Effects of the invasive polychaete Marenzelleria spp. on benthic processes and meiobenthos of a species-poor brackish system

The effects of the invasive polychaete Marenzelleria spp. on sediment processes and meiobenthos with an emphasis on free-living nematodes of the Vistula Lagoon (southern Baltic Sea) were investigated in a laboratory microcosm experiment. Marenzelleria occupies an open niche and its deep burying behaviour and feeding strategy represent a new function in the study area. Halos of oxidized sediment along Marenzelleria burrow walls indicated oxygen penetration into the burrows but the polychaete had no significant effect on porewater nutrient concentrations. The results showed, however, the density dependent effects of Marenzelleria on ammonium transport. An enhanced ammonium efflux was recorded at high polychaete densities (2000 ind. m− 2) but not at low polychaete densities (300 ind. m− 2). was no observable impact of the polychaete on total meiobenthic numbers. There was, therefore, no indication that Marenzelleria caused meiofauna mortality. On the contrary, the polychaete significantly affected vertical distribution of meiofauna facilitating the colonization of deeper sediment depths and thus extending the habitat to be used by meiobenthos. In addition, Marenzelleria had a positive impact on the survival of turbellarians. heless, there was no effect of Marenzelleria on nematode assemblage structure and diversity, indicating that neither the physical presence nor the biological activity of the worm affected the nematode community. This suggests either 1. the limited impact of Marenzelleria on nematodes, resulting from the creation of simple, narrow and un-branched burrows, 2. poor response of nematode community resulting from their low abundance and diversity in the study area, or 3. the overriding role of the harsh chemical environment typical of sediments of the Vistula Lagoon, masking the effect of the bioturbator.