Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 266

9

Regions of freshwater influence (ROFIs)

As we saw in Chapter 2 , the input of buoyancy over much of the shelf seas is
dominated by heating and cooling through the sea surface. There are additional
exchanges of buoyancy at the sea surface through the processes of evaporation
and precipitation, both of which modify the salinity of surface layers, but in
temperate latitudes their contribution is generally small in comparison with heat
exchange. In areas of the shelf adjacent to estuaries, however, freshwater
discharge from rivers can make a dominant contribution to buoyancy input
( Section 2.3 ) and maintain strong horizontal gradients of salinity.
In this chapter, we shall consider these Regions Of Freshwater Influence (ROFIs)
(Simpson, 1997 ) , the suite of processes which operate within them and the
distinctive environment that results. In terms of physical processes, ROFIs have
much in common with estuaries, but they differ insofar as estuaries are confined by
land barriers and are generally of a smaller horizontal scale than that which is
characteristic of ROFIs. In many cases, the circulation in an estuary is predominantly
an along-channel flow while transverse motions are relatively small and the effects
of the Earth's rotation can be neglected. We shall initially develop the theory of
density-driven circulation for such a simplified, non-rotating system before proceed-
ing to consider how rotation modifies the density-driven circulation in ROFIs. We
shall next examine the way in which the density-driven circulation is involved in
determining the water column structure of ROFIs in competition with stirring and
then explore the implications for the biology and environmental health of ROFIs.

9.1 Freshwater buoyancy and estuarine circulation

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The competition between buoyancy input and stirring in estuaries and ROFIs
is somewhat analogous to that between surface heating and stirring discussed in
Chapter 6 , but there is an important difference. Whereas buoyancy input due to heat
exchange is more or less spatially uniform, freshwater input from rivers enters
through the lateral boundaries and is distributed by buoyancy-driven flows in a
manner which is common to estuaries and ROFIs. This re-distribution of buoyancy