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Many arid and semiarid regions are likely to become warmer and drier by the end of this century, due to human-induced climate change. We hypothesize that a reduction in water level caused by droughts will aggravate eutrophication, leading to higher cyanobacteria biomass and dominance in tropical regions. To test this hypothesis, we analyzed physical and chemical variables and plankton communities of 40 man-made lakes in warm semiarid northeastern Brazil at the end of the wet and dry seasons. We also constructed a predictive model of cyanobacteria biovolume in these lakes. The lakes had significantly lower water volume, transparency, and CO 2 concentrations but higher water temperature, water column stability, electrical conductivity, pH, suspended solids, ammonium, total nitrogen concentrations, bacteria biomass, phytoplankton biomass, and cyanobacteria biomass and dominance in the dry than in the wet season. Our regression model suggested that cyanobacteria biovolume was positively related to water column stability, pH, and total nitrogen and negatively related to water transparency and concentrations of inorganic suspended solids. These results suggest that the projected warmer and drier climate in the future will reduce water quantity and quality of man-made lakes in the region, increasing the risks of salinization, anoxia, eutrophication, and cyanobacteria blooms.

Understanding how species are structured in space and time and how they are functionally related to environmental conditions is still a challenge in ecology. In this study, we assessed the predictive power of lake morphometry, physical and chemical conditions of the water, and zooplankton density in relation to phytoplankton taxonomic and functional diversity in Amazonian floodplain lakes during low- and high-water periods. We also examined to what extent taxonomic and functional indexes were coupled. Taxonomic diversity was evaluated by species richness and Shannon index, and functional diversity by functional richness (FRic) and community-weighted mean trait value (CWM). The relative importance of predictive factors was evaluated by model selection, multi-model inference and correlations. We found that phytoplankton taxonomic and functional diversity indexes were mostly related to the same factors within the low- and high-water periods. Total phosphorus was the main driving factor in the low water, while euphotic zone depth and zooplankton density were the main driving factors in the high water. Taxonomic and functional diversity indexes were weakly coupled in the low water, but strongly coupled in the high water. Our results highlight that phytoplankton taxonomic and functional diversity can differ between periods, but respond similarly to environmental driving factors.

Phytoplankton is widely recognized as being regulated mainly by resources (nutrients and light) and predation by higher trophic levels. In reservoirs, these controls also can be modulated by hydrology, for example through the influence of flow pulses generated by the operation of the dam. In this study, we tested the influence of light, nutrients, and zooplankton grazing pressure, and also hydrology (as water residence time) on the phytoplankton biomass in eight tropical hydroelectric reservoirs, which differ in size, morphometry, location, trophic state, and water residence time. Our hypothesis was that, as these reservoirs are used for hydroelectric purposes, the control that would otherwise be exerted on phytoplankton biomass primarily by resource availability and grazing will also be modulated by hydrology. Low phytoplankton biomass (range of system medians = 12–299 μg C l −1 ) occurred in most systems, except for one highly eutrophic reservoir (median = 1331 μg C l −1 ). Our data showed that phosphorus was more often likely to be the limiting nutrient in these systems, as assessed through nutrient limitation indexes (nitrogen and phosphorus), based on concentrations and ratios. For most reservoirs, excluding the eutrophic system with high cyanobacteria biomass, seasonal water residence time was the variable that best explained phytoplankton variation among the several environmental variables analyzed in this study ( P  < 0.0001; adjusted r 2  = 0.38). Hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability.

Based on a literature survey, we evaluated the periods of cyanobacterial dominance in Brazil. We hypothesized that variability of environmental forces along the country will promote or facilitate temporal and spatial mosaic in cyanobacterial dominance. The most striking outcomes are related to the dominance of Cylindrospermopsis , Dolichospermum , and Microcystis. Although they share important adaptive strategies (e.g., aerotopes, large size and toxins production), our findings suggest that they have different environmental preferences. Dolichospermum and Microcystis dominated mainly in warm-rainy periods whereas Cylindrospermopsis was more common during dry periods and in mixed systems, or formed perennial dominance. Maximum phosphorus concentrations were observed in reservoirs dominated by Cylindrospermopsis . Although the main genera reached high biomass levels individually, different abilities to form dominance and co-dominance were observed. The number of co-dominance of Chroococales and Nostocales was almost the same as the individual occurrence of the main genera from these groups. This dataset reveals patterns of dominance of these cyanobacteria and also indicates that physiological features will cause differences in the mechanisms of interactions between species. The understanding of these processes and their relationship to environmental conditions will promote better understanding of cyanobacterial dominance and increase our ability to predict and manage these events.

Reservoirs emit significant amounts of greenhouse gases. An analysis of data from 85 globally distributed hydroelectric reservoirs indicates that about 48 Tg carbon is emitted as carbon dioxide and 3 Tg carbon as methane, and that carbon emissions are correlated with reservoir age and latitude. Hydroelectric reservoirs cover an area of 3.4×10 5  km 2 and comprise about 20% of all reservoirs. In addition, they contain large stores of formerly terrestrial organic carbon. Significant amounts of greenhouse gases are emitted 2 , especially in the early years following reservoir creation, but the global extent of these emissions is poorly known. Previous estimates of emissions from all types of reservoir indicate that these human-made systems emit 321 Tg of carbon per year (ref.  4 ). Here we assess the emissions of carbon dioxide and methane from hydroelectric reservoirs, on the basis of data from 85 globally distributed hydroelectric reservoirs that account for 20% of the global area of these systems. We relate the emissions to reservoir age, location biome, morphometric features and chemical status. We estimate that hydroelectric reservoirs emit about 48 Tg C as CO 2 and 3 Tg C as CH 4 , corresponding to 4% of global carbon emissions from inland waters. Our estimates are smaller than previous estimates on the basis of more limited data. Carbon emissions are correlated to reservoir age and latitude, with the highest emission rates from the tropical Amazon region. We conclude that future emissions will be highly dependent on the geographic location of new hydroelectric reservoirs.

Omnivorous filter-feeding fish are common in tropical lakes and reservoirs, and can potentially reduce phytoplankton biomass in eutrophic systems. The goal of this study was to evaluate direct grazing or indirect increase in phytoplankton biomass through the trophic cascade and fish-mediated nutrient recycling produced by Nile tilapia. Natural phytoplankton assemblages were incubated in permeable chambers placed inside mesocosms with and without fish. Outside these chambers (mesocosms), phytoplankton was exposed to effects from nutrient recycling by zooplankton and fish, and to grazing by these consumers. Inside the permeable chambers, phytoplankton was exposed only to nutrient recycling by zooplankton and fish. Our results showed that in mesocosms, cyanobacteria biomass was significantly reduced by fish; water transparency and ammonium concentrations also increased, but did not affect soluble reactive phosphorus concentrations or zooplankton biomass. Fish-mediated nutrient recycling did not enhance phytoplankton growth inside permeable chambers, because phytoplankton growth was limited in this study by phosphorus availability. The estimated grazing rates showed that tilapia were able to reduce approximately 60% of phytoplankton biomass (mostly cyanobacteria). Our data indicated that fish grazing was the mechanism controlling cyanobacteria biomass. This study provides evidence that Oreochromis niloticus has the potential to reduce cyanobacteria community in eutrophic reservoirs.

Although some efforts have been made to understand the role of hydrology in structuring aquatic systems, little is known about how rainfall affects the relative roles of local environmental and spatial processes in structuring plankton metacommunities in tropical drylands. We hypothesize that the role played by spatial variables is more important in the dry than in the rainy season, because drought increases lakes isolation in semiarid regions. To test our hypothesis, we compared the variation in plankton structure between seasons attributable to local and spatial predictors by using variation-partitioning techniques. We used data for phytoplankton and zooplankton communities as a whole and separately by their size groups (nanoplankton, microplankton and mesoplankton) from 40 man-made lakes in northeastern Brazil. Our results showed that rainfall homogenized limnological variables and reduced total phytoplankton and nanophytoplankton beta diversity, but no effect of season was observed for microphytoplankton and zooplankton communities. Overall, in the dry season, both environmental and spatial variables were important structuring factors for the total phytoplankton, total zooplankton and microzooplankton communities, concurring with both niche- and neutral-based models. In the rainy season, spatial variables were neither important for shaping the phytoplankton nor the zooplankton metacommunities, confirming our hypothesis. The main ecological implication of our findings is that both niche- and neutral-based processes might play an important role on phytoplankton and zooplankton metacommunities dynamics in a future warmer and drier climate in tropical semiarid regions.

Understanding the patterns of biological diversity between different regions (beta diversity) and how diversity is generated and maintained is a central goal in ecology. The relative roles of niche differentiation and geographical distances in shaping the phytoplankton and zooplankton structures are not yet fully determined, especially considering the subsets of species that share particular traits. We evaluated whether dissimilarity in phytoplankton and zooplankton assemblages depends on environmental distances and geographical distances on the whole communities and for subsets. We sampled eight hydroelectric reservoirs over a large scale (1500 km) in central Brazil, during three climatological periods. Phytoplankton composition was correlated with niche differentiation; that is, environmental similarity would account for assemblage similarity, due to the high dispersal abilities and low turnover rates of these organisms. For zooplankton, geographical distances were more important, which resulted in a stronger spatial structure with a greater dispersal limitation. For phytoplankton, the Morphologically Based Functional Group formed by diatoms, and for zooplankton, the three major taxonomic groups appeared as spatially structured. Therefore, phytoplankton composition was mostly correlated to niche differentiation and zooplankton to geographical distances; the communities deconstructed into subsets of species with similar traits refined the responses of the community structures.

Cyanobacteria blooms are a risk to environmental health and public safety due to the potent toxins certain cyanobacteria can produce. These nuisance organisms can be removed from water bodies by biomass flocculation and sedimentation. Here, we studied the efficacy of combinations of a low dose coagulant (poly-aluminium chloride-PAC-or chitosan) with different ballast compounds (red soil, bauxite, gravel, aluminium modified zeolite and lanthanum modified bentonite) to remove cyanobacterial biomass from water collected in Funil Reservoir (Brazil). We tested the effect of different cyanobacterial biomass concentrations on removal efficiency. We also examined if zeta potential was altered by treatments. Addition of low doses of PAC and chitosan (1-8 mg Al L-1) to the cyanobacterial suspensions caused flock formation, but did not settle the cyanobacteria. When those low dose coagulants were combined with ballast, effective settling in a dose-dependent way up to 99.7% removal of the flocks could be achieved without any effect on the zeta potential and thus without potential membrane damage. Removal efficacy was influenced by the cyanobacterial biomass and at higher biomass more ballast was needed to achieve good removal. The combined coagulant-ballast technique provides a promising alternative to algaecides in lakes, ponds and reservoirs.

Eutrophication is considered the most important water quality problem in freshwaters and coastal waters worldwide promoting frequent occurrence of blooms of potentially toxic cyanobacteria. Removal of cyanobacteria from the water column using a combination of coagulant and ballast is a promising technique for mitigation and an alternative to the use of algaecides. In laboratory, we tested experimentally the efficiency of two coagulants, polyaluminium chloride (PAC) and chitosan (made of shrimp shells), alone and combined with two ballasts: red soil (RS) and the own lagoon sediment, to remove natural populations of cyanobacteria, from an urban brackish coastal lagoon. PAC was a very effective coagulant when applied at low doses (≤8 mg Al L⁻¹) and settled the cyanobacteria, while at high doses (≥16 mg Al L⁻¹) large flocks aggregated in the top of test tubes. In contrast, chitosan was not able to form flocks, even in high doses (≥16 mg L⁻¹) and did not efficiently settle down cyanobacteria when combined with ballast. The RS itself removed 33-47 % of the cyanobacteria. This removal was strongly enhanced when combined with PAC in a dose-dependent matter; 8 mg Al L⁻¹ was considered the best dose to be applied. The lagoon sediment alone did not promote any settling of cyanobacteria but removal was high when combined with PAC. Combined coagulant and ballast seems a very efficient, cheap, fast and safe curative measure to lessen the harmful cyanobacteria bloom nuisance in periods when particularly needed, such as around the 2016 Olympics in Jacarepaguá Lagoon.