Seasonal Hydrochemical Variability and Mangrove Water Quality Assessment in Vembanad Lake, Southwest India

Mangrove ecosystems are strongly influenced by estuarine hydrochemical dynamics, and their resilience depends on maintaining ionic balance and oxygen availability. This study investigates seasonal water-quality variability in the mangrove-fringed sectors of Vembanad Lake, a monsoon-regulated tropical estuary on the southwest coast of India. Ten hydrochemical variables were monitored across monsoon, pre-monsoon, and dry seasons in 2023. Salinity showed pronounced seasonality, lowest during monsoon (6.30 ± 1.87 PSU) and highest in the dry season (13.24 ± 9.71 PSU), while dissolved oxygen (DO) followed the opposite pattern, indicating saline intrusion and hypoxia outside monsoon. Nitrate and phosphate reflected flushing during monsoon and stagnation-driven accumulation during pre-monsoon. Principal Component Analysis revealed a dominant ionic stress gradient (37.6% variance), with negative association to DO, and a nutrient–thermal gradient (21.2% variance). A mangrove-specific water quality index (MWQI) indicated Good–Moderate conditions during monsoon (0.07–0.32) but Moderate–Poor quality in pre-monsoon and dry seasons (0.14–0.60). No season achieved “Good” status throughout, suggesting persistent ecological stress linked to reduced flushing, saline mixing, and anthropogenic inputs. Collectively, the results demonstrate monsoon-driven freshwater inflow as a critical recovery mechanism supporting estuarine–mangrove integrity, emphasizing the need for hydrological restoration and nutrient load control to enhance resilience under intensified salinity intrusion and climate variability.