Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 195

in enough light to allow net phytoplankton growth. Such a calm period can
potentially generate thin, short-lived near-surface stratification sufficient to drive
a brief surface bloom of these opportunist phytoplankton. Even without stratifica-
tion, the weak winter sunshine may at least be able to reduce the vigorous
convective mixing in the surface layer, leading to the possibility of turbulence in
the photic zone dropping below the critical mixing threshold (Huisman et al., 1999 )
(see Section 5.1.5 ). Indeed, we could conjecture that such short-lived events of
primary production in winter may be important in establishing the phytoplankton
community that eventually responds to the spring stratification. However, at the
moment the transient, unpredictable nature of the winter meteorology and the
difficulty in observing winter time series in temperate regions means that our
understanding of phytoplankton dynamics prior to the spring bloom is limited.
There are important consequences of spring blooms being dominated by diatoms.
As a source of food, the relatively large size of diatom cells might seem to make them
particularly useful to the rest of the ecosystem. Their silicate armouring tends to limit
grazing by microzooplankton, while the mesozooplankton (e.g. copepods) have the
strength and appendages suitable for cracking open the cells (Smetacek, 2001 ). Thus
direct grazing of diatoms is carried out mainly by organisms that will themselves feed
the higher trophic levels of the ecosystem, rather than by organisms that contribute
more to the re-cycling of organic material. Indeed, the diatoms are generally con-
sidered to provide the basis for food chains that support important commercial
fisheries (Irigoien, Harris, et al., 2002 ). At the same time, however, diatoms tend to
be negatively buoyant, and so the cells in the surface layer may not be available to the
grazers for too long. This downward flux of diatom cells, combined with that of
the rapidly sinking fecal pellets produced by the copepods, could mean that much of
the spring bloom biomass may be exported to the deeper water and the seabed and so
play an important role in supplying the benthos with food (Maar et al., 2002 ) and
exporting carbon. Modelling experiments have suggested that the proportion of the
spring production that reaches the seabed can be increased if the pelagic grazing is
reduced by colder water temperatures (Townsend et al., 1994 ).

6.3.3 Variability in the timing of the spring bloom

With such strong tidal control we might expect the timing of the bloom to be almost
fixed each year. However, year to year variations in weather conditions leading to
differences in the air-sea exchange of heat and in wind stirring provide considerable
potential for inter-annual variability in the timing of the spring bloom. Because of
the bloom's importance as the first supply of significant concentrations of new
organic material for the rest of the marine ecosystem, many organisms have evolved
to time their breeding to take advantage of it. For instance, fish larvae may be
dependent on bloom timing either directly through planktivorous fish larvae feeding
on phytoplankton, or indirectly with larvae feeding on zooplankton that are them-
selves responding to the bloom (Platt et al., 2003 ). Higher trophic levels, dependent