Predicting the occurrence of rocky reefs in a heterogeneous archipelago area with limited data

The lack of spatial distribution data on marine habitats often presents an obstacle to their protection. The Annex I of the Habitats Directive (European Council Directive 92/43/EEC) lists habitats that are important in biodiversity protection and should be maintained (or restored) to a favourable conservation status. The habitats listed should be protected within an ecological network of protected areas, the Natura 2000 network. However, in the past the establishment of the marine Natura 2000 network has been largely based on insufficient knowledge on the distribution of the habitats. Annex I habitat type reefs are defined as formations of hard compact biogenic or geogenic substrata, which arise from the seafloor in the sublittoral and littoral zone. As obtaining marine data is time-consuming and costly, the bathymetric and substratum data needed for their identification on a larger scale are often scarce. Furthermore, the use of data may be limited due to e.g. national security reasons. This study identifies reefs in a complex archipelago area in the northern Baltic Sea using the best, although limited, data currently available. In the area reefs are elevated rocky outcrops and the associated algal communities and blue mussel beds are vital in maintaining biodiversity in the relatively species poor Baltic Sea. In addition to identifying the physical reef structures, an estimate of their ecological value is obtained by modelling the distribution of four key species occurring on reefs. The results are encouraging, as 55 out of 68 of the potential reefs ground-truthed were confirmed to be reefs. Furthermore the number of predicted species occurring on the reefs, correlated significantly with the number of species observed. The presented maps serve as a valuable background for more detailed mapping of the species diversity occurring on reefs as well as for monitoring their ecological status. Map-based information on important habitats is essential in conservation and marine spatial planning to minimize human impact on marine ecosystems.