Effect of Lake‐Wide Internal Waves on Near‐Bottom Oxygen Concentrations

Abstract

In deep temperate lakes, large portions of the hypolimnion can become anoxic during summer stratification. Anoxia develops at the deepest point of the lake, and the oxycline gradually rises over the course of the summer. In the presence of internal waves, the oxycline oscillates, creating periodically reoccurring hypoxic conditions above the sediments at affected depths. To assess the effect of oxycline oscillations on oxygen, we measured dissolved oxygen (DO) concentrations at the sloping boundary of a deep stratified lake (Lake Stechlin) close to the sediment surface during two summers with varying meteorological conditions and phosphorus concentrations. DO concentrations 1 m above the sediments were lower compared to the same depth in the lake interior. Duration and extent of anoxia varied significantly between years. In 2023, the year with higher phosphorus concentrations, DO depletion rates were higher, and a larger fraction of the sediment area was affected by permanent or periodically occurring anoxia (>30%, compared to approximately 15% in 2024). Current measurements from 2024 reveal that DO concentrations and temperatures are correlated with velocities directed toward the lake interior. Velocities in the bottom boundary layer were higher during downwelling compared with upwelling and led to higher turbulence levels. Higher DO concentrations coinciding with higher transfer velocities between water and sediment led to increased sediment oxygen uptake (SOU). Averaged over the whole measurement period, internal seiches enhanced SOU compared with conditions without internal seiche oscillations. These findings demonstrate that variable SOU driven by internal waves substantially influences DO depletion even on seasonal timescales.