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In this study, we have shown that nutrient input in aquatic ecosystems has different impacts on the various facets of phytoplankton and zooplankton diversity. We designed an experiment with eleven levels of nutrient addition replicated four times. Alpha, beta, and gamma taxonomic and functional diversity indices were calculated based on organisms' occurrences and abundances in each microcosm, among microcosms of the same treatment and for the set microcosms of a same treatment. There was an increase in species richness and a decrease in equitability along the gradient for both groups. Taxonomic beta diversity based on occurrences was positively related to nutrient gradient, but a hump-shaped relationship between nutrient enrichment and beta diversity was observed when abundance data were considered, showing greater variation in species composition at intermediate levels of nutrient. Alpha functional richness increased for both groups, whereas alpha and gamma RaoQ, and gamma functional divergence decreased, indicating that nutrient enrichment promotes functional trait richness, but also the dominance of species. At small spatial scale and at a short period of time, we showed that taxonomic and functional diversities respond in similar ways to nutrient enrichment. We conclude that nutrient input may modify community composition and affect ecological processes overtime. [ABSTRACT FROM AUTHOR]

The oligotrophication of freshwater is the reduction of the nutrient concentration in the water column, which causes the depletion of organic matter and diminishes the biological production of the ecosystem. To elucidate the zooplankton response to nutrient depletion (oligotrophication), an experiment was performed in mesocosms simulating real scenarios. Phytoplankton and zooplankton inocula were collected in the Upper Paraná River Floodplain and subjected to combinated nutrient and transparency treatment. We evaluated whether resource limitation and increased transparency (both associated with oligotrophication) affect the size structure of planktonic crustaceans. The effects of predation were corroborated in the treatments with low nutrient concentrations. Thus, the oligotrophic scenario (high transparency and low nutrient concentration) indicated a decreased size structure of zooplankton, probably because of the predation pressure, supporting the predictions of the size-efficiency hypothesis (SEH). When resources were abundant (in treatments with high nutrient concentrations), the decrease in the size structure indicated that the enrichment of nutrients favoured small individuals. Our results showed that nutrient and transparency manipulation affected species richness, the abundance of individuals, and the zooplankton community size structure. Therefore, we suggest that oligotrophication affects predation and competition dynamics in zooplankton. [ABSTRACT FROM AUTHOR]