Impact of a borrowing deposit-feeder, Monoporeia affinis, on viable zooplankton resting eggs in the northern Baltic Sea

We investigated the effect of different densities of the burrowing deposit- feeding amphipod Monoporeia affinis on the potential for recruitment of zoo plankton from benthic resting eggs. Hatching of resting eggs was induced in the laboratory on sliced and resuspended 1-cm depth-sections of sediment c ores, collected at six stations ill an archipelago area of the Gulf of Both nia, Baltic Sea. The uppermost 5 cm of the sediment was studied. The most c ommon species that hatched was Eurytemor affinis (Copepoda). Individuals fr om another copepod genus, Acartia, hatched in significant numbers only in t he cores from two stations with low amphipod abundance. Cores from stations with high amphipod densities showed a deeper distribution of emerging E. a ffinis nauplii compared with stations with few amphipods: the oxidised sedi ment layer was also deeper at high M. affinis densities than at low. Total (0 to 5 cm strata pooled) number of hatched E. affinis nauplii was independ ent of amphipod density. This indicates that the effect of M. affinis on E. affinis eggs involves deeper burial due to bioturbation, rather than preda tion. Decreased benthic recruitment of zooplankton at localities with high M. affinis density is suggested, since more deeply positioned eggs are less likely to hatch. When hatching was induced in intact, non-sliced cores fro m one station, the number of E. affinis nauplii that hatched was on average 43% of the number that hatched in the upper centimetre of the sliced cores from the same station. This fraction (43%), if applied to the other statio ns, implied a potential for benthic recruitment of up to 80000 ind m(-2) fo r E. affinis. Due to its high abundance, M. affinis is likely to greatly re duce benthic recruitment of zooplankton in this system.