Determination of heavy metal bioaccumulation in selected species of seaweeds and seagrasses found on the Nasese seashore, Suva, Fiji

Aim Conventional environmental monitoring in Fiji relies on discrete water and sediment sampling, which fails to capture integrated bioavailability or food-web contaminant transfer. To address this gap, this study evaluated the species-specific heavy metal (HM) bioaccumulation capacities of co-occurring marine macrophytes along the Nasese foreshore (Suva, Fiji) to establish a cost-effective coastal biomonitoring framework. Method Two species of seaweeds ( Ulva intestinalis, green; and Sargassum polycystum, brown) and three species of seagrasses ( Halodule pinifolia, Halodule uninervis, and Halophila ovalis, all green) were collected from the Nasese intertidal zone and prepared by washing, drying, homogenization, and acid digestion. Heavy metals (Cd, Cu, Cr, Pb) were quantified using Flame Atomic Absorption Spectrometry after multi-acid digestion, while Hg was determined separately by Cold Vapour Atomic Absorption Spectrometry following oxidative digestion. Bioaccumulation factors (BAFs) and geoaccumulation indices (Igeo) were calculated to assess environmental risk. Principal findings The concentration of HMs in the biota were in the range: 0.34–0.73, 67.75–111.61, 6.33–15.88, 7.18–76.01, and 3.79–44.37 mg/kg for Hg, Pb, Cd, Cr, and Cu, respectively. The overall trend in HMs concentration in all species followed the order Hg < Cd < Cu < Cr < Pb . The seagrass H. ovalis had a consistently high concentration of Hg, Pb, and Cd, while the two seaweed species U. intestinalis and S. polycystum had the highest concentration of Cr and Cu, respectively. In seawater, Pb (0.03–0.07 mg/L) and Cd (0.014 mg/L) exceeded WHO permissible limits (0.01 and 0.003 mg/L, respectively) at all sites. High Bioaccumulation Factor (BAF) values (>1,000) identify U. intestinalis , S. polycystum , and H. ovalis as effective sentinel biomonitors. Implication The recorded levels of bioaccumulation factor classify these organisms as moderately to high bioaccumulators. This finding demonstrates that accumulated HMs by investigated species pose an unmonitored risk of trophic transfer to subsistence-harvested marine fauna (e.g., rabbitfish and sea urchins).