SCOPE 42 - Biogeochemistry of Major World Rivers

4.1 INTRODUCTION

The transport of carbon and minerals by major world rivers gives essential information on the rate of erosion of continents, the cycling of major and minor elements upon the Earth, and the contribution of terrestrial materials to the ocean. Rivers transport a variety of materials to the sea, and in some cases fluvial inputs are the major source of materials found in the ocean. These materials respond rapidly to changes induced by natural causes or by human agency and may thereby be useful 'monitors' of events that take much longer to manifest their effects in the ocean. Nevertheless, studies of anything other than water discharge (e.g. of C, N and P; Meybeck 1982) have been sporadic on major rivers. Infrequent sampling of rivers gives an inadequate and possibly erroneous idea of the material composition and its changes in time (Walling and Webb 1985). 

In order to obtain reliable carbon transport data, the SCOPE/UNEP International Carbon Unit at the University of Hamburg in early 1980 initiated worldwide year-round qualitative and quantitative studies of both dissolved and particulate carbon in the world's major rivers (Degens 1982). As part of this program, some of the major rivers of North America and Soviet Russia were studied for the carbon and mineral transport. The rivers studied were: the St Lawrence River (Canada); the Mackenzie River (Canada); the Arctic Alaskan rivers (USA); and the Arctic basin rivers and the Yenisei River in the Soviet Union (Figure 4.1). This chapter summarizes the mean concentrations and yearly fluxes of major ions in the dissolved state, and of carbon and nitrogen in the dissolved and particulate state in the above rivers (Figure 4.1).

4.1.1 REGIONAL SETTINGS

Except for the St Lawrence River and the Yukon rivers, all of the above North American and Russian rivers discharge their waters either directly or eventually into the Arctic Ocean. The St Lawrence River discharges its water into the Atlantic Ocean and the Yukon into the Bering Sea which is a transitional region between the Arctic and the North Pacific Ocean. All of the above rivers are located in the northern hemisphere within 45° to 75° latitude. The climate in these regions is characterized by long cold winters and short cool summers. The geology and mineralogy of the hemisphere differ from region to region.

The St Lawrence River is the second largest river in North America (total annual flow 413 km³/year) after the Mississippi-Missouri. The drainage basin (1.153 X 10⁶ km² to Quebec City; Pocklington and Tan 1987) contains the Great Lakes Basin (0.77 x 10⁶ km²; Anon. 1985), from which the river emerges to traverse the St Lawrence Lowland (sedimentary strata), and draw tributaries from portions of the Canadian Shield (igneous and metamorphic rocks) and the Appalachian terrain (folded rocks and intrusives). The material load at Quebec City consists of the combined contributions from these disparate sources, plus the products of such biogeochemical processes as occur within the river. 

The Mackenzie River, the longest river in Canada, flows across the North-west Territories for a distance of about 1600 km from the Great Slave Lake in the north to the Beaufort Sea in the northwest. The river discharges about 249 km³/year at Arctic Red River. Water enters the Mackenzie River from the Great Slave Lake. The lake collects water from the Slave River which in turn is fed with water by the Peace and Athabasca Rivers in Alberta. Many tributaries, such as the Liard, Great Bear, Arctic Red, and Peel Rivers, discharge their waters into the Mackenzie River. The Mackenzie River drains an area of 1.805 x 10⁶ km² (Anon. 1985). Its headwaters arise in the Rocky Mountains about 4141 km south of the Arctic mouth of the river.  The Mackenzie River system basin supports agricultural, forest, petroleum, natural gas and mining practices.

 Figure 4.1 Location of North American and Russian rivers studied for carbon and mineral transport

The climate of the basin is characterized by moderately warm summers, with an average July temperature of 12 °C, and long, cold winters, with an average January temperature of -25 °C. Annual precipitation averages 325 mm, most of it falling as snow.

The drainage basin of the fluvial systems of the north Alaskan Arctic rivers comprise the Brooks Range, Foothills and Coastal Plain Provinces (Payne et al. 1952). The Brooks Range is a complex geanticlinal belt composed mainly of sedimentary and metasedimentary rocks with subordinate igneous rocks. The Foothills and Coastal Plain Provinces, which together are known as the 'North Slope', are underlain chiefly by sedimentary rocks; recent sediments overlie the Coastal Plain.

The catchment area of the North Slope rivers lies in the Arctic climatic zone (temperature: summer, 18 °C; winter, -38 °C), while the upper and lower drainage basins of the Yukon and Kobuk are situated in the Continental and Transitional climatic belts, respectively (Selgkregg 1974).

The most striking climatic feature of the Alaskan Arctic and adjacent subarctic is the presence of long, severely cold winters with ice cover for seven to eight months and short, cool summers for the rest of the year. The mean annual temperature for the North Slope is -12 °C and the mean annual precipitation is about 120 mm, which is comparable with semi-arid regions. The drainage basins of the north Alaskan Arctic rivers arid of the Yukon are in continuous and discontinuous permafrost terrain, respectively (Walker 1974).

The north Alaskan rivers of these regions are partly or wholly frozen for six to nine months of the year, with the result that almost all of the yearly flow of the North Slope rivers and a major proportion of the Yukon is restricted to short spring and summer periods. The great seasonality of the water and suspended sediment flow regimes is reflected in the North Slope rivers. Arnborg et al. (1967) reported that 43% of the annual flow and 73% of the total inorganic suspended load of the Colville River were discharged during a three-week period at spring breakup (late May-early June). In the Yukon River, approximately 60% of the mean annual discharge occurs in June to August (Roden 1967).

The catchment area of the Soviet Arctic basin rivers (12.6 x 10⁶ km²) makes up about 56% of the area of the Soviet Union. According to multiannual data, the annual runoff of rivers into the Arctic basin from Soviet territory totals 2808 km³ of which 85% is supplied by Siberian rivers (2394 km³; Maltseva et al. 1987; Romankevich and Artemyev 1985).

The largest rivers falling into the Arctic Ocean are Ob, having the largest catchment area and length (together with Irtish); Yenisei, having the greatest discharge volume; Lena; and Kolyma. These four rivers together provide 73% of annual Siberian runoff.

The thermal regime of the Arctic basin rivers is dependent on climate, subsoil water temperatures, areas of permafrost, the distribution of lakes and glaciers. Mean water temperature in July is + 14 °C in Soviet European rivers and +6 °C in rivers of northeastern Siberia. The freezing and breaking up of rivers always has a zonal character and can have a duration of up to three months. The rivers of the European part of the Arctic basin become free of ice in April-May (North Dvina, Pechora), whereas in the Asian territory (lower Lena, Kolyma, Khatanga, etc.) this happens in late Mayor June. The meridionally directed Siberian rivers have a characteristic flood surge wave moving northwards, breaking up ice and causing ice dams and high water levels. Variations in annual runoff of the Arctic basin rivers are sometimes observed. During years of abundant water the discharge of some rivers (Pechora, North Dvina, Ob, Yenisei, Lena, Kolyma) can increase by a factor of 1.2-1.5; in dry years it can dwindle its long-term average to 0.5-0.8. The variation coefficients of the annual runoff of Siberian rivers increase in the northward direction (Yenisei at the town of Yeniseisk = 12% , Tobol at the town of Kurgan = 100%).

Based on water content, Ob is the third largest river of the Soviet Union. The river is formed by the confluence of mountain torrents Biya ( outflowing from Teletskoye Lake) and Katun (taking its source from Belukha Mountain glaciers), both of them originating in the Altai Mountains. Ob drains an area of 2.99 x 10⁶ km² (including internal closed-drainage regions of 0.53 X 10⁶ km²), and extends 3650 km in length (from the source of Irtish 5410 km, and together with Ob inlet¾6370 km). Mean annual runoff averages 439 km³. Although high precipitation occurs in the basin (1630 km³), the runoff of Ob is relatively small (runoff coefficient 0.24) compared to Yenisei (runoff coefficient 0.42) and Kolyma (runoff coefficient 0.50) (World Water Balance and Water Resources of the 'Earth 1974). This is because eastward from Ob runoff conditions improve due to growing hardness of climate, predominance of mountainous terrain and deeply frosted rocks.

After the confluence of Biya and Katun, Ob flows between low banks built of easily eroded rocks, and forms branches and floodplain lakes. The bottom is mostly sandy, with rapids. Ob is a typical river of the plains, its mean gradient being 0.000042. From the fall of Tom until Irtish, Ob flows through taiga forest, and after that through the forest tundra zone and tundra. The delta of Ob is nearly 100 km long and comprises 50 isles. The breakup of Ob takes place in April in the upper course and in June in the lower reaches.

Lena, the second largest river of the Soviet Union by its water content, begins its flow at a point only 10 km distant from Lake Baikal, at a western slope of the Baikal Ridge and at 930 m above sea level. The largest tributaries are Vitim, Aldan and Viliuy. Lena's catchment area is 2.47 X 10⁶ km², its length is 4337 km. Mean annual runoff is 505 km3.

In the upper reaches (down to Vitim), Lena is a mountain torrent in a rock- walled canyon and with numerous rapids. In the middle course (between Vitim and Aldan) , Lena flows most of the way in a narrow limestone valley, covered by taiga forest. Upstream from Yakutsk the valley of Lena broadens to some 30 km. In the lower part, Lena, a typical river of the plains with the valley bottom as much as 35 km wide, flows across a spacious alluvial lacustrine swamp plain, which narrows down drastically in the area of Kharaulakh Mountains. Lena's delta has a surface of 30000 km², a length of nearly 120 km and comprises more than 800 branches, totalling over 6500 km in length, about 1500 inlets, and 60000 lakes (Mostakhov 1972).

The breakup of Lena starts in the beginning of May in the upper course and at the beginning of June in the lower section. Of the incoming water, 40% is supplied annually by melting snow cover, 35% by rain, and 25% by subterranean discharge.

The Yenisei is the largest river of the USSR. Its water catchment area equals 2.58 x 10⁶ km²; the length of the river is 3844 km. The mean annual discharge is estimated as 19 800 m³/s; the total annual discharge is about 600 km³. In this respect, the Yenisei takes ninth place among world rivers. In its upper and middle stream the river passes the Sayan and the Yeniseiskii Kryage Mountains. The banks in this region are rocky, covered with taiga, and small strips of forest steppe. Two large reservoirs, the Krasnoyarsk's and the Sayano-Shushenskaya's, are situated in the upper reaches of the Yenisei. Filling of the latter has not yet been achieved. The character of the river here is mountain-like, i.e. it has a high speed of flow, a small width of bed and considerable depth, alternating with rapids.

The lower reaches of the river pass the West-Siberian Plain in the region of permafrost. Here the Yenisei has the features of a plain river. Its flow becomes smooth, the width of bed reaching several kilometers. The taiga is gradually replaced by forest tundra along the banks.

The Yenisei is fed by thawing snow (50% of the annual flow), by rain (35- 38%) and by groundwaters (16%) (Zhadin and Gerd 1961). The largest tributaries of the Yenisei are the Angara (25% of flow) flowing out from the Lake Baikal and the Nizhnjaja Tunguska (20% of flow). The specific discharge amounts to 15-501/s/km² in mountainous districts and equals 9-141/s/km² in the West Siberian Plain (Bakhtin 1961).

4.2 METHODS

4.2.1 FIELD SAMPLING AND ANALYTICAL TECHNIQUES

The St Lawrence River water was sampled every two weeks over four years (Aug. 1981-Aug. 1985) where it discharges to its estuary at Quebec City. Details of the sampling techniques and location are described in Tremblay (1985). Water samples were analysed as follows: total organic carbon (TOC) according to the method of Pocklington (1982); dissolved ions (Cossa and Tremblay 1983); particulate organic carbon (POC) and particulate organic nitrogen (PON) (Pocklington and Kempe 1983); and suspended particulate matter (SPM) and dissolved organic carbon (DOC) (Michaelis and Ittekkot 1982).

The Mackenzie River water sampling program began in May of 1981, and water samples were collected at regular intervals until May 1982. After that, the sampling program was interrupted for five months. It restarted in October 1982 and lasted until September 1983. In all, 36 water samples were collected over a period of approximately two and a half years. Details of the sampling techniques and location are given in Telang et al. (1982). Water samples were analysed for physical parameters, major dissolved ions and DOC, as described by Telang et al. (1982). POC and PON were determined by methods described by Pocklington and Kempe (1983).

The water quality data on Alaskan rivers was gathered mainly from annual reports. The index published by Still and Jones (1985) served as a primary guide for checking the status of water quality data available from the US Geological Survey (USGS) for the various years (1978-85). The US Geological Survey's Water Resources Data Reports were considered the primary source of data on the dissolved chemical constituents and flow rates for the Alaskan Arctic rivers and Yukon River. Other published and unpublished chemical data were also taken into consideration to supplement the above data bases (Hufford 1974; Alexander 1974; Hamilton et al. 1974; Alexander et al. 1975; Schell 1975). For the purpose of this chapter, only water quality data reported for the station nearest to the lowest reach of the fresh water alluvial channel of a river were collated. Therefore, the data presented here presumably provide an average measure of the content of chemicals and their annual fluxes for the fresh water end member of a particular river entering into its estuary.

For the Soviet Russian Arctic basin rivers and the Yenisei, details of the investigation area and techniques are given by Romankevich and Artemyev (1985) and Gitelson et al. (1985).

4.2.2 DATA ANALYSIS

The total flow of water in the St Lawrence River was calculated from a gauging system operated by the Federal Government of Canada and by the provinces of Ontario and Quebec. The discharge of the river is measured daily at LaSalle, Montreal, and monthly totals are published (Anon. 1986). From this, the volume discharged in each month at Quebec City was calculated by adding the contributions of the tributaries downstream of Montreal. The mass of materials transported in each month was calculated by multi- plying the volume of water discharged in the month by the mean concentration of each variable (e.g. DOC) during that month. Those variables which did not show a Gaussian frequency distribution (POC, PON and SPM) were transformed to their logarithms and the arithmetic mean of the logtransformed data was used.

Water sampling of the St Lawrence River began in August 1981 and continued through August 1985, therefore 'years' in this study run from September through August. In all years except the second, there was one month when, for logistic reasons, no sample was taken. This made some form of interpolation necessary. In the case of December in the third year and May in the fourth, this was done by linear interpolation between the preceding and the following months. April of the first year posed more of a problem as this was the month with highest volume discharge of the year, and linear interpolation between March and May gave unrealistically low values which could have led to false interpretation, e.g. that the fall enhancement of discharge was more important for the transport of POC, DOC and SPM, than the spring flood. This illustrates the danger of describing the annual cycle of a river on the basis of only one year of sporadic data. A conservative estimate based on the mean discharge for the month of April in the three subsequent years was used and gave more reasonable values.

For the Mackenzie River above the Arctic Red, daily discharge was obtained from the Water Survey of Canada. To be sure that all significant ions were analyzed, the calculation of a mass charge balance was carried out on water samples on the assumption that the water is electrically neutral. If the analysis is complete and accurate, the sum of anions and the sum of cations calculated in milliequivalents must balance. The mass of material transported was calculated by multiplying discharge for the sampling day by the concentration of material in the water on that day. The mean fluxes of the chemical constituents reported for the Mackenzie River, therefore, may show occasional wide variation in the concentration of dissolved and particulate materials.

For the estimations of mean annual fluxes of dissolved chemical constituents for each of the Alaskan rivers, the concentrations of the chemical constituents were first tabulated on a monthly basis. Invariably the published time-series data for the North Slope rivers and Yukon River related to selected months confined to spring breakup and summer. Moreover, data for one or two months of the summer were generally not available. For these months chemical data for a river were interpolated by prorating the data corresponding to the nearest months with similar water discharge. Following this, the fluxes (concentration x discharge) of individual chemical constituents were calculated by adopting the following steps. First, the flux for each month of a calendar year was obtained, then the annual flux for that year was arrived at by adding the monthly fluxes. Finally, the long-term annual flux of a chemical constituent for a particular river was computed by averaging the annual fluxes over several years. As water discharge data for the Canning River were not available, it was not possible to calculate the chemical fluxes for that river. The annual discharge for all rivers, except the Colville River, were taken from the US Geological Survey Water Resources Data Reports. For the Colville River, the discharge for various months was prorated based on data in Arnborg et al. (1967) and Wright et al. (1974). The fluxes of the chemical constituents reported for the Alaskan rivers must be considered very tentative in light of the fact that occasionally the dissolved concentrations of some of the ions varied widely. The summary presented for the Alaskan rivers simply reflects the current status of knowledge which, needless to say, is expected to be progressively modified as long-term time-series quantitative data are gathered in the future.

The mass transport rates of organic and inorganic constituents in the Soviet Russian rivers were calculated by multiplying the individual concentrations of chemical constituents by river discharge.

4.3 RESULTS AND DISCUSSION

The mean monthly discharge for the St Lawrence River at Quebec City ranged from 24.5 km³ to 50.9 km³ about a mean of 33.8 km³. It was consistently highest in the spring (April/May), with a secondary enhancement in the fall (November/December), and lowest in the winter (January/ February), with low values also in the summer (July-September). Over the years of our study, the volume discharged annually increased from 406 km³ to 418 km³ (Table 4.1). This is not an exceptional occurrence: patterns of above- average and below-average discharge on the St Lawrence have a persistence of 2-5 years and are part of longer term (15-16 year) cycles in the water balance of Eastern Canada (Pocklington 1982).

Monthly loads of POC and PON (Table 4.1) followed closely the pattern of variation in discharge, being above the mean (21.5 x 10³ t and 2.44 x 10³ t, respectively) in fall (November-December) and spring (April-June), and below it in winter (January-February) and summer (July-September). The fall peak in particulate organic matter (POM = POC + PON) transport is more obvious than the fall enhancement of discharge alone would allow. This is because the concentrations of POC and PON are higher in the fall than the spring. The progressive increase in annual discharge noted above was followed by an increase in POC and PON transported. This has necessitated a constant upward revision of our estimates of the annual total from 236 x 10³ t and 28.0 x 10³ t in the first year to 366 x 10³ t and 41.5 X 10³ t in the fourth. Again, the potential weakness of studies based on a single year of data is demonstrated.

The proportion of carbon to nitrogen in the fluvial POM (C/N, Table 4.1) ranged from 8.4 to 12.7 about a mean of 10.4. The ratio was consistently highest for the year in the fall (November/December) and lowest in the summer (July). The high ratios in the fall coincided with the enhanced POC loads at that season. Our interpretation is that more POM of terrestrial origin (high C/N) is added to the river by runoff during the fall. In contrast to this, the C/N ratio is lowest in summer when conditions are optimal for within- river organic production (low C/N). The annual mean C/N ratio increased progressively over the first three years as discharge and POM increased. This implies that the fall contribution of terrigenous POM became more significant to the annual total.

Table 4.1 Monthly mass fluxes of the St Lawrence River to the St Lawrence Estuary at Quebec City, 1981-85

The monthly load of DOC ranged from 82 x 10³ t to 220 X 10³ t about a mean of 130 x 10³ t. Though consistently higher in the spring and fall than in summer and winter, the seasonal cycle was less strongly defined for DOC than for the other variables. The annual transport of DOC was not positively related to annual discharge as was POM, but on average declined over the course of our study from 1724 x 10³ t/year in year 1 to 1291 X 10³ t/year in year 4. The two effects to some extent compensated each other, so that the total organic carbon (TOC = POC + DOC) load was less variable between years (coefficient of variation = 12% ). The proportion of the TOC which was  particulate increased regularly from 12% in year 1 to 22% in year 4 as the importance of the particulate load to the total increased.

Monthly SPM ranged from 126 x 10³ t to 1100 x 10³ t about a mean of 353 x 10³ t. It was consistently higher in spring than in fall, and higher in fall than in summer or winter ( except in year 2 when there appeared to be little fall enhancement of SPM) .Over the first three years the annual total SPM increased concomitantly with discharge, but in the fourth year it declined due to low loadings in the spring months. The proportion of the SPM that was organic averaged 6% annually (range 5.2% in year 2 to 7.4% in year 4). This is well above the mean for world rivers of 1% (Meybeck 1982) but is more the result of lower than average SPM rather than greater than average POC in the St Lawrence. The concentration of suspended matter in the St Lawrence is as low as that in the largest 'blackwater' river in the world, the Rio Negro (Brinkmann 1986).

Table 4.2 Physical measurements, average concentrations and mass flux of inorganic ions, carbon and nitrogen in the Mackenzie River above the Arctic Red (1981-83)

The daily discharge for the Mackenzie River above the Arctic Red River averaged 249 km³/year during 1981-83 which were years of below-average annual discharge. It was highest in late May and early June, and lowest in winter (January to March), with low values in the late summer (August). The averages of physical and inorganic parameters of nitrogen and of organic carbon, measured in the Mackenzie River waters, are given in Table 4.2. Details on the sampling dates and analytical results for the above parameters on those dates are given in Telang et al. (1982, 1983) and Telang (1985). The pH of the river water averaged 7.5, with high pH values occurring in July to September water samples (pH 8.3). In other months pH ranged from 7.2 to 7.9.

The dissolved inorganic constituents in the Mackenzie River totalled 199.7 mg/l. This value compares well with that of 219 mg/l reported earlier by Livingstone (1963) for the Mackenzie River near the Arctic Red River. Total dissolved solids in the Mississippi River, the largest river in North America, average 221 mg/l.

In the case of the Mackenzie River water the ratio of anion to cation ranged between 0.86 to 1.2 indicating that all significant ions were analysed. Although ratios show small discrepancies, these are within analytical uncertainties.

The most abundant anion in the river was bicarbonate, and it accounted for 67% of the total anions. The next most abundant anion was sulphate, accounting for 22% of the total anions. Chloride was the next most abundant anion forming 7% of the total anions. The remaining 4% of the total anions were accounted for by phosphate, nitrite-nitrate, and silica. It has been suggested by Levinson et al. (1969) that the nitrogen content of the northern rivers is to some extent organically controlled as these rivers drain muskeg and tundra regions. The silica content of the Mackenzie River, of 4.0 mg/l as it entered the Beaufort Sea, is much less than the world river average of 13.1 mg/l (Livingstone 1963) .The low abundance of silica is suggested to be due to the high abundance of carbonate rocks in the basin, and lack of easily soluble silica (Levinson et al. 1969). Silica showed no appreciable variation in concentration with time. In general, all the anions show increased abundance with decreased flow rate. Levinson et al. (1969) have attributed this phenomenon to the relative importance of contributions to the Mackenzie River from groundwater inflow and lake storage.

The most abundant cations associated with the bicarbonate ion appear to be calcium and magnesium. Together they accounted for 82% of the total cations. Sodium formed 16% of the total cations. Potassium accounted for the remaining 2% .Like the anions, cations also exhibited increased abundances with decreased flow rate. Potassium, like silica, did not show any appreciable variation in concentration with time.

The high abundances of calcium, magnesium and bicarbonate ions in the Mackenzie River suggest that these are the weathering products of carbonate rocks, such as limestone or dolomite, and calcareous materials in arenaceous rocks and argillaceous rock, together with associated gypsum (Reeder et al. 1972). This could also account for the high abundance of sulphate ions in the Mackenzie River.

Mass flow data of dissolved inorganic substances indicated that the major ions transported by the Mackenzie River are: bicarbonate, calcium and sulphate ions, constituting 83% of the total mass flow of dissolved inorganic substances (Table 4.2). The mass flow of magnesium, sodium, chloride and silica ions accounted for 16% .The remaining 1% of the total mass flow was due to phosphate, nitrite-nitrate and potassium. The total transport of dissolved inorganic substances in the Mackenzie River amounted to 1386 kg/s or 43.71 x 10⁶ t/year. When the mass flow of the total inorganic constituents of the Mackenzie River is compared with those of some of the largest rivers of the world, such as the Amazon (7357 kg/s or 232 x 10⁶ t/year), Congo (3123 kg/s or 98.5 x 10⁶ t/year) and Mississippi (3741 kg/s or 118 x 10⁶ t/year) (Levinson et al. 1969), it is apparent that the Mackenzie River carries a comparably low load of dissolved substances. However, it is important to note that the above-mentioned rivers have high flow rates (two to twenty times) compared with the Mackenzie River.

Three forms of carbon were measured in the Mackenzie River (Table 4.2). They were total inorganic carbon (TIC), dissolved organic carbon (DOC) and particulate organic carbon (POC) .The abundance of total carbon averaged 30.3 mg/l. The most abundant form of carbon was inorganic carbon with an average concentration of 22.6 mg/l. It accounted for 75% of the total carbon. Inorganic carbon showed slight variability in concentration with a standard deviation of ± 4.9 mg/l. Seasonal variations in concentration showed higher values in the winter (average 25 mg/l) and lower in the summer months (average 17 mg/l). The higher values in the winter months reflect the groundwater and lake discharges into the Mackenzie River during the cold wintry period (Levinson et al. 1969). When the concentration data were interpreted in terms of mass flow (Table 4.2), the transport of inorganic carbon totalled 154 kg/s or 4.9 x 10⁶ t/year and accounted for 60% of the total carbon load transported in the Mackenzie River.

The next most abundant form of carbon was DOC with an average value of 4.5 mg/l (standard deviation ± 1.45 mg/l), amounting to one-fifth the concentration of inorganic carbon and 15% of the total carbon. Seasonal variations in DOC were of a minor nature. Higher values were observed during the winter period (average 5.6 mg/l) and lower values during the summer time (average 3.3 mg/l). Mass flow of DOC totalled 41.4 kg/s or 1.3 x 10⁶ t/year, amounting to 15% of the carbon load transported in the Mackenzie River (Table 4.2).

The abundance of POC averaged 3.2 mg/l which was 10% of the total carbon. Seasonal trends of POC were quite different from those of dissolved organic and inorganic carbon. Higher values were observed in the summer months with values ranging from 2 to 7 mg/l. The higher values may be attributed to the introduction of allochthonous material due to surface runoff and to the resuspension of riverbed sediments due to high river flow. During the winter months POC values ranged from 0.2 to 0.7 mg/l. In mass flow terms, the transport of POC totalled 57.6 kg/s or 1.82 x 10⁶ t/years amounting to 23% of the total carbon load transported by the river. The DOC: POC ratio in the Mackenzie River averaged 1.45. In rivers of North America, this ratio ranges from 1 to 20 (Malcolm and Durum 1976; Naiman and Sedell 1979), so the DOC: POC ratio in the Mackenzie River was within this range, but much lower than the DOC/POC ratio in the St Lawrence River.

The abundance and mass flow of nitrogen in the Mackenzie River was very different from that of carbon. The concentration of total dissolved nitrogen (TDN) and particulate nitrogen (PN) was very similar: it averaged 0.2 mg/l (Table 4.2). When these values were compared with those of organic carbon, DOC at 4.5 mg/l was about 22 times greater than TDN (0.2 mg/l); and POC at 3.2 mg/l was about 16 times greater than PN (0.2 mg/l). When the concentration data of nitrogen were interpreted in terms of mass flow, the transport of TDN amounted to 1.0 kg/s or 0.03 X 10⁶ t/year, and that of PN 3.1 kg/s or 0.1 X 10⁶ t/year (Table 4.2). TDN accounted for 25% and PN 75% of the total nitrogen transported.

As indicated in Table 4.2, the concentration of nitrite-nitrate averaged 0.1 mg/l. However, as nitrogen it amounted to only 0.026 mg/l, only one-tenth of the TDN at 0.2 mg/l. The majority of nitrogen, therefore, appears to be organic in nature as suggested by Levinson et al. (1969). The relationship between carbon and nitrogen indicated by the C/N ratio averaged 22.5 for DOC and TDN, 16.0 for POC and PN and 19.3 for TOC and TN.

For the Alaskan rivers, the main annual concentrations and fluxes of selected chemical constituents from the North Slope rivers of Arctic Alaska and adjacent subarctic Yukon River are shown in Table 4.3. Table 4.4 illustrates the dissolved concentrations of selected nutrients in fresh waters of the North Slope rivers for spring and summer months.

In north Arctic Alaska considerable research has continued on the dynamics of nutrient chemistry of the Colville River and adjacent coastal waters (Hufford 1974; Alexander 1974; Alexander et al. 1975; Hamilton et al. 1974; Schell1974, 1975). The dissolved nutrient input by the Colville River into the nearshore occurs as a large pulse during a short period in late May-early June at spring breakup (Hamilton et al. 1974). The primary production in Beaufort Sea coastal waters appears to be a strongly nitrogen-limited system (N : p ratios 5 : 1 to 7 : 1) with the maximum standing stocks of inorganic nutrients occurring immediately prior to the onset of the spring algal bloom (Table 4.3). Unlike the offshore areas, the lagoons and bays adjacent to the Colville River provide strong evidence of being highly phosphate deficient. This phosphate deficiency is most likely related to the extremely low phosphate concentrations ( often undetectable levels) in the river runoff from the tundra (Table 4.4). The inorganic nitrogen which is in moderate amounts in the summer fluvial flow (Table 4.4), is quite quickly assimilated by nearshore phytoplankton. In winter, active regeneration of nutrients occurs beneath the ice in the relatively deep and saline distributary of the Colville River (Schell 1974, 1975). This is accompanied by the biologically mediated mineralization of high organic nitrogen loads to ammonia and nitrate (Schell 1974, 1975).

Table 4.3 Dissolved concentrations (mg/l) and fluxes (x 10⁶ t/year) of selected chemical constituents for fresh waters of north Arctic rivers and the Yukon River, Alaska

Table 4.4 Time-series dissolved concentrations (µg/I) of selected nutrients in fresh waters of Colville, Kuparuk, Sagavanirktok and Canning rivers of north Arctic Alaska 

From the viewpoint of the chemical content of the Soviet Arctic rivers, the riverine waters of the Arctic basin are mostly hydrocarbonates (28-24% equiv. ). Also, they are noted for their weak mineralization rates (less than 50 mg/l during flood time and up to 100-200 mg/l during low flow). The degree of mineralization and hardness of river water increases in the southward direction, whereas the organic carbon (C_org) content decreases (Emelianova and Danilova 1979).

The average ion content and average perennial discharges of C_org (un-filtered waters) and some chemical elements into the Arctic basin from Soviet territory are shown in Tables 4.5 and 4.6. The flux of organic matter for the period 1936-75 and micro elements for the period 1954-6 ( determined separately for winter, spring flood, and summer-autumn periods with recourse to the water discharge statistics for the same periods) has been calculated for 35 major Soviet rivers. Annual flux was obtained by totalling these data. The total ion discharge into the Arctic basin from Soviet territory was found to be 172.2; that of Corg, 28.3; Si, 5.73; and Fe, 0.50 (each figure representing 10⁶ t/year). From the Asian part of Soviet territory the amounts supplied to the Arctic Ocean are (in thousand t/year): B, 32.15; F, 392.2; I,15.7; Cu, 9.20; Zn, 36.80; V, 2.06; Mn, 18.2; Ni, 6.87; Mo, 1.95 (Rousanov 1986; Romankevich and Artemyev 1985; Konovalov and Koreneva 1979).

The amount of organic matter brought by rivers to the Arctic Ocean is estimated to be 56.7 x 10⁶ t/year, which makes up 33% of the total ion runoff. C_org concentrations in river estuaries of Arctic seas vary between 3 and 29 mg/l, the maximum figure being typical for Soviet European rivers and the minimum value for the ones in the extreme north-east of the country. Seasonal variation in the discharge of C_org is observed with spring high flood, accounting for 58-78%; summer-autumn period, 21-34%; and winter time, 1-11% of the total C_org discharge (Maltseva et al. 1987).

In the Soviet rivers the flux of microelements in waters depends either on water runoff or on their concentration. For Siberian rivers, the principal factor governing the discharge of micro elements is the water runoff. In the case of European Soviet rivers, the discharge of micro elements appears to be greatly affected by their concentrations. The discharge of individual microelements has its specifics for each of the rivers and is strongly influenced by landscape. High F discharge with Ob waters is due not only to its powerful runoff, but also to elevated F concentrations in some tributaries (such as Tobol Basin with rocks abounding in fluorites), as well as to alkaline environmental properties (as in Ishim Basin). A significant Mn discharge with Ob waters is caused by its high concentration in water owing to acid reaction of bog soils and microbiological reduction of Mn⁴⁺ to Mn²⁺, as well as to a high migration mobility of its organo-mineral complexes. The outflow of large amounts of Fe into the Arctic Ocean from Ob, Yenisei, Lena, Pechora, North Dvina and other rivers results from a high concentration and migration mobility of Fe³⁺ in an organo-mineral colloid form. Low mobility of V and Mo in acid landscapes, combined with pronounced reduction potential of the medium, results in small inputs of these elements into the Arctic Ocean (Konovalov and Koreneva 1979).

Table 4.5 Ion content of water of some Siberian rivers (mg/l) (Alekin and Brazhnikova 1964; Zhadin and Gerd 1961) 

Table 4.6 Average perennial runoff, total ion discharge, C_org, Si (in 10⁶ t/year) and microelements (in 10³ t/year) into the Arctic Ocean (Maltseva et al. 1987; Konovalov and Koreneva 1979; Rousanov 1986)

In Ob waters, the annual discharge of suspended sediments amounts to 13.4 x 10⁶ t, and that of dissolved matter to 34 x 10⁶ t. The mean mineralization rate is 77 mg/l. The turbidity in the area of Novosibirsk is 245 g/m³; below the mouth of Irtish, 60 g/m³; and near Salekhard, 34 g/m³. The chemical composition of water undergoes variations from the source of the mouth of the river and from spring high-flood to the drought period (Table 4.5).

The content of C_org in Ob varies from 6 to 23 mg/l (the perennial average for the region of Salekhard nears 10.0 mg/l). The high concentration of C_org occurs during spring flood. During this time (as well as during continuous rains) large amounts of dissolved and colloidal humic substances reach Ob waters from surrounding forests, bogs and lakes. Suspended organic matter and aquatic organisms play a small role in the overall balance of organic matter. However, quantitative estimates of the ratios of allochthonous and autochthonous components are not available (Skopintsev 1950; Maltseva et al. 1987). The expenditure of O₂ for the oxidation of large amounts of organic matter and ferrous oxide (the average value being 6 mg Fe²⁺/l), as well as the inflow of oxygen-free water of Ob tributaries, bring about the decrease of O₂ content and even its complete disappearance. Cases of mass mortality of organisms are a rather common feature in the Ob's lower course. This phenomenon is associated with natural processes and, therefore, in most instances it is not associated with the anthropogenic pollution of water .

The plankton of Ob is mostly autotrophs (diatoms, blue-greens) with a small number of animal species (predominantly rotifers) and considerable admixture of demersal forms (ticks, nematodes). In the lower and middle course of Ob, the principal element limiting the development of diatom plankton is Si, which is brought by lower Ob tributaries in very small quantities. The optimum development of phytoplankton takes place at iron contents between 0.2 and 0.1 mg/l (Smagin et al. 1980).

Ob and its flood-plain lakes have a diversified bottom life: Potamorgetonaceae and other hydrophytes, Spongia, Bryozoa, Oligochaeta and Mollusca (Gastropoda, Bivalvia) (Zhadin and Gerd 1961). The biomass of benthos in the middle and lower course of the river is between 0.01 and 0.2 g/m², with patches of up to 27 g/m².

The annual flux of suspended sediments in Lena is 11.7 X 10⁶ t, that of dissolved substances is 41.3 X 10⁶ t. The mean turbidity value for the river is 24 g/m³: the turbidity in the upper reaches attains 100 g/m³; in the middle course, 50 g/m³. Average mineralization in the lower flow is 85 mg/l, average C_org concentration is 9.5 mg/l with variations from the source to the mouth from 6 to 9.5 mg/l. Each year Lena brings to the Laptev Sea about 5 x 10⁶ t of C_org.

In Lena waters more than 50% of phytoplankton are diatom algae. In second and third place are green (35% ) and blue-green (15% ) algae. Within zooplankton, the dominant part belongs to water fleas and Copepoda. The demersal fauna was found to include Spongilla, Turbellaria, Oligochaeta, Hirudinea and Mollusca; they play a significant role in shaping the content of bottom sediments.

The main hydrochemical peculiarities of the Yenisei River are a low turbidity and a weak mineralization. It is accounted for by the fact that the river flows across the mountains and the permafrost zones. The total suspended solid discharge of the Yenisei amounts to 14.5 x 10⁶ t/year, ionic discharge is 43.2 x 10⁶ t/year. The Yenisei carries far less total suspended solids and ions than the largest European river, the Volga. The mean annual flow of the Volga before the construction of dams was estimated as 243 km³/ year, i.e. it was 2.5 times lower than the Yenisei. Yet the total suspended solid discharge of the Volga was 27.4 x 10⁶ t/year, and that of ions 54.4 x 10⁶ t/year (Bogoslovsky 1974). Thus the transport of these constituents was much higher for the Volga than that for the Yenisei River.

The Yenisei water is characterized mainly by the same chemical composition as its reservoirs (Table 4.7). Both in the river and in its reservoirs, high concentrations of dissolved oxygen -(about 100% of saturation) and low concentrations of phosphorus (0-10 µg/I in summer) are observed. The main consequence of the erection of the Krasnoyarsk's electric power station dam was the decrease in water temperature in the 200 km region of the river downstream of the dam in summer. The temperature falls due to the fact that a sharply defined temperature stratification is set up in summer in the reservoir up to 100 m deep. Surface water temperature reaches 20-22 °C, but stays at 4-6 °C at depth. The cold deep waters of the reservoir are drained by the intake holes of the power station.

Table 4.7 Water chemical composition in the Yenisei River and its reservoirs (mg/l) (Bakhtin 1961; Volkova 1975; Sorokovikova 1981)

In spite of a relatively low temperature and low concentrations of nitrogen and phosphorus, the organic carbon content in the Yenisei ranges from 7 to 10 mg/l (Maltseva 1980). This value is higher than the mean index for the rivers of the USSR. The total annual carbon outflow of the Yenisei is estimated to be 10 x 10⁶ t/year (Gitelson et al. 1985) and the total carbon outflow of all the USSR rivers is estimated to be 82 x 10⁶ t/year (Maltseva 1980).

In order to explain the high carbon outflow of the Yenisei it is essential to evaluate the combined contribution of allochthonous and autochthonous organic matter from the ecosystem of the Yenisei to this outflow. This has not been done sufficiently, especially as far as the quantitative side is concerned.

The flora and the fauna of the Yenisei were investigated in detail. There are more than 500 algae species in the river, most of them diatoms. The river fauna contains about 500 species, primarily benthic ones: Ephemeroptera, Plecoptera and Trichoptera. There are close to 40 species of food fish, 50% of which belong to the salmon family and 5% are sturgeons (Zhadin and Gerd 1961). The ecosystem's functions on the whole, and its role in the formation of water chemical composition need to be investigated.

The functioning of the Yenisei ecosystem is conditioned by the contradictory environments. In the upper and lower parts of the river the high flow rate inhibits the development of pelagic plankton, but at the same time the stony bottom serves as a good substrate for the attachment of benthic organisms. The high transparency is favorable for phytobenthos photosynthesis, while the low concentrations of phosphorus can be the limitation. gt is suggested that the local high concentrations of phosphorus and nitrogen can occur at the water-bottom boundary.

Pelagic planktonic communities begin to form at a flow rate lower than 0.3 m/s ( Greze 1957) , mainly in the downstream of the river. The river pools and back waters are very rare in the upper and the middle parts of the Yenisei. The study of pelagic communities' role in the make-up of chemical composition of water is hindered by methodical difficulties. The degree of turbulence is supposed to play the main role in the vital activity of pelagic communities. But, the study of a primary production by the method of light and dark bottles does not take into consideration the influence of this fact. Hence, the investigation of the processes of production and destruction of the organic substance in a river requires new methodic principles, differing from the limnological ones.

The type of the river ecosystem is likely determined by the combination of three main factors: the flow rate, the depth and the bottom fauna and flora. During its course the river is not uniform ecologically. 11 is a system of pools and shoals forming an ecological 'mosaic'. Thus, various ecosystems have different influences on the chemical composition of their water and the ability for self-purification. For example, it has been reported that the specific rate of pollutant degradation differed by a factor of 2-5 for two sites on Yenisei which were 1 km apart and had different flow rates (Gladyshev et al. 1987).

Table 4.8 Average concentration and transport of carbon in some South. Central, North American and Soviet Russian rivers

Thus, due to the unique ecological system of the Yenisei, a considerable flux of carbon from Yenisei into the Arctic Ocean requires a detailed investigation with the help of specific methods.

4.3.1 COMPARISON OF RIVERS

Table 4.8 shows the most recent data on discharge and mass flow in some North American and Soviet Russian rivers.

Comparison of data on organic carbon abundance and transport in the two Canadian rivers, the Mackenzie River and the St Lawrence River, showed that total organic carbon (TOC) abundance and transport in the Mackenzie River was higher than that in the St Lawrence River. These differences were attributed to the POC. The abundance of POC was higher in the Mackenzie River (3.2 mg/l) than in the St Lawrence River (0.48 mg/l); and this was reflected in the transport of POC through their respective rivers. The major transport of organic carbon in the Mackenzie River occurred as POC (1.82 x 10⁶ t/year). In the St Lawrence River, transport of POC was only 0.31 x 10⁶ t/ year. The abundance and transport of DOC was similar in both rivers. The abundance of TIC was also similar in both rivers (20 mg/l). But, the TIC transport in the St Lawrence River was quite high (7.02 x 10⁶ t/year) compared with that in the Mackenzie River (4.9 x 10⁶ t/year). When the TC transport was compared, it was apparent that the rivers transport about the same annually. When consideration was given to the area of the drainage basin, it was observed that the rate of TC transport in the St Lawrence River was almost twice that of the Mackenzie River, the majority of which was due to high TIC transport in the St Lawrence. The transport rate of TOC per unit area of drainage basin however, was similar in both rivers.

Comparison of abundance and transport of TOC in all rivers including Russian rivers indicates that TOC in all rivers averages 8.1 mg/l and the abundance of TOC in individual rivers falls within 20% of this value, the exception being the Yukon River (Table 4.8). The TOC transport rate for all rivers averaged 3.5 x 10⁶ t/year and in Russian rivers 4.3 x 10⁶ t/year. It appears that the transport rate of TOC in the North American rivers is below the average value, and in the Russian rivers it is close to the average value. However, when consideration is given to the area of the drainage basin, the annual TOC load in all rivers (Table 4.8) is remarkably similar in the rate of organic transport considering the differences in landforms, climate and land use of their catchments.

ACKNOWLEDGEMENTS

The authors thank associates of the SCOPE/UNEP International Carbon Unit, University of Hamburg for having encouraged our initial participation and for continued logistic support, specifically, for the analytical determinations. The St Lawrence River study was an identified project under the Canada/Federal Republic of Germany Bilateral Science and Technology Agreement (â1.1.2.5).

The authors also thank:

• colleagues of the Centre Champlain des Sciences de la Mer, Quebec City, for having maintained the high quality of the sampling program in the St Lawrence River over the years;

• Mr Herb Wood (Water Survey of Canada), and R. J. Scott (Inland Waters Directorate) for their help in the Mackenzie River project;

• Water Survey of Canada for providing flow rates for the Mackenzie River; 

• M. Baskarn for helping in the compilation of analytical data and calculation of the chemical fluxes in the Alaskan rivers;

• Dr N. S. Abrosov for his participation in the Yenisei River study; and

• Lynn Ewing for typing the final copy of the manuscript.

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