Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 192

The resulting appearance of an initially weak thermocline, separating the warming
surface layer from the colder, deeper water, is the key to the spring phytoplankton
bloom. Thermoclines inhibit the vertical turbulent transfer of water and its constitu-
ents between the surface and bottom layers (see Turner's experiment described
in Section 6.1.1 ). 2 As stratification develops, the phytoplankton are split into two
separate communities. The 'luckier' cells are in the surface water as the thermocline
develops, and so they become trapped in the new surface mixed layer. As long as the
thermocline develops above the phytoplankton critical depth, then the surface layer
phytoplankton will now receive enough light to achieve net photosynthesis exceeding
the losses due to respiration and grazing. Growth rates are initially not reduced by a
lack of dissolved inorganic nutrients, because, at the onset of stratification, nutrient
concentrations are uniformly high throughout the water column. The less fortunate
phytoplankton cells are trapped within the bottom mixed layer; the amount of light
they receive drops dramatically, and unless they are able to re-access the surface
layer, respiration and grazing losses will gradually reduce their numbers.
The rapid surface growth of phytoplankton is, however, short-lived. The thermo-
cline isolates the surface layer, severely restricting the re-supply of nutrients from the
deep water. The surface layer phytoplankton use up the finite store of nutrients which
was trapped with them above the thermocline, and then their growth becomes
nutrient-limited and the surface population decreases. The rate at which the cell
concentration in the surface layer decreases is partially determined by the mortality
of the nutrient-starved cells, and also by the increasing impacts of the grazers as they
take advantage of this flush of food in the bloom.
This spring pattern of rapid phytoplankton growth, a sharp biomass peak, followed
by a decrease towards a low-biomass surface layer in summer is a well-recognised
feature of temperate and high-latitude oceans. The introduction of in situ measure-
ments of chlorophyll fluorescence as a proxy for phytoplankton biomass in the 1970s
provided a method of recording the development and decay of blooms at high temporal
resolution. An example of a spring bloom in the North Sea, observed by instruments on
a mooring, is shown in Fig. 6.10 . Notice that the bloom peaks when surface-bottom
temperature differences are still only about 0.5 C. The daily signal in the chlorophyll
fluorescence during and after the bloom is a result of non-photochemical quenching
(see Section 5.1.1 ). Regional pictures of the formation and decay of spring blooms are
available from satellite imagery (e.g. Thomas et al., 2003 ).
Due to the sudden high light, and the abundant nutrients, carbon fixation rates
during shelf sea spring blooms can be high. However, the transient nature of the
spring bloom, combined with the difficulty in predicting it, means that there are few
reliable in situ observations of carbon fixation rates during a bloom. 3 Based on
2 It is remarkable how apparently weak the stratification can be and still have an appreciable effect on vertical
turbulent mixing. As a rule of thumb a cross-thermocline temperature difference of only 0.5 C(adensity
contrast of about 0.06 kg m 3 ) is sufficient to severely inhibit mixing and trigger a biological response.
3 The difficulties of observing such a transient phenomenon were underlined for one of us while modelling
the inter-annual variability of primary production off the west coast of New Zealand. The local
biological oceanographers claimed to have never seen a spring bloom despite several 'spring' cruises to