Influência de fatores ambientais locais e regionais na composição das assembleias de micro e mesozooplâncton em reservatórios do semiárido

Abstract

Brazil's semi-arid region is characterized by great spatio-temporal variation in rainfall. These conditions lead to changes in the physical and chemical composition of the water, such as increased turbidity and eutrophication of reservoir water. These changes can lead to changes in the composition of zooplankton assemblages, but restrictions on dispersal associated with zooplankton body size can lead different size classes (microzooplankton: 20-200 μm and mesozooplankton: > 200 μm) to respond differently to environmental and seasonal changes, depending on the geographical scale. To test this hypothesis, we used both linear and generalized linear models to test the effects of space, seasonality and environmental variables associated with eutrophication (e.g. nitrogen, phosphorus and chlorophyll a) and water turbidity (e.g. water transparency, turbidity and total suspended solids) on the beta diversity of micro- and mesozooplankton from 16 reservoirs in the Brazilian semi-arid region. Of these, eight are located in a more semi-arid region, while the other eight are located in a less semi-arid region, both belonging to the same river basin. We evaluated the relative importance of turbidity, eutrophication, spatial and seasonal variation in explaining variations in the beta diversity components (substitution and nesting) of microzooplankton, mesozooplankton and total zooplankton. The results revealed that increasing concentrations of total phosphorus increased the difference in composition of total zooplankton assemblages (i.e. caused by species loss), both locally and regionally. At the local scale (within reservoirs), species replacement and total beta diversity were higher in the dry season, while regionally beta diversity was higher in the less semi-arid region. However, the direction and magnitude of the effects are contingent according to the size class of the zooplankton. For example, the results for mesozooplankton are similar to those for the total zooplankton, with the effects of phosphorus, region and season. Meanwhile, water turbidity and region were important in determining the beta diversity of microzooplankton, with a positive effect of turbidity and greater total beta diversity and species substitution in the more semi-arid region. Our findings give us a better insight into how changes in environmental variables, affected by a period of prolonged drought, influence the structure of zooplankton assemblages at different geographical scales in semi-arid regions.