Mercury concentration in six fish guilds from a floodplain lake in western Amazonia: Interaction between seasonality and feeding habits

Floodplain systems are widespread in the Amazon and several studies have evaluated mercury biogeochemistry in these systems. The Amazon region can be considered a hotspot for the study of mercury (Hg), due to the naturally high concentration of the pollutant in its old soils and the anthropogenic emissions from small-scale gold mining activities. The periodic flooding influences the availability and distribution of Hg contamination. Therefore, it is possible that flooding can influence accumulation of Hg in fish. The aim of this study was to evaluate total Hg (THg) concentration in four seasons (rising water, high water, falling water and low water) in six fish guilds in a lentic part of a floodplain system (Puruzinho Lake). We hypothesized that fish guilds would show different seasonal patterns of THg concentration variation. A total of 2016 fish specimens were sampled over the four seasons. Two-way ANOVA indicated an interaction between guilds and seasonality (F = 7.3; p < 0.001), suggesting different seasonal patterns of THg concentration variation among the guilds. Two major patterns were identified. Herbivorous, detritivorous, carnivorous and piscivorous fish guilds showed higher concentrations during the rising and low water seasons, while planktivorous and omnivorous guilds showed lower THg levels during the low water season. There were no significant differences in THg levels between the early and late periods of each season.