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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Aim: The objective of this study was to evaluate the efficacy of phytoplankton functional groups as indicators of environmental conditions in floodplain rivers and lakes with different trophic state and connectivity degree to the Paraná River. Phytoplankton functional groups (FGs) cluster sensitive species to environmental variation and can be an alternative for environmental monitoring. Methods: Samples were performed quarterly from 2010 to 2013 in the Ivinhema, Paraná and Baia Rivers and in three lakes permanently connected to each river. Results: 419 taxa were identified, and those taxa that had values greater than 5% of the total biovolume were classified into FGs (P, C, A, B, MP, H1, W1, J, TD, L O , and N). The lakes presented higher biovolume and were more productive than rivers, especially in the dry periods. The rivers presented light limitation and low phytoplankton development. The FG L O was an indicator in rainy seasons. Both rivers and lakes were mostly oligotrophic. We registered FG indicators only for the lakes (A, B, C, E, L O , P, and W1) and mesotrophic environments (A, B, C, E, J, L O , and P). Conclusion: The FGs reflected the seasonal variation and the trophic state of environments in the upper Paraná River floodplain, mainly in the lentic environments. The species-environment relationship (FGs as indicators) was clearer in the lakes, probably because of the longer water retention that allows evidencing the response of the phytoplankton community to environmental factors. On the other hand, the absence of FGs as indicators in the rivers could be due to the high water flow that promotes dispersal stochasticity and masks the relationship between the environmental factors and the phytoplankton community. Thus, phytoplankton FGs proved to be a viable tool to evaluate the environmental conditions in a floodplain.

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The upper Paraná river floodplain is a strategic area for the Brazilian biological and environmental patrimony and contains several conservation units. We aimed to record the occurrence and the geographic distribution of microalgae in this floodplain during 30 years of research carried out by the Center for Research in Limnology, Ichthyology and Aquaculture - State University of Maringá. We consulted 80 publications (national and international journals and books published from 1986 to 2016). We considered only published works that referenced algae at the generic and infrageneric levels. The results indicated 938 species of algae (562 periphytic, 482 planktonic), which 103 co-occur in the two habitats. Cosmarium was the richest genera for periphyton and Traquelomonas for phytoplankton. This study increased the knowledge of microalgae biodiversity in Brazil and provided data for future ecological and biogeographic studies.