Community-based monitoring reveals the impacts of the permanent river drought imposed by the Belo Monte Hydroelectric Power Plant at Volta Grande do Xingu, Amazonia

doi:10.21203/rs.3.rs-5090322/v1

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Community-based monitoring reveals the impacts of the permanent river drought imposed by the Belo Monte Hydroelectric Power Plant at Volta Grande do Xingu, Amazonia

https://doi.org/10.21203/rs.3.rs-5090322/v1

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Hydroelectric dams, once seen as clean and renewable energy sources, have been the subject of extensive research, particularly concerning their socio-environmental impacts. The Belo Monte Hydroelectric Power Plant (HPP) relies on the operation of two dams, diverting water from a 130 km stretch of the Xingu River to generate energy, disrupting the natural flooding cycle along the Volta Grande do Xingu (VGX) and creating a riverine hydrological condition equivalent to a prolonged extreme dry season in the watershed. Before the Belo Monte HPP, local communities relied on the predictability of the seasonally flooded forest and the synchrony of the reproductive cycle of a highly diverse fish assemblage. VGX residents observed the transformation firsthand, noting the water discharge shortage that critically undermined the river's capacity to sustain vital ecosystem processes that support local people's lives. The Belo Monte HPP environmental licensing process ignored the vital link between the river's flood pulse, the aquatic and seasonally flooded ecosystems, and the traditional lifestyles of VGX residents. The local communities reacted and sought partnerships, establishing the Independent Territorial Environmental Monitoring Program (MATI-VGX). Data acquired by the MATI-VGX evidenced the drastic transformations of traditional lifestyles, shifts in fishing practices, and a significant decline in fishing yield, jeopardizing food sovereignty and security. Here we present evidence of hydrological-induced disruptions in ecological cycles along the VGX, which are intrinsically connected to the modes of life of local populations. To ensure the ecological sustainability of the VGX, the Belo Monte HPP operation must change to support key spawning areas, maintain water quality, avoid short-term water discharge fluctuations, and emulate natural interannual discharge variability, mitigating the flood pulse disruption. Local ecological knowledge should never be ignored in projects where local communities are the most affected. These communities should be central in decision-making regarding socio-environmental impact assessment, mitigation, and monitoring.

Conservation Biology

Socio-environmental impacts

Indigenous and ribeirinhos communities

Spawning

Fishing productivity

Belo Monte HPP

The South American Monsoon System (Marengo et al. 2010) gives origin to a seasonal variation in water and sediment discharge of the large Amazonian rivers generating a yearly "flood pulse", which supports the world’s largest seasonally flooded environments (Junk 1989). Seasonally flooded forests account for at least 9% of the Amazon basin (Hess et al. 2015) and host distinct fauna and flora, often exclusive to these habitats (Myster 2016; Ramalho et al. 2016). The yearly flood pulse is the primary determinant of ecological processes in these environments (Junk 1989). Most trees of seasonally flooded forests synchronize the flowering and fruiting with the rising water level, which is part of an ecological strategy that efficiently disperses propagules by water or animals (Gottsberger 1978; Goulding 1980). This hydrological synchrony in plant reproduction also provides fishes, chelonians, and other aquatic animals with a rich and abundant food source. Many fish species migrate across freshwater habitats along their life cycle to reproduce, feed, and seek refuge (Goulding 1980; Lucas et al. 2001b, 2001a). There is an important link between the timing of riverine hydrological cycles and species migrations, with reproduction as the primary driver. Fish species migrations are crucial for the ecological integrity of Amazonian ecosystems and to support Indigenous and ribeirinhos (local river dweller) communities, which are deeply dependent on river wildlife for subsistence and cultural survival (Begossi et al. 2018). Approximately 223 fish species are known to perform freshwater migrations in the Amazon basin, with the majority categorized as lateral migrants (Herrera-R et al. 2024). Thus, the biological and ecological cycles of fishes are fine-tuned with the Amazonian flood pulse duration, amplitude, and predictability, which means that hydrological changes that disrupt the phenology of the seasonally flooded forest will have negative cascade impacts on the populations of aquatic animals, leading to disturbances in fisheries activities and yield (e.g. Forsberg et al. 2017), inducing loss of important ecological relations (e.g. Correa et al. 2022) and disrupting ecosystem integrity (Pelicice et al. 2023).

Beyond being home to a unique segment of the Amazonia's biodiversity, seasonally flooded ecosystems sustain the traditional ways of life of Indigenous and ribeirinhos populations, intricately synchronized with the river’s annual discharge variation (Goulding et al. 1996zéquel et al. 2020; Junk et al. 2020). However, the high discharge combined with the significant slope in some sectors of Amazonian rivers that drain the Central Brazil and Guianas plateaus is also attractive for the construction of large hydroelectric power plants (HPP), like the Belo Monte HPP, a complex of two consecutive dams concluded in 2016 in the Xingu River (Sawakuchi et al. 2015). Several studies have pointed out the large, far-reaching, and interlinked impacts of damming Amazonian rivers, disrupting the annual flood pulse and reducing hydrological connectivity (Lees et al. 2016; Latrubesse et al. 2017; Zuanon et al. 2019; Bertassoli Jr et al. 2021; Pezzuti et al. 2024). Yet, hydropower is still often considered clean energy, and run-of-the-river dams are considered even less harmful (Chowdhury et al. 2024). However, impact assessment and monitoring of impacts, which in Brazil are carried out by the companies responsible for the construction and/or operation of hydropower complexes through third-party consultants that work for them, often under confidentiality rules, are inadequate and fail to account for impacts on the most affected ecosystems that depend on seasonal flooding (Ritter et al. 2017; RTAC/USAID 2020; Martins et al. 2024).

The disruptions in the natural flooding patterns caused by dams have harsh social impacts, including the displacement of communities, loss of land, livelihoods, and cultural heritage, as well as heightened territorial conflicts and health issues (Moran et al. 2018; Ribeiro & Morato 2020; Schapper & Urban 2021; Mayer et al. 2023; Ty et al. 2023). The decline in fisheries activities and yield caused by altered community structures and functional traits of fish directly affects food sovereignty and security, and economic stability for local human populations (Reis et al. 2016; Santos et al. 2018; Duponchelle et al. 2021). In addition, the feasibility of hydroelectric projects in rivers of eastern Amazonia, such as the Xingu, has been questioned due to predicted energy insecurity arising from intensified droughts caused by a combination of deforestation and climate change (Stickler et al. 2013; Prado Jr et al. 2016; Almeida et al. 2021; Legg 2021).

The Belo Monte HPP, operated by the Norte Energia consortium, features a run-of-the-river design aimed at minimizing reservoir size. However, it stands out due to its unique configuration, which includes two dams. The first dam diverts a significant portion of the Xingu River's water to an off-channel reservoir (Intermediate Reservoir) that supplies water to the main powerhouse, located at the second dam (Fig. 1). As a result, the stretch of 130 km of the Xingu River between the two dams, known as the Volta Grande do Xingu (VGX), experiences reduced water flow, controlled by the HPP's operation. This operation follows two diversion hydrographs (based on the monthly median of water discharge), referred to as A and B, which significantly decrease the water flow at VGX (Pezzuti et al. 2018; Utsunomiya et al. 2024), creating a hydrological condition akin to a permanent state of severe drought (Fig. 2A).

The VGX region hosts high and endemic biodiversity (Winemiller et al. 2016; Zuluaga-Gómez et al. 2016; Evers et al. 2019), which historically sustains and has been protected by local indigenous and ribeirinhos communities (Pezzuti et al. 2018; Zuanon et al. 2019). The construction and operation of the Belo Monte HPP imposed a sudden shift in the ecosystems and traditional lifestyles in VGX, causing conflicts between local communities and representatives of the Belo Monte HPP. The essence of this conflict is the water sharing between maintaining the VGX ecosystems or maximizing energy production by the Belo Monte HPP. The dispute for water in the VGX has become a crucial element for the maintenance of the way of life of Indigenous Peoples, such as the Juruna Yudjá of the Paquiçamba Indigenous Territory (IT), the Arara of the Arara of the VGX IT, and the Mebengokre-Xikrin of the Trincheira-Bacajá IT, as well as ribeirinhos communities. The biocultural diversity of the communities in the Volta Grande do Xingu is rooted in their resilience and determination to defend and protect the river and its flood pulse. These Indigenous and ribeirinhos peoples share a profound connection with the Xingu River, viewing it as a lifeline essential to their culture and subsistence. This deep sense of belonging motivates them to seek to advocate for a more equitable water-sharing system that ensures the socio-environmental conditions necessary for sustaining life. The conflict has increased since November of 2019 with the installation of the last turbine, allowing full operation of the Belo Monte HPP, and reached a new level in 2021, amid the outbreak of the COVID-19 pandemic, when the HPP reached full operation (Fig. 2A).

Since 2014, the Juruna Yudjá people of the Paquiçamba IT, through the Associação Yudja Mïratu da Volta Grande do Xingu (AYMIX), have been denouncing the underestimation of the impacts of the operation of the Belo Monte HPP, based on the independent production and analysis of socio-environmental data (Pezzuti et al. 2018). The trade-off between energy generation and the conservation of socio-ecological systems has created an atmosphere of lack of trust in the official impact monitoring conducted by Norte Energia, which triggered the expansion of the independent monitoring to include more Indigenous communities within the Paquiçamba IT and ribeirinhos communities along the VGX. In this context, local VGX inhabitants joined forces with academic researchers to carry out a unique collaborative and intercultural alliance: the Independent Territorial Environmental Monitoring of the Volta Grande do Xingu (MATI-VGX) in 2020. This monitoring constitutes a unique capacity of data production for impact assessment at the VGX, which demands spatially representative of the complex and heterogeneous environment of the VGX and daily monitoring of coupled hydrological and ecological variables, which is underrepresented in Norte Energia monitoring programs.

Here we present the results obtained by the MATI-VGX, on the monitoring of fish spawning sites and fishing dynamics, which complement and quantify local communities' perceptions about the environmental impacts caused by the Belo Monte HPP, revealing unforeseen ecosystem impacts. These results emphasize the fundamental role of local communities as protagonists in monitoring environmental impacts on their territories. Together, local and academic researchers act as an essential alliance in the defense of human rights and ecosystem integrity in the VGX's socio-environmental landscape. This research, grounded in local knowledge and daily observations, offers an integrated view of the disruption caused by the Belo Monte HPP on fishing practices, food security, and community life, and provides important information for developing a water-sharing scheme that is less detrimental to ecosystems, sustaining fundamental environmental conditions for supporting the life of Indigenous and ribeirinhos communities at the VGX.

Piracemas are the places between the river channels and the widespread seasonally flooded habitats that fish and other aquatic animals use, throughout their life cycle, to reproduce, seek shelter, and feed. Thus, the term "piracema", which usually refers to the movement of fish upriver to reproduce at the headwaters, is also understood in the Xingu as a biocultural landscape and territory. The piracemas are the places where VGX fishers fish throughout their lives, and where they learn and teach their children traditional fishing techniques. The fishers monitor the piracemas, usually the ones closest to their homes, or where they find their most prized fish.

Changes in the flood pulse imposed by the operation of the Belo Monte HPP disrupt processes in the aquatic and seasonally flooded ecosystems, and thus directly affect the piracemas. A previous mapping survey of piracemas ranging from the confluence of the Xingu River with the Iriri River to the waterfalls complex upstream of the Belo Monte dam (Fig. 1) recorded 198 sites historically recognized as piracemas by the local fishers, highlighting the importance of seasonally flooded habitats for fish reproduction (Camargo et al. 2004).

Monitoring the timing, duration, and magnitude of flooding in piracemas in terms of operational discharge variations is challenging because the water flows over a complex array of channels along the VGX. The Brazilian National Water Agency (ANA), and Norte Energia have fluviometric monitoring stations along the Xingu River (https://www.gov.br/ana/pt-br), but very spatially restricted (Fig. 1). Independent monitoring of the piracemas began in 2021, with the installation of water level scales to monitor daily flooding of the Zé Maria Island piracema, near the Community Mïratu (Paquiçamba IT). In the following years, other water level scales were installed in eight additional piracema locations with different characteristics (Table S1). The water scale measurements aim to detect the timing of the onset and duration of flooding of specific piracemas along with observations of the minimum water level allowing hydrological connectivity needed for fish to enter the piracema, and the water level at which spawning occurs (see Supplementary material for detailed methods description).

The selection of piracema sites for monitoring was made by the local researchers, and monitoring stations were built strictly following the protocol used for water level monitoring adopted by ANA. The minimum water level at which fish enter each piracema and the water level at which spawning occurs were determined based on direct field observations by the Indigenous and ribeirinhos researchers. The data obtained show that the onset of flooding of VGX piracemas is delayed by one to four months due to the artificial discharge reduction applied by the operation of the Belo Monte HPP (Fig. 2; Figures S1-S5). This hydrological perturbation is preventing fish spawning.

For example, fish entered the “Zé Maria” island piracema when the water column reached a height of at least 103 cm from the soil, which corresponds to a discharge of approximately 13,000 m³/s. This discharge should be reached in December if the Xingu water was not diverted for energy generation. The MATI-VGX monitoring shows that, due to the diversion of water and consequent reduction of discharge at the VGX, this discharge and thus this corresponding water level was only reached almost three months later in 20222, on 24th February, and five months later in 2023, on 05th April (Fig. 2).

The onset of flooding of the Xingu River floodplains occurs in November as a result of the South American monsoon activity (Marengo et al. 2010). This is crucial to trigger the environmental signals for fish spawning starting in December. The delay documented by the MATI-VGX monitoring hinders spawning completely or leads to the death of fish larvae due to insufficient flooding, food, and shelter. Furthermore, and still, due to the delayed flooding, the fruits produced by trees in the seasonally flooded forests fall before flooding begins, and thus do not reach the water and fail to disperse and feed aquatic fauna. Thus, the water discharge diverted to fuel the Belo Monte HPP energy generation promoted a severe impact on fish reproduction, feeding, and life cycles along the VGX, with predictable impacts on fisheries yields in the following years.

Based on local communities' knowledge and perceptions of the changes in fishing dynamics at the VGX after the dam operation, MATI-VGX has gathered data using methodologies compatible with the fishing monitoring performed by Norte Energia (Isaac et al. 2008). Here we present MATI-VGX data collected between 2020 and 2023 (after HPP construction and full operation), comparing it with information collected by Norte Energia between 2001 and 2008 (before HPP construction) (Isaac et al. 2008). Using "fishing schedules" (see Supplementary material for specific methods, Figure S6), MATI-VGX focused on Catch Per Unit Effort (CPUE) values as the main indicator, calculated by dividing the amount of fish caught (in kilograms) by the fishing effort (number of fishers per day). Thus, the higher the CPUE index, the higher the food provision and income associated with fishing activity at that time.

The results revealed a marked decrease in fishing yield during the rising, high, falling, and low water level stages after water diversion compared to the pre-diversion period (Table 1). The MATI-VGX data showed a significant difference between the water level stages (chi-squared = 16.52, p < 0.001), although only the high water level stage was different from the other stages (Table 1). Significant changes were observed in the frequencies of use of boat types and fishing gear, showing adaptations to environmental changes, particularly the reduction in the discharge of the VGX and the lower abundance of fish (Table S2 and S3). Rabeta canoes (small boats with adapted small outboard motors), for example, which used to be predominant, became less used, while rowing canoes increased. Before the dam, there were significant differences in productivity between the three types of boats assessed, with voadeiras (speed boats with more powerful outboard motors) having the highest productivity and rowing canoes the lowest. In contrast, the data collected by MATI-VGX shows that rabetas now have the highest productivity (chi-squared = 26.416, p < 0.001), although the frequency of use of rabetas has decreased from 75–46%, and canoes have increased from 7–41% (Table S2). Voadeiras recorded lower performance than rowing canoes (Table S2). This change is related to the discharge reduction in the VGX, since the use of motorboats is hampered in shallow areas, if not prevented. This impact can also be seen in the need to use more than one type of tool to help move around on some occasions, such as the use of paddles and sticks in motorboats and rabetas (Table S2).

Table 1

Average Catch Per Unit Effort (CPUE) and the standard deviation (SD) in kg.fisher.day-1 according to the period of the hydrological cycle. For the period from 2001 to 2008, data from the "Environmental Diagnosis of the Belo Monte HPP - Middle and Lower Xingu River (ICTIOFAUNA AND PESCA)" is presented. The 2020–2023 data was collected in the Communities of Yapukaka (Furo Seco), Lakarika, Mïratu, and Paquiçamba, as part of MATI-VGX. Differences were computed using multiple comparison tests, where different letters indicate significant differences (p < 0.05).
	2001–2008 (pre-dam)					2020–2023 (post-dam)
Period	CPUE	N	SD	Median	Diff.	CPUE	N	SD	Median	Diff.
Dry	13.53	148	12.2	10	A	4.89	158	4.525	3.47	A
Rising	7.54	83	9.037	6	C	4.49	141	3.806	3.34	A
Flood	10.4	536	11.611	7.5	B	3.28	116	2.752	2.41	B
Falling	12.14	298	14.388	8.33	B	5.42	109	6.156	3.40	A
ALL	11.1	1065	12.442	8		4.53	524	4.480	3.12

Hook and line was the most used fishing gear in catches by the commercial fleet that landed in Altamira (the main city in the area impacted by Belo Monte HPP) during the period sampled in the official pre-damming diagnosis (2001–2008) and accounted for 47% of landings, while gillnets accounted for 22%. This pattern was reversed after water diversion (2020–2023), with an increase in the use of gillnets to 47% and a reduction in the use of hook and line to 35% (Table S3). There was a significant increase in the use of "other fishing gear", which accounted for 16% of all fishing records. There were differences in CPUE values between fishing gears in the pre-damming period (2001 to 2008) (LogCPUE: F = 19.15, p < 0.0001; Table S3). After water diversion (2020–2023), there was also a significant difference (LogCPUE: F = 27.89, p < 0.0001) and a drop in overall yield, with an average of only 6.29 kg⋅fisherman⋅day-1 with gillnets, in contrast to 16 kg⋅fisher⋅day-1 in the pre-damming period (Table S3).

The differences in fishing yield between the monitored communities indicated unequal impacts, with Furo Seco Community being the most affected (Table S4). Analysis of catches by fish species revealed changes in the composition of catches, with species such as tucunaré (peacock bass, Cichla spp.) showing a notable reduction (Table 2). The official diagnosis of fishing before the construction of the Belo Monte HPP did not provide CPUE data by species and stretch of the river, but only data on the average mass (kg) landed in Altamira, which makes comparisons difficult. Additionally, the data presented refers to commercialization, an activity practically extinguished in the VGX after the Belo Monte HPP operation. In the pre-damming period, the composition of the catches highlights the production of tucunaré (Cichla spp.) (29%), aracu/piau (Anostomidae, mainly Leporinus spp.) (20%), and pescada (Plagioscion spp.) (20%), followed by various species of pacu (Myleinae, Serrasalmidae) (13%) and curimatá (Prochilodus nigricans) (8.5%), as well a non-native species to the region, such as pirarucu Arapaima gigas (Isaac et al. 2008). In the MATI-VGX data (2020–2023), the tucunaré (Cichla spp.) accounted for only 5.5% of catches. The average catch in kg contrasts sharply among fish species, with species such as pescada (Plagioscion spp.), which had an average of almost 100 kg in the pre-damming period, reaching a maximum average of 1.66 kg in the post-damming period. A similar situation occurred with curimatá (Prochilodus nigricans), which fell from 68 kg to 5.4 kg after the Belo Monte HPP operation (Table 2). The consequences of these changes strikingly affected the food sovereignty of all families, and the effects were spread in other areas directly and indirectly influenced by Belo Monte HPP. Lopes et al. (2024) showed an overall decline of 58.5% in fish consumption, although their analysis was not restricted to the VGX.

Table 2

Number of fishes (N), average capture (kg) and the standard deviation (SD) per fishing trip for the main species landed in Altamira and comparison with catches in the post-dam period.
	2001–2008 (pre-dam)			2020–2023 (post-dam)
	N	Capture	CI	N	Capture	SD
Aracu/Piau	1617	51.04	6.31	662	1.63	1.9
Ariduia	104	60.95	17.64	275	2.35	2.73
Curimatá	526	68.04	6.21	1465	5.40	7.48
Fidalgo	189	45.67	7.18	42	1.24	0.67
Filhote	28	90.19	57.15
Flecheira	23	92.17	62.74	120	0.51	0.37
Pacus	1617	34.15	4.34	4314	3.09	3.69
Pescada	834	98.95	11.32	290	1.66	1.59
Pirarucu	10	64.9	20.91
Surubim	170	41.3	12.86	27	1.83	1.11
Trairão	59	42.74	13.38	7	2.74	1.77
Tucunaré	1617	76.03	8.32	583	2.29	2.16
*Scientific names in Table S6.

The decrease of fish reproduction in the piracemas documented by MATI-VGX between 2022 and 2023 affects areas that previously attracted many species of high socio-environmental importance, such as the piaus (Anostomidae), trairão (Hoplias aimara), ariduia (Semaprochilodus brama), fidalgo (Ageneiosus inermis), pirarara (Phractocephalus hemioliopterus) and curimatá (Prochilodus nigricans). Besides the immediate impacts on local fisheries and food security, the lack of spawning in the piracemas indicates current and future negative impacts on the diversity and abundance of fish populations, with potential impacts on the entire aquatic trophic chain and human livelihoods.

The change in the amount and timing of flooding affects the synchrony between fruit production in the seasonally flooded forest and the flooding period. As a result, the fruit ripens in a period when the seasonally flooded forest is not flooded yet, and thus most fruits fall into the dry ground and are not available for feeding frugivorous fish. Deterioration of aquatic and seasonally flooded ecosystems and reduction of food availability may have also made fish more vulnerable to diseases, as evidenced by the increase in the number of sick, contaminated, wormy fish (authors' observations) and the appearance of fish with spinal deformities (Palm 2011; Karling et al. 2013; Lehun et al. 2023), a phenomenon already documented for the VGX (Montag et al. 2023). The decrease in the quantity of fish prompts the need for increased fishing efforts, such as a reduction in the mesh size of gillnets and more time spent fishing, to obtain results that were regularly obtained before the construction of the Belo Monte HPP. As an example, in the past, there were piracemas where it was possible to catch fish with arrows, and this is not possible anymore, as, due to HPP operation, now these places are not even flooded during piracema’s periods (authors’ observations). A drastic reduction in the number of frugivorous fish has also been observed by the authors (Table 2).

The observation that fish species reproducing in the piracemas, such as pacus (Myleinae), jaraqui, and ariduia (Semaprochilodus spp.), are becoming increasingly scarce indicates that their reproductive cycle has been disrupted locally. Combined with a lack of food for the fish and an increase in diseases, this disturbance probably explains the general decline in fish populations in the region. This situation is similar to that reported by (2022), who found significant reductions in fish abundance, richness, and functional diversity in the VGX after five years of Belo Monte HPP operation, and other studies also conducted in rivers impounded by HPPs (Pyron et al. 1998; Agostinho et al. 2008). The decrease in the quantity and quality of fish not only affects the communities' food source, impairing food sovereignty and security but also has significant economic impacts. Fishers who traditionally relied on fishing for subsistence and income face increasing challenges. Sick and wormy fish are not attractive for consumption and sale, resulting in financial losses. The perception of the imminent local extinction of fish such as the fidalgo (Ageneiosus inermis) and the significant decline in species abundances such as the pacus (Myloplus spp., Myleus spp.), and the ariduia (Semaprochilodus brama) raises concerns about the sustainability of fishing activities in the region. The challenge of finding fish for consumption has become an arduous task, and traditional fishing strategies have become ineffective in the face of environmental transformations, as we can see from the significant and continuous increase in the use of gillnets as a fishing tool.

The MATI-VGX results evidenced strong and overlooked impacts on ecosystem processes caused by water discharge reduction and disruption of duration and amplitude of the flood pulse at the VGX, requiring the urgent implementation of measures to preserve local sociobiodiversity. Adopting a strongly needed precautionary approach, we suggest that a provisional hydrograph should be applied while further studies are carried out (Fig. 3). The criteria used to propose this provisional solution are based on analysis of water level, fish spawning, and fishing dynamics data combined with local knowledge about specific locations and periods of spawning:

1 - The timing and rate of raising and falling water levels must be maintained to keep the duration of the flood pulse in synchrony with the regional climate: the historical discharge series (1971–2019) for the Xingu River shows that water rising starts in November, reaching a peak level in April-May and falling phase finishing in August. The flooding of the seasonally flooded forest from December to February is critical to allow fish reproduction in piracemas and for aquatic animals to find a rich supply of food. However, the average discharge and discharge rate increase for these months in hydrographs applied by Norte Energia (A and B) are excessively low to maintain ecological processes in the seasonally flooded forests. The flooding rate driven by rainfall patterns in these months is essential for natural fish reproduction cycles to occur since the gradual increase in the river level is responsible for triggering the piracema process along the VGX.

2 - The critical discharge for overcoming upstream barriers for fish migration should also be considered. This includes consideration of the backwater effects in the igarapés (small rivers) and tributaries of VGX. In this case, the VGX discharge must be synchronized with the annual rainfall pattern in tributary watersheds, which also relies on the activity of the South American monsoon (Marengo et al. 2010).

3- The level of flooding of at least part of the seasonally flooded forest must be reached concurrently with the fruiting period of the seasonally flooded forest trees so that at least part of the fruit falls into the water and serves as food for the aquatic fauna, which in turn will disperse seeds and contribute to the regeneration of the flooded habitats. The sarobal (a specific kind of vegetation that is proportionally smaller and develops over rock and sand riverbeds at higher flood levels) must be flooded for long enough to maintain its viability, as it is an essential resource for aquatic fauna.

4- The river level falling phase after the flood peak must be gradual and with duration determined by watershed rainfall. After spawning, at least three months are needed for fish to develop in lakes and seasonally flooded forests, whose early drying leads to the consequent death of the developing fish due to desiccation or more intense predation than usual.

5- The VGX discharge variation must be proportional to the natural upstream discharge variation. As currently applied, monthly discharge means allow large discharge variation within each month, disrupting the gradual cycle of flooding and leading to operational hydropeaks. Fish spawn once the river reaches certain water levels in different parts of the VGX. Thus, an abrupt drop in water level amidst the process can dewater spawning sites, hindering the survival of eggs and larvae, and may also interfere with feeding patterns.

6- The interannual rainfall variability must be represented in the VGX discharge. This is important to keep the habitat diversity and dynamism across VGX. The suppression of the interannual discharge variability can favor the expansion of specific habitats and the suppression of others. The reduction of peak discharge, for example, can induce sediment accumulation in channels, reducing hydrological connectivity and lotic environments.

The operation of the Belo Monte HPP must adhere to the criteria above to restore a flooding cycle that resembles the natural one, thus supporting some ecological sustainability for the VGX ecosystem. We recommend establishing a multidisciplinary working group, with the participation of local community representatives, tasked to design and verify hydrographs that balance the viability of VGX’s ecosystems and the traditional ways of life of the ribeirinhos and Indigenous populations, alongside the energy production needs of the Belo Monte HPP. The group should also assess the impact of inter-annual climate variability and climate change on the Xingu River’s flow and plan for continuous monitoring of the effects of reduced flow. These studies should guide periodic revisions of hydrographs. The interannual variability of the flow includes, for example, the climate phenomena El Niño/La Niña and the Pacific Decadal Oscillation. On an internal to decadal scale, this variability may be important in maintaining the heterogeneity of the VGX's habitats.

The true magnitude of the ongoing impacts of the Belo Monte HPP on the VGX region has only been revealed through the initiative of local communities who started an independent monitoring program of their territories. This was driven by the perceived disconnection between the official monitoring conducted by Norte Energia and the communities’ daily experiences, perceptions, and changes in their livelihoods. The situation at VGX highlights the underestimation of hydropower dam impacts in megadiverse regions like Amazonia, where ecosystems are intricately linked to seasonal cycles.

To protect the ecological processes and the biocultural landscapes in VGX, it is essential to adopt a strongly precautionary approach by implementing a provisional hydrograph while further studies are conducted, by a multidisciplinary working group, including local community representatives, to establish socioecological hydrograph. These hydrographs must balance the needs of both the ecosystems and the traditional ways of life of the ribeirinhos and Indigenous populations, as well as the production of energy by the Belo Monte HPP. Periodic revisions of these hydrographs are necessary, considering future water availability scenarios for the Xingu Basin and the combined threats of climate change and deforestation.

Belo Monte HPP's operation has violated the rights of Indigenous and ribeirinhos populations and disregarded Brazil’s National Water Resources Policy (https://www.planalto.gov.br/ccivil_03/leis/l9433.htm), which advocates for equitable water sharing. The current operational system must be revised, as the licensing process failed to honor the right to free, prior, and informed consultation, as mandated by the Brazilian Federal Constitution (Article 231) and by the Convention 169 of the International Labour Organization. Despite numerous environmental recommendations, many remain unfulfilled, and the HPP has continued to operate without a renewed license since 2020.

The unique experience and local wisdom incorporated into MATI-VGX not only contribute to the understanding of the challenges faced by traditional communities but also reinforce the urgent need to rethink development strategies that respect and preserve local ecosystems and ways of life. More effective coordination among governmental, academic, and local leadership is essential to establish a water management scheme that truly represents a consensus and minimally meets the requirements to maintain the socio-environmental system (river, local Indigenous and ribeirinhos populations, biota, energy generation). Collaboration between local communities, academic researchers, and decision-makers is essential to forge sustainable solutions that ensure not only the survival of communities but also the preservation of the rich biodiversity and natural cycles of the VGX.

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The authors declare no competing interests.

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Community-based monitoring reveals the impacts of the permanent river drought imposed by the Belo Monte Hydroelectric Power Plant at Volta Grande do Xingu, Amazonia

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Introduction

Monitoring the timing and duration of flooding at fish spawning sites ( piracemas )

Impacts of the Belo Monte HPP on Fishing Dynamics

Integration of MATI-VGX results and contribution to the development of a new water-sharing framework for the survival of the socio-ecological system at the VGX

Conclusion

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