Hydroacoustic Mapping of Geogenic Hard Substrates: Challenges and Review of German Approaches

Abstract

Subtidal hard substrate habitats are unique habitats in the marine environment. They provide crucial ecosystem services that are socially relevant, such as water clearance or as nursery space for fishes. With increasing marine usage and changing environmental conditions, pressure on reefs is increasing. All relevant directives and conventions around Europe include sublittoral hard substrate habitats in any manner. However, detailed specifications and specific advices about acquisition or delineation of these habitats are internationally rare although the demand for single object detection for e.g., ensuring safe navigation or to understand ecosystem functioning is increasing. To figure out the needs for area wide hard substrate mapping supported by automatic detection routines this paper reviews existing delineation rules and definitions relevant for hard substrate mapping. We focus on progress reached in German approval process resulting in first hydroacoustic mapping advices. In detail, we summarize present knowledge of hard substrate occurrence in the German North Sea and Baltic Sea, describes the development of hard substrate investigations and state of the art mapping techniques as well as automated analysis routines.

Dynamics of Stone Habitats in Coastal Waters of the Southwestern Baltic Sea (Hohwacht Bay)

“…Other characterizations of physical habitat structures were not carried out [24,26,71]. Prior investigations focused on lag deposits and stone habitats were described as varying unsorted coarse-grained sediments ranging from gravel to boulders [14,30,61]. This is in accordance with results from SA2, where the habitat-forming cobbles and boulders are part of an area dominated by lag deposits.…”

“…In the Baltic Sea, they belong to those habitats with the most diverse community types, providing various ecosystem services [11,12]. Conservation, protection, and suitable management of these environments require precise knowledge about their occurrence, structure and spatial diversity, their status, and their environmental conditions [13,14]. This system knowledge also informs reporting to guidelines such as the Marine Strategy Framework Directive (MSFD, 2008/56/EC) [15] and the Habitat Directive (HD 92/43/EEC annex 1 1170-reefs) [16,17].…”

A State-Of-The-Art Perspective on the Characterization of Subterranean Estuaries at the Regional Scale

“…There are several techniques available to map the seafloor. Sidescan sonar data can reveal the location of pockmarks (e.g., Schlüter et al, 2004;Virtasalo et al, 2019), and echo sounders generally can also be applied for this task (Feldens et al, 2018;Papenmeier et al, 2020). Multibeam echo sounders provide high lateral coverage across-track on the seafloor, which is several times the water depth, enabling a fast mapping of larger scales.…”

Super Resolution by Deep Learning Improves Boulder Detection in Side Scan Sonar Backscatter Mosaics

“…In such backscatter intensity mosaics, boulders are recognized by a characteristic backscatter pattern. The typical resolution of backscatter mosaics in practical applications in the German Baltic Sea and North Sea is 0.25 m to 1.0 m. This is in contrast to the aims of European habitat mapping guidelines to describe all objects exceeding a diameter of 0.063 m, the sedimentological transition from gravel to cobbles, as potential hard ground that needs to be detected [5]. Obviously, objects of such size cannot be detected in currently available datasets, and the widespread application of new technology such as synthetic aperture sonar [7], strongly increasing the resolution of bathymetric grids and backscatter intensity mosaics especially in the along-track direction, is required.…”

“…The need for improving the resolution of images is widespread, including tasks in medical applications, object detection, or remote sensing [1]. In marine habitat mapping by acoustic remote sensing specifically, a recent topic of interest is the detection of individual boulders for purposes of hard ground delineation for marine spatial planning purposes as well as ecosystem research [2][3][4][5]. The detection of boulders is typically based on the interpretation of backscatter intensity mosaics derived from acoustic remote sensing by side scan sonars or multibeam echo sounders [6].…”