Integrated Assessment of Coastal Water Quality and Trophic Status Along the Aegean Coast of Türkiye

Increasing anthropogenic pressures and nutrient inputs threaten the ecological status of coastal ecosystems along the Aegean coast of Türkiye. This study evaluated coastal water quality and trophic conditions using physicochemical parameters, nutrient concentrations, chlorophyll-a, national eutrophication criteria, the Trophic Index (TRIX), and multivariate statistical analyses across 25 coastal stations monitored during 2022–2023. While basic physicochemical variables showed limited spatial variability, nutrient concentrations, chlorophyll-a, and TRIX values exhibited pronounced spatial differences among stations (p < 0.05), identifying Bostanlı (Inner İzmir Bay) as the principal eutrophication hotspot. Principal Component Analysis (PCA) revealed two dominant environmental gradients: a Nutrient Enrichment Gradient associated with total phosphorus (TP), ammonium, nitrite, and nitrate nitrogen (32.6% of total variance), and a Hydrographic–Thermal Gradient associated with salinity, conductivity, total dissolved solids, temperature, and dissolved oxygen (30.4%). Hierarchical and k-means clustering analyses further distinguished environmentally coherent coastal sectors and confirmed the clear separation of Bostanlı from the remaining sampling stations. The close agreement among nutrient distributions, chlorophyll-a concentrations, TRIX values, and multivariate analyses indicates that localized anthropogenic nutrient enrichment, rather than basin-scale hydrographic variability, drives trophic differentiation along the coast. These findings demonstrate the value of integrated monitoring approaches combining physicochemical, trophic, and multivariate indicators for identifying eutrophication hotspots and supporting ecosystem-based management in Mediterranean coastal ecosystems.