EFFECTS OF PREDATION AND OXYGEN DEFICIENCY ON DIFFERENT AGE CLASSES OF THE AMPHIPOD MONOPOREIA-AFFINIS

Predation by the big brackish-water isopod Saduria entomon on two age- classes (juveniles and adults) of the small deposit-feeding amphipod M onoporeia (syn. Pontoporeia) affinis was studied at different oxygen c onditions (normoxia >11 mg/l O-2, and moderate hypoxia 4 mg/l O-2) Vul nerability of juvenile and adult M. affinis was studied in single and mixed age-class treatments. The proportions of juveniles and adults we re varied at one total numerical density. The predatory effects were m easured as numbers of individuals consumed and of total biomass intake (mg ash-free dry weight). The highest predation rate (in numbers) was detected in the single-prey treatment with juvenile M. affinis, both in normoxia and moderate hypoxia. No preference for any of the two siz e classes was found in the treatment with equal numbers of adults and juveniles. In normoxia, the total number of prey consumed decreased wi th increasing proportions of adults, while intake increased in terms o f biomass. In moderate hypoxia, high proportions of adults reduced the predation rate of S. entomon to such an extent that also the intake i n terms of biomass was significantly negatively affected.