SUMMER DIAPAUSE IN CYCLOPOID COPEPODS - ADAPTIVE RESPONSE TO A FOOD BOTTLENECK

1. Summer diapause in freshwater cyclopoid copepods is often regarded as an adaptation to avoid fish predation. We tested the alternative hypothesis that the ultimate reason for fourth instar copepodites to enter the sediment during summer is to avoid a food bottleneck for herbivorous juvenile stages caused by the grazing of competing cladocerans. 2. Seasonal life cycles of three co-existing cyclopoid copepod species, Cyclops kolensis, C. vicinus and C. abyssorum, which spend the summer in anoxic sediments, were studied in a eutrophic lake. 3. All three species undergo an ontogenetic shift in their feeding habits from herbivory in the naupliar stages to omnivory in the later juvenile and adult stages. Nauplii depend on flagellates as food. 4. Copepods were cultured in the laboratory without a diapause. Nauplii were subjected to lake water containing seston that was freshly collected during the different seasons, and their growth and development was recorded. 5. Nauplii developed quickly into copepodites in lake water that was drawn in late winter (February/March), when they were abundant in the lake. In late spring and early summer, when cladocerans dominated the plankton, lake water did not support naupliar growth. Development of C. abyssorum was retarded and nauplii from the other two species died before they reached the first copepodite stage. 6. However, all species developed nearly optimally during spring and summer if the water was supplemented with flagellates. We conclude that all species must pass a serious bottleneck as they need high concentrations of flagellates for their naupliar development. 7. Hence, summer diapause may be an adaptation to avoid poor food conditions for nauplii. Fish predation may be important for shaping the timing of diapause in C. abyssorum, as this is the largest species and is least affected by food limitation. This hypothesis is supported by a comparison of diapause behaviour of cyclopoids in lakes of varying trophy.