The zebra mussel (Dreissena polymorpha) and the benthic community in a coastal Baltic lagoon: another example of enhancement?

Abstract

AbstractThe zebra mussel (Dreissena polymorpha), a sessile suspension feeder, has been known to enhance the benthos in the sediment around mussel beds and druses. The enhancement is mainly due to biodepositing activity of the mussel and the resultant organic enrichment of the sediment. In addition, mussel beds increase habitat complexity. In June 2007, we studied the distribution of D. polymorpha beds and their effects on the benthos in Kleines Haff, the western part of the Szczecin Lagoon, a Southern Baltic coastal water body. We expected that: (i) the sediment near a Dreissena bed will support meio‐ and macrobenthic assemblages more abundant than those living away from the bed, and (ii) the meio‐ and macrobenthic assemblages close to the Dreissena bed will show a stronger dominance of the taxa relying on the organic enrichment (nematodes among the meiofauna; oligochaetes and chironomids among the macrobenthos). Meiobenthos was sampled at two localities, one (MB4) featuring D. polymorpha druses and the other (MB5) supporting a distinct zebra mussel bed. The two localities differed in their meiobenthic communities: whereas no difference in abundance and composition was observed between the sets of samples collected at MB4, MB5 showed a distinctly more abundant meiobenthos in the uppermost sediment layer near the zebra mussel bed, but the overall abundance was lower than that away from the bed. We conclude that the meiobenthic response to the presence of D. polymorpha was, at least in part, mediated by site‐specificity of sediment characteristics. Macrozoobenthos was sampled at four stations in the vicinity of MB5 and around another Dreissena locality (MB10), and was also identified in the sediment samples collected for the meiobenthos. The macrobenthic abundance was higher by a factor of 2.4 (sandy sediment) to 4.9 (silty bottom) near the Dreissena beds than away from them. Irrespective of their situation relative to Dreissena aggregations, both meio‐ and macrobenthos were dominated by the taxa associated with organic enrichment. The macrobenthos associated with Dreissena beds included Gammarus tigrinus, a non‐indigenous amphipod, whereas another non‐native species, the polychaete Marenzelleria neglecta, was present in the macrobenthos of sandy sediments of the area.