55
Dissolved Organic Matter in Natural Waters
C in North America (Morris et al. 1995; Pace and Cole 2002; Hudson et al. 2003;
Wu et al. 2005; Cammack et al. 2004; McKnight et al. 1997; Allard et al. 1994;
Waiser and Robarts 2000; Biddanda and Cotner 2002; Repeta et al. 2002; SchwedeThomas et al. 2005; Curtis and Prepas 1993; Curtis and Schindler 1997; Arts et al.
2000; Biddanda et al. 2001; Kelly et al. 2001; Xenopoulos et al. 2003; Molot et
al. 2004; Brooks et al. 2007). The general results of DOC concentrations studies
in lakes show some characteristic phenomena (Table 2): (i) relatively low DOC
concentrations have been found in Asia, but lakes situated Indonesia and NorthEast China shows high contents of DOC. (ii) Lakes situated in Europe and North
America show relatively high DOC values. DOC concentrations in Saline lakes are
significantly higher than in non-Saline lakes situated in the Prairies and Alberta in
Canada. Boreal lakes often show high contents of DOC. This is interesting because
the boreal region contains roughly 30 % of the global lakes and their water is rich
in OM (Molot and Dillon 1996; Downing et al. 2006; Benoy et al. 2007).
A large lake database (7,514 lakes from 6 continents) shows that mean DOC
concentrations are 632 ± 16 μM C, ranging from 8 μM C to 27667 μM C (Sobek
et al. 2007). In 87 % of the lakes DOC was between 83 and 1667 μM C, whilst
8.3 % of the lakes had concentrations lower than 83 μM C. Lakes between 1667
and 3333 μM C are relatively few (4.2 %), and only 0.4 % of the lakes had DOC
concentrations above 3333 μM C (Fig. 2a). In 55 % of the lakes DOC concentration was above 417 μM C, which is suggested to be a threshold value for the transition between net autotrophy and net heterotrophy in lakes (Sobek et al. 2007;
Jansson et al. 2000; Prairie et al. 2002). Several hypotheses are considered based
on DOC concentration and terrestrial vegetation (Sobek et al. 2007). First, arctic
lakes are generally characterized by low DOC concentrations where land-cover
types are wooded tundra to bare desert and correspond to an annual mean temperature of <−4 °C (Fig. 2b). Second, boreal lakes display a tendency toward higher
DOC concentration if the land-cover types are deciduous and mixed boreal forest
to deciduous conifer forest, which corresponds to an annual mean temperature of
0.5– 4 °C (Fig. 3b). Third, lakes on the northern Great Plains in Saskatchewan
and Canada show the highest DOC concentrations (~80 to ~10500 μM C) and
the land-cover types are cool grasses, shrubs, cool crops and towns (Fig. 3c).
Fourth, Fig. 2b shows that DOC concentrations for lakes situated in the warmer
climate zones (average mean annual temperature >17.4 and <22.3 °C) reach relatively high values (approximately 80–3300 μM C) for land-cover types including
broadleaf crops, corn and cropland and conifer forests (Fig. 2b). Correspondingly,
DOC concentrations for lakes situated in the low warmer climate zones (average
mean annual temperature >8.0 and <13.3 °C) are relatively low (approximately
50–2000 μM C). Here typical land-cover types are crops, mixed and deciduous
broadleaf forests, cool and cool rain forests, grass, and shrubs. Fifth, Fig. 2b suggests that DOC concentrations for lakes that experience relatively low annual
temperature (>1.5 and <4.3 °C) are relatively high (approximately 50–2100 μM
C), and the land-cover types are mostly woods, deciduous and mixed boreal forest, cool mixed forest, conifer boreal forest, narrow conifers and deciduous conifer forests.
Dissolved Organic Matter in Natural Waters
C in North America (Morris et al. 1995; Pace and Cole 2002; Hudson et al. 2003;
Wu et al. 2005; Cammack et al. 2004; McKnight et al. 1997; Allard et al. 1994;
Waiser and Robarts 2000; Biddanda and Cotner 2002; Repeta et al. 2002; SchwedeThomas et al. 2005; Curtis and Prepas 1993; Curtis and Schindler 1997; Arts et al.
2000; Biddanda et al. 2001; Kelly et al. 2001; Xenopoulos et al. 2003; Molot et
al. 2004; Brooks et al. 2007). The general results of DOC concentrations studies
in lakes show some characteristic phenomena (Table 2): (i) relatively low DOC
concentrations have been found in Asia, but lakes situated Indonesia and NorthEast China shows high contents of DOC. (ii) Lakes situated in Europe and North
America show relatively high DOC values. DOC concentrations in Saline lakes are
significantly higher than in non-Saline lakes situated in the Prairies and Alberta in
Canada. Boreal lakes often show high contents of DOC. This is interesting because
the boreal region contains roughly 30 % of the global lakes and their water is rich
in OM (Molot and Dillon 1996; Downing et al. 2006; Benoy et al. 2007).
A large lake database (7,514 lakes from 6 continents) shows that mean DOC
concentrations are 632 ± 16 μM C, ranging from 8 μM C to 27667 μM C (Sobek
et al. 2007). In 87 % of the lakes DOC was between 83 and 1667 μM C, whilst
8.3 % of the lakes had concentrations lower than 83 μM C. Lakes between 1667
and 3333 μM C are relatively few (4.2 %), and only 0.4 % of the lakes had DOC
concentrations above 3333 μM C (Fig. 2a). In 55 % of the lakes DOC concentration was above 417 μM C, which is suggested to be a threshold value for the transition between net autotrophy and net heterotrophy in lakes (Sobek et al. 2007;
Jansson et al. 2000; Prairie et al. 2002). Several hypotheses are considered based
on DOC concentration and terrestrial vegetation (Sobek et al. 2007). First, arctic
lakes are generally characterized by low DOC concentrations where land-cover
types are wooded tundra to bare desert and correspond to an annual mean temperature of <−4 °C (Fig. 2b). Second, boreal lakes display a tendency toward higher
DOC concentration if the land-cover types are deciduous and mixed boreal forest
to deciduous conifer forest, which corresponds to an annual mean temperature of
0.5– 4 °C (Fig. 3b). Third, lakes on the northern Great Plains in Saskatchewan
and Canada show the highest DOC concentrations (~80 to ~10500 μM C) and
the land-cover types are cool grasses, shrubs, cool crops and towns (Fig. 3c).
Fourth, Fig. 2b shows that DOC concentrations for lakes situated in the warmer
climate zones (average mean annual temperature >17.4 and <22.3 °C) reach relatively high values (approximately 80–3300 μM C) for land-cover types including
broadleaf crops, corn and cropland and conifer forests (Fig. 2b). Correspondingly,
DOC concentrations for lakes situated in the low warmer climate zones (average
mean annual temperature >8.0 and <13.3 °C) are relatively low (approximately
50–2000 μM C). Here typical land-cover types are crops, mixed and deciduous
broadleaf forests, cool and cool rain forests, grass, and shrubs. Fifth, Fig. 2b suggests that DOC concentrations for lakes that experience relatively low annual
temperature (>1.5 and <4.3 °C) are relatively high (approximately 50–2100 μM
C), and the land-cover types are mostly woods, deciduous and mixed boreal forest, cool mixed forest, conifer boreal forest, narrow conifers and deciduous conifer forests.