Differences in hydroacoustic backscatter signals and epifauna growth in a stony and coarse grain habitat ("Helgolaender Steingrund", German Bight, North Sea)

Abstract

There are a variety of methods used to observe stony and coarse grain habitats. Many of these, like video recordings by scuba divers, in situ recordings of macrofauna and photographic documentation, are expensive and complex. For geological analysis, grab samples are quite common, but represent just a small range of the actual habitat. Quantitative sampling of macrofauna is difficult. Thus is it important to develop new methods that are less expensive and can cover larger areas. This study presents the first results of a combination of side-scan sonar data and drop camera records to study the presence and absence of macrofauna as well as the sediment grain size distribution in a stony and coarse grain habitat in the "Helgolaender Steingrund". The "Helgolaender Steingrund" is a stony and natural coarse grain habitat of ~159 ha, 11 km east-northeast of Helgoland island in the German Bight (North Sea). It is included in the Flora-Fauna-Habitat guidelines and is part of the German Natura 2000 areas. Sampling was carried out during three different cruises from May to August 2016 with RV "SENCKENBERG". Side-scan sonar and drop camera records show a quite heterogeneous habitat. Several areas comprising of different living communities were correlated to sediment grain size. Larger organisms, such as the European lobster (Homarus gammarus), the brown crab (Cancer pagurus) and the edible sea urchin (Echinus esculentus) were identified. Additionally, a cover of Bryozoan species such as Flustra foliacea and Alcyonidium diaphanum could clearly be discerned. In conclusion, the drop camera and side-scan sonar proved to be reliable methods for the analysis of stony and coarse grain habitats such as the "Steingrund". Further analysis on the gathered data has yet to be done.