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Kotta, J., H. Orav-Kotta, and I. Kotta, 2003. Effects of the Introduced Polychaete (Marenzelleria viridis) on the Simple Ecosystem of the Northern Baltic Sea, Estonian Marine Institute. Proceedings of the Third International Conference on Marine Bioinvasions, La Jolla, California, March 16-19, 2003, p. 74. Effects of the Introduced Polychaete (Marenzelleria viridis) on the Simple Ecosystem of the Northern Baltic SeaOwing to isolation, short developing time, low salinity and large fluctuations in temperature only a limited number of species have been adapted to the conditions of the Baltic Sea. Low species diversity together with the extensive anthropogenic impact make the region potentially invasion prone. A number of benthic animals presently living in the Baltic have only recently invaded the area, some only in the last decades or years. The North-American polychaete (Marenzelleria viridis) was introduced to Europe at the beginning of the 1980s. The polychaete has quickly spread to most parts of the North and Baltic seas. The species prevails in many coastal areas of the northern Baltic Sea. The establishment of M. viridis has been more successful either in more eutrophicated regions or in more uniform biotopes. Concurrent with this invasion the densities of the amphipods (Corophium volutator and Monoporeia affinis), the polychaete (Nereis diversicolor) have dropped considerably. M. viridis has become an important food item for some benthophagous fishes. Field experiments combining natural densities of native macrofauna and the introduced polychaete showed that the effects of M. viridis on the sublittoral community ecology were many and varied. The presence of M. viridis increased benthic production. M. viridis reduced the growth and survival of N. diversicolor and M. affinis. This effect decreased with the increasing density of adult specimens of the bivalve Macoma balthica. Competitive interactions between M. viridis and M. balthica appear a key factor limiting the distribution of M. viridis in the northern Baltic Sea. Competitive superiority of M. balthica over M. viridis is likely due to more efficient feeding regime of the bivalve. To conclude, owing to its unprecedented invasion success, dominance in many biotopes and effects on the native fauna the polychaete may be ranked among the most influential exotics in the northern Baltic Sea. Contact: Jonne Kotta, Estonian Marine Institute, Marja 4d, Tallinn 10617,
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