High-resolution Multibeam Echosounder Research Articles

Marine Habitat Mapping using Multibeam Echosounder Survey and Underwater Video Observations: A Case Study from Tioman Marine Park

In recent years, there has been an increasing trend of utilizing high-resolution multibeam echosounder (MBES) datasets and supervised classification via machine learning to create marine habitat maps. The purpose of current study was threefold: (1) to extract bathymetric and backscatter derivatives from a multibeam dataset, (2) to measure the correlation between bathymetric and backscatter derivatives, and (3) to generate a marine habitat map using the Random Forest (RF). Tioman Marine Park (TMP), which is situated Southeast China Sea. MBES surveyed area are encompassed an area of 406 km² and served as the location for the study. Based on results and analysis, fourteen (14) derivative were derived from bathymetry map and backscatter mosaic. The second step involved integrating variables and a total of 152 of habitat ground-truth data were used, derived from underwater imageries, and sediment samples, into an RF model to generate a map of the marine habitat. Based on marine habitat map, six habitat classes including sand, rock, gravel and sand, coral rubble, coral and rock, and coral were classified. The distribution of coral habitat was found to be correlated with the depth of the bathymetry in the shallow water region. Therefore, the study has reached the conclusion that the integration between MBES derivatives, ground-truth data, and RF machine learning algorithm is an effective in classifying the distribution of marine habitats, specifically the coral habitat.

Character of advance and retreat of the southwest sector of the British-Irish Ice Sheet during the last glaciation

Relict landforms and sediments across former glaciated settings provide information about ice-sheet dynamics and can contribute to the understanding of the behaviour of contemporary ice masses, for which observations are limited in spatial and temporal extent. In this study, we focus on the shelf offshore southwest Ireland, in the Celtic Sea, which was once occupied by the Irish Sea Ice Stream (ISIS), the largest ice stream draining the southern portion of the marine-terminating British-Irish Ice Sheet (BIIS). Newly acquired high-resolution multibeam echosounder, sub-bottom and core data enabled the investigation of the shelf geomorphology and of the sedimentology and chronology of glacial and glacimarine sediments. A suite of drumlins records ice sheet flow from the coastline towards the central part of the shelf in southwest Ireland. Pre-existing highs in the seafloor topography promoted the formation of arcuate and transverse landforms interpreted as a grounding-zone wedge and moraines and they document episodic retreat of the ISIS across this portion of the shelf. Observed lithofacies show consolidated subglacial till and laminated fine muds. The sediments provide evidence of ice grounded ca. 30 km off the south-west Irish coastline with subsequent deglaciation occurring under glacimarine conditions. These new data refine the current reconstructions of the dynamics of the southern BIIS. They reveal for the first time the interplay of marine- and land-based ice and the presence of grounded ice offshore SW Ireland. This study highlights the importance of high-resolution data in revealing palaeo-landscapes as valuable analogues to test possible scenarios of modern ice sheet changes.

Sidescan Only Neural Bathymetry from Large-Scale Survey.

Sidescan sonar is a small and low-cost sensor that can be mounted on most unmanned underwater vehicles (UUVs) and unmanned surface vehicles (USVs). It has the advantages of high resolution and wide coverage, which could be valuable in providing an efficient and cost-effective solution for obtaining the bathymetry when bathymetric data are unavailable. This work proposes a method of reconstructing bathymetry using only sidescan data from large-scale surveys by formulating the problem as a global optimization, where a Sinusoidal Representation Network (SIREN) is used to represent the bathymetry and the albedo and the beam profile are jointly estimated based on a Lambertian scattering model. The assessment of the proposed method is conducted by comparing the reconstructed bathymetry with the bathymetric data collected with a high-resolution multi-beam echo sounder (MBES). An error of 20 cm on the bathymetry is achieved from a large-scale survey. The proposed method proved to be an effective way to reconstruct bathymetry from sidescan sonar data when high-accuracy positioning is available. This could be of great use for applications such as surface vehicles with Global Navigation Satellite System (GNSS) to obtain high-quality bathymetry in shallow water or small autonomous underwater vehicles (AUVs) if simultaneous localization and mapping (SLAM) can be applied to correct the navigation estimate.

Deep learning-assisted high resolution mapping of vulnerable habitats within the Capbreton Canyon System, Bay of Biscay

The Capbreton Canyon System is an area currently under study for its proposal as a Site of Community Importance under the EU Habitats Directive in the context of the LIFE IP INTEMARES project. Identifying and mapping benthic Vulnerable Marine Ecosystems (VMEs) plays a key role in this process. Although obtaining information on species distribution in deep sea rocky habitats has traditionally been a complicated task, the development of underwater remote sensing techniques resulted in a massive increase in the collection of digital imagery; however, processing all this information has led to another bottleneck due to the time-consuming nature of biota manual annotation. At this point, the use of computer vision and deep learning to automate image processing has substantial benefits but has rarely been adopted within the field of marine ecology. This study presents the integration of deep learning techniques for benthic fauna identification, high resolution multibeam echosounder (MBES) data and Species Distribution Models (SDMs), to map the potential habitat of the yellow coral Dendrophyllia cornigera, a representative species of the VME 1170 Reef habitat, on the circalitoral area of the Capbreton Canyon System. The localization and identification of the coral colonies was based on more than 7500 photographs taken during the INTEMARES-CapBreton 0619 and 0620 surveys using the photogrammetric ROTV Politolana. For the automatic annotation of the image set a deep learning based framework was developed by testing two different deep neural networks architectures; a FasterRCNN+Resnet101 model, accomplishing a precision of 100% over human expert annotation for presence/absence discrimination, was selected. Environmental data included different quantitative terrain attributes derived from high resolution MBES bathymetry data. A presence-only species distribution model, Maximum Entropy (MaxEnt), was used to infer the spatial distribution of D. cornigera over the study area. Predicted occurrences corresponded mainly to relevant topographic structures with significant slope, mainly associated to the edge of the continental shelf. These results are consistent with the ecological knowledge on the species and validate the use of deep learning tools to assist in the identification and mapping of VME for management and conservation purposes. This study provides a baseline for the protection of vulnerable habitats of the Capbreton Canyon System in the context of the Natura 2000 Network.

Sediment budget and riverbed deformation in the uppermost part of the lower Yangtze River, China

The Yangtze River (YR), similar to most large rivers in the world, has experienced significant changes in its depositional environment due to anthropogenic disturbances and climatic influences in recent decades. However, knowledge of how the river channel and bed deformation respond to these changes in the uppermost part of the lower YR, a 200-km-long branched channel, is limited. In the current study, historical bathymetric data collected from 1992 to 2013 and high-resolution multibeam echo sounder (MBES) data obtained from 2015 to 2016 were analyzed to address the changes in the depositional environment. The results indicate that the whole riverbed in the Hukou to Datong reach (HDR) has been scouring over the past two decades and this trend is expected to continue in the future due to decreasing sediment supply from the upper watershed. The amount of net erosion from the HDR during 1992–2013 approximately accounted for 7.2% of total sediment load in the Yangtze River (5.02 billion metric tons at Datong). The evolution of the riverbed is trending towards stabilization even though the patterns of erosion and/or deposition in the HDR have dramatically changed in the last two decades. The part of the channel from the upper 100 km to Datong was found to be a sensitive reach that first showed erosion from 1992 to 2003 and showed deposition during 2003–2008, and then mild erosion during 2008–2013 in response to environmental changes. Thus, the findings indicate that riverine channel deformation in response to anthropogenic disturbance and climate influence is dynamic in different reaches, and this finding has both scientific and practical significance in terms of understanding channel stability and its response to external stressors.

Using difference modelling and computational fluid dynamics to investigate the evolution of complex, tidally influenced shipwreck sites

The large number of historic metal-hulled shipwrecks on the seabed is a major consideration for the marine environment, heritage management and spatial planning. Their stability is driven by linked hydro- and sediment-dynamics, which in turn influence chemical corrosion and biological encrustation. The dynamism at underwater sites is frequently expressed in elaborate patterns of depositional and erosional features developed due to seabed scour. These settings are complex, due to the variable morphologies of the wrecks, and diverse types of seabed geology and geomorphology. Not only are the flow patterns and geomorphic changes at shipwreck sites not fully understood, but how these influence the wreck structures remains under-researched. Here we combine high-resolution multibeam echosounder, ocean current and sediment data with 3D Computational Fluid Dynamics (CFD) to investigate interrelations between hydro- and sediment-dynamics and the deterioration of two complex, fully submerged tidally-influenced shipwrecks. Intricate patterns of wake and horseshoe vortices are observed, and modelled wall shear stresses predict geomorphic changes recorded in 4-year and one-week interval difference models. Moreover, substantial damage is detected on the wrecks, correlated with areas of elevated wall shear stress and pressure in CFD simulations. The combined approach aids site management and provides analogies for offshore engineering.

Sea bottom characteristics and geochemistry of oil and gas seeps in the Gulf of Mexico

From 2015 to 2018, TGS conducted a comprehensive multiclient oil and gas seep mapping survey in the Gulf of Mexico (GOM). The basis for identifying seeps on the sea bottom was a high-resolution multibeam echosounder survey, mapping approximately 880,000 km2 of the sea bottom deeper than 750 m water depth, at a bathymetric resolution of 15 m, and a backscatter resolution of 5 m. We identified more than 5000 potential oil and/or gas seeps, and of those, we cored approximately 1500 for hydrocarbon geochemical analysis. The sea bottom features best related to hydrocarbon seepage in the GOM are high backscatter features, mud volcanoes, pock marks, brine pools, “popcorn” texture, fault traces, and anticlinal crests. We also tracked gas plumes in the water column back to the sea bottom to provide an additional criterion for hydrocarbon seepage. Cores from sea bottom targets recovered liquid oil, tar, and gas hydrates. Oil extract and gas analyses of samples from most target types produced values substantially higher than background in both oil and gas.

Subtidal Natural Hard Substrate Quantitative Habitat Mapping: Interlinking Underwater Acoustics and Optical Imagery with Machine Learning

Subtidal natural hard substrates (SNHS) promote occupancy by rich benthic communities that provide irreplaceable and fundamental ecosystem functions, representing a global priority target for nature conservation and recognised in most European environmental legislation. However, scientifically validated methodologies for their quantitative spatial demarcation, including information on species occupancy and fine-scale environmental drivers (e.g., the effect of stone size on colonisation) are rare. This is, however, crucial information for sound ecological management. In this investigation, high-resolution (1 m) multibeam echosounder (MBES) depth and backscatter data and derivates, underwater imagery (UI) by video drop-frame, and grab sediment samples, all acquired within 32 km2 of seafloor in offshore Belgian waters, were integrated to produce a random forest (RF) spatial model, predicting the continuous distribution of the seafloor areal cover/m2 of the stones’ grain sizes promoting colonisation by sessile epilithic organisms. A semi-automated UI acquisition, processing, and analytical workflow was set up to quantitatively study the colonisation proportion of different grain sizes, identifying the colonisation potential to begin at stones with grain sizes Ø ≥ 2 cm. This parameter (i.e., % areal cover of stones Ø ≥ 2 cm/m2) was selected as the response variable for spatial predictive modelling. The model output is presented along with a protocol of error and uncertainty estimation. RF is confirmed as an accurate, versatile, and transferable mapping methodology, applicable to area-wide mapping of SNHS. UI is confirmed as an essential aid to acoustic seafloor classification, providing spatially representative numerical observations needed to carry out quantitative seafloor modelling of ecologically relevant parameters. This contribution sheds innovative insights into the ecologically relevant delineation of subtidal natural reef habitat, exploiting state-of-the-art underwater remote sensing and acoustic seafloor classification approaches.

High-Resolution Vertical Habitat Mapping of a Deep-Sea Cliff Offshore Greenland

Recent advances in deep-sea exploration with underwater vehicles have led to the discovery of vertical environments inhabited by a diverse sessile fauna. However, despite their ecological importance, vertical habitats remain poorly characterized by conventional downward-looking survey techniques. Here we present a high-resolution 3-dimensional habitat map of a vertical cliff hosting a suspension-feeding community at the flank of an underwater glacial trough in the Greenland waters of the Labrador Sea. Using a forward-looking set-up on a Remotely Operated Vehicle (ROV), a high-resolution multibeam echosounder was used to map out the topography of the deep-sea terrain, including, for the first time, the backscatter intensity. Navigational accuracy was improved through a combination of the USBL and the DVL navigation of the ROV. Multi-scale terrain descriptors were derived and assigned to the 3D point cloud of the terrain. Following an unsupervised habitat mapping approach, the application of a K-means clustering revealed four potential habitat types, driven by geomorphology, backscatter and fine-scale features. Using groundtruthing seabed images, the ecological significance of the four habitat clusters was assessed in order to evaluate the benefit of unsupervised habitat mapping for further fine-scale ecological studies of vertical environments. This study demonstrates the importance of a priori knowledge of the terrain around habitats that are rarely explored for ecological investigations. It also emphasizes the importance of remote characterization of habitat distribution for assessing the representativeness of benthic faunal studies often constrained by time-limited sampling activities. This case study further identifies current limitations (e.g., navigation accuracy, irregular terrain acquisition difficulties) that can potentially limit the use of deep-sea terrain models for fine-scale investigations.

Mapping 3D structure of aSargassumforest with high-resolution sounding data obtained by multibeam echosounder

AbstractSargassum forests play an important role in coastal waters as habitats for marine organisms, including commercial species. However, human activities have negatively affected their distribution causing a worldwide decline of Sargassum forests. Mapping and monitoring the distribution and biomass of these habitats using acoustic remote sensing techniques is key for their conservation. Nonetheless, most researches based on acoustic remote sensing methods focus on estimations of macrophyte area and its canopy height, and less researches reporting 3D visualization of these habitats. This study demonstrates the use of high-resolution multibeam echosounder (MBES) bathymetric data to visualize the 3D structure of Sargassum forests. Comparing acoustic data and underwater camera photos collected in field surveys, we identified Sargassum individuals as vertical clusters of contiguous sounding points with a base close to the sea bottom in the sounding data of the MBES. Using this criterion, we could distinguish Sargassum echoes, visualize the 3D structure of Sargassum forests and estimate the number of Sargassum individuals in the survey area. Using the relation between thallus length and dry weight of sampled Sargassum plants, standing stock and biomass could be estimated assuming the thallus length was the height of Sargassum plants identified with the MBES.

Quantifying Marine Sedimentary Carbon: A New Spatial Analysis Approach Using Seafloor Acoustics, Imagery, and Ground-Truthing Data in Scotland

Marine sediments are important repositories of particulate organic matter, effectively burying organic carbon (OC) over geological timescales thus providing a climate regulation service. However, the spatial distribution of this marine sedimentary OC store is not well constrained. In this study we leverage a high resolution multibeam echosounder (MBES) survey taken at Loch Creran, a model fjordic site on the west coast of Scotland, to develop a new methodology for predicting the distribution of OC in surface sediments. Using an integrated approach, we combine use MBES survey, video imagery and ground-truthing data to produce a high-resolution (2 m x 2 m) map of surficial carbon and calculate a 10 cm stock. We find that the backscatter survey reliably uncovers a heterogeneous seabed and that OC correlates strongly with the MBES backscatter signal as a function of sediment composition. We estimate that there are approximately 12,346 ± 2,677 tonnes of OC held within the top 10 cm of mixed sediments across the MBES survey area (7.96 km2; 60% of the total area), upscaled to 20,577 ± 4,462 t of OC across Loch Creran (13.27 km2). Normalised by area, we find that mixed fine sediments with small fractions of sand and gravel are more effectivehold more carbon OC stores than homogenous fine sediments. This initial work proposes a novel methodological approach to using high resolution MBES surveys to improve the spatial mapping of sedimentary C carbon (C) and identification of C hotspots, enabling consideration of this resource in sedimentary carbon accounting, seabed management and climate mitigation strategies.

Faulting within the San Juan–southern Gulf Islands Archipelagos, upper plate deformation of the Cascadia subduction complex

Abstract The San Juan–southern Gulf Islands Archipelago of Washington State, USA and western Canada is located on the upper plate of the Cascadia subduction zone, in the forearc between the trench and volcanic arc. Onland and island investigations show many faults within the region that primarily represent old, inactive faults associated with transport, subduction and accretion of tectonostratigraphic terranes. However, until recently little geologic investigation and mapping have been done in the offshore. From these narrow straits, channels and sounds we have collected and interpreted high-resolution multibeam echosounder bathymetric data, 3.5 kHz sub-bottom and Huntec seismic-reflection profiles, and piston-cores to identify and date recently active faults. Previous studies by us focused on the earlier recognized active Devils Mountain fault zone that bounds the southern part of the Archipelago and the recently reported newly mapped active Skipjack Island fault zone that bounds the northern part. These transcurrent fault zones appear to be deforming and rotating the Archipelago. We concentrate on the unique deformation occurring within the seaways to determine the relationship and styles of faulting associated with these active bounding fault zones and relate the fault geometry and kinematics to one other subduction complex, the New Hebrides island arc of Vanuatu.

Residual relief modelling: digital elevation enhancement for shipwreck site characterisation

Scour processes play a critical role in the preservation status of submerged historic shipwrecks. Erosion of sediment leads to enhanced exposure of archaeological sites to physical, chemical and biological processes. Current methods for identifying erosional and depositional features at wreck sites are based primarily on visual interpretation of data, which is labour-intensive and entirely subjective. The increasing availability of high-resolution multibeam echosounder–derived digital elevation models (DEMs) of historic wreck sites allows for an entirely new level of detailed interrogation and analyses of the geomorphological features associated with these. In this study, we present a residual relief modelling method for the semi-automated extraction of such depositional and erosional features at wreck sites. Relief modelling is supplemented with a breakpoint classification approach, with final separation supported by DEM visualisation enhancement techniques. We applied the method to three World War I shipwreck sites and evaluated it against traditional manual vectorisation techniques. The results suggest that the semi-automated modelling method is robust, time-effective and capable of quantifying the products of scour processes with increased objectivity. Our method holds great potential for the objective characterisation of erosional and depositional patterns and processes at wreck sites, which have important implications for site formation studies and in situ preservation of underwater cultural heritage.

Sonar and LiDAR investigation of lineaments offshore between central New England and the New England seamounts, USA

High-resolution multibeam echosounder (MBES) and light detection and ranging (LiDAR) data, combined with regional gravity and aeromagnetic anomaly maps of the western Gulf of Maine, reveal numerous lineaments between central New England and the New England seamounts. Most of these lineaments crosscut the NE-SWtrending accreted terranes, suggesting that they may be surface expressions of deep basement-rooted faults that have fractured upward through the overlying accreted terranes or may have formed by the upward push of magmas produced by the New England hotspot. The 1755 Cape Ann earthquake may have occurred on a fault associated with one of these lineaments. The MBES data also reveal a NW-SE-oriented scarp just offshore from Biddeford Pool, Maine (Biddeford Pool scarp), a 60-km-long, 20-km-wide Isles of Shoals lineament zone just offshore from southeastern New Hampshire, a 50-m-long zone of mostly low-lying, WNW-ESE-trending, submerged ridge-like features and scarps east of Boston, Massachusetts, and a ~180-km-long, WNW-ESE-trending Olympus lineament zone that traverses the continental margin south of Georges Bank. Three submarine canyons are sinistrally offset ~1–1.2 km along the Thresher canyon lineament of the Olympus lineament zone.

Techniques for Classifying Seabed Morphology and Composition on a Subtropical-Temperate Continental Shelf

In 2017, the New South Wales (NSW) Office of Environment and Heritage (OEH) initiated a state-wide mapping program, SeaBed NSW, which systematically acquires high-resolution (2–5 m cell size) multibeam echosounder (MBES) and marine LiDAR data along more than 2000 km of the subtropical-to-temperate southeast Australian continental shelf. This program considerably expands upon existing efforts by OEH to date, which have mapped approximately 15% of NSW waters with these technologies. The delivery of high volumes of new data, together with the vast repository of existing data, highlights the need for a standardised, automated approach to classify seabed data. Here we present a methodological approach with new procedures to semi-automate the classification of high-resolution bathymetry and intensity (backscatter and reflectivity) data into a suite of data products including classifications of seabed morphology (landforms) and composition (substrates, habitats, geomorphology). These methodologies are applied to two case study areas representing newer (Wollongong, NSW) and older (South Solitary Islands, NSW) MBES datasets to assess the transferability of classification techniques across input data of varied quality. The suite of seabed classifications produced by this study provide fundamental baseline data on seabed shape, complexity, and composition which will inform regional risk assessments and provide insights into biodiversity and geodiversity.

Semi-Automatic Versus Manual Mapping of Cold-Water Coral Carbonate Mounds Located Offshore Norway

Cold-water coral reefs are hotspots of biological diversity and play an important role as carbonate factories in the global carbon cycle. Reef-building corals can be found in cold oceanic waters around the world. Detailed knowledge on the spatial location and distribution of coral reefs is of importance for spatial management, conservation and science. Carbonate mounds (reefs) are readily identifiable in high-resolution multibeam echosounder data but systematic mapping programs have relied mostly on visual interpretation and manual digitizing so far. Developing more automated methods will help to reduce the time spent on this laborious task and will additionally lead to more objective and reproducible results. In this paper, we present an attempt at testing whether rule-based classification can replace manual mapping when mapping cold-water coral carbonate mounds. To that end, we have estimated and compared the accuracies of manual mapping, pixel-based terrain analysis and object-based image analysis. To verify the mapping results, we created a reference dataset of presence/absence points agreed upon by three mapping experts. There were no statistically significant differences in the overall accuracies of the maps produced by the three approaches. We conclude that semi-automated rule-based methods might be a viable option for mapping carbonate mounds with high spatial detail over large areas.

Seabed domes with circular depressions in the North Yellow Sea

Numerous isolated shallow seabed domes with depressions were discovered in the North Yellow Sea in a high-resolution multibeam echosounder survey. Twelve domes were located within a 11 km by 17 km survey area. The domes in this area were between 0.1 m and 1.0 m high with diameters ranging from 250 to 1 700 m. They were surrounded by corresponding depressions and displayed characteristic shapes ranging from circular to elliptical. The acoustic anomalies displayed on the high resolution sub-bottom profile images demonstrated that shallow gas accumulated just beneath these domes, suggesting that the migration and accumulation of the shallow gas caused the formation of these features. Accumulation atop the seabed domes and erosion within the depressions, as indicated by the lithological features of the sediment cores, suggested that seabed bottom currents also played a role in the formation and evolution of these features. In addition, the simultaneous decrease in the chlorite and sulfate content in core d pore water samples confirmed the presence of submarine fresh/brackish groundwater, which is of interest for hydrological budgets and may be a significant component of the regional hydrologic balance. Our findings suggested that shallow gas accumulation and migration are likely coupled to localized groundwater discharge at this domed site.

Ring-shaped morphological features and interpreted small seamounts between southern Quebec (Canada) and the New England seamounts (USA) and their possible association with the New England hotspot track

Enhancements of recently available high-resolution multibeam echosounder data from the western Gulf of Maine and Atlantic continental margin and light detection and ranging (LiDAR) and digital elevation model data from southeastern Quebec (Canada) and the northeastern United States have revealed numerous ring-shaped morphological features and interpreted small seamounts between the Monteregian Hills igneous province and the New England seamounts. The morphological features onshore are mainly ring-shaped depressions, several of which surround mapped igneous intrusions in the Monteregian Hills igneous province and White Mountain magma series. Most of the rings offshore are also depressions, although a few rings are curved ridges above the seafloor. The largest ring in the western Gulf of Maine is the 30-km-diameter Tillies ring that lies 20 km east of Cape Ann, MA. Several small (<3 km in diameter) round, flat-topped submerged hills that we interpret to be volcanic necks are also present beneath the western Gulf of Maine. The rings between Cape Cod and the continental slope are more subtle because of thicker sediments and poorer spatial resolution of the sonar data in this area. The southernmost ring-shaped features are located on the continental slope and upper continental rise and coincide with the northwestern end of the New England seamount chain. The concentration of these features between the Monteregian Hills igneous province and the New England seamounts suggests that they are igneous features that may be associated with the New England hotspot track.

Advances in assessing Sabellaria spinulosa reefs for ongoing monitoring.

Standardized and repeatable data acquisition and analyses are required to enable the mapping and condition monitoring of reefs within Marine Protected Areas (MPAs). Changes in habitat condition must be reliably identified and reported to best support evidence‐based management. Biogenic reefs in temperate waters, that is, hard matter created by living organisms and raised above the seabed, provide food and shelter for many plant and animal species. This article explores the feasibility of habitat mapping, using remote sensing datasets, as well as metrics for repeatable and suitable assessment of areas of Sabellaria spinulosa for their status as biogenic reef. Data were gathered within the North Norfolk Sandbanks and Saturn Reef candidate Special Area of Conservation/Site of Community Importance in the southern North Sea. Six study areas were identified as potential locations of biogenic reef using previously acquired data, and these were targeted for further investigation using a combination of high resolution multibeam echosounder and sidescan sonar. Where potential S. spinulosa was identified from the acoustic data, a drop‐down camera system was employed for visual verification. Areas of known and potential S. spinulosa reef were mapped successfully at two of the six study areas, although future approaches should take careful consideration of the seabed morphology and predominant habitat backdrop to successfully interpret such data. Camera tows from S. spinulosa reef areas were broken up into 5‐s segments, with each segment scored for (a) average tube elevation; (b) average percentage cover; and (c) for the presence or absence of S. spinulosa. These metrics were utilized to create summary statistics, including a value of patchiness derived from presence/absence data, that is recommended for application as part of future monitoring programs. The application of this methodology could benefit wider assessments of similar threated or declining habitats such as intertidal Mytilus edulis beds on mixed and sandy sediments, Maerl beds, Modioulus modiolus beds, Ostrea edulis beds, and Zostera beds where patchiness may also be considered of environmental importance.

Morphological characterization of submarine slope failures in a semi-enclosed fjord, Frobisher Bay, eastern Canadian Arctic

AbstractSubmarine slope failures in the nearshore waters of SE Baffin Island, eastern Canadian Arctic, present a challenge to coastal and seabed development. Submarine slope failures are a known geohazard in fjords in Norway, Chile, Alaska, British Columbia and elsewhere, but have received little attention in the coastal waters of Arctic Canada. Over the past 6 years, there has been a rapid expansion of multibeam echosounder (MBES) mapping in Canadian Arctic fjords, leading to the discovery of many submarine slope failures. One area that has been mapped in detail is inner Frobisher Bay. This macrotidal, seasonally ice-covered, semi-enclosed embayment has a glacially scoured bed, ice-contact deposits, including recessional moraines, and stratified glaciomarine and post-glacial silts and clays with abundant dropstones. The prevalence of submarine slope failures in the inner bay (one per 20 km2) appears to be anomalous. To date, MBES mapping has imaged at least 246 failures, ranging in size from 0.007 to 2.1 km2 and all within the glaciomarine and post-glacial succession. Morphometric analysis of these features based on high-resolution MBES bathymetry provides an insight into their spatial distribution, relative chronology, triggers and flow characteristics; factors essential to understanding the mechanisms underlying their abundance in this Canadian Arctic fjord.