Marine diffusive boundary layers at high latitudes

The thickness of marine diffusive boundary layers (DBLs) can be calculated from the friction velocity and the water density (a function of temperature and salinity). However, DBL thickness scales differently with temperature, depending on whether free-stream or friction velocity is used. We show that there are advantages to using frictional velocity for experimental scaling. Low seawater temperatures in polar areas cause DBLs to be up to 32 % thicker than in temperate or tropical areas. This will have a significant effect on biological processes such as photosynthesis and respiration. Boundary layers are regions of fluid, adjacent to a bound-ary, where some property of the fluid has been influenced by the boundary, compared to areas unaffected by the boundary (fluid properties in these unaffected areas are re-ferred to as ‘‘free-stream’ ’ properties). The two properties of interest to this paper are fluid velocity and chemical concen-tration. Typical velocity and chemical concentration profiles for a boundary layer are shown in Fig. 1. Diffusive boundary layers (DBLs) are boundary layers where chemical concen-tration gradients are of interest. One fundamental property of boundary layers of signifi-cance to this paper is the friction velocity (u*), which is a measure of the fluid shear at the boundary and is defined by the formula u * 5 Ïv(du/dy) and has the units of velocity. Characterization of the physical and chemical properties of DBLs associated with microbial biofilms has become crit-ical for quantifying metabolic processes such as photosyn