Benthic communities in relation to wave exposure and spatial positions on sublittoral boulders in the Baltic Sea

Boulders are an important substrate for Baltic Sea benthic communities. Although previous studies have examined benthic species on sublittoral boulders in the Baltic Sea, information on benthic assemblages and how they vary in relation to structuring factors is limited. The aim of the present study was to describe the benthic communities, including both algae and animal taxa, on sublittoral boulders in relation to spatial positions on the boulders (4 levels) and differences in wave exposure (2 levels). The multivariate results, including 29 algae and 25 animal taxa, from the field sampling showed a significant interaction between the 2 main factors. Changes in the biomass of both annual an perennial macroalgae as well as Mytilus edulis and Balanus improvisus, were found among different positions on wave-exposed boulders, a pattern which was less clear on boulders from sheltered sites, showing that water movement influences small-scale spatial patterns on sublittoral boulders. Apart from water movement, the physical factors substrate slope and light intensity were also associated with the small-scale patterns found in benthic communities. The difference in these factors may influence both settlement and post-settlement processes for both algae and animals, and possible mechanisms leading to the patterns found are discussed.