Testing theoretical frameworks of zooplankton longitudinal distribution in a large reservoir

Reservoirs are thought to contain three zones (riverine, transition and lacustrine) emerging from environmental gradients from the river to the dam where zooplankton are predicted to peak in the transitional zone. Few high spatial resolution studies, however, have surveyed zooplankton distribution in reservoirs, leading to limited knowledge regarding longitudinal abundance and composition of zooplankton. Using 44 samples from river to dam in a 35.7 km long reservoir, we explore spatial variability in zooplankton density, biomass, composition and relative bottom-up to top-down control. Broadly, density and biomass increased from the river to the dam. Variation occurred in the location and frequency of peaks among groups. Divergence between per unit volume and area within groups highlighted the importance of depth. Richness and turnover between sites were both greatest near the river. The ratio of zooplankton to phytoplankton biomass varied longitudinally, with a low near 20% of the distance from the river to the dam indicating spatial variability in bottom-up versus top-down control. This study highlights the complexity of longitudinal zooplankton composition and distribution in reservoirs and provides insight into reservoir ecosystem complexity and function, with important consequences for water quality, fish distribution, and productivity.