Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 300

covers 40% of the entire USA, achieving an adequate reduction in nutrient loading
will be a major challenge.

Summary

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ROFIs are host to a wide range of physical processes; they combine many of the
characteristics of both the central shelf and estuarine regimes in a dynamic system of
considerable complexity. They occur where the flux of freshwater from one or more
estuaries inputs significant amounts of buoyancy to a coastal region. The resulting
horizontal gradients of density tend to drive a circulation analogous to that which
occurs in an estuary, but the flow in a ROFI is modified by the Coriolis forces which
tend to generate coast parallel currents moving in the direction of Kelvin wave
propagation (i.e. with the coast to the right of the flow direction in the northern
hemisphere). The competition between the stratifying effect of the freshwater buoy-
ancy and stirring by wind and tide is more subtle than that between heating and
stirring (explored in Chapter 6 ) because the buoyancy is injected through specific
sources at the coast rather than being distributed uniformly over the sea surface.
Variations in tidal stirring over the spring-neap cycle, along with more irregular
changes in wind stirring and freshwater input, produce a highly variable regime
which can change from strongly stratified to vertically well mixed on a variety of
time scales. Periodic variations in stratification may occur on a semi-diurnal time
scale in response to tidal straining, a process which results from the interaction
between vertical shear in the tidal currents and the horizontal density gradient. The
form of the interaction depends on whether the tide is predominantly a standing or a
progressive (Kelvin) wave. Wind stress may be important in both cases, both by
adding extra stirring and by driving the along-coast flow towards, or away from, the
land boundary.
Given this dynamic complexity, and particularly the range of time scales on which
buoyancy and mixing processes compete, it is not surprising that primary production
in ROFIs does not operate with the seasonal consistency that we see in deeper shelf
sea regions. Rather than a single bloom of surface productivity in spring, production
in ROFIs can peak at any time of the year. Spring-neap modulation of stratification
in many ROFIs forces similar periodicity in phytoplankton growth. There may be
evidence that phytoplankton uptake of nutrients can respond to the faster semi-
diurnal stratification of tidal straining, but the rapid switching between mixed and
stratified sites is unlikely to result in a measurable response in phytoplankton
biomass.
An ecologically important feature of the physics of ROFIs is the mean, density-
driven circulation set up by the horizontal density gradient. Outflow at the sea
surface, and shoreward flow near the seabed, can be exploited by organisms that
release larvae into the water column but then require some method for transport of
older, developed larvae back towards the adult habitat. The slow density-driven
flows (O(cm s 1 )) and the typical horizontal scale of ROFIs are often well suited to