The potential impact of new Andean dams on Amazon fluvial ecosystems

Mercury concentrations and differences in isotopic niches of fish from upstream and downstream of an Amazon reservoir dam

Reservoir construction promotes many environmental impacts, including the enhancement of mercury concentrations in fish. The processes that can influence mercury concentrations in fish in Amazonian reservoirs are still little explored in depth, especially when we consider the possible particularities of the ecosystems in question. This study aims to investigate how mercury concentrations in fish could be influenced by the Tucuruí dam, considering possible changes in their feeding and trophic position according to the dam position (up or downstream). Fish were sampled upstream and downstream of the Tucuruí reservoir, and total mercury (THg) and stable isotopes of carbon and nitrogen (δ15N and δ13C) were measured in muscles. We observed three different Hg bioaccumulation patterns influenced by the dam. These differences occurred due to species trophic niche changes corroborated by the isotope analysis. Higher THg concentrations downstream compared to those upstream ones were only observed for Geophagus proximus. On the contrary, Plagioscion squamosissimus, from downstream, presented lower concentrations than upstream ones. The isotopic niche of these two species presented different changes according to the sampled site. THg biomagnification was higher upstream compared to downstream, considering that the regression slope was approximately two times higher upstream versus downstream. THg concentrations in fish were explained by the differences in their feeding habits according to their location in relation to the dam. The difference in THg biomagnification was able to reflect differences in structure of the food web chain in ecosystems under the dam's influence.

Adapted Sequential Extraction Protocol to Study Mercury Speciation in Gold Mining Tailings: Implications for Environmental Contamination in the Amazon

Artisanal small-scale gold mining (ASGM), an increasingly prevalent activity in South America, generates mercury-contaminated tailings that are often disposed of in the environment, leading to the introduction of mercury into ecosystems and the food web, where it bioaccumulates. Therefore, studying the geochemical processes involved in the desorption and dissolution of mercury in these tailings is essential for critical risk evaluations in the short and long term. For this purpose, sequential extraction procedures (SEPs) can be useful because they help to identify the phases to which Hg is associated, although they also have limitations such as a lack of selectivity and specificity. In this work, we propose a modified four-step SEP: exchangeable mercury (F1), oxidizable mercury (F2), mercury bound to Fe oxides (F3), and strongly bound mercury (F4). To test this adapted sequential extraction method, we evaluated the Hg contamination in mercury-contaminated tailings of the Amazon basin. The results revealed a total mercury concentration of 103 ± 16 mg·kg-1 in the tailings, with a significant portion in F1 (28% of the total), where Hg was bioavailable. The large Hg concentration in F3 (36%) suggested that Fe oxides likely contribute to mercury retention. Together, the SEP results emphasize the urgent need for improved surveillance of gold mining activities and responsible tailings management practices to mitigate environmental contamination and safeguard the health of the Amazon ecosystem.

Temporal analysis of genetic diversity and gene flow in the threatened catfish Pseudoplatystoma magdaleniatum from a dammed neotropical river

The striped catfish Pseudoplatystoma magdaleniatum is a large-sized migratory species from the north Andes region, endemic to Magdalena basin and one of the major fishery resources. Despite the estimated reduction of over 80% of the fisheries production of this species throughout the basin in recent decades, its population in the lower Magdalena-Cauca basin showed healthy genetics after molecular analyses. However, the current conservation status of this species and several habitat disturbances demand the re-evaluation of its population genetics to infer evolutionary risks and assess potential changes. This work analyzed a total of 164 samples from the Cauca River collected downstream the Ituango Dam between 2019-2021 using species-specific microsatellite markers to compare the genetic diversity and structure in samples collected between 2010-2014 from the lower Magdalena-Cauca basin, previously analyzed. Our results showed a relatively stable panmictic population over time (4 to 10 years), with high genetic diversity and evidence of recent bottleneck. Promoting habitat connectivity to conserve gene flow, characterizing diversity and genetic structure over the entire basin, and integrating the results with future monitoring are important aspects for the management planning for P. magdaleniatum in the Magdalena-Cauca basin.

Does a hydropower reservoir cascade really harm downstream nutrient regimes

River damming is believed to largely intercept nutrients, particularly retain more phosphorus (P) than nitrogen (N), and thus harm primary productivity, fishery catches, and food security downstream, which seriously constrain global hydropower development and poverty relief in undeveloped regions and can drive geo-political disputes between nations along trans-boundary rivers. In this study, we investigated whether reservoirs can instead improve nutrient regimes downstream. We measured different species of N and P as well as microbial functions in water and sediment of cascade reservoirs in the upper Mekong River over 5 years and modelled the influx and outflux of N and P species in each reservoir. Despite partially retaining total N and total P, reservoirs increased the downstream flux of ammonium and soluble reactive phosphorus (SRP). The increase in ammonium and SRP between outflux and influx showed positive linear relationships with the hydraulic residence time of the cascade reservoirs; and the ratio of SRP to dissolved inorganic nitrogen increased along the reservoir cascade. The lentic environment of reservoirs stimulated algae-mediated conversion of nitrate into ammonium in surface water; the hypoxic condition and the priming effect of algae-induced organic matter enhanced release of ammonium from sediment; the synergy of microbial phosphorylation, reductive condition and sediment geochemical properties increased release of SRP. This study is the first to provide solid evidence that hydropower reservoirs improve downstream nutrient bioavailability and N-P balance through a process of retention-transformation-transport, which may benefit primary productivity. These findings could advance our understanding of the eco-environmental impacts of river damming.

Habitat specialization predicts demographic response and vulnerability of floodplain birds in Amazonia

The annual flooding cycle of Amazonian rivers sustains the largest floodplains on Earth, which harbour a unique bird community. Recent studies suggest that habitat specialization drove different patterns of population structure and gene flow in floodplain birds. However, the lack of a direct estimate of habitat affinity prevents a proper test of its effects on population histories. In this work, we used occurrence data, satellite images and genomic data (ultra-conserved elements) from 24 bird species specialized on a variety of seasonally flooded environments to classify habitat affinities and test its influence on evolutionary histories of Amazonian floodplain birds. We demonstrate that birds with higher specialization in river islands and dynamic environments have gone through more recent demographic expansion and currently have less genetic diversity than floodplain generalist birds. Our results indicate that there is an intrinsic relationship between habitat affinity and environmental dynamics, influencing patterns of population structure, demographic history and genetic diversity. Within the floodplains, historical landscape changes have had more severe impacts on island specialists, making them more vulnerable to current and future anthropogenic changes, as those imposed by hydroelectric dams in the Amazon Basin.

Influence of a hydroelectric dam on fish mercury contamination along the Sinnamary River (French Guiana)

The Petit Saut hydroelectric dam and the upstream and downstream areas of the Sinnamary River in French Guiana (Amazon basin) have been studied from 1993 to 2020. The nearly thirty-years-long study of the monitoring of total mercury concentration in fish and the physicochemical survey of the environment made it possible to demonstrate the impact of the flooding of the forest and the role of the hydroelectric dam on the methylation of mercury. Results show that the physicochemical modifications generated by the construction of the dam led to a significant production of methylmercury (MeHg) in the anoxic part of the reservoir and downstream of the river leading to a strong spatio-temporal impact of the dam. Seven species of fishes are studied and their mercury concentrations vary according to many parameters: fish diet, position in the water column, site, lake oxycline level and time.

Emergy-based sustainability evaluation model of hydropower megaproject incorporating the social-economic-ecological losses

The sustainable development of the hydropower megaproject (HM) is one of the critical components of sustainable water resources management. Hence, an accurate assessment of the impacts of social-economic-ecological losses (SEEL) on the sustainability of the HM system is of utmost importance. This study proposes an emergy-based sustainability evaluation model incorporating the social-economic-ecological losses (ESM-SEEL), which integrated the inputs and outputs during HM's construction and operation into an emergy calculation account. The Three Gorges Project (TGP) on the Yangtze River is selected as a case study to comprehensively evaluate the HM's sustainability from 1993 to 2020. Subsequently, the emergy-based indicators of TGP are compared with several hydropower projects in China and worldwide to analyze the multi-impacts of hydropower development. The results showed that the river chemical potential (2.35 E+24sej) and the emergy losses (L) (1.39 E+24sej) are the primary emergy inflow sections (U) of the TGP system, accounting for 51.1% and 30.4% of the U, respectively. The flood control function of the TGP produced tremendous socio-economic benefits (1.24 E+24sej), accounting for 37.8% of the total emergy yield. The resettlement and compensation, water pollution during operation, fish biodiversity loss, and sediment deposition are the main L of the TGP, accounting for 77.8%, 8.4%, 5.6%, and 2.6%, respectively. Based on the enhanced emergy-based indicators, the assessment reveals that the sustainability level of the TGP falls in the middle range compared to other hydropower projects. Thus, along with maximizing the benefits of the HM system, it is necessary to minimize the SEEL of the HM system, which is a critical approach to promote the coordinated development of the hydropower and ecological environment in the Yangtze River basin. This study helps to understand the complex relationship between human and water systems and provides a novel framework that can be used as an evaluation index and insights for hydropower sustainability assessment.

Large-scale hydropower impacts and adaptation strategies on rural communities in the Amazonian floodplain of the Madeira River

Understanding social and environmental impacts and household adaptation strategies in the face of expansions in energy infrastructure projects is essential to inform mitigation and interventions programs that promote well-being. Here we conducted surveys in seven communities distributed across varying degrees of proximity to a hydropower dam complex in the Brazilian Amazon along about 250 km of the floodplain of the Madeira River. Based on interviews with 154 fishers from these communities, we examine how fishers perceived changes in fisheries yields, changes in the composition of fish species, and whether and how adaptation strategies had evolved 8-9 years after the dams' construction. Most respondents (91%) indicated declines in yields after the dams for both upstream and downstream zones. Multivariate analyses revealed statistically significant differences in the composition of species yields in pre-and post-dam periods for all communities and in both upstream and downstream zones (p < 0.001). The composition of yields diversified after the dams, with an apparent decline in yields of species of greatest market value (e.g., catfishes Brachyplatystoma spp., Pseudoplatystoma spp., and jatuarana Brycon spp.), and increases in yields of a set of other smaller bodied and faster growing species (e.g., 'branquinhas' Psectrogaster spp., Potamohinna spp., and sardines Triportheus spp.). Both downstream and upstream fishers indicated that fishing profits decreased since the dams' construction (76.8% and 67.9%, respectively). To cope with these changes, the majority of both upstream and downstream fishers (>70%) stated they have had to devote more time to fishing after the dams were built. The time fishers spend traveling to fishing locations also increased for upstream communities (77.1%), but not for downstream communities. Thirty-four percent of the interviewees changed the gear they use to fish after the dams construction, with twice as many mentioning uses of non-selective gear, such as gillnets, and declining use of traditional fishing gears such as castnets and a trap ("covi"). Fish consumption overall decreased: fish was consumed 'everyday' before the dams, but 1-2 times per week or rarely after the dams were built. Although the species that declined were those of high economic value, 53% of fishers stated fish prices have increased overall after the dams. These results shed light on the potential challenges faced by fishers and which adaptation strategies they have evolved to maintain livelihoods since the construction of the dams.

Isotopic niche alteration of a predator fish in a dammed Amazonian black water river

Although many upstream effects of a dam on the trophic ecology of fish have been reported, little is known about their downstream effects on the isotopic niche of Amazonian predator fish. The authors used stable isotope analysis of δ¹³ C and δ¹⁵ N to determine the downstream effects of damming of the Uatumã River on the niche width, carbon energy sources and trophic position of peacock bass Cichla temensis comparing with a free-flowing river in the Amazon basin, Brazil, during the peak flood and early falling water period of 2020. They found that the C. temensis population of the undammed river had a smaller niche width than the C. temensis population of the dammed river, despite the greater number of prey trophic levels utilized and the higher trophic position of C. temensis individuals. The results demonstrate that in both rivers there is a gradual shift in the contribution of prey fish sources to the diet of C. temensis throughout its growth, even among adult individuals. They conclude that the isotopic niche of C. temensis was altered by damming during the period of late high water to early low water in the Uatumã River.

Health Risk Assessment Attributed to Consumption of Fish Contaminated with Mercury in the Rio Branco Basin, Roraima, Amazon, Brazil

The aim of this study was to assess the health risk attributable to the consumption of mercury-contaminated fish for the urban and non-urban populations living in the Roraima state, Amazon, Brazil. Seventy-five fish specimens distributed across twenty different species, comprising four trophic levels (i.e., herbivore, omnivore, detritivore, and carnivore), were collected at four locations in the Branco River Basin. The fish samples were sent to the Toxicology Laboratory at Evandro Chagas Institute to determine the total-Hg levels by using the cold vapor atomic system (CVAAS). The total-Hg levels ranged from 0 to 3.159 µg/g. The average concentration in non-carnivorous species (n = 32) was 0.116 µg/g, and among carnivorous fish (n = 43), it was 0.869 µg/g. The weighted average of contamination levels for all samples was 0.545 µg/g. The health risk assessment was conducted according to the methodology proposed by the World Health Organization and different scenarios of human exposure were considered, based on three levels of fish consumption (low: 50 g/day; moderate: 100 g/day and high: 200 g/day). Women of childbearing age ingest 5 to 21 times more mercury than the dose considered safe by the U.S. EPA and intake a dose from 2 to 9 times higher than the safe dose proposed by FAO/WHO. Children under 5 years of age ingest from 18 to 75 times the dose proposed by the U.S. EPA and from 8 to 32 more mercury than the limit proposed by FAO/WHO. In summary, regardless of the level of fish consumption, type of residency (urban or non-urban), and the subset of the population analyzed, anyone who consumes fish from the locations sampled is at high risk attributable to mercury ingestion, with the only exception of adult men, who consume an average of 50 g of fish per day.

Fish consumption habits of pregnant women in Itaituba, Tapajós River basin, Brazil: risks of mercury contamination as assessed by measuring total mercury in highly consumed piscivore fish species and in hair of pregnant women

The Tapajós River basin in the Amazon region, Brazil is one of the most active gold mining areas in the world. In this study, we evaluated fish consumption habits and mercury exposure in 110 pregnant women in the city of Itaituba by measuring their total hair mercury concentrations. In addition, we investigated seasonal differences in mercury concentrations in two highly consumed piscivorous fish species, tucunaré (Cichla spp.) and pescada (Plagioscion squamosissimus). Total fish mercury concentrations (THg) during the dry season were 0.62±0.07 mg/kg for Cichla spp. and 0.73±0.08 mg/kg for P. squamosissimus. During the rainy season they were 0.39±0.04 and 0.84±0.08 mg/ kg, respectively. Of our participants 44 % declared that they ate Cichla spp. and 67 % P. squamosissimus. Mean mercury concentration in their hair was 1.6±0.2 mg/kg and was above the US Environmental Protection Agency (US EPA) reference dose of 1 mg/kg in 48 % of them. Mean fish THg concentrations were also above the joint Food and Drug Administration and US EPA safety limit of 0.5 mg/kg for P. squamosissimus during both seasons and for Cichla spp. during the dry season only. These results show that pregnant women should avoid consumption of these piscivorous fish species during pregnancy and call for a regular programme to monitor Hg levels in that area.

Reducing adverse impacts of Amazon hydropower expansion

Proposed hydropower dams at more than 350 sites throughout the Amazon require strategic evaluation of trade-offs between the numerous ecosystem services provided by Earth's largest and most biodiverse river basin. These services are spatially variable, hence collective impacts of newly built dams depend strongly on their configuration. We use multiobjective optimization to identify portfolios of sites that simultaneously minimize impacts on river flow, river connectivity, sediment transport, fish diversity, and greenhouse gas emissions while achieving energy production goals. We find that uncoordinated, dam-by-dam hydropower expansion has resulted in forgone ecosystem service benefits. Minimizing further damage from hydropower development requires considering diverse environmental impacts across the entire basin, as well as cooperation among Amazonian nations. Our findings offer a transferable model for the evaluation of hydropower expansion in transboundary basins.

Unveiling biogeographical patterns of the ichthyofauna in the Tuichi basin, a biodiversity hotspot in the Bolivian Amazon, using environmental DNA

To date, more than 2400 valid fish species have been recorded in the Amazon basin. However, some regions remain poorly documented. This is the case in the Beni basin and in particular in one of its main sub-basins, the Tuichi, an Andean foothills rivers flowing through the Madidi National Park in the Bolivian Amazonia. The knowledge of its ichthyological diversity is, however, essential for the management and protection of aquatic ecosystems, which are threatened by the development of infrastructures (dams, factories and cities), mining and deforestation. Environmental DNA (eDNA) has been relatively little used so far in the Amazon basin. We sampled eDNA from water in 34 sites in lakes and rivers in the Beni basin including 22 sites in the Tuichi sub-basin, during the dry season. To assess the biogeographical patterns of the amazonian ichthyofauna, we implemented a metabarcoding approach using two pairs of specific primers designed and developed in our laboratory to amplify two partially overlapping CO1 fragments, one of 185bp and another of 285bp. We detected 252 fish taxa (207 at species level) among which 57 are newly identified for the Beni watershed. Species compositions are significantly different between lakes and rivers but also between rivers according to their hydrographic rank and altitude. Furthermore, the diversity patterns are related to the different hydro-ecoregions through which the Tuichi flows. The eDNA approach makes it possible to identify and complete the inventory of the ichthyofauna in this still poorly documented Amazon basin. However, taxonomic identification remains constrained by the lack of reference barcodes in public databases and does not allow the assignment of all OTUs. Our results can be taken into account in conservation and management strategies and could serve as a baseline for future studies, including on other Andean tributaries.

OUP accepted manuscript

Riverine floodplains are biologically diverse and productive ecosystems. Although tropical floodplains remain relatively conserved and ecologically functional compared to those at higher latitudes, they face accelerated hydropower development, climate change, and deforestation. Alterations to the flood pulse could act synergistically with other drivers of change to promote profound ecological state change at a large spatial scale. State change occurs when an ecosystem reaches a critical threshold or tipping point, which leads to an alternative qualitative state for the ecosystem. Visualizing an alternative state for Amazonian floodplains is not straightforward. Yet, it is critical to recognize that changes to the flood pulse could push tropical floodplain ecosystems over a tipping point with cascading adverse effects on biodiversity and ecosystem services. We characterize the Amazonian flood pulse regime, summarize evidence of flood pulse change, assess potential ecological repercussions, and provide a monitoring framework for tracking flood pulse change and detecting biotic responses.

Amazon river dolphins Inia geoffrensis are on the path to extinction in the heart of their range

Riverine cetaceans are threatened globally by increasing anthropogenic impacts, and the Yangtze river dolphin Lipotes vexillifer is already extinct. The Amazon river dolphin Inia geoffrensis is widely distributed, but is categorized as Endangered on the IUCN Red List. Drowning in gillnets and being hunted for fish bait are probably the main causes of its decline. A long-term (25-year) study of the biology and ecology of I. geoffrensis in and near the Mamirauá Reserve in Amazonas state, Brazil, has produced estimates of survival and reproductive rates based on the individual recognition of > 650 animals. Concurrently, monthly standardized counts of dolphins along a fixed transect in the Reserve over 22 years demonstrated a sustained, profound decline. In this study we tested whether the measured rate of population decline is consistent with population parameters estimated from independent data, using population viability analysis. With the known parameters as input, programme Vortex predicted population extinction in all scenarios and a 95% loss of dolphins within 50 years. With default parameters, the model predicted a 5.5% annual decline, which is consistent with that observed. We conclude that the Amazon river dolphin population is in severe decline because of human pressures, even in one of the better protected parts of its range. Unless the accidental and deliberate killing of this species is rapidly reduced, it is highly likely that the Amazon river dolphin will follow its Yangtze counterpart to extinction.

Mercury Exposure in Munduruku Indigenous Communities from Brazilian Amazon: Methodological Background and an Overview of the Principal Results

The Amazonian indigenous peoples depend on natural resources to live, but human activities' growing impacts threaten their health and livelihoods. Our objectives were to present the principal results of an integrated and multidisciplinary analysis of the health parameters and assess the mercury (Hg) exposure levels in indigenous populations in the Brazilian Amazon. We carried out a cross-sectional study based on a census of three Munduruku indigenous villages (Sawré Muybu, Poxo Muybu, and Sawré Aboy), located in the Sawré Muybu Indigenous Land, between 29 October and 9 November 2019. The investigation included: (i) sociodemographic characterization of the participants; (ii) health assessment; (iii) genetic polymorphism analysis; (iv) hair mercury determination; and (v) fish mercury determination. We used the logistic regression model with conditional Prevalence Ratio (PR), with the respective 95% confidence intervals (CI95%) to explore factors associated with mercury exposure levels ≥6.0 µg/g. A total of 200 participants were interviewed. Mercury levels (197 hair samples) ranged from 1.4 to 23.9 μg/g, with significant differences between the villages (Kruskal-Wallis test: 19.9; p-value < 0.001). On average, the general prevalence of Hg exposure ≥ 6.0 µg/g was 57.9%. For participants ≥12 years old, the Hg exposure ≥6.0 µg/g showed associated with no regular income (PR: 1.3; CI95%: 1.0-1.8), high blood pressure (PR: 1.6; CI95%: 1.3-2.1) and was more prominent in Sawré Aboy village (PR: 1.8; CI95%: 1.3-2.3). For women of childbearing age, the Hg exposure ≥6.0 µg/g was associated with high blood pressure (PR: 1.9; CI95%: 1.2-2.3), with pregnancy (PR: 1.5; CI95%: 1.0-2.1) and was more prominent among residents in Poxo Muybu (PR: 1.9; CI95%: 1.0-3.4) and Sawré Aboy (PR: 2.5; CI95%: 1.4-4.4) villages. Our findings suggest that chronic mercury exposure causes harmful effects to the studied indigenous communities, especially considering vulnerable groups of the population, such as women of childbearing age. Lastly, we propose to stop the illegal mining in these areas and develop a risk management plan that aims to ensure the health, livelihoods, and human rights of the indigenous people from Amazon Basin.

Patterns of Pelagic Fish Diversity in Floodplain Lakes of Whitewater and Blackwater Drainage Systems Within the Central Amazon River Basin of Brazil

The Amazon River Basin, one of the world’s most threatened ecosystems, has an enormous diversity of fish species, a result of temporally and spatially complex habitat containing biogeochemically different river systems. The annual hydrologic cycle results in floodplain lakes during low water and inundates forests during high water, exposing fish to different resources and environmental conditions. The two principal river systems in the central Brazilian Amazon are blackwater, with nutrient-poor acidic water, and nutrient-rich whitewater. Although species-rich, the Amazon Basin is data-poor in terms of comparative studies on a regional scale. We analyzed data sets from independent sampling studies of pelagic fish in 16 floodplain lakes, nine whitewater (Rio Solimões) and seven blackwater (Rio Negro), in the central Amazon Basin of Brazil. Our findings suggest striking similarities in pelagic fish diversity patterns. Species richness was virtually equal (165 in whitewater and 168 in blackwater). Both species richness, and number of migratory species, per lake increased toward the confluence of the rivers in both systems in our study. The proportion of unique species was also similar in whitewater lakes and blackwater (41 and 43%, respectively), boosting total regional richness to 237 species. However, species composition in whitewater lakes was more homogenous (lower β diversity), and species composition was associated with conductivity and pH in whitewater, but with dissolved oxygen and transparency in blackwater. Therefore, regional fish diversity cannot be represented by sampling one lake or even one drainage system, but must include multiple lakes from both systems. These two systems may differ in sensitivity to anthropogenic stressors such as damming and deforestation.

Home range and movements of Amazon river dolphins Inia geoffrensis in the Amazon and Orinoco river basins

Studying the variables that describe the spatial ecology of threatened species allows us to identify and prioritize areas that are critical for species conservation. To estimate the home range and core area of the Endangered (EN) Amazon river dolphin Inia geoffrensis, 23 individuals (6&female;, 17&male;) were tagged during the rising water period in the Amazon and Orinoco river basins between 2017 and 2018. The satellite tracking period ranged from 24 to 336 d (mean ± SE = 107 ± 15.7 d), and river dolphin movements ranged from 7.5 to 298 km (58 ± 13.4 km). Kernel density estimates were used to determine minimum home ranges at 95% (K95 = 6.2 to 233.9 km2; mean = 59 ± 13.5 km2) and core areas at 50% (K50 = 0.6 to 54.9 km2; mean = 9 ± 2.6 km2). Protected areas accounted for 45% of the K50 estimated core area. We observed dolphin individuals crossing country borders between Colombia and Peru in the Amazon basin, and between Colombia and Venezuela in the Orinoco basin. Satellite tracking allowed us to determine the different uses of riverine habitat types: main rivers (channels and bays, 52% of recorded locations), confluences (32%), lagoons (9.6%), and tributaries (6.2%). Satellite monitoring allowed us to better understand the ecological preferences of the species and demonstrated the importance of maintaining aquatic landscape heterogeneity and spatial connectivity for effective river dolphin conservation.

The Upper Juruá Extractive Reserve in the Brazilian Amazon: past and present†

Amazonian livelihoods are largely dependent on rivers, with local protein consumption mainly relying on several species of fish. The UJER (Upper Juruá Extractive Reserve - Reserva Extrativista do Alto Juruá) is located in the state of Acre, bordering Peru and several indigenous areas. Here we summarize the data we collected in 1993/1994 on the population living along the banks of the Juruá, Tejo, Bagé, Igarapé São João and Breu rivers on crop cultivation, animal husbandry, and use of game and fish resources. We interviewed 133 individuals (94 on the Juruá and Tejo, 16 on Bagé, 16 on Igarapé São João and 7 on Breu rivers). Our results include a comprehensive description on local livelihoods, including the most important fish species for local subsistence considering gender and seasonality, the main husbandry and game species, and the items cultivated on the local agriculture. Whenever more recent information was available in the literature, we compared changes in livelihoods over time in the same region and also with the recent patterns observed in the Lower and in the Middle Juruá River. We hope to provide useful information to understand temporal changes in local livelihoods, which can help adapt and shape the ecological management in the region.

How green can Amazon hydropower be? Net carbon emission from the largest hydropower plant in Amazonia

The current resurgence of hydropower expansion toward tropical areas has been largely based on run-of-the-river (ROR) dams, which are claimed to have lower environmental impacts due to their smaller reservoirs. The Belo Monte dam was built in Eastern Amazonia and holds the largest installed capacity among ROR power plants worldwide. Here, we show that postdamming greenhouse gas (GHG) emissions in the Belo Monte area are up to three times higher than preimpoundment fluxes and equivalent to about 15 to 55 kg CO₂eq MWh^-1 Since per-area emissions in Amazonian reservoirs are significantly higher than global averages, reducing flooded areas and prioritizing the power density of hydropower plants seem to effectively reduce their carbon footprints. Nevertheless, total GHG emissions are substantial even from this leading-edge ROR power plant. This argues in favor of avoiding hydropower expansion in Amazonia regardless of the reservoir type.

Response of vegetation to submergence along Jingjiang Reach of the Yangtze River

This work explores the changes in vegetation coverage and submergence time of floodplains along the middle and lower reaches of the Yangtze River (i.e., the Jingjiang River) and the relations between them. As the Three Gorges Dam has been operating for more than 10 years, the original vegetative environment has been greatly altered in this region. The two main aspects of these changes were discovered by analyzing year-end image data from remote sensing satellites using a dimidiate pixel model, based on the normalized difference vegetation index, and by calculating water level and topographic data over a distance of 360 km from 2003–2015. Given that the channels had adjusted laterally, thus exhibiting deeper and broader geometries due to the Three Gorges Dam, 11 floodplains were classified into three groups with distinctive features. The evidence shows that, the floodplains with high elevation have formed steady vegetation areas and could hardly be affected by runoff and usually occupied by humans. The low elevation group has not met the minimal threshold of submerging time for vegetation growth, and no plants were observed so far. Based on the facts summed up from the floodplains with variable elevation, days needed to spot vegetation ranges from 70 to 120 days which happened typically near 2006 and between 2008 and 2010, respectively, and a negative correlation was detected between submergence time and vegetation coverage within a certain range. Thus, floods optimized by the Three Gorges Dam have directly influenced plant growth in the floodplains and may also affect our ability to manage certain types of large floods. Our conclusions may provide a basis for establishing flood criteria to manage the floodplain vegetation and evaluating possible increases in resistance caused by high-flow flooding when these floodplains are submerged.

Species-level ichthyoplankton dynamics for 97 fishes in two major river basins of the Amazon using quantitative metabarcoding

The Amazon basin holds the world's largest freshwater fish diversity. Information on the intensity and timing of reproductive ecology of Amazonian fish is scant. We use a metabarcoding method by capture using a single probe to quantify species-level ichthyoplankton dynamics. We sampled the Marañón and the Ucayali rivers in Peru monthly for 2 years. We identified 97 species that spawned mainly during the flood start, the flood end or the receding periods, although some species had spawning activity in more than one period. This information was new for 40 of the species in the Amazon basin and 80 species in Peru. Most species ceased spawning for a month during a strong hydrological anomaly in January 2016, demonstrating the rapidity with which they react to environmental modifications during the breeding season. We also document another unreported event in the Amazon basin, the inverse phenology of species belonging to one genus (Triportheus). Overall larval flow in the Marañón was more than twice that of the Ucayali, including for most commercial species (between two and 20 times higher), whereas the Ucayali accounts for ~80% of the fisheries landings in the region. Our results are discussed in the light of the main anthropogenic threats to fishes, hydropower dam construction and the Hidrovía Amazónica, and should serve as a pre-impact baseline.

Modeling the Ecological Responses of Tree Species to the Flood Pulse of the Amazon Negro River Floodplains

The large flood pulse of the Amazon basin is a principal driver of environmental heterogeneity with important implications for ecosystem function and the assembly of natural communities. Understanding species ecological response to the flood pulse is thus a key question with implications for theories of species coexistence, resource management, and conservation. Yet these remain largely undescribed for most species, and in particular for trees. The large flood pulse and high tree diversity of the Negro River floodplain makes it an ideal system to begin filling this knowledge gap. We merged historical hydrologic data with 41 forest inventories under variable flooding conditions distributed across the Negro River basin, comprising a total area of 34 ha, to (i) assess the importance of flood duration as a driver of compositional variation, (ii) model the response curve shapes of 111 of the most frequent tree species in function of flood duration, and (iii) derive their niche properties (optima and tolerance). We found that flood duration is a strong driver of compositional turnover, although the majority site-to-site variation in forest composition still remains unexplained. About 73% of species responded to the flood duration gradient, exhibiting a diversity of shapes, but most frequently skewed. About 29% of species were clearly favored by flood durations &gt;120 days year–1, and 44% of species favored by shorter floods. The median niche breadth was 85 flood days year–1, corresponding to approximately 30% of the flood duration gradient. A significant subset of species (27%) did not respond to flooding, but rather exhibited wide tolerance to the flood gradient. The response models provided here offer valuable information regarding tree species differential capacity to grow, survive, and regenerate along an ecologically important gradient and are spatially valid for the Amazon Negro basin. These attributes make them an appealing tool with wide applicability for field and experimental studies in the region, as well as for vegetation monitoring and simulation models of floodplain forest change in the face of hydrologic alteration.

The Downstream Impacts of Hydropower Dams and Indigenous and Local Knowledge: Examples from the Peace-Athabasca, Mekong, and Amazon

There has been much written about the negative social and environmental impacts of large hydropower dams, particularly the impacts on people and the environment caused by flooding linked to the creation of large reservoirs. There has also long been recognition of the importance of Indigenous and local knowledge for understanding ecological processes and environmental impacts. In this paper, however, we focus on a topic that has received insufficient consideration: the downstream impacts of dams, and the role of Indigenous and local knowledge in assessing and addressing these impacts. Using examples from three river basins in different parts of the world: the Peace-Athabasca in Canada, the Mekong in mainland Southeast Asia, and the Amazon in Brazil, we demonstrate that the downstream impacts of hydropower dams are often neglected due to the frequently long distances between dams and impacted areas, jurisdictional boundaries, and the less obvious nature of downstream impacts. We contend that Indigenous or local knowledge, if applied consistently and appropriately, has important roles to play in understanding and addressing these impacts, with the goal of avoiding, reducing, and appropriately compensating for the types of environmental injustices that are frequently associated with the downstream impacts of dams.

Influence of environmental conditions on the mercury levels of the sediment along the Balbina Reservoir, Brazilian Amazon

Mercury dynamics in hydroelectric reservoirs have been studied worldwide. In tropical reservoirs, especially in those of the Amazon, the influence of geochemistry on Hg levels along this aquatic system is not well understood. The main objective of the present study was to assess the influence of environmental conditions (physical and chemical water parameters, trace element concentrations of sediment and sediment geochemistry) on mercury levels of sediment along the Balbina Reservoir (Amazon basin, Brazil). Sediment was collected along the reservoir and in the main tributaries in May 2015 (n = 10). These samples were assessed for labile iron (LFe), manganese (LMn), aluminum (LAl) and mercury (LHg) concentrations, total mercury (THg) concentrations, organic matter (OM) content, and granulometry. Concentrations in the sediment were 4-212 (LFe), 2-460 (LAl), 180-613 (LMn), < detection limit-256 (LHg), and 12-307 μg kg^-1 dry weight (THg). In general, these concentrations decreased along the reservoir from upstream to downstream, reaching the lowest concentrations in the middle of the lake, and they increased in the sampling points near the dam. The lability and concentrations of mercury were influenced by the concentrations of LFe, LMn, LAl, and the granulometry and OM content of the sediment. Altogether, THg concentrations of sediments of the Balbina Reservoir encompass the range of concentrations of other reservoirs or natural lakes in the Amazon basin (from ten to a few hundred μg kg^-1 dry weight).

The most important fishery resource in the Amazon, the migratory catfish Brachyplatystoma vaillantii (Siluriformes: Pimelodidae), is composed by an unique and genetically diverse population in the Solimões-Amazonas River System

ABSTRACT The migratory catfish Brachyplatystoma vaillantii is one of the most important fishery resources in the Amazon. Intense capture occurs associated to its life cycle. In order to know the genetic status, we sequenced the mitochondrial DNA control region from 150 individuals of B. vaillantii, collected in five fishing landing locations, covering the length of the Solimões-Amazonas River in Brazil. Genetic diversity parameters suggest there is no genetic differentiation between the five localities. Population’s expansion indicated by R 2 and Fu’s Fs tests was also confirmed by the high number of unique haplotypes found. The Analyses of molecular variance indicated that nearly all variability was contained within locations (99.86%), and estimates of gene flow among B. vaillantii were high (F ST = 0.0014). These results suggest that Brachyplatystoma vaillantii forms a panmitic population along the Solimões-Amazonas River and, has greater genetic variability than other species of the Brachyplatystoma genus available so far. Although the influence of different tributaries on B. vaillantii migration patterns remains uncertain, a single population in the main channel should be consider in future policies for management of this resource. However, since the species’ life cycle uses habitats in several countries, its management and conservation depend greatly of internationally joined efforts.

A cascade of dams affects fish spatial distributions and functional groups of local assemblages in a subtropical river

Abstract Dams reduce the longitudinal connectivity of rivers and thereby disrupt fish migration and the spatial distribution of species, impacts that remain poorly studied for some Neotropical rivers from mega-diverse basins. We investigated the spatial distribution of fish species with different trophic and movement/reproductive/size characteristics to assess how functional groups have responded to a cascade of dams on the Uruguai River in southern Brazil. Fish abundance, biomass, and species composition were evaluated at eight locations along the longitudinal gradient. The fish assemblage in the upper stretch was mainly characterized by small and medium-sized species at higher trophic levels, whereas the sites located furthest downstream displayed more medium and large-sized species, including many carnivorous species. Species with high fecundity, seasonal migrants, and catfishes with internal fertilization were common in the river´s middle and lower reaches. Detritivorous species dominated areas distant from the dams. Overall, functional diversity of local fish assemblages was greater in lower reaches. The cascade of dams has impacted the distribution of functional groups of local fish assemblages of Uruguai River. The alteration of functional groups in upper reaches of the river has potential consequences for ecosystem processes and services, such as nutrient cycling and fisheries.

Mercury: What can we learn from the Amazon?

Mercury is among the ten most dangerous chemicals for public health, and is a priority concern for the 128 signatory countries of the Minamata Convention. Mercury emissions to the atmosphere increased 20% between 2010 and 2015, with South America, Sub-Saharan Africa and Southeast Asia as the main contributors. Approximately 80% of the total mercury emissions in South America is from the Amazon, where the presence of the metal is ubiquitous and highly dynamic. The presence of this metal is likely increasing, with global consequences, due to events of the last two years including extensive biomass burning and deforestation, as well as mining activities and the construction of large-scale projects, such as dams. Here we present a concise profile of this mobilization, highlighting the human exposure to this metal in areas without mining history. Mercury reaches the food chain in its most toxic form, methylmercury, intoxicating human populations through the intake of contaminated fish. Amazonian populations present levels over 6 ppm of hair mercury and, according to the 175:250:5:1 ratio for methylmercury intake : mercury hair : mercury brain : mercury blood, consume 2-6 times the internationally recognized reference doses. This exposure is alarmingly higher than that of other populations worldwide. A possible biphasic behavior of the mercury-related phenomena, with consequences that may not be observed in populations with lower levels, is hypothesized, supporting the need of improving our knowledge of this type of chronic exposure. It is urgent that we address this serious public health problem in the Amazon, especially considering that human exposure may be increasing in the near future. All actions in this region carry the potential to have global repercussions.

The combined effects of climate change and river fragmentation on the distribution of Andean Amazon fishes

Upstream range shifts of freshwater fishes have been documented in recent years due to ongoing climate change. River fragmentation by dams, presenting physical barriers, can limit the climatically induced spatial redistribution of fishes. Andean freshwater ecosystems in the Neotropical region are expected to be highly affected by these future disturbances. However, proper evaluations are still missing. Combining species distribution models and functional traits of Andean Amazon fishes, coupled with dam locations and climatic projections (2070s), we (a) evaluated the potential impacts of future climate on species ranges, (b) investigated the combined impact of river fragmentation and climate change and (c) tested the relationships between these impacts and species functional traits. Results show that climate change will induce range contraction for most of the Andean Amazon fish species, particularly those inhabiting highlands. Dams are not predicted to greatly limit future range shifts for most species (i.e., the Barrier effect). However, some of these barriers should prevent upstream shifts for a considerable number of species, reducing future potential diversity in some basins. River fragmentation is predicted to act jointly with climate change in promoting a considerable decrease in the probability of species to persist in the long-term because of splitting species ranges in smaller fragments (i.e., the Isolation effect). Benthic and fast-flowing water adapted species with hydrodynamic bodies are significantly associated with severe range contractions from climate change.

Dynamics of Hg and MeHg in the Madeira River basin (Western Amazon) before and after impoundment of a run-of-river hydroelectric dam

The impact of a hydroelectric run-of-river (RoR) dam construction on the dynamics of total mercury (THg) and methylmercury (MeHg) is of interest to the environment and health of human and wild life. We monitored (May 2010 to October 2018) THg and MeHg in the waters and in the suspended particulate matter (SPM) of the Madeira River and its tributaries (before and after dam construction) to evaluate changes upstream and downstream from the Santo Antonio Hydroelectric Dam (SAHD). We collected 2826 samples of water and SPM at sampling stations upstream (UPMD-01, UPMD-02 and UPMD-03) and downstream the Madeira River (DWMD-04, DWMD-05 and DWMD-06), and Tributaries upstream (Branco River, Jaci-Parana River, Jatuarana-I Igarapé, Contra River, Caripunas Igarapé, Ceara Igarapé, and Teotonio Igarapé) and downstream (Jatuarana-II Igarapé and Belmont Igarapé) from the SAHD and monitored water and the total load of SPM. SPM was significantly higher in the Madeira River (median: 178.63 mg.L^-1) than in upstream and downstream tributaries (median: 15.30 mg.L^-1); however, the THg and MeHg concentrations were significantly higher in the tributaries (median: 161.14 ng g^-1 and 9.03 ng g^-1, respectively) than in the mainstream Madeira River (median = 57.06 ng g^-1 and 1.63 ng g^-1, respectively). THg concentration in the water was significantly higher in the mainstream Madeira River (median = 6.51 ng.L^-1) than in the tributaries (median = 2.57 ng.L^-1). However, the percentage of methylation in the tributaries (median = 4.9%) was 4-times higher than in the Madeira River (median: 1.3%). The significantly higher MeHg percentages in the tributaries may indicate natural (hydro-bio-geochemical factors) still predominant in this changing landscape of the Western Amazon. So far, the data suggest that this RoR has not yet impacted the THg and MeHg concentrations.

Changes in taxonomic and functional diversity of fish communities after catastrophic habitat alteration caused by construction of Three Gorges Dam

Habitat alterations that result from anthropogenic disturbance impact both the abiotic and biotic conditions of ecosystems, causing changes in biodiversity in many parts of the world. Recently, the use of functional diversity has been suggested as an approach to better evaluate the effects of such disturbance on particular communities. Here, we investigated the temporal changes in species and functional diversities of fish communities in the downstream area of the Three Gorges Dam (TGD) before, during, and after impoundment. We found two regime shifts in the fish community in 2004 and 2013 following impoundment. Although taxonomic diversity declined sharply at the first regime shift, it increased at the second shift. On the other hand, functional diversity declined throughout the same period, indicating the loss of functional diversity despite increased species diversity. Our analysis also showed that the fish communities shifted from under-dispersion to over-dispersion due to both a decrease in the relative abundance of migratory fish and an increase in the number of fish adapted to the new hydrologic conditions. Our results indicated that the impacts of dams on downstream fish communities may change over time. Interactions between species may become more important when the environment is stable.

Analysis of Suspended Sediment in the Anavilhanas Archipelago, Rio Negro, Amazon Basin

This article analyzes the flows of water and total suspended sediment in different reaches in the lower course of the Negro River, the largest fluvial blackwater system in the world. The area under study is the Anavilhanas Archipelago, which is a complex multichannel reach on the Negro River. Between the years 2016 and 2019, data about water discharge, velocity, and concentration of total suspended solids (TSS) were acquired in sample sections of the Negro River channels located upstream, inside, and downstream of the Anavilhanas Archipelago. In the study area, the Negro River drains an area greater than 700,000 km2, and the mean water discharge observed before the Anavilhanas was about 28.655 m3·s−1, of which 97% flows through two channels of the Archipelago close to the right and left banks. The mean TSS concentration of the Negro River upstream and downstream the Archipelago was 3.28 mg·L−1 and 1.63 mg·L−1, respectively. Within the Archipelago, we observed more TSS in the channel on the left bank of the Negro River (mean of 4.50 mg·L−1). The total suspended sediment discharge of the Negro River before (3.14 Mt·year−1) and after (1.43 Mt·year−1) the Anavilhanas Archipelago indicates a 55% retention of the suspended load due to the low water slope and reduced flow velocity caused by the backwater effect of Solimões River on the Negro River. The hydro-sedimentary scenario of the low course of the Negro River characterized in this study indicates a slow and continuous sedimentation process in the Anavilhanas Archipelago. The results presented will serve as a baseline to assess the impacts of the dams on the Branco River, the main tributary for both water and sediment in the Negro River basin.

A global perspective on tropical montane rivers

Tropical montane rivers (TMR) are born in tropical mountains, descend through montane forests, and feed major rivers, floodplains, and oceans. They are characterized by rapid temperature clines and varied flow disturbance regimes, both of which promote habitat heterogeneity, high biological diversity and endemism, and distinct organisms' life-history adaptations. Production, transport, and processing of sediments, nutrients, and carbon are key ecosystem processes connecting high-elevation streams with lowland floodplains, in turn influencing soil fertility and biotic productivity downstream. TMR provide key ecosystem services to hundreds of millions of people in tropical nations. In light of existing human-induced disturbances, including climate change, TMR can be used as natural model systems to examine the effects of rapid changes in abiotic drivers and their influence on biodiversity and ecosystem function.

Dams, Chinese investments, and EIAs: A race to the bottom in South America?

The political economy of dam development in South America is changing as a result of a resurgence in water infrastructure investments. The arrival of Chinese-funded projects in the region has altered a context traditionally dominated by multilateral development banks. Tensions are escalating around new dam projects and the environmental impact assessment process is increasingly the site of politicization around water in the region. In this perspective, we examine the most recent surge in dam development in South America, the resulting environmental and social impacts, and the mobilization of civil society and environmental groups that have developed in response to these projects. In the absence of regionally shared standards for environmental assessment and regional mechanisms to mitigate the emerging conflicts-primarily occurring between companies, states, and civil society-we argue there is a risk of a race to the bottom to finance infrastructure projects with laxer environmental and social standards.

The use of dead stand trees by birds: a prospective study in a Brazilian hydroelectric dam

Abstract: Hydropower constructions cause severe disturbances to biodiversity. Arboreal plants die after flooding; however, they expose their emerged branches (also called paliteiros) as a new habitat for animals and plants. Nothing is known about how paliteiros influence the presence of aquatic birds. This study verified which species of birds use the paliteiros, what are their behaviors and probably consequences to ecological processes. In November 2018, we sampled 5 km of paliteiros recording perched birds and their behaviors in the Três Marias dam, a 1040 km2 flooded area in southeastern Brazil. In total, 5.4% of paliteiros were used by 14 bird species. The Neotropic Cormorant Nannopterum brasilianus (Gmelin, 1789) was the most frequent species. Birds mainly preened feathers; they also rested (with no apparent activity), defecated, hunted, fed, vocalized and drank water. Attention must be focused to some behaviors such as resting and defecation, which may influence long-term ecological processes (e.g., incorporating additional organic matter and changing aquatic community) in the dam.

Mercury in Populations of River Dolphins of the Amazon and Orinoco Basins

In the Amazon and Orinoco basins, mercury has been released from artisanal and industrial gold mining since the Colonial time, as well as a result of deforestation and burning of primary forest, that release natural deposits of methyl mercury, affecting the local aquatic vertebrate fauna. This study reports the presence of mercury in river dolphins' genera Inia and Sotalia. Mercury concentrations were analysed in muscle tissue samples collected from 46 individuals at the Arauca and Orinoco Rivers (Colombia), the Amazon River (Colombia), a tributary of the Itenez River (Bolivia) and from the Tapajos River (Brazil). Ranges of total mercury (Hg) concentration in muscle tissue of the four different taxa sampled were: I. geoffrensis humboldtiana 0.003-3.99 mg kg^-1 ww (n = 21, M_e = 0.4), I. g. geoffrensis 0.1-2.6 mg kg^-1 ww (n = 15, M_e = 0.55), I. boliviensis 0.03-0.4 mg kg^-1 ww (n = 8, M_e = 0.1) and S. fluviatilis 0.1-0.87 mg kg^-1 ww (n = 2, M_e = 0.5). The highest Hg concentration in our study was obtained at the Orinoco basin, recorded from a juvenile male of I. g. humboldtiana (3.99 mg kg^-1 ww). At the Amazon basin, higher concentrations of mercury were recorded in the Tapajos River (Brazil) from an adult male of I. g. geoffrensis (2.6 mg kg^-1 ww) and the Amazon River from an adult female of S. fluviatilis (0.87 mg kg^-1 ww). Our data support the presence of total Hg in river dolphins distributed across the evaluated basins, evidencing the role of these cetaceans as sentinel species and bioindicators of the presence of this heavy metal in natural aquatic environments.

On the Relationship between Suspended Sediment Concentration, Rainfall Variability and Groundwater: An Empirical and Probabilistic Analysis for the Andean Beni River, Bolivia (2003–2016)

Fluvial sediment dynamics plays a key role in the Amazonian environment, with most of the sediments originating in the Andes. The Madeira River, the second largest tributary of the Amazon River, contributes up to 50% of its sediment discharge to the Atlantic Ocean, most of it provided by the Andean part of the Madeira basin, in particular the Beni River. In this study, we assessed the rainfall (R)-surface suspended sediment concentration (SSSC) and discharge (Q)-SSSC relationship at the Rurrenabaque station (200 m a.s.l.) in the Beni Andean piedmont (Bolivia). We started by showing how the R and Q relationship varies throughout the hydrological year (September to August), describing a counter-clockwise hysteresis, and went on to evaluate the R–SSSC and Q–SSSC relationships. Although no marked hysteresis is observed in the first case, a clockwise hysteresis is described in the second. In spite of this, the rating curve normally used ( SSSC = aQ b ) shows a satisfactory R2 = 0.73 (p < 0.05). With regard to water discharge components, a linear function relates the direct surface flow Qs–SSSC, and a hysteresis is observed in the relationship between the base flow Qb and SSSC. A higher base flow index (Qb/Q) is related to lower SSSC and vice versa. This article highlights the role of base flow on sediment dynamics and provides a method to analyze it through a seasonal empirical model combining the influence of both Qb and Qs, which could be employed in other watersheds. A probabilistic method to examine the SSSC relationship with R and Q is also proposed.

Limnological effects of a large Amazonian run-of-river dam on the main river and drowned tributary valleys

Run-of-river dams are often considered to have lower environmental impacts than storage dams due to their smaller reservoirs and low potential for flow alteration. However, this has been questioned for projects recently built on large rivers around the world. Two of the world's largest run-of-river dams-Santo Antônio and Jirau-were recently constructed on the Madeira River, a major tributary to the Amazon River in Brazil. Here we evaluate the effects of the creation of the Santo Antônio dam on the water chemistry and thermal structure of the Madeira River mainstem and back-flooded valleys of tributaries within the reservoir inundated area. In contrast to the mainstem river, some back-flooded tributaries periodically developed thermal stratification, which is associated with higher water residence times. Additionally, biochemical oxygen demand, partial pressure of CO2, and organic carbon all increased in the tributary valleys inundated by the reservoir, possibly due to increased input of allochthonous organic matter and its subsequent mineralization upon back-flooding-a common feature of newly flooded impoundments. The mainstem did not show detectable dam-related changes in water chemistry and thermal structure. Although the majority of the reservoir area maintained riverine conditions, the lateral valleys formed upon back-flooding-corresponding to ~30% of the Santo Antônio reservoir area-developed lake-like conditions akin to a typical reservoir of a storage dam.

Reducing greenhouse gas emissions of Amazon hydropower with strategic dam planning

Hundreds of dams have been proposed throughout the Amazon basin, one of the world's largest untapped hydropower frontiers. While hydropower is a potentially clean source of renewable energy, some projects produce high greenhouse gas (GHG) emissions per unit electricity generated (carbon intensity). Here we show how carbon intensities of proposed Amazon upland dams (median = 39 kg CO2eq MWh-1, 100-year horizon) are often comparable with solar and wind energy, whereas some lowland dams (median = 133 kg CO2eq MWh-1) may exceed carbon intensities of fossil-fuel power plants. Based on 158 existing and 351 proposed dams, we present a multi-objective optimization framework showing that low-carbon expansion of Amazon hydropower relies on strategic planning, which is generally linked to placing dams in higher elevations and smaller streams. Ultimately, basin-scale dam planning that considers GHG emissions along with social and ecological externalities will be decisive for sustainable energy development where new hydropower is contemplated.

Quantifying the impacts of dams on riverine hydrology under non-stationary conditions using incomplete data and Gaussian copula models

Across the world, the assessment of environmental impacts attributable to infrastructure and development projects often require a comparison between observed post-impact outcomes with what "would have happened" in the absence of the impact (i.e., the counterfactual). Environmental impact assessment (EIA) methods traditionally determine the counterfactual based on strong assumptions of stationarity (e.g., using before and after comparisons) and can be particularly challenging to use in the context of substantial data gaps, a vexing problem when combining several time-series data from different sources. Here we propose and test a widely applicable statistical approach for quantifying environmental impacts that avoids the stationarity assumption and circumvents issues associated with data gaps. Specifically, we used a Gaussian Copula (GC) model to assess the hydrological impacts of the Tucuruí dam on the Tocantins River in the Brazilian Amazon. Using multi-source water level and climate data, GC predictions of pre-dam hydrology for the validation period were excellent (Nash-Sutcliffe coefficients of 0.83 to 0.98 and 93-96% of observations within the 95% predictive intervals). In the post-dam period, the river had higher dry-season water levels both upstream and downstream relative to the predicted counterfactual, and the timing and duration of wet-season drawdown was delayed and extended, substantially altering the flood pulse. These impacts were evident as far as 176 km away from the dam, highlighting widespread hydrological impacts. The GC model outperformed standard multiple regression models in representing predictive uncertainty while also avoiding the stationarity assumption and circumventing the issue of sparse and incomplete data. We thus believe the GC approach has wide utility for integrating disparate time-series data to quantify the impacts of dams and other anthropogenic phenomena on riverine hydrology globally.

Genetics and telemetry indicate unexpected movements among structured populations for Brachyplatystoma platynemum in the Amazon

The genetic analysis of Brachyplatystoma platynemum individuals sampled from the lower Madeira River reinforces the existence of two structured populations in the Amazon Basin (Madeira and Amazon populations). However, the recapture of an individual from the Amazon population in the Solimões River, which was telemetry-tagged in the Madeira River after the damming, indicates that fish from the Amazon population move between the two river systems. This has not yet been observed, however, in the Madeira River population, which is currently divided and isolated in the lower and upper Madeira River by the construction of two dams.

Retrieving Total and Inorganic Suspended Sediments in Amazon Floodplain Lakes: A Multisensor Approach

Remote sensing imagery are fundamental to increasing the knowledge about sediment dynamics in the middle-lower Amazon floodplains. Moreover, they can help to understand both how climate change and how land use and land cover changes impact the sediment exchange between the Amazon River and floodplain lakes in this important and complex ecosystem. This study investigates the suitability of Landsat-8 and Sentinel-2 spectral characteristics in retrieving total (TSS) and inorganic (TSI) suspended sediments on a set of Amazon floodplain lakes in the middle-lower Amazon basin using in situ Remote Sensing Reflectance (Rrs) measurements to simulate Landsat 8/OLI (Operational Land Imager) and Sentinel 2/MSI (Multispectral Instrument) bands and to calibrate/validate several TSS and TSI empirical algorithms. The calibration was based on the Monte Carlo Simulation carried out for the following datasets: (1) All-Dataset, consisting of all the data acquired during four field campaigns at five lakes spread over the lower Amazon floodplain (n = 94); (2) Campaign-Dataset including samples acquired in a specific hydrograph phase (season) in all lakes. As sample size varied from one season to the other, n varied from 18 to 31; (3) Lake-Dataset including samples acquired in all seasons at a given lake with n also varying from 17 to 67 for each lake. The calibrated models were, then, applied to OLI and MSI scenes acquired in August 2017. The performance of three atmospheric correction algorithms was also assessed for both OLI (6S, ACOLITE, and L8SR) and MSI (6S, ACOLITE, and Sen2Cor) images. The impact of glint correction on atmosphere-corrected image performance was assessed against in situ glint-corrected Rrs measurements. After glint correction, the L8SR and 6S atmospheric correction performed better with the OLI and MSI sensors, respectively (Mean Absolute Percentage Error (MAPE) = 16.68% and 14.38%) considering the entire set of bands. However, for a given single band, different methods have different performances. The validated TSI and TSS satellite estimates showed that both in situ TSI and TSS algorithms provided reliable estimates, having the best results for the green OLI band (561 nm) and MSI red-edge band (705 nm) (MAPE < 21%). Moreover, the findings indicate that the OLI and MSI models provided similar errors, which support the use of both sensors as a virtual constellation for the TSS and TSI estimate over an Amazon floodplain. These results demonstrate the applicability of the calibration/validation techniques developed for the empirical modeling of suspended sediments in lower Amazon floodplain lakes using medium-resolution sensors.

Massive tree mortality from flood pulse disturbances in Amazonian floodplain forests: The collateral effects of hydropower production

Large dams built for hydroelectric power generation alter the hydrology of rivers, attenuating the flood pulse downstream of the dam and impacting riparian and floodplain ecosystems. The present work mapped black-water floodplain forests (igapó) downstream of the Balbina Reservoir, which was created between 1983 and 1987 by damming the Uatumã River in the Central Amazon basin. We apply remote sensing methods to detect tree mortality resulting from hydrological changes, based on analysis of 56 ALOS/PALSAR synthetic aperture radar images acquired at different flood levels between 2006 and 2011. Our application of object-based image analysis (OBIA) methods and the random forests supervised classification algorithm yielded an overall accuracy of 87.2%. A total of 9800 km² of igapó forests were mapped along the entire river downstream of the dam, but forest mortality was only observed below the first 49 km downstream, after the Morena rapids, along an 80-km river stretch. In total, 12% of the floodplain forest died within this stretch. We also detected that 29% of the remaining living igapó forest may be presently undergoing mortality. Furthermore, this large loss does not include the entirety of lost igapó forests downstream of the dam; areas which are now above current maximum flooding heights are no longer floodable and do not show on our mapping but will likely transition over time to upland forest species composition and dynamics, also characteristic of igapó loss. Our results show that floodplain forests are extremely sensitive to long-term downstream hydrological changes and disturbances resulting from the disruption of the natural flood pulse. Brazilian hydropower regulations should require that Amazon dam operations ensure the simulation of the natural flood-pulse, despite losses in energy production, to preserve the integrity of floodplain forest ecosystems and to mitigate impacts for the riverine populations.

Decline of Fine Suspended Sediments in the Madeira River Basin (2003–2017)

The Madeira River is the second largest Amazon tributary, contributing up to 50% of the Amazon River’s sediment load. The Madeira has significant hydropower potential, which has started to be used by the Madeira Hydroelectric Complex (MHC), with two large dams along the middle stretch of the river. In this study, fine suspended sediment concentration (FSC) data were assessed downstream of the MHC at the Porto Velho gauging station and at the outlet of each tributary (Beni and Mamoré Rivers, upstream from the MHC), from 2003 to 2017. When comparing the pre-MHC (2003–2008) and post-MHC (2015–2017) periods, a 36% decrease in FSC was observed in the Beni River during the peak months of sediment load (December–March). At Porto Velho, a reduction of 30% was found, which responds to the Upper Madeira Basin and hydroelectric regulation. Concerning water discharge, no significant change occurred, indicating that a lower peak FSC cannot be explained by changes in the peak discharge months. However, lower FSCs are associated with a downward break in the overall time series registered at the outlet of the major sediment supplier—the Beni River—during 2010.