EFFECT OF VERTEBRATE PREDATION ON THE SPATIOTEMPORAL DISTRIBUTION OF CLADOCERANS IN A TEMPERATURE EUTROPHIC LAKE

We analysed the spatio-temporal distribution of zooplankton along a pr ofile of 10 stations from the shore to the pelagic zone from April to September 1988, the period when the larvae and juveniles Rutilus rutil us, the most abundant species in the Lake, are in the littoral zone. T he digestive tracts of the young roach were analysed. They fed essenti ally on rotifers and on cladocerans. For comparison, zooplankton was a lso analysed at one littoral area without fish fry. There was an incre ase of cladoceran density from the vegetated nearshore zone to the off shore zone. Considering the density of Bosmina longirostris, Daphnia l ongispina, Chydorus sphaericus and Ceriodaphnia quadrangula, we observ ed a different distribution pattern in the course of the year. In the nearshore zone, the relative abundance of small species, Bosmina and C hydorus, was much higher than that of the larger Daphnia. From April t o September, predation pressure mainly affected the smallest species: in contrast to the inshore station without fish fry, the density of Bo smina decreased in May in the littoral with fish. Chydorus was concent rated in the littoral between February and April, then grew into the p elagic zone, where predation pressure obviously was low during the war m season. The number of Daphnia, which was eaten by the fish fry at an y time, remained low in the nearshore zone, which suggests that the pr esence of fish may cause Daphnia to avoid this zone. Ceriodaphnia whic h was not affected by this predation, was scarce in the nearshore zone during mid-summer. The low density of the cladocerans in the nearshor e zone is likely associated with vertebrate predation by roach fry and juveniles, the result of such a process being either a depletion in d ensity of the prey, or an avoidance behaviour.