Impact of Sparse Benthic Life on Seafloor Roughness and High-Frequency Acoustic Scatter

Schönke Schönke,Wilken Wilken,Schulze Schulze,Papenmeier Papenmeier,Feldens Feldens,Darr Darr,Wiesenberg Wiesenberg

doi:10.3390/geosciences9100454

Schönke Schönke, Wilken Wilken + Show 5 more

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https://doi.org/10.3390/geosciences9100454

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Abstract

Quantitative acoustic marine habitat mapping needs to consider the impact ofmacrobenthic organisms on backscatter data. However, the sensitivity of hydroacoustic systems toepibenthic life is poorly constrained. This study explores the impact of a benthic community withsparse abundance on seafloor microroughness and acoustic backscatter at a sandy seafloor in theGerman North Sea. A multibeam echo sounder survey was ground-truthed by landermeasurements combining a laser line scanner with sub-mm resolution and broad-band acoustictransducers. Biotic and abiotic features and spatial roughness parameters were determined by thelaser line scanner. At the same locations, acoustic backscatter was measured and compared with anacoustic scatter model utilizing the small-roughness perturbation approximation. Results of thelander experiments show that a coverage with epibenthic features of 1.6% increases seafloorroughness at spatial wavelengths between 0.005–0.03 m, increasing both spectral slope andintercept. Despite the fact that a strong impact on backscatter was predicted by the acoustic modelbased on measured roughness parameters, only a minor (1.1 dB) change of backscatter was actuallyobserved during both the lander experiments and the ship-based acoustic survey. The results of thisstudy indicate that benthic coverage of less than 1.6% is insufficient to be detected by currentacoustic remote sensing.

Highlights

Habitat mapping using acoustic remote sensing has become an important topic for marine spatial management and research purposes [1,2,3,4], increasingly including the detection of benthic life
The results of an optical–acoustic lander experiment in a sand environment offshore Sylt island (German North Sea) show that sparse benthic life dominated by tubeworms, shell fragments, and brittlestars (1–5% seafloor coverage) impacts seafloor roughness at spatial wavelengths of 0.005 to
Backscatter strengths modeled by the small-roughness perturbation approximation are strongly influenced by sparse benthic organism abundance

Summary

Introduction

Habitat mapping using acoustic remote sensing has become an important topic for marine spatial management and research purposes [1,2,3,4], increasingly including the detection of benthic life. The acoustic backscatter intensity is affected by the sensor geometry relative to the seafloor and physical characteristics of the sediment–water interface, the water column characteristics such as stratification, the applied acoustic frequency, and the shallow subsurface [5]. The physical properties of the seafloor interface are influenced by the presence of benthic life. Benthic life, including macrofauna [6,7] and macroflora [8], are biological activities that change physical sediment properties [9,10,11]. The impact of benthic life on acoustic backscatter is both a challenge and an opportunity for environmental remote sensing.

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