Definition and Relevance of the Benthic Boundary Layer

The benthic boundary layer (BBL) of lakes, reservoirs, and rivers constitutes that part of the water column that is directly influenced by the presence of the sediment-water interface. Similar to the surface mixed layer, it represents a hot spot not only of dissipation of kinetic energy, but also of biological activity and of geochemical transformation processes. These different processes are strongly coupled and interact with each other: While the hydrodynamic conditions are modified by biological activity, which changes the structure of the sediment surface, the release of dissolved solids from the sediment can modify the density stratification in the BBL. Moreover, the actual sediment surface cannot always be regarded as rigid since the BBL flow does not only modify the structure of the sediment surface, but it can also bring sediment particles into suspension, whereas at other sites or at other times, the particles resettle.

The BBL definition provided here and the more detailed discussions later explicitly refer to direct influences of the sediment surface. From a more general point of view, the BBL is of great importance for the entire water body, almost independent of the dimensions of the basin. Strong turbulence and mixing along the boundaries are known to be important for vertical mixing, and transport on a basin scale and biogeochemical processes at the sediment surface or within the sediment effect the distribution of relevant water constituents on scales much larger than the actual dimensions of the BBL. These larger-scale effects, however, require additional transport processes for energy and matter into or out of the BBL and are considered elsewhere.

A major characteristic of the BBL is the magnitude and the temporal dynamics of the physical forcing, i.e., the current velocity at the top of the BBL. Although in most rivers this forcing can be regarded as a steady-state unidirectional flow, its nature in deep and stratified lakes and reservoirs is more complex. In these, usually stratified, water bodies the major energy is provided by surface waves in the shallow littoral zone, by high-frequency internal waves at the depth of the thermocline and by basin-scale internal waves (seiches or Kelvin and Poincare waves) in the hypolimnion. Hence, the magnitude and temporal dynamics of the different forcing mechanisms range from current velocities of some 10 cm s_1 and time scales of seconds for surface waves, to typical current velocities of a few centimeters per second and time scales of several hours to days for basin-scale internal waves (Figure 1).

Structure of the BBL

The BBL can be structured vertically according to the physical processes governing the vertical transport of momentum and solutes (Figure 2). In an outer layer (turbulent BBL) up to several meters above the sediment surface, this transport is governed by turbulent eddies and the associated mixing rates are high. While approaching the sediment surface down to scales where viscous forces suppress overturning turbulent motions, the vertical transport of momentum is governed by molecular viscosity and a viscous sublayer with a typical height of O (1 cm) develops. The exchange of heat and dissolved solids and gases is eventually controlled by a diffusive sublayer with a height of O (1 mm) directly at the sediment-water interface.

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