Trophic status analysis and nutrient source allocation in urban lakes of Dhaka, Bangladesh: a comprehensive approach to eutrophication monitoring.

Urban lakes are vital to ecosystems, providing essential services and recreational spaces in densely populated megacities. However, rapid urbanization and anthropogenic activities, particularly eutrophication driven by macronutrient accumulation, severely threaten these water bodies. This study underscores the critical need for continuous trophic state monitoring to sustain fish, wildlife, and plant ecosystems. The trophic status of Dhanmondi, Gulshan, and Banani Lakes in Dhaka City, Bangladesh, was assessed using Carlson's Trophic State Index (CTSI) and Burn's Trophic Level Index (BTLI), based on chlorophyll a (Chl-a), total phosphorus (TP), Secchi disc depth (SD), and total nitrogen (TN). Water samples from five sites per lake were analyzed for physicochemical parameters from June 2023 to May 2024, revealing monthly and seasonal variations. The study revealed that Dhanmondi Lake's CTSI ranged from 69.3 to 79.5 (June 2023 to March 2024), indicating initial "Eutrophic" conditions progressing to "Hypereutrophic." Gulshan Lake consistently showed "Hypereutrophic" conditions, with CTSI values between 84.1 and 97.3. Banani Lake was "Eutrophic" in June and July 2023, transitioning to "Hypereutrophic" from August 2023 to May 2024 (84.1-97.7). The Trophic Level Index (TLI) showed the "Hypereutrophic" status with a progressive monthly escalation for all the lakes. The Water Quality Index (WQI) categorized the lakes as "Poor" to "Very Poor" from June to August 2023, becoming "Unsuitable" from September 2023 to May 2024, indicating significant anthropogenic stress. Principal component analysis (PCA) identified nutrient infiltration, soil erosion, waste discharge, and organic residue accumulation as key pollution drivers. The study advocates for a multi-sectoral strategy to regulate nutrient loading and mitigate eutrophication, emphasizing best management practices for urban lake conservation.