The Paraíba do Sul River Basin and its coastal area as a study model of the mercury cycle: A meta-analytical review of three decades of research

The Paraíba do Sul River (PSR) Basin is a crucial drainage basin in Southeast Brazil, being the most industrialized and densely populated region in the country. Over the last three decades, the basin has been the subject of numerous studies due to its long history of mercury (Hg) contamination. This makes the PSR Basin an excellent model to evaluate Hg cycling, which is a priority for signatory countries of the Minamata Convention, which includes Brazil. This review compiled data on Hg from five environmental compartments (animals, plants, sediment, suspended particulate matter (SPM), and water), three different sectors (upper, middle, and lower) and five different ecosystems (reservoir, mangrove, fluvial, estuarine, and coastal) from 60 published studies, aiming to understand the Hg cycling through meta-analyses. The highest concentrations of Hg were observed in the upper and middle sectors of the basin, which are areas with high industrialization and urbanization levels. Among the evaluated ecosystems, hydropower reservoirs showed the highest medians and were also frequent in the upper and middle portions of the PSR basin. Over the years, all environmental compartments showed a decline in Hg concentrations due to the implementation of federal and state environmental policies. The main source of Hg for the basin was Hg-rich soils from past activities (such as artisanal small-scale gold mining and the use of organomercurial fungicides in sugarcane plantations). These results can assist decision-makers in the management of the basin's ecosystems and can also be used to evaluate the effectiveness of implementing the Minamata Convention in the region.

Geochemical behavior of rare earth elements (REE) in urban reservoirs: the case of Funil Reservoir, Rio de Janeiro State, Brazil

Rare earth elements (REE) have unique chemical properties, which allow their use as geochemical tracers. In this context, the present study aims to assess the role of Funil Reservoir on REE biogeochemical behavior. We collected water samples upstream of the reservoir (P-01) in the city of Queluz, inside the reservoir (P-02), and downstream of Funil Reservoir (P-03) in the city of Itatiaia, RJ. In the field, physicochemical parameters were measured using a probe (pH, temperature, electrical conductivity, and dissolved oxygen). In the laboratory, water samples were filtered (0.45 µm) and properly packed until chemical analysis. Chlorophyll a concentrations were determined by a spectrophotometric method and suspended particulate matter (SPM) by a gravimetric method. Ionic concentrations were determined by ion chromatography technique and REE concentrations were determined by ICP-MS. Chlorophyll a concentrations were higher in Funil Reservoir. Ionic concentrations in Queluz (P-01) suggest anthropic contamination. The sum of REE in the dissolved fraction ranged from 2.12 to 12.22 µg L^-1. A positive anomaly of La in Queluz indicates anthropic contamination. The observed patterns indicate that Funil Reservoir acts as a biogeochemical barrier, modifying the fluvial transport of REE. Nonetheless, another factor that probably influences REE behavior is the algal bloom that occurs in reservoirs during the rainy season. The seasonal behavior of algae can influence REE biogeochemistry through the incorporation and release of trace metals.

Effects of dams on As and Hg concentrations in three southeastern Brazil fluvial systems: Ocean inputs, sources and seasonal dynamics among environmental compartments

Damming rivers for hydropower generation interferes in the flow of water bodies and, consequently, in trace element biogeochemistry and transport. This study focused on the effects of dams on the seasonal dynamics of arsenic (As) and mercury (Hg) in three fluvial systems (Itabapoana River, Flechas Channel and Paraíba do Sul River) and on the input of these elements to the ocean. The elements were evaluated in suspended particulate matter (SPM), surface sediments, and the roots of the aquatic macrophyte Eichhornia crassipes. Our results showed that the proximity of the dams in the Flechas Channel and Rio Itabapoana in relation to their mouth (distances of 4 km and 58 km, respectively) changed As and Hg seasonal dynamics in the environmental compartments analyzed, with the Flechas Channel being the most impacted area. In contrast, the lower part of the Paraíba do Sul River that has more distant dams to its mouth (distance of 184 km) did not show such marked changes, suggesting that most of the inputs are coming from the long river reach. The isotopic and elemental composition of organic matter indicated the soils of native forests as the main source of trace elements to the water bodies. Flow rate was the main driver controlling As and Hg ocean inputs, with higher inputs observed in the Paraíba do Sul River (9.69 and 0.59 t∙year^-1, respectively) compared to the other assessed fluvial systems. Also, this study demonstrated that, if a dam is closer to the mouth of a river, less contaminants will be input.

Effects of illegal gold mining on Hg concentrations in water, Pistia stratiotes, suspended particulate matter, and bottom sediments of two impacted rivers (Paraíba do Sul River and Muriaé River), Southeastern, Brazil

Recent reports of illegal small-scale alluvial gold mining activities (locally called garimpo) by miners working on rafts in the Paraíba do Sul River (PSR) and in one of its tributaries (Muriaé River (MR)) have raised concerns about Hg contamination. This study aimed to evaluate the impact of garimpo activities on Hg contamination in three environmental compartments. Water, sediment, and aquatic macrophytes (Pistia stratiotes) were sampled during the rainy season in PSR, forming a 106-km transect from the point where garimpo rafts were seen and/or seized by the Federal Police. They were also sampled in the MR. Total and dissolved mercury (Hg) concentrations in water and total Hg in the suspended particulate matter (SPM) sampled in the PSR increased by 1.7, 1.5, and 2.1 times at the points where the rafts were seen compared to the point immediately upstream. In the MR, Hg concentrations were higher than those in the PSR, but most values in the environmental compartments were below the safe limits (174-486 ng∙g^-1, threshold and probable effect level, respectively), with the exception of Hg in the SPM of one of the MR sampling points (256 ng∙g^-1) and the mining tailings (197 ng∙g^-1). Sediment granulometry was exponentially associated with Hg concentrations in the sediment (R² = 0.75, p < 0.0001) and is also essential to understand the physical impacts of garimpo on PSR. Future studies should focus on assessing the seasonal variability of Hg concentrations in the studied compartments, especially if garimpo is identified during the dry season.

Arsenic in the lower drainage basin of the Paraíba do Sul River (Southeast Brazil): dynamics between the water column and sediment

The presence of arsenic (As) in the sediment and the particulate and dissolved fractions of the water column determines its behavior and bioavailability. The main geochemical parameters responsible for As mobility are organic matter and oxide-forming metals such as Fe. The As distribution and its concentration were evaluated in the dissolved fraction, suspended particulate matter (SPM) and sediment of the lower Paraíba do Sul River (PSR), its main tributaries (Muriaé, Pomba, and Dois Rios rivers), and flooded and estuarine areas. As was not detected in the dissolved fraction. The river flow influenced the As concentration in the SPM, which was higher in the dry season than in the rainy season (2.6 ± 0.69 and 1.98 ± 0.29 mg kg^-1, respectively). The Fe oxyhydroxides, organic carbon, and surface area measured in the sediment were positively related with As concentration (R² = 0.11, 0.34, and 0.30; p < 0.05). The highest As concentrations in the sediment occurred in the secondary estuary and flooded areas (5.16 ± 4.78 and 1.23 ± 0.44 mg kg^-1, respectively), in which finer granulometric fractions (silt and clay) predominated (64% and 71%, respectively), in addition to greater surface area. The measurement of stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) and atomic ratio (C:N)a indicated the existence of a combination of autochthonous and allochthonous sources of organic matter composing the SPM. In general, the As concentrations in the sediment and SPM were low, with values below that permitted by Brazilian regulations (5.9 mg kg^-1), which suggests that the As sources in the lower basin of the PSR are diffuse and natural.

Genetic Diversity of an Imperiled Neotropical Catfish and Recommendations for Its Restoration

The long-whiskered catfish Steindachneridion parahybae (Family Pimelodidae) is endemic to the Paraíba do Sul River basin in southeastern Brazil. This species was heavily exploited by artisanal fisheries and faces challenges posed by dams, introduced species, and deterioration of critical habitat. The remaining populations are small and extirpated from some locales, and the species is listed as critically endangered in Brazil. Screening variation at a partial mitochondrial control region sequence (mtCR) and 20 microsatellite loci, we: (i) describe the patterns of genetic diversity along its current distributional range; (ii) test the null hypothesis of panmixia; (iii) investigate the main factors driving its current population structure, and (iv) propose management of broodstock for fostering recovery of wild populations through genetically cognizant restocking. Our microsatellite data for 70 individuals from five collections indicate moderate levels of heterozygosity (H_O = 0.45) and low levels of inbreeding (F_IS = 0.016). Individual-based cluster analyses showed clear genetic structure, with three clusters of individuals over the collection area with no mis-assigned individuals, suggesting no recent migration among the three clusters. Pairwise D_EST values showed moderate and significant genetic differentiation among all populations so identified. The MUR population may have suffered a recent demographic reduction. mtCRs for 70 individuals exhibited 36 haplotypes resulting from 38 polymorphic sites. Overall, mitochondrial haplotype diversity was 0.930 (±0.023) and nucleotide diversity was 0.011 (±0.002). Significant population structure was observed, with ϕ_ST = 0.226. Genetic markers could be used in a hatchery-based restoration program emphasizing breeding of pairs with low kinship values in order to promote retention of genetic diversity and avoid inbreeding. Individual average kinship relationships showed 87.3% advised matings, 11.0% marginal matings, and 1.7% advised against. While these results comprise a contribution toward planning better breeding management and monitoring, parallel actions to be undertaken include surveying healthy riverine habits for reintroduction and continued searching for wild individuals to introduce new variation into the captive broodstock to avoid adaptation to captivity and to minimize inbreeding.

Genetic divergence among invasive and native populations of the yellow peacock cichlid Cichla kelberi

This study used the hypervariable domain of the mitochondrial DNA (mtDNA) control region (CR) to assess the genetic divergence among native and invasive populations of Cichla kelberi, which is considered the first peacock cichlid introduced and established throughout Brazil and is among the most invasive populations of this genus worldwide. The maximum likelihood tree based on 53 CR sequences with strong bootstrap support revealed that C. kelberi forms a monophyletic clade, confirming that all 30 C. kelberi studied belong to this morphotype. Additionally, the haplotype analysis of the C. kelberi sequences from 11 sampling sites revealed that invasive populations are much less diverse than native ones and largely dominated by a single haplotype that prevailed in reservoirs at the Paraíba do Sul River basin. Two haplotypes were recorded exclusively in an invasive population at Porto Rico, southern Brazil, and one private haplotype was detected in two reservoirs from Paraíba do Sul (Pereira Passos and Paracambi), suggesting more than one introduction event and that native populations should be better evaluated to encompass the entire genetic diversity of native C. kelberi. The possible route and pathways of C. kelberi introduction are also briefly discussed.

First assessment of trace metal concentration in mangrove crab eggs and other tissues, SE Brazil

The mangrove crab Ucides cordatus is widespread in the Brazilian coast, which has an important role in nutrient cycling. This species reproduces in summer and females carry eggs about a month, when they maintain contact with water and sediments. It remains unclear if trace metals can be absorbed or adsorbed by the eggs during development. The present study aims to investigate, for the first time, trace metal concentrations in ovigerous female tissues and eggs of U. cordatus in two areas with different metal pollution levels in the Southeastern Brazil. Samples were collected in two different mangroves, Guanabara Bay (GB) highly polluted environment and Paraíba do Sul River (PSR). In both populations, we observed significant increase of V, Cr, and Mn concentrations along eggs maturation. The higher metals averages were found in PSR population. This trend was reported since the 1990s and lower concentrations in GB marine organisms were attributed to reducing conditions, high organic load, and the presence of sulfide ions. These conditions restrict the bioavailability of metals in the bay, with exception of Mn. No significant differences were observed in gills and muscles. In both populations of the present study, V, Zn, As, and Pb were higher in eggs of initial stage, whereas Mn, Ni, Cu, and Cd were higher in hepatopancreas. Beside this, V, Cr, Mn, As, and Pb showed an increase concerning egg development. Thus, V, As, and Pb in eggs come from two sources previous discussed: females and environment. Zinc came mainly from females due to essential function. Those new information should be considered as one of the mechanisms of trace metal transfer to the trophic chain, between benthonic and pelagic environment.