Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 173

6

Seasonal stratification and the spring bloom

We have now provided the foundations in physics and biology that can be used to
understand how shelf sea ecosystems are driven by the underlying physics. In this
chapter we will begin our journey through the links between physics and biology
in shelf seas by identifying the role of physics in partitioning the shelf seas into
biogeochemically contrasting regimes. Recall from Chapter 5 that the vertical struc-
ture of the water column is critical in determining the availability of both light and
nutrients to phytoplankton. In this chapter we first consider the fundamental physical
controls that determine whether or not a region will thermally stratify in spring.
We derive a condition for seasonal stratification to occur which indicates that, during
the summer season, the shelf seas will be dividedintostratifiedandmixedregimes.
We then consider the spring stratification as the physical trigger for the spring bloom
of phytoplankton. This highlights the spatial partitioning into mixed and stratifying
regions as controlling whether or not regions experience a spring bloom.

6.1 Buoyancy inputs versus vertical mixing:

the heating-stirring competition
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The vertical structure of the water column is the result of an ongoing competition
between the buoyancy inputs, due to surface heating and freshwater input on the one
hand and stirring by the tides and wind stress on the other. Our focus here is on
temperate shelf sea regions where the dominant buoyancy input is a seasonally
varying surface heat flux. We will deal with freshwater as a source of buoyancy in
Chapter 9 . During the winter months, when heat is lost from the surface, the
buoyancy term contributes to stirring by increasing surface density and making all
or part of the water column convectively unstable. As a result, apart from limited
areas influenced by large positive buoyancy inputs from estuaries, the shelf seas are
vertically well mixed during the winter months. This vertically mixed regime con-
tinues until the onset of positive heating at, or close to, the vernal equinox (21 March/
September in the northern/southern hemisphere) after which the increasing input
of positive buoyancy tends to stabilize the water column. Whether or not the water
column stratifies is then dependent on the relative strengths of the surface heating