SCOPE 42 - Biogeochemistry of Major World Rivers

8.1 INTRODUCTION

Europe is the second smallest continent. It measures (without Iceland and Spitzbergen, but including other small marginal islands) 10330.5 X 10³ km²; this is equivalent to 6.94% of the total continental surface. Morphologically, Europe is highly structured and features the longest coastline of all continents relative to its area. At its southern margin this diversity arises from the ongoing collision of the African with the European plate which caused the uplift of the Alpine mountain ranges. At the northern margin, the glaciers of Quaternary ice ages cut deep scars into the sea level fringes of Fennoscandia and of the northern British Islands.

As a consequence of the geologically young morphology, Europe is occupied by a multitude of small basins drained by relatively short rivers (Figure 8.1; Table 8.1). The two largest rivers, the Wolga and the Danube (3694 and 2850 km long), do not rank among the top dozen world rivers in terms of length, basin area or discharge. Only six rivers (Wolga, Danube, Northern Dvina, Pechora, Neva and Rhine) qualify for the top 40 world rivers by discharge (e.g. Kempe 1982; Lerman 1981) which comprise some 48% of the total river discharge. Cumulatively the European rivers discharge some 2800 km³/year, i.e. 7.4% of the world total discharge of 37700 km³/year (Baumgartner and Reichel 1975), slightly more than one would expect from the relative size of Europe's area.

Europe, nevertheless, plays an important part in the global cycles of matter. Due to its temperate, humid climate and its high percentage of

Figure 8.1 Map of major European river systems

Table 8.1 European rivers ordered counterclockwise and their basic parameters, according to various sources (tributaries indented)

Table 8.2 Absolute and relative total dissolved solid (TDS). bicarbonate and dissolved inorganic carbon (DIC) transport from continents (after Kempe 1979)

limestones in surface rocks, it has the highest chemical weathering rate of all continents. Table 8.2 (Kempe 1979) shows that the average total dissolved ion (TDI) concentration as calculated from Livingstone's (1963) compilation of European rivers amounts to 182 ppm, much higher than on any other continent. Of all dissolved solids reaching the ocean 12.6% derive from Europe, i.e. double the amount one would expect from the relative area. With this portion Europe surpasses Africa and s. America in spite of their much higher water discharges. In the case of bicarbonate, the single most important ion, Europe delivers slightly less than S. America, but still more than Africa to the world ocean. The same is naturally true for the discharge of dissolved inorganic carbon (DIC) which is calculated from the HCO₃^- load (Table 8.2).

If one wants to estimate, however, the total flux of biogeochemically important compounds such as dissolved, particulate or total organic carbon (DOC, POC, TOC), dissolved nitrate (NO₃^-), nitrite (NO₂^-) and ammonia (NH₄⁺), organically bound nitrogen (N_org), dissolved ionic or total phosphate (PO₄^3-, T-PO₄) or particulate phosphate (PP), then the Livingstone data base does not suffice. Also, fluxes alone do not provide information about sources, transformations and sinks for these compounds within the river basin, nor do they allow the evaluation of time trends in concentrations which are valuable indications for changing levels of pollution in rivers and lakes. Because of the high population density in some of the European river basins (Table 8.3) and because of intensive agriculture and the highly developed industry in Europe, the input of these compounds to river systems has most likely changed dramatically since the last century and is probably still changing.

Table 8.3 Size of population in large European river basins (compiled from Helmer 1989; Pet tine et al. 1985)

 8.2 THE DATA BASE

Discharge of rivers has been monitored since the last century because the data were important for planning channels for shipping, reservoirs, power stations and irrigation schemes. Regular water quality monitoring, however, started only after the Second World War. In fact, even the longest hydrochemical records, those of the Elbe and of the Rhine, cover only some 30 years. The Elbe record of the Hamburgian Water Works covers the years 1954-81 and is still largely unpublished (Kempe 1982). Water quality monitoring of the Rhine started in 1963 (with some of the stations extending further into the past) under the auspices of the International Commission for Protection of the River Rhine against Pollution (Intern. Comm., since 1972; Intern. Comm., since 1976; Deutsch. Komm. since 1976). Results of monitoring of Spanish rivers are published by the Minister of Public Works since 1974 (Ministerio de Obras Publicas, since 1974) and French rivers are monitored for various parameters by the Minister of the Environment and the data are published since 1975 (Ministere de l'Environnement et al., since 1975). The Danube was sampled daily at Vienna between 1978 and 1981 and analysed for a large variety of constituents, also micropollutants, yielding a data set largely unpublished (Reuschel and Forster 1982).

Today, almost all European countries operate national or regional discharge and water quality monitoring networks. Publication and scientific evaluation of these data are, however, limited. European participants of the SCOPE/UNEP Project, 'Transport of Carbon and Minerals in Major World Rivers', were therefore asked to evaluate these existing records and to study specific regions or biogeochemical problems, rather than to set up new sampling programs.

Lugo (1983) and Cauwet and Martin (1982) evaluated the Spanish and French records, respectively, for total organic carbon transport. Kempe (1982) reviewed the data for the four largest French rivers, and of the Rhine, Weser and Elbe for long-term trends, long-term average transports and biogeochemical interactions. Pet tine et al.(1983,1985, 1987) gave accounts of Italian rivers and Reuschel and Forster (1982) reviewed some results derived from the Danube record. Pocklington and Pempkowiak (1983) and Pempkowiak (1985) calculated the organic carbon transport of the Vistula, and Romankevich and Artemyev (1985) did the same for the Russian rivers. Skoulikidis (1989) sampled within the SCOPE/UNEP River Project ten Greek rivers and discussed their chemistry in a doctoral thesis.

Headwater basins of various characteristics were studied in the Federal Republic of Czecho-Slovakia, Germany and Yugoslavia by Moldan (1987) (Elbe River), Hartmann (1983) (organic output of a Harz mountain bog) and Kempe and Emeis (1985) (carbonate chemistry and formation of travertine at Plitvice).

Much work has been devoted also to trace the fate of organic matter in estuaries. The Elbe, Weser and Ems Estuaries were studied by a cruise of the R/V Valdivia (Degens et al.1982). Several authors (Cadee and Laane 1983; Cadee 1987; Eisma et al.1983; Laane and Ittekkot 1983, 1985) studied the Ems Estuary and Eisma et al.(1985) compared it to the Gironde Estuary. Eisma et al.(1983) and Lindeboom and Merks (1983) described results obtained in parts of the Rhine Estuary and Cauwet and Meybeck (1987) studied the Loire and Gironde Estuaries, while Pet tine et al.(1987) investigated the situation of the Tiber Estuary. Recently, Artemyev and Romankevich (1988) studied organic carbon transport through the Northern Dvina Estuary.

Another point of gravity in the SCOPE/UNEP River Project is formed by the many studies dealing with the chemical characterization of the organic matter in rivers and estuaries. Seifert (1982, 1985) compared the composition with regard to carbohydrates in several European rivers and at various times of the year in the Elbe Estuary. Particulate carbohydrates were analysed in the Elbe Estuary by Lohse and Michaelis (1983) and Lohse (1983). Pempkowiak (1985) studied the fractionation into labile and stable organic matter in the Vistula Estuary and Mycke (1982, 1985) analysed Elbe River water samples for dissolved phenolic compounds.

Parallel to the SCOPE/UNEP Project, the UNEP Global Environmental Monitoring System (GEMS) was launched in 1977. Under the auspices of the World Health Organization (WHO) the Global Freshwater Quality Monitoring Project collects basic hydrochemical and health-related data from 43 lake, 61 groundwater and 240 river stations, 86 of which are in Europe (GEMS 1983). Meybeck (1987) described the project and gave first results. A more detailed account is given in 'Global Freshwater Quality¾A First Assessment' (GEMS 1989). Additional data compilations are also available from the OECD (1982, 1985).

8.3 TOTAL EXPORT

Rivers gain and lose water and dissolved or particulate matter along their course. They also experience substantial annual and interannual variations in their discharge, sediment load and concentration of the various chemical compounds transported. In large river systems, which derive their water from regions different in climate, high and low water stages and therefore the mobilization of material may occur in different seasons. The Rhine is such an example (Figure 8.2). The upper Rhine, which receives melt water from the Alps has the highest water discharge in June/July, while the lowland rivers in Germany have their highest runoff in February/March when the snow melts at the end of the winter. At Cologne, the hydrograph of the Rhine still shows a double peak, while the average discharge curve at Lobith (the Dutch/German border) hides the alpine signal under a smooth shoulder of decreasing discharge.

Figure 8.2 Long-term average monthly discharge of the Rhine at three stations: Basel, Cologne and Lobith (German/Dutch border) (Eisma et al. 1982)

Tributaries may play a more important role for the transport of water or a certain compound than the main stream itself. This is the case for the Danube, where the Inn contributes more water than the Danube and where more than 50% of the water and suspended matter is derived from the tributaries downstream of Budapest, i.e. the Drava, Sava and Tisza (Figure 8.3).

Total transports can therefore only be defined for a certain station. In most cases even the station closest to the mouth excludes some of the coastal tributaries and it does not give any information of how much matter really passes the estuary. In fact, certain estuaries may import more marine matter than they export terrigenous matter. Calculating river transports even for a specific station is, furthermore, not a straightforward task. Water discharge is mostly derived from daily readings of a gauge. Thus runoff is the best known mass transport in rivers. Other physical and chemical parameters are, however, mostly monitored in relatively large intervals. In the case of the Rhine stations, many important parameters are measured twelve or eight times per year only. The most simple way to obtain an estimate of the average transport (F_x) is to use the arithmetic mean (M_x) of the parameter (X) and to multiply it with the arithmetic mean of the discharge measured (M_Q) during the sampling days:

where the arithmetic mean is defined as:

(8.2)

(n = number of measurements, Xi individual measurement of parameter).

If no simultaneous discharge measurement is available, the otherwise available annual average discharge is also often used. This method of calculation is common and has been used with the GEMS data in Table 8.4 because only annual or long-term arithmetic mean concentrations have been published. If the samples are not equally spaced in time, the method becomes even more unreliable. Also, the widely spaced measurements may then miss a major flooding event, which perhaps could mobilize 50% of the total annual load. It would therefore be very important to study a daily chemical record and compare its calculated transport with values obtained from more widely spaced sampling events.

Figure 8.3 Longitudinal profiles of the Danube for discharge (right) and suspended matter transport (left) (data after  Lászlóffy 1967) 

Table 8.4. European rivers, ordered counterclockwise, and their transports of carbon and nutrients according to various sources (tributaries indented) (updated after Kempe et al. 1985)

Using arithmetic means gives an equal weight to each of the measurements. The concentration measured at low discharge has the same importance as the concentration measured at peak discharge. This introduces a serious bias in favour of low discharge concentrations in the transportation calculation. It is therefore much better to use discharge (Q) weighted means for the calculation of total transport:

In order to take into account irregular sampling intervals, the weighting should also be introduced for time ( expressed as numbers of days in a year, D_i) . An annual time-weighted concentration can be calculated from a set of measurements beginning not with the first day in the year and ending not with the last day in the year by:

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