Introduction to the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 3

Figure 1.2 (a) The area of
seabed in different depth
ranges; if shelf seas are taken to
be less that 500 metres depth
they cover 9% of the ocean's
total area. (b) Net primary
production (gigatonnes of
carbon per year) within
different depth ranges; water
shallower than 500 metres
accounts for 16% of global
production. The plots are
based on the data in Fig. 1.1 .

(a) Shelf

8

6

4 9%

2

0

0 1000 2000 3000 4000 5000

(b)

8

16%

6

4

2

0

0 1000 2000 3000

Depth (m)

4000 5000

As a result of these external inputs and the direct action of the local wind at the
surface, the shelf seas are physically energetic areas with vigorous stirring. They also
receive large inputs per unit volume of solar energy which act to modify the density
and thus create horizontal density gradients. Similarly, freshwater river discharge
from the adjacent land, as well as rainfall, lower the density near the coast and thus
contribute to density forcing of the shelf seas, which we will describe in Chapter 9 .
Because of the strength and variety of the forcing, the shelf seas are, in many ways,
the most dynamic regions of the ocean and are host to much of the biogeochemical
action. They play a major role in the growth of phytoplankton which constitutes the
primary production of the oceans. A recent collation of available observations
has suggested the annual primary production within the world's shelf seas is about
11Gt C a 1 (Jahnke, 2010 ), which compares with a global total of between 45 and
60 Gt C a 1 (Longhurst et al., 1995 ; Behrenfeld et al., 2005 ). The distribution of net
phytoplankton production in the ocean can be estimated from satellite imagery
(Behrenfeld and Falkowski, 1997 ), though care needs to be taken in accounting
for the contribution from subsurface phytoplankton growth (invisible to a satellite
sensor) and the difficulty of linking ocean colour to chlorophyll concentrations in
coastal regions where suspended sediments and dissolved organic material can influ-
ence the data (Longhurst et al., 1995 ) . Notice in the example of such a satellite-based
estimate in Fig. 1.1b that the highest values of annual carbon fixation are located in
narrow bands around the continents. Fig. 1.2b provides an analysis of that data
showing that we can attribute 16% of global marine primary production to the shelf

seas. That amount of production occurs in 5% of the ocean surface sampled by the
satellite (note in Fig. 1.1b that the satellite coverage misses the high latitudes, including

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