Geological Structure and Natural Dynamics of Stone Habitats in Shallow Waters of the Southwestern Baltic Sea

Stone habitats belong to the most valuable habitat types, as they provide three-dimensional settlement space, food, and nursery grounds for many organisms. The state of coastal waters, and especially within the Baltic Sea, has deteriorated due to increasing human pressures, resulting in dramatic changes in coastal ecosystems, impacting habitat types and associated benthic communities. Due to the ongoing deterioration of natural habitats, preservation, protection, and improvement of the environmental quality are at the forefront of research and policy agendas. The conservation and protection of habitat types listed in the European NATURA 2000 network, Annex І of the Habitats Directive (HD) (Council Directive 92/43/EEC) is one of the main goals of the Biodiversity Strategy of the European Union, aiming at improving the status of the marine environment and particularly of the coastal waters. To achieve these goals, efficient protection of important marine habitats and communities, knowledge on their spatial distribution, natural dynamics and ecosystem interrelationships are mandatory. Understanding how shallow water habitats change in response to natural environmental variations is necessary to reliably assess how habitats change due to additional human-induced pressures. This is especially important in areas such as the Baltic Sea, where the natural variability creates substantial environmental gradients over different spatial and temporal scales. This thesis explores the variability of stone habitats within the southwestern Baltic Sea and their physical characteristics, which are considered as important drivers of benthic assemblages living within these habitats. The relationship of geological and biological diversity is examined at local and regional scales. By recognizing and explicitly characterizing spatial heterogeneity in marine systems, distinct relationships between geological and biological processes can be defined and explained. Improved detection and identification of the heterogeneous habitat structures yields reliable habitat maps. The related findings, concerning methodological limitations in cobble and boulder detection, using high-resolution sidescan sonar images in shallow water environments, reveal considerable discrepancies in the numbers and distribution patterns of detectable cobbles and boulders. Results on the detection of individual stones and boulders in high-resolution images indicate, if methodological constraints are known and considered, detailed information about individual stones and boulders, and potential settlement space for marine organisms, can be derived. Through multidisciplinary approaches, combining hydroacoustic data, sediment composition analysis, in situ diver measurements, benthic community analysis and the characterisation of the local wave climate, deeper insights are revealed into the function and dynamics of stone habitats. Determining changes in spatial distribution and habitat structure provide a practical approach to understand ecosystem dynamics, which has useful applications in conservation and resource management. Investigations on the origin, physical shape, and structure of habitat-forming cobbles and boulders and an assessment of their dynamics in coastal environments of the southwestern Baltic Sea were conducted. Dynamic processes act on the physical habitat structure and induce a change in the colonization potential of cobbles and boulders. Results show, that within the shallow waters of the southwestern Baltic Sea, stone habitats are not limited to lag deposits, as previously thought, and need to be described as dynamic environments. This thesis highlights the need to consider geological structures and natural dynamics shaping stone habitat complexity at multiple scales, providing tools and framework to do so. These improvements in understanding the relationships between biological and geological habitat features support actively the management of habitats.