The objective of the present work is to evaluate the seasonal patterns of species richness and abundance of the zooplankton community in a semi-arid river, Northeastern of Brazil. Zooplankton samples were taken in four hydrological periods along the Upper Parnaíba River: April (low), August (dry), November (rising) 2013 and January (flood) 2014. The zooplankton community consisted of 125 species; the testate amoebae was the most species-rich (56 species) and the most abundant group (71.5%) of total zooplankton abundance. Season-specific differences were highly significant. The overall zooplankton richness and abundance was significantly higher during the low (71 species) and flood (878.47 ind.m ) period, respectively. The hydrological regime was important in structuring the zooplankton community, emphasising the importance of the Parnaíba River and its seasonal variation for biodiversity conservation in the Brazilian semi-arid region.Keywords: potamoplankton, large rivers, semi-arid, environmental variables, seasonality. Comunidade zooplanctônica do alto rio Parnaíba (Nordeste, Brasil)Resumo O objetivo deste trabalho foi avaliar os padrões sazonais da riqueza de espécies e abundância da comunidade zooplanctônica em um rio do semi-árido, Nordeste do Brasil. As amostragens do zooplâncton foram realizadas em quatro períodos hidrológicos ao longo do alto rio Parnaíba: Abril (vazante), Agosto (seca), Novembro (enchente) de 2013 e Janeiro (cheia) de 2014. A comunidade zooplanctônica foi constituída por 125 espécies, sendo as amebas testáceas o grupo mais rico em espécies (56 espécies) e o grupo mais abundante (71,5%) do zooplâncton. Diferenças significativas foram registradas entre os períodos. A riqueza e a abundância do zooplâncton foram significativamente maiores durante o período de vazante (71 espécies) e cheia (878,47 ind.m -3 ), respectivamente. Os resultados mostraram ainda, que o regime hidrológico foi importante na estruturação da comunidade zooplanctônica, ressaltando a importância do rio Parnaíba e sua variação sazonal para a conservação da biodiversidade do semi-árido brasileiro.

Background In reservoirs, zooplankton strongly interact with the physical and chemical characteristics of water, and this interaction is mainly influenced by climate variation and the different methods used to manage the dam water level. Therefore, the aim of this study was to evaluate how the distinct operating modes of two cascade reservoirs affected the richness, abundance, and composition of zooplankton, both spatially (intra and inter-reservoirs) and temporally (annual and seasonal). In this study, the upstream reservoir (Salto Santiago) operates using the storage method, with a water retention time (WRT) of 51 days, whereas the downstream reservoir (Salto Osório) operates using the run-of-river method, with a WRT of 16 days. Methods Zooplankton samples were collected for 16 consecutive years from the two reservoirs located on the Iguaçu River, Brazil. A total of 720 samples were collected. Four-way ANOVAs were used to determine the differences in richness and abundance of the zooplankton among years, periods, reservoirs, and environments. Multidimensional non-metric scaling (NMDS) and an analysis of similarities (ANOSIM) were used to describe similarity patterns in species composition. Finally, a canonical correspondence analysis (CCA) was used to select the environmental predictors that best explained the variation in zooplankton abundance data. Results We identified a total of 115 taxa in this study, and rotifers were the richest group. In contrast, the copepods were the most abundant. The four-way ANOVA results showed significant differences in the species richness and abundance of the zooplankton among years, periods, reservoirs, and environments. The NMDS ordination and ANOSIM test indicated that the largest differences in zooplankton species composition were annual and seasonal differences. Finally, the CCA showed that these differences were mainly associated with changes in water transparency, temperature, and the chlorophyll a, phosphorus, and total dissolved solids concentrations. Discussion Inter-annual changes in zooplankton species composition showed that over time, large filters-feeders (e.g., large daphinids and calanoid copepods) were replaced by small cladocerans (e.g., bosminids) and generalist rotifers. The highest species richness was associated with the fluvial environment, whereas the highest abundance was associated with the transitional and lacustrine reservoir environments. Variations in water temperature, nutrients, and food availability explained the annual and seasonal changes in community structure, whereas variations in the water flow characteristics of the environments explained the longitudinal changes in the richness and abundance of zooplankton in reservoirs. The differences in zooplankton structure between the two reservoirs can be explained by the functional differences between the two systems, such as their WRTs and morphometrics.

Zooplankton are widely recognised as being regulated primarily by predators and food availability. In reservoirs, the quantity and quality of food resources are generally affected by the characteristics of the water, which in turn are controlled by the flow pulse generated by operation of the dams. In this study, we investigated the relationship between zooplankton, water quality and food availability (phytoplankton) in eight hydroelectric reservoirs located in Brazil. Samples were collected during the rainy and dry periods between 2008 and 2009. In general, the reservoirs exhibited mesotrophic conditions and Cyanobacteria were the predominant phytoplankton. The results showed that the rotifers Kellicottia bostoniensis, Hexarthra mira, Keratella spp., and Polyarthra vulgaris were present, indicating nutrient-rich environments. In addition, the copepod Thermocyclops decipiens occurred in eutrophic environments. In contrast, the cladoceran Daphnia gessneri and copepod Notodiaptomus henseni were considered indicators of more desirable water quality, owing to their relationship with waters with lower levels of nutrients and suspended solids. The results support the use of these organisms as a useful tool for understanding changes in water quality and in the ecosystem processes involved.

We investigated changes in the species richness, abundance, and composition of the zooplankton community in response to the formation of a small reservoir in the Caveiras River, southern Brazil. Zooplankton were collected using a motor-pump and a plankton net (68 μm mesh), with 600 L of water filtered per sample. Sampling occurred during the pre- (April, August, and December 2011) and post-impoundment (July and October 2013, and January 2014) phases of the Caveiras River. We identified 86 taxa in this study, and rotifers were the predominant group. The species richness and abundance of the zooplankton increased after the filling of the reservoir. Furthermore, the zooplankton community showed a clear change in the species composition between the phases before and after the formation of the reservoir, with the emergence of typical planktonic species. Changes in the structure of the zooplankton community were related to changes in limnological characteristics due to the impoundment of the river, mainly in the availability of food and in the concentration of nutrients.

Aim: This study aimed to characterize the zooplankton structure in highland streams in southern Brazil, Pelotas River sub-basin, as well as the respective relationships with the environmental conditions in the different seasons. Methods Zooplankton was collected using a plankton net (68 µm mesh) and concentrated into a volume of 50 ml for later analysis in the laboratory. Sampling occurred during the winter (August 2013), spring (November 2013), summer (February 2014) and autumn (May 2014). Results Rotifers and testate amoebae were the predominant groups. Mean abundance, diversity and evenness varied considerably between the different seasons. The variability of zooplankton was correlated to environmental conditions (conductivity, suspended solids, pH, temperature and dissolved oxygen). Conclusions The seasonal variation of the zooplankton community made it possible to elucidate some questions involving the ecology of these organisms in highland streams in southern Brazil. These unique ecosystems warrant further research and monitoring efforts, including studies of their hydrological patterns, biological diversity and adaptive mechanisms of the zooplankton community.

Recentemente no Brasil começaram a ser construídas pequenas centrais hidrelétricas em rios de pequena ordem para aumentar a capacidade energética, e com isso, o rio São Francisco Verdadeiro foi barrado para a formação do reservatório da PCH São Francisco. O objetivo deste trabalho foi verificar se a formação do reservatório da PCH São Francisco, alterou positivamente a estrutura populacional de Psalidodon aff. fasciatus. As coletas trimestrais dos peixes foram realizadas em três locais amostrais, com redes de espera, nas fases pré e pós enchimento. Foram verificadas diferenças significativas na abundância, nos locais do reservatório e jusante. Na estrutura em tamanho, observou-se maior frequência de indivíduos de menor tamanho a jusante e no reservatório. Com relação à atividade reprodutiva, houve redução da intensidade no reservatório no período imediatamente após o represamento. Quanto ao fator de condição no reservatório e na jusante foram verificadas alterações positivas. Deste modo, concluímos que a implantação da PCH São Francisco influenciou positivamente a abundância, estrutura em tamanho e fator de condição de P. aff. fasciatus, o que está relacionado à maior disponibilidade de alimento e habitats após o barramento.

Effect of slackwater areas on the establishment of plankton communities (testate amoebae and rotifers) in a large river in the semi-arid region of northeastern BrazilThe increase in discharge of rivers over time and space poses challenges to the establishment of some biotas, because of higher turbulence, turbidity, hydraulic forces and limited food sources, which may directly affect the community structure. In this context, slackwater areas play an important role in the establishment of organisms, acting as refuges, reducing turbulence, turbidity and providing greater availability of food. This study investigated the changes in community structure of testate amoebae and rotifers in relation to the appearance and disappearance of slackwater areas formed by sandbars on a stretch of the Parnaíba River. The appearance of slackwaters promoted suitable environmental conditions for the establishment of rotifers. On the other hand, slackwaters acted as a buffer for testate amoebae, allowing them to colonize the plankton due to a lower suspension of sediment in these areas. We suggest that the intermittent presence of slackwaters in the Parnaíba River channel is an essential driver of abundance and succession patterns of species in the plankton, through the alternation of washing-out effects, taxon-specific potential of reproduction and biological interactions.

Aim Our objective was to describe the structure and spatial patterns of the zooplankton community in a small Brazilian river system (Paranaíba River sub-basin) and evaluate the role of an oxbow lake in the dynamics of this community. Methods Zooplankton samples were collected from sites located in an oxbow lake and the Aporé River bed during the rainy and dry seasons. We collected these samples from the surface and bottom levels of two different zones (littoral and channel). For each hydrological season, analysis of variance was conducted to determine the differences in species richness and abundance of zooplankton among different sites (longitudinal), zones (lateral), and depths (vertical). Cluster analysis was performed to assess the similarity in species composition. In addition, a canonical correspondence analysis was used to select the environmental variables that best explained the variation in zooplankton abundance data. Results We recorded 152 species: 70 rotifers, 53 testate amoebae, 20 cladocerans, and 9 copepods. We observed changes in the species richness and abundance of zooplankton along the longitudinal axis of the river in both the rainy and dry seasons. However, we did not detect any lateral or vertical changes in these attributes. Cluster analysis also indicated that the greatest differences in species composition were in the longitudinal axis of the river. The longitudinal variability of zooplankton was mainly associated with the influence of an upstream oxbow lake, as well as the changes in water flow velocity, turbidity, and concentrations of suspended solids downstream. Conclusions Our results demonstrate the importance of a heterogeneity of hydrological characteristics as a structuring factor of the zooplankton community in the Aporé River. Thus, we highlight the relevance of actions to protect and maintain the natural characteristics of these environments.