Mercury Concentration in Two Estuarine Fish Populations across a Seasonal Salinity Gradient

Abstract

AbstractMercury (Hg) cycling in estuaries is complex, and little is known about the extent of Hg bioaccumulation in the ecologically diverse fishes that inhabit coastal areas. Using tissue analysis combined with otolith microchemistry, analysis of Hg concentration in common prey species, diet analysis, and age‐and‐growth analysis, we investigated seasonal and spatial trends of Hg accumulation in largemouth bass Micropterus salmoides and southern flounder Paralichthys lethostigma inhabiting the Mobile—Tensaw River Delta, Alabama. The Hg concentrations in largemouth bass (which are known to have high site fidelity) were lowest at downstream, low‐salinity estuarine areas and highest at upstream freshwater areas. Conversely, southern flounder, which have a highly variable migratory life history across salinity gradients, had Hg concentrations that were similar across the study area and lower than those for largemouth bass. Both largemouth bass and southern flounder had negative relationships between individual salinity exposure (as indicated by otolith strontium: calcium ratios) and tissue Hg concentration, although considerable variability existed. Concentrations of Hg in two common prey species, the bluegill Lepomis macrochirus and blue crab Callinectes sapidus, suggested that vertebrate prey had higher Hg concentrations than invertebrate prey. Largemouth bass upstream consumed a higher proportion of vertebrates than those downstream, so differences in prey consumption may partially explain Hg concentration differences across the salinity gradient. However, additional research is required to understand the mechanism responsible for the negative relationship between Hg concentration and salinity exposure.