Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 291

to be advected offshore and alongshore. Nearer the seabed, things are likely to drift
shoreward, possibly even returning to the estuary. We have already seen the import-
ance of persistent flows to the maintenance of some shelf animal populations ( Section
8.6.3 ); mean flows in ROFIs and estuaries can play a similar role. Second, cycles in
mixing and stratification have a great impact on phytoplankton growth, which we
have seen in the context of the annual spring bloom ( Section 6.3.1 ) and the modula-
tion of primary production by the spring-neap tidal cycle at shelf sea fronts ( Section
8.6.1 ) and within the shelf sea seasonal thermocline (Section 7.3.2). ROFIs can
exhibit stratification-mixing cycles ranging from the semi-diurnal and spring-neap
tidal cycles, through to seasonal responses to changes in freshwater supply. We might
ask if the phytoplankton are seen to respond to any or all of these? Finally, a ROFI is
a coastal zone where nutrients and pollutants borne by the freshwater will have their
first impact on the shelf sea system. The physics of the ROFI regime is crucial in
determining where these inputs go and what will be their impact on the ROFI
ecosystem.

9.8.1 Density-driven flows: export and return

Any buoyant or actively surface-dwelling organism in an estuary or ROFI will tend
to experience a net movement out of the estuary and off- and alongshore in the
coastal zone due to the density-driven circulation. Conversely, anything living down
near the seabed will tend to be moved back towards the coast, possibly re-entering
the estuary. You can imagine that a combination of these two density-driven mean
flows could be exploited by an organism in order to remain roughly in one place or to
limit its dispersion. All an organism needs is some capacity for controlling its vertical
position and hence which component of the density-driven flow it experiences.
Organisms which can swim vertically have the potential to react on short time
scales to their environment, by using different stages of the tidal cycle to maintain
their horizontal position. Utilisation of the much slower density-driven exchange
tends to occur over time scales longer than tidal, with changes in the depth of an
organism occurring at the very low frequency associated with different life stages (e.g.
the ontogenic behaviour of some mesozooplankton introduced in Section 5.2.1 ). We
have already seen the patterns of low salinity water outside Delaware Bay on the
northeast shelf of the United States ( Fig. 9.13 ). Several organisms have been recorded
apparently taking advantage of the exchange flows between the Bay and the shelf.
For instance early stage fiddler crab larvae, which are buoyant, are thought to be
flushed from the estuaries of Delaware Bay out onto the shelf. Later stages are found
near the seabed on the shelf, where the return flow of the density-driven circulation
brings them back into the Bay (Epifanio et al., 1988 ) . This ontogenic shift in the
depth distribution of larval stages, combined with later stages being found further
offshore, is illustrated in Fig. 9.14 . Notice how the later stages are found in the
seaward sampling stations outside the Bay ( Fig. 9.14a , b ), while at the same time they
have a more even, vertical distribution compared to the surface bias of the earlier
stages ( Fig. 9.14c ).