There is an ongoing debate on the fate of mercury (Hg) in areas affected by artisanal and small-scale gold mining (ASGM). Over the last 30 years, ASGM has released 69 tons of Hg into the southeastern Peruvian Amazon. To investigate the role of suspended matter and hydrological factors on the fate of ASGM-Hg, we analysed riverbank sediments and suspended matter along the partially ASGM-affected Malinowski-Tambopata river system and examined Hg accumulation in fish. In addition, local impacts of atmospheric Hg emissions on aquatic systems were assessed by analysing a sediment core from an oxbow lake. Hg concentrations in riverbank sediments are lower (20–53 ng g−1) than in suspended matter (∼400–4000 ng g−1) due to differences in particle size. Elevated Hg concentrations in suspended matter from ASGM-affected river sections (∼1400 vs. ∼30–120 ng L−1 in unaffected sections) are mainly driven by the increased amount of suspended matter rather than increased Hg concentrations in the suspended matter. The oxbow lake sediment record shows low Hg concentrations (64–86 ng g−1) without evidence of any ASGM-related increase in atmospheric Hg input. Hg flux variations are mostly an effect of variations in sediment accumulation rates. Moreover, only 5% of the analysed fish (only piscivores) exceed WHO recommendations for human consumption (500 ng g−1). Our findings show that ASGM-affected river sections in the Malinowski-Tambopata system do not exhibit increased Hg accumulation, indicating that the released Hg is either retained at the spill site or transported to areas farther away from the ASGM areas. We suspect that the fate of ASGM-Hg in such tropical rivers is mainly linked to transport associated with the suspended matter, especially during high water situations. We assume that our findings are typical for ASGM-affected areas in tropical regions and could explain why aquatic systems in such ASGM regions often show comparatively modest enrichment in Hg levels.

For the first time, we revealed the life-history traits including growth pattern (length–weight relationships, LWRs), condition factors, form factor (a₃.₀), first sexual maturity (Lₘ), age at first sexual maturity (tₘ), life span (tₘₐₓ), natural mortality (Mw), asymptotic length (L∞), and optimum catchable length (Lₒₚₜ) of ten commercially important small indigenous fish species (SIFS) in the Oxbow lake (Baor), southwestern regions of Bangladesh. A total of 1651 specimens were sampled during January to December 2020 with traditional fishing gears including seine nets, gill nets, and lift nets. Individual total length (TL) and body weight (BW) were measured by digital slide calipers and digital balance, respectively. To calculate the Lₘ, empirical maximum length-based model was considered, and Lₒₚₜ was calculated based on L∞. The TL vs. BW relationship indicated positive allometric growth for Chanda nama (Hamilton 1822), Channa punctata (Bloch 1793), Channa striata (Bloch 1793), Lepidocephalichthys guntea (Hamilton 1822), Macrognathus pancalus (Hamilton 1822), and Puntius sophore (Hamilton 1822), but negative allometric growth for Badis badis (Hamilton 1822), Gudusia chapra (Hamilton 1822), Glossogobius giuris (Hamilton 1822), and Hyporhamphus limbatus (Valenciennes, 1847). All r² values exceed 0.910 that indicated all LWRs were highly significant (P < 0.001). According to Spearman correlation test, Fulton’s condition factor (KF) vs. BW was highly correlated (P < 0.001), indicating better well-being for these species. Moreover, a₃.₀ indicates B. badis, C. punctata, C. striata, G. giuris, H. limbatus, L. guntea were elongated; C. nama, P. sophore, were short and deep; G. chapra was fusiform, and M. pancalus was eel-like body shape respectively. The minimum tₘ and tₘₐₓ were obtained as 0.74 year and 2.66 year for C. striata and maximum were 0.93 year and 3.31 year for B. badis, respectively. This study provided information on tₘ and tₘₐₓ for ten SIFS that is globally absent. From empirical models, the smallest mean value of Lₘ was found for B. badis (3.98 cm), and the greatest was found for C. striata (16.96 cm). The minimum Lₒₚₜ was obtained as 3.78 cm TL for B. badis and maximum was 14.09 cm TL for C. punctata. The minimum Mw was documented as 1.39 for B. badis and maximum was 1.73 for C. striata. The output of this research will be helpful for developing sustainable management policies and protection of SIFS through the application of mesh size based on Lₘ and Lₒₚₜ in the Oxbow lakes, Bangladesh and neighboring countries.

The aim of this study is to investigate complex pollution history and spatial trends of pollution (heavy metals and organic pollutants) in the selected oxbow lake (at the border between the Czech Republic and Poland) along the Odra River, located in the heavily polluted Ostrava urban agglomeration. Contaminant distribution is affected by position in the lake; the highest heavy metal concentrations were found in the central parts. Depth trends in ¹³⁷Cs mass activity, hexachlorobenzene, and DDT metabolites were used as additional date levels. Depth profiles of heavy metals and their enrichment factors and multi-proxy stratigraphic analysis allowed two stages of oxbow lake evolution to be recognized. The initial stage was characterized by higher sediment accumulation rates in the whole oxbow lake, while during the second stage, sediment was deposited in a plug bar and in proximal parts of the lake, providing a more complete pollution record. Coal-rich layers were found in the deeper parts of the sedimentary record, related to coal mining activities. Samples rich in coal revealed a negative correlation with CIEL* (brightness) and conversely a positive correlation with specific biomarkers (homo hopanes and 16 α(H)-phyllocladane) and PAHs (secondary sorption to coal-rich strata). The sedimentary record since the oxbow lake cut-off event in 1966 showed a distinct vertical distribution of all pollutants. Maximum concentrations were attained during 1970s and 1980s, while a decreasing trend occurred after 1989 due to political and socio-economical changes in the former Czechoslovakia (now the Czech Republic).

Geothermal waters exploited in the southeastern region of Hungary are alkali-hydrogen-carbonate type, and beside the high amount of dissolved salt, they contain a variety of aromatic, heteroaromatic, and polyaromatic hydrocarbons. The majority of these geothermal waters used for heating are directed into surface waters following a temporary storage in reservoir lakes. The aim of this study was to gain information about the temporal and spatial changes of the water quality as well as the bacterial community composition of an alkaline and saline oxbow lake operated as reservoir of used geothermal water. On the basis of the water physical and chemical measurements as well as the denaturing gradient gel electrophoresis (DGGE) patterns of the bacterial communities, temporal changes were more pronounced than spatial differences. During the storage periods, the inflow, reservoir water, and sediment samples were characterized with different bacterial community structures in both studied years. The 16S ribosomal RNA (rRNA) gene sequences of the bacterial strains and molecular clones confirmed the differences among the studied habitats. Thermophilic bacteria were most abundant in the geothermal inflow, whereas the water of the reservoir was dominated by cyanobacteria and various anoxygenic phototrophic prokaryotes. In addition, members of several facultative anaerobic denitrifying, obligate anaerobic sulfate-reducing and syntrophic bacterial species capable of decomposition of different organic compounds including phenols were revealed from the water and sediment of the reservoir. Most of these alkaliphilic and/or halophilic species may participate in the local nitrogen and sulfur cycles and contribute to the bloom of phototrophs manifesting in a characteristic pink-reddish discoloration of the water of the reservoir.