Although estuaries are critical habitats for many aquatic species,the spatial trends of toxic methylmercury (MeHg) in biota from fresh to marine waters are poorly understood.Our objective was to determine if MeHg concentrations in biota changed along a salinity gradient in an estuary.Fourspine Stickleback (Apeltes quadracus),invertebrates (snails,amphipods,and chironomids),sediments,and water were collected from ten sites along the Saint John River estuary,New Brunswick,Canada in 2015 and 2016,with salinities ranging from 0.06 to 6.96.Total mercury (proxy for MeHg) was measured in whole fish and MeHg was measured in a subset of fish,pooled invertebrates,sediments,and water.Stable sulfur (δ34S),carbon (δ13C),and nitrogen (δ15N) isotope values were measured to assess energy sources (S,C) and relative trophic level (N).There were increases in biotic δ13C and δ34S from fresh to more saline sites and these measures were correlated with salinity.Though aqueous MeHg was higher at the freshwater than more saline sites,only chironomid MeHg increased significantly with salinity.In the Saint John River estuary,there was little evidence that MeHg and its associated risks increased along a salinity gradient.