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(a)
0.4
0
-0.4
April 23 April 30
May 7
May 14 May 21
(b)
0
12
6.25
-20
8
-40
4
-60
0
April 23 April 30
May 7
May 14 May 21
Date 1983
Figure 6.12
See
colour plates version
of (b). The potential for the interruption of the spring
bloom caused by the spring-neap tidal cycle, based on model results for the northeastern North
Sea in 1983. (a) Tidal current velocity, with neap tides on April 24 and spring tides on May 3;
(b) Depth-time series of temperature (line contours) and chlorophyll (colours). Adapted from
(Sharples et al.,
2006
), with permission from Elsevier.
6.3.4
Surface layer phytoplankton after the spring bloom
What happens to the phytoplankton production and population in the surface layer
after the spring bloom has utilised all of the available nutrients? A typical summer
vertical profile of nutrients in a stratified shelf sea shows nitrate to be a limiting nutrient
in surface waters (e.g. Fig. 5.8), so how does this influence (e.g. Fig. 5.8) primary
production? The source of nitrogen for growth switches from nitrate to ammonium,
with the ammonium supplied by the action of the microheterotrophs (ciliates
and flagellates) and bacteria recycling organic matter. During this post-bloom period,
there is an equilibrium in which the uptake rate of ammonium balances the rate at
which ammonium is produced (Le Corre et al.,
1993
). The system becomes almost
entirely dependent on a supply of locally regenerated nitrogen. In our own work off the
southwest of the United Kingdom, we often find the summer surface layer to have
the clear blue-ness of the open ocean, andmany of the attributes of the surface waters in
the an oligotrophic subtropical gyres, including low attenuation of PAR (typically
0.08-0.09m
1
) and low f-ratios. The phytoplankton population in this nutrient-starved
surface layer shifts from the large cells of the spring bloom to much smaller cells (e.g.
picoeukaryotes and the cyanobacterium Synechococcus) as size becomes an important
factor in the competition for the limited supply of nitrogen.
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