Environmental Engineering Reference
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et al. 2010 ; Midorikawa and Tanoue 1998 ; Carlson et al. 1985 ; Sugimura and
Suzuki 1988 ; Guo et al. 1994 , 1995 , 1996 ; Santschi et al. 1995 ; Guo and Santschi
1996 , 1997a ; Mopper et al. 1996 ). These results demonstrate that the contribution
of the lower MW fraction (<1-10 kDa) is relatively low in rivers and that it sig-
nificantly increases in lakes, coastal waters and the open ocean. Comparison of
molecular fractions between surface (epilimnion) and deep (hypolimnion) waters
shows that the molecular size fraction of <1-5 kDa in deep water is often more
important than in the surface waters of lakes and oceans (Table 1 ) (Yoshioka et al.
2007 ; Wu and Tanoue 2001 ; Wu et al. 2003 ; Mopper et al. 1996 ). It is suggested
that either microbial degradation of DOM or new releases of DOM from micro-
bial respiration of organic matter in deeper waters are responsible for the high
contents of the low molecular size fractions of DOM in natural waters. An addi-
tional implication is that significant microbial or biological degradation of DOM
and organic matter occurs in deep waters. The high percentage of colloidal DOC
or colloidal organic carbon included in the >1 kDa to 0.45 μ m range suggests that
colloids are the predominant phase in bulk DOC transported by rivers (Guéguen
et al. 2006 ; Benner and Hedges 1993 ; Guo and Santschi 1997b ; Guéguen and
Dominik 2003 ).
The optical and chemical characteristics of the molecular size fractions of
DOM show that truly dissolved DOM (<1-10 kDa) includes fulvic acid (59-96 %
on the basis of fluorescence), total hydrolyzed amino acids (51-63 %), tryp-
tophan (free tryptophan has a molecular weight of 0.2 kDa) and total dissolved
carbohydrates (10-20 %). In contrast, the DOM fraction between >1-10 kDa and
0.2-0.45 μ m or 0.1-GF/F includes fulvic acid (5-22 % on the basis of fluores-
cence), total dissolved carbohydrates (80-90 %) and total hydrolyzed amino acids
(29-42 %). The DOM fraction of 0.1 μ m-GF/F (0.45-0.7 μ m) includes protein-
like or tryptophan-like or bacterial cells or phytoplankton cells, total hydrolyzed
amino acids (7-11 %) and fulvic acid (2-8 % on the basis of fluorescence) (Liu
et al. 2007 ; Guéguen et al. 2006 ; McCarthy et al. 1996 ; Midorikawa and Tanoue
1998 ; Wu and Tanoue 2001 ; Wu et al. 2003 ; Pakulski and Benner 1992 ; Skoog and
Benner 1997 ; Boehme and Wells 2006 ). The contributions to the molecular size
fractions of sedimentary fulvic acid extracted from Tokyo Bay sediment samples
are 44.8 % for <1 kDa, 3.5 % for 10 kDa, 31.8 % for 50 kDa, 14.6 % for 100 kDa
and 5.3 % for 300 kDa. The corresponding contributions of humic acid are 2.4 %
for <1 kDa, 0.8 % for 10 kDa, 5.3 % for 50 kDa, 16.1 % for 100 kDa and 75.4 %
for 300 kDa (Hayase and Tsubota 1983 , 1985 ). This suggests that allochthonous
fulvic acid is mostly composed of low molecular size fractions (<1-10 kDa) whilst
allochthonous humic acid is mostly composed of high molecular size fractions,
>300 kDa (Hayase and Tsubota 1983 , 1985 ; Rashid and King 1969 ; MacFarlane
1978 ). Therefore, molecular size fractions could be a useful indicator to distin-
guish between fulvic and humic acids in DOM in a variety of natural waters.
These results also imply that allochthonous fulvic acid of terrestrial origin or
the autochthonous fulvic acid (C-like) of algal or phytoplankton origin can primar-
ily undergo photoinduced and microbial in situ degradation, which can decrease
the molecular size and increase as a consequence the low molecular size fraction
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