Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 221

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Figure 7.9 See colour plates version . A transect of chlorophyll (colours) and temperature (lines)
over Jones Bank in the Celtic Sea in July 2005, carried out with the Seasoar vehicle (see
Fig. 1.5 ). The data in (a) and (b) are the same, but in (b) the chlorophyll colour scale has
been skewed to highlight the lower chlorophyll concentrations.

vertical flux of nitrate into the base of the thermocline of 2 mmol m 2 d 1 , while over
the bank slopes we have seen the flux reach over 20 mmol m 2 d 1 . Note in Fig. 7.9a
that there are two patches of elevated chlorophyll concentration over the slopes of
the bank. The shift in the chlorophyll scale used in Fig. 7.9b highlights the increase in
bottom layer chlorophyll underneath these patches which contrasts with the very low
chlorophyll concentrations in bottom water away from the bank. The mixing process
is thought to be the generation and breaking of internal waves as the stratified tidal
flow moves on- and off-bank, similar to the mixing seen over similar seabed features
elsewhere (Moum and Nash, 2000 ).
Observational evidence of a subsurface biochemical response to inertial mixing
(mechanism (ii) in Section 7.2.2 ) is more difficult to capture, largely because of the
intermittent nature of the meteorological forcing and the challenge of identifying a
response against the slow mean advection of horizontal patchiness. The need to
incorporate a reasonable amount of natural variability in wind stress has been identi-
fied in numerical models as driving possibly-important contrasts in annual primary
production (Radach and Moll, 1993 ) , but observational evidence of phytoplankton
growth in response to wind variability is currently lacking. We could imagine that a
pulse of wind might supply nutrients into the SCM and increase growth. On the other
hand, we might also expect the pulse of wind to deepen the base of the thermocline and
so disrupt and reduce the light received by the phytoplankton. This is a good example
of the kind of problem amenable to carefully designed numerical model experiments.