Oxygen dynamics in a stratified clearwater lake: hourly to decadal timescales

Lake Stechlin (A = 4.3 km², mean depth = 23.3 m, max depth = 69.5 m, V = 96.9 × 106 m³) is a deep clearwater lake situated in northern Germany. Formerly known for its excellent water quality, the lake experienced severe increases in total phosphorus (P) concentrations between 2014 and 2020, most likely related to shifts in the macrophyte communities in the shallow sediments and associated P mobilisation. After 2020, P levels began to decrease again. Dissolved oxygen (DO) concentrations followed these changes: the DO depletion rate increased from 0.4-0.6 g/m²/d in the pre-eutrophication period to more than 1 g/m²/d during 2019-2021, before starting to decrease again afterwards. Consequently, large areas of the deep hypolimnion became anoxic.

Here we present long-term monitoring data combined with results from two high-resolution measurement campaigns of temperature, dissolved oxygen (DO), and current velocities near the lake sediment at three different depths (45 m, 50 m, and 55 m). During the beginning of the measurements in summer, DO concentrations were comparable or even slightly higher in 2024 compared to 2023. In the fall, anoxic conditions occurred 1 meter above the sediment at all three depths in 2023 but not in 2024, when phosphorus concentrations were considerably lower. We attribute the higher DO concentrations during summer 2024 compared to 2023 to an earlier onset of summer stratification and the lower concentrations during fall 2023 to lower depletion rates in response to decreasing nutrient concentrations. In both years, oxygen fluctuations with amplitudes of up to 4 mg/L were observed and caused by internal waves with periods of approximately 24 h and 6-8 h. Especially in 2023, the sediments experienced periodically changing redox conditions during fall at all three measurement depths. The impact of these fluctuations is still unknown although large fractions of the lake sediments are situated in areas that can potentially become periodically anoxic in Lake Stechlin and other lakes worldwide.

The results illustrate extensive areas of Lake Stechlin experience periodic anoxia, which cannot be detected by monthly routine measurements. The also indicate that oxygen depletion rates respond quickly to changes in the nutrient concentrations. Oxygen concentrations are influenced by multiple factors such as varying nutrient concentrations and differences in the stratification phenology due to meteorological conditions.  For reliable future prediction of oxygen budgets, a good mechanistic understanding of their influence is desirable.