Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 217

bottom layer of a tidally energetic shelf region could have diffusivities of
10 1 -10 2 m 2 s 1 .

Comparison of model predictions with observations of turbulent dissipation
generally shows that the models do well at reproducing boundary-driven
turbulence; e.g. turbulence generated by friction between water flow and the
seabed and between the wind and the sea surface. However, away from the
boundaries the models do far less well, potentially underestimating turbulent
dissipation by several orders of magnitude.

Model failure in the water column interior arises because of physical processes
that the models do not resolve. Spring-neap changes in tidal mixing at the base
of the thermocline, wind-driven motion of the surface layer at the inertial
frequency and short-wavelength (1 - a few km) internal waves are all important
potential sources of interior mixing that either need to be resolved or adequately
parameterised.

More complex methods involve limitations imposed on the eddy diffusivity and
viscosity or on the turbulent dissipation as a function of the gradient Richardson
number (Kantha and Clayson, 1994 ; Canuto et al., 2001 ).

7.3 Phytoplankton growth, distribution and survival in pycnoclines

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In regions which stratify, the spring bloom ( Chapter 6 ) often receives particular attention
as the most conspicuous event of the primary production cycle. After the spring
bloom, surface layer chlorophyll is usually seen to be very low throughout the summer
and dominated by small cells in an efficient recycling system as the effect of the
thermocline in inhibiting mixing prevents the surface from being replenished with
nutrients. The dominant small cells are not efficient in supplying organic material to
the rest of the marine food web, and the primary production of large cells during the
spring bloom is not sufficient to fuel the known demands of the pelagic and benthic food
webs during the rest of the year. We have also seen that, once stratification has become
established, it is very hard to re-mix the water columnwithwinds. In these circumstances,
we might reasonably ask where and how is new primary production (i.e. production that
utilises nutrients from below the thermocline) supported after the spring bloom.
Understanding phytoplankton survival within pycnoclines requires knowledge of
the often-intermittent interior mixing, which we described earlier in this chapter, and
how phytoplankton respond to it. We will first describe how turbulent mixing across
the base of a pycnocline can both supply a phytoplankton layer with nutrients and
entrain phytoplankton into the bottomwaters. We will then demonstrate that primary
production within the SCM is a significant contributor to the total annual production
of a shelf sea. Some phytoplankton attempt to control their own resource needs by
swimming vertically; we will describe how the success of this strategy depends on the
strength of the turbulence in the bottom water and within the pycnocline. Finally, we