| Abstract: | In the eutrophic L. Hiidenvesi, the spring biomass maximum of cladoceran zooplankton is missing and the highest biomass takes place in July–August. The factors behind the delayed biomass peak were studied in four different basins of the lake with concomitant data on cladocerans assemblages, density of the predatory cladoceran Leptodora kindti and food composition of fish. In all the basins, the abundance of Leptodora peaked in June, being highest (up to 800 ind. m–3) in the two most shallow basins (max depth < 4 m). The duration of the high population density was short and in July‐August Leptodora density stayed below 200 ind. m–3, although the water temperature was still favourable. The collapse of the Leptodora population coincided with the change in the feeding habits of fish. In early summer, fish predation was targeted mainly on copepods and zoobenthos, while in high summer Leptodora was one of the main preys of perch, white bream and bleak. The biomass of herbivorous cladocerans was below 10 μg C l–1 in June, and climbed to a maximum in August in the two most shallow basins (34 and 76 μg C l–1), in July in the deepest basin (27 μg C l–1), and in September in the intermediate basin (55 μg C l–1). In the two most shallow basins, the death rate of the dominating cladoceran, Daphnia cristata, closely followed the food consumption rate by the Leptodora population. In the deeper basins, the agreement was not so close, smelts (Osmerus eperlanus) and chaoborids being important predators of herbivores. The duration of the period of high Leptodora density thus depended on the predation pressure by fish, while the increased fish predation on Leptodora in July–August allowed the elevation of the biomass of herbivorous cladocerans. |
