Mercury Bioaccumulation in Mangrove Oysters (Crassostrea rhizophorae) (Guilding, 1828) and Associated Human Exposure from the Parnaíba River Delta, Equatorial Coast of Brazil

The present study quantifies mercury (Hg) concentrations in mangrove oysters (<i>Crassostrea rhizophorae</i>) and assesses their potential as biomonitors of Hg contamination in the Parnaíba River Delta (PRD), located on the equatorial coast of Brazil (ECB). The highest Hg concentrations occurred in the smallest individuals' size class (20-40 mm) from the main channel of the Parnaíba River (52.1 to 195.4 ng g^-1 w.w.), whereas the largest individuals' size class (larger than 60 mm) exhibited the lowest Hg concentrations (35.2-114 ng g^-1 w.w.). There was a significant correlation between Hg concentrations and shell sizes, either when considering all size classes or when considering only individuals of size classes smaller than 40 mm. Oysters larger than 40 mm did not present any significant correlation between Hg concentrations and size. In addition to size, higher concentrations were observed at the freshwater-seawater transition in the main channel sites. These maximum suspended particulate zones, with bottom sediment resuspension, can favor Hg adsorption to fine particles, increasing the bioavailability of Hg. A regional comparison of Hg concentrations in mangrove oysters from the ECB suggests they are efficient biomonitors at a regional level. In contrast, the environmental dynamics of the PRD, with high variability within sites, hampered its use at the local level. Mangrove oysters from the PRD are shown to be safe for human consumption, as far as Hg exposure is concerned, and the presented risk assessment shows no excessive exposure, even at high-frequency consumption rates.