The Biogeochemistry of Lena River: Organic Carbon and Nutrients Distribution
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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 im- pregnated 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).