Geoscience Reference
In-Depth Information
7.3. The Ozmidov length is a measure of the scale of the largest over-turning eddies
in stratified turbulence. It is defined as:
r
e
N
3
L
0
¼
:
Using the data shown in
Fig. 7.6
, estimate L
0
(i) in the pycnocline at 75 mab,
and (ii) in the bottom boundary layer at 10 mab. Explain the significance of your
result for plankton in the two situations.
7.4. A vertically mixed shelf seawater column has an initial spring concentration of
8 mmol N m
3
. In late spring it stratifies with a surface layer of about 30 metres
thickness. When the water is stratified (May-September) there is a mean nitrate
supply rate of 2 mmol N m
2
d
1
into the SCM.
Estimate the contributions that the spring bloom and the SCM make to annual
carbon fixation. (Hint: remember the Redfield ratio.)
7.5. Consider the possible biological response to the mixing event described in
Problem 6.4.
(i) Prior to the mixing event the water column has negligible surface layer
nitrate and a uniform bottom layer nitrate concentration of 7 mmol m
3
.
Following a wind event that vertically mixes the whole water column,
estimate the potential total carbon that can be fixed by phytoplankton
(expressed as g C m
2
).
(ii) Compare the additional carbon fixed as a result of the wind event to phyto-
plankton growth in the SCM through 5 months of seasonal stratification.
[Assume a mean nitrate supply rate to the thermocline of 2 mmol m
2
d
1
.]
[Hint: remember the Redfield ratio.]
10
4
ms
1
, a tidally averaged bottom
layer diffusivity for the Celtic Sea of K
z
¼
7.6. Taking a cell swimming speed of v
c
¼
1
10
2
m
2
s
1
,andL
2
¼
distance
travelled in 16 hours of daylight, calculate the Pe´ clet number.
Do the same for the bottom water of Monterey Bay where the lower layer
diffusivity is 10
5
m
2
s
1
.
What do the answers tell you about the likely role of cell swimming in the two
environments?
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