Life in the shelf seas - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 143

Figure 5.7 Results from a numerical
model investigation of the depth or
turbulent mixing requirements for
bloom development. Blooms can
develop in all of the white areas.
After Huisman et al., 1999 , with
permission from ASLO.

Critical
depth

10 -1

No blooms

Critical
mixing

10 -3

Blooms

10 -5

10 -7

0.1 1 10 100 1000

Depth (metres)

now, remember from Section 4.4 that vertical turbulent mixing will be suppressed in
parts of the water column where there is a strong vertical density gradient. The
development of a thermocline will limit the vertical transport of phytoplankton cells.
The key point of Sverdrup's hypothesis is that if a thermocline develops above the
phytoplankton critical depth, then the cells in the mixed layer above the thermocline
will be able to grow and accumulate biomass.
Now think about phytoplankton cells in a mixed water column, but one in which
the strength of the mixing is very weak. The water column may be deeper than the
critical depth, but the weak turbulence could result in the residence time of the
phytoplankton above the critical depth being long enough to allow growth, and so
the phytoplankton population could accumulate biomass in the upper water column.
This idea has been formulated as the critical mixing hypothesis: a bloom can develop in
a mixed water column if the timescale for phytoplankton growth is less than the cells'
residence time in the portion of the water column where net growth can be achieved
(Huisman et al., 1999 ). This concept is particularly pertinent to shelf seas as the
proximity of the seabed provides a limit to the deep mixing of the phytoplankton cells.
We can estimate this residence time as h crit = K z , with h crit the critical depth. The roles of

critical depth and critical mixing in bloom formation are summarised in Fig. 5.7 based
onmodel experiments. At high levels of vertical turbulent mixing, a bloomwill develop
when the depth of water column, or mixed layer, decreases below the critical depth.
Alternatively, in a deep water column a bloom can develop if the level of vertical
mixing drops below some critical value. The common feature of both hypotheses is a
mechanism that allows phytoplankton to receive sufficient light for enough time to
allow them to grow.

5.1.6 Nutrient requirements and nutrient sources

Within the phytoplankton cell, nutrients are required in order to synthesise amino
acids and nucleic acids (DNA and RNA) (see Falkowski and Raven, 2007 ). The
principal requirements are for nitrogen and phosphorus, with silica also needed by