The Thane Creek-Ulhas estuary confluence region acts as a naturally active infiltration system, crucial for altering water quality in the area and Arabian Sea. Particle-water exchange, hydrodynamics, and anthropogenic discharge influence nutrient transfer and transformation, highlighting the need for effective water quality management in this urbanized ecosystem. We analyzed monthly hydrography and nutrient data in water, and particulates from April 2021 to March 2022, including period of Cyclone Gulab at four locations along the inflow and outflow sectors. Our results revealed prevalent hypoxic conditions in the confluence waters, driven by microbial oxygen demand that surpassed the chemical load due to indiscriminate sewage inflow. Phosphorus emerged as the limiting nutrient, inhibited by adsorption/desorption equilibrium. Dissolved phosphate was transformed into particulate form under intermediate suspended load, with further transform into organic phosphorous during non-monsoon season. Excessive ammonium from sewage during the cyclone and elevated urea during non-monsoon indicated regenerated nitrogen forms, rendering the region eutrophic and hypereutrophic, with intermittent organic pollution. Engineering interventions may help mitigate water quality alterations that pose significant ecological risks in this mangrove-dominant confluence ecosystem.
