Connectivity and complexity of coastal lakes as determinants for their restoration – A case study of the southern Baltic Sea

Abstract To understand functional mechanisms and ecological conditions of the Baltic coastal lakes (BCL) for the need of their restoration, the current state of knowledge on environmental, ecological, and evolutionary processes responsible for biological diversity is presented. This has allowed the classification of coastal lakes along the southern Baltic Sea coast as freshwater, transitional, and brackish ones. Principles of their functioning are explained here on the basis of the theory of alternative stable states in shallow lake ecosystems (brackish vs. freshwater) separated by adaptive cycles (transitional lakes). Successful concepts for the design and application of restoration of degraded coastal aquatic ecosystems is conditioned by both marine and terrestrial factors. Thus, the restoration plans of the southern Baltic coastal lakes, contrary to freshwater inland lakes, require some specific criteria to be applied due to (i) various connectivity of lakes with the sea, (ii) intrusion of seawater of low salinity as well as (iii) genetic heterogeneity of lake basins. Awareness of these findings makes it possible to trace and predict the threats posed to coastal lakes, as well. As an example, recommendations on the holistic restoration of Lake Jamno are presented. The presented data is useful for determining the state of preservation and methods of management of coastal aquatic habitats under high human pressure.