Photosynthesis
Most of the oxygen in a lake comes from photosynthesis of producers (plants and algae). These
organisms use up carbon dioxide and water during photosynthesis. They also produce oxygen and
organic compounds such as glucose. Photosynthesis occurs only when light and chlorophyll are present.
The oxygen which is produced dissolves in the water.
chlorophyll
glucose + oxygen
Photosynthesis produces oxygen and uses up carbon dioxide.
Respiration
Du
sp
p uc s
c sum s ( m s) us
“ u up” f
s k uc s .
Most decomposers (bacteria and fungi) do the same thing. Respiration gives the organisms the energy
they need for life processes. Respiration produces two by-products, carbon dioxide and water.
Glucose + oxygen
carbon dioxide + water + heat energy
Respiration produces carbon dioxide and uses up oxygen.
Review questions
1. Write a word equation for photosynthesis.
2. Write a word equation for respiration.
The Balance between Photosynthesis and Respiration
Organisms occur throughout a lake-from top to bottom. As a result, respiration takes place throughout
the lake. Therefore oxygen is used up and carbon dioxide is produced at all depths. Often, though,
respiration is greatest near the bottom. This is because dead organisms sink to the bottom where
numerous decomposers feed on the. Thus the lower part of the hypolimnion often has high carbon
dioxide and low oxygen values. This part of the lake is called the tropholytic zone (Troph- means feeding
and –lytic means decomposition). The organisms in this zone feed by decomposing organic matter.
If a lake is oligotrophic (“ ck f
” c
),
c c
c s s
s
p . If,
v ,
k s u p c (“
qu
f
”
u
-rich), the oxygen concentration
decreases a great deal in the bottom of the hypolimnion, that is, in the tropholytic zone.
Oligotrophic Lake
Eutrophic Lake
Littoral
Zone
Limnetic Zone
Producing
Littoral
Zone
Trophogenic Zone
Compensation Depth
Tropholytic Zone
Feeding
Decomposing
Organisms occur throughout a lake-from top to bottom. As a result, respiration takes place throughout
the lake. Therefore oxygen is used up and carbon dioxide is produced at all depths. Often, though,
respiration is greatest near the bottom. This is because dead organisms sink to the bottom where
numerous decomposers feed on the. Thus the lower part of the hypolimnion often has high carbon
dioxide and low oxygen values. This part of the lake is called the tropholytic zone (Troph- means feeding
and –lytic means decomposition). The organisms in this zone feed by decomposing organic matter.
If a lake is oligotrophic (“ ck f
” c
),
c c
c s s
s
p . If,
v ,
k s u p c (“
qu
f
”
u
-rich), the oxygen concentration
decreases a great deal in the bottom of the hypolimnion, that is, in the tropholytic zone.
In some lakes no oxygen occurs in the tropholytic zone. In such lakes the bacteria in the bottom ooze
respire anaerobically (without oxygen). The forms methane
su f (“
– ”) s s.
Methane is the natural gas many people burn in their homes for cooking and heating. You can smell
hydrogen sulfide if you push a stick into the black muck around a pond, lake or marsh.
Although respiration occurs at all depths, photosynthesis does not. Photosynthesis requires light. Light
can penetrate only so far into the water before it becomes too weak to cause much photosynthesis.
Thus oxygen is not given off at all depths. Most of it is produced in an upper layer called the trophogenic
zone. (Troph-means feeding and –genic means producing).the organisms in this zone produce food by
photosynthesis.
Between the trophogenic zone (at the top) and the tropholytic zone (at the bottom) is a zone called the
compensation depth. In this zone oxygen production and use are equal. Or, if you like, carbon dioxide
production and usage are equal. The trophogenic zone produces more oxygen than it uses. In contrast,
the tropholytic zone uses more oxygen that it produces. Therefore, between these zones there will be a
zone where oxygen production and usage are equal. That is, oxygen production compensates for oxygen
usage. Why are carbon dioxide production and usage also equal at the compensation depth?
The trophogenic zone can be divided into two subzones. These are the littoral zone and the limnetic
zone. The littoral zone (littoral means shore) is the region where light reaches the bottom. Thus
submerged plants grow in this zone. Of course, it is normally around the edges of a lake. The limnetic
zone (limn-means lake) is that part of the open water in which effective light penetration occurs.
Phytoplankton are the main producers in this zone. In fact, they are the main producers of the lake.
Note that the bottom of this zone meets the bottom of the littoral zone. Why is this so?
Review questions
3. Describe the tropholytic zone.
4. Compare the oxygen-depth profiles of oligotrophic and eutrophic lakes.
5. What happens when the tropholytic zone runs out of oxygen?
6. Describe the trophogenic zone.
7. What does compensation depth mean?
8. Describe the two subzones of the trophogenic zone.