Abstract
The reduced zooplankton abundance at the outlet sections of lakes depends on the occurrence of preying fry. Therefore, light conditions can play a major role in the drift of zooplankton along river outlets. The aim of this study was to determine the influence of diurnal light conditions on the decline of zooplankton densities at lake outlets. Photosynthetically active radiation (PAR) levels were measured to determine their effect on the rate of reduced zooplankton abundance. Cladocerans and copepods showed a significantly greater reduction in abundance than rotifers and nauplii. A significant positive relationship was observed between the PAR levels and the reduced abundance of Asplanchna sp., small cladocerans, large cladocerans and Copepoda at the lake outlets. Among the rotifers, small pelagic rotifers drifted the farthest at all hours of the day. Large crustaceans, especially the large cladocerans and copepodites and adult copepods, had the lowest chance of dispersing over a wide area. Our results indicate that light conditions play an important role in the reduction of zooplankton abundance at lake outlets and have an indirect influence on the downstream food web.
Highlights
Flow-through lakes and reservoirs provide a rich source of zooplankton for river ecosystems
The aim of this study was to determine the influence of diurnal light conditions on the decline of zooplankton densities at lake outlets
The greatest zooplankton community decline occurred in the 0.2 km long section downstream from the outlet [1, 7, 8]
Summary
Flow-through lakes and reservoirs provide a rich source of zooplankton for river ecosystems. Zooplankton assemblages reach high densities with high biomasses because the organisms wash out of the upper sections of lakes into the river outlets. Because of this high abundance, high densities of fry that prey on plankters at lake outlets are observed, especially cyprinid fish [1], which make the most significant contribution to the reduction in zooplankton communities [2,3,4,5,6]. The highest reductions occur in the first 0.2 km downstream section from the lake outlet [1, 7, 8]. The main components of the drifting zooplankton are small rotifers and the nauplii of Cyclopoida
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