Biogeochemical characterization, phosphorus sources and intrinsic drivers to its speciation within the Nyanza Gulf of Lake Victoria

Abstract

Lake Victoria, like many African great lakes, was formed through tectonic activity that formed East African Rift Valley. The lake's ecology has undergone dramatic reorganization dating back to the 1920s, before agricultural mechanization and high urban populations were observed, transforming from it from a desirable to less desirable state. The present study was conducted to better understand Nyanza Gulf's biogeochemical characteristics, phosphorus sources and the driving forces to their speciation. Five littoral‐limnetic‐littoral transects were sampled for water‐associated phosphorus (soluble reactive phosphorus, SRP; total phosphorus, TPW); sediment‐associated phosphorus (non‐apatite inorganic phosphorus. NAIP); apatite phosphorus, AP; inorganic phosphorus, IP; organic phosphorus, OP; total phosphorus, TPS); elemental compositions (calcium, Ca; iron, Fe; aluminium, Al; manganese, Mn); organic matter (OM); and organic carbon (OC) contents in the sediment. The SRP and TPW concentrations ranged from 22.9 to 142.9 μg/L, and 57.1 to 277.1 μg/L, respectively. The littoral sampling sites exhibited relatively higher TPW concentrations than the limnetic sites. Sheltered bays and sites located off sewerage discharge points had higher NAIP concentrations exceeding 400 mg/kg, compared with strong current areas. The AP and TPS concentrations ranged from 136.7 to 1,511.3 mg/kg, and 512.5 to 2,254.4 mg/kg, respectively. The AP and TPS concentrations were generally higher (>500 mg/kg) within the littoral zones compared to the limnetic zones; with the littoral sites located close to documented carbonatite rock substrates manifesting exceptionally high concentrations. Nyanza Gulf's eutrophic/hypertrophic status is derived highly from shoreline erosions of phosphorus‐enriched carbonatite rocks associated with rift valleys and from municipal sewerage discharges. Re‐afforestation, fringing wetland restorations and tertiary treatment of municipal waste waters are vital for its ecological restoration.