Metal retention in two semi-arid mangroves under contrasting environmental pressures

This study examines metal interactions in sediments from two semi-arid mangroves in North East Brazil, Jaguaribe (impacted) and Pacoti (preserved). Sediment cores (up to 80 cm) were collected from upstream, middle, and downstream areas, with an additional core near shrimp farm effluents in Jaguaribe. Granulometric parameters (% fine particles, Al, Fe), organic matter (OM) characteristics (Loss on ignition, %C, %N), total sulfur (TS%), and metals contents (Zn, Co, Cr, Hg) were analyzed every 5 cm. Metal excess inventories and OM complexation constants (K) were determined. In Jaguaribe, Hg and Zn strongly correlate with fine particles (R >0.9), OM (R>0.87), and N (R >0.82), while Co primarily associates with fine particles (R=0.8). In Pacoti, Zn and Co concentrations are low, preventing clear pattern identification, and Hg correlates with sulfur (R=0.84) rather than OM and N. These results suggest that in Jaguaribe, Hg and Zn interact with nitrogen-rich OM, influenced by anthropogenic effluents, whereas in Pacoti, they associate with sulfur compounds such as pyrite, a typical feature of preserved mangroves. Cr shows no correlation with sediment parameters but exceeds Brazilian limits of 75 mg.kg -1 (Jaguaribe: 184 mg.kg -1 , Pacoti: 216 mg.kg -1 ). Co and Zn metal inventories are increasing downstream in Jaguaribe with highest levels near the shrimp farm, suggesting a higher risk of erosion-induced metal release. Despite OM accumulation, weak Cu complexation (LogK∼4.95) suggests limited metal retention, challenging the assumption of mangroves as metal sinks. High Cr inventories in both mangroves warrant further investigation. • Granulometry strongly controls both metal and organic matter retention • Zn and Hg interact with nitrogen-rich OM in impacted site, not with sulfur compounds • Land use influences metal–sediment interactions more than tidal position in mangroves • Impacted mangrove stores more metals downstream