The invasive amphipod Gammarus tigrinus was found for the first time and in abundant populations in shallow-water habitats in coastal areas of the city of Helsinki (Gulf of Finland, Baltic Sea). In summer-autumn 2007 the species occurred at four out of seven study sites, dominating almost exclusively at one site and comprising a progressively increasing share of the local gammarid community at another site with a reducing portion of the native Gammarus zaddachi during the study period. The

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... es did not occur at the least polluted archipelago stations, which are also most exposed to wave action. With only single previous observations of this species in Finland the current findings show that G. tigrinus has now firmly established itself into the northern Baltic littoral ecosystem. As this exceedingly omnivoric species is able to outcompete and replace native herbivorous Gammarus species its environmentally detrimental effects may include lesser consumption of e.g. filamentous macroalgae, which are already highly abundant in the study region due to eutrophication.