Bioconcentration And Biomagnification of Organochlorine Pesticides by Aquatic Macroinvertebrates and Fish in Estuarine Ecosystems, South Coast, Kenya

Abstract

The persistence of organochlorine pesticides (OCPs) in estuarine ecosystems remains a major ecological and public health concern, particularly in tropical regions with intense agricultural activity and weak regulatory enforcement. This study investigated the bioconcentration and biomagnification dynamics of legacy OCPs in estuarine systems along Kenya’s South Coast. Sampling was conducted across multiple stations varying in land use and hydrology, and analyses were performed on water, sediments, and biota including benthic macroinvertebrate trophic guilds and fish (Penaeus monodon) to assess trophic transfer and ecological risk. Sixteen OCPs were detected across matrices, with significantly higher concentrations in biota than in sediments or water. P. monodon consistently exhibited the highest burdens (mean > 200 ng/g), approximately four times higher than in macroinvertebrates: Saccostrea cucullata and Nerita undata. Compounds such as alpha-HCH, o,p′-DDD, Mirex, and p,p′-DDE displayed high biomagnification factors (2.8–3.1), indicative of strong lipophilicity and persistence. Site-specific differences revealed elevated OCP levels in estuaries, suggesting ongoing anthropogenic inputs. Principal Component Analysis (PCA) showed that water parameters such as salinity, conductivity, and total dissolved solids (TDS) were negatively correlated with OCPs, while pH and dissolved oxygen were positively associated with p,p′-DDE and Mirex. These relationships highlight how physicochemical properties influence pesticide transport and bioavailability. Bioconcentration Factor (BCF) analysis confirmed higher accumulation in P. monodon, with moderate BCFs in N. undata and S. cucullata, likely due to sediment-particle interactions. Rhagovelia species acted as intermediate vectors, facilitating contaminant trophic transfer up the food web. Sediment-based ecotoxicological indices further confirmed substantial risk. All OCPs recorded Hazard Quotients (HQs) > 1.0, and compounds like HCB, Heptachlor, and p,p′-DDE exceeded HQ = 1.75, indicating high probability of toxic effects. The Contamination Factor (Cf) and Geo-accumulation Index (Igeo) classified most sediments as extremely polluted, particularly with p,p′-DDD, Mirex, and Cis-chlordane. The Pollution Load Index (PLI) was 2.57, while the Nemerow Pollution Index (PN) reached 420.03; both indicative of widespread non-point and point pollution from sewerage facilities. Hakanson’s Potential Ecological Risk Index (RI) was exceptionally high (31,352.42), far above the critical threshold (RI > 600), with gamma-HCH and p,p′-DDD posing the greatest threat. The Mean Effect Range- Median Quotient (ERM-Q) of 0.119 suggested a 21% probability of adverse effects on benthic fauna. Notably, sediment-bound pesticide levels consistently exceeded ecotoxicological thresholds, reinforcing their potential to drive chronic toxicity in estuarine habitats and disrupt benthic food webs. These results show that legacy OCPs to pose serious ecological threats despite regulatory bans. Elevated residues in P. monodon raise concerns for food safety among coastal human populations reliant on estuarine fisheries and international communities. The study recommends strengthened environmental monitoring, enforcement of pesticide bans, public education, and integrated pesticide and watershed management. Future research should focus on trophic markers, broader contaminant profiling, and ecological restoration to support remediation efforts.