A Snapshot of Species Composition and Mercury Bioaccumulation in Fish from Natural and Constructed Wetlands

We compared fish assemblage structure and total mercury (THg) bioaccumulation between a natural floodplain lake and a constructed irrigation canal in central Brazil. A total of 473 individuals representing 34 species were recorded, and dorsal muscle samples from 62 specimens representing shared species or species occupying comparable trophic positions were analyzed for THg (Curimatella immaculata, Hemiodus microlepis, Astyanax aff. bimaculatus, Triportheus albus, Geophagus sveni, Pimelodus blochii, Pygocentrus nattereri, Lycengraulis batesii, and Cichla kelberi). The floodplain lake exhibited higher species richness, diversity, and evenness, whereas the irrigation canal supported a simplified assemblage dominated by fewer species. Total Hg concentrations were significantly higher in the lake than in the irrigation canal; however, this pattern was observed only for the carnivorous guild (t = 5.384, p < 0.0001) and the detritivorous guild (t = 4.183, p = 0.0001). THg increased significantly with trophic level in both systems, from detritivores to carnivores (F2,4 = 15.127, p = 0.009), yielding comparable trophic magnification slopes (lake: 1.46, 95% CI: 1.11–1.81; canal: 1.36, 95% CI: 0.94–1.77). Despite lower diversity and THg concentrations in the irrigation canal, Hg transfer efficiency across trophic levels was conserved between systems.