164 Ecology and Applied Environmental Science
fix atmospheric molecular nitrogen; for them, therefore, the limiting factor
would be phosphorus or some other element (e.g. iron) or even the intensity
of solar radiation.
Unlike what occurs during photosynthesis, during the process of algae
respiration O 2 is consumed and CO 2 is liberated into the water. The rate of
photo synthesis depends on the intensity of the radiation, which decreases with
depth. For a critical value of intensity (about 10 × 10 2 erg/(cm 2 sec)), the rate of
photosynthesis of algae is just enough to produce the amount of O 2 consumed
in the respiration of both producers and consumers. The depth below the
surface at which this phenomenon occurs is called the compensation point.
For intensities of radiation lower than the critical intensity, the amount of O 2
dissolved in the water tends to decrease due to predominance of respiration
over photosynthesis, and for intensities higher than the critical intensity the
water is enriched with O 2 . The layer of water above the compensation point
is called the euphotic zone. The oxygen produced in this zone exceeds that
consumed by the photosynthetic organisms in the aquatic environment.
Eutrophication is natural in some cases. As a rule, however, it is caused
by human activities and constitutes a serious disturbance of aquatic ecosystems. Its main adverse consequences are the following:
• Oxygen depletion: Excessive production of organic matter in the form
of algae may cause severe oxygen depletion in the receiving water.
When there is less sunlight (e.g. at night), photosynthesis decreases,
and as a result the demand for O 2 for the respiration of the existing
quantity of algae and other organisms is greater than the production of O 2 . If the phenomenon lasts for a significant amount of time,
it may lead to oxygen depletion. In addition, when the algae die, their
protoplasm is used as food by aerobic heterotrophs that consume
oxygen . The nutrient elements (mainly N and P) from the dead cells
of the algae are liberated into the water, and as a result the phenomenon of eutrophication becomes self-perpetuating. Excessive growth
of algae may lead to the creation of a thick layer at the surface. This
layer of algae impedes the transfer of atmospheric O 2 to the water
(lower oxygenation coefficient) and prevents sunlight from passing
through, resulting in a decrease in the photosynthetic activity of the
algae underneath. Thus the danger of oxygen depletion increases.
• Less clarity: Excessive concentrations of algae increase water turbidity,
and this may have extremely adverse effects on the aesthetic value of
recreational waters. This phenomenon is of particular importance for
tourist coastal areas that are attractive for their crystal-clear waters.
• Emergence of toxicity: It has been observed that certain (fresh water
or seawater) algae may cause illness in humans and animals due to
the toxic substances they produce. Cases of poisonings have been
fix atmospheric molecular nitrogen; for them, therefore, the limiting factor
would be phosphorus or some other element (e.g. iron) or even the intensity
of solar radiation.
Unlike what occurs during photosynthesis, during the process of algae
respiration O 2 is consumed and CO 2 is liberated into the water. The rate of
photo synthesis depends on the intensity of the radiation, which decreases with
depth. For a critical value of intensity (about 10 × 10 2 erg/(cm 2 sec)), the rate of
photosynthesis of algae is just enough to produce the amount of O 2 consumed
in the respiration of both producers and consumers. The depth below the
surface at which this phenomenon occurs is called the compensation point.
For intensities of radiation lower than the critical intensity, the amount of O 2
dissolved in the water tends to decrease due to predominance of respiration
over photosynthesis, and for intensities higher than the critical intensity the
water is enriched with O 2 . The layer of water above the compensation point
is called the euphotic zone. The oxygen produced in this zone exceeds that
consumed by the photosynthetic organisms in the aquatic environment.
Eutrophication is natural in some cases. As a rule, however, it is caused
by human activities and constitutes a serious disturbance of aquatic ecosystems. Its main adverse consequences are the following:
• Oxygen depletion: Excessive production of organic matter in the form
of algae may cause severe oxygen depletion in the receiving water.
When there is less sunlight (e.g. at night), photosynthesis decreases,
and as a result the demand for O 2 for the respiration of the existing
quantity of algae and other organisms is greater than the production of O 2 . If the phenomenon lasts for a significant amount of time,
it may lead to oxygen depletion. In addition, when the algae die, their
protoplasm is used as food by aerobic heterotrophs that consume
oxygen . The nutrient elements (mainly N and P) from the dead cells
of the algae are liberated into the water, and as a result the phenomenon of eutrophication becomes self-perpetuating. Excessive growth
of algae may lead to the creation of a thick layer at the surface. This
layer of algae impedes the transfer of atmospheric O 2 to the water
(lower oxygenation coefficient) and prevents sunlight from passing
through, resulting in a decrease in the photosynthetic activity of the
algae underneath. Thus the danger of oxygen depletion increases.
• Less clarity: Excessive concentrations of algae increase water turbidity,
and this may have extremely adverse effects on the aesthetic value of
recreational waters. This phenomenon is of particular importance for
tourist coastal areas that are attractive for their crystal-clear waters.
• Emergence of toxicity: It has been observed that certain (fresh water
or seawater) algae may cause illness in humans and animals due to
the toxic substances they produce. Cases of poisonings have been