Araguaia Research Articles

Chemical and mineralogical constitution of redoximorphic features and mechanism of formation of Plinthosols from the Araguaia River plain, Brazil

ABSTRACT Currently in Brazil, large grain cultivation projects on Plinthosols are a reality, however, there is little or no knowledge of the real mechanism of formation of the plinthite feature, in addition to what is reported in the literature as being a product of oxidation-reduction processes of iron element. This study evaluates iron redoximorphic features and investigates their chemical and mineralogical composition in two profiles of Plinthosols from the Araguaia River plain (P1 and P2). The study strengthens the understanding of the pedogenetic processes involved in the formation of mottles and plinthite. In this sense, it assesses whether the formation mechanisms corroborate the literature. Soil features were sampled in the upper right and left position at the initial plinthic horizon, upper right and left position at the main plinthic horizon, and lower right position at the base horizon of the plinthite zone in the profile. Separated samples comprising the soil matrix, mottles, and plinthite under natural moisture conditions were ground into powder form for chemical determinations by X-ray fluorescence (XRF), sulfuric acid attack (H 2 SO 4 ), sodium dithionite-citrate-bicarbonate (DCB), and ammonium acid oxalate; and mineralogical determinations by X-ray diffraction. Iron contents in all determined forms were always higher in the plinthite feature, intermediate in the mottle feature, and lower in the soil matrix feature. Most of the Fe in all redoximorphic features is included in the structure of primary minerals and their derivatives (vermiculite, illite, and VHEs). Only part of the iron present (about 35.40 % in P1 and 41.98 % in P2) is detected in the form of oxides such as goethite and hematite, which could be formed in redox processes. The mottle and plinthite features under study are not the product of the classic process of segregation, mobilization, and accumulation of iron as a consequence of redox processes. These features were formed or emerged as a result of a relatively slow and constant weathering process of their source material, which is gradually decomposed in an aqueous medium, releasing most of its components. These components include iron and more mobile elements such as bases and silicon, which leave the system through drainage water, and of which a small part may eventually recombine to form new less complex minerals such as kaolinite and oxides.

Ectoparasitary Analysis in the Native Ichthiofauna of the Araguaia River Basin in the Cocalinho/MT Region

Introduction: Parasitism can be used as a highly sensitive monitor to detect changes occurring in degraded habitats and can be an indicator of environmental contamination and environmental health. Objetive: This work aimed to investigate the diversity of parasite species in the area, to verify its potential as an indicator of environmental quality, as well as to analyze biochemical parameters of the water at each collection point. Method: The study area comprises part of the Araguaia river basin, in the Cocalinho-MT region. This region has environments with different levels of anthropic impacts (pasture, agriculture, urban occupation, farm condominiums, native vegetation, etc.), in addition to several connected lakes. Result and conclusion: The average prevalence of Protozoa varied between 7.69% and 41.67%, with an overall average of 21.63 ± 10.95%. The mean intensity ranged from 1 to 3 parasites/parasitized fish, with an overall mean of 2.16 ± 0.58 parasites/parasitized fish. The average abundance ranged from 0.15 to 0.75 parasites/examined fish, with an overall average of 0.46 ± 0.23 parasites/examined fish. The present study found ectoparasitism of Protozoa, Monogenea and Branchiura in specimens of fish in the region of the Araguaia River basin, in Cocalinho/MT.

Static load test, up to 6000 kN, carried out on a bridge under construction over the Araguaia River

Static Load Tests are foundation control tools used to assess the load capacity versus displacement behavior of piles. This article a challenging Static Load Test on the foundations of the new bridge under construction over the Araguaia River, in São Miguel do Araguaia-GO. The bridge has a total length of more than 1.1 km and during the load test, almost all of its structure was already executed in the central region of the river channel - place where one of the masts with 104m free span is located. To assess the load capacity and safety of the piles, a static load test was carried out, where the biggest challenge was the need to run it underneath the structure of the bridge, using it as a reaction system. There were 15 tubular steel piles in the reaction/foundation block, and the load test was successfully performed to 6000 kN. That test represented a significant challenge and required a high level engineering work. The paper presents all the work constraints, along with assembly and executive challenges to the achieved results.

Impact of Nest Parameters and Vegetation on Turtle Reproductive Success in the Araguaia River

Objective: The objective of this work is to verify whether these factors influence the hatching of Amazon turtle eggs in this region. Method: For this, data from reproductive periods during the years 2013 and 2019 were used within the selected areas. Only the depth of the nests was related to the total number of eggs, on both beaches, in both years, and with the number of eggs hatched, only on Praia da Saudade in 2013. Results and Conclusion: In 2019, Praias da Saudade and Thaís showed differences between incubation temperature, percentage of finer grains in the sand, distance from vegetation to nests and hatching rate. However, none of the evaluated parameters were related to reproductive success. Factors such as the incubation period, the amount of clay in the sand and the percentage of riverine vegetation may also be possible predictors of reproductive success that should be investigated.

Mercury Bioconcentration and Translocation in Rooted Macrophytes (Paspalum repens Berg.) from Floodplain Lakes in the Araguaia River Watershed, Brazilian Savanna

Macrophytes are fundamental photosynthetic organisms for functioning freshwater ecosystems, identified as potential bioindicators of mercury (Hg) in the environment. We quantified the concentrations of total Hg (THg) in water and macrophytes (Paspalum repens Berg.) from 17 lakes on the Araguaia River floodplain, aiming to compare the bioconcentration factor (BCF) in the aerial tissues and roots; evaluate the translocation factor (TF) between plant tissues; and assess the influence of environmental factors and land use on THg concentrations in water and macrophytes. The BCF was significantly higher in roots (1.29 ± 0.32) than in aerial tissues (0.41 ± 0.34), with low TF between plant tissues (0.14 ± 0.06). The highest concentrations of THg in water were determined in lakes with higher land use intensity and a pH close to neutral, indicating the transport of particulate-bound Hg and the immobilization in the water column. In contrast, wetlands were priority areas for the bioconcentration of THg in macrophytes, associated with sulfate, dissolved oxygen, and oxidation–reduction potential in the water. Thus, although P. repens is not a suitable bioindicator of Hg mobilization by anthropogenic land use in our study area, our results suggest the potential of macrophytes as bioindicators of sites that are favorable to Hg methylation.

Mercury distribution, bioaccumulation, and biomagnification in riparian ecosystems from a neotropical savanna floodplain, Araguaia River, central Brazil

Litterfall is the main source of dry deposition of mercury (Hg) into the soil in forest ecosystems. The accumulation of Hg in soil and litter suggests the possibility of transfer to terrestrial invertebrates through environmental exposure or ingestion of plant tissues. We quantified total mercury (THg) concentrations in two soil layers (organic: 0–0.2 m; mineral: 0.8–1 m), litter, fresh leaves, and terrestrial invertebrates of the Araguaia River floodplain, aiming to evaluate the THg distribution among terrestrial compartments, bioaccumulation in invertebrates, and the factors influencing THg concentrations in soil and invertebrates. The mean THg concentrations were significantly different between the compartments evaluated, being higher in organic soil compared to mineral soil, and higher in litter compared to mineral soil and fresh leaves. Soil organic matter content was positively related to THg concentration in this compartment. The order Araneae showed significantly higher Hg concentrations among the most abundant invertebrate taxa. The higher Hg concentrations in Araneae were positively influenced by the concentrations determined in litter and individuals of the order Hymenoptera, confirming the process of biomagnification in the terrestrial trophic chain. In contrast, the THg concentrations in Coleoptera, Orthoptera and Hymenoptera were not significantly related to the concentrations determined in the soil, litter and fresh leaves. Our results showed the importance of organic matter for the immobilization of THg in the soil and indicated the process of biomagnification in the terrestrial food web, providing insights for future studies on the environmental distribution of Hg in floodplains.

A systematic assessment of Leporinus tigrinus (Characiformes: Anostomidae) using morphological and molecular data

Leporinus tigrinus was described based on specimens collected in the rio Tocantins basin, and was only recently recorded at the rio Xingu, rio Tapajós and upper rio Paraná basins. So far, no study has compared specimens of all these distinct drainages focusing on understanding whether all these populations truly belong to a single species or may consist instead in a species complex. Our study focused on investigating the intraspecific variation within Leporinus tigrinus using morphological and molecular data. Both morphology and molecular analyses support that all samples belong to a single species widespread in the rio Araguaia, rio Tocantins, rio Xingu and rio Tapajós. In addition, the population occurring at upper rio Paraná is considered to be allochthonous, originating from the rio Araguaia/Tocantins. A comprehensive morphological description of the species, including osteological illustrations and descriptions are provided, as well as a comparative diagnosis with similar-looking species.

Hydropower Plants and Ichthyofauna in the Tocantins–Araguaia River Basin: Challenges for Planning and Approaches to Ichthyofauna Conservation

The management of water resources is a complex issue, and the conservation of fish and fishery resources is a growing challenge given the increase in the number of hydroelectric projects in Brazilian river basins. This study describes the fragmentation of the Tocantins–Araguaia River Basin resulting from the installation of hydroelectric plants in operation and planned by the electricity sector until 2050, as well as its relationship with the conservation of fish fauna, especially long-distance migratory species. The hydroelectric projects in operation and planned were analyzed using data obtained from the National Electric Energy Agency. A literature review was carried out to assess the fish species of the Tocantins–Araguaia ecoregion, with an emphasis on long-distance migrants. In general, 75 hydroelectric dams are in operation, and a further 119 projects are in the electricity sector’s plans for construction by 2050 in the Tocantins–Araguaia ecoregion, including a stretch of the basin above the Tucurui dam, which will accentuate the fragmentation in the area. Of the 702 species found in the region, 31.1% are endemic, 6.1% are endangered, and several long-distance migratory species have had their populations restricted. Analysis of this information highlights the widespread impact on the ichthyofauna, affecting both species with a restricted distribution and migratory species with a long-distance distribution. The studies associated with the projects contribute to the advancement of knowledge, but they are only carried out after the works have been defined, which makes it difficult to plan conservation in advance. Transformations in the basin are imminent given the current changes and those expected in the coming years because of the electricity sector’s planning for the region. Considering the inseparable relationship between biodiversity and socio-environmental and cultural diversity, fish conservation is intrinsically linked to the conservation of socio-diversity and the effective participation of local communities from the start of the process. Fish depend on water, and people need both water and aquatic diversity. In conclusion, a well-structured and adaptative conservation plan, combined with the integration of effective fish routes, can contribute to the sustainable development of hydroelectric projects while safeguarding the biodiversity and ecological integrity of the Tocantins–Araguaia Basin.

Mercury distribution in organisms, litter, and soils of the Middle Araguaia floodplain in Brazil.

Mercury (Hg) is a chemical element that, depending on its concentration, may become toxic to living organisms due to the ability of Hg to bioaccumulate in food chains. In this study, we collected samples of soil, litter, and organisms in the Middle Araguaia floodplain, Brazil. Total mercury (THg) concentrations in litter were significantly higher (p < 0.0001) than that in soil, ranging from 10.68 ± 0.55 to 48.94 ± 0.13 and 20.80 ± 1.07 to 55 .19 ± 1.59 ng g-1, respectively. Total mercury concentration levels in soil showed a linear, inversely proportional relationship with soil organic matter (SOM) contents and soil pH, consistent with the geochemical behavior of chemical elements in flooded environments. Ten orders of organisms were identified, and the average THg concentrations determined in their bodies were up to 20 times higher than those in soil and litter. We found a significant linear relationship between the levels of THg in litter and those found in soil organisms, thereby allowing the prediction of THg concentration levels in soil organisms through the analysis of litter at the sample units. The different dynamics and feeding habits of soil organisms and the concentration of THg in these organisms may be influenced by the river's course. This study provides evidence of the bioaccumulation of THg in soil organisms in the floodplain of the Middle Araguaia River, an important river basin in the Brazilian savanna.

Exploring the relationship between land-use and pesticides in freshwater ecosystem: A case study of the Araguaia River Basin, Brazil

Freshwater aquatic ecosystems are vulnerable to human activities within their surroundings, presenting a concerning scenario for aquatic organisms and human subsistence. Therefore, understanding the occurrence of pesticide contamination is a necessary condition to safeguard aquatic biodiversity and human health. We analysed the pesticide residues in water samples, using the solid-phase extraction (SPE) technique and determination by ultra performance liquid chromatography coupled with tandem mass spectrometry (UPLC‒MS/MS). We also we investigated whether land use influences water contamination by pesticides in the middle and upper Araguaia River basin, situated in the central region of Brazil in the Cerrado biome. We evaluated the effect of land use and land cover type on pesticide occurrence in nine subbasins using mixed linear models. We registered the occurrence of pesticides in all subbasins. After water analysis a total of eight pesticides were found: atrazine, carbendazim, cyanazine, imidacloprid, 2,4-D, clomazone, chlorpyrifos-ethyl, and imazalil. Atrazine and carbendazim were detected during both sampling years. The active ingredient atrazine and clomazone were registered in all studied subbasins, with concentrations ranged from 0.006–0.207 µg L−1 and 0.183–0.373 µg L−1, respectively. There were no significant relationships observed between the number of pesticides and the type of land use and land cover, a result possibly related to the short sampling period, as these pesticides are largely used for fungi and insect control in agricultural areas. The presence of pesticides in the water may have potentially detrimental effects on aquatic biodiversity because sampled pesticides have been associated with anomalies in ontogenetic development, diseases, and mortality in organisms. Based on European legislation, it was evident that most of the subbasins had pesticide concentrations in the water exceeding the safety levels for human consumption. Thus, it is crucial for Brazilian legislation to establish maximum limits for pesticide concentrations in water to ensure safe drinking water for the Brazilian population. Additionally, a permanent monitoring system should be established to understand contamination trends in this basin and to take appropriate actions to mitigate adverse impacts.

Evidence of water surface and flow reduction in the main hydrographic basin of the Brazilian savannah (Cerrado biome): the Araguaia river

Evidence of water surface and flow reduction in the main hydrographic basin of the Brazilian savannah (Cerrado biome): the Araguaia river

Temporal trends in sand bars and water bodies on the right side of the world's largest fluvial island - Javaés River

Bananal Island is the largest river island in the world, located in the Araguaia River basin in central-western Brazil. Studies on the Araguaia River have shown that changes in the main channel are linked to the increase in sediment input due to agricultural expansion in the Cerrado. However, very little is known about the Javaés River, on the opposite margin of Bananal Island. This study analyzed the temporal trends of sand bars and water masses in the Javaés River from 1985 to 2021. Annual mosaics of Landsat images were generated to identify the classes of water masses and sand bars. The Mann Kendall and Sen's Slope tests were used to identify trends and statistical significance of changes over time. The results indicated an increase of 57% (6.5 km²) in the areas of sand bars and a reduction of 39% (15.2 km²) in water bodies. The tests revealed significant trends with an increase in bars and a decrease in water bodies. It was observed that changes in the distribution and rates of regional precipitation, along with the expansion of anthropogenic occupation, are contributing to an increase in sediment input, in a cascade effect, affecting the balance between erosion and deposition in the Javaés River.

Local and landscape factors influencing mercury distribution in water, bottom sediment, and biota from lakes of the Araguaia River floodplain, Central Brazil

Mercury (Hg) is a chemical element widely present in the Earth's crust. However, its high toxicity and ability to accumulate in organisms and biomagnify through food chains characterize it as a global pollutant of primary control. We assessed total mercury concentrations ([THg]) in abiotic and biotic compartments from 98 floodplain lakes associated with the Araguaia River and six tributaries (Midwest Brazil). [THg] quantification in water was performed by cold vapor atomic fluorescence spectroscopy. [THg] in bottom sediment was assessed using cold vapor generation atomic absorption spectrophotometry, while [THg] in macrophyte, periphyton, and plankton were quantified by thermal decomposition atomic absorption spectrometry. Hotspots of [THg] in water, bottom sediment, and macrophytes were determined in areas impacted by pasture and urban areas. In contrast, hotspots of [THg] in periphyton and forest fires were determined in preserved areas downstream. [THg] in plankton did not show a clear spatial distribution pattern. The mean bioaccumulation factor order was plankton (2.3 ± 1.8) > periphyton (1.3 ± 0.9) > macrophytes (0.7 ± 0.4) (KW = 55.09, p < 0.0001). Higher [THg] in water and bottom sediment were associated with high pH (R2adj = 0.118, p = 0.004) and organic matter (R2adj = 0.244, p < 0.0001). [THg] in macrophytes were positively influenced by [THg] in water (R2adj = 0.063, p = 0.024) and sediment (R2adj = 0.105, p = 0.007). [THg] in periphyton are positively related to forest fires (R2adj = 0.156, p = 0.009) and [THg] in macrophytes (R2adj = 0.061, p = 0.03) and negatively related to lake depth (R2adj = 0.045, p = 0.02). The transfer of Hg from water and sediment to the biota is limited. However, the progressive increase of the bioaccumulation factor between macrophyte, periphyton, and plankton may indicate Hg biomagnification along the food chain of the Araguaia River floodplain.

Mercury Contamination as an Indicator of Fish Species' Trophic Position in the Middle Araguaia River, Brazil.

This study evaluates the use of mercury (Hg) concentrations in fish muscle tissue to determine a species' trophic position (TP) in its environment. A campaign conducted in 2019 along 375 km in the middle Araguaia River basin, Brazil, resulted in 239 organisms from 20 species collected. The highest total mercury (THg) concentrations were found in Pellonacastelnaeana (6.93 µg·g-1, wet weight) and in Triportheus elongatus (3.18 µg·g-1, wet weight), whose TPs were different according to the FishBase database. However, they occupied the same trophic level in this study. The intra-specific comparison showed a difference in Hg concentrations between individuals captured in distinct sites. The study of the biota-sediment accumulation factor (BSAF) showed that spatiality interferes with a species' TP. Statistical analyses revealed that when we used a predicted species' TP based on each individual's size, it explained 72% of the variability in THg concentration across all fish species. Multiple regression analysis confirmed that standard length and FishBase values are positively associated with THg (R2 = 0.943). These results point to Hg as a viable indicator of a fish species' TP since it reflects regional, biological, and environmental factors, as demonstrated here for the middle Araguaia River.

Sediment Mercury, Geomorphology and Land Use in the Middle Araguaia River Floodplain (Savanna Biome, Brazil)

In order to assess the influencing factors of the presence of mercury in a river within the Savanna biome (Cerrado), we surveyed total mercury (THg) in bottom sediment from 50 lakes along 750 km of the Middle Araguaia floodplain. The sampling sites included non-urban and urban surroundings over three distinct geomorphologies. We measured water physicochemical parameters at each site and tested statistically if land use nested within the geological formation influenced the THg concentration in bottom sediments and related water parameters. Multivariate results indicate that the interaction between geological groups and land use is statistically significant (p &lt; 0.05). Nested ANOVA and Tukey HSD tests confirmed that the geological formation with its nested land use influences the THg, pH, DO, conductivity, and TDS (p &lt; 0.05). THg was significantly lower in Quaternary terrains (p &lt; 0.05) and differed significantly between non-urban and urban areas in Neoproterozoic terrains (p = 0.02). The spatial projections of the THg eigenvector on the main axes with the scoring factors of the Neoproterozoic/Paleoproterozoic terrains, and urban/non-urban, confirmed the spatial correlations. These results indicate that the association of land use and geology could be the main driver of THg in the bottom sediments of lakes from the Middle Araguaia floodplain.

The Role of Geomorphology on Flood Propagation in a Large Tropical River: The Peculiar Case of the Araguaia River, Brazil

In large rivers, floods are affected by the mosaic of geomorphic and geologic settings of the fluvial corridor. Here, we assess the role of geomorphology on the downstream flood dynamics of the Araguaia River, the largest free-flowing river in Central Brazil. The study integrates and advances existing flood-type classifications. We assess the factors that govern flood hydrograph properties and their downstream propagation by using flow time series, conducting statistical analysis, and evaluating geomorphic and flood metrics. Our findings highlight the role of geomorphology in the transmission of floods. In the upper and lowermost fluvial segments, the geological characteristics of the valley are a major factor. In the intermediate section, two main factors modulate the floods. The wide and complex floodplain plays a major role through storage and buffer effect for floods, and water diversion from the main system to a huge abandoned channel by avulsion governs seasonal flow transfers. The Araguaia is the most geodiverse floodplain of the Amazon–Cerrado ecotone, and floods play a fundamental ecological role in the river–floodplain environments. The combination of diverse factors controlling the flood mechanisms has to be considered when implementing conservation plans for the fluvial corridor and effective river management strategies.

Ecological risk of mercury in bottom sediments and spatial correlation with land use in Neotropical savanna floodplain lakes, Araguaia River, Central Brazil

The Araguaia River floodplain is an important biogeographic boundary between the two largest South American biomes: the Cerrado (Brazilian Savanna) and the Amazon rainforest. The large-scale degradation due to land use conversion experienced in the Araguaia River watershed represents a potential source of mercury (Hg) transport to aquatic ecosystems. However, more information is needed about the dynamics of Hg distribution in savanna floodplains, including the Araguaia River floodplain. We analyzed total mercury (THg) concentrations in the bottom sediments of 30 lakes connected to the Araguaia River and four tributaries, aiming to evaluate the environment's integrity based on the geoaccumulation index (Igeo) and the ecological risk index (ERI). The principal component analysis was applied to examine associations between Hg concentrations, environmental conditions, and land use intensity among lakes associated with different river systems. We used indicator cokriging to identify areas with a greater probability of Hg pollution and ecological risk associated with land use intensity. The land use data showed the predominance of areas used for pasture in the Araguaia River basin. THg concentrations in the sediments varied between 22.6 and 81.9 ng g−1 (mean: 46.5 ± 17.7 ng g−1). Sediments showed no significant pollution (Igeo: 1.35 - 0.50; Classes 1 and 2) and low to considerable ecological risks (ERI: 23.5–85.1; Classes 1 to 3). THg in bottom sediments was associated with land use, water turbidity and electrical conductivity, and sediment organic matter. The indicator cokriging indicates a moderate to strong spatial dependence between land use intensity and Hg, confirming the contribution of anthropic sources to the increment of ecological risk but also the influence of extrinsic factors (such as environmental conditions, geology, and hydrology). Integrating sediment assessment and land use indices with geostatistical methods proved a valuable tool for identifying priority areas for Hg accumulation at a regional scale.

Reevaluation of Rhipidomys emiliae (J.A. Allen 1916) and description of a new Rhipidomys (Rodentia: Cricetidae) species from Amazonia and Cerrado.

We present the revalidation of the sigmodontinae rodent species R. emiliae, as well as the description of a new species for the genus Rhipidomys. The maximum likelihood analysis recovers R. emiliae as sister species of the clade with Rhipidomys sp. nov. and R. ipukensis, with high bootstrap values. Comparisons between these species based on the external, cranial, and dental morphology identified several unique characters in Rhipidomys sp. nov., including more grayish brown color of the dorsal coat, subsquamosal fenestra wide and long, angular process ends in the same position of the end of condyloid process, conspicuous protostyle and enterostyle. We describe a new karyotype (2n = 44 and FN = 64) for the genus and, based on an integrative analysis together with morphology and molecular phylogeny, assign it to R. emiliae, and assign the karyotype with 2n = 44 and FN = 52 to Rhipidomys sp. nov.. The analysis integrating data indicated that R. emiliae has a geographic distribution restricted to the lowlands of eastern Amazonia, whereas Rhipidomys sp. nov. occurs in the central Amazonia and Cerrado. The data showed that some Rhipidomys species have its distribution currently limited by rivers, as Rhipidomys sp. nov. occurring west of the Araguaia-Tocantins interfluve, R. emiliae east of the Tocantins River, and R. ipukensis between the Tocantins and Araguaia rivers. This work, in addition to revealing a still unknown biodiversity describing a species, brings a new understanding to the genus, and shows how integrating different markers helps in the correct association between the nominal form and the karyotype.

New record of Spot-Legged Turtle, Rhinoclemmys punctularia (Daudin, 1801) (Reptilia, Testudines, Geoemydidae), from an Amazon ecotonal zone in the Tocantins&amp;ndash;Araguaia river basin, Brazil

We report a new record of Rhinoclemmys punctularia (Daudin, 1801), extending the geographic dis-tribution of this species by 450 km towards the south to the Araguaia River in the state of Par&amp;aacute;, Brazil. The new record comes from an ecotonal area of the Cerrado and Amazon biomes, which indicates a possible adaptability of this species in fragmented environments and in agricultural areas. These new findings help enable future investi-gations better understand the distribution of R. punctularia in the southern Amazon Basin and in ecotones.

On a new genus and four new species of the subfamily Cyprettinae (Crustacea, Ostracoda) from Brazilian floodplains.

We describe the new genus Triangocypretta gen. nov. and four new species from Brazilian floodplains. Triangocypretta angustus gen. et spec. nov. and Triangocypretta labiata gen. et spec. nov. were described from the Amazon floodplain only, while Triangocypretta nates gen. et spec. nov. was described from Amazon, Araguaia, and Paran River floodplains. Triangocypretta hirsuta gen. et spec. nov. was recorded from all four floodplains: Amazon, Araguaia, Pantanal and Paran. The new genus is characterized by the triangular shape of the carapace in lateral view, the absence of teeth on the posteroventral inner list in the right valve and the presence of anterior marginal septa in both valves, as well as by the relatively short and thin and -setae on the mandibular palp. All populations found were asexual. Owing to the clear differences in valve anatomy and limb chaetotaxy as compared to species of Cypretta s.s., the four species were allocated to a new genus in the subfamily Cyprettinae.