Water Quality Assessment and Trophic State Evaluation of Foy’s Lake, Bangladesh

In recent years, human activities have significantly altered Foy's Lake, shifting it from a clear, pristine state to a turbid, eutrophic condition. Thus, this study analyzed seasonal variations of physicochemical parameters, chlorophyll-a levels, and the trophic state index (TSI) to assess water quality and trophic state fluctuations in Foy’s Lake, Chittagong. Sampling was conducted in September 2023 (as wet season) and January 2024 (as dry season) to assess the key water quality parameters including transparency, temperature, pH, dissolved oxygen (DO), electrical conductivity (EC), total dissolved solids (TDS), total hardness (TH), total alkalinity (TA), and chlorophyll-a (Chl-a). Secchi depth (SD) and Chl-a concentration were used to determine trophic conditions based on the Carlson Trophic State Index (CTSI). Results showed that in wet season, temperature, pH, DO, EC, TDS, transparency, TA, TH, and Chl-a ranged from 30.25–30.9°C, 5.4–5.5, 7.21–7.55 mg/L, 123.6–136.3 µS/cm, 51.5–78.48 mg/L, 109.06–126.94 cm, 49.6–68.4 mg/L, 36.09–41.75 mg/L, and 0.824–1.072 µg/L, respectively. In dry season, these values ranged from 28.45–29.02°C, 5.11–5.37, 6.36–6.92 mg/L, 143.7–156.3 µS/cm, 43.24–83.12 mg/L, 183.82–218.98 cm, 95.95–130.71 mg/L, 39.03–43.35 mg/L, and 0.885–0.933 µg/L, respectively. The mean TSI values based on SD and Chl-a were 59.58–48.54 and 31.23–28.75, respectively. Overall, most parameters decreased in the dry season except for transparency, EC, total alkalinity, and total hardness. While temperature, EC, DO, TH, TA, and Chl-a remained within standard limits, transparency exceeded the standard for aquaculture in both seasons. TSI values classified the lake as oligotrophic (Chl-a) and eutrophic (SD) in both seasons. Pearson’s correlation analysis showed strong positive relationships among most parameters. To preserve the ecological integrity and aesthetic value of Foy’s Lake, the study recommends the necessity to develop guidelines for the trophic status monitoring of Foy’s Lake since results suggest the deteriorating lake condition.