Environmental Microbiology of Aquatic and Waste Systems

6.2 Some Properties of Sea Water 129

water column. Warmer water is less dense and thus
tends to stay on top of colder water. During winter,
storm winds mix the water column, and the tempera-
ture is more uniform in the top – several 100 m. As
spring approaches, increasing solar radiation warms
the surface waters and this warmer, buoyant water
stays on top. This increases the stability of the water
column, preventing deeper, nutrient-rich water from
being mixed into the surface from below. The stable
surface layer keeps the planktonic cyanobacteria and
algae near the surface where there is plenty of light.
Under this situation, nutrients brought to the surface
by winter mixing encourages the rapid growth of the
organisms and spring bloom may occur.
Marine organisms are also adapted to live at differ-
ent temperatures, which can thus determine the diver-
sity or the numbers of organisms at different levels of
the water column. As temperature changes with season

and location, the diversity and numbers of organisms

also change.

Temperature data are recorded in situ with instru-

mentation lowered from a ship; the temperature is

instantaneously recorded at various depths of the water

column (Anonymous 2010b).

6.2.3 Light

Light has a major influence on the distribution of pho-

tosynthetic organisms. Light is generally limited to the

upper 150–200 m or the photic region. Blue light has

the deepest penetration in sea water and photosynthetic

organisms at the lowest level of the photic region have

mechanisms with which they are able to collect blue

light. The relationship between light in the ocean with

temperature and depth are depicted in Fig. 6.4.

Aphotic

zone

Photic

zone

0

5

Light

Metalimnion

(thermocline)

Temporary

thermoclines

Epilimnion

0 5 10 15 20

0.1 50 100

Incident light l 0 , %

Temperature, °C

Temperature

Hypolimnion

10

15

Compensation

point for

photosynthesis

20

Depth, m

Fig. 6.4 Relationship between light, temperature and increasing depth in the ocean (Modified from http://www.windows2universe.
org/earth/Water/temp.html; Anonymous 2010b)