SCOPE 42 - Biogeochemistry of Major World Rivers

10.1 INTRODUCTION

Organic matter in natural waters represents a continuous size spectrum, beginning with free small molecules, macromolecules and aggregates and ending with organisms. By arbitrary definition, the fraction retained on a filter with a pore size of 0.5 to 1.0 µm is defined as particulate matter. The upper limit of particulate matter is determined by the sampling device used, and will be around a few millimetres.

 In rivers, particulate organic matter (POM) can originate from autochthonous production of phytoplankton and from allochthonous sources like soils, waste water and atmosphere. It can also be produced in situ from dissolved organic matter by physico-chemical and biological processes.

The particulate organic matter is subjected to different processes in the fluvial, estuarine and coastal marine environment. These processes such as mineralization, disaggregation and sedimentation determine its role, behaviour and fate in the aquatic ecosystem. Available information on these sources and processes is mostly qualitative in nature. In order to understand the role of POM in the aquatic ecosystem information is required on the quantities involved.

The total river flux of POM has been estimated by various authors and is in the range of 0.07¾0.2 x 10¹⁵ g C/year (Meybeck 1982 and references therein). However, this flux does not give any information about the role, behaviour and fate of riverine POM in the estuarine and marine environment: Such information can be obtained from the chemical composition of POM and the sediment behaviour .

Particulate organic matter can be divided into a living (i.e. bacteria and plankton) and non-living (detritus) part. In general, most of the living part is recycled rapidly together with a minor part of the detritus (Laane 1982). That part of organic matter which is decomposed is often called labile, and here defined as the fraction of carbohydrates and proteins in POM. One of the problems associated with the calculation of the actual flux of riverine carbon to the ocean is the uncertainty concerning the amount of organic matter decomposed in estuarine and coastal environment. Estimates of this labile part of POM, either indirectly by oxygen consumption and loss of carbon in the estuaries or directly by chemical characterization, showed it to be in the range of 30-75% (Richey et al. 1980; Eisma et al. 1985; Ittekkot and Arain, 1986; Laane et al. 1987).

In this chapter results of riverine POM from all the major rivers studied during the SCOPE Project are presented. For 13 of these world rivers the labile data are given. The POM data cover more than 50% of the sediment flux to the ocean and encompass rivers draining from various climatic zones, from low to high latitudes. For extrapolation of the data to calculate global fluxes, information from the papers by Meybeck (1982) and Thurman (1985) is included.

All the data obtained from individual samples (irrespectively of the rivers) such as total sediments, dissolved and particulate organic carbon were treated together. Subsequently, relationships between the different parameters were determined. The samples were grouped into various classes according to the measured total suspended matter (TSM) concentrations (see Meybeck 1982).

10.2 PARTICULATE ORGANIC MATTER

Particulate organic matter in the rivers is expressed as a percentage of total suspended matter (%) and as concentrations (milligrams carbon per litre) (Table 10.1). Expressed as a percentage of TSM the figures vary between 1.3% and 8.4% and the concentrations are between 0.6 mg C/l and 14.2 mg C/l in the different classes. There is an inverse relationship between the content and concentration of POC. Included are only TSM concentrations measured during the SCOPE study.

However, according to Meybeck (1982) there are also rivers where suspended matter concentrations exceed these values. These rivers together contribute c. 37% of total annual sediment discharge to the sea and have POC contents between 0.5% and 0.6% .

10.2.1 RELATIONSHIP WITH SUSPENDED MATTER

Figure 10.1 gives the relationship between the particulate organic carbon content and the concentration of suspended matter grouped together in the various classes. The major trend is a decrease in the percentage of POM with

Table 10.1 Mean concentration of particulate organic carbon in world rivers expressed as percentage of total suspended matter (%) and as concentrations per litre (mg C/l) in different ranges of concentrations of suspended matter (TSM, mg/l) 

Figure 10.1 Relationship between total concentration of suspended matter (TSM) and particulate organic carbon (POC) for different rivers. The bars indicate means with standard deviation at mean suspended matter values for samples with different concentration ranges (mg/l) given above each bar

logarithmically increasing concentrations of suspended matter. This trend is very similar to the one observed by Meybeck (1982). This could be a consequence of a reduction in primary production due to high suspended matter concentrations (Thurman 1985).

10.2.2 RELATIONSHIP WITH DISSOLVED ORGANIC MATTER

To estimate the global riverine input of organic matter to the oceans different authors have used, for extrapolation purposes, the ratio between dissolved and particulate organic carbon in different rivers (e.g. Meybeck 1982).

The results from the SCOPE study show that the ratio changes with increasing concentrations of suspended matter (Table 10.2). The ratios are 10.8 at concentrations of suspended matter smaller than 15 mg/l and decrease to less than 1 at suspended matter concentrations more than 500 mg/l.

Table 10.2. Relationship between the ratio of dissolved and particulate organic carbon and different classes of suspended matter from different rivers 

Previous studies indicate that rivers carry 10 times more DOM than POM (Wetzel 1975). The values in Table 10.2 show that this is only valid for concentrations of suspended matter smaller than 15 mg/l. Significant variations in these ratios, which are strongly dependent on the concentrations of suspended matter, are apparent from Table 10.2. The low values are characteristic for highland rivers and the high values for lowland rivers, suggesting an inverse relationship between the suspended matter and the DOC/POC ratios (Meybeck 1982).

10.3 CHEMICAL COMPOSITION

 From an ecological and geochemical point of view it is important to distinguish between labile and refractory components of POM to provide a general picture of the fate of POM in the estuarine and marine environment. The fraction that may undergo mineralization (labile) is an important food source for aquatic organisms. For the sake of simplicity the labile part of POM is defined as the sum of carbohydrates and proteins (Laane 1982; Degens and Ittekkot 1985; Ittekkot et al. 1985; Laane et al. 1987). The non-mineralizable fraction (refractory) ultimately ends up in the coastal marine sediments and is in this sense of geochemical significance.

10.3.1 CARBOHYDRATES AND PROTEINS

The average percentage of carbohydrates and proteins to POC is given for the different classes together with the labile fraction of POC (Table 10.3). Remarkable is that the contribution of proteins is higher than for carbohydrates in POC, with values varying between 7% and 29% for proteins and 5% to 17% for the carbohydrates.

A decrease in labile POM is found when the concentration of suspended matter is high. This is probably due to the decreased light penetration in highly turbid waters, which decreases primary production resulting in a low contribution of labile POC.

Table 10.3 Percentage of carbohydrate carbon (PCHO-C, %) and protein carbon (PAA-C, %) contents and the labile fraction of particulate organic carbon (LPOC, %) in different ranges of concentration of suspended matter (TSM, mg/l) in world rivers

10.4 FLUXES

For the calculations of fluxes of POM it is necessary to have reliable and representative TSM data. However, sampling a river system to obtain representative TSM samples is difficult. Their concentrations have been found to vary at a chosen location both laterally and with depth (Curtis et al. 1979; Nordin and Meade 1981). New sampling devices have been developed which allow the collection of representative samples using depth-integrated samples (Richey et al. 1986). During the SCOPE Project logistical difficulties prevented

Figure 10.2 The total suspended matter (TSM) flux (a) in grams and (b) as percentage of the total annual TSM flux, (c) the POC flux (in grams) in world rivers expressed in relationship with the TSM classes

the use of such techniques. The approach taken here minimizes the errors in flux estimates, resulting from inadequate sampling of the rivers, by covering all ranges of suspended matter concentrations (see above).

The annual POC transport in the world rivers from the data collected from the SCOPE study (i.e. those presented in Table 10.1) is 190 x 10¹² g C. These calculations include suspended matter concentrations between 1 and 1500 mg/l. Inclusion of the rivers with higher TSM (> 1500 mg/l) in the calculations increases the total annual POC transport by the rivers to 230 x 10¹² g C. This value is similar to that reported previously (e.g. Schlesinger and Melack 1981).

In Figure 10.2(a) and (b), the pattern of TSM and POC transport in the rivers is given for the different classes. The maximum transport of TSM occurs in the class of TSM concentrations of 500-1500 mg/l in absolute quantities and as a percentage of total annual TSM transport from the rivers to the sea. The figure also shows the relationship between percentage POC contents and TSM, and the annual fluxes of POC expressed in absolute quantities and as a percentage of the total annual POC flux from rivers.

The data on the labile fraction of the POC show that the TSM concentration in the range between 1 and 150 mg/l has an average labile content of about 35%, whereas at concentrations above 150 mg/l the labile content drops to c. 15%. The total POC transport in these two fractions is, respectively, 29 x 10¹² g C/year and 202 x 10¹² g C/year, which means about 10 x 10¹² g C/year and 30 x 10¹² g C/year, respectively, are transported by these two groups. Thus the total transport of labile POC is 40 x 10¹² g C/year or 17% of the annual POC transported by the major world rivers.

However, it must be mentioned that the labile content, described here by the data on carbohydrates and proteins, is not the only fraction available for organisms; an example is the lipid fraction of the POM. Uncertainties also exist as to how far carbohydrates and proteins serve as food source for organisms (van Es and Laane 1982). In order to account for labile constituents which are not covered by carbohydrates and proteins, Ittekkot (1988) doubled the labile fraction and obtained a labile POC riverine flux of 81 x 10¹² g C/year or 35% of the total annual POC transport.

10.5 ENVIRONMENTAL IMPLICATIONS

It is seen from the results that a certain part of the riverine POC is available for heterotrophic utilization in the estuaries and the adjacent coastal seas (Figure 10.3a). This labile fraction of POC shows regional differences (Figure 10.3b). What becomes immediately apparent is a temperature effect. Rivers of the temperate regions (e.g. the Mackenzie, the St Lawrence and the Paraná) have relatively high labile POC fractions and in the tropical and subtropical rivers (e.g. Ganges, Brahmaputra, Zaïre) there is a labile fraction which is much lower: 40% and 20% respectively.

This implies that in tropical and subtropical rivers most of the labile POC has already been decomposed due to the relatively higher temperature. This is also reflected in the distribution of the C/N ratios of POM: at higher temperatures the C/N ratios are relatively lower than at moderate temperatures (Ittekkot, unpublished results).

The river input of labile organic matter represents only 13% of the primary production in the estuaries (630 x 10¹² g C/year, Romankevich 1984), and if the primary production in the coastal zone is added it is just 1% .This means that the in situ estuarine primary production is the major food source for heterotrophic organisms in estuaries and coastal zones.

The residual fraction of the riverine POC flux is 150 X 10¹² g C/year. This is 30% of the refractory part of the estuarine and coastal primary production being about 10% (van Es and Laane 1982), i.e. 480 x 10¹² g C/year. Together they may account for a large part of the organic matter being trapped in the coastal zone. This amount of POM is considerably higher than the amount of organic matter buried in deep-sea sediments which is c. 6 X 10¹² g C/year (Berner 1982).

It must be borne in mind that river fluxes are sensitive to man's activities. These changes, such as constructions of dams and barrages in the drainage areas of some of the major world rivers, have led to a significant reduction in 

Figure 10.3 Labile particulate organic carbon (POC), calculated as the total percentage contribution to total particulate organic carbon by carbohydrates and proteins, in some major world rivers: (a) From Ittekkot (1988); and (b) in relationship with the total suspended matter ranges

the river transport of organic matter and nutrients. On the other hand, deforestation in the drainage basins has brought about increased topsoil erosion and subsequently an increase of the sediment transport and the associated organics. Whether these two processes offset each other is not yet known. The data presented here thus represent a general picture of the nature of organic matter transported by the rivers, but say less about the dynamics in the river systems as such. For this more detailed studies of selected river systems and the structures within the rivers such as man-made lakes are necessary.

REFERENCES

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