Hydroclimatic controls on nearshore pollution and ecosystem stress along a tropical urban coastline

Urbanizing coasts face escalating pollution from anthropogenic sources and climate change. This study provides an integrated seasonal assessment of coastal water quality along the Visakhapatnam coast, India, using a multi-index framework combining the Canadian Council of Ministers of the Environment Water Quality Index (CCME-WQI), Trophic State Index (TSI), and Pollution Index (PI). Surface water samples were collected from eleven representative locations including fishing settlements, recreational beaches, leisure beaches, and industrial-port zones. Physicochemical factors, nutrients, chlorophyll-a, and microbial indicators were assessed using multivariate statistical analysis to determine significant environmental stressors. Results revealed clear spatial and seasonal differences driven by monsoon runoff interacting with localized urban pollution sources. Jalaripeta exhibited the lowest water quality, with low WQI scores (<40), eutrophic to hypereutrophic status (TSI >56), and elevated microbial contamination associated with untreated sewage and inadequate flushing. In comparison, RK Beach and the Harbour area showed moderate degradation, characterized by intermittent nutrient influx, spikes in microbes, localized hypoxia associated with tourism pressure and port activities. Principal Component Analysis also revealed that nutrient-microbe enrichment, thermal-biological productivity, and organic matter breakdown are major environmental gradients influencing water quality variability. The findings indicate that the interplay between urbanization and seasonal hydroclimate intensifies environmental stress along the Visakhapatnam coast. This integrated multi-index framework provides as a valuable decision-support tool for identification pollution hotspot and adaptive management of tropical urban coasts under increasing climate and anthropogenic pressures.