Oxygen Dynamics in a Complex Climate Change: Investigating Thermocline and Hypoxia in Lake Długie Wigierskie, Poland

Complex climate change exacerbates variability in bottom oxygen availability, posing serious threats to aquatic ecosystems. This study investigates the interrelationships among meteorological, thermocline, oxycline variables, and the Kjeldahl nitrogen ratio in Lake Długie Wigierskie, Poland, using long-term monitoring data (2008–2022). Results show a decline in surface and bottom %saturation, but an increase in dissolved oxygen (DO) concentrations. Air temperature and Secchi depth primarily influenced surface oxygen dynamics, while wind speed drove bottom oxygen variability. Thermocline depth and thickness positively correlated with oxycline depth and hypoxic thickness, revealing that stable stratification restricts vertical mixing and shapes oxygen distribution. Air temperature significantly affected Schmidt Stability (SS), with warmer periods promoting stronger stratification, greater hypoxic thickness, and lower hypolimnetic oxygen minimum (HOM). Interestingly, DO levels and their variability showed significant correlation with the Kjeldahl nitrogen ratio (TKN/TN), suggesting that oxygen fluctuations may influence nitrogen cycling more strongly than average DO concentrations. These findings imply that warming may worsen bottom hypoxia by elevating respiration rates, thereby altering organic nitrogen mineralization. Overall, the study highlights the need for effective management strategies to alleviate hypoxia and protect water quality in deep lakes under climate change.