Widespread dissolved oxygen supersaturation in Chinese warming lakes

Dissolved oxygen (DO) supersaturation long considered an ecological curiosity, is now emerging as a widespread and systematic phenomenon in lakes. Through long-term time series analysis, we find an average DO saturation (DO_sat%) of 101.6%, with 45% of observations exceeding 100%. DO supersaturation (DO_sup) events predominantly occur from June to September, and diurnal fluctuations follow a "roller-coaster" pattern, peaking between 12:00-16:00 and plummeting at dawn. Non-parametric statistical tests and GAMM analysis reveal the dual influence of temperature on DO_sat%. While increased temperature reduces DO by lowering oxygen solubility, it simultaneously boosts DO through enhanced photosynthesis by aquatic primary producers, leading to DO_sup, which constitutes the primary biological mechanism. The pH acts as a direct metabolic signal for biologically driven oxygen production, while algal biomass-supported by nutrients-promote DO_sup under favorable conditions. DO_sup is characterized by strong seasonal and diurnal oscillations, with daytime DO_sup alternating with nocturnal oxygen depletion, indicating that surface DO_sup may coexist with hypoxic conditions or oxygen-depleted waters in deeper water layers rather than reflect whole-lake oxygen improvement. These findings reveal the complexity of DO_sup and enhance our understanding of DO dynamics, with the aim of providing crucial support for the development of science-based lake management and conservation strategies.