Benthic competition and population dynamics of Monoporeia affinis and Marenzelleria sp. in the northern Baltic Sea

• Monoporeia affinis is superior of Marenzelleria at low resources of high quality.
• Monitoring data show that M. affinis is declined significantly from 1998 to 2001.
• Marenzelleria used a density gap to increase its abundance in the Gulf of Bothnia.
• The abundance of M. affinis and primary production are strongly correlated.

During the last two decades major changes of the benthic fauna have occurred in the northern Baltic Sea, the Gulf of Bothnia. The native amphipod, Monoporeia affinis, has shown a large-scale abundance decrease, while polychaetes, Marenzelleria spp. have invaded the system. Marenzelleria co-exist with the native fauna in the southern Baltic Sea, but in the north the pelagic production might be too low to allow co-existence. Thus, M. affinis might have been out-competed by Marenzelleria in the Gulf of Bothnia. This hypothesis was tested in a competition experiment with a high and a low fresh phytoplankton food supply. When exposed to high food supply both species showed stable or increased biomass over the four week test period. In low food supply, however, M. affinis was found to have a competitive advantage. The experimental data were also related to Baltic Sea monitoring data on primary production, sedimentation and invertebrate abundances. Data from the northern Baltic Sea show that the dominance in the benthic community by M. affinis was replaced by Marenzelleria around 2001. The amphipod decrease might be explained by a marked decrease in primary production during this period. Combining monitoring and experimental data suggests that the invasion of Marenzelleria did not cause the decrease of M. affinis in the northern Baltic Sea; it rather took advantage of the density gap that had occurred. A shift may thus have been established in the Bothnian Sea benthic community.

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