The shelf edge system - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 332

Figure 10.19 The specific uptake rate of
nitrate in recently upwelled water off
the coast of Peru. The different symbols
are for data collected by different
drogue buoys, with all of the
measurements put onto a common time
base with 0 days the time when
upwelled water first reached the
euphotic zone. After MacIsaac et al.
( 1985 ), with permission from Elsevier.

0.12

0.08

0.04

0.0

0 5 10

Days

Californian upwelling (Wilkerson et al., 2006 )), often involving an upwelling-driven
diatom bloom being succeeded by smaller flagellates and dinoflagellates as the
upwelling relaxes (Smayda and Trainer, 2010 ; Mitchellinnes and Walker, 1991 ). This
initial bloom of diatoms, responding to the high nutrient concentrations, is an
example of the opportunistic nature of these relatively large, fast-growing cells, as
discussed in Chapter 5 .
In an environment where the upwelling-favourable winds are more infrequent (e.g.
the time series in Fig. 10.6 ), vertical turbulent mixing will be a relatively more
important part of any biogeochemical response. A short burst of winds may bring
the pycnocline closer to the sea surface, but as the winds relax the pycnocline will
return towards its previous depth. In order to produce a significant response in the
primary production, the necessary net transfer of nutrients into the photic zone
requires mixing across the pycnocline (Hales et al., 2005 ).

Secondary production and fish
So far we have focused on the autotrophic community of photosynthesising phyto-
plankton which require the upwelled nutrients. Populations of heterotrophic phyto-
plankton are often much larger (Joint et al., 2001 ), and they can have an important
grazing impact on the diatoms (Kiørboe et al., 1998 ). However, as we described in
Chapter 5 , these microzooplankton grazers respond on the same time scales as those
on which the photosynthetic phytoplankton respond to the supplies of nutrients and
light. How do the larger grazers, such as copepods, fish larvae and the planktivorous
fish, which have longer response times to changes in their environment, cope with the
variability of food supply?
Copepods usually dominate the zooplankton communities in upwelling systems.
They have life cycles with intervals of several days to weeks, depending on species,
between eggs being produced, hatching, and adulthood. This delay between egg
production and the appearance of the adults will decouple copepod numbers from
the conditions that triggered their initial production. We have noted similar decoup-
ling in the context of the lack of a clear copepod response to shelf sea fronts in