Dynamics and secondary pollution risk assessment of heavy metals during Sargassum polycystum floating and decomposing process

Abstract

Sargassum ranks among the most prolific seaweed species in coastal regions across the globe. Its swift biomass accumulation is predominantly propelled by heightened nutrient concentrations. In coastal waters, a substantial amount of Sargassum has amassed and undergone decomposition, resulting in the release of carbon (C), nitrogen (N), phosphorus (P), and heavy metals into the environment, thereby exerting detrimental effects on coastal habitats. The present study implemented a 72-day laboratory controlled experiment utilizing litter bag technology to simulate the floating stage of S. polycystum. The results showed that the decomposition rate of the low-density S. polycystum group was significantly higher than that of the medium-density, high-density, natural, dried, and frozen S. polycystum groups. The heavy metals contents of S. polycystum were in the following order: Fe > Zn > Mn > As > Cr > Cu > Ni > Pb > Cd > Se. Fe predominantly exhibits a release effect (MAI < 100 %). In contrast, Zn, Mn, and Se primarily demonstrate an accumulation effect (MAI > 100 %), As, Cd, Cr, Cu, Pb, and Ni display both release and accumulation effects concurrently. At least Fe (35.12 %–50.22 %), Cd (30.76 %–49.44 %), Ni (4.63 %–6.37 %), Cu (2.51 %–10.71 %), Cr (75.64 %–92.3 %), As (17.44 %–20.02 %), and Pb (37.09 %) in S. polycystum were released back to the water. The C contents during the decomposition process of S. polycystum were stably maintained, the N and P contents increased during the experiment. The present study revealed the decomposition characteristics of S. polycystum and its environmental impact, providing a theoretical basis for marine ecological environmental protection of seaweed bed.