Environmental Engineering Reference
In-Depth Information
Approximate Velocities of Various Organisms in Water
a
TABLE 18.1
Relative velocity
Velocity (m s
1
)
(body lengths s
1
)
Organism
10
6
Desmids
1
0.01
10
6
Amoebae
6
0.03
10
6
Bacteria (gliding)
2.5
0.1
10
5
Diatoms
2.5
0.1
10
5
Cyanobacteria (gliding)
1
0.1
10
5
Cyanobacteria (floating)
2
0.2
Carp
0.4
1
Eels
0.5
1
Trout
4
8
Salmon
6
10
10
4
Ciliate protozoan
5
10
10
6
Bacteria (flagellar movement)
20
20
Copepod
0.1
200
a
Data from various sources; maximum speed generally presented.
spectively. A negative tactic response is called a
phobic
response; for ex-
ample, negative phototaxis is also a photophobic response.
Perhaps the simplest tactic behavior involves moving when the envi-
ronment is not suitable and remaining in place when it is. An example of
this is the formation and synthesis of gas vesicles used for flotation in
planktonic cyanobacteria. Under low light and high nutrients, vesicles are
synthesized. When light is high, synthesis stops (Walsby, 1994). This adap-
tation for movement allows the cyanobacteria to dominate in eutrophic
waters. At the surface of a lake, in a dense cyanobacterial bloom, ample
light may be available from above. However, a large, actively growing pop-
ulation of algal cells will locally deplete nutrient concentrations. In this
case, the synthesis of gas vesicles stops, and the cells sink. At depth, nutri-
ents are high and light is low. Nutrients are assimilated and the rate of gas
vesicle synthesis increases again. Concentration of CO
2
may also be in-
volved in buoyancy regulation, but the specific mechanism is less clear
(Klemer
et al.,
1996). This “behavior” may allow the cyanobacteria to
compete well in eutrophic waters by moving up and down to avoid light
and nutrient limitation, respectively, but field validation of the adaptive
value of this behavior is lacking (Bormans
et al.,
1999).
Geotaxis,
movement with respect to gravity, may be useful for benthic
organisms. These organisms need to remain in the benthos to survive
(Hemmersbach
et al.,
1999). They may seek the bottom by avoiding light,
sensing gravity, or using the earth's magnetic field. Magnetotactic bacteria
(Fig. 18.2) are examples of organisms that use magnetic fields to move
downward (Blakemore, 1982). In the Northern Hemisphere, the electro-
magnetic field toward magnetic north also has a downward component.
Thus, bacteria that move toward the north in the Northern Hemisphere
tend to move down into sediments. In the Southern Hemisphere, benthic
magnetotactic bacteria must move toward the magnetic south to move
down into the sediments.
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