Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 263

The evidence to support a simple link from the mixing, through the primary produc-
tion, to the zooplankton and on up to the larger organisms is not strong. For
zooplankton there is evidence for increases in reproduction rates (e.g. eggs produced
per female) in response to the elevated food supply, but observations of increased
densities of adults at fronts seem to be infrequent. This is probably because the
combination of frontal circulation and the developmental time between zooplankton
egg and a new breeding adult leads to a de-coupling between the primary producers
and the adult zooplankton population. It should also be recognized that aggregation
of zooplankton at a front could be occurring mainly within the small region of
convergent flow, and could therefore have been missed by ship-based sampling
procedures. Animals from fish larvae and juvenile fish, to basking sharks, and to
seabirds have been seen to forage at fronts, particularly at the convergence zones
where their zooplankton prey aggregate.
Fronts are also recognised as important for many commercially exploited fish
stocks. The underlying reason may be linked, not so much to food supply at a front,
as to the need for a mean flow from a spawning ground back to the adult feeding
area, so that the free-drifting eggs and early stage fish larvae are not lost within the
otherwise dispersive marine environment. The role of the baroclinic jets and the
cross-frontal circulation in this context is illustrated in examples from the Irish Sea
and Georges Bank.

FURTHER READING

Circulation and Fronts in Continental Shelf Seas, ed. J. C. Swallow, R. I. Currie, A. E. Gill
and J. H. Simpson. The Royal Society, London, 1981.
Simpson, J. H., and J. Sharples. Does the Earth's rotation influence the location of the shelf
sea fronts? Journal of Geophysical Research, 99(C2), 3315-3319, 1994.
Franks, P. J. S. Phytoplankton blooms at fronts: patterns, scales and physical forcing
mechanisms. Reviews in Aquatic Sciences, 6(2), 121-137, 1992.
Loder, J.W., and T. Platt. Physical controls on phytoplankton production at tidal fronts. In
Proceedings of the 19th European Marine Biology Symposium, 3-21, 1985, Cambridge
University Press.

Problems

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8.1. The density distribution on a section perpendicular to a shelf sea front located in
latitude 50 N is shown in the figure below. Given that the currents are in steady
geostrophic balance with the density field, estimate the speed and direction of
the along-front flow in the region of strongest horizontal density gradients and
present your result as a sketch of the velocity profile assuming that the velocity is
zero at the seabed.

If the flow were to become unstable, what would you expect to be the dominant
scale of the instability?