Geoscience Reference
In-Depth Information
heterotrophs live at very low Reynolds numbers. Remember from Chapter 4 that we
defined the Reynolds number as:
flow speed
length scale
ð
5
:
15
Þ
with n the kinematic viscosity of seawater (about 10 6 m 2 s 1 ). A 10
m cell swimming
at 0.1 mm s 1 would have a Reynolds number of 10 3 . By contrast, a 10 cm fish
swimming at O(1 m s 1 ) has a Reynolds number of 100 000. The practical result of this
is that a fish (or a human) swims through water by pushing water in the opposite
direction to their motion (an example of conserving linear momentum). A plankton
instead grabs hold of the water and pulls itself past it (and then has the tricky problem
of how to release the water in order to prepare for the next 'stroke'). In the case of a
flagellum the cell is not so much swimming as twisting itself through the viscous water,
similar to when you drive a screw into a block of wood. Only at the upper end of the
plankton size range are some organisms able, at least temporarily, to shift themselves
out of the control of fluid viscosity. As an escape response to an approaching predator,
a copepod (e.g. Fig. 5.13d ) can put on a burst of speed of a few 10s of mm s 1 , briefly
raising the Reynolds number to 100 or so (van Duren and Videler, 2003 ).
A good deal of attention has been focused on the role that turbulence might play in
altering the encounter rates between predators and prey. Imagine a completely
quiescent water column with non-motile predators and prey suspended within it;
encounters between the predators and their prey would be rare. If we add some
turbulent mixing into the water, then we would expect predators and prey to
encounter each other much more often. So, as we gradually increase turbulence we
might expect predator-prey encounter rates to increase. That is half of the story.
During an encounter, a prey particle needs to remain in detection range of the
predator long enough for the predator to respond and attempt to catch the particle.
Also, once a prey item has been caught it needs to be hung on to and transferred to
the predator's mouth. So, increasing turbulence might be expected to have a negative
impact on feeding rate. Figure 5.17 summarises the net effect of these processes,
based on a modelling study of cod larvae feeding on copepod nauplii (Mackenzie
et al., 1994 ). Figures 5.17a and 5.17b show the separate effects of turbulence on
encounter rates and on catch success, with Fig. 5.17c showing the classic dome-
shaped response when the two separate processes are combined. Notice the wind
speeds in Fig. 5.17a , based on the wind required to generate the turbulent velocities at
a depth of 20 metres in the surface mixed layer. Gale force winds have a speed of
about 20 m s 1 , so the peak in successful predation occurs at typical wind speeds over
the ocean. If we equate wind stress with a stress generated at the seabed by a current,
then a 20 m s 1 wind is about the same as a 0.5 m s 1 current. So moderate tidal flows
can lead to an increase in successful predation, but stronger tides will hinder feeding.
Further work has shown that the effect of turbulence depends on the feeding strategy
employed by the predator, and that for realistic values of turbulence the enhance-
ment of predator-prey interactions only works for predators that are close to the
Kolmogorov lengthscale (see Section 4.4.3 ) (Kiørboe and Saiz, 1995 ).
m
Search WWH ::




Custom Search