Of humic substances in lake sediments

Ishiwatari, R. 1985. Geochemistry of humic substances in lake sediments. In Humic Substances in Soil, Sediment and Water Geochemistry, Isolation, and Characterization (G. R. Aiken, D. M. McKnight, R. I. Wershaw, and P. MacCarthy, Eds.), pp. 147-180. 

Humic substances are clearly the major constituent of organic matter in lake sediments and therefore should play the central role in the geochemical cycle of carbon compounds in lakes, lake sediments, and sedimentary rocks. The characteristics and geochemistry of humic substances in lake sediments is described in this work by citing research done by the author and his coworkers, some of which is unpublished. 

TABLE 1. Abundance of Humic Substances in Lake Sediments"... 

GEOCHEMISTRY OF HUMIC SUBSTANCES IN LAKE SEDIMENTS 153 TABLE 3. Average Values of Elemental Composition of Humic Substances from Lake Sediments" ... 

Chemical reactions of formation of humic substances in lake sediments may be essentially the same as those in marine environments. Lake sediment humic substances are expected to represent an earlier stage of humification than those from marine sediments. 

The composition of humic substances in lake sediments appears to be a function of depth. Sampling of cores from recent lake sediments in Japan (Koyama, 1966) shows that total carbon increased while total oxygen decreased with depth. Ishiwatari (1975a) isolated humic and fulvic acids from lake sediments and found a similar trend in the elemental composition. He summarized the changes in humic acid composition which occur with increased depth as follows ... 

Since most chemical studies on humic substances in lake sediments have been reported on samples without HCl hydrolysis, it should be kept in mind that these humic substances contain a relatively large amount of biochemicals. 

Isotopic Analysis. Isotopic study of humic substances is important for obtaining information on precursors and processes involving formation and diagenesis (Nissenbaum and Kaplan, 1972 Stuermer et al., 1978). However, little work has been carried out for humic substances in lake sediments. Table 6 lists isotopic data of lake sediment humic substances. 

The formation reaction of humic substances in lakes proceeds rapidly and is almost completed in decaying phytoplankton and/or in surface sediments. The major characteristics of humic acid and humin in sediments are determined by this formation stage, reflecting primarily organic constituents of phytoplankton. 

Environmental organic matter is a composite of humic and nonhumic substances, which is formed through operation and interactions of various biotic and abiotic processes. Humic substances are formed through both selected preservation (residue) and catalytic synthesis mechanisms. Both enzymatic and mineral catalyses contribute to the formation of humic substances in the environment. The relative importance of these catalytic reactions would depend on vegetation, microbial population and activity, enzymatic activity, mineralogical composition and surface chemistry of environmental particles, management practices, and environmental conditions. Selective preservation pathways would play a more important role in humification processes in poorly drained soils and lake sediments, compared with more aerated environmental conditions. 

Golebiowska, D., Mielnik, L., and Gonet, S. (1996). Characteristics of humic acids in bottom sediments of lobelia lakes. Environ. Int. The HUMEX/HUMOR Project and Humic Substances 22(5), 571-578. 

TABLE 8. Distribution of Amino Acids in Sediment and Sedimentary Humic Substances in Lake Haruna ... 

Vertical Changes of Extractable Humic Substances in Lake Biwa Sediments"... 

Transformation of Humic Substances Chemical Characteristics. Ishiwatari (1975b) studied chemical characteristics of humic acid in Lake Haruna sediment (0-24 cm) and recognized the following trends with depth ... 

A gradual decrease of carbohydrates with depth in humic substances in Lake Haruna sediments was shown by Uzaki (unpublished). Uzaki analyzed carbohydrates (neutral sugars) in humic acid, fulvic acid, and humin [Fig. 7(B)-(D)]. 

The chemical characteristics of humic substances in productive lake sediments are determined primarily by organic composition of aquatic organisms (e.g., phytoplankton) as precursors. Humic substances (humic acid and humin) remain in sediments because they are relatively hydrophobic, causing them to be more rapidly deposited than other hydrophilic organic materials and to be less easily attacked by microorganisms (Vallentyne, 1962). 

Ishiwatari, R. (1966). Structural characteristics of humic substances in recent lake sediments. In Advances in Organic Geochemistry (G. D. Hobson and G. C. Speers, eds.). Pergamon Press, Oxford, pp. 283-311. 

Figure 19 Ecosystem-level differences in the ability of humic substances to accept electrons. Samples were taken from the water column of freshwater lakes and rivers, aquatic sediments and soils (FA = fulvic acid, HA = humic acid) (after Scott et al., 1998).

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