Organochlorine pesticides and polycyclic aromatic hydrocarbons in marine sediments and polychaete worms from the west coast of Unguja island, Tanzania

Abstract

The coastal areas of Stone Town, on the island of Unguja that is part of Zanzibar, are subjected to different anthropogenic activities and receive untreated municipal wastes that are likely to carry organic pollutants that can bio-accumulate in marine organisms. Sediments and polychaete worms Capitella capitata collected from coastal sites north and south of Stone Town were analysed for organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs) using GC–MS to assess their pollution status in the coastal environment and uptake by the food web. The analysis revealed quantifiable concentrations of DDTs, cyclodienes, and HCHs, and six of the 11 USEPA-priority PAHs. The concentration ranges of ΣOCPs were 0.1–100 ng/g dm (dry mass) and 8.5–96 ng/g lm (lipid mass) in sediments and worms, respectively. The concentrations of ΣPAHs6 ranged from non-detectable to 27 ng/g dm in sediments and non-detectable to 18 ng/g lm (lipid mass) in worms. Malindi harbour was the most contaminated of all sites with respect to the pollutants we measured in both sediments and worms. The relative compositions of the OCPs suggest legacy use of DDT and technical HCH, and more recent use of lindane. The relatively higher proportion of low molecular mass PAHs to high molecular mass PAHs suggests major contributions from petrogenic sources in the study area. The results demonstrated bioavailability and uptake of OCPs and PAHs to the food web via C. capitata. Concentrations of DDTs, HCHs, and dieldrin exceed sediment quality guidelines, indicating risk to sediment-dwelling organisms, as well as organisms that eat them. It is likely that other toxic compounds are present, suggesting that the current assessment may underestimate the risk. Due to the indicated risk, further work is needed to look at coastal distributions, terrestrial concentrations, and accumulation of pollutants into higher trophic level organisms of Zanzibar and associated seas.