Consequences of winter salinization in urban freshwater reservoirs – its role in microbial diversity decline and water quality deterioration

Despite increasing winter road-salt application, its effects on microbial functioning in urban freshwater sediments are not fully understood. This study analysed seasonal changes in microbial functional diversity in four urban ponds exposed to different chloride regimes. Microbial metabolic capacity was assessed using Biolog® EcoPlates™, based on average well color development (AWCD), substrate richness and Shannon diversity. All indices exhibited strong seasonal variation, with significantly lower functional diversity in winter, coinciding with peak chloride concentrations. In pond S3, characterised by prolonged salinity exposure, microbial functioning remained suppressed into spring, indicating persistent effects of chronic stress. Redundancy analysis identified chloride concentration as the main driver of functional changes, independent of organic carbon content (Corg), Corg/nitrogen (N) ratio and nutrient availability. Organic carbon enhanced microbial activity, whereas high Corg/N ratios reduced substrate utilisation. These findings demonstrate that winter salinization impairs microbial metabolic potential, with chronic chloride exposure posing the greatest risk to ecosystem functioning.