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The interannual variation of phytoplankton communities in the three main rivers of the Upper Paraná River floodplain is evaluated in relation to changes in the hydrosedimentological regime. These changes are a result of climatic variability and the formation of Porto Primavera Reservoir, located at the upper Paraná River. Phytoplankton species richness and density were investigated in rivers during a prior period (1993)(1994) and eight years after reservoir impoundment (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007). Multiple analyses were conducted to test the differences between these time periods in order to find predictor variables for phytoplankton attributes. A total of 454 phytoplanktonic taxa were found. The regression analysis revealed significant differences between periods. In the years following construction of the Porto Primavera dam, species richness was lower in the Paraná River and density was higher in the three rivers. In general, the algal density decreased from 2005 to 2007. Diatoms and cyanobacteria contributed significantly to the total density during the period from March 1993 to February 1994. The years 2000-2007 presented the lowest diatom contribution to species richness and the highest cyanobacteria contribution. From 2000 on, cryptomonads and cyanobacteria dominated. The interannual variability of phytoplankton was probably influenced by changes in hydrosedimentological regime due to climatic variations (La Niña and El Niño -Southern Oscillation events -ENSO) and the operational procedures associated with an upstream reservoirs. Studies on climatic variability and its effects on hydrosedimentological regimes of the Paraná, Baía and Ivinhema rivers and the biota therein are necessary to obtain subsidies for management, including decisions related to the operation of dams upstream and downstream of the study area, with the purpose of minimizing risks to the Environmental Protection Area.

Aim Evaluating how groups of organisms vary in dispersal capability and how environmental, spatial and temporal signals vary across multiple scales is critical to elucidating metacommunity theory. We examined whether the relative contributions of environmental, spatial and hydrological factors have different effects on organismal groups with different dispersal abilities at three spatial scales, and how this knowledge contributes to our understanding of metacommunity dynamics. Location Four major Brazilian floodplains, with the largest distance among them of 2,300 Km. Taxon 10 aquatic organismal groups, ranging from bacterioplankton to fish. Methods We sampled lakes connected to the main river in the low‐ and high‐water periods of each floodplain between 2011 and 2012. Different biological groups were analysed across three hierarchical spatial scales (fine, intermediate and subcontinental) within and between floodplain systems. We applied a series of partial redundancy analyses to estimate the relative contributions of environmental factors, spatial factors and hydrological period for each biological group. Results At the fine spatial scale, predominantly environmental factors and hydrological period structure metacommunities, although less so for microorganisms than for micro‐invertebrates and macro‐organisms. The relative importance of environmental factors increased at the intermediate spatial scale. At the subcontinental scale, the relative importance of spatial factors increased for all biological groups, but environmental factors remained the primary regulators of microorganisms even at the largest scale. Main conclusions This study design allowed us to make more robust inferences about the mechanisms responsible for regulating community structure of distinct biological groups at different spatial scales. Our results suggest that biological groups displaying distinct body size likely determine the spatial extent at which environmental, spatial and hydrological processes prevail as the primary regulators of community structure. These findings are important in guiding the conservation and management of floodplain biodiversity because these systems are naturally highly heterogeneous in space and time.

Evaluation of the relative influences of environmental and spatial processes in structuring aquatic metacommunities is an essential first step for understanding how these factors govern species distributions and affect spatial and temporal variability in community structure. Such variability has many causes and consequences at different scales. In the case of phytoplankton metacommunities, both species sorting and mass effect processes are likely to be important in structuring the patterns of biomass distribution on large spatial scales. We investigated the influences of environmental and spatial components on the phytoplankton community on a large spatial scale, consisting of a reservoirs‐river‐floodplain lakes gradient in a Neotropical region. Using partial redundancy analysis, we partitioned the relative role of environmental (environmental filters) and spatial (asymmetric eigenvector maps) processes structuring phytoplankton biomass. A clear difference of the limnological conditions was observed between the reservoirs and river–floodplain lakes system. High species richness and higher mean values of phytoplankton biomass were recorded in the floodplain lakes. Variation partitioning demonstrated the importance of both environmental and spatial process in phytoplankton biomass structuring. However, the relative importance of these processes may vary over time. Our results suggest that from a metacommunity perspective, the phytoplankton biomass of upstream reservoirs and a downstream river–floodplain lakes system is determined by environmental conditions (species sorting process), mainly in the floodplain lakes; and by high dispersion rates favoured by the connectivity and unidirectional flow (mass effect process), mainly between the reservoirs and the river.

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