Ecological risks of nutrient and metal contamination in Calabar River sediments, Nigeria

The Calabar River in southeastern Nigeria faces severe pollution from industrial discharges, agricultural runoff, and urban wastewater, threatening its ecological integrity and the livelihoods of dependent communities, yet comprehensive risk assessments integrating nutrient enrichment and metal contamination remain limited. This study examined water and sediment quality at 16 locations along a 10.78 km stretch of the river. Water and surface sediment samples were analyzed for key physicochemical parameters, nutrients, and metals. The Potential Ecological Risk Index (PERI) was used to assess the risk status of the river based on the metals analyzed, categorized as low to moderate, high, and extremely high. Water quality varied across sites, with dissolved oxygen ranging from 3.80 ± 0.10 to 6.50 ± 0.14 mg L⁻¹ and total nitrogen from 5.07 ± 0.03 to 9.17 ± 0.11 mg L⁻¹. Sediment analysis showed elevated levels of lead (27.5 ± 0.9 to 34.0 ± 1.8 mg kg⁻¹), zinc (85.3 ± 3.8 to 138.3 ± 6.7 mg kg⁻¹), and nickel (24.7 ± 0.6 to 94.5 ± 0.6 mg kg⁻¹). PERI results identified sites S9, S10, and S4 as having extremely high ecological risk, mainly due to lead, zinc, and molybdenum. Lead alone contributed up to 29% of the total risk at some locations. Sites S7 and S16 showed low to moderate risk. The findings reveal serious ecological threats from nutrient and metal pollution in the Calabar River, especially near industrial and agricultural zones. These results highlight the need for targeted pollution control and better environmental management to protect the river’s ecosystem and the communities that depend on it.