Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 223

increasing turbulent mixing towards spring tides has been observed (Sharples et al.,
2001b ) , and a modelling study with a more complex microbial system has suggested
possible spring-neap changes in the community structure within the thermocline
(Allen et al., 2004 ) . Phytoplankton growth rates at the depth of the SCM, typically
0.25 d 1 - 0.5 d 1 , mean that growth or community responses to a physical perturb-
ation will require a few days to become significant. This is evident, for instance, in the
2-3 day lag in the biomass response relative to the timing of neap tides that you can see
comparing Fig. 7.10a and 7.10b . The time scales of physical change driven by the
spring-neap cycle tend, therefore, to favour phytoplankton growth.
It is worth noting at this stage that in many cases the vertical mixing of nutrients is
not strong enough to exceed the nutrient uptake capacity of the phytoplankton within
the SCM. Thus little or none of the nutrient flux reaches the sea surface (e.g. look at all
of the nitrate profiles in Fig. 7.9 ), suggesting that the nutrient supply is an important
limiter of growth in the SCM. Consequently, direct measurements of vertical turbulent
fluxes of nitrate into the base of the thermocline have been used to infer the potential
new phytoplankton carbon fixation rate within the SCM (Sharples et al., 2001 b) .
How much primary production occurs in the seasonal thermocline? Much of our
knowledge of annual primary production rates globally is now based on the use of
satellite images of surface chlorophyll (e.g. Fig. 1.1b ). Such a method will capture
the major production event of the spring bloom, but clearly there is phytoplankton
growth continuing within the thermocline after the bloom. Light at the thermocline
is generally very weak, typically 1-10% of the surface PAR, so primary production
rates tend to be low. Studies in the thermocline of the Western English Channel in
summer have yielded rates of primary production over the whole stratified water
column of 0.5 0.8 g C m 2 d 1 (Moore et al., 2003 ) , which is far less than the value

of about 6 C m 2 d 1 reported for the spring bloom in the same area (Pingree et al.,
1976 ). The differences are due to the low light experienced by cells at the thermo-
cline, though the full impact is offset by the cells' ability to acclimate to the low
light environment. Values of I k are significantly lower within the SCM, so that
production rates are able to approach their maximum values (Moore et al., 2006 )
(see Fig. 5.6 ).
While the production rates appear small compared to the spring bloom, we should
remember that they are maintained for many weeks. The 1D numerical model can be
used to provide a rough answer to the primary production question, or we can make
use of estimates of nutrient supplies to the spring bloom and to the thermocline
during summer (see Problem 7.4). Observations of surface and subsurface primary
production rates over a whole year are limited. Estimates based on observations of
primary production rates within the thermocline near the Dogger Bank in the North
Sea have suggested that subsurface carbon fixation contributes about 37% of the
annual primary production of the stratified North Sea (Weston et al., 2005 ), and
could even exceed the production achieved during the spring bloom (Richardson
et al., 2000 ) . Moreover, the SCM is likely to play a vital role in providing organic fuel
for the pelagic food web during summer (Richardson et al., 2000 ). Our own work in
the Celtic Sea thermocline arose from estimates of spring primary production