Spatio-temporal Variations of Zn, Cu, Cd, Pb, Se, and Sn in Sediments Around a Kappaphycus Seaweed Farm and Its Associated Risks

Coastal development and maritime activities continue to introduce metals into nearshore environments, including seaweed farms. Despite the rapid expansion of tropical seaweed aquaculture, sediment metal, naturally and anthropogenic in origin, dynamics within active Kappaphycus farming areas remain poorly characterized, particularly with respect to the bioavailable metal fraction that controls ecological exposure. This study quantified bioavailable Zn, Cu, Cd, Pb, Se, and Sn in sediments surrounding a Kappaphycus farm in Concepcion, Iloilo, Philippines, from September to November 2024 and evaluated their associated risks. Sediments showed low bioavailable concentrations, with 2.26 to 13.1 mg kg⁻¹ Zn, 0.258 to 2.22 mg kg⁻¹ Cu, 0.0455 to 0.0718 mg kg⁻¹ Cd, 0.268 to 3.00 mg kg⁻¹ Pb, and 1.12 to 4.61 mg kg⁻¹ Sn, but considerably high Se of about 3.32 to 14.3 mg kg⁻¹. Spatial patterns showed higher metal concentrations in fine grained sediments with higher moisture content and organic matter. Temporal observations during the cultivation period indicated declining metal concentrations. Most metals in sediments were below commonly used guideline thresholds, suggestive of low potential ecological concern, except for Se, which warrants attention due to trophic relevance. These findings show low ecological risk from most metals under present conditions, while elevated selenium concentrations warrant continued monitoring due to its known bioaccumulative behavior. The study provides information on bioavailable metals in sediments of tropical seaweed farms and demonstrates the influence of sediment characteristics on metal distribution within mariculture environments.