Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 205

7

Interior mixing and phytoplankton survival
in stratified environments

In the last chapter we developed an understanding of the basic seasonal
competition between heating and stirring, in which the mixing was driven by
frictional stresses at the water column boundaries. In this chapter we shall
describe the generally far weaker mixing which occurs across density interfaces
within the interior of the water column. We will illustrate the physics involved
using more detailed models of the interaction between buoyancy input and
vertical mixing processes in the seasonally stratified regime. We will show where
the models fail in their descriptions of mixing and how correcting these failings is
vital if we are to understand and model the survival and growth of phytoplank-
ton in stratified waters.
Pycnoclines often separate biochemically distinct regimes in the water column:
high light and low nutrients near the sea surface, low light and high nutrients
near the seabed. The inherent stability of a pycnocline can provide a niche for
phytoplankton that contains both sufficient light and nutrients for survival. We
will describe the links between physical and biological processes that lead to the
survival of phytoplankton; you will see that understanding the processes that
drive turbulence within and across pycnoclines lies at the heart of the growth
and distribution of the primary producers.

7.1 A 1D model of vertical mixing with turbulence closure

(the TC model)
......................................................................................................................
In order to overcome the limitations of the simple mixed-layer model of
Section 6.2.1 , we need a fuller representation of processes in the water column
which permits more complex vertical structures and allows the mixing rate to
depend on density and velocity structure in the flow. Such a detailed model of
the vertical structure is required to give us a more complete picture of where and
when vertical mixing is taking place, information which will be vital when we
come to understand and use the model of the seasonal cycle of primary
production.