Sub-diurnal (daytime) and vertical profile variations of the physicochemical characteristics of Lake Rukwa: Implications for ecological state

• The results revealed that Lake Rukwa has low clarity, affecting light attenuation. • Chlorophyll concentrations decreased with an increase in water depth. • Daytime and water depth variations influenced the variations of pH and DO. • The CTSI assessment reveals that Lake Rukwa is experiencing hyper-eutrophication. This study investigates the daytime and vertical variations in the physicochemical properties of Lake Rukwa and their implications for the lake’s ecological status. In this study, field measurements included pH, temperature, dissolved oxygen (DO), oxygen saturation (O2 (%)), electrical conductivity (EC), salinity, total dissolved solids (TDS), and turbidity. Additionally, TP and Chlorophyll-a (Chl-a) concentrations were analyzed. The trophic status of the lake was evaluated using the Carlson Trophic State Index (CTSI). The study revealed that the lake had low water clarity (0.42 ft), indicating high suspended particle loads that limit light penetration, reduce photosynthesis, and lower DO concentrations. Chl-a significantly declined sharply with depth (P < 0.05), reflecting higher surface photosynthetic activity. Both pH and DO exhibit notable variations with depth and time of day (P < 0.05), while O2 (%) also showed a strong vertical variation. CTSI values ranged from 82.49 to 89.9, classifying the lake as hypereutrophic and highlighting severe ecological stress. These conditions are likely driven by nutrient enrichment from agricultural runoff and land-use change within the lake basin. To mitigate further degradation, management interventions, continuous monitoring, and stricter regulation of anthropogenic inputs are urgently required.