Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 191

6.3 Primary production in seasonally stratifying shelf seas

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The tides impose an almost fixed pattern of stirring on the shelf that is dictated by the
dynamical response of the shelf to forcing at the shelf edge imposed by the oceanic
tidal wave. We have just seen how this pattern is reflected in the physical partitioning
of the shelf into regions that experience seasonal stratification and those that remain
vertically mixed throughout the year. We will now look at how the primary produ-
cers respond to the seasonal cycle of thermal stratification, addressing the phenom-
enon of the spring bloom and the pivotal role of the thermocline in inhibiting vertical
turbulent mixing.

6.3.1 The spring bloom

One of the most important events in the annual cycle of primary production of
temperate shelf seas is the sudden flush of phytoplankton growth in spring known as
the spring bloom. The rapid growth of near-surface phytoplankton provides the
first substantial food supply of the year to much of the rest of the marine food chain.
The development of the bloom is dependent on the changes in the vertical profile
of turbulent mixing in the water column in response to the development of stratifica-
tion. These changes in the physical environment control the availability of resources
(light and nutrients) to the phytoplankton. This important link between the physical
environment and the growth of the phytoplankton was first recognised by G. A. Riley
in the early 1940s (Riley, 1942 ), and led to Sverdrup's formulation of the concept of
the critical depth (Sverdrup, 1953 ) that we described in Section 5.1.5 .
As we have seen already, during winter the water column of most temperate
shelves is kept vertically homogeneous by convective overturning, as the surface
ocean loses heat to the atmosphere. This convective overturning is a whole-depth
turbulent motion that maintains vertical homogeneity of water constituents, includ-
ing phytoplankton cells and inorganic nutrients. The vertical movement of the
phytoplankton cells between sea surface and seabed combined with weak winter
sunlight leads to the phytoplankton not receiving enough light to allow them to
grow significantly. The phytoplankton will still try to grow, but the photosynthesis
that they can achieve while close to the sea surface cannot compete against the losses
due to respiration and mortality in the darker bottom water and the impacts of
grazing. In other words, the critical depth for positive net phytoplankton growth
lies somewhere within the interior of the water column, and the turbulent mixing
prevents the phytoplankton from staying above that critical depth.
As the sun's elevation increases through late winter and spring, the water
column eventually begins to receive more heat than it loses back to the atmosphere.
Initially the tidal (and wind) mixing may prevent a warm surface layer from becom-
ing established; notice in Fig. 6.8a how the water column begins to warm when the
heat flux switches to net warming, but that there is a delay before stratification
begins. In many areas the condition described in Equation (6.18) eventually is met.