Regions of freshwater influence (ROFIs) - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 302

at low water which would result
from tidal straining given that the density gradient along the principal axis of the
tide has a magnitude of 2.1

0.65ms 1 , estimate the water column stability F

10 -4 kg m 4 and the depth is 25 m. what would be

the reduced value of if tidal stirring is included?

(Hint: Assume the velocity profile has the form given in ( Equation 9.23 ) and
integrate Equation ( 9.24 ) over the ebb half cycle. Use Equation ( 9.25 )to
estimate the contribution of tidal stirring with e

F

¼ 0.004; k ¼ 0.0025.)

9.3. The data from the Liverpool Bay anchor station shown in Fig. 9.18c looks very
similar to that shown in Fig. 9.9c , suggesting that rather than advection of a
front, the data in Fig. 9.18c reflect the semi-diurnal process of straining-induced
stratification. If this is true, there are interesting implications for the biochemis-
try of a ROFI arising from the observations shown in Fig. 9.18a ,b.
Write down any arguments you can think of that support the 'frontal advection'
and the 'SIPS' hypotheses as explanations for the physical and biochemical data
presented in Floodgate et al., 1981 . Do you favour one or other of the hypoth-
eses, and why? Are there other data that you would collect to allow you to
separate the possible explanations?

9.4. At high water a ROFI has a typical vertically mixed chlorophyll profile with
concentration 1 mg Chl m 3 . Using reasonable values for the net phytoplankton
growth rate, estimate:

(i) The change in near-surface chlorophyll in response to one semi-diurnal cycle
of SIPS.
(ii) The change in near-surface chlorophyll in response to a neap-tide stratifica-
tion period (similar to that in Fig. 9.7 ).

Specify what assumptions you make.

9.5. Crustacean larvae in the surface waters of a ROFI need to develop to a final
stage where they can then swim downward towards the onshore mean flow of
the bottom water. Depending on rainfall, the horizontal density gradient in the
ROFI ranges between 1

10 6 m 1 , depth is about 40 metres and
the typical offshore extent of the ROFI is 50 km.

10 7 and 1

What limits do the physical characteristics of the ROFI place on the develop-
mental time scale of the larvae?
(Hint: Use the offshore component of the mean ROFI circulation in Heaps,
1972 , and assume N z ¼ 10 3 m 2 s 1 .) 5