Mercury and flooding cycles in the Tapajós river basin, Brazilian Amazon: The role of periphyton of a floating macrophyte (Paspalum repens) Original Research Article

Methylmercury (MeHg) increases mercury (Hg) toxicity and is biomagnified in the trophic chain contaminating riverine Amazon populations. Freshwater macrophyte roots are a main site of Hg methylation in different Brazilian environments. Paspalum repens periphyton was sampled in four floodplain lakes during the dry, rainy and wet seasons for measurement of total Hg (THg), MeHg, Hg methylation potentials, %C, %N, δ13C, δ15N and bacterial heterotrophic production as 3H-leucine incorporation rate. THg concentration varied from 67 to 198 ng/g and the potential of Me203Hg formation was expressive (1–23%) showing that periphyton is an important matrix both in the accumulation of Hg and in MeHg production. The concentration of MeHg varied from 1 to 6 ng/g DW and was positively correlated with Me203Hg formation. Though methylmercury formation is mainly a bacterial process, no significant correlation was observed between the methylation potentials and bacterial production. The multiple regressions analyses suggested a negative correlation between THg and %C and %N and between methylation potential and δ13C. The discriminant analysis showed a significant difference in periphyton δ15N, δ13C and THg between seasons, where the rainy season presented higher δ15N and the wet period lighter δ13C, lower THg values and higher Me203Hg formation. This exploratory study indicates that the flooding cycle could influence the periphyton composition, mercury accumulation and methylmercury production.