Mercury dynamics in the mangrove-influenced estuary of the Parnaíba Delta, Brazil

The Parnaíba River Delta (PRD), located in northeastern Brazil, is an ecologically crucial estuarine system little impacted by anthropogenic activities. This study aimed to assess mercury (Hg) contamination levels in the water and sediments and their link to changes in the hydrographic parameters across the delta to evaluate spatial distribution patterns and Hg speciation. Water and surface sediment samples were collected from 12 stations throughout the PRD. Results showed that total Hg (tHg) levels ranged from 4.27 to 39.01 pM, with the majority associated with particles (pHg: 16.03 ± 9.95 pM). Dissolved methylmercury concentrations (dMeHg: 0.043 ± 0.015 pM) were low and represented a minor fraction of Hg. Seawater intrusion during flood tide was associated with lower levels of all Hg species. Particulate Hg was sequestered and stored in the fine-grained mangrove sediments (0.14-28.2 ng g^-1 dry weight). Our study provides baseline data on Hg cycling in the PRD, highlighting its pristine condition and function as a buffer between terrestrial and marine environments. Pristine mangrove systems are effectively sequestering carbon and mercury and should, therefore, be considered as blue carbon and blue mercury ecosystems for mitigation strategies.