The effects of bioturbation and predation by the amphipod Monoporeia affinis, the bivalve Macoma balthica and the polychaetes Marenzelleria spp. were studied on the number, vertical distribution and hatching of benthic eggs (ephippia) of the cladoceran Bosmina longispina maritima. It was hypothesised that the 3 functionally different macrobenthic species affect Bosmina ephippia to different degrees. In addition, it was hypothesised that macrofaunal bioturbation either inhibits or enhances predation by the necto-benthic mysid Mysis mixta on cladoceran benthic eggs. M. affinis and M. balthica caused a 48 and 23% decrease in the total number of Bosmina ephippia, respectively, indicating egg predation. In addition, M. balthica extended the distribution of eggs into deeper sediment layers. Both M. affinis and M. balthica suppressed the daily hatching rate of Bosmina, and the effect of the bivalves (reduction 92%) was stronger than that of the amphipods (34 to 78%). None of the benthic species promoted the predation of mysids on Bosmina eggs. Instead, both M. affinis and M. balthica reduced the number of ephippia in the sediment surface (by 79 and 43%, respectively), implying that these species decrease the ability of mysids to feed on cladoceran benthic eggs. M. balthica was the most harmful species for the recruitment of Bosmina, whereas the nonindigenous Marenzelleria spp. did not affect ephippia.

No abstract

The abundance of Bosmina longispina maritima (Cladocera), the most common water flea in the Baltic Sea, shows considerable interannual fluctuations.The number of Bosmina resting eggs (ephippia) in the sediment also fluctuates from year to year. Biotic as well as abiotic factors have been suggested to contribute to these fluctuations, but the impact of benthic predation by pelagic mysids has not yet been considered. The objective of this study was to investigate the effect of benthic predation and bioturbation by Mysis mixta and M. relicta on the ephippia of B. longispina maritima on the SW coast of Finland. The results show that mysids can significantly affect the number of Bosmina eggs in the sediment. All size groups of both mysid species preyed upon ephippia of B. longispina maritima both in the water column and directly from the top layer of sediment, whereas they rejected the eggs of another cladoceran species, Pleopsis polyphemoides. The predation rate on B. longispina maritima ephippia was significantly lower when the eggs were buried under a sediment layer of ca. 1 cm. This suggests that factors affecting the vertical distribution of ephippia in the sediment determine the magnitude of the impact that mysids have on B. longispina maritima benthic eggs. KEY WORDS: Bosmina longispina maritima · Resting eggs · Mysis mixta · M. relicta · Benthic predation · Baltic SeaResale or republication not permitted without written consent of the publisher

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.

No abstract