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Abstract

We investigated the effects of two functionally different deposit feeders, the amphipod Monoporeia affinis and the bivalve Macoma balthica, on the benthic emergence of copepods (Acartia spp., Eurytemora affinis, Temora longicornis), cladocerans (Bosmina longispina maritima, Daphnia spp.), and rotifers (Synchaeta spp., Keratella spp., Notholca spp.) in the northern Baltic Sea. We performed 3.5-week laboratory experiments in June-July and in September-October using surface sediment containing zooplankton eggs and various combinations of amphipod and bivalve densities. Both zoobenthic species increased egg mortality rates in Synchaeta spp. and in calanoids. Each amphipod consumed a maximum of one Synchaeta spp. egg and 31 calanoid eggs per day, while the bivalves consumed six Synchaeta and 143 calanoid eggs per day. Monthly consumption throughout the community matches or even exceeds the field densities of eggs. In summer, M. affinis decreased the daily emergence rate of Synchaeta spp. by 31-53% and of M. balthica by 57%. The effect on calanoids was less severe: the emergence rate of Acartia spp. and E. affinis was significantly suppressed only by M. balthica (the reduction was 48% and 28% in Acartia spp. and in E. affinis, respectively). T. longicornis was the only species that benefited from macrofauna: together the two zoobenthic species resulted in a 2.7-fold increase in the emergence of this calanoid. Bioturbation significantly influences the benthic emergence of zooplankton, with effects varying among species, and may thus affect the dynamics of pelagic populations.