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Endocrine-disrupting contaminant (EDC) loads in rivers, and the associated risk to wildlife, may be linked to different anthropogenic stressors occupying river catchments. The aims of this study were to evaluate seasonal and spatial variation in steroid estrogen loads, and (anti)estrogenic and (anti)androgenic activity in a river catchment (upper Olifants River, South Africa), subject to a diversity of anthropogenic impacts. In addition, Mozambique tilapia, Oreochromis mossambicus, was applied as African-endemic sentinel and source of in vivo biomarkers. In particular, the expression of selected genes linked with reproductive, thyroid and adrenal signalling were determined in juveniles exposed to organic compounds extracted from surface water. Estradiol and ethinylestradiol were detected at all locations, yet the highest concentrations were observed during summer at a waste water treatment plant (WWTP)-impacted site (30.8 and 10.83 ng.l⁻¹, respectively). Moreover, in vitro estrogenic and androgenic activity was detected solely at the aforementioned locality. Anti-androgenic activity was more widespread, and detected at four of the localities sampled, with the highest potency at WWTP-impacted sites. The expression of the aromatase coding gene, cyp19a1b, was significantly downregulated in O. mossambicus representing a site dominated by agricultural land use. Moreover, the thyroid-linked type 2 deiodinase (dio2) was upregulated in fish representing three of the five localities, although with no clear link to a specific land use area. The present study suggests that different land cover areas contribute differentially to endocrine disruptive activity, and that a combinational approach (in vitro and in vivo biomarkers) is required to screen for EDC risk.
