Box 14.6 (continued)
mixing in spring and autumn); (3) polymictic lakes
(several successive annual periods of mixing and
stratification); (4) meromictic lakes in which the
upper layers of water are never mixed with the deep
layers.
Lakes also present a stratification of oxygen
concentrations that is the consequence of a dynamic
balance of gas exchanges between atmosphere, photosynthesis, and respiration of organisms. The stratification can be temporary like in dimictic lakes where the
deep water layer is anoxic in summer time or permanent as in meromictic lakes where the hypolimnion is
always anoxic. The mixing of water is a major event in
the life of lakes because at the stratification of abiotic
factors due to stratification matches a stratification of
biological processes involved in biogeochemical
cycles.
Major biotic components of freshwater lake
ecosystems. The organisms are distributed in three
main zones. The littoral zone is colonized by the
primary producers that include submerged or floating
high-sized plants (the macrophytes: hydrophytes
and helophytes) and various types of aquatic metazoa.
The macrophytes belong to Streptobiontes and
Chlorobiontes (cf. Sect. 7.5.5); they can be attached
to different types of surfaces (periphyton). The pelagic
zone harbors the plankton and the nekton. As in
oceans, the plankton comprises the zooplankton and
the phytoplankton. The phytoplankton is constituted
by oxygenic photosynthetic prokaryotes (cyanobacteria), by photosynthetic eukaryotes (diatoms,
Chlorophyceae) and by non-photosynthetic eukaryotes
such as Chytridiomyctes (Fungi). The zooplankton
is composed of eukaryotes belonging to a large diversity of taxonomic units (ciliates, rotifers, crustaceans
such as cladocerans and copepods). It also includes
insect larvas. The nekton includes animals that
are able to freely swim (fishes, amphibians). The
benthic zone comprises organisms living on or nearby
the bottom of the lake (insect larvas, molluscs,
worms, crustaceans). According to the trophic level
(mineral and intact organic matter concentrations),
different types of lakes are distinguished: the oligotrophic lakes that are poor in biogenic mineral
materials and in plankton, the eutrophic lakes that are
rich in nutrients and the mesotrophic lakes that represent an intermediate between oligotrophic and
eutrophic lakes.
(continued)
Box 14.6 (continued)
Forms of Carbon in Lakes
• Inorganic carbon is the result of both physical
(exchanges of CO 2 with atmosphere and dissolution from chalky rocks) and biochemical processes
(primary production and respiration). The atmospheric CO 2 , very soluble in water, is hydrated to
form carbonic acid (H 2 CO 3 ) that is then dissociated
in bicarbonate (HCO 3
À ) and carbonate (CO 3
À
)
according to the pH. In most of the freshwater
ecosystems the pH varies from 6 to 9, so the bicarbonate is usually dominant. The carbonic acid
attacks the superficial rocks, and transforms the
calcium carbonate (CaCO 3 ) in calcium bicarbonate
[Ca(CO 3 ) 2 ] that is more soluble.
• Organic carbon is located in the dissolved organic
matter (DOM) and in the particulate organic matter
(POM) and constitutes the dissolved organic carbon
(DOC) and the particulate organic carbon (POC),
respectively. In lacustrine systems, the DOC
storage is 5–10 times higher than that of POC and
remains more stable with time. The POM and DOM
origin can be autochthonous or allochthonous.
The autochthonous POM comprises the living
organisms mentioned above and dead organic matter. The living fraction usually represents only a
small part of total organic carbon. DOM storage is
supplied by the compounds excreted by organisms
and by the intermediary products of microbial degradation. In non-anthropic lakes, the allochthonous
organic matter is brought by interstitial and run-off
water. When the basin side is wooded and is of
small surface, leaves are one of the main source of
this type of organic matter. In most of cases, this
matter arrives in the dissolved form, but the
allochthonous POC can be dominant in strongly
eroded basins. The allochthonous organic matter
is essentially composed of recalcitrant compounds
(lignin). DOM and POM are constituted of complex
but non-well characterized molecules.
Food webs. In pelagic zones of lakes, carbon and
energy circulates within a complex web of interactions
between the organisms and their biotopes, and between
organisms themselves. This web is summarized in
Box Fig. 14.11: The organic carbon (primary production) is synthetized from mineral carbon (CO 2 ) by photosynthesis performed by macrophytes, phytoplankton
and at a lesser extent by photo-and chemosynthetic
bacteria. Some heterotrophic bacterial species are also
(continued)
552
J.-C. Bertrand et al.
mixing in spring and autumn); (3) polymictic lakes
(several successive annual periods of mixing and
stratification); (4) meromictic lakes in which the
upper layers of water are never mixed with the deep
layers.
Lakes also present a stratification of oxygen
concentrations that is the consequence of a dynamic
balance of gas exchanges between atmosphere, photosynthesis, and respiration of organisms. The stratification can be temporary like in dimictic lakes where the
deep water layer is anoxic in summer time or permanent as in meromictic lakes where the hypolimnion is
always anoxic. The mixing of water is a major event in
the life of lakes because at the stratification of abiotic
factors due to stratification matches a stratification of
biological processes involved in biogeochemical
cycles.
Major biotic components of freshwater lake
ecosystems. The organisms are distributed in three
main zones. The littoral zone is colonized by the
primary producers that include submerged or floating
high-sized plants (the macrophytes: hydrophytes
and helophytes) and various types of aquatic metazoa.
The macrophytes belong to Streptobiontes and
Chlorobiontes (cf. Sect. 7.5.5); they can be attached
to different types of surfaces (periphyton). The pelagic
zone harbors the plankton and the nekton. As in
oceans, the plankton comprises the zooplankton and
the phytoplankton. The phytoplankton is constituted
by oxygenic photosynthetic prokaryotes (cyanobacteria), by photosynthetic eukaryotes (diatoms,
Chlorophyceae) and by non-photosynthetic eukaryotes
such as Chytridiomyctes (Fungi). The zooplankton
is composed of eukaryotes belonging to a large diversity of taxonomic units (ciliates, rotifers, crustaceans
such as cladocerans and copepods). It also includes
insect larvas. The nekton includes animals that
are able to freely swim (fishes, amphibians). The
benthic zone comprises organisms living on or nearby
the bottom of the lake (insect larvas, molluscs,
worms, crustaceans). According to the trophic level
(mineral and intact organic matter concentrations),
different types of lakes are distinguished: the oligotrophic lakes that are poor in biogenic mineral
materials and in plankton, the eutrophic lakes that are
rich in nutrients and the mesotrophic lakes that represent an intermediate between oligotrophic and
eutrophic lakes.
(continued)
Box 14.6 (continued)
Forms of Carbon in Lakes
• Inorganic carbon is the result of both physical
(exchanges of CO 2 with atmosphere and dissolution from chalky rocks) and biochemical processes
(primary production and respiration). The atmospheric CO 2 , very soluble in water, is hydrated to
form carbonic acid (H 2 CO 3 ) that is then dissociated
in bicarbonate (HCO 3
À ) and carbonate (CO 3
À
)
according to the pH. In most of the freshwater
ecosystems the pH varies from 6 to 9, so the bicarbonate is usually dominant. The carbonic acid
attacks the superficial rocks, and transforms the
calcium carbonate (CaCO 3 ) in calcium bicarbonate
[Ca(CO 3 ) 2 ] that is more soluble.
• Organic carbon is located in the dissolved organic
matter (DOM) and in the particulate organic matter
(POM) and constitutes the dissolved organic carbon
(DOC) and the particulate organic carbon (POC),
respectively. In lacustrine systems, the DOC
storage is 5–10 times higher than that of POC and
remains more stable with time. The POM and DOM
origin can be autochthonous or allochthonous.
The autochthonous POM comprises the living
organisms mentioned above and dead organic matter. The living fraction usually represents only a
small part of total organic carbon. DOM storage is
supplied by the compounds excreted by organisms
and by the intermediary products of microbial degradation. In non-anthropic lakes, the allochthonous
organic matter is brought by interstitial and run-off
water. When the basin side is wooded and is of
small surface, leaves are one of the main source of
this type of organic matter. In most of cases, this
matter arrives in the dissolved form, but the
allochthonous POC can be dominant in strongly
eroded basins. The allochthonous organic matter
is essentially composed of recalcitrant compounds
(lignin). DOM and POM are constituted of complex
but non-well characterized molecules.
Food webs. In pelagic zones of lakes, carbon and
energy circulates within a complex web of interactions
between the organisms and their biotopes, and between
organisms themselves. This web is summarized in
Box Fig. 14.11: The organic carbon (primary production) is synthetized from mineral carbon (CO 2 ) by photosynthesis performed by macrophytes, phytoplankton
and at a lesser extent by photo-and chemosynthetic
bacteria. Some heterotrophic bacterial species are also
(continued)
552
J.-C. Bertrand et al.