Large-scale species invasion into a low-diversity system: spatial and temporal distribution of the invasive polychaetes Marenzelleria spp. in the Baltic Sea

Invasive species are considered to be a major threat to both terrestrial and aquatic ecosystems worldwide due to their negative effects on biodiversity, ecosystem functioning and species interactions. This study investigates with the help of large-scale monitoring datasets the spatial and temporal distribution of a successful invader, the polychaete species complex Marenzelleria spp. Mesnil 1896 into a species-poor, brackish water ecosystem in northern Europe, the Baltic Sea. The species complex has spread throughout the entire Baltic Sea and has become a dominant member of the communities especially in deeper, hypoxia-affected areas. The dynamics of the invasion follow a boom-and-bust pattern, with abundances peaking almost simultaneously throughout the Baltic Sea in 2005-2006. Possible mechanisms behind the successful establishment are changes in hydrography and species composition. Hypothesized and observed ecological impacts are reviewed. Evidence suggests a multitude of potential, sometimes contrasting, impacts. Due to the widespread distribution and high abundances, the biology and ecology of Marenzelleria spp. in the Baltic Sea need to be investigated further in order to assess its impact on ecosystem structure and functioning.