Chlor-alkali plants (CAP) are recognized as major sources of mercury (Hg) in the environment. In this work, Hg concentration, speciation and isotopic signature were determined in sediments and biota (fish and oyster) from Sagua La Grande River (SG River) and the adjacent coastal zone in the vicinity of a CAP (Cuba). High Hg concentrations in surface sediments (up to 5072 ng g(-1)), mainly occurring as inorganic Hg, decrease with the distance from the CAP along the SG River and seaward. Meanwhile, Hg concentration and speciation in riverine catfish (Claria gariepinus) muscle (1093 +/- 319 ng g(-1), (similar to)70% as MeHg) and coastal oysters (Crassostrea rizophorae) (596 +/- 233 ng g(-1) (similar to)50% as MeHg) indicate a direct impact from CAP. Hg isotopic signature in sediments, following both mass dependent (MDF) and mass independent fractionation (MIF), exhibits a clear binary mixing between CAP pollution (+0.42 parts per thousand, delta Hg-202;-0.18 parts per thousand, Delta Hg-201) and regional background end-member ((similar to)0.49 parts per thousand, delta Hg-202; +0.01 parts per thousand, Delta Hg-201). The combination of speciation and isotopic information in biota and sediments allows to trace Hg contamination pathways from contaminated sediments to the biota, establishing the importance of both methylation and demethylation extent in both river and coastal sites before Hg species bioaccumulation.