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Wind
Wind
PAR
ocean
PAR
shelf
PAR
Thermocline
Thermocline
High nitrate flux
Low nitrate flux
Tides
z
Seabed
Seabed
Open ocean
Shelf
Figure 7.11
Schematic illustration of the role of tides on the shelf in altering the light and
nutrient environment experienced at the seasonal thermocline. Tidal mixing increases both
the light and the supply of nitrate in the shelf sea thermocline compared to the non-tidal
environment of the open ocean.
suggesting that the bloom was insufficient to support the demands of the pelagic and
benthic foods webs in the shelf sea. Also, remember from
Chapter 6
that diatom-
dominated blooms may rapidly sink out of the water column to the sediments, so the
bloom is possibly only briefly accessible to pelagic organisms. The SCM provides
pelagic organisms with an alternative strategy for acquiring food, with low phyto-
plankton concentrations available for several months in contrast to the concept of
needing to match the timing of the spring bloom discussed in Section 6.3.3.
The role that the shelf sea SCM plays in augmenting annual primary production
contrasts markedly with the SCM found in the open ocean. We noted earlier that the
chlorophyll layer found in the thermocline of the major oceanic gyres is generally
seen to be the result of changes in the cell chlorophyll content, not additional cell
numbers. We can suggest two reasons for this arising from the different physics on
the shelf, illustrated in
Fig. 7.11
. First, the depth of the seasonal thermocline in the
open ocean is set by the amount of wind mixing acting to deepen the surface mixed
layer. On the shelf, the thermocline depth is a balance between wind mixing trying to
deepen it, and tidal mixing attempting to push it back upward. Thus the shelf
thermocline tends to be shallower and so the SCM receives more light. Second, tidal
mixing, or internal wave mixing originating from tidal flows over an uneven seabed,
leads to relatively high fluxes of nitrate upward into the shelf sea SCM. Typically we
see fluxes of 1-10 mmol m
2
d
1
. In the open ocean gyres the lack of such a strong
source of turbulence at the thermocline leads to far lower nitrate fluxes of the order
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