Nutrient Enrichment and Eutrophication Potential of Agricultural Runoff in Otuoke, Bayelsa State, Nigeria
Umueni, Uchenna E.
Department of Environmental Management and Toxicology, Dennis Osadebay University, Asaba, Nigeria.
Rasheed Habib O.
Department of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria.
Edodi Iyam O.
Department of Science Technology, University of Calabar, Nigeria.
Aningo Gloria N.
Department of Chemistry, Kogi State College of Education (Technical), Kabba, Nigeria.
Okpoji, Awajiiroijana U. *
Department of Pure and Industrial Chemistry, University of Port Harcourt, Choba, Nigeria.
Etesin Monday U.
Department of Chemistry, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Okonkwo Chidiebere C.
Department of Pure and Industrial Chemistry, Nnamdi Azikiwe University, Awka, Nigeria.
Ekwere Ifiok O.
Department of Chemistry, Akwa Ibom State University, Ikot Akpaden, Nigeria.
Okeke Chioma F.
Department of Chemistry, Alex Ekwueme Federal University, Ndufu-Alike, Nigeria.
Anarah Samuel E.
Department of Agricultural Economics and Extension, Nnamdi Azikiwe University, Awka, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
The present study evaluated the extent of nutrient enrichment and eutrophication potential of the Otuoke River in Bayelsa State as influenced by agricultural runoff. Nine surface water samples were collected from three monitoring stations located upstream, midstream, and downstream during the 2025 wet season and analysed according to the APHA (1995) standard method. Selected physicochemical parameters—pH, dissolved oxygen (DO), biological oxygen demand (BOD₅), nitrate, phosphate, ammonium, and chlorophyll-a—were determined using a Hanna HI98194 multi-parameter probe and Shimadzu UV-1800 spectrophotometer. Results showed a decrease in DO, which dropped from 6.8 mg/L at the upstream site to 4.1 mg/L at the downstream site, with a concurrent increase in BOD₅, rising from 2.8 to 5.4 mg/L, indicating organic enrichment occurred. Nutrient concentrations showed downstream increases in nitrate and phosphate, with nitrate ranging, and ammonium, indicators of runoff and fertiliser, increasing from 0.31 to 0.68 mg/L. Chlorophyll-a concentration ranged from 8.4 to 23.5 µg/L, while the Secchi depth decreased from 1.8 to 0.7 m. The Trophic State Index (TSI) recorded values ranging from 45.2 to 67.5, classifying the river from mesotrophic to eutrophic. The study revealed that strong positive correlations between nutrient concentration and chlorophyll-a (r ≥ 0.88) indicate nutrient loading was the primary factor controlling algal productivity and consequently reducing oxygen levels in the Otuoke River. The study concluded that agricultural practices are significant contributors to the trophic status of the river.
Keywords: Nutrient enrichment, eutrophication, agricultural runoff, Otuoke River, Bayelsa State