Annual Research & Review in Biology

Lower Lake, Bhopal, is an urban freshwater system subject to marked spatial variation in anthropogenic influence. The present research offers an exhaustive ecological evaluation of Lower Lake, Bhopal, a critical urban freshwater ecosystem, by delineating the complex interactions between planktonic diversity and spatial physicochemical gradients. Conducted over a six-month period from September 2025 to February 2026, the investigation captures the crucial post-monsoon to mid-winter transition. Monthly sampling across three distinct ecological zones—an open water zone (Station I), a littoral zone (Station II), and a heavily polluted sewage influx zone (Station III)—revealed severe spatial heterogeneity driven by anthropogenic nutrient loading. Qualitative taxonomic assessments identified 46 genera, highlighting a shift towards pollution-tolerant cyanobacteria such as Microcystis sp. and Oscillatoria sp. in the degraded zones. Quantitative biodiversity indices indicated a productive but stressed ecosystem. Phytoplankton diversity peaked in the post-monsoon month of October (Shannon-Wiener Index H' = 2.8988 at Station II), while zooplankton diversity exhibited a classic one-month ecological lag, reaching maximum diversity in November (H' = 2.7571). Concurrently, physicochemical analyses exposed critical hypoxic and hypertrophic conditions at Station III, where Dissolved Oxygen plummeted to 4.12 mg/L and Biochemical Oxygen Demand (BOD-5) peaked at 8.65 mg/L, exceeding permissible thresholds. The sustained elevations of Ammonia-Nitrogen (0.685 mg/L) and Phosphate Phosphorus (0.385 mg/L) underscore severe cultural eutrophication. Furthermore, the integration of historical and parallel environmental data exposes overlapping crises, including declines in macrozoobenthic diversity, the emergence of microplastic contamination, and physical habitat fragmentation due to illicit encroachments. This assessment confirms that, while Lower Lake maintains a robust biological buffering capacity in its open waters, localised municipal effluent discharges threaten systemic ecological failure, necessitating immediate remedial intervention, decentralised wastewater management, and strict regulatory enforcement.