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Detection of Kelp Vegetation off Helgoland (SE North Sea)Using the Acoustic Ground-Discrimination System RoxAnn

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Citation:
Mielck, F. , Bartsch, I. , Bürk, D. and Hass, H. C. (2012): Detection of Kelp Vegetation off Helgoland (SE North Sea)Using the Acoustic Ground-Discrimination System RoxAnn , GV & Sediment Meeting, Hamburg, 23 September 2012 - 28 September 2012 .
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Abstract:

The seafloor of the SE North Sea is mostly characterized by unconsolidated sandy sediments of Tertiary, Pleistocene and early Holocene origin. However, Helgoland and the surrounding coastal waters are different. Hardrock ridges crop out and in between the ridges sandy to gravelly sediments occur. The rocky seafloor forms an ideal environment for macroalgae and associated organisms which dominate the vegetation along a depth gradient down to 10 - 12 m below sea chart. Kelps provide habitat and shelter for a wide variety of associated seaweeds and invertebrates and serve as natural coastal protection. Hence, observation of these habitats is of great interest. The aim of this study is (1) to determine the acoustic signatures of the kelp vegetation for automated mapping purposes using the single-beam seafloor-classification system RoxAnn (Model GD-X) and (2) to map the spatial distribution and variable densities of the kelp populations and other seaweed dominated communities within two areas in the North and in the South off Helgoland. The hydroacoustic survey was performed in June 2011 during times of high tide. Altogether 32 transects across the investigation area were recorded. RoxAnn works with a frequency of 200 kHz and measure hardness (soft to hard) and roughness (smooth to rough) properties of the seafloor as well as water depth. For positioning a Leica 1200 differential GPS was used. A Kongsberg underwater camera was utilized to ground-truth the acoustic data on 13 video transects. Additionally to the video transects ground truthing was achieved via georeferenced diving transects which provided detailed information on the percentage ground coverage of kelp species, red algae vegetation or other substrates. The whole acoustic data set consists of ~ 45,000 measurements. On the basis of the acoustic and ground-truth data different habitats were distinguished: Gravel fields and hardrock outcrops are characterized by high hardness parameters while sandy seafloor illustrates rather smooth signatures. However, the signature of kelp vegetation is rough and soft while a high density of the kelps correlates to high roughness value. Hence, the occurrence of vegetation was classified in four categories including (1) no, (2) sparse, (3) medium and (4) dense vegetation. Since RoxAnn only provides point data and no spatial information areas between the surveyed transects were interpolated. The resulting map reveals a number of small elongated kelp fields south off Helgoland Dune and two big dense kelp fields in the North as well as several smaller kelp accumulations. In conclusion, a rapid habitat mapping was carried out by hydroacoustic means showing an accurate spatial distribution of the kelp population. It was even possible to differentiate between the varying densities of the kelp fields.

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