SYNERGISTIC EFFECTS OF NATURAL AND ANTHROPOGENIC IMPACTS ON ZOOPLANKTON DIVERSITY IN A SUBTROPICAL FLOODPLAIN: A LONG-TERM STUDY

Large biological diversity can be observed in floodplains, which is linked to several ecosystem processes, such as productivity and nutrient cycling; among the species involved in these processes are zooplanktonic organisms. Here, we aimed to quantify the temporal trend of zooplankton, through changes in species richness and composition (β-diversity) and the abundance of individuals, in preserved and non-preserved sites of the Upper Paraná River floodplain during the last 19 years, and 617 species (321 rotifers, 135 testate amoebae, 107 cladocerans, and 54 copepods) were recorded. The highest species richness was verified in 2002 and the lowest in 2016. Zooplankton richness and abundance in non-preserved sites show clear temporal homogenization, while preserved sites show cyclical fluctuations over time. The rarefaction curve showed the biodiversity potential in this floodplain. Three invasive species were recorded (Kellicottia bostoniensis, Daphnia lumholtzi, and Mesocyclops ogunnos). The variation in the hydrologic regime, the oligotrophication, and the reduced connectivity among the sites were the main factors interfering in the species composition over the years and leading to a reduction of the species richness and abundance. The preservation of the tributaries with pristine characteristics is very important for the mitigation of these synergistic effects under the community structure