Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 204

r ¼ 1026 kg m 3 , k ¼ 2.5 10 3 , r a ¼ 1.3 kg m 3 , C d ¼ 0.0012, g¼ 0.02, e ¼ 0.004

and e s ¼ 0.023).

6.4. A water column of 80 metres depth is thermally stratified. The density profile can
be approximated as a 20-metre surface layer with a density of 1025.0 kg m 3 ,
above a 60-metre bottom layer of density 1026.0 kg m 3 .

(a) Calculate the amount of mixing energy required to vertically mix this water
column.
(b) Assume the energy is supplied by winds. For the following wind strengths,
estimate the amount of time needed to achieve a vertically mixed water
column:

20 m s 1 ).
(ii) A severe storm (wind speed

(i) A gale (wind speed ¼

30 m s 1 ).

¼

40 m s 1 ).

(iii) A hurricane (wind speed ¼

Comment on your results.

6.5. If we assume that a phytoplankton population approximately maintains a Red-
field C:N of 6.6 during a spring bloom, write down an equation that allows you
to estimate the total carbon fixed (g C m 2 ) during a spring bloom in a surface
layer of thickness h s (metres) and with an initial pre-bloom nitrate concentration
of S 0 (mmol m 3 ).
Estimate the total carbon fixed rate during the spring bloomwith S 0 ¼ 7mmol m 3

and h s ¼ 20 m.

6.6. Using an estimate of the timing of a spring bloom from Fig. 6.10 , calculate the
rate of carbon fixation by the bloom in the previous problem (g C m 2 d 1 ).