Abteilung Oberflächengewässer

Seit gut hundert Jahren weckt das Phänomen der Sauerstoffzehrung in Seen das Interesse von Forschern und Gewässerschutzverantwortlichen. Die Sauerstoffzehrung im Tiefenwasser wird durch den Abbau von organischem Material - vorwiegend Algen - verursacht. Zwischen Nährstoffangebot, Algenwachstum, Sedimentation und Abbau herrscht ein komplexes Zusammenspiel. Neue wissenschaftliche Arbeiten zeigen heute ein deutlich klareres Bild der verschiedenen Komponenten der Sauerstoffzehrung als zur Zeit der ersten Seenschutz-Massnahmen in den 1980er-Jahren.

Periodic winds acting on a stratified waterbody can amplify normal modes of motion and enhance the basin-scale circulation via resonance. Here, we use idealized large-eddy simulations to investigate the flow features and quantify the horizontal transport in periodically wind-forced stratified basins. Motivated by observations in lakes, we focus on systems in which daily winds either resonate with the second vertical basin-scale internal mode, V2H1 (case 1), or the first vertical basin-scale internal mode, V1H1 (case 2). In particular, we analyze the case when strong nonlinearities affect the evolution of the V2H1 mode (case 3). To achieve these three resonance scenarios, we hold the basin morphology and the periodic forcing invariant, but change the background stratification. Our results show that a quasilinear V2H1 modal response has more active mass transport in the boundary regions than does the quasilinear V1H1 case. This difference arises from the lack of midlayer horizontal transport in the V1H1 mode, whereas the midlayer current in the V2H1 mode intensifies the transport along the slopes in both directions by splitting the flow into two branches, one running upslope and one running downslope. Nonlinear dynamics further amplify the along-slope transport in case 3, in which a second mode, an undular borelike wave, emerges from the periodic forcing. This study shows that the horizontal transport under wind-induced resonance is sensitive to the amplified mode of motion in the stratified basin and that nonlinear flow dynamics can considerably enhance mass transport in sloping regions.