The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 333

Figure 10.20 Production of
phytoplankton carbon and copepod
eggs in the Benguela upwelling system.
The shaded bar on the date axis
indicates when upwelled water
arrived at the fixed sampling station.
a Phytoplankton growth rate (solid
circles) and phytoplankton biomass
(open circles); (b) Copepod fecundity
(egg production rate per female).
Adapted from Armstrong et al. ( 1991 ),
with permission from Elsevier.

(a)

4

160

3

120

2

80

1

40

8 10 12 14

(b)

40

30
20
10

0

8 10 12 14

Date April 1987

Section 8.6.3 . The earliest stage of the copepod life cycle can be linked to upwelling.
Consider the example in Fig. 10.20 from the Benguela upwelling system off the
coast of southwest Africa, which shows the initial pulse of phytoplankton growth,
the subsequent increase in phytoplankton biomass and a correlated increase in the
copepod eggs produced per female. Longer term cycles in copepod biomass and
community composition will be the result of adding together a sequence of such
upwelling responses (Peterson and Keister, 2003 ). We can view longer-term vari-
ability in copepod populations as a damped response to changes in the upwelling
region, driven by inter-annual changes in, for instance, the mean winds, the number
of upwelling wind events and the broader oceanographic environment.
In general, larger phytoplankton cells are more efficient at transferring energy
up though the food chain (Legendre and Rivkin, 2002 ), and, in terms of
fisheries, we noted in Chapter 5 that it is recognised that fish larvae and,
planktivorous fish target food particles larger than about 5

m in size (Cushing,
1995 ). We can see, therefore, why diatom-producing upwelling regions support
some of the world's largest commercial fish stocks. Fish species with the highest
biomass in upwelling systems are plankton-eating fish, e.g. anchovies, sardines
and mackerel.

m

Environmental variability and fisheries in upwelling regions
The transfer of energy from sunlight through to the protein harvested by fishing
vessels in upwelling areas is through a very short food chain, with the important
result that the success of the fisheries is very responsive to changes in the primary
production. Inter-annual changes in the oceanographic environment, for instance as
a result of changes to the wind or large-scale basin-wide changes in ocean currents,
can modulate the growth of the phytoplankton and very rapidly (i.e. the same year or
1 year later) affect the success of the fish stocks. The clearest example of a basin-wide
change in the environment having an impact on fisheries is that of the El Nin˜ o