This review examines the various strategies and methods used to produce benthic habitat maps using acoustic remote sensing techniques, coupled with in situ sampling. The applications of three acoustic survey techniques are examined in detail: single-beam acoustic ground discrimination systems, sidescan sonar systems, and multi-beam echo sounders. Over the past decade we have witnessed the nascence of the field of benthic habitat mapping and, on the evidence of the literature reviewed in this paper, have seen a rapid evolution in the level of sophistication in our ability to image and thus map seafloor habitats. As acoustic survey tools have become ever more complex, new methods have been tested to segment, classify and combine these data with biological ground truth sample data. Although the specific methods used to derive habitat maps vary considerably, the review indicates that studies can generally be categorized into one of three over-arching strategies; 1) Abiotic surrogate mapping; 2) Assemble first, predict later (unsupervised classification); 3) Predict first, assemble later (supervised classification). Whilst there is still no widely accepted agreement on the best way to produce benthic habitat maps, all three strategies provide valuable map resources to support management objectives. Whilst there is still considerable work to be done before we can answer many of the outstanding technological, methodological, ecological and theoretical questions that have been raised here, the review concludes that the advent of spatial ecological studies founded on high-resolution environmental data sets will undoubtedly help us to examine patterns in community and species distributions. This is a vital first step in unraveling ecological complexities and thus providing improved spatial information for management of marine systems.

Benthic habitat mapping: A review of progress towards improved understanding of the spatial ecology of the seafloor using acoustic techniques

Image-based classification of multibeam sonar backscatter data for objective surficial sediment mapping of Georges Bank, Canada

Marine habitat mapping provides information on seabed substrata and faunal community structure to users including research scientists, conservation organizations, and policy makers. Full-coverage acoustic data are frequently used for habitat mapping in combination with video groundtruth data in either a supervised or unsupervised classification. In this investigation, video ground-truth data with a camera footprint of 1 m2 were classified to level 4 of the European Nature Information System habitat classification scheme. Acoustic data with a horizontal resolution of 1 m2 were collected over an area of 130 km2 using a multibeam echosounder, and processed to provide bathymetry and backscatter data. Bathymetric derivatives including eastness, northness, slope, topographic roughness index, vector rugosity measure, and two measures of curvature were created. A feature selection process based on Kruskal–Wallis and post hoc pairwise testing was used to select environmental variables able to discriminate ground-truth classes. Subsequently, three datasets were formed: backscatter alone (BS), backscatter combined with bathymetry and derivatives (BSDER), and bathymetry and derivatives alone (DER). Two classifications were performed on each of the datasets to produce habitat maps: maximum likelihood supervised classification (MLC) and ISO Cluster unsupervised classification. Accuracy of the supervised habitat maps was assessed using total agreement, quantity disagreement, and allocation disagreement. Agreement in the unsupervised maps was assessed using the Cramer’s V coefficient. Choice of input data produced large differences in the accuracy of the supervised maps, but did not have the same effect on the unsupervised maps. Accuracies were 46, 56, and 49% when calculated using the sample and 52, 65, and 51% when using an unbiased estimate of the population for the BS, BSDER, and DER maps, respectively. Cramer’s V was 0.371, 0.417, and 0.366 for the BS, BSDER, and DER maps, respectively.

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An evaluation of supervised and unsupervised classification techniques for marine benthic habitat mapping using multibeam echosounder data

Coastal marine environments are contaminated globally with a vast quantity of unexploded ordnance and munitions from intentional disposal. These munitions contain organic explosive compounds as well as a variety of metals, and represent point sources of chemical pollution to marine waters. Most underwater munitions originate from World Wars at the beginning of the twentieth century, and metal munitions housings have been impacted by extensive corrosion over the course of the following decades. As a result, the risk of munitions-related contaminant release to the water column is increasing. The behavior of munitions compounds is well-characterized in terrestrial systems and groundwater, but is only poorly understood in marine systems. Organic explosive compounds, primarily nitroaromatics and nitramines, can be degraded or transformed by a variety of biotic and abiotic mechanisms. These reaction products exhibit a range in biogeochemical characteristics such as sorption by particles and sediments, and variable environmental behavior as a result. The reaction products often exhibit increased toxicity to biological receptors and geochemical controls like sorption can limit this exposure. Environmental samples typically show low concentrations of munitions compounds in water and sediments (on the order of ng/L and μg/kg, respectively), and ecological risk appears generally low. Nonetheless, recent work demonstrates the possibility of sub-lethal genetic and metabolic effects. This review evaluates the state of knowledge on the occurrence, fate, and effect of munition-related chemical contaminants in the marine environment. There remain a number of knowledge gaps that limit our understanding of munitions-related contaminant spread and effect, and the need for additional work is made all the more urgent by increasing risk of release to the environment.

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Spread, Behavior, and Ecosystem Consequences of Conventional Munitions Compounds in Coastal Marine Waters

Military activities have been conducted on land and at sea. Both during conflicts and in peace time, some regions served as a military training ground which included firing positions and bunkers. Mercury fulminate has been used in ammunition primers and detonators. Certain amount of ammunition was dumped into the Baltic Sea after the Second World War. Because of corroded containers, mercury can be released into the marine environment. The soil and sediment samples were taken from military training grounds, southern Baltic in 2014 and 2015. The concentration of mercury was determined by AMA-254 analyzer. Hg concentration was higher in the places of military activities, as compared to other areas. Ten times increased concentration of Hg was determined in soil sample collected in area of active gun range compared to the reference station. The significant higher concentration of mercury was detected in stations where chemical warfare agents were found.

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The impact of military activities on the concentration of mercury in soils of military training grounds and marine sediments

The Irish Sea, like many marine areas, is threatened by anthropogenic activities. In particular the Pisces Reef system, a series of smothered rocky reefs are subject to fishing pressures as a result of their position within a Nephrops norvegicus fishery. In an area of sediment deposition and retention the reefs modify the environment by increasing the energy of near-bottom currents which results in localised scouring. This is the first study to attempt to characterise and investigate the ecological functioning of the Pisces Reef system. A multidisciplinary approach was essential for accurate investigation of the area. To facilitate more effective management of the benthic habitats of the Reef system, this study integrates acoustic, seismic, grab sampling and video ground-truthing methods for benthic habitat discrimination. Orientation of the scour hollows also suggest that seabed features could be used to infer dominant flow regimes such as the Irish Sea Gyre. The data revealed significant geology–benthos relationships. A unique biotope was described for the reef habitat and it was demonstrated that scouring may influence community composition through disturbance mechanisms. This study provides preliminary information required for management of a unique habitat within a uniform region.

The impact of scour processes on a smothered reef system in the Irish Sea

Beaufort's Dyke is a submarine depression located in the North Channel of the Irish Sea. With a maximum depth of 312 m, the dyke is one of the deepest areas within the European continental shelf. Integration and interpretation of 450 km of sparker seismic data and full-coverage bathymetric data derived from multi-beam echo sounder surveys allow for the investigation of the formation processes of Beaufort's Dyke and the evolution of geomorphological features within it. The dyke, formed by composite subglacial processes dominated by subglacial meltwater discharge, is interpreted as a tunnel valley. The regional isolation of Beaufort's Dyke may be explained by the bounding of the North Channel by the bedrock masses of Ireland and Scotland, coupled with the exploitation of structural weakness along a fault plane and presence of halite in the eroded substrate enhancing the erosive potential of the overlying glacier. Beaufort's Dyke has probably been maintained as an open feature by strong rectilinear tidal currents. The morphology of lunate sediment waves and a large parabolic bedform towards the south of the dyke contradict the observed dominant S–N mean hydrodynamic flow recorded within the North Channel, suggesting an alternative hydrodynamic regime either within the dyke or during bedform creation

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Alexander Callaway

Papers

The impact of scour processes on a smothered reef system in the Irish Sea

The formation and evolution of an isolated submarine valley in the North Channel, Irish Sea: an investigation of Beaufort's Dyke

Trace metal contamination of Beaufort’s Dyke, North Channel, Irish Sea: A legacy of ordnance disposal