Electroanalytical Investigation of the Speciation and Redox Cycling of Iron in Tropical Aquatic Systems of South East Nigeria: Implications for Phosphorus Availability and Carbon Cycling

Iron speciation and its redox dynamics profoundly govern the biogeochemical cycling of phosphorus and carbon in aquatic systems, yet in-situ electroanalytical characterisation of iron speciation in tropical Nigerian water bodies remains a significant gap in the literature. This dissertation presents an original electroanalytical investigation of iron speciation, redox cycling rates, and coupling with phosphorus and dissolved organic carbon dynamics in three contrasting aquatic systems of South East Nigeria, including a pristine forest stream in Cross River State, an agricultural impoundment in Anambra State, and a eutrophic urban lake in Enugu State. Voltammetric microelectrode sensors were deployed in-situ for continuous measurement of dissolved ferrous and ferric iron across redox gradients in the water column and sediment-water interface. Electrodeposition methods were applied for labile colloidal iron quantification. Iron oxidation and reduction rate constants were determined from kinetic experiments under controlled pH and dissolved oxygen conditions. Phosphorus fractionation in sediment pore waters was correlated with iron speciation dynamics across seasonal cycles. Dissolved organic matter characterisation by excitation-emission fluorescence spectroscopy and size-exclusion chromatography assessed ligand controls on iron speciation. Results reveal that iron redox cycling at the sediment-water interface in the eutrophic lake drives seasonal phosphorus release fluxes of 4.2 milligrams per square metre per day during thermal stratification. Humic-bound iron represented 35 to 55 percent of total dissolved iron in the forest stream, reducing iron(III) reduction kinetics. Original rate parameterisations for tropical aquatic iron chemistry, distinct from temperate literature values, are derived and proposed for integration into biogeochemical models. Keywords: iron speciation, electroanalytical chemistry, redox cycling, phosphorus, South East Nigeria