MERCURY BIOGEOCHEMICAL CYCLING IN A STRATIFIED ESTUARY

Total Hg in the permanently stratified Pettaquamscutt estuary was <25 pM throughout the water column, even in highly sulfidic bottom waters. Particulate Hg was typically >40% of the total Hg. Reactive Hg (Hg(R) ) was generally <3 pM and decreased with depth, but there is Hg(R) eve n in the anoxic bottom waters. Elemental Hg (Hg0) was highest in the m ixed layer and below the detection limit at depth. Demethylation is no t an important source of Hg0 in this estuary. Dimethylmercury was not detected. Monomethylmercury (MMHg) was near the detection limit in the mixed layer and increased rapidly in the low oxygen region. Dissolved MMHg correlated with bacteriochlorophyll pigments, suggesting that th e microbial community plays an important role in MMHg production in th is estuary. The overall distributions of dissolved and particulate Hg species result from the interaction with Fe and Mn redox cycling, part iculate scavenging and sinking, and MMHg production in the pycnocline. The estimated rate of MMHg production from Hg(R) in the pycnocline re gion is 1.7% d-1. Hg0 and MMHg are formed principally in the mixed lay er and in the pycnocline region, respectively. Particulate scavenging is important, and sedimentation, methylation, and Hg0 production are t he principal sinks for Hg(R).