Spatiotemporal Variations of Temperature, Oxygen, and Salinity in a Typical Constructed Wetland for Source Water Protection in a Plain River Network

Pond-wetland-reservoir complexes serve as critical buffers in the pretreatment of micro-polluted source water, yet their spatiotemporal hydrochemical dynamics-and the ecological implications thereof-remain poorly quantified across functional units. This study characterizes the seasonal and diurnal variability of water temperature (WT), dissolved oxygen (DO), and electrical conductivity (EC) in a multi-stage source-water constructed wetland (Yanlong Lake, Yancheng, eastern China) by integrating seven published studies (2013-2019) with two high-density in situ spring monitoring campaigns (116 sites, 2023). Results suggest that high-temperature seasons amplify inter-unit thermal heterogeneity (maximum ΔWT up to 1.5°C), whereas the emergent vegetation zone appears to attenuate both seasonal and diurnal temperature fluctuations-exhibiting seasonal amplitude (DifMaxMin) below ~21°C, suggesting an indicative thermal-buffering threshold in this system. The wetland maintained a net oxygenation effect throughout the year, elevating yearly mean DO concentration and saturation by ~74% relative to source water, with the strongest enrichment occurring during winter and spring. Electrical conductivity showed a progressive upstream-to-downstream increase during spring, with littoral margins of the submerged and deep-water zones emerging as ionic accumulation hot spots. On the diurnal scale, the distributary channels of the emergent zone remained near-isothermal (~20°C-21°C) and relatively resistant to atmospheric forcing, whereas littoral zones behaved as temperature-sensitive hot spots with pronounced DO supersaturation and elevated EC. We introduce the concepts of "thermal-buffer zones" and "temperature-sensitive zones" to characterize wetland thermal heterogeneity and discuss the implications of the ~21°C threshold for biogeochemical processes and algal ecology. These findings advance understanding of eco-hydrochemical coupling in constructed wetlands and may inform climate-adaptive wetland design and source-water quality management.