Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 200

Figure 6.14 Vertical profiles of density
and chlorophyll from Massachusetts
Bay, showing increased surface
chlorophyll despite the lack of any
clear stratification above the critical
depth, Z cr . From Townsend et al.,
1992 , courtesy Nature Publishing
Group.

t (kg m -3 )

25.0 25.5 26.0 26.5 27.0

-10

-30

Z cr

-50

-70

-90

012345678

Chlorophyll ( μ g l -1 )

vertical extent of the water column within which photosynthesis is possible. In the
Irish Sea, for example, attenuation of PAR in mixed waters can be 0.3-0.4 m 1
(Bowers et al., 1998 ). Phytoplankton growth is, however, still often possible in
summer if the high surface irradiance results in a critical depth that reaches the
seabed. You can investigate this using the physics-primary production model associ-
ated with this topic. The mixed waters of, for instance, Georges Bank and the Irish
Sea often have chlorophyll concentrations of 1-2 mg m 3 . Note that this chlorophyll
concentration is seen throughout the depth (40-70 m on Georges bank, 60-80 m in
the Irish Sea), so the integrated biomass through the whole water column is high
(around 100 mg m 2 ).
The phytoplankton population in mixed environments is invariably observed to be
dominated by diatoms. This is probably because there is usually an abundant supply
of nutrients available from re-mineralisation in bottom sediments so that large,
fast-growing cells with an ability to counter some of the grazing pressures will out-
compete small cells that are better suited to low nutrient environments. One interest-
ing feature of mixed-water phytoplankton is that they tend to have low Chl:Carbon
ratios (

0.02 mg Chl (mg C) 1 ) (Holligan et al., 1984 ). This perhaps seems paradox-
ical. Surely, given the low average light received by a cell as it is mixed from the
surface to the seabed and back again, the photosystem would optimize to the low
light? However, the rapid mixing means that the cells are re-introduced to the surface
of the water column frequently and unavoidably, experiencing low average light but
with intermittent very high light. Figure 6.15 illustrates the light histories of cells

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