Interior mixing and phytoplankton survival in stratified environments - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 231

layers often concentrate on detecting chlorophyll, more detailed assessments of the
community of phytoplankton show different phytoplankton groups exploiting dif-
ferent parts of a pycnocline.
Variations in turbulence, combined with the distribution of nutrients and modifi-
cation of the light spectrum with depth, sets up an array of niches within which
phytoplankton can achieve significant growth and where the community of phyto-
plankton species competes. Grazers and larger marine predators are often seen to be
associated with patches of high phytoplankton biomass. Zooplankton and fish are
able to position themselves within the water column to take advantage of phyto-
plankton layers. Larger predators may be correlated with elevated phytoplankton
biomass, but rather than responding to a series of trophic links between their prey
and the primary production they may instead be taking advantage of localized prey
distributions driven by the interior turbulence.

FURTHER READING

Burchard, H. Applied Turbulence Modelling in Marine Waters, Springer-Verlag,
Berlin-Heidelberg, 2002.
Cullen, J. J., The deep chlorophyll maximum - comparing vertical profiles of chlorophyll a.
Canadian Journal of Fisheries and Aquatic Sciences, 39(5), 791-803, 1982.
Sharples J., et al., Phytoplankton distribution and survival in the thermocline, Limnology
and Oceanography, 46(3), 486-496, 2001.

Problems

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7.1. Estimate the total rate of kinetic energy dissipation per square kilometre
of water in a layer 20 metres thick for (i) the stable pycnocline region
where the mean dissipation 10 5 Wm 3 , and (ii) in the bottom boundary

e ¼

10 3 Wm 3 . Express your results in terms of the number of
hand-held 100W kitchen mixers required to produce an equivalent stirring
effect.

layer where e ¼ 3

7.2. Using the TML model of heating vs. stirring, investigate the dependence of the
amplitude and phase of the annual cycle of heat storage for the following cases:

(i) Strong tidal stirring (log 10 SH ¼ 1 : 5) in a range of water depths e.g. 10, 20,

40, 80, 160 metres.
(ii) Weak tidal stirring (log 10 SH

¼ 3 : 5) in the same range of depths.

Plot amplitude and phase versus depth and explain the form of the curves and
the marked differences between mixed and stratified conditions.