The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 340

Mixing nutrients towards the sea surface, and the apparent increase in sea surface
chlorophyll seen in observations such as in Fig. 10.16 and Fig. 10.24 , suggests that
the shelf edge should be found to be a region of significantly enhanced primary
production. However, we need to be careful in making the leap from biomass to
production. The mixing that supplies the nutrients to the surface will also be redis-
tributing the chlorophyll, so the surface chlorophyll could simply be the result of
upward turbulent transport of chlorophyll from within the thermocline. Also, while
mixing might be thought to promote phytoplankton growth by supplying nutrients,
it will also hinder photosynthesis by disrupting the light regime experienced by the
phytoplankton. Remember also that when interpreting satellite images we have no
information in the images about what is happening beneath the sea surface. Meas-
urements of carbon fixation rates at the Celtic Sea shelf edge at neap tides have been
found to be 400-800 mg C m 2 d 1 , and 300-600 mg C m 2 d 1 at spring tides
(Sharples et al., 2007 ). In both cases the range shows the effect of typical cloudy and
sunny days, while the lower rates at spring tides could be an indication of the effect of
increased mixing on the light received by the phytoplankton. These rates are about a
factor of 2 greater than the rates of carbon fixation on the adjacent shelf and in the
open ocean (Sharples et al., 2009 ).

10.8.2 Phytoplankton community gradients across the shelf edge

While the shelf edge primary production rates are increased above those on either
side, a more dramatic contrast is seen when we consider the detailed characteristics of
the shelf edge productivity. Measurements of nutrient assimilation in the region have
shown that the shelf edge band of elevated chlorophyll is associated with an f-ratio of
about 0.7, compared to typically 0.3 away from the shelf edge (Joint et al., 2001 ),

indicating that the shelf edge primary production increase is the result of new
production responding to the supply of deep nitrate. So how is this injection of
new nutrients affecting the phytoplankton community structure?
A detailed picture of the chlorophyll distribution and the phytoplankton commu-
nity structure across the Celtic Sea shelf edge is shown in Fig. 10.25 , based on data
collected on a research cruise aboard the RRS Charles Darwin in 2005 (Sharples
et al., 2009 ). This was the same cruise that collected the data in Fig. 10.12d . Fig.
10.25a shows a subsurface chlorophyll peak associated with the thermocline either
side of the shelf edge. At the shelf edge the effects of internal tide mixing are evident,
with a broader thermocline and cooler, high chlorophyll water outcropping at the sea
surface corresponding to the typical surface satellite imagery of temperature and
chlorophyll (e.g. Fig. 10.16 ). Figure 10.25b shows that when considering all of the
chlorophyll in the upper 100 metres of the water column there is no clear evidence
that the shelf edge has higher phytoplankton biomass than anywhere else on the
section. Interesting contrasts begin to appear when we consider what the chlorophyll
is comprised of. Figure 10.25c shows that the shelf edge phytoplankton population
contains proportionally more diatoms than either the shelf or the open ocean, and
that 60% of the chlorophyll is held within phytoplankton cells greater than 5

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