Seasonal stratification and the spring bloom - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 198

An obvious way of triggering further new phytoplankton growth would be to rapidly
but temporarily re-mix the water column with a strong burst of wind, re-supplying
nutrients to the surface water. Subsequent surface heating after the wind event would
then re-stabilise the water column and trigger a bloom analogous to the spring bloom.
With well-established stratification in midsummer, is this feasible? A simple calculation
based on the wind impulse required to homogenise a two-layer shelf sea water column
(see Problem 6.4) suggests that very strong and/or prolonged winds are required. In
temperate shelf seas in summer, we would normally expect such meteorological events
to be very rare. The most likely regions to experience such summer winds are those in
the tropical and subtropical hurricane zones, where there are examples of hurricanes re-
supplying nutrients to the surface water and fuelling post-hurricane phytoplankton
blooms (Fogel et al., 1999 ; Sharples et al., 2001a ; Shi andWang, 2007 ). The example of
a wind mixing event based on mooring data from the northeast shelf of New Zealand
shown in Fig. 6.13 illustrates how the passage of a tropical cyclone completely mixed
the shelf water column. A nitrate analyser attached to the mooring near the sea surface
recorded the mixing of the nitrate profile, and the replenishment of nitrate in the surface
waters ( Fig. 6.13c ). The utilization of the surface layer nitrate took about 2 days as
stratification became re-established after the cyclone. Note the semi-diurnal signal in
the nitrate in Fig. 6.13c . This is caused by an internal tide driving the thermocline (and
nitracline) up and down, a topic which will be addressed in Chapter 10 .

6.4 Primary production in mixed water

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So far we have concentrated on surface phytoplankton growth in regions that
undergo seasonal stratification, where stratification plays the pivotal role in control-
ling primary production by modifying the profile of vertical turbulent mixing. What
happens in the areas of the shelf seas that remain well mixed all year? The main
problem for the phytoplankton here is that they are being continuously mixed from
the sea surface to the seabed. If the depth of the water column is greater than the
critical depth, then we would expect there to be no net phytoplankton growth.

6.4.1 Surface phytoplankton blooms in the absence of stratification

There have been instances where surface blooms appear to have begun prior to the
onset of any stratification. An example fromMassachusetts Bay is shown in Fig. 6.14
where a surface concentration of phytoplankton appeared to develop in contradic-
tion to Sverdrup's critical depth hypothesis. How can biomass become concentrated
in surface waters without the aid of stratification?
This apparent paradox can be resolved by noting the concept of critical mixing
(Huisman et al., 1999 ) that we discussed in Section 5.1.5 . Remember that if we have
estimates of the vertical diffusivity within the surface water of a mixed water column,
K z , and some knowledge of the critical depth of the phytoplankton population, Z cr ,we
can estimate a residence time for cells above the critical depth as T res Z cr =

K z . If this