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

Abstract

Lay AbstractSediments 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.