Effect of food and sediment pre-treatment in experiments with a deposit-feeding amphipod, Monoporeia affinis

We experimentally investigated the effects of different pre-treatments of t he sediment, and the effect of daily addition of fresh phytoplankton, on th e growth and survival of 1-year-old (1 +) individuals of the deposit feeder Monoporeia affinis (Amphipoda). We used three different types of sieved se diment: pre-frozen muddy clay, non-pre-frozen muddy clay, and fine sand. Th e muddy clay contained phytoplankton originating from the surface sediment sampled in the field during the late spring bloom. No phytoplankton was ini tially present in sand. The experiment lasted for 18 days. M. affinis respo nded to the daily phytoplankton addition by increasing growth. Phytoplankto n addition had no significant effects on the survival of M. affinis. Upon p hytoplankton addition, the sandy and non-frozen muddy clay gave similar gro wth and survival responses. In contrast, the pre-frozen sediment resulted i n significantly lower growth and survival. The growth was negative in all t reatments without phytoplankton. Thus, the high initial chlorophyll content in the muddy clay was not of sufficient quality or concentration to allow a positive growth response in M. affinis. The growth of M. affinis was sign ificantly correlated with the reduction of the chlorophyll. Our results ind icated that M. affinis is capable of assimilating settled phytoplankton wit h no, or only a few days' time delay. (C) 2000 Elsevier Science BN: All rig hts reserved.