Thriving reefs in the Baltic? Diversity and small-scale variability of hard-bottom assemblages along natural and anthropogenic gradients in the German Baltic Sea

Since the vast majority of global seafloor habitats are characterized by soft sediments, hard substrata represent rarities hosting species and functional groups not found elsewhere. The same holds true for the enclosed and brackish Baltic Sea, where hard substrata occur patchy and infrequent. Subtidal low-relief terrains with homogenous mud and sand flats form the predominant benthic substrate and the associated infaunal communities are well described. In contrast, the diverse, primarily epibenthic assemblages on subtidal hard bottoms received far less attention.

Since 2011 a team of geologists and biologists aims to map the subtidal habitats and biotopes. On joint cruises, geologists use side scan sonar to map the seafloor sediments while ground truthing is performed in collaboration with biologists. Biogenic concretions like mussel beds and hard structures of geogenic origin like stones and glacial boulders form the predominant hard substrata in the German Baltic Sea. These habitats are subsequently investigated using frame sampling by SCUBA diving, dredges or towed photo/video platforms. The type of hard substratum, salinity and light availability seem to be the most important natural factors that determine the epibenthic community composition.

Identified geological substrata and biological communities are matched with biotopes of the HELCOM Underwater biotope and habitat classification system. Predictive modeling approaches are used to generate biotope specific distribution patterns, based on biological point samples, area-wide sediment distribution maps and measured/modeled environmental parameters.

The resulting hard-bottom biotope maps, combining geological and biological information, complement the existing area-wide biotope map of soft sediment communities. The maps are important tools both for the scientific community to understand the functioning of marine ecosystems as well as for nature conservation, e.g. for the implementation of MPA management plans and for the development of monitoring concepts and the red list classification of biotopes.