Bioaccumulation of Heavy Metals in Fish and Invertebrates of the Imo River, South East Nigeria: Trophic Transfer and Human Health Risk

This study quantifies heavy metal bioaccumulation in fish and macroinvertebrates of the Imo River in Imo and Abia States, South East Nigeria, and assesses trophic transfer efficiency and human health risk through fish consumption. The Imo River receives effluents from the Aba industrial corridor and agricultural runoff, and while water quality has been assessed intermittently, a trophic transfer analysis of heavy metals through the aquatic food web is absent from the literature. Fish (four species representing different trophic levels), benthic macroinvertebrates, and sediment samples were collected from five stations over two seasons. Lead, cadmium, arsenic, mercury, and zinc concentrations were measured in tissues by ICP-MS. Bioaccumulation factors and trophic magnification factors were calculated using stable nitrogen isotopes to establish trophic positions. Estimated daily intake and carcinogenic and non-carcinogenic risk indices were calculated for an adult consumer consuming locally typical fish quantities. Cadmium showed significant trophic magnification (TMF = 2.8) across trophic levels. Lead bioaccumulation factors in sediment-dwelling invertebrates exceeded 600 at downstream industrial effluent stations. Non-carcinogenic hazard quotients for arsenic exceeded 1.0 at two stations for regular adult fish consumers, indicating health risks. Mercury concentrations in piscivorous Hemichromis species at the most downstream station exceeded WHO fish consumption guidelines. The study provides a risk assessment framework applicable to Imo River fisheries management and recommends health advisories for frequent consumers of fish from the downstream industrial corridor. Keywords: heavy metal bioaccumulation, trophic transfer, Imo River, human health risk, fish contamination.