Organisation at the Ecosystem Level 77
4.5 FunctionS oF aquatic EcoSyStEMS
Aquatic ecosystems are distinguished into the marine ones, which include
open and closed seas, and the freshwater ecosystems, which include lakes,
rivers, springs and most of the wetlands. A special case of marine ecosystems
are coastal aquatic ecosystems; they generally are wetlands. Underground
waters are not considered to be autonomous ecosystems. The study of
aquatic ecosystems is especially important for the thorough examination
of most of the biogeochemical cycles as well as the various water pollution
phenomena (Chapter 8).
4.5.1 trophic and other characteristics
of aquatic Ecosystems
The main producer organisms in aquatic ecosystems are algae (phytoplankton); plants with roots are confined in the shallow areas, and for this reason
usually play a secondary role in photosynthesis and therefore in the trophic
chains of aquatic ecosystems; the contribution of some photosynthetic
cyanobacteria is generally marginal.
Consumer organisms are usually ranked in 3−4 trophic levels. Primary
consumers include a great variety of organisms, such as protozoans, jellyfish, molluscs, worms as well as some small fish. The rest of the consumers
are mainly bigger fish, as well as some mammals and birds. The category of
decomposer organisms includes bacteria and fungi.
Aquatic ecosystems’ organisms are distinguished in plankton, nekton
and benthos. Plankton is the total of organisms that float, with a mainly
passive movement caused by waves and currents. It consists of algae
( phytoplankton), herbivorous animals (zooplankton) and decomposers.
Nekton consists of organisms that have an active movement in the water.
Benthos are organisms that live in the bottom of the aquatic space and seek
their food in silt.
Contrary to many terrestrial ecosystems, the availability of nutrients
usually determines the aquatic ecosystems’ productivity. The concentration
of nutrient salts is generally low in not polluted water ecosystems. Typically,
the concentration of nitrates in sea water is only 1/10,000 of their concentration in fertilized soil. For this reason, when pollution adds nutrients to
the water bodies, a dramatic production increase is observed. The phenomenon is called eutrophication and shows the significance of nutrients
as a limiting factor of aquatic primary production. Since the atmosphere
transfers CO 2 to the aquatic ecosystems, carbon as a rule does not constitute a limiting factor for eutrophication. This role is usually claimed by
nitrogen or phosphorus (Chapter 8). Untreated liquid waste and surface
runoffs from fertilized fields usually contain nitrates or phosphates. They
4.5 FunctionS oF aquatic EcoSyStEMS
Aquatic ecosystems are distinguished into the marine ones, which include
open and closed seas, and the freshwater ecosystems, which include lakes,
rivers, springs and most of the wetlands. A special case of marine ecosystems
are coastal aquatic ecosystems; they generally are wetlands. Underground
waters are not considered to be autonomous ecosystems. The study of
aquatic ecosystems is especially important for the thorough examination
of most of the biogeochemical cycles as well as the various water pollution
phenomena (Chapter 8).
4.5.1 trophic and other characteristics
of aquatic Ecosystems
The main producer organisms in aquatic ecosystems are algae (phytoplankton); plants with roots are confined in the shallow areas, and for this reason
usually play a secondary role in photosynthesis and therefore in the trophic
chains of aquatic ecosystems; the contribution of some photosynthetic
cyanobacteria is generally marginal.
Consumer organisms are usually ranked in 3−4 trophic levels. Primary
consumers include a great variety of organisms, such as protozoans, jellyfish, molluscs, worms as well as some small fish. The rest of the consumers
are mainly bigger fish, as well as some mammals and birds. The category of
decomposer organisms includes bacteria and fungi.
Aquatic ecosystems’ organisms are distinguished in plankton, nekton
and benthos. Plankton is the total of organisms that float, with a mainly
passive movement caused by waves and currents. It consists of algae
( phytoplankton), herbivorous animals (zooplankton) and decomposers.
Nekton consists of organisms that have an active movement in the water.
Benthos are organisms that live in the bottom of the aquatic space and seek
their food in silt.
Contrary to many terrestrial ecosystems, the availability of nutrients
usually determines the aquatic ecosystems’ productivity. The concentration
of nutrient salts is generally low in not polluted water ecosystems. Typically,
the concentration of nitrates in sea water is only 1/10,000 of their concentration in fertilized soil. For this reason, when pollution adds nutrients to
the water bodies, a dramatic production increase is observed. The phenomenon is called eutrophication and shows the significance of nutrients
as a limiting factor of aquatic primary production. Since the atmosphere
transfers CO 2 to the aquatic ecosystems, carbon as a rule does not constitute a limiting factor for eutrophication. This role is usually claimed by
nitrogen or phosphorus (Chapter 8). Untreated liquid waste and surface
runoffs from fertilized fields usually contain nitrates or phosphates. They