The Biogeochemistry of Lena River: Organic Carbon and Nutrients Distribution

Marine Chemistry 53 (I 996) 2 1I-227

The biogeochemistry of Lena River: organic carbon and nutrients distribution

G. Cauwet ‘, I. Sidorov ‘.’

” Centre National de la Recherche Scientfiyue. Gmupement dr Rrcherchrs lnteractioru Continent-O&n. Ohsewrrtoire OcPanologique. BP 44, 66651 Barguls sur Mer. France h Tiksi Department of Roscomhydmmrt, Akademiku Fedororcr, 27. 678400 Tiksi, Republic Sukha f YcrkutiwL Ruxsitr Received 25 May 1994: accepted 5 January 1995

Abstract

The Lena River is one of the most important rivers flowing to the Arctic Ocean. Draining the Siberian forest and tundra, it is characterized by black waters enriched in organic matter. Compared to other Arctic or subarctic rivers, the Lena River is very similar in the content of ammonia, phosphates, organic nitrogen and phosphorus, but three times richer in silica and nitrate. The distribution of POC, DOC, DIC and suspended matter during two cruises in September 1989 and 1991 was comparable and was influenced by the water input from the river. DOC and DIC exhibit a very conservative behaviour to salinity. The TOC discharge, is on a yearly basis directly connected to water discharge with a maximum during the flood time in June-July. From about 330 pM during the low stage period (November to April), the TOC concentration increases up to 1200 pM during the flood. The organic carbon content of suspended matter depends upon the level sampled and decreases with the suspended load. Surface samples range between 4 and 209 while samples collected in bottom waters are less rich (6 to 3%). Waters from the Buor-Khaya Bay are richer (20 to 10%7F). The concentrations of the nutrients (SiO,, PO,, TDP, TDN, NH,, NO,) are different in surface and bottom waters, and vary from summer to winter. Plotted against chlorinity, these parameters exhibit a characteristic behaviour. Silica is always more concentrated in bottom water, decreasing with salinity. Phosphate and nitrate are more concentrated in bottom water, suggesting mineralization of organic matter and regeneration of nutrients. On the contrary, ammonium is more concentrated in surface water. Total dissolved nitrogen, mainly represented by organic nitrogen (DON),is decreasing rapidly in summer at low salinities (O-2%), and slowly increases seawards. In winter the concentration is not lower but slowly decreases all along the salinity gradient. The behaviour of organic carbon and nutrients are linked to the inputs by the river and marine production and to the degradation step in the sediment.

1. Introduction zone must be considered as a specially important area (Wollast. 1991). Though representing only a Considering the global carbon cycle and the major small surface of marine realm (about S%), it is the role played by the Ocean, it seems that the coastal most productive area of the Ocean (more than 25% of total marine production, Nienhuis. 1981). This

’ Present address: Forward Marine Agency. 12 Shevchenko high productivity of the coastal zone is mainly re- Ave.. 270058 Odessa, Ukraine. lated to the influence of the river inputs, enriching

0304.4203/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved. SSDI 0304-4203(95)00090-9 the coastal waters in nutrients and organic matter, cium ions predominate in the low stream of the Lena and to the close coupling between the water and River. In winter time, when water mineralization sediment, assuring a rapid reutilization of regener- exceeds 250 mg l-‘, river waters changed to the ated elements. chloride class, sodium and potassium predominating This explains why a strong interest was put for a over calcium. This change seems to be due to the long time on the carbon and nutrient inputs by world increasing role of ground water input (Gordeev and river (Degens, 1982; Degens et al., 1983, 1985, Sidorov, 1993). 1987, 1988) and the biogeochemistry of the most important ones (Degens et al.. 1991). Considering the data bank represented by the 2. Methods series of books published by Degens and his team (see above), it appears that the information is signifi- Total organic matter in Lena River was deter- cant for European and North American rivers and mined on unfiltered river waters by means of dichro- some of the major world rivers (Amazon, Zaire etc.). mate and permanganate oxidations in an acidic but quite limited for large East Asian and Siberian medium (Semenov, 1977). TOC was calculated by rivers. The lack of data for reliable carbon inputs the dichromate oxidation and the ratio between was recently partly filled for Chinese rivers (Cauwet dichromate and permanganate oxidations (Skopint- and Mackenzie, 1993) but data concerning large sev and Goncharova, 1988). Siberian rivers is still rare in literature. Three main The samples for the determination of dissolved rivers are draining the Asian continent from west to organic carbon (DOC) and particulate organic carbon east: the Ob, Ienissei and Lena. (POC) have been collected in the framework of the Among the largest Russian Arctic rivers, the Lena international program SPASIBA, in September 1989 River ranks first with regard to the total suspended and 1991 in the coastal zone of Lena River and the matter (TSM) and total organic carbon (TOC) export southeastern part of the Laptev Sea (Fig. I). Surface and second (after Ienissei) for water and total dis- water samples were collected with Teflon pumping solved solids (TDS) export. The contribution of the and Niskin sampling bottles; bottom water samples Lena River to Arctic Ocean in terms of water, TDS, were collected with GO-FL0 and Niskin sampling TSM and TOC is about 20% of the total flux from bottles and transferred to glass bottles. All samples the Eurasian territory (Gordeev et al., 1996). were filtered under reduced pressure, with an all-glass Mon~ly water and total suspended matter dis- filter holder (Milli~re) on 47-mm pre-weighed glass charges from the Lena River range respectively from fibre filters (Whatman GF/F, 0.7 pm), precom- 1220 m3 s.- ’ and 6.4 kg s ~ ’ (April) to 73 700 mi busted overnight at 450°C. After filtration, filters S - ’ and 4360 kg s- ’ (June). The turbidity of water were washed with distilled water to eliminate the in the lower reaches of the river is maximal in remaining salt, and dried for 24 h at 50°C. The dry June-July (50-70 mg I_ ’ >, decreasing rapidly after weight of suspension collected was used to calculate the flood time (IO-20 mg l- ’ in August-September), the total suspended matter and the filters were anal- while minimum turbidity occurs in November-April ysed for POC. Four aliquots of the filtrate were (3-6 mg 1-i) wh en surface waters are frozen (Fig. collected into lo-ml glass tubes and poisoned with 2). During the flood time and the summer-autumn mercury chloride (HgC12) to avoid any bacterial period (June-September~ the Lena River provides development and stored until DOC analyses. 67% of the annual TDS export, 83% of the annual POC is measured by dry combustion of the filters water discharge and 96% of the annual TSM export. in a LECO CS 125 carbon analyser. After being Average mineralization of Lena River water dried and weighed the filters were folded into cru- changes during the year from 60-70 mg 1-l during cibles and impregnated with 2 N HCI in order to the flood time (June-July) up to 300-330 mg 1-l in destroy carbonates. They were dried at 60°C to low discharge (Ap~I-May). At the same time, the eliminate the inorganic carbon and most of the re- class (type) of water is also changing. During the maining acid and water. The analysis was performed greatest part of the year hydrocarbonates and cal- by combustion in an induction furnace and CO, G. Cauwet, I. Sidoror/Marine Chemistry 53 (19961211-227 213 formed was quantitatively measured by infrared ab- carbon. Then it is pumped from an automatic sam- sorption. pler, mixed with a potassium persulphate solution DOC analysis was previously described (Cauwet, buffered with sodium borax, and UV-irradiated in a 1984). The sample is acidified to pH 3 with HCl, and quartz coil. Under these conditions the oxidation of bubbled with nitrogen to eliminate the inorganic organic matter is achieved and the CO, is swept by

b Kotelny island WJ

A20 A21 n s

l ne

Are A22

Al8 A23

n 34 LAPTEV SEA

A17 A24 DMITRIYLAPTE”

STRAIT

AIS l

l ml

Al5 l 2 k Al4 YANSKYPAY

Fig. 1. Location of sampling stations: Tiksi Hydrometeorological Survey (1989-1991) (0); SPASIBA 1 (A 1, September 1989; SPASIBA 2 ( w ), September 199 1. 214 G. Cauwct, I. Sidoroc /Marine Chemistry 53 (19961 211-227

(high temperature catalytic oxidation) method with a Shimadzu TOC 5000 equipment. After removal of carbonates from samples by acidification and bub- bling with pure air, aliquots of 100 pl are injected in a vertical furnace on a catalyst made of silica impregnated by 1.2% Pt at 680°C. Organic matter is oxidized into CO, which is measured with a non-dis- persive infrared (NDIR) detector (Cauwet, 1994; Sugimura and Suzuki, 1988). After addition of chloroform, the samples were kept at 4°C and analysed by classical calorimetric methods for nutrient determination.

3. Results

3.1. Total organic carbon and nutrients

Lena delta has a surface of 30000 km’, a delta front of more than 400 km and comprises more than 800 branches, totalling over 6500 km in length, about 1500 inlets, and 60000 lakes (Antonov, 1967). Minimum TOC concentration in the lower reaches of the Lena River occurs in winter time (November- May), with mean values in the range 170-400 p.M, while the maximum TOC concentration is observed in June during the flood (800-1200 FM). The mean IO annual value was estimated at 850 pM (Fig. 2). In June, more than 50% of annual TOC export of Lena River enters in the delta, while only 4% of the 3 annual TOC export is discharged during winter (Ta- ble 1). During winter, climatological conditions prevent jE almost any biological activity and physical weather- Q ing so that the discharge remains at a minimum level and the concentration and the composition of OM does not vary. During the flood, TOC concentrations in the delta decreases about lo-15%, which is influenced by the dilution of a huge volume of water issued from melting ice. In this period, the organic

Fig. 2. Seasonal variations of water discharge and TSM (A). matter discharge represents 30-50% of the total dissolved oxygen and carbon dioxide (B), TOC and SiO, concen- dissolved solid. After the high water period, TOC trations (C) in lower reaches of the Lena River. concentrations in the delta increased to IO-15%. which is caused by the input of soluble organic matter from soils, rocks and bottom sediments. As a pure air (after acidification) and determined with an result the annual Lena TOC flux is 5.3 X lo6 t a-’ infrared detector. The samples collected in Septem- and the average concentration of TOC is 850 FM. ber 1991 were analysed for DOC by a new HTCO During the year the Lena River water has a deficit G. Cauwet, I. Sidoroc /Marine Chemist? 53 (I 996) 2 I l-227

!S 130 135 140 145 125 130 13% 140 14s I T I 1 1 I 7% 78

77 77

7% T%

75 1%

14 74

73 ?3

72 T2

71 71 ll 15

I25 130 135 140 14% 325 130 133 i4a t45 i I I I I 1 7% . 78 l 7% c l . b . 77 . . Ii . . . 7% * 0 75

\@ \ l h \\=o 74

-.iKi.. 73

Fig. 3. ~~~~ri~~(ion of salinity (g kg- ’ ) in the Laptev Sea in September 199 I : surface (a), 5.0 m fb), 10.0m (cl and bottom cdl. 216 G. Cauwrt, 1. Sidoroc~/Marine Chemistry53 (19961 211-227 in oxygen (Fig. 2). The oxygen concentration is same time maximum carbon dioxide concentrations minimal during the winter (April-May), about 220 (270-320 p_M) are observed. After the high water PM, which corresponds to 50-55% saturation. In the period, oxygen concentrations reach 380-440 FM

n 4 : I b + . I

04 ...... 4 04 . . . I . . . . 1 0 2 4 6 14 16 16 20 0 2 4 6 6 10 12 14 16 16 20 Chkrinily (g kg-‘)

l . q .n - . . q

1.1 n . . 3 0

I .

04 ...... 0 2 4 6 14 16 16

0.04 ...... J 04 . . . . 3 I . . 0 2 4 6 14 16 16 20 0 2 4 6 C:btin;(gk;; 14 16 16

Fig. 4. Nutrient variations (p.M) and chlorinity (g kg ’ ) in swnmer time in the southeastern part of the Laptev Sea: surface ( 0) and bottom (ml. G. Cauwet, I. Sidorov/Marine Chemist? 53 (1996) 211-227 217

(90-95% saturation). During the summer-autumn Table 1 also shows the seasonal variation of all period the carbon dioxide concentration does not dissolved forms of nutrients in the lower reaches of exceed 70 p,M. the Lena River. During the flood time the content of

0 10 12 14 16 18 0 110 2 4 6 8 10 12 14 16 18 O.do 2 4 6 Chbrinity (g kg’ ) Chkrinky (Q kg’)

0. 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 16 Chbdnity (g kg” ) Chbrinity (g kg.‘)

Fig. 5. Nutrient variations &M) and chiorinity (g kg- ’ f in winter time in the southeastern part of the Laptev Sea: surface ( q ) and bottom t=j. 218 G. Cauwet, I. Sidoror/Marine Chemistry 53 (19961211-227

Table 1 total concentration. Maximum concentrations of Average concentration of nutrients in the lower reaches of the phosphates and DOP are observed during the sum- Lena River and fluxes to the Laptev Sea mer-autumn period, whereas in the winter time the Head of the delta Mouth of the delta concentration of both forms reaches a minimum. 1 2 3 4 1 2 3 4 The relationship between TOC, DON and DOP is Concentration, pM given by the C/N ratio (average 221, C/P ratio TOC 1050 620 300 850 980 700 310 830 (950) and N/P ratio (431, what is very similar to the average world’s rivers ratios (Meybeck, 1982). Mini- NO,zSiO 632.1 722.9 10816 703.6 431.4 584.3 10316 523.6 mum values of these ratios were observed in the NH, 2.1 2.9 2.9 2.1 2.9 2.9 2.9 2.9 DIN 4.2 5.8 19 5.7 4.3 7.2 19 6.5 summer-autumn period. In the delta, C/N and C/P DON 48 31 5.7 39 39 30 6.4 34 ratios were higher. Maximum C/N and C/P ratios TDN 52 37 25 45 43 37 25 40 were observed in winter period and N/P ratio in PO, 0.3 0.8 0.1 0.4 0.2 0.7 0. I 0.4 flood time. DOP 0.7 1.2 0.1 0.9 0.5 1.3 0.1 0.8 TDP 1.0 2.0 0.3 1.3 0.7 2.0 0.3 1.2 Seasonal variations of concentration and dis- C/N 22 20 53 22 25 32 48 25 charge of dissolved silica (SiO,) in the lower reaches C/N 1500 520 3000 950 2000 540 3100 1040 of the Lena River are shown in Table 1 and Fig. 2. N/P 69 26 57 43 78 23 64 43 During the flood time, melting ice waters decrease the concentration of SiO, by 25-30%. In the sum- Flux, Mt a- ’ TOC 3930 1300 130 5360 3680 1480 140 5300 mer-autumn period the SiO, concentration also de- SiOz 1200 760 230 2190 800 610 230 1640 creases by H-20%. NO, 9.6 7.0 7.8 24 6.3 11 7.8 25 The biggest branches of the Lena delta are situ- NH, 9.6 7.0 1.4 18 13 7.0 1.4 21 ated in a way that the major mass of the water (more DIN 19 14 9.2 42 19 18 9.2 46 DON 209 76 2.8 288 168 74 3.2 245 than 90%) entering the sea moves towards east and TDN 228 90 12 330 187 92 12 291 northeast (Fig. 3), in accordance with that, the main PO, 2.5 4.2 0.1 6.8 1.6 4.1 0.1 5.8 influence of Lena River waters is observed in the DOP 7.2 6.7 0.1 14 5.3 7.0 0.2 13 eastern part of the Laptev Sea. TDP 9.7 11 0.2 21 6.9 1 I 0.3 18 Fig. 4 shows the distribution of dissolved nutri-

1, flood; 2, summer and autumn: 3, winter; 4, average (sum for ents in summer time in the southeastern part of the fluxes). Laptev Sea. For all nutrients, except TDN and ammonia, higher concentrations were observed in near- bottom waters than in surface water masses. The stratification existing in the coastal zone of the Laptev dissolved forms of nutrients (except ammonium) are Sea during the year prevents the mixing of surface decreasing within the Lena delta (15-40%). The and near-bottom water masses and preserves the high ammonium concentration in this time is increased by concentration of nutrients in the near-bottom waters. 30-40%. In the summer-autumn period the concen- The same situation occurs in the winter period (Fig. tration of nitrate in the Lena delta increased by 51. 40-60%, but the ammonium concentration remains almost unchanged. 3.2. Particulate organic carbon The main part of total dissolved nitrogen (TDN) in the Lena River is the dissolved organic nitrogen The particulate matter carried by the river can be (DON), which is about 90% of TDN as a yearly divided into four parts: detrital inorganic matter, average. A maximum of DON is discharged during non-algal organic matter, phytoplanktonic organic the flood time, which is related to the supply of material and autochtonous calcite particles (in the superficial waters, enriched with terrestrial organics, Lena River basin, carbonate weathering is prevailing into the river. The dissolved organic phosphorus over the silicate process (Gordeev and Sidorov, (DOP) is also prevailing over the inorganic form of 1993)). The phytoplanktonic material is character- this element, contributing to 70% of the mean annual ized by the low ratio POC/total-pigments, where G. Cauwet,I. Sidoror/Marine Chemistry53 (1996) 211-227 219 total pigments are the sum (chlorophyll-u + cently, the same relation was established in turbid phaeopigments) based on Lorenzen equations or ra- Chinese estuaries like the Yangtze and Hoanghe tio POC/chlorophyll-a, based on the SCOR-UN- (Yellow) Rivers (Cauwet, 1989; Cauwet and ESCO equations. Mackenzie, 1993) and the Rhone estuary (Cauwet et In September 1989 and 199 1, POC concentrations al., 19901, giving a more general sense to this rela- in lower reaches of the Lena River were in the range tion. For the Lena estuary, the POC content was 0.86-1.43 mg I-‘, representing 3.1-4.3% of total plotted against suspended matter, in surface (area 1 suspended matter. The chlorophyll-u concentrations and 2) and near-bottom samples (Fig. 7). In bottom were between 3 and 6 pg 1-l (Heiskanen and Keck, samples (area 31, the carbon values are generally 19961, while the ratio POC/chlorophyll-a in river lower than in surface waters. The individual group of waters was in the range 100-200, showing insignifi- points (area 2) characterizes the relation between cant influence of the primary productivity. TSM and POC for water in the central part of the Fig. 6 shows the variation of TSM and POC in Yanskiy Bay and in northern part of the Buor-Khaya surface water on a river-sea transept in southeastern Bay. part of the Laptev Sea. The relation between TSM Fig. 8 shows the distribution of POC in the and the organic content of particles in rivers was surface layer in the Laptev Sea during the September described by Meybeck (1982). To the higher turbid- 199 1 survey. A maximum POC content (in percent ity corresponds the lower carbon content. More re- of SM) of 16.1-20.8% was measured in surface water in the central part of the Yanskiy Bay and in northern part of the Buor-Khaya Bay. This water mass is characterized by a salinity of 3.3-23.0%0, a 25, (1.0 relatively high chlorophyll-u concentration (1.3- 1.7 p.g l- ’ > and low POC/chlorophyll-a ratios (140- 180). At station 29, the POC/chlorophyll-a ratio was 46, the turbidity was 0.3-0.8 mg l- ‘, the total content of POC was 0.06-o. 14 mg 1-l) and in the Buor-Khaya waters 1.2-2.1 mg 1-I and 0.19-0.36 mg ll’, respectively. A minimum POC content (in percent of SM) of 2.7-4.3% was measured in bottom samples, in the same area, where the presence of the Buor-Khaya waters was marked. This near-bottom water masses are characterized by a high salinity, high turbidity and total content of POC of 21 .O- 33.0%0, 3.0-11.0 mg Il’ and 0.14-0.39 mg I-‘, respectively. The same characteristics were observed in the Dmitry Laptev and Sannikov straits.

3.3. Dissolved organic and inorganic carbon

The organic-rich character of the Siberian rivers was verified with the determination of dissolved organic carbon (DOC) during the two SPASIBA cruises (September 1989 and 1991, Tables 2 and 3). Concentrations in the river reached 600-700 FM, which is among the highest values reported in world’s rivers. Few higher values were recorded during the Fig. 6. Variation of suspended matter and particulate organic flood time, approaching 1000 pM or more. With carbon in surface water on a river-sea transect. such concentrations, and taking in account the high G. Cauwet, 1. Sidoror/Marine Chemistv 53 (1996) 211-227

I 1 I II111 I I 1 I ,,,,I / I l_ 0.4 0.5 0.6 0.8 l.0 2 3 4 5 6 7 8 9 10 20 30 4050 Load (mg I-1)

Fig. 7. Variation of POC (%‘c)and suspended matter (mg 1-l ): surface (I), bottom (2) and Buor-Khaya Bay surface water (3).

125 130 135 140 145 125 130 135 140 145 I I I I 1 I I I I 78 I - a

l . .

75 -0.1-

I 74 I IO i74n

‘I

I 71 ,1”t- 7 125 130 135 140 145 125 130 135 140 145 Fig. 8. Distribution of POC (%)(a) and (mg 1-l) (b) m surface layer of the Laptev Sea: SPASIBA 2, September I99 I. G. Cauwet, I. Sidoroc/Marine Chemist? 53 (19961211-227 221 water discharge in flood periods (around 70000 30%0) on the edge of the continental slope, the DOC m3/s) the DOC flux in these periods is in the range concentration remains around 200 p,M which is two of 0.8 tons of carbon per second (about 69000 times more than in most of the coastal sea waters. tons/day). In the period considered (September), The prevailing impression is that the whole Laptev where the discharge was 17 000 m3/s, it is about 7 Sea, and possibly the Arctic Ocean, keep a stock of times less but still considerable. This high carbon carbon from the input of the Lena River (and proba- input must have a great influence on the whole bly from the Ob and Ienissei in the western Arctic coastal zone. seas). Looking at the distribution of DOC in the delta One important question is if this DOC input from and in the Laptev Sea in surface and bottom waters the river is transferred to the sea without transforma- (Fig. 9) shows clearly the influence of river water in tion or undergoes some exchange with the particles the surface layer. DOC concentrations remain impor- or some partial degradation during the transfer. Plot- tant, from 600 to 300 pM, with a wide extension ting the DOC content against salinity (Fig. 10) shows towards the open sea. In bottom waters, where the a very linear relationship, suggesting a pure dilution marine character (salinity) is more pronounced, con- process. A slight dispersion of data is observed in the centrations are lower but still high compared to other middle part, which can be attributed to variability of marine environments. In marine waters (salinity > sampling rather than to a biological or physical

Table 2 Results from cruise SPASIBA-1, September 1989 Station Total depth, m Sample depth, m Load, mg/l POC. mg/l POC, % DOC, (LM Salinity, g/kg

24 0.5 21.4 0.93 4.35 608 0.06 12.0 39.7 1.43 3.60 495 0.06 2 0.5 15.7 0.57 3.61 478 0.06 9 0.5 5.4 0.60 11.10 616 1.71 6.5 11.0 0.95 8.60 493 13.77 14 17 3.0 4.2 0.58 13.82 592 2.31 13.0 11.0 0.39 3.53 401 23.84 15 14 1.0 7.3 0.39 5.31 478 9.70 11.0 4.9 0.27 5.49 418 18.44 16 23 4.0 1.7 0.18 10.95 376 17.93 14.0 3.1 0.17 5.44 353 25.50 17 25 4.0 1.8 0.17 9.92 379 19.00 14.0 1.8 0.13 7.24 348 28.60 19 25 5.0 1.1 0.19 16.98 362 19.65 13.0 1.1 0.20 18.32 351 28.95 20 42 5.0 1.1 0.13 11.43 349 20.77 13.0 1.3 0.22 16.08 266 28.61 21 16 5.0 2.0 0.15 7.23 293 24.03 13.0 1.5 0.11 7.14 267 29.58 22 25 5.0 5.2 0.99 19.14 308 22.92 12.5 0.5 0.09 18.84 306 23.66 23 12 4.0 0.8 0.14 17.08 444 17.00 12.0 9.0 0.33 3.69 329 23.38 24 18 5.0 1.5 0.22 14.69 417 14.50 25 12 3.0 3.0 0.36 11.68 480 9.49 26 22 5.0 4.6 0.30 6.35 512 8.90 27 1.5 4.0 4.9 0.42 8.5 1 501 4.15 28 15 3.0 4.4 0.39 8.89 633 2.36 12.0 17.2 0.6 1 3.52 388 21.74 30 7 3.0 5.4 0.59 10.67 588 3.78 222 G. C&wet, I. Sidonx /Marine Chemistry 53 f 1996) 21 I-227 process. The environmental conditions during the obtained with two different methods are very compa- two cruises (river flow, production, turbidity etc.> rable after a careful estimation of blanks, destroying were so similar that we can assume that we found definitely the idea that UV-persulphate and HTCO the same situation and we may compare the results. methods give very different results. If the conserva- This is obvious for DOC, the plots from both cruises tivity of DOC is obvious along the salinity gradient, being almost superposed. We must note that data we must notice that in the riverine part (from river to

Table 3 Results from cruise SPASIBA-2 September 1991

Station Sample depth, m Load, mg/l POC, mg/l POC, o/c DOC, FM DIC, FM Sal., g/kg L-O 1 0.5 21.7 0.89 3.9 639 541 0.10 3.0 28.8 1.00 3.5 661 523 0.10 10.0 30.9 1.14 3.7 608 537 0.10 18.0 32.0 1.20 3.1 628 528 0.10 L-09 0.5 10.4 0.61 5.9 615 669 0.10 2.5 9.1 0.53 5.8 513 669 0.10 L-15 3.5 2.9 0.39 13.2 587 789 3.13 L-16 3.0 18.5 0.57 3.1 555 567 0.26 L-23 2.0 30.5 1.14 3.7 555 132 0.82 L-25 2.0 9.1 0.52 5.1 515 820 2.06 L-21 1.0 2.0 0.36 18.0 593 741 3.21 10.0 2.9 0.19 6.6 451 1788 18.55 20 3.5 2.5 0.32 12.6 574 853 4.48 10.0 0.9 0.11 12.5 354 1718 19.15 21 3.0 2.1 0.35 11.2 415 1083 7.73 14.0 3.1 0.16 4.3 247 2123 28.29 22 6.0 0.4 0.09 20.8 312 1375 16.40 23 2.5 3.0 0.33 11.0 468 1115 11.18 9.0 0.8 0.11 12.7 367 1682 20.40 24 2.5 1.1 0.2 1 20.2 313 1235 13.40 7.5 2.9 0.17 5.7 307 1814 20.98 10.0 3.9 0.10 2.7 290 1972 21.03 25 5.0 0.4 0.07 19.8 301 1385 16.67 20.0 3.1 0.09 3.0 203 2169 33.21 26 6.5 5.8 0.20 3.5 300 1619 21.05 21 7.0 3.4 0.14 4.3 292 1483 18.70 28 2.5 0.6 0.13 20.6 428 1302 13.19 25.0 2.5 0.09 3.7 262 1880 25.40 29 4.0 0.3 0.10 19.7 441 1182 I 1.36 30.0 1.o 0.06 6.1 233 1995 3 I .49 30 4.0 0.6 0.10 11.1 446 1698 20.59 35.0 11.8 0.11 0.9 201 2193 32.63 32 3.0 1.5 0.30 20.0 497 1028 8.18 33 5.0 3.6 0.18 5.1 399 1518 15.84 20.0 10.8 0.32 3.0 218 2171 29.88 34 6.0 1.2 0.2 1 17.4 460 1101 9.18 35 5.0 0.6 0.10 16.3 379 1712 30 5.5 167 2228 36 6.0 2.3 0.24 10.7 276 2108 20.0 4.9 0.24 4.9 278 2107 31 3.5 4.7 0.47 10.1 518 958 9.0 3.1 0.28 8.7 443 1469 38 5.0 5.6 0.30 5.4 413 1814 G. Cauuet, I. Sidorot, / Marine Chemistc 53 (1996121 l-227 223

Fig. 9. Distribution of DOC (FM) in surface (a) and bottom layers(b) of the Laptev Sea: SPASIBA 2. September 1991. the end of the delta, salinity = 0) some lower values upper in the river than for SPASIBA 1. To try to appear, like if during this transport some DOC is verify if there is an aggregation mechanism, we removed. It is more evident for SPASIBA 2, the made an ultrafiltration on a few samples in the river most riverine samples having been collected more and in the deltaic environment. The first interesting result was the existence of a large fraction of col-

700, 1

0 SPASIBA 1

- SPASIBA 2

100-l I 5 10 15 20 25 SO 35 0 d Lb1 L:16 LJC9 Lb L&i Ll15 Salinity StEltiOllS

Fig. 10. Variation of DOC (FM) and salinity (g kg-’ ); SPASIBA Fig. I 1. Ratios of colloidal organic and inorganic carbon on total 1 and SPASIBA 2. dissolved concentrations atong a river section. 224 G. Cauwet, I. Sidoror/Marine Chemist? 53 (1996) 211-227

duction or production by the oxidation processes are not enough consequent to be visible here. It is interesting to notice that the input from the river in DOC is similar to that in DIC (Fig. 12b).

4. Discussion

The Lena River drains the Siberian forest and tundra and is characterized by “black” waters highly enriched in organic matter (OM) as compared to other major world rivers. When compared to the world average of subarctic rivers (Meybeck, 19821, the Lena River is very similar concerning ammonia,

I__ phosphate, organic nitrogen and phosphorus, but three times richer in silica and nitrate. A* According to Rosswall (1976), the high pH soils * DC -1 favour ground waters with high nitrate concentrations, whereas tundra and subalpine forests have more ammonium. In winter time, when the Lena River is fed by ground waters, the nitrate content is maximal. In flood time and summer and autumn periods, ammonium and nitrate concentrations are I. . 8 -_ ; : DOC similar. Maximum nitrite concentrations occur dur- . I n . ing the flood time, and the ammonium varies little

“, 0 5 lb 15 !%I is 30 i5 over the year. Salinity To explain the increase of ammonium concentra- Fig. 12. Variation of DOC and DIC (FM) and salinity (g kg-’ ), tion during the flood time, it should be noted, that SPASIBA 2. snow and river ice are characterized by high ammonium concentrations (20-30 ~.LM). During the flood, one can observe a considerable flux of ammonium in the lower reaches of the Lena River and as a result, loidal carbon (between 0.7 and 0.01 km), which can the ammonium concentration in Lena delta is in- represent of the so-called “dissolved” fraction (Fig. creased. These results are in connection with the 11). According to the morphology of the area, the existence of phytoplanktonic and zooplanktonic succession of the stations chosen does not represent species. a straight transept; this can be seen with the CIC/DIC According to the data of the Tiksi Hydrometeoro- ratio which is “globally” constant but within large logical Survey, in the Lena delta and coastal waters limits. Anyway, the COC/DOC ratio is clearly de- of the Laptev Sea more than 100 species of bacillar- creasing from more than 50% to about 25%. This iophyta were identified, from which more than 60 high colloid content and its decrease could explain species are diatoms, 20 are green algae, 1.5 are the deficit observed in the upper part of the blue-green algae, and 6 species are flagellates. In DOG/salinity curve. summer period, more than 90% of the total amount Dissolved inorganic carbon (DIG) is low in the and 95% of the phytoplankton biomass was consti- river water (about 500 ~.LM) and increases towards tuted by diatoms. The dominant fresh water species the sea. It also exhibits a very conservative be- are Melosira granulata, Asterionella formosa and haviour (Fig. 12a), with a good correlation coeffi- Diatoma elongatum, and seawater species are Tha- cient CR2 = 0.985, n = 31). Uptake by primary pro- lassiosira baltica, Achnanthestaeniata, Chaetoceros G. Camvet. I. Sidomw/ Marine Chemistp 53 ilY!Xi 21 l-227 225 wighamii and Nitzshia ,frigida. Within zooplankton, pigments, terrestrial POC detritus mostly carried by the dominant groups were Daphniae and Copepods. the river during the flood time. and marine plank- However, it is necessary to remark, that the Lena tonic POC (POC up to 20%). River water is characterized by a very small total The total carbon brought by the river. in particu- planktonic biomass (Table 4). late, colloidal or dissolved form, represents an im- The coastal water masses with salinity l.O-3.0% portant discharge. especially during the flood period. are characterized in summer time by the intense One of the questions is what the fate is of this carbon short term bloom of bacillariophyta and a significant pool, if it is consumed more or less rapidly or if it decrease of all nutrients, except ammonia, was ob- accumulates in the coastal zone. Because of the low served in these water masses. In the mixing zone temperatures registered most of the year, we did not with salinity more than 3.0%, river diatoms disap- expect a very intense microbial activity, with the pear but marine diatoms do not appear. Autotrophic exception of the summer period when fresh organic plankton was present not as seaweeds but as symbi- matter is produced by the primary productivity and otic infusoria mesodimium. the surface temperature higher than the rest of the The POC concentrations depend primarily on the year. The nutrient distribution in surface and bottom amount of suspended matter (SM), and then on the waters. in summer and winter (Figs. 4 and 5) gives origin and age of the particulate material. If we plot us some information about the recycling of nitrogen, the variation of turbidity and particulate organic phosphorus and, consequently, carbon. In winter. we carbon, in surface water, from Station 1 and Ll (in can observe that nitrate in surface water is decreas- the Lena River) to more marine environments (Sta- ing from the river (12 p,M) to the sea, showing some tions 22 and 29), we can observe that total and uptake and a production of ammonium. On the con- organic loads (mg 1-I) are decreasing at the same trary, close to the bottom NH, decreases while NO, rate, while the organic content of suspensions is increases. reaching concentrations higher than in sur- considerably increasing (Fig. 6). These results sug- face water which is indicative of nitrification. A very gest that an important fraction of suspended matter is important increase of phosphate also occurs in bot- rapidly sinking in the estuary and that particulate tom waters, while it is constant in surface samples. organic matter is involved in this phenomenon as In summer, nitrates decrease very rapidly at low much as inorganic particles. Nevertheless, the in- salinity in surface waters and remain about constant crease in organic carbon percentage corresponds to in the estuarine zone. In the bottom, on the contrary. an increased colonization by marine organisms with the nitrate regeneration is intense, the concentration an increasing production found in brackish waters increasing from I.5 to 5 PM. Ammonium is pro- and coastal marine waters. duced in surface as well as in bottom levels, repre- Buor-Khaya waters were formed early in bloom senting the general biological activity. It seems that time (June-July), as a result of mixing the Lena in winter. there is a slow mineralization process flood water and Laptev Sea water and are character- going on despite of the temperature in bottom wa- ized by higher turbidity and POC concentrations. ters, regenerating phosphate and nitrate. In summer, The POC pattern in the coastal waters of the Laptev kinetics are faster, and uptake of nitrate is higher Sea is complex due to its three different origins: than regeneration in surface water, the deficit being riverine plankton, living and detrital, which is rich in compensated by that produced in bottom water. In

Table 4 Abundance of phytoplankton and zooplankton in waters of the Lena delta Period Phytoplankton Zooplankton Amount. 1000 cells/l Biomass, mg/l Amount, rib/l Biomass, kg/l

Flood 400- 1000 0.6-I .4 0.46-0.70 1.2-21.1 Summer, autumn 1000-4000 1.o-4.0 0.50-0.76 I7.0-55.1 Winter 40-200 0.1-0.3 0.07-0.23 0.05-9. I 226 G. Cauwet. I. Sidorm~/Marine Chemisttyc53 (199rjl 211-227

terms of budget, total nitrogen is decreasing in the Cauwet, G.. 1984. Automatic determination of dissolved organic low salinity range but about constant in the Laptev carbon in seawater in the sub-ppm range. Mar. Chem., 14: 297-306. Sea. The system is then reaching a dynamic equilib- Cauwet, G.. 1989. Distribution and behaviour of organic and rium. All these observations are not proving that inorganic carbon in the Changjiang estuary. In: G. Yu, H. riverine organic matter is consumed, not even partly. Zhou and J.M. Martin (Editors). Biogeochemistry of The POC content of bottom sediment is very vari- Changjiang Estuary and Adjacent East China Sea. China Ocean able, due to dynamics and differential sedimentation. Press. Beijing, pp. 570-588. Cauwet. G., 1994. HTCO method for dissolved organic carbon Because of this, it cannot be easily compared to that analysis in seawater: influence of catalyst on blank estimation. of river suspensions. We have seen that DOC is Mar. Chem., 47: 55-64. almost conservative along the salinity gradient. The Cauwet, G. and Mackenzie. F.T., 1993. Carbon inputs and distri- relatively high DOC concentrations encountered on bution in estuaries of turbid rivers: the Yang Tze and Yellow the shelf breakdown (about 200 ~_LM)suggest that rivers (China). Mar. Chem., 43: 235-246. Cauwet. G.. Gadel. F.. de Souza. M.M.. Donard. 0. and Ewald. DOC is accumulating in coastal water on a long term M.. 1990. Contribution of the Rhane River to organic carbon basis, like a stock of accumulated carbon input from inputs to the northwestern Mediterranean Sea. Cont. Shelf the Lena. Though we do not have any direct evi- Res.. 10(9-11): 1025-1037. dence, it seems consistent to think that most of the Degens, E.T.. 1982. Transport of carbon and minerals in major marine production is recycled, in surface waters in world rivers. Part 1. Mitt. Geol:Palaont. Inst. Univ. Hamburg. SCOPE-UNEP, Sonderbd. 52. 766 pp. summer or in bottom waters in winter, and that only Degens. E.T.. Kempe, S. and Soliman. S.. 1983. Transport of a small fraction of the terrestrial organic matter carbon and minerals in major world rivers. Part 2. Mitt. undergoes degradation on a yearly basis. A more Geol.-PalPontol. Inst. Univ. Hamburg, SCOPE-UNEP. Son- precise budget would need further studies directed to derbd. 55, 535 pp. this problem, with a tentative appreciation of carbon Degens, E.T.. Kempe, S. and Herrera, R.. 1985. Transport of carbon and minerals in major world rivers, Part 3. Mitt, accumulated in Laptev Sea. a better estimation of the Geol.-Palaontol. Inst. Univ. Hamburg, SCOPE-UNEP. Son- total primary production and more studies on bacte- derbd. 58. 645 pp. rial degradation (Saliot et al.. 1996). Degens, ET.. Kempe. S. and Can Weibin, 1987. Transport of carbon and minerals in major world rivers. Part 4. Mitt. Geol.-Palaontol. Inst. Univ. Hamburg, SCOPE-UNEP. Son- derbd. 64. 515 pp. Acknowledgements Degens, E.T.. Kempe. S. and Naidu, A.S., 1988. Transport of carbon and minerals in major world rivers, lakes and estuaries. This work was performed in the frame of the Part 5. Mitt, Geol:Pallontol. Inst. Univ. Hamburg, SCOPE- French Russian cooperation program SPASIBA, sup- UNEP. Sonderbd. 66, 422 pp. ported by CNRS (GDR ICO and PICS-99). We Degens. E.T.. Kempe. S. and Richey, J.E., 1991. Biogeochemistry thank Prof. Savostin, Director of the Institute of of major world rivers, SCOPE 42. Wiley. New York, 356 pp. Gordeev, V.V. and Sidorov. IS.. 1993. Concentrations of major Oceanology for the invitation to participate in his elements and their outflow into the Laptev Sea by the Lena expedition in the Laptev Sea. The authors are in- river. Mar. Chem.. 43(1-4): 33-45. debted to the captains and crews of Russian research Gordeev. V.V.. Martin, J.M. and Sidorov. IS., 1996. A reassess- vessels Mezen, Okeanolog, Olikhon and Alexandr ment of the Eurasian Rivers input of water, sediment. major Smimiskiy and the members of the Laboratory of elements and nutrients to the Arctic Ocean. In press. Heiskdnen, A.S. and Keck. A., 1996. Distribution and sinking rate Regional Geodynamics (LARGE), for their helpful of algal pigments. particulate silica and phytoplankton in the participation. I. Sidorov was supported for nine Laptev Sea and Lena River (Arctic Siberia). Mar. Chem., 53: months by the University of Perpignan with a fel- 229-235. lowship from the French Ministry of Research. Meybeck, M.. 1982. Carbon, nitrogen. and phosphorus transport by world rivers, Am. J. Sci., 282: 401-450. Nienhuis, P.H., 1981. Distribution of organic matter in living marine organisms. In: E.K.Duursmd and R. Dawson (Editors), References Marine Organic Chemistry. Elsevier, Amsterdam, pp. 31-69. Rosswall, T., 1976. Nitrogen. phosphorus and sulphur - global Antonov. VS.. 1967. The mouth region of Lena River (hydrologi- cycles. SCOPE Report 7. Swedish National Research Council, cal assay). Gidrometeoizdat, Leningrad, 107 pp. (in Russian). Stockholm. pp. 157-167. G. Cauwet. I. Sidoror/Marine Chetnistryv 53 (19%) 21 l-227 227

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