Geoscience Reference
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will illustrate how changes in the light spectrum through a pycnocline can lead to
layering of individual species within a single chlorophyll layer, again showing how the
weakly turbulent environment provided by pycnoclines plays a vital role.
7.3.1
The subsurface chlorophyll maximum (SCM)
Below the sea surface, phytoplankton are often found to be located in well-defined
layers where they are actively growing. In regions where mixing is dominated by the
barotropic tide, these phytoplankton are generally found in a thin layer within
the base of the seasonal thermocline, such as in the example from the Celtic Sea in
Fig. 7.7a . In areas with weak barotropic tides, where internal mixing plays a more
dominant role in subsurface structure, broader temperature and subsurface chloro-
phyll distributions are seen. Two examples are shown in Fig. 7.7b, c , from the
northeast shelf of New Zealand and from the edge of Georges Bank, northeast
United States. In regions of weak barotropic and internal mixing, highly concen-
trated chlorophyll peaks can be found, often dominated by a single species of motile
phytoplankton, as seen off Monterey shown in Fig. 7.7d . A layer of phytoplankton
situated within a region of vertical density gradient below the surface is often referred
to as a subsurface chlorophyll maximum (SCM).
These layers can be very extensive, covering entire stratified regions as seen in a 500
km transect with a towed CTD through the Celtic Sea shown in Fig. 7.8 . Several
candidate mechanisms have been proposed as the cause of such layers of phytoplank-
ton. Cells may sink to form a layer at a location within a pycnocline where they are
neutrally buoyant, or alternatively they may sink until a supply of nutrients within the
nitracline (the region of rapid changes in nitrate within the pycnocline) allows them to
grow and increase their buoyancy (Steele and Yentsch, 1960 ) . The ability of some
species (particularly dinoflagellates) to swim in response to resource needs may allow
them to actively form the layer (Margalef, 1978 ). Alternatively, growth of a layer of
phytoplankton could be driven by a weak flux of nutrients into the thermocline from
the bottom layer (Pingree et al., 1977 ) , as long as there is sufficient light to drive
photosynthesis. It has also been argued that the SCMmay not represent a true biomass
maxima, but instead may be a result of changes in pigment concentration per cell
(Kiefer and Kremer, 1981 ). Indeed, for SCMs in the open ocean it has been found that
the entire signal of the chlorophyll peak can be attributed to higher pigment concen-
trations, with cells in similar numbers to the surface waters (Veldhuis and Kraay,
2004 ) . In shelf seas, however, we generally find that both increased pigment per cell and
increased cell numbers are evident.
7.3.2
Nutrient supply and primary production within the SCM
Let us focus first on a two-layer stratified system in a region with moderate or strong
barotropic tides, ignoring the possibility of significant cell swimming so that phyto-
plankton growth requirements are dominated by the turbulent environment. For
growth of the phytoplankton within a midwater layer, nutrients must be mixed into
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