Assessment of Toxic Elements Accumulation in Seagrasses and Commonly Edible Fishes from Chwaka Bay, Zanzibar: A Threat to Marine and Human Health

Anthony OO, Rashid RJ and Ali MY

Published on: 2025-07-01

Abstract

Four types of sea grasses Enhalus Acoroides, Halophila Ovalis, Cymodocea Rotudanta and Thalassia Hemprichii found in Chwaka regions of Unguja Island in Zanzibar were investigated on bioaccumulation of heavy metals. Using inductively coupled plasma mass spectrometry (ICP- MS), significant concentrations of heavy metals were detected, with notably high levels of lead (Pb: 344.22 mg/kg), arsenic (As: 266.10 mg/kg), copper (Cu: 74.23 mg/kg), and chromium (Cr: 8.85 mg/kg). Additionally, elevated levels of aluminum (Al: 1362.50 mg/kg), boron (B: 742.13 mg/kg), manganese (Mn: 106.07 mg/kg), silicon (Si: 455.36 mg/kg), molybdenum (Mo: 6.04 mg/kg), nickel (Ni: 23.11 mg/kg), and lanthanum (La: 3.03 mg/kg) were primarily found in E. acoroides and H. ovalis. These concentrations exceed the safety thresholds established by the World Health Organization (WHO), the European Union (EU), the International Program on Chemical Safety (IPCS), and the United States Environmental Protection Agency (USEPA), indicating significant pollution in the marine ecosystem. Furthermore, analysis of the eight most commonly consumed fish species in the region revealed Estimated Daily Intake (EDI), Cumulative Pollution Index (CPI), and Total Target Hazard Quotient (TTHQ) values exceeding the limits, suggesting potential health risks. Particularly high levels of lead and arsenic in Rastrelliger kanagurta, Hemiramphus nigricans, and Siganus gracilis contributed to high values of EDI and TTHQ above safe limits thus leading to their decline including seagrasses. These findings highlight the urgency of monitoring and regulating effluent discharge into the Chwaka Bay to mitigate ecological and human health risks.