Elena Ezhova¹, Ludwik Żmudziński² and Krystyna Maciejewska^3
1 P. P. Shirshov Institute Oceanology RAS, 1 Pr. Mira, Kaliningrad, 236000, Russia
² Pomeranian Pedagogical Academy in Słupsk, 76-200 Słupsk, Poland
³ Sea Fisheries Institute, Kołłątaja 1, 81-322 Gdynia, Poland

Key words: Vistula Lagoon, zoobenthos, species composition, biomass distribution, eutrophication, Marenzelleria cf. viridis.

Abstract.
Drastic changes of the macrobenthos species composition was noted in the Vistula Lagoon during the past century. Some species of bottom invertebrates, previously widely distributed in the lagoon, are either now rare or have disappeared. The biomass of zoobenthos increased several-fold this period. Until the end of the 1980s, the zoobenthos of the open waters of the lagoon did not exceed 20-36 g � m^-2, but it increased to 81-103 g � m^-2 in the 1990s. Simultaneously, the thickness of the inhabited sediment layers increased from 10 to 20-25 cm. The spatial patterns of benthos biomass, both of vertical distribution in the sediment layer and horizontal distribution, changed significantly, mainly due to the introduction of the North American spionid polychaete Marenzelleria cf. viridis. The relations of these trends to hydrological changes, eutrophication, and bioinvasion are discussed.