Search

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]

In this study, we evaluated the effects of nutrient enrichment on primary and secondary productivity, the relationship between these ecosystem processes, and the relationship between the resources of zooplankton community in response to ecosystem processes, experimentally. The experimental design was randomized, with three levels of nutrient replicated three times, giving a total of nine experimental units, for a period of 30 days (November-December, 2013). We measured primary productivity, secondary productivity, and zooplankton community structure, at the beginning and after every seven days of the experiment. Primary and secondary productivity increased with nutrient enrichment. Secondary productivity was supported by a few dominant species with ecologically selected traits, especially large-bodied zooplankton (copepods and large cladocerans). Nutrient enrichment turned the relationship between primary and secondary productivity from positive to negative and intensified the biotic process of competition because of exploitation by zooplankton populations, favoring an increase in the dominance of large microcrustaceans. This reflects the initial effects of a moderate eutrophication. At a larger temporal scale and at higher nutrient concentrations, effects of higher magnitude are expected, which may disrupt the whole community, resulting in the loss of ecosystem processes. [ABSTRACT FROM AUTHOR]