damps turbulence in the flow. Away 0.5 to 1 cm, velocity increases more rapidly as
inertial effects become more important, reaching an asymptote several dm out, or, in
the case of the seabed, several dm up. Velocity in this range plots linearly vs. a
logarithmic scale of distance from the surface. With distance from a surface, the
potential for turbulence in the flow increases.
Fig. 1.4 A vertical profile of average velocity upward from a sandy-silt seabed at 199
m through the viscous boundary layer, in which velocity increases linearly with
distance above bottom (inset), and then on upward (larger graph) through the “buffer
layer”. In the latter the velocity approaches that at a distance, ∼10.5 cm s−1 here,
within about 20 cm. Measurements were with a heated-thermistor velocity probe by
Caldwell and Chriss (1979). Very mild turbulence (±0.5 cm s−1 at 20 cm, much less in
the viscous layer; see Chriss & Caldwell 1984) has been averaged out.
Boundary layers have many ecological effects that are well reviewed by Mann and
Lazier (2006). They require that a swimming animal, particularly a small one,
effectively must push along a mass somewhat larger than itself. It adds further
dominance to the role of molecular diffusion for final transport of molecules to gills
and cell surfaces. It means that small hairs side-by-side in a palisade, such as cilia on
a ctenophore or setae on a euphausiid’s (krill) leg, will have intersecting boundary
layers and effectively form a solid paddle. Similarly, animals filter-feeding with setal
or mucus meshes must generate substantial pressure to force water through their webs