78 Ecology and Applied Environmental Science
often cause pollution to the corresponding water receptors, especially in
lakes and closed marine bays. The phenomena of materials’ transfer from
the terrestrial to the aquatic environment and gases’ exchange between the
atmosphere and water are significant for the aquatic ecosystems’ function.
The availability of basic nutrients is an especially important factor for
lakes; thus, limnologists classify them into the categories of oligotrophic
and eutrophic lakes. In the geological and ecological history of lakes, there
is a general tendency from oligotrophism to eutrophism. As centuries go by,
debris are deposited in the lakes and, according to the nature of the parent
rock, nutrients are continuously accumulated and this favours an increasing
primary production. This natural course has often been accelerated by man
through wastewater disposal as well as the wide use of fertilizers.
Oxygen is also a significant factor of aquatic ecosystems’ function. The
maximum concentration in which it may be diluted in the water is low, one
milligram per litre, while the speed by which it is absorbed by the atmosphere is also small. A usually secondary oxygen contribution comes from
the aquatic producer organisms. Algae, as well as aquatic plants, enrich the
water with oxygen through photosynthesis, while they consume a smaller
quantity of oxygen through respiration. When dissolved oxygen is absent,
aerobic respiration is not possible, and therefore the aquatic environment only can support lower forms of heterotrophic organisms, basically
anaerobic bacteria. Thus O 2 can be the limiting factor for many aquatic
populations, because of aerobic respiration.
4.5.2 temperature layering and Seasonal inversions
In the summer, the phenomenon of temperature layering or stratification
is observed in deep lakes and seas (Figures 4.8 and 4.9). A surface layer of
water, a few meters thick, acquires a relatively high and uniform temperature. This layer is called epilimnion. It is characterized by an almost stable
density and its masses move easily by the wind and transport currents.
Thus the epilimnion tends to maintain a high degree of mixture.
Under the epilimnion we find the thermocline or mesolimnion, a layer of
water of a relatively small thickness, characterized by a rapid decrease of
temperature with the increase of depth.
The water layer found under the thermocline reaches as far as the bottom;
it is called hypolimnion and is characterized by a lower temperature that
slowly decreases as it goes deeper and it can get as low as 4°C, that is, the
temperature of maximum density. The horizontal movements in the hypolimnion are weak, while the vertical are almost non-existent.
In the fall, the surface water masses get colder, and they become more
dense and sink, whereas warmer and thinner water masses rise to the surface. This movement of water masses is called autumn or great inversion
and tends to establish a stable temperature in the whole depth of the water.
often cause pollution to the corresponding water receptors, especially in
lakes and closed marine bays. The phenomena of materials’ transfer from
the terrestrial to the aquatic environment and gases’ exchange between the
atmosphere and water are significant for the aquatic ecosystems’ function.
The availability of basic nutrients is an especially important factor for
lakes; thus, limnologists classify them into the categories of oligotrophic
and eutrophic lakes. In the geological and ecological history of lakes, there
is a general tendency from oligotrophism to eutrophism. As centuries go by,
debris are deposited in the lakes and, according to the nature of the parent
rock, nutrients are continuously accumulated and this favours an increasing
primary production. This natural course has often been accelerated by man
through wastewater disposal as well as the wide use of fertilizers.
Oxygen is also a significant factor of aquatic ecosystems’ function. The
maximum concentration in which it may be diluted in the water is low, one
milligram per litre, while the speed by which it is absorbed by the atmosphere is also small. A usually secondary oxygen contribution comes from
the aquatic producer organisms. Algae, as well as aquatic plants, enrich the
water with oxygen through photosynthesis, while they consume a smaller
quantity of oxygen through respiration. When dissolved oxygen is absent,
aerobic respiration is not possible, and therefore the aquatic environment only can support lower forms of heterotrophic organisms, basically
anaerobic bacteria. Thus O 2 can be the limiting factor for many aquatic
populations, because of aerobic respiration.
4.5.2 temperature layering and Seasonal inversions
In the summer, the phenomenon of temperature layering or stratification
is observed in deep lakes and seas (Figures 4.8 and 4.9). A surface layer of
water, a few meters thick, acquires a relatively high and uniform temperature. This layer is called epilimnion. It is characterized by an almost stable
density and its masses move easily by the wind and transport currents.
Thus the epilimnion tends to maintain a high degree of mixture.
Under the epilimnion we find the thermocline or mesolimnion, a layer of
water of a relatively small thickness, characterized by a rapid decrease of
temperature with the increase of depth.
The water layer found under the thermocline reaches as far as the bottom;
it is called hypolimnion and is characterized by a lower temperature that
slowly decreases as it goes deeper and it can get as low as 4°C, that is, the
temperature of maximum density. The horizontal movements in the hypolimnion are weak, while the vertical are almost non-existent.
In the fall, the surface water masses get colder, and they become more
dense and sink, whereas warmer and thinner water masses rise to the surface. This movement of water masses is called autumn or great inversion
and tends to establish a stable temperature in the whole depth of the water.