Heavy Metal-induced Oxidative Stress and Antioxidant Defence Responses in Aquatic Macrophytes of the Qua Iboe River Estuary

Heavy metal contamination of freshwater ecosystems is a major environmental concern because metallic pollutants are persistent, toxic, bioaccumulative and capable of inducing oxidative stress in aquatic organisms. Aquatic macrophytes exposed to metal pollution may experience excessive reactive oxygen species (ROS) production, lipid peroxidation, membrane instability and metabolic disturbance. However, information on oxidative stress biomarkers and antioxidant enzyme responses in indigenous African aquatic macrophytes remains limited. This study investigated heavy metal accumulation, oxidative stress biomarkers and antioxidant enzyme responses in selected aquatic macrophytes from the Qua Iboe River Estuary, Niger Delta, Nigeria. Samples of Vossia cuspidata, Crinum natans and Nymphaea lotus were collected from Onna, Atabong and Mariner stations. Concentrations of copper (Cu), cadmium (Cd), zinc (Zn), lead (Pb), nickel (Ni) and aluminium (Al) were determined using flame atomic absorption spectrophotometry following acid digestion. Reactive oxygen species, malondialdehyde (MDA), superoxide dismutase (SOD) and catalase (CAT) were quantified using standard biochemical methods. Analyses were performed in triplicate, and results were expressed as mean ± standard deviation and tested using one-way ANOVA with Tukey's post hoc comparisons at p < 0.05. Significant spatial and species-dependent differences were observed. Copper recorded the highest concentration, whereas aluminium recorded the lowest concentration. Crinum natans from Onna accumulated the highest Cu concentration (724.85 ± 36.24 mg/g), while Vossia cuspidata from Onna recorded the lowest Cu concentration (65.65 ± 3.28 mg/g). Cadmium ranged from 36.97 ± 1.85 to 70.51 ± 3.53 mg/g. ROS ranged from 4.24 ± 0.17 to 5.34 ± 0.21 IU/L, and MDA ranged from 5.64 ± 0.23 to 6.44 ± 0.26 mg/dl. Elevated oxidative stress biomarkers were associated with increased antioxidant enzyme activities, indicating activation of defence responses in the studied macrophytes.