Towards sustainable urban estuary health: Modeling microbial pollution sources and mitigation strategies in the Golden Horn.

In support of the United Nations Ocean Decade goals for sustainable pollution reduction and improved ocean health, this study develops a science-based prognostic framework to assess and manage microbial contamination in urban estuarine systems. Taking the heavily urbanized Golden Horn (Istanbul) as a primary study site, a coupled two-dimensional (2D) hydrodynamic-fecal indicator bacteria (FIB) model was developed and rigorously validated using field observations from contrasting dry (December 2022) and wet (April 2023) conditions. Scenario analyses revealed that a theoretical best-case of eliminating FIB inputs from the Alibey and Kağıthane Creeks and a localized shoreline point source produces near-complete reductions in microbial concentrations, with this idealized zero-load scenario suggesting that 'excellent' bathing water quality standards could theoretically be achieved within approximately 15 days. Conversely, increasing seawater inflow-a strategy often proposed to enhance circulation-did not improve microbial conditions and, under wet-weather scenarios, resulted in further contamination. These findings suggest that source control serves as a more fundamental requirement than hydrodynamic manipulation for restoring microbial water quality in semi-enclosed urban estuaries. By integrating transferable modeling methods with actionable management guidance, this study contributes to global efforts towards sustainable estuarine health and pollution mitigation.