Seafloor geomorphology and benthic habitat of the German Bank glaciated shelf, Atlantic Canada

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Seafloor geomorphology and benthic habitat of the German Bank glaciated shelf, Atlantic Canada Craig J. Brown1, Brian J. Todd2, Stephen J. Smith3 and Jessica A. Sameoto3 1

Nova Scotia Community College, Ivany Campus, Dartmouth, NS, Canada 2Geological Survey of Canada Atlantic, Natural Resources Canada, Dartmouth, NS, Canada 3Fisheries and Oceans Canada, Dartmouth, NS, Canada

Abstract An area of 5320 km2 in water depths of 30 250 m has been mapped on German Bank on the southern Scotian Shelf in Atlantic Canada. The Scotian Shelf is a formerly glaciated continental margin characterized by a topographically rugged inner shelf. Bedrock is exposed at the seafloor on much of German Bank. Ice-contact sediment (till) was deposited beneath or at the margins of the ice sheet directly onto bedrock during the Wisconsinan glaciation and occurs as a widespread sediment blanket. Ice-distal glaciomarine silt overlies the older till and is primarily confined to small basins on the bank. Limited accumulations of postglacial sediments are composed of well-sorted sand, grading to rounded and subrounded gravel. Analysis of seafloor photographs and underwater video revealed broadscale gradients in benthic fauna composition across the bank. Surficial geology, broad biophysical (“benthoscape”), and modeled scallop habitat suitability maps are presented for the area, generated using a variety of methods. The combined use of these different maps is offering benefits for use in fisheries management in this area.

Keywords: German Bank; Canada; Atlantic Ocean; Nova Scotia; Gulf of Maine; glaciated shelf

Introduction This chapter is an updated edition of Todd et al. (2012). It provides an overview of the geology and benthic habitat characteristics of German Bank, off southern Nova Scotia, including methods developed since the publication of the first edition of this case study (Todd et al., 2012) to integrate biological information into seafloor maps for fisheries management purposes. German Bank is located off southern Nova Scotia on the Scotian Shelf in the eastern Gulf of Maine (Fig. 40.1) and is the offshore extension of the southern Nova Scotia landmass. Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00040-3 © 2020 Elsevier Inc. All rights reserved.

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Figure 40.1 Multibeam sonar bathymetric map of German Bank. Locations of Figs. 40.4, 40.5, 40.6, and 40.7 are indicated by labeled white boxes.

Much of German Bank is exposed bedrock comprising Cambro Ordovician metasedimentary rocks intruded by Late Devonian Carboniferous granitoid plutons (Pe-Piper and Jansa, 1999). Bedrock has been modified by glacial erosion and is separated by a rugged erosional surface from the discontinuous overlying quaternary sediments (Drapeau and King, 1972). German Bank lies within the photic zone, with water depths on the shallowest parts of the bank shoaling to 30 m. The southern and western margins of German Bank are demarcated approximately by the 100 m isobath. Consequently, the bank has an irregular outline, but extends roughly 90 km seaward from land at its greatest extent. The channel to the south of German Bank and the basin to the west both attain depths greater than 200 m. Circulation on German Bank is tidally dominated with a persistent westward and northwestward flow toward the Gulf of Maine (Smith, 1983; Lynch, et al., 1996). Tidal current speeds reach 70 cm s21 over eastern parts of the bank. The northwest flow of Scotian Shelf water across German Bank contributes to the broadscale counterclockwise ocean circulation within the Gulf of Maine. The average yearly bottom water temperature on German Bank increases from 4 C in the east to 8 C in the west, varying seasonally from less than 2 in the

Seafloor mapping of German Bank, Atlantic Canada 677 deeper part of the bank ( . 100 m) to almost 10 in the shallow inshore zone (Hannah et al., 2001). Average bottom salinity is 32 ppm in shallow eastern waters increasing to 34 ppm in the deeper part of the bank to the west (Hannah et al., 2001). Based on the summer water density difference between surface and 30 m depth, water masses on the bank are well mixed, with stratification slightly higher in the eastern nearshore part of the study area. Spring phytoplankton bloom production reaches 6 µg mL21 (estimated from SeaWifs, G. Harrison, personal communication, 2004). The bottom waters of German Bank are strongly saturated with oxygen, with July average saturation of 100% to the east near the Nova Scotia shore, decreasing to 60% saturation to the southwest (Fisheries and Oceans Canada, 2018). The German Bank benthic environment has experienced, in places, very high impacts from human activities (Todd et al., 2006; Halpern et al., 2008; Kostylev and Todd, 2010; Todd et al., 2011; Brown et al., 2012; Smith et al., 2017). The region supports both commercial sea scallop (Placopecten magellanicus) (Smith and Lundy, 2002) and lobster (Homarus americanus) fisheries (Fisheries and Oceans Canada, 2006). Scallop fishing intensity is spatially concentrated in areas with relatively high scallop habitat suitability with low to no fishing in areas of low scallop habitat suitability (e.g., outcropping bedrock and moraines; Brown et al., 2012; Smith et al., 2017). A significant proportion of German Bank comprises low scallop habitat suitability (Brown et al., 2012). Lobster fishing practices that overlap with German Bank use fixed fishing gear which has minimal impact on seafloor habitat. An improved understanding of the benthic environment was required to assist fisheries management in reducing potential conflict between these two colocated fisheries (DFO, 2006). Understanding the different physical habitats (both geological and oceanographic) of the bank and their associated benthic assemblages is necessary information to yield improved fisheries management practice. To this end, a multibeam sonar survey was conducted over 5320 km2 of German Bank from 1997 to 2003, accompanied by 2133 km of geophysical profiles, 86 sediment samples, and 4324 seafloor photographs (Todd, 2007, 2009; Brown et al., 2012).

Geomorphic features and habitats If a strict interpretation of the definition of a bank were applied (International Hydrographic Organization, 2008), German Bank would not qualify as it is not an isolated elevation of the seafloor. Rather, it is an area of the Canadian Atlantic continental shelf, bathymetrically contiguous with the adjacent landmass, that has been designated a bank since at least 1812 (Holland, 1812). Regionally, at horizontal distances of tens of kilometers (regional scale), German Bank can be characterized as bathymetrically smooth glaciated continental shelf, with a regional gradient of less than 1 to the southwest. At distances of tens to hundreds of meters (local scale), geomorphic features are evident in outcropping bedrock and within the overlying glacial and postglacial sediments.

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Bedrock Metasedimentary and igneous rocks outcrop over the central portion of the map area (designated as IM on Fig. 40.2). Seabed relief on bedrock reaches 40 m in places (over a horizontal distance of B500 m) but is generally less than 6 m (Fig. 40.3). At regional scale the igneous bedrock exhibits two jointing directions (approximately 40 and 190 ); these zones of inherent weaknesses have been exploited by erosion lending a distinctive crosshatch pattern to the bedrock topography (Fig. 40.4). At local scale areas mapped as bedrock outcrop are in fact a complex of substrates including exposed rock, accumulations of shell hash, sand, and gravel within interstices in the bedrock (Todd and Valentine, 2010).

Glacial sediment Till is widespread on German Bank and was deposited directly on bedrock beneath and at the margins of the ice sheet during the late Wisconsinan substage of the Pleistocene epoch (Todd et al., 2007). The till (designated as T on Fig. 40.2) is unsorted, unstratified, and unconsolidated glacial drift consisting of a mixture of mud, sand, and gravel and displays a

Figure 40.2 Surficial geology map of German Bank. Locations of Figs. 40.3, 40.4, 40.5, 40.6, and 40.7 are indicated by labeled white boxes.

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Figure 40.3 Seismic-reflection profile (upper) and interpreted geological cross-section (lower) showing typical seismostratigraphy of German Bank. See Fig. 40.2 for location of profile (CCGS Hudson 76016, Day 173, 0356 0450).

characteristic chaotic to transparent seismic-reflection configuration (Fig. 40.3). Glacial landforms of till are widespread on German Bank and include drumlins and moraines (see paragraph later). They were subjected to modification during sea level transgression but were not buried after glacial retreat because of the low volume of postglacial sediment transported offshore. Drumlins were formed by deposition beneath the active ice sheet on German Bank and are streamlined, oval-shaped mounds of glacial deposits with long axes oriented northwest southeast, aligned parallel to the direction of glacial ice flow (Fig. 40.5; Todd et al., 2016). The drumlins are typically 500 800 m in length and 100 300 m in width, with elevations 10 15 m above the surrounding seafloor. Seismic-reflection profiles reveal that the mounds are constructed of acoustically incoherent sediment, interpreted as till, with intermound areas deeply draped by stratified sediment ( . 10 m thick in places) characterized by continuous coherent reflections and interpreted as glaciomarine sand and silt. De Geer moraines on German Bank form fields of numerous parallel ridges of cobbles and boulders; each ridge displays a simple or curved line in planform (Fig. 40.6) and strike

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Figure 40.4 Rugged German Bank bedrock topography to the southeast juxtaposed with smooth glacial and postglacial sediment topography to the northwest. Position of topographic cross-section AB is indicated by white line. Vertical exaggeration of the cross-section is 5.8 3 .

approximately west-southwest east-northeast (Todd, 2016). Individual moraine crests can be traced horizontally from short segments of 2 km up to almost 10 km. De Geer moraines are formed at or close to the grounding lines of water-terminating glaciers (Benn and Evans, 1998) and are the most ubiquitous glacial landforms on German Bank. The moraines are generally symmetrical in cross-section and the horizontal distance between moraines, normal to strike, varies from 30 to 200 m. Moraine height varies from 1.5 to 8 m, and width varies from 40 to 130 m. Cobbles and boulders were observed and recovered from De Geer moraines (Todd et al., 2004). In places De Geer moraines are subdued in relief or are absent, resulting from partial to complete burial by sediment (sandy gravel) transported from glacial fronts and/or reworked during sea level transgression of

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Figure 40.5 German Bank drumlinized topography showing northwest southeast long axis strike. Artificial sun illumination is from the north, and the vertical exaggeration is 15 3 . Position of topographic cross-section AB is indicated by white line. Vertical exaggeration of the cross-section is 13.8 3 .

German Bank. This sediment blanket is generally thin (a few meters) but can reach an appreciable thickness (B10 m) in places. Ice-distal glaciomarine silt (Gm on Fig. 40.2) is widespread in basins on German Bank. This sediment is poorly sorted clayey and sandy silt with some gravel. At the map scale of

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Figure 40.6 German Bank De Geer moraine swarm showing approximate west-southwest east-northeast strike of moraine crests. Artificial sun illumination is from the north, and the vertical exaggeration is 15 3 . Position of topographic cross-section AB is indicated by white line. Vertical exaggeration of the cross-section is 6.3 3 .

Fig. 40.2, there are only two mapped seafloor exposures of glaciomarine silt in the extreme east of the study area. However, the glaciomarine silt is commonly observed in the subsurface in seismic profiles (Fig. 40.3). This unit exhibits a characteristic parallel to subparallel seismic-reflection configuration; the reflectors conformably drape the irregular top of the underlying till and bedrock. In places the glaciomarine silt is interbedded with the till.

Unconsolidated sediment Postglacial sediments, composed mainly of sand with minor gravel (PG on Fig. 40.2), are derived from current and wave reworking of glaciogenic material. In the shallow, eastern

Seafloor mapping of German Bank, Atlantic Canada 683 region of German Bank, postglacial sediments occur in elongated mounded deposits reaching hundreds of meters in length and 17 m in thickness, oriented southeast northwest, aligned with the current flow direction. In deeper water of western German Bank, postglacial sediments are deposited mainly in a basin-fill external morphology (Fig. 40.3) up to 20 m thick in places.

Biological communities and surrogacy In this updated edition of this GeoHab Atlas chapter on German Bank, we summarize the results from a recent underwater imaging survey conducted in 2010. We describe how information on the biology and substrata extracted from the imagery data is integrated using a variety of methods with the multibeam sonar bathymetric and backscatter data to generate a series of maps, which complement the surficial geology map presented in Fig. 40.2. These methods explore the statistical and geospatial relationships between the physical surrogates measured by the multibeam sonar and the benthos. Through this process of applying multiple methods to generate multiple maps for management applications (primarily fisheries stock assessment and management), we demonstrate the efficient use of survey data sets to maximize the benefit to a wide number of potential end users, and to facilitate the move toward an ecosystem-based approach to management. Seafloor imagery was obtained using Towcam, a towed camera platform fitted with forwardlooking video and downward looking digital still camera (Gordon et al., 2007; Brown et al., 2012) from a total of 52 camera stations in 2010 (Brown et al., 2012). Postsurvey, 3190 georeferenced photographs from the 52 survey stations were analyzed to classify the broad biophysical characteristics of the seafloor into classes (“benthoscapes”—see later). The video footage from the forward-facing camera was also analyzed for the presence of sea scallop (P. magellanicus) on the seafloor. Positional data for each scallop observation were extracted using the software ClassAct Mapper, a utility developed within Fisheries and Oceans Canada to assist in the postprocessing and analysis of video data (Sameoto et al., 2008). Finally, detailed biological community data were extracted from a subset of 521 still images selected from the 52 survey stations. Each image was carefully examined and all visible taxa were counted and identified using authoritative taxonomic keys [see Brown et al. (2012) for details on methodology]. To examine the statistical relationships between physical surrogates and benthos, methods conventionally used for classification of remotely sensed satellite multispectral data were tested for segmentation and classification of the multibeam data set. The resulting maps (Fig. 40.7A,B) summarize the broad biophysical characteristics of the seafloor, which we call “benthoscape maps” (Zajac et al., 2003; Zajac, 2008; Brown et al., 2012) due to the analogies with generating terrestrial landscape maps from optical data sets in a similar

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Figure 40.7 Examples of surficial geology, benthoscape, and scallop habitat suitability maps for two areas of German Bank. Source: Figure location is shown in Figs. 40.1 and 40.2.

way. This can be considered complimentary to the surficial geology interpretation (Fig. 40.2), incorporating both surficial geological and biological information where it can be resolved from the acoustic remote sensed data at a relevant scale. An unsupervised classification was performed within the GIS software Idrisi (Eastman, 2008), using a combination of bathymetry, derived geomorphology, and backscatter layers to objectively segment the data into 15 statically derived acoustic classes [see Brown et al. (2012) for details]. Photographs from the 2010 Towcam surveys were classified into five benthoscape classes based on biophysical characteristics (e.g., dominant sediment type, the presence/absence of

Seafloor mapping of German Bank, Atlantic Canada 685 any sediment bedforms, and the presence of characterizing biological elements). Table 40.1 summarizes the five benthoscape classes, and lists the primary substrate characteristics and common biota associated with each seafloor class. The positional information for the 3190 images categorized into the five benthoscape classes was subsequently overlaid on the unsupervised multibeam segmentation, and an error matrix was derived to examine the relationship between the multibeam classes and in situ benthoscape classes. Assessment of the best match between in situ and map classes was used to facilitate merging of the 15 segmented multibeam classes into five classes, and a new five-class benthoscape map was produced (Fig. 40.7A,B), with an estimated accuracy of 70% (Brown et al., 2012). The benthoscape map (Fig. 40.7A,B) shows similarities in the spatial segmentation to the surficial geology map (Fig. 40.2), but with the distinction that the geological origin of the seafloor substrata is not incorporated into the classes, and additional biological descriptors are used where relevant (e.g., the PG and T classes in Fig. 40.2 are spatially and categorically divided differently into Till and Silt benthoscape classes in Fig. 40.7A and B). The biological relevance of this distinction is that faunal distribution patterns are commonly driven by substrate properties (e.g., grain size and percentage composition) and seabed morphology irrespective of geological origin of the material or seafloor form. Spatial patterns in the biology therefore correlate to a higher degree to the benthoscape map versus the surficial geology map (Brown et al., 2011, 2012). A species-specific habitat map for the sea scallop P. magellanicus was also generated from the multibeam data by applying a Species Distribution Modeling (SDM) method to predict spatial habitat suitability [see Brown et al. (2012) for a detailed description of these methods]. Maximum Entropy (Maxent) SDM was applied to the bathymetry, derived geomorphology, and backscatter data layers using a training subset of the scallop observations positions extracted from the video to generate a predictive scallop habitat suitability map. Area under curve (AUC) statistics derived from threshold independent receiver operating characteristic curves were used to validate the success of the scallop distribution model using the remaining validation subset of scallop observations (Phillips et al., 2006), with a resulting AUC score of 0.743 (Brown et al., 2012). Habitat suitability (calculated using a logistic scale for the predictive plot) revealed higher habitat suitability for scallop concentrated in the north-eastern, shallower water regions of the survey area (Fig. 40.7). The eastern region of the survey area displayed the highest proportion of suitable habitat, with areas of high suitability also found in patches to the southwest, and in isolated patches in deeper water to the west of German Bank. Community data extracted from the subset of 521 still images were analyzed using multivariate statistical methods within the software package PRIMER v6 (Clarke and Warwick, 2001). Images with fewer than three taxa were removed from the data set prior to analysis in order to reduce the variability caused by these low abundance images,

686 Chapter 40 Table 40.1: Summary of Benthoscape classes on German Bank, summarizing surficial sediment characteristics and common benthic fauna. Example seafloor photograph

Substrata Hard substrata: comprising .50% coverage of boulders or bedrock

Mixed substrata: various fractions of cobbles, gravel, and sand. ,50% coverage of bedrock and boulders

Biology Cnidaria: Metridium senile Porifera: Polymastia spp; Hymedesmia sp.; Haliclona sp. Bryozoa: Encrusting spp. Flustra foliacea Hydrozoa: Multiple species Crustacea: Balanus sp. Porifera: Hymedesmia sp. Bryozoa: Encrusting sp. Flustra foliacea Hydrozoa: Multiple species Crustacea: Balanus sp.

Unconsolidated substrata: silt

Annelida: Polychaeta spp. Brachiopoda: Terebratulina septentrionalis

Unconsolidated substrata: silt with bedforms

Amphipoda: Unidentified— possibly multiple species

Unconsolidated substrata: sand with bedforms

Echinodermata: Echinarachnius parma

Benthoscape description

Annelida: Filograna implexa Mollusca: Anomia sp.; Modiolus modiolus Echinodermata: Henricia sp. Asterias sp. Chordata: Didemmum sp.

Reef—images with more than 50% boulders or bedrock with frequent, patchy epifauna visible on the surface of the hard Substrata

Mollusca: Anomia sp.; Modiolus modiolus; Placopecten magellanicus Echinodermata: Asterias sp. Chordata: Didemmum sp.

Till—images with mixed sediments cobbles, gravel, and sand; frequent and diverse patchy fauna.

Silt with frequent evidence of bioturbation in the form of small infaunal tubes on the surface of the sediment Silt with sediment bedforms

Sand with sediment bedforms and highly abundant sand dollars (Echinarachnius parma)

Seafloor mapping of German Bank, Atlantic Canada 687 leaving a total of 339 images. The assemblage composition from the still images was assessed by nonmetric multidimensional scaling (MDS) ordination using the Bray Curtis similarity measure on presence absence transformed data, and each sample points in the ordination assigned a label for the corresponding benthoscape class in which the image coincided. Analysis of similarities (ANOSIM; Clarke, 1993) was also performed to describe the extent of the differences in assemblage composition between the final benthoscape classes. The MDS plot revealed no distinct faunal patterns (Fig. 40.8), with considerable overlap between benthoscape classes. There was a gradual shift from hard substrata assemblages concentrated to the right in the ordination (i.e., Reef and Till benthoscape classes) to soft substrata assemblages concentrated to the left in the ordination (i.e., Silt and Silt with sediment bedforms classes; Fig. 40.8). Average similarity of assemblage compositions within each benthoscape class was relatively low, ranging between 24% and 34% [see Brown et al. (2012), for further details]. This is reflected in the diffuse spread of points within each benthoscape class in the MDS (Fig. 40.8) and the low Global R value (0.244) which denotes that groups within the ordination overlap are not clearly separable from one another. Nonetheless, ANOSIM did reveal that there were significant differences in assemblage composition between benthoscape classes (Brown et al., 2012). The results from these multivariate analyses agree with previous studies on German Bank which have collected, analyzed, and described benthic communities from underwater

Figure 40.8 MDS ordination of epifaunal assemblage data derived from seafloor photographs, labeled by benthoscape class. Source: Modified from Brown C.J., Sameoto J.A. and Smith S.J., Multiple methods, maps and management applications: purpose made seafloor maps in support of Ocean Management, J. Sea Res. 7, 2012, 1 13.

688 Chapter 40 imaging surveys (see Todd et al., 2004; Kostylev and Todd, 2010; Todd et al., 2012). These studies suggest that the scale of observation of the seafloor biology from the underwater imagery is much finer than the scale at which the surficial geology has been mapped (Fig. 40.2—1:50,000; Todd et al., 2012), and that fine scale sediment heterogeneity can be misleading when associating community information with the surficial geology interpretation. These findings are in agreement with the results presented here (Fig. 40.8 and Table 40.1), and which are discussed in detail in Brown et al. (2012). Nonetheless, both survey data sets report similar, general patterns in faunal distributions over the bank driven by the local, complex geology and oceanographic conditions.

Conclusion The derived surficial geology, benthoscape, and scallop SDM maps are proving valuable for incorporating spatial information on benthic habitat into management of the scallop fishery on German Bank (Smith et al., 2009, 2017). The SDM output, in particular, has recently been incorporated into the scallop stock assessment process, providing significant benefits to the management of this stock (Smith et al., 2017). The use of multiple thematic maps, interpreted in different ways, has therefore been demonstrated to provide advantages over the use of a single interpretation for marine spatial planning applications.

Acknowledgments We thank Vicki Gazzola for support in production of the figures for this manuscript. Natural Resources Canada contribution number 20180234.

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