Planktonic ciliate response to initial filling of a Neotropical water reservoir

Abstract Reservoir construction causes marked changes in hydrological, physical and chemical conditions, with rapid effects on aquatic communities. The initial filling phase, characterised by abrupt changes over days to weeks and the onset of vertical stratification, is particularly critical, yet its effects on planktonic ciliate communities remain poorly understood. Here, we combined high-frequency sampling with a two-strata design (surface and bottom) to investigate how the initial filling, and consequent vertical stratification, of a tropical reservoir affect the density, biomass and taxonomic groups composition of the planktonic ciliates community. We observed pronounced temporal and spatial changes, with physicochemical differences between strata and filling phases. We observed that the ciliate community responded to these changes with consistently higher density and biomass at the surface and in the middle of the filling period. Linear mixed-effects and generalised additive models supported our findings, with significant temporal variation in all attributes, with trajectories over the filling period characterised by an initial increase followed by a decline, and by marked shifts among ciliate groups that indicate a clear successional pattern. Our results suggest that the surface strata of the reservoir work as a dynamic and productive environment, whereas the bottom strata remain limiting for ciliate development, mainly due to lower oxygen concentrations. By providing one of the few high-resolution assessments of planktonic ciliates during reservoir filling, integrating vertical structure, temporal dynamics and taxonomic succession, this study extends the trophic-surge perspective to microbial consumers and highlights the value of ciliates as bioindicators during dam construction and early reservoir operation.