Determinations ofdissolved reactive and total dissolved mercury, particulate and sedimentary mercury, dissolved organic carbon
(DOC), particulate organic carbon (POC) and suspended particulate matter (SPM) have been made in the estuary ofriver Douro, in
northern Portugal. The estuary was stratified by salinity along most ofits length, it had low concentrations of SPM, typically <20 mg dm-3, and concentrations of DOC in the range <1.0–1.8 mg dm-3. The surface waters had a maximum dissolved concentration of reactive mercury of about 10 ng dm-3, whereas for the more saline bottom waters it was about 65 ng dm-3. The surface waters had maximum concentrations oftotal suspended particulate mercury of ~7 ug g-1 and the bottom waters were always
< l ug g-1. Concentrations of mercury in sediments was low and in the range from 0.06 to 0.18 ug g-1. The transport of mercury
in surface waters was mainly associated with organic-rich particulate matter, while in bottom waters the dissolved phase transport
ofmercury is more important. Lower particulate organic matter, formation ofchlorocomplexes in more saline waters and eventually the presence of colloids appear to explain the difference of mercury partitioning in Douro estuarine waters.
