Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 202

histories. Cell A in Fig. 6.15 spent quite a lot of time near the sea surface, but mostly
during times of low surface irradiance so that it received low light over the whole day.
Cell B spent much more time away from the surface; around noon it did manage to
get to about 10 metres from the sea surface, but the strong light attenuation still
limited the total light it received. Cell C was mixed to the top of the water column
briefly close to noon, and as a result received a blast of very high irradiance only
30 minutes after it had been in total darkness at the bottom of the water column. It is
possible that phytoplankton may bias their acclimation status to cope with the
extremes in light history such as those experienced by cell C in Fig. 6.15 (Moore
et al., 2006 ) .

Summary

..................................................................................
A competition between heating and stirring controls the occurrence of seasonal
stratification of the water column in the shelf seas. When tidal stirring is dominant,
the competition can be conveniently represented in an energetics model which leads
to the parameter Q i h/u 3 where h

depth (m) and u (ms 1 ) is a measure of the tidal
stream velocity. For a region where the net heating rate Q i

¼

constant, control rests
simply with h/u 3 . For large h/u 3 (deep water or weak tidal currents) the water column
will become stratified, while for low values (shallow water or strong tides) it will
remain vertically mixed throughout the season. Hence, during the summer months,
the shelf seas are divided into a two-layer stratified regime and a continually mixed
regime separated by tidal mixing fronts. Wind stirring can also contribute to mixing
the water column and its inclusion in the energetics model improves the fit to

observations of stratification.
The two regimes provide contrasting environments for primary production.
In regions of high h/u 3 the development of stratification is a key event in the growth
of phytoplankton. The new thermocline confines the phytoplankton to the surface
layer where they enjoy high levels of light. Moreover, at the onset of stratification,
nutrients are initially abundant in the surface layer. With neither light nor nutrients
limiting their capacity for growth, the phytoplankton grow rapidly in a spring bloom
which is dominated by large cells, mainly diatoms. Production has a high f-ratio,
utilising the supply of winter nitrate trapped in the surface layer. This bloom
continues for a period during which the nutrients in the surface layers are rapidly
depleted; as well as inhibiting removal of phytoplankton from the surface layer, the
thermocline blocks replenishment of the depleted nutrients in the surface layer by
the still high nutrient pool in the deeper water. Thereafter, there is much reduced
growth in the nutrient-poor surface layers, in which diatoms are replaced by dinofla-
gellates and smaller cells as the surface layer production becomes dominated by
the recycling of organic material; primary production here has low f-ratios. The
onset of stratification and with it the timing of the spring bloom varies considerably
from year to year because of variations in surface heat exchange, wind stirring and
phase of the spring-neap cycle in the tides. In the lower layers of the stratified water