Seasonal rainfall affects occurrence of organohalogen contaminants in tropical marine fishes and prawns from Zanzibar, Tanzania

Ane Haarr,Eliezer B Mwakalapa,Aviti J Mmochi,Jan L Lyche,Anders Ruus,Halima Othman,Martin M Larsen,Katrine Borgå

doi:10.1016/j.scitotenv.2021.145652

Abstract

Seasonal differences in precipitation may affect contaminant dynamics in tropical coastal regions due to terrestrial runoff of contaminants to the marine environment after the rainy seasons. To assess the effect of seasonal rainfall on occurrence of organohalogen contaminants in a coastal ecosystem, marine fishes and prawns were collected off the coast of Zanzibar, Tanzania in January and August 2018, representing pre- and post-rainy season, respectively. Samples were analyzed for organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), brominated flame retardants (BFRs), including polybrominated diphenyl ethers (PBDEs) and emerging BFRs, as well as the dietary descriptors stable isotopes of carbon (δ13C) and nitrogen (δ15N). Across all species and seasons, mean contaminant concentrations ranged from below limit of detection (LOD) to 129 ng/g lipid weight (lw) ΣPCBs; 5.6–336 ng/g lw ΣOCPs; and < LOD –22.1 ng/g lw ΣPBDEs. Most of the emerging BFRs were below LOD. Contaminant concentrations generally increased with higher pelagic carbon signal (δ13C) and higher relative trophic position (δ15N). The ratio of DDE/ΣDDTs in fishes and prawns was lower in August than in January, suggesting runoff of non-degraded DDT into the marine system during or after the seasonal rainfall. Contaminant patterns of OCPs and PCBs, and concentrations of BFRs, differed between seasons in all species. A higher relative concentration-increase in lower halogenated, more mobile PCB and PBDE congeners, compared to higher halogenated congeners with lower mobility, between January and August aligns with a signal and effect of terrestrial runoff following the rainy season.

Highlights

Environmental fate, food web dynamics and spatial, and temporal variations of persistent organic pollutants (POPs) have been relatively understudied in tropical ecosystems (Kidd et al, 2001; Verhaert et al, 2013)
Samples were analyzed for organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), brominated flame retardants (BFRs), including polybrominated diphenyl ethers (PBDEs) and emerging BFRs, as well as the dietary descriptors stable isotopes of carbon (δ13C) and nitrogen (δ15N)
Muscle samples from 4 to 8 individuals per fish species and 3–4 pooled samples of approximately 100 g prawn muscle and whole herring were analyzed for organic contaminants, including polychlorinated biphenyls (PCBs), legacy and emerging brominated flame retardants (BFRs), organochlorine pesticides (OCPs), lipid content, and the dietary descriptors stable isotopes of carbon (δ13C) and nitrogen (δ15N)

Summary

Introduction

Environmental fate, food web dynamics and spatial, and temporal variations of persistent organic pollutants (POPs) have been relatively understudied in tropical ecosystems (Kidd et al, 2001; Verhaert et al, 2013). In Africa, local sources of pollution including discharge from landfills and waste incineration plants, sewage, wastewater, industry and agriculture play an increasingly important role in environmental pollution pressure (Gioia et al, 2014; Mmochi and Francis, 2003; UNEP/Nairobi Convention Secretariate, 2009). The Zanzibar islands of Tanzania are representative of a developing region with major identified pressures for the marine environment in the Western Indian Ocean region, including increasing anthropogenic activity and urbanization, overexploitation of fish stocks, destruction of sensitive ecosystems and habitats, and pollution (Johnstone et al, 1998; Mmochi and Francis, 2003; UNEP et al, 1998). Commercial fisheries, small-scale artisanal fishing and an increasing aquaculture industry are important for the economy and food security in Tanzania (Mwakalapa et al, 2018; Wetengere et al, 2008). The occurrence of organochlorine pesticides in the Tanzanian environment has been more extensively documented (Elibariki and Maguta, 2017; Kishimba et al, 2004; Lema et al, 2014; Machiwa, 2010; Mahugija et al, 2017, 2018; Mmochi and Mberek, 1998; Mwakalapa et al, 2018; Mwevura et al, 2020; Müller et al, 2017; Nonga et al, 2011; Polder et al, 2016; Polder et al, 2014), and is still considered a significant pollution issue due to indoor residual spraying, unregulated use and large pesticide stockpiles still residing in the country

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