Limitations of Boulder Detection in Shallow Water Habitats Using High-Resolution Sidescan Sonar Images

Gitta Von Rönn,Christian Winter,Hans-Christian Reimers,Klaus Schwarzer

doi:10.3390/geosciences9090390

Gitta Von Rönn, Christian Winter + Show 2 more

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https://doi.org/10.3390/geosciences9090390

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Abstract

Stones and boulders in shallow waters (0–10 m water depth) form complex geo-habitats, serving as a hardground for many benthic species, and are important contributors to coastal biodiversity and high benthic production. This study focuses on limitations in stone and boulder detection using high-resolution sidescan sonar images in shallow water environments of the southwestern Baltic Sea. Observations were carried out using sidescan sonars operating with frequencies from 450 kHz up to 1 MHz to identify individual stones and boulders within different levels of resolution. In addition, sidescan sonar images were generated using varying survey directions for an assessment of range effects. The comparison of images of different resolutions reveals considerable discrepancies in the numbers of detectable stones and boulders, and in their distribution patterns. Results on the detection of individual stones and boulders at approximately 0.04 m/pixel resolution were compared to common discretizations: it was shown that image resolutions of 0.2 m/pixel may underestimate available hard-ground settlement space by up to 42%. If methodological constraints are known and considered, detailed information about individual stones and boulders, and potential settlement space for marine organisms, can be derived.

Highlights

Stones and boulders in shallow waters form important geo-habitats in which many species coexist.they are important contributors to coastal biodiversity and high benthic production.These habitats provide valuable feeding areas, and spawning and nursing grounds for fish [1,2,3].Human impact on coastal habitats is increasing and benthic communities living in these environments are affected [4,5,6,7]
We show the limitations using high-resolution SSS data to identify individual stones and boulders in shallow water, in order to contribute to the discussion on: (a)
The backscatter mosaic of the study area combined with the ground truthing and sediment analysis indicates two sedimentological zones

Summary

Introduction

Stones and boulders in shallow waters form important geo-habitats in which many species coexist.they are important contributors to coastal biodiversity and high benthic production.These habitats provide valuable feeding areas, and spawning and nursing grounds for fish [1,2,3].Human impact on coastal habitats is increasing and benthic communities living in these environments are affected [4,5,6,7]. Stones and boulders in shallow waters form important geo-habitats in which many species coexist They are important contributors to coastal biodiversity and high benthic production. The need for mapping and understanding small-scale seabed features like stone and boulder assemblages, combined with habitat assessment, is growing significantly, as European legislations and directives are demanding the documentation and monitoring of environmental states [9]. Due to their valuable characteristics, hard-bottom substrates (e.g., stones and boulders) are protected in the Baltic Sea by the European Habitats Directive

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