Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 262

While the above summary suggests that the basic requirements of the fish stock are
being provided by the tidal environment over Georges Bank, there are likely to be
large inter-annual variations associated with changes to the density field by the
transport of the nearby Scotian Shelf water (Townsend and Pettigrew, 1996 ).
The examples from the western Irish Sea and Georges Bank make no mention of the
impact of frontal primary production on the survival of the fish stocks. Food must of
course be important. In the case of Nephrops they are likely to be dependent on organic
material reaching the seabed from the surface spring bloom and the SCM over the
stratified region. For the Georges Bank, haddock and cod appear to use the mixed
shoals over the Bank as the feeding area. The absolute food concentration is thought to
be less than would normally be required, but the effective concentration is increased by
the turbulent mixing over the bank leading to greater predator-prey contact rates
(Werner et al., 1996 ), a mechanism described in Section 5.2.3 (see Fig. 5.17 ). Tidal
mixing fronts appear to be most important to larger organisms via the circulation
which they induce, rather than the primary production that they support.

Summary

..................................................................................
Shelf sea fronts, or tidal mixing fronts, mark the boundaries between regions that are
always vertically well-mixed and those that stratify (thermally) during spring and
summer. The positions of the tidal mixing fronts can be explained by assessing the
relative impacts of the supply of heat through the sea surface and of mixing generated
by the friction between tidal currents and the seabed. This leads to the prediction that
fronts should follow contours of a critical value of h/u 3 , with h the water depth and
u some measure of the strength of the tidal currents. Analysis of frontal positions in
ship surveys of temperature structure and in satellite images of sea surface tempera-
ture confirms the validity of this prediction. The density field at a front leads to the
development of an along-front jet with a weak attendant cross-frontal circulation.
For some fronts, the jet is unstable and this causes the development of baroclinic
meanders and eddies, which can transfer water constituents between the front and the
mixed water.
As well as marking the physical boundary between the biologically very distinct
mixed and stratified regions, the fronts have their own biological characteristics
arising from processes particular to the frontal zone. The enhanced growth of
phytoplankton frequently observed at fronts results from a number of mechanisms,
the most important of which are probably:

(1) A nutrient supply, greater than in strongly stratified regions, brought about by
enhanced vertical mixing in the frontal zone.
(2) A fortnightly movement of the frontal boundary due to variation in stirring over
the spring-neap cycle. This adjustment of the front involves the alternate mixing
and re-stratification of a band, typically 2-4 km, in width extending along the front.
(3) Eddy transfers of properties across the front.