Deposit-feeding amphipods (Monoporeia affinis) reduce the recruitment of copepod nauplii from benthic resting eggs in the northern Baltic Sea

We experimentally investigated the effect of different densities of the bur rowing, deposit-feeding amphipod Monoporeia affinis on the recruitment of z ooplankton from benthic resting eggs. Intact sediment cores with in situ de nsity and species composition of zooplankton resting eggs and benthic fauna were collected in the northern Bothnian Sea, part of the Baltic Sea. We re moved as many M, affinis as possible from the cores and then added differen t numbers of ill. affinis to the cores to generate a range of densities. Th e cores were exposed to different densities of M. affinis for either 3 or 4 0 days, after which the hatched zooplankton was registered. One subset of t he cores were initially incubated under low temperature (2-3 degreesC, to p revent hatching) for 37 days (the resting phase), to allow for effects of M . affinis on unhatched resting eggs. These cores were then incubated under higher temperature (13 degreesC) for 3 days (the hatching phase), to induce hatching and allow for effects on hatching or hatched specimens. In a seco nd subset of cores with the same time and temperature schedule, the M. affi nis density was experimentally reduced at the start of the hatching phase, to evaluate the effect of M. affinis during the hatching phase. To a third subset of cores, we immediately initiated the hatching phase, without an ex perimental resting phase, to evaluate the effects induced during the restin g phase. The most common zooplankton species that hatched was Eurytemora af finis (Copepoda), followed by Bosmina longispina maritima (Cladocera). In a ll cores that were subjected to a resting phase, the numbers of hatched E. affinis were log-linearly negatively related to density of M. affinis. An i ncrease of M. affinis density from 1,000 to 5,000 individuals m(-2), normal field densities, reduced the hatching by 60-70%. The negative impact was m ainly exerted during the hatching phase, suggesting predation on, burial of or physical injury of hatching nauplii or eggs in a late development stage as likely mechanisms. Also, the number of B. longispina maritima that hatc hed was reduced by M. affinis during the hatching phase, but no clear relat ion to density of M. affinis could be identified. The results show that M. affinis can reduce recruitment to zooplankton from benthic resting eggs. Su ch impact by the benthos on resting stages of zooplankton is therefore a po tentially significant link between the benthic and pelagic systems.