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The conversion of riparian forests into agricultural land results in the loss of water quality and aquatic biota health. The objectives of this study were therefore to determine the proportion of land use with emphasis on the type of vegetation cover; evaluate the limnological parameters and concentration of inorganic elements in the water of the São José stream in the Dourados River Basin, Mato Grosso do Sul, Brazil; correlate land use with the concentrations of inorganic elements in water; and evaluate the risk to preservation aquatic biota and in situ toxic effect on Astyanax lacustris. We collected samples from the São José Stream in 2020 and evaluated land use with high resolution aerial images. The inorganic elements in water samples were quantified using inductively coupled plasma optical emission spectrometry. In the surroundings of the São José stream, a reduction in riparian forest was observed with a proportion of only 16.32% of the area and the predominance of agricultural areas with 75.06%. The concentrations of dissolved oxygen (1.510 mg L-1) and P (> 0.235 mg L-1) in the water did not comply with the national legislation. In addition, Al and P indicated risks regarding the conservation of aquatic biota (risk quotient >1). The in situ evaluation of A. lacustris also revealed toxicity in the water. The results indicate environmental imbalance in the São José stream, requiring mitigation measures for its restoration and the sustainable use of its resources. Keywords: anthropic action, inorganic contaminants, risks for aquatic biota.

Colomesus asellus (Amazonian Puffer) is endemic to the Amazon Basin. It uses channels and streams to reproduce, spawn and feed. In areas close to urban centers, these fish may be exposed to anthropogenic residues containing mixtures of metals that can be bioaccumulated, resulting in genetic alterations. Therefore, we aimed to determine the extent of nuclear alterations in erythrocytes of female C. asellus resulting from the bioaccumulation of metals in tissues and organs. Metal concentrations were analyzed by atomic absorption spectrophotometry. Tissue metal concentrations decreased in the following order: Fe > Zn > Cu > Mn > Hg > Pb > Ni > Cd > Cr. In fish tissues/organs, the concentration of metals followed, in decreasing order, liver > skin > gonads > musculature > bones. The most frequent nuclear alterations were nuclear invagination, nuclear budding and lobulated nucleus. Metal concentrations and resultant nuclear alterations observed can damage C. asellus, thus compromising the conservation of this species at the mouth of the Amazon River.