURP), and Office of Ocean Exploration Research (OER), a series of cruises to the New England and Corner Rise seamounts were undertaken. In 2003 bathymetric mapping using the R/V Atlantis and dives with the submersible Alvin were completed on Bear, Kelvin, and Manning seamounts under the program Mountains in the Sea I; in 2004, the NOAA ship Ronald H. Brown was used for mapping and support of sampling dives with the ROV Hercules during the Mountains in the Sea II cruise with samples obtained from Balanus, Bear, Kelvin, Manning, and Retriever seamounts. In 2005 the Deep Atlantic Stepping Stones expedition used the same ship and ROV system, mapping and exploring Yakutat, Caloosahatchee, and Corner seamounts in the Corner Rise group, and Nashville, Manning, and Rehoboth seamounts in the New England chain. Later in 2005 mapping with the
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R/V Atlantis and one dive with Alvin focused on Picket Seamount during the CanyBal expedition. In all, 35 ROV or submersible dives were completed on 13 seamounts in those programs. In 2013 and 2014 NOAA’s Office of Ocean Exploration Research (OER) program conducted two cruises, Okeanos New England Seamount Exploration, and Our Deepwater Backyard, Exploring Atlantic Canyons and Seamounts, respectively, resulting in a series of ROV dives on several seamounts using the ship Okeanos Explorer and ROV Deep Discoverer. Seamounts explored in those programs included Mytilus, Physalia, Atlantis II, Gosnold, and Kelvin, adding six ROV dives and four additional seamounts.
Geomorphic features and habitats Three-dimensional reconstructions of the seamounts were compiled from multibeam bathymetry data collected during the cruises from 2003 to 2005, along with data from the US Law of the Sea cruise from the slope of the northeastern United States (Gardner, 2004) and ETOPO2 bathymetric data as baseline, are presented in Figs. 57.1 and 57.3. Seamount reconstructions were made using the software Fledermaus ver. 6 (www.qps.nl). Because of their extreme ages, few of the seamounts have retained their conical shape and most show features of slumping and resultant debris flows to the abyssal plain (Mitchell, 2001), for example, Fig. 57.3C and D. In addition, many of the seamounts are large structures with multiple peaks. This is especially true for Kelvin, Manning, and Nashville seamounts in the New England chain, and Corner and Caloosahatchee seamounts in the Corner Rise group (Fig. 57.3F and G). The Shipboard Scientific Party (1979) for Leg 43 of the Deep-Sea Drilling Project suggested that the elongate shape of Nashville Seamount could have resulted from a chain of closely spaced volcanos, or from a series of fissure eruptions. Both explanations may apply to most of the New England and Corner Rise seamount groups. Also, as noted by Taylor et al. (1975) for Gilliss Seamount, none of the seamounts in these two groups exhibit a caldera at the summit or moat at the base. Some of the seamounts show carbonate caps, most likely of Eocene age, and a few could have been emergent at some time in their history (Uchupi et al., 1970). Auster et al. (2005) proposed a hierarchical classification scheme for characterizing seamount landscapes as fish habitat. The two major habitat classes on these seamounts are basalt and fine-grained sediment, which can each then be divided into habitat subclasses and microhabitats. The substrate on the New England and Corner Rise seamounts is mainly basalt, covered by varying levels of biogenic sand. Several of the seamounts contain numerous habitat subclasses and microhabitats, including ridges, walls, boulders, and ledges in some areas and fine-grained sediment in others. A few seamounts may also contain clastic debris, compressed ash, or carbonate crust (Vogt and Tucholke, 1979). Houghton et al. (1977) noted that all surfaces were encrusted with manganese oxides and we saw varying levels of encrustation on subfossil and fossil corals. The average rate of deposition
Figure 57.3 Three-dimensional images of representative seamounts from the New England and Corner Rise Seamount chains: (A) from back to front, Bear, Physalia, and Mytilus Seamounts; (B) from left to right, Retriever, Picket, and Balanus Seamounts; (C) Manning Seamount viewed from the south; (D) Kelvin Seamount viewed from southeast; (E) Nashville Seamount viewed from the south; (F) Corner Seamount, from the south, showing “Kukenthal Peak” (left) and “Goode Peak” (right) and with Rockaway Seamount in the left background; other seamounts unnamed; (G) Caloosahatchee Seamount, facing north, with “Milne-Edwards Peak” (left) and “Verrill Peak” (right); other seamounts unnamed; (H) Yakutat Seamount, facing north and unnamed seamount in the background. Scale for elevation included in (D).
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Figure 57.4 Bray Curtis, group average cluster dendrogram of seamount relationships using presence absence data. Dashed lines in clusters indicate no significant difference among included seamounts.
of the manganese was estimated to be about 3 mm million years21 (Houghton et al., 1977) indicating that some of the dead coral we observed could be quite old.
Biological communities Analysis of the benthic communities of the seamounts was based on data compiled from greater than 200 collected specimens, 38,433 in situ images, and video footage obtained during the NOAA sponsored cruises detailed above. Images were analyzed for a total of 33 dives on 17 peaks, sampled at depths ranging from 713 to 3000 m. There were two short duration dives conducted at 3900 m on Kelvin and Retriever seamounts, but the area surveyed was very limited and the fauna was sparse, and so those dives are not included in this paper. The major factors influencing the structure of the communities on the New England and Corner Rise seamounts are depth, position along the chain (coded as “longitude”), and substrate composition. It is likely that slope is also an important factor but we have not yet analyzed that variable. Cluster analysis of all seamounts using species presence/absence data, not accounting for depth, produced three main groups (Fig. 57.4): (1) the three deepest sampled seamounts—Mytilus, Physalia, and Atlantis—all sampled below 2400 m; (2) the New England Seamounts—Retriever, Balanus, Kelvin, Gosnold, and Rehoboth; and (3) the Corner Rise Seamounts—Ku¨kenthal, Milne-Edwards, Verrill Peaks, and Yakutat Seamount.
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Figure 57.5 Total taxa found on each seamount/peak in the New England and Corner Rise Seamount groups.
Five seamounts or peaks did not fall into any group—Bear, Goode Peak, Picket, Nashville, and Manning. Overall, the New England Seamounts east from Retriever to Nashville are more diverse than those at Corner Rise (Fig. 57.5). The lowest diversity was on the three seamounts located closest to the continental shelf—Bear, Mytilus, and Physalia. Yakutat, located farthest east, is the most diverse seamount in the Corner Rise chain. Although the two seamount chains share several taxa, Bamboo C1, Paragorgia sp. cf. coralloides, and Corallium bathyrubrum are widespread throughout the New England Seamounts but were not recorded at Corner Rise. Anthomastus sp. and Iridogorgia magnispiralis are also common and widespread on the New England Seamounts, but only one and two specimens, respectively, were recorded at Corner Rise. On Ku¨kenthal Peak, 11 colonies of a purple plexaurid were recorded, as well as one colony on Yakutat Seamount, but this species was not found on the New England Seamounts. The most widespread coral taxa in this portion of the Northwest Atlantic, observed frequently on the New England Seamounts and Corner Rise, are Bathypathes sp. and Metallogorgia melanotrichos. Bear is the oldest of the New England Seamounts, located at the edge of the continental shelf southeast of Georges Bank. Uchupi et al. (1970) described Bear Seamount as being covered by Globigerina sand and ooze, with basalt pebbles dispersed throughout. In our survey from 1428 to 1780 m, the most abundant coral species are Bamboo C1 and M. melanotrichos, with patchy distributions of Anthomastus sp., Chrysogorgia tricaulis,
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Desmophyllum dianthus, Keratoisis grayi, Calyptrophora antilla, Bamboo D1a, and Leiopathes sp. The dominant sponge on the seamount is Euplectella. Goode Peak, located on Corner Seamount, was sampled from 1858 to 2128 m. The substrate on Goode is basalt rock protruding from varying depths of biogenic sand. The most abundant species is a sponge in the family Rossellidae (24 colonies). The bamboo coral Bamboo D1c occurs below 1900 m, and was most abundant around 2100 m. Candidella imbricata and Parantipathes larix were found between 1800 and 2000 m, but were not observed below 2000 m. Conversely, M. melanotrichos was recorded below 2000 m, but was not observed at shallower depths. Additional corals recorded include Bathypathes sp., Paramuricea sp., C. tricaulis, Corallium niobe, and an unidentifiable solitary coral. Picket Seamount is dominated by an undescribed bramble bamboo coral, Bamboo D2f for which 60 1 colonies were recorded. It is possible that this coral is found on other seamounts throughout the Northwest Atlantic; however, genetic analysis was only conducted on specimens collected from Picket Seamount and image data alone was not sufficient to determine which of the several bramble bamboo species are occurring on other seamounts. Additional abundant species on Picket include M. melanotrichos, Acanthogorgia armata, Corallium bayeri, and sponges in the genus Hertwigia. On Nashville Seamount, the farthest east seamount of the New England chain, the substrate is mainly basalt rock ledge with biogenic sand sediment (Fig. 57.6). The benthic community is abundant and diverse around 2100 m, and the landscape is densely populated with colonies of the corals M. melanotrichos, I. magnispiralis, Paramuricea sp., Bamboo J3a, Calyptrophora microdentata, and sponges in the genera Farrea and Hertwigia. With increasing depth, biomass and diversity decreases, and below 2500 m C. bathyrubrum and C. bayeri dominate the habitat with patchy distributions of other species in low abundances. The southwest peak on Manning Seamount between 1400 and 1500 m hosts a unique benthic community not observed on central Manning or on any of the other seamounts in the region. The substrate is biogenic sand with small stones, basalt rock, and compressed ash ledges, as well as botryoidal surfaces (manganese crust) and fossil coral debris consisting of Lophelia, Enallopsammia, Desmophyllum, and bamboo coral skeletons. The benthic community is densely populated with the primnoid whip coral Calyptrophora clinata, as well as a large fan-shaped Calyptrophora sp. which could not be identified from images and no specimens were collected. The most diverse of all of the seamounts sampled was Kelvin, located in the New England Seamount chain (Fig. 57.6). Kelvin was sampled from 1715 to 2606 m and throughout those depths the water temperature was in the 3 C 2 4 C range (Fig. 57.2). The majority of the substrate on Kelvin is basalt rock with varying levels of biogenic sand. However,
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Figure 57.6 Some representative seamount communities. (A) Manning Smt., SW peak, 1820 m, basalt ledge with unbranched bamboo corals, and the scleractinian corals Enallopsammia (yellow) and Lophelia pertusa (pink). (B) Manning Smt., SW Peak, 1850 m, basalt outcrop with abundant sponges, octocorals, and brittle stars, surrounded by a large death assemblage of Desmophyllum dianthus. (C) Balanus Smt., 1900 m, small basalt outcrop covered with octocorals Corallium, on the right, and Metallogorgia melanotrichos on the left, in a sea of small pebbles and biogenic sand. (D) Nashville Smt., 2230 m, colonies of Iridogorgia magnispiralis on a nearly vertical basalt wall with biogenic sediment drape. (E) Kelvin Smt., summit area, 1750 m, current swept biogenic sand with abundant xenophyophores. (F) Nashville Smt., 2200 m, sea urchin on heavily burrowed biogenic sand. Source: Courtesy NOAA Ocean Exploration and are publicly available.
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Figure 57.7 MDS of taxa analyzed by depth of occurrence. Similarity values from Bray Curtis cluster analysis.
a region of the seamount around 1950 m was characterized by a thick cover of fossil coral skeletons, mainly from the genus Desmophyllum. The benthic communities show distinct variation with depth, both within and among seamounts (Fig. 57.7). Multidimensional Scaling (MDS) and cluster analysis of the benthic communities arranged by depth produced six groups at 40% similarity (Bray Curtis, presence absence): (1) 700 2 999 m; (2) 1100 1199 m; (3) 1200 2 1399 m; (4) 1400 2 2199 m; (5) 2200 2 2699; and (6) 2700 2 2999 m and three groups at 20%: (1) 700 1199 m; (2) 1200 2699 m; and (3) 2700 2999 m. The major fauna breaks where the communities noticeably change structure are around 1200 m, and then again around 2600 m. MDS and cluster analysis of the communities arranged by water temperature produced four distinct groups (Fig. 57.8): (1) ,3 C; (2) 3 C 2 5 C: (3) 5 C 2 6 C; and (4) 9 C 2 13 C. The highest taxon diversity of the megabenthic fauna was observed between 1700 and 2100 m (Fig. 57.9). The temperature at these depths is mainly between 3 C and 4 C (Fig. 57.2). Diversity was lowest at the depths sampled shallower than 1200 m, as well as at depths below 2700 m (Fig. 57.9). However, it should be noted that sampling effort was minimal in the shallowest and deepest depth ranges and species accumulation curves suggest that the deepwater community could possess as many as 80 megafaunal species. Only one location, Ku¨kenthal Peak on Corner Seamount, was sampled at depths shallower
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Figure 57.8 MDS of taxa analyzed by occurrence in water of various temperatures at 1 C increments.
than 1000 m, and this was an area that had been previously trawled by commercial fisheries (Vinnichenko, 1997, Waller et al., 2007, Watling et al., 2007). The only data from the 1100 m depth range was based on less than 5 minutes at the end of a dive on Verrill Peak, Caloosahatchee Seamount. Sampling below 2700 m occurred at Mytilus and Atlantis Seamounts in the New England Seamount chain. Below 2700 m, the temperature decreases to below 3 C. All of the New England Seamounts were sampled at depths below 1300 m. At Corner Rise, Ku¨kenthal, Milne-Edwards, and Verrill peaks were all sampled above 1300 m, and the deepest sample site was on Yakutat Seamount at 2400 m. The substrate on Ku¨kenthal Peak between 700 and 1000 m is a carbonate crust, possibly of Eocene age, with isolated areas of exposed basaltic bedrock. There is a steep temperature gradient between around 9 C to over 12 C at the shallowest depth sampled. The summit area had been heavily trawled, most likely in the 1970s to 1990s (Vinnichenko, 1997), and the basal structures of a variety of corals and sponges were found but most were dead, although there were several young colonies of P. larix and a few colonies of the plexaurid gorgonian coral, Placogorgia sp., were observed. The latter species was not found at deeper depths of the New England or Corner Rise seamounts. Other corals in this community include the scleractinian Lophelia pertusa, Acanella arbuscula, and Muriceides sp. However, above 1000 m, the community was dominated by sponges. The 1100 m community on Verrill Seamount was dominated by several colonies of A. arbuscula, with P. larix and Placogorgia sp. also observed on a basalt rock.
Figure 57.9 Left panel, total species, cumulative hours spent, and average species recorded per hour spent in each 100 m depth interval across all seamounts. Right panels, observed species accumulation curves (Sobs) and Chao-1 estimates of species likely to be found with increasing sample sites (seamount/peak) at 1400 and 1900 m, and for the deep interval, 2400 3000 m.
930 Chapter 57 From 1200 to 2299 m the communities are much more diverse (Fig. 57.9), even accounting for the increased time spent at those depths. The corals A. arbuscula and P. larix are still present at these depths, but were not found below 2300 and 2400 m, respectively. Forty additional species of corals appear below 1200 m. On all of the seamounts sampled at depths above 2300 m, coral taxa dominated the overall benthic megafaunal community diversity, with sponges and other invertebrates, mainly anemones, contributing to varying degrees (Fig. 57.5). However, on the deepest seamounts sampled at depths below 2400 m, sponges were the most diverse on Mytilus Seamount while other invertebrates were most diverse on Physalia, and on Atlantis the diversity was similar between corals and sponges (14 and 13 taxa, respectively). Below 2300 m, there were only four species of corals observed that were not found at shallower depths—Chrysogorgia averta, Chrysogorgia agassizii, and two species of undescribed bamboo corals. Below 2700 m, the temperature decreases to less than 3 C and there were no corals found that were not recorded in shallower water. Certain species, including the black corals Telopathes magna and Bathypathes sp., as well as Paragorgia johnsoni, were found at these deep depths and also as shallow as 1200 m. These findings indicate that the corals at the deeper depths ( . 2700 m) are extending their range from shallower habitats, rather than being sourced from the abyss. Total bottom time varied at the different depths sampled (Fig. 57.9). Although a pattern can be seen between bottom time and the total number of taxa recorded, the diversity observed is not simply an artifact of sampling effort. At 1900 m, diversity was highest, although the most sampling effort was in the 2000 m depth range (38 hours). The bottom time at 1300 and 1900 m was the same (36.5 hours), although many fewer taxa were recorded in the 1300 m depth range. At 1200 and 1300 m, the hours spent on the bottom varied substantially (3 and 36.5 hours, respectively) although the number of taxa recorded were similar. Conversely, the bottom time at 2400 and 2500 m was similar (11 and 10.5 hours), but more taxa were observed at 2500 m. Species accumulation curves at intermediate and deep depths approach an asymptote, indicating a saturation point where more sampling will not produce more taxa. The saturation point is reached by adding more sampling sites rather than total bottom time. Sampling effort was lowest, in terms of both bottom time and sample sites, at depths shallower than 1300 m. Future research at these depths on the New England and Corner Rise seamounts will help to further delineate the benthic community structure. We observed a distinct difference in the community structure of the benthic communities on the deepest (,2700 m) and shallowest ( . 1000 m) seamounts and peaks in the bathyal Northwest Atlantic. At intermediate depths, the community structure varies depending on location along the chain (longitude), depth (temperature), and substrate. We hypothesize that the benthic communities are structured largely due to the water masses in which they
Deep-sea benthic megafaunal communities on the New England
931
reside, and the precise nature of these relationships is currently being explored in more detail and will be the subject of a future publication.
Acknowledgments We gratefully acknowledge the NOAA NURP, and Office of OER for the many years of support for cruises to the New England and Corner Rise Seamounts. Each cruise involved detailed mapping of the seamounts, as well as dives with submersibles and ROVs, requiring the expertise of a host of crew and technicians on the R/Vs Atlantis and Ronald H. Brown, as well as pilots and technicians for the HOV Alvin and ROV Hercules. We sincerely appreciate the time and effort of all involved. The ROV Hercules was made available by the Institute for Exploration, University of Rhode Island, but we would also like to express our appreciation to Robert Ballard for his invention of the “telepresence” concept, which we tested on our 2004 and 2005 cruises, and which is now standard operating procedure for Okeanos cruises. We are especially indebted to D. Scheirer of the US Geological Survey, Menlo Park for compiling the grids on which the 3D reconstructions are based and to K. Scanlon, US Geological Survey, Woods Hole, for early help with the Geographic Information System (GIS) analysis of National Oceanographic Data Center. (NODC) data. We also greatly appreciate the help and camaraderie of our many collaborators on these cruises and for help with species identifications, especially S. France, P. Auster, R. Waller, C. Kelley, and T. Shank.
Abbreviations ATL BAL BEA GIS GOO GOS KEL KUK MAN NODC PHY MIL MYT NAS MDS PIC REH RET VER YAK
Atlantis II Seamount, New England Seamounts Balanus Seamount, New England Seamounts Bear Seamount, New England Seamounts Geographic Information System Goode Peak, Corner Seamount, Corner Rise Gosnold Seamount, New England Seamounts Kelvin Seamount, New England Seamounts Kukenthal Peak, Corner Seamount, Corner Rise Manning Seamount, New England Seamounts National Oceanographic Data Center Physalia Seamount, New England Seamounts Milne-Edwards Peak, Caloosahatchee Seamount, Corner Rise Mytilus Seamount, New England Seamounts Nashville Seamount, New England Seamounts Multidimensional Scaling Picket Seamount, New England Seamounts Rehoboth Seamount, New England Seamounts Retriever Seamount, New England Seamounts Verrill Peak, Caloosahatchee, Corner Rise Yakutat Seamount, Corner Rise
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