Annual flooding and fish-mercury bioaccumulation in the environmentally impacted Rio Madeira (Amazon).

Regular annual flooding of the Amazonian rivers changes the aquatic environment affecting fish feeding strategies. The Rio Madeira has been greatly impacted by deforestation for agricultural projects, damming for a hydroelectric power plant, and alluvial gold extraction. We studied fish-Hg concentrations within defined weight ranges of representative species at the top of the food web, comparing high and low water seasons. Selected piscivorous species (Cichla spp, Hoplias malabaricus, Pinirampus pirinampu, Serrasalmus spp) showed a large variation of Hg concentrations but only "traíra" (Hoplias malabaricus) showed a statistically significant difference between seasons. However, the bioaccumulation trends during high and low waters were similar for "tucunaré" (Cichla spp) and "traíra" (Hoplias malabaricus) but different for "piranhas" (Serrasalmus spp), "barba chata" (Pinirampus pirinampu) and the detritivorous Prochilodus nigricans. Fish-Hg bioaccumulation is species specific; changes in feeding strategies brought by flooding seasons can change the bioaccumulation pattern without systematically affecting the overall accrual of methylmercury in tertiary consumer species. It appears that naturally occurring Hg and the high sediment load of the Rio Madeira are secondary factors in the Hg bioaccumulation pattern of fish species at the top of the food chain.