Seasonal and multi-annual patterns of colonisation and growth of sessile benthic fauna on artificial substrates in the brackish low-diversity system of the Baltic Sea

Abstract

Although benthic succession is well understood, the growth of assemblages does not follow the same progression across environmental variables and differs among coastal ecosystems. This study investigates the seasonal and multi-annual patterns of development of sessile invertebrate assemblages and the effects of environmental variables and substrate orientation (topsurface vs. undersurface) on this process. Perspex panels deployed on the seafloor horizontally were monitored seasonally from March 2008 to March 2010 (two locations) and yearly from March 2010 to April 2015 (one location) in the southern Baltic Sea. All faunal taxa occurred simultaneously in the first six months of immersion, but no clear sequence of colonising species was detected. Seasonal occupation of free space coincided with increased primary production in the water column and was driven by recruitment timing and intensity, and the growth rates of recruits. More diverse and numerous assemblages developed on the panel undersurfaces presumably because of reduced physical disturbance. After 3 years of continuous immersion, the assemblage composition, but not its abundance, became stable and convergent towards the natural surrounding communities, which indicated the advanced successional stage. The rate of assemblage development was fast which can be attributed to weak interspecific competitive interactions and reduced feeding interferences among benthic fauna.