Multivariate assessment of outflow water quality in highly urbanized creek system: implication of natural and anthropogenic processes

Nutrient source and transport study in tropical creeks adjacent to megacities are sparse on a regional and global scale. High-frequency chemical data collected during 2017–2018 in the Thane creek system, the largest micro-tidal urbanized creek in Asia, were analysed to assess the chemical water quality, with its linkage to different hydrological stages (southwest monsoon, post- and pre-monsoon) and ongoing anthropogenic activities. Cluster analysis indicates typical zonation between creek outflow and nearshore waters with distinctive physicochemical properties. The creek outflow upholds substantial amounts of nutrient and suspended sediment due to turbid water movement from the ephemeral mudflats and anthropogenic dredging. The year-round hyper-turbid condition in the bottom water triggers the addition of disproportionate nutrients (9% dissolved inorganic nitrogen (DIN) and 14% reactive phosphorous (PO₄³⁻) in the outflow region. The DIN and PO₄³⁻ removal up to 10 and 35%, respectively, occurs in the nearshore region; sedimentation, which acts as a sinking interface for nitrogen and phosphorous, also causes shifting in their limiting conditions. The hyper-turbid condition causes removal of dissolved silicate (DSi) by 5% in the entire region. Ammonium (NH₄⁺) is mainly sourced from the sewage in outflow waters and efficiently mineralized. Chemical indexing of water highlights that the bottom water column remained un-supportive to the balanced ecology. The findings of this study have implications for other tropical creek-estuary concerning management strategies against inadequate flushing. The stalled nutrient export affected balance ecology in tropical Thane creek, which need attention in order to accurately understand the impact on adjacent marine environment and to form effective mitigation policies.