Export and bioaccumulation of methylmercury in streams draining distinct soils in the Central Brazilian Amazon, 2012-2013

Abstract

Background: Upland forest streams of the Central Amazon are unique aquatic systems.These small streams have short, unpredictable and multimodal flood-pulses that are influenced by local rainfall. The biogeochemical cycles in these systems are almost unknown. This study investigated the role of soil type on the export and bioaccumulation of methylmercury (MeHg) in upland forest streams of the Central Amazon during a complete annual cycle. Basic Procedures: Limnological measurements and water samples for MeHg analyses were collected monthly from October 2012 to October 2013 in two headwater streams drained by different soil types, spodzol (Campina Stream) and oxisol (Barro Branco Stream), in natural forest reserves. Shrimp and fish were collected for total mercury (THg) in April 2013 in both streams. Main findings: MeHg in water varied from 0.04 to 0.50 ng/L in the Campina Stream and between <detection limit and 0.04 ng/L in Barro Branco Stream. Physical characteristics of soils and the interaction between mercury and organic matter influenced the methylation of mercury and its export to the streams. In both streams, MeHg export was highest during the rainy season, probably due to soil leaching, lateral flooding and soil hydromorphism during this period, processes conducive to mercury methylation and export. THg in shrimps (omnivores) and carnivorous fish, which fed on aquatic food items, reflected the MeHg differences between streams while those in insectivorous fish, feeding on allochthonous food items, did not. Principal Conclusions: MeHg dynamics in streams were influenced by the physical characteristics of basin soils and by local seasonal rainfall.

Keywords

• Bioaccumulation
• Methylation
• Oxisol
• Seasonality
• Spodzol