Establishment of new gammarid species in the eastern Gulf of Finland (Baltic Sea) and their effects on littoral communities

During the last ten years two new species of gammarids, Gmelinoides fasciatus (Stebb.) and Pontogammarus robustoides (Sars), invaded the Gulf of Finland. We studied the distribution and abundance of these species and their effects on littoral communities in the Neva estuary in 1998-2002. The study was performed in three types of habitats: Phragmites australis beds, exposed stone littoral, and Cladophora glomerata mats on stones. Feeding rates of the gammarids consuming filamentous algae and benthic invertebrates were estimated experimentally. Intensive consumption of plant food (Cladophora, roots and debris of Phragmites) by gammarids resulted in the production of faeces that increases organic matter availability for benthic detritivores. At the same time, gammarid predation resulted in a decrease of benthic invertebrates. The magnitude of the effects depended on the density of the established species and the type of habitat. In addition, an experimental study of interactions between G. fasciatus and P. robustoides revealed the existence of intraguild predation, which was the strongest in the case of P. robustoides.