SCOPE 42 - Biogeochemistry of Major World Rivers

7.1 INTRODUCTION

The Himalayan rivers, i.e. Ganges, Brahmaputra, Indus and Irrawady, contribute one-third of the global sediment transport to the world oceans (Milliman and Meade 1983). The major portion of these rivers drain one of the most densely populated regions in the world. Hence, elemental transport by these rivers assumes global importance in continent-ocean mass balance studies.

Table 7.1 summarizes the basic hydrological data for these rivers, and Figure 7.1 shows the location of various stations discussed in this chapter. With the exception of the Irrawady, they drain predominantly Tertiary or younger rock types of variable chemical composition. The common watersheds for these rivers lie on either side of an axis running a distance of more than 1500 km west to east. Due to lack of data, the Irrawady is not considered for our present discussion.

7.2 WATER CHEMISTRY AND THE PARTICULATE LOAD

The water chemistry of the Indus is known in detail only at the river mouth (Arain 1985); the Brahmaputra is reported in detail by Hu-Ming Hui et al. (1982) for the upper reaches in Tibet; Sarin and Krishnaswami (1984) and Subramanian (1987) in the mid-reaches in India and Safiullah et al. (1985) in the lower reaches in Bangladesh. Subramanian (1979, 1987) and Sarin and Krishnaswami (1984) reported the water chemistry of the Ganges at several locations in the drainage basin. Table 7.2 summarizes the chemical composition of river waters in the Himalayan drainage system. While all these rivers exhibit temporal and spatial variations, their chemistry broadly reflects the world average river water composition of Meybeck (1981). Exceptions to this are the very low value for the Brahmaputra in Bangladesh, and the very high value for the lndus at Karachi.

Figure 7.1 Location map of stations discussed for the Himalayan rivers (not to scale). H, Haridwar; K, Kanpur; A, Allahabad; P, Patna; B, Brahmaputra; G, Gauhati; D, Dibrugarh; L, Lhasa

The particulate loads given for the various rivers indicate the relative predominance of sediment vs solute transport, the ratio of which varies from

Table 7.1 Hydrological data for the Himalayan drainage systems

Table 7.2a Water chemistry of the lndus (in mg/l)

Table 7.2b Water chemistry of the Brahmaputra (in mg/l)

                                       Table 7.2c Water chemistry of the Ganges (in mg/l)

1 (Indus at Karachi) to 10 (Brahmaputra at Gauhati). This region of the world deviates therefore sharply from the global average value of 4.5 to 5 reported by Garrels and Mackenzie (1971). Hence, for transport of individual elements such as carbon, partitioning behaviour will be important to understand sinks and sources in these riverine systems.

7.3 CARBON TRANSPORT

Table 7.3 summarizes the carbon values (DOC and POC) for the Himalayan rivers. Figure 7.2 shows the variation in POC for these rivers. These are average values, and pronounced temporal variation for POC, at least for the Indus at Karachi (Ittekkot and Arain 1986) and the Ganges (Subramanian et al. 1985) have been reported. They observed that the POC values can vary by a wide range from year to year and season to season. Ittekkot and Arain (1986) observed an almost 15-fold change in POC values within a year and three- to four-fold changes from year to year for the Indus. Ittekkot et al. (1985) also reported a five-fold change in the POC values within a single year for the Ganges in Bangladesh, where¾as reported by Subramanian et al. (1985)¾up to an eight-fold change in POC was observed between 1980 and 1982 for the Ganges in India. Spatial variation for the Ganges over its 2200 km stretch is less pronounced.

Based on an average non-discharge weighted POC value of 2.3% (2.1% for River Indus, 2.6% for the Ganges and 2.5% for the Brahmaputra) and considering the water and sediment discharge (given in Table 7.1), the organic carbon transport from the Himalayan drainage systems to the world oceans amounts to 57 x 20¹² g/year. At an average DOC content of 4.5 mg/l, these rivers transport 12 x 10¹² g/year so that a total annual transport of organic carbon of the order of 69 x 10¹² g can be arrived at. However, it must be mentioned that more than 90% of sediment transport in these rivers occurs within three to four months (Ittekkot et al. 1986), and these sediments have an average POC content of c. 0.7%. If this discharge-weighted is taken for POC transport calculations, then the annual transport will be c. 18 x 10¹² g C. The total organic carbon transport in these rivers will then amount to 30 x 10¹² g/year.

Table 7.3 Organic carbon concentrations in the Himalayan rivers

Figure 7.2 (a) DOC, POC variation in the Indus River, Pakistan (from Arain 1985); (b) in the Ganges and the Brahmaputra, Bangladesh (from safiullah et al. 1985) 

Figure 7.3 Chlorinity-particulate organic carbon relationship for the Ganges Estuary

Figure 7.4 (a) Seasonal variations in the percentage of sugars (DCHO) and amino acids (DAA) in the total dissolved organic carbon (DOC) in the River Ganges and Brahmaputra (from lttekkot et al. 1986); (b) distribution of dissolved amino acids (top), of b-alanine and g-aminobutyric acid (middle), and of glucose and total dissolved carbohydrates (bottom) in the Indus River, Pakistan (from Arain 1985)

The POC contents in these rivers show temporal and spatial variations as demonstrated by Ittekkot et al. (1985, 1986) and Subramanian et al. (1985). For example, the POC values (Figure 7.3) in the Ganges (Hoogly) estuary in India show an increase seaward suggesting carbon fixation there. Increased carbon fixation could occur due to increased nutrient input into the estuaries in both dissolved and particulate forms. An additional supply of POC along with sediments could be expected from the two tributaries of the Ganges between Calcutta and the open ocean (Bay of Bengal) draining a high-density industrial belt.

7.4 NATURE OF POC

The characterization of organic matter in the Himalayan rivers' sediment has only recently been attempted (Figures 7.4a and b). Ittekkot et al. (1985) studied the seasonal variability of various compounds in POC in the mouth of the River Ganges in Bangladesh at a single location. While the POC values

Figure 7.5 (a) Seasonal variations in particulate organic carbon (POC) and (b) the contribution of sugars and amino acids to POC in the Indus (Ittekkot and Arain 1986)

Fig 7.6 Relationship between total suspended matter (TSM) and particulate organic carbon (POC) in; (a) the Ganges (from Subramanian et al. 1985); and (b) the Indus (from Ittekkot and Arain 1986)

varied from 1% to 6% in one year, the individual constituents such as sugar and amino acids sharply differed during low discharge as against high discharge periods. Both in the dissolved and in the particulate carbon fraction, the sugars and amino acids showed corresponding changes¾peak values in June and low values in March. The peak values of dissolved sugar (DCHO) at 1100 µg/l, particulate sugar (PCOH) at 1700 µg/l, dissolved AA (DAA) at 620 µg/l and particulate AA (P AA) at 2400 µg/l correspond to the initial phases of rising water. The relative distribution of various types of AA such as b-alanine and g-aminobutyric acid indicate the presence of microbially degraded organic matter in DOC and POC. Both before and after the peak carbon values in June, the AA and CHO decrease in both the dissolved and particulate phases. Thus, the nature of organic carbon in the Ganges at low discharge and low sediment periods differs from that at high discharge and high sediment load periods.

Similar studies have been carried out on the other end of the Himalayan belt¾the Indus River mouth¾at Karachi in Pakistan (Figure 7.5). Ittekkot and Arain (1986) reported a systematic variation in POC values with peaks (ranging from 8% to 15% of POC) around AugustlSeptember in a three-year period. The labile organic matter, namely sugars and amino acids, constituted up to 70% of POC; generally, the contribution of sugars and amino acids to POC showed maximum values during the low discharge and low sediment period (June) during the three-year period. While Subramanian et al. (1985) reported a positive relationship between POC and total particulates for the fresh water region of the Ganges and Brahmaputra in India, Ittekkot and Arain (1986) observed a negative trend for the Indus (Figures 7.6a and b). They reported extensive biodegradation of labile organics at higher TSM levels as responsible for low POC values in these periods. Thus, the organic matter transported from the Himalayan region to the marine environment is predominantly non-labile.

7.5 GENERAL DISCUSSION

The current estimate of organic carbon transport to world oceans is about 0.1 x 10¹⁵ g C/year (Meybeck 1982). This is largely based on data from low-sediment rivers of Europe as well as the Amazon. The Himalayan rivers carry a very large amount of labile organics such as sugars and amino acids during low-sediment discharge periods. The bulk of sediments from these rivers reaches the ocean when the discharge is high; but at these peak sediment discharge periods, the POC of these rivers is poor in labile components.  Hence the large proportion of fine particles carried by these rivers to mid-ocean regions will have POC with a high percentage of non-labile groups.

The fractionation behaviour of individual compounds such as sugars and amino acids in the Indus and Ganges-Brahmaputra River mouth shows that tropical seas might be subjected to terrestrial influence to a larger degree than other marine regions. Ittekkot et al.(1986) suggested that the high-sediment rivers draining the Himalayas may even contribute organic matter to the deeper part of oceans away from nearshore regions. Assessing the nature and extent of this influence and its implications for the chemistry of tropical marine systems in general should be a major goal for future research activities.

REFERENCES

Arain, R. (1985) Carbon and mineral transport of the Indus River. In: Degens, E. T., Kempe, S. and Herrera, R. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt. 3. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 58, pp. 487-93.

Garrels, R. M. and Mackenzie, F. T. (1971) Evolution of Sedimentary Rocks, W. W. Norton and Co., New York, 397pp.

Hu Ming-Hui, Stallard, R. F. and Edmond, J. M. (1982) Major ion chemistry of some large Chinese rivers. Nature 298, 550-3.

Ittekkot, V. and Arain, R. (1986) Nature of particulate organic matter in the River Indus, Pakistan. Geochim. Cosmochim. Acta 50, 1643-56.

Ittekkot, V., Safiullah, S., Mycke, B. and Seifert, R. (1985) Seasonal variability and geochemical significance of organic matter in the River Ganges, Bangladesh. Nature 317, 800-3.

Ittekkot, V., Safiullah, S. and Arain, R. (1986) Nature of organic matter in rivers with deep-sea connections: The Ganges-Brahmaputra and Indus. Sci. Tot. Environ. 58, 93-107.

Meybeck, M. (1981) River transport of organic carbon to the ocean. In: Likens, G. E., Mackenzie, F. T. et al. (Eds) Fluxes of Organic Carbon by Rivers to Oceans, Carbon Dioxide Research Assessment Program, US DOE, Washington, DC, pp. 219-69.

Meybeck, M. (1982). Carbon, nitrogen and phosphorus transport by world rivers. Amer. J. Sci. 282, 282-450.

Milliman, J. D. and Meade, R. H. (1983) Worldwide delivery of river sediments to ocean. J. Geol. 91, 1-19.

Rao, K. L. (1975) India's Water Wealth, Oxford Univ. Press, 475pp.

Safiullah, S., Chowdhury, M. I., Mafizuddin, M., Ali, I. and Karim, M. (1985) Monitoring of the Padma (Ganges), the Jamuna (Brahmaputra) and the Baral in Bangladesh. In: Degens, E. T., Kempe, S. and Herrera, R. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt. 3. Mitt. Geol.-Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 58, pp. 519-24.

Sarin, M. M. and Krishnaswami, S. (1984) Major ion geochemistry of the Ganges-Brahmaputra river system, India. Nature 312, 538-41.

Subramanian, V. (1979) Chemical and suspended matter characteristics of rivers of India. J. Hydrol. 44, 37-55.

Subramanian, V. (1987) Environmental geochemistry of Indian river basins¾a review. J. Geol. Soc. India 29, 205-20.

Subramanian, V., Richey, J. E. and Abbas, N. (1985) Geochemistry of river basins of India, Pt II: Preliminary studies on the particulate C and N in the Ganges-Brahmaputra river system. In: Degens, E. T., Kempe, S. and Herrera, R. (Eds) Transport of Carbon and Minerals in Major World Rivers, Pt.3, Mitt. Geol.- Paläont. Inst. Univ. Hamburg, SCOPE/UNEP Sonderbd. 58, pp. 513-18.

Subramanian, V., Van Grieken, R. and Van't Dack, L. (1987) Heavy metals distribution in the sediments of Ganges and Brahmaputra Rivers. Environ. Geol. 9, 93-105.