TROPHIC STATE ASSESSMENT OF TROPICAL LOTIC ECOSYSTEMS IN BENIN CITY, NIGERIA
TROPHIC STATE ASSESSMENT OF TROPICAL LOTIC ECOSYSTEMS IN BENIN CITY, NIGERIA

Summary/Abstract: Freshwater ecosystems provide essential ecological, economic, and social services, yet urbanization and agricultural intensification threaten their integrity through nutrient enrichment and eutrophication. In tropical rivers, phosphorus-driven productivity changes can alter biodiversity, water clarity, and oxygen dynamics. Despite widespread nutrient impacts in West African waters, standardized trophic assessments using indices such as the trophic state index (TSI) remain scarce, limiting effective water quality management. Objectives: This study systematically evaluates trophic states in urban water bodies of Benin City, Nigeria, using TSI integrated with physicochemical measurements. Objectives include quantifying eutrophication risk, identifying nutrient drivers, assessing spatial variability, and linking trophic states to anthropogenic pressures. Methods: Eighteen lotic water bodies across urban, peri-urban, and relatively undisturbed sites in Benin City, Nigeria, were sampled during the wet season (May–September 2024). Triplicate surface water samples (20–30 cm depth) were collected mid-channel, preserved on ice, and analysed for physicochemical parameters including pH, temperature, dissolved oxygen, conductivity, turbidity, total phosphorus, nitrate, and other nutrients following APHA (2017) protocols. The trophic state index was calculated based on total phosphorus to classify water bodies. Statistical analyses, including ANOVA, Tukey's HSD, effect size (η²), and Pearson correlation, were conducted in Python 3.10 to assess spatial variability, nutrient drivers, and relationships between TSI and environmental variables. Results: River water quality varied across the 18 studied sites. Water temperatures were relatively stable (25–30°C), while turbidity and colour showed notable variation, with the Ogba River exhibiting the highest values (141 NTU; 135 PtCo units), indicating elevated suspended solids. pH ranged from 4.5 to 6.8, while conductivity, total hardness, and total dissolved solids were low, reflecting soft freshwater conditions. Dissolved oxygen was generally high (mean 8.52 mg L⁻¹), although some rivers displayed low DO (2.7–3.0 mg L⁻¹), suggesting localized oxygen stress. Nutrient concentrations were low, with phosphate (8–13 µg L⁻¹) and nitrate (<0.5 mg L⁻¹). The trophic state index ranged from 34.65 to 41.14, classifying most rivers (94.4%) as oligotrophic, while the Ogba River reached mesotrophic conditions. Spatial differences in TSI were significant, with phosphorus, conductivity, turbidity, and colour identified as the main drivers of trophic variability. Conclusion: Rivers in the study area are largely oligotrophic, chemically soft, and sensitive to anthropogenic inputs. The Ogba River's mesotrophic shift highlights vulnerability from sediment and nutrient loading. Conductivity, sulphate, and turbidity are effective proxies for early trophic state monitoring.

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