Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 230

picture of the evolution of water column structure and flow is given by more
advanced models incorporating a turbulence closure scheme. Such schemes provide
a more explicit representation of turbulent processes, including the inhibiting effect
of stratification on mixing. The models give a fuller account of the heating-stirring
problem and furnish detailed information on the distribution of vertical mixing.
In particular they provide profiles of the TKE dissipation rate and hence of the
eddy diffusivity, a parameter which is needed for the estimation of vertical fluxes of
nutrients and other properties. Turbulence closure schemes of varying degrees of
complexity are now widely used in shelf-wide coupled physics-biogeochemistry-ecol-
ogy models.
The period of 4-7 months during which many shelf sea regions are strongly
stratified provides a stringent test of the capabilities of our models. Comparing
the models with observations of, for instance, surface and bottom layer tempera-
tures or profiles of currents generally shows good agreement. However, the more
challenging test of comparison with observed turbulent dissipation rates indicates
that the models fail to reproduce bursts of mixing activity in the interior of the
stratified water column which is of fundamental importance to the biochemistry.
The reason for the model-observation discrepancies is most probably that the
models are failing to adequately represent one or more important sources of interior
turbulence, such as internal waves, spring-neap changes in bottom layer turbulence
and inertial oscillations.
Throughout the stratified period, subsurface layers of phytoplankton (subsur-
face chlorophyll maxima, or SCMs) associated with pycnoclines are a common
feature of shelf seas. Turbulent mixing within and at the base of pycnoclines
plays a fundamental role in the survival of the phytoplankton. Turbulence
can supply bottom water nutrients to the pycnocline, where they support phyto-
plankton growth, or it can remove phytoplankton from the base of the pycno-
cline into the bottom water. Nitrate profiles frequently show a sharp nitracline
within the phytoplankton layer, with negligible nitrate above the layer and in the
surface waters, suggesting that the turbulent flux of nitrate into the base of a
phytoplankton layer is a limiting factor to phytoplankton growth. SCMs are not
visible from above the sea surface, i.e. they are not accessible to mapping by
satellites. Estimates of carbon fixation rates within SCMs, either inferred from
the supply of nitrate or from direct measurements using C 14 incubations, suggest
that they make an important contribution to annual primary production in shelf
seas, potentially fixing the same amount of carbon as the spring phytoplankton
bloom.
The low turbulent region within the pycnocline can be exploited by motile phyto-
plankton that are capable of using swimming to balance their resource requirements.
In some weak tidal regions, turbulence is also low below the pycnocline, and swim-
ming phytoplankton can aggregate to form concentrated layers far more rapidly than
they could solely as a result of in situ growth. While observations of phytoplankton