Geoscience Reference
In-Depth Information
column, nutrients remain high throughout the season but growth is inhibited by the
lack of light.
In the continuously mixed (low h/u 3 ) regimes, phytoplankton are transported
through the full depth of the water and so experience a highly variable and erratic
light regime. Here growth is still possible during the high surface irradiances of
summer, but the limited light reduces growth rates even though nutrients are abun-
dant. In addition to continuous vertical motions, the average light supply may be
further restricted by high levels of re-suspended sediments maintained by the strong
vertical mixing which characterises these areas.
FURTHER READING
Circulation and Fronts in Continental Shelf Seas, ed. John C. Swallow, et al., The Royal Society
of London, 1981.
The Dynamics of Marine Ecosystems: Biological-Physical Interactions in the Oceans,by
Kenneth H. Mann and John R. N. Lazier, Blackwell Publishing, 2006.
Problems
....................................................................................................................
6.1. Consider an initially depth-uniform water column of depth h which receives an
input of heat
Q per unit area which is absorbed in a thin near-surface layer of
thickness h 1 <<
D
h. Calculate the potential energy of (i) this stratified water
column and (ii) its value if the water column becomes fully mixed. Hence show
that the energy required to bring about mixing is
agh
Q
ð
P
:
E
:Þ¼
2c p
where
a
is the volume expansion coefficient and c p is the specific heat of seawater.
6.2. Observations at the summer solstice (June 21), at a location where the depth
h
95 m and the tidal stream amplitude is 0.5 m s 1 , indicate that the potential
energy anomaly
¼
55 Jm 3 . If the net seasonal heat input to the sea surface is
F ¼
140 sin(2pt/365) Wm 2 , where t is time measured from the vernal
equinox (March 21), determine the total heat input up to the time of the solstice
and hence the value of
given by Q i ¼
which would occur in the absence of mixing. Assuming
that all mixing is due to tidal stirring, estimate the efficiency of mixing.
F
6.3. For a region of the shelf seas, the peak summer heat input is 105 Wm 2 and
average cubed wind speed W 3
520 m 3 s 3 . Estimate the maximum depth to
which complete mixing can be maintained if the tidal stream amplitude is (a) 0.6
ms 1 and (b) 0.1 m s 1 using the combined
¼
F
model for tidal and wind stirring.
(You may use the numerical values of constants and estimates of mixing efficiency
given in Section 6.3 and 6.4 ,i.e.a
10 4 C 1 , c p ¼
10 3 Jkg 1 C 1 ,
¼
1.67
4.0
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