Assessment of pollution status using Water Quality Index (WQI) and hydrochemical ındicators in the Gemlik Gulf, Marmara Sea, Türkiye: a spatial and temporal perspective

Coastal regions, including the Gemlik Gulf in Turkey, face increasing threats from environmental pollution, driven by industrial activities, urban expansion, and wastewater discharges. Despite its ecological and economic significance, limited studies have comprehensively assessed the spatiotemporal variations in seawater quality in this region. This study aims to fill this gap by evaluating the key pollution sources affecting seawater quality in the Gemlik Gulf, particularly following the 2021 Marmara Sea mucilage crisis. The research investigates the distribution of various seawater quality parameters in the Gemlik Gulf (GG), including pH, temperature, density, fluorescence, NH₄-N, NOx (NO₂ + NO₃), total N (TN), total P (TP), salinity, chlorophyll-a (Chl-a), and dissolved oxygen (DO). Samples were collected from 31 stations between January 2022 and November 2022, covering the Spring, Summer, Autumn, and Winter seasons. During the investigation, the temperature, salinity, and dissolved oxygen levels of the seawater varied from 10.3-27.3 °C, 23-38.54 psu, and 2.63-10.57 mg L^-1, respectively. The amounts of nitrate + nitrite-N (0.01-3.356 µM), TN (125.72-271.1 µg L^-1), and TP (4.62-28.88 µg L^-1) were measured. Chl-a levels ranged from 0.259 to 6.44 µg L^-1. Surface water quality can be investigated and classified based on contamination levels using the Water Quality Index (WQI) and trophic status. The WQI classed seawater samples as medium water quality. However, the WQI was specifically calculated for surface waters discharging into the sea. The results indicated that Engürücük River (WQI = 78) and Susurluk River (WQI = 77) were classified as "very poor," while Nilüfer River (WQI = 50) and Karsak River (WQI = 57) were categorized as "poor." This situation indicates that the rivers carry significant pollution to the Sea of Marmara. According to the Surface Water Quality Management Regulation, the Gemlik Gulf was classified as hypertrophic-eutrophic in terms of TP and NOx, between January and April varied by location, but was predominantly determined as oligotrophic-mesotrophic. Based on Chl a and trophic index, it was determined that the gulf generally had low water quality-high mesotrophic and poor water quality-eutrophic conditions during winter.