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

Abstract

Sediments at the bottom of lakes are active biogeochemical zones where settled particulate organic matter is decomposed into dissolved compounds. The latter serve as building materials and energy sources for lake organisms. The amount of the dissolved matter released back to the water column determines, in part, the production and activity of lake biota and directly affects dissolved oxygen levels. However, low‐mixing conditions at the water–sediment interface limit the upward flow of dissolved matter from the sediment to the water. Standing internal waves (seiches), which occur, for example, after changes in wind speed or direction and are inherent for thermally stratified lakes, produce horizontal motions of lake water so that cold deep waters climb over warmer sediment along sloped lake bottoms. This process may initiate pore‐water convection—vertical flow of the interstitial water in the sediment—and accelerate the release of dissolved matter into the water column. We investigated pore‐water convection by using vertical heat flux within the upper sediment as an indicator of convective motions. This was achieved using fine‐resolution temperature measurements within the upper sediment of Lake Arendsee, Germany. Our field study demonstrated that cold water flowing over warmer sediment amplified the vertical heat flux within the sediment. The rate of heat flow increase agreed with theoretical estimates, suggesting that it originates from vertical convective motions. The average velocities of these motions were estimated as ∼0.5 mm s −1 , penetrating up to 9 cm deep in the sediments and lasting for 2–4 h. The consequences of this hydrodynamic phenomenon for benthic microorganisms and for transport of solutes have yet to be quantified. 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.