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N
S
N
S
(a)
0
-6
-20
-40
-7
-60
-8
-80
180
185
190
195
200
205
210
(b)
0
120
-20
90
-40
60
-60
30
-80
0
180
185
190
195
200
205
210
(c)
0
0
-20
-40
-80
-40
-120
-60
-160
-80
-200
180
185
190
195
200
205
210
Figure 7.10 Model simulation of the effects of spring-neap modulated mixing on the base of
the thermocline and the SCM, based on (Sharples , 2008 ) . (a) Daily averaged vertical
distribution of the turbulent diffusivity, K z ; (b) phytoplankton carbon concentration; (c) daily
averaged vertical turbulent flux of phytoplankton carbon. In (a)-(c) the line contours are
isotherms. Along the upper axis in (a) N show neap tides and S spring tides.
Spring-neap changes in bottom layer turbulent mixing (mechanism (iii) in Section
7.2.2 ) leading to erosion of the base of the thermocline and exporting any biomass in
the base of the thermocline into the bottom mixed layer, is amenable to our 1D
numerical model (Sharples, 2008 ) . The model simulation is shown in Fig. 7.10 .As
turbulent mixing increases towards spring tides ( Fig. 7.10a ), the base of the thermo-
cline is eroded and phytoplankton carbon is mixed downward into the bottom layer
( Fig. 7.10c ). As the tidal currents decrease towards the next neaps, solar heating and
wind stirring push the stratification back downward and incorporate bottom-layer
nutrients into the base of the thermocline. These nutrients are then available for
primary production within the base of the thermocline; in the model results
( Fig. 7.10b ) this produces fortnightly pulsing of thermocline phytoplankton biomass.
Carbon export from the thermocline to the bottom mixed layer as a result of
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