Periodic convection within littoral lake sediments on the background of seiche-driven oxygen fluctuations

Abstract

This study examines the effects of internal seiches on heat transport through the sediment-water interface, and the internal seiche-related temperature and oxygen fluctuations above the sediment, in the littoral zone of a stratified lake. High-resolution temperature profiles were taken within the upper sediment, accompanied by temperature and oxygen measurements within the overlying water. Heat transport in the upper sediment alternated between diffusion and convection at the periodicity of the internal seiches, with the strongest oscillations at a period of 2.4 h. During long-duration events (>30 min) of seiche-driven cooling of the sediment surface, the thermal instability extended as much as 9 cm down into the sediment, followed by free convective transport in the upper sediment. The vertical convective heat fluxes were close to those of Rayleigh–Bénard convection for pure fluid flow. The convective heat fluxes were, on average, three times higher than the diffusive heat fluxes, and the maximum convective heat fluxes of 50–100 W m⁻² were 10–20 times higher than the maximum diffusive heat fluxes. Internal seiches caused advective oxygen fluctuations above the sediment that can potentially reinforce the effect of convection on biochemical processes within the lake sediments. Periodic temperature and oxygen variations due to internal seiching can cover ∼10% of the sediment area, depending on seasonal stratification and lake morphometry. In these areas, convection intensifies the transport of heat, nutrients, and oxygen through the sediment surface and represents an important feature of the ecology of lakes.