Zooplankton communities show contrasting productivity variables thresholds in dammed and undammed systems

Reservoir construction is one of the main threats to large rivers because they alter various ecological aspects downstream of the dam such as hydrodynamics and the concentration of productivity variables. In this context, our study aimed to i) compare zooplankton community responses (composition) to the concentration of productivity variables (chlorophyll a, phosphate and nitrate concentrations) in dammed and undammed systems, ii) indicate which concentrations of the productivity variables represent thresholds for changes in the community and iii) verify if the body sizes of the species with greater magnitudes of environmental community change are different between these systems. Samples were collected in six habitats of the upper Parana River floodplain between 2000 and 2015. We tested for variation in the abundance and frequency of the zooplankton community in both systems using a TITAN analysis. We registered 256 taxa in the undammed system and 239 in the dammed system. A PERMANOVA analysis showed that the composition of zooplankton species differed significantly (p = 0.001) among the studied systems. The TITAN analysis identified significant positive and negative thresholds for phosphate and nitrate in higher concentrations in the unpreserved system, contrary to what was found for chlorophyll a. According to the size spectrum, smaller species are indicators of thresholds in the undammed system. In general, our results indicate that productivity variables differentially affect the frequency and abundance of the community according to the type of system evaluated.