The biogeochemistry of Lena River: organic carbon and nutrients

Marine Chemistry
The biogeochemistry
53
(I 996) 2 1I-227
of Lena River: organic carbon and nutrients
distribution
G. Cauwet ‘, I. Sidorov ‘.’
” Centre National
de la Recherche Scientfiyue.
Gmupement
dr Rrcherchrs
BP 44, 66651 Barguls
h Tiksi Department
of Roscomhydmmrt,
Akademiku
lnteractioru
Continent-O&n.
Ohsewrrtoire
OcPanologique.
sur Mer. France
Fedororcr,
27. 678400 Tiksi, Republic
Received 25 May 1994: accepted 5 January
Sukha
f YcrkutiwL
Ruxsitr
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
Considering the global carbon cycle and the major
role played by the Ocean, it seems that the coastal
’ Present address: Forward Marine
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12 Shevchenko
zone must be considered as a specially important
area (Wollast. 1991). Though representing
only a
small surface of marine realm (about S%), it is the
most productive area of the Ocean (more than 25%
of total marine production, Nienhuis.
1981). This
high productivity of the coastal zone is mainly related to the influence of the river inputs, enriching
0 1996 Elsevier Science B.V. All rights reserved.
the coastal waters in nutrients and organic matter,
and to the close coupling between the water and
sediment, assuring a rapid reutilization
of regenerated elements.
This explains why a strong interest was put for a
long time on the carbon and nutrient inputs by world
river (Degens, 1982; Degens et al., 1983, 1985,
1987, 1988) and the biogeochemistry
of the most
important ones (Degens et al.. 1991).
Considering
the data bank represented
by the
series of books published by Degens and his team
(see above), it appears that the information is significant for European and North American rivers and
some of the major world rivers (Amazon, Zaire etc.).
but quite limited for large East Asian and Siberian
rivers. The lack of data for reliable carbon inputs
was recently partly filled for Chinese rivers (Cauwet
and Mackenzie,
1993) but data concerning
large
Siberian rivers is still rare in literature. Three main
rivers are draining the Asian continent from west to
east: the Ob, Ienissei and Lena.
Among the largest Russian Arctic rivers, the Lena
River ranks first with regard to the total suspended
matter (TSM) and total organic carbon (TOC) export
and second (after Ienissei) for water and total dissolved solids (TDS) export. The contribution of the
Lena River to Arctic Ocean in terms of water, TDS,
TSM and TOC is about 20% of the total flux from
the Eurasian territory (Gordeev et al., 1996).
Mon~ly
water and total suspended matter discharges from the Lena River range respectively from
1220 m3 s.- ’ and 6.4 kg s ~ ’ (April) to 73 700 mi
S - ’ and 4360 kg s- ’ (June). The turbidity of water
in the lower reaches of the river is maximal in
June-July
(50-70 mg I_ ’ >, decreasing rapidly after
the flood time (IO-20 mg l- ’ in August-September),
while minimum turbidity occurs in November-April
(3-6 mg 1-i) wh en surface waters are frozen (Fig.
2). During the flood time and the summer-autumn
period (June-September~
the Lena River provides
67% of the annual TDS export, 83% of the annual
water discharge and 96% of the annual TSM export.
Average
mineralization
of Lena River water
changes during the year from 60-70 mg 1-l during
the flood time (June-July)
up to 300-330 mg 1-l in
low discharge (Ap~I-May).
At the same time, the
class (type) of water is also changing. During the
greatest part of the year hydrocarbonates
and cal-
cium ions predominate in the low stream of the Lena
River. In winter time, when water mineralization
exceeds 250 mg l-‘, river waters changed to the
chloride class, sodium and potassium predominating
over calcium. This change seems to be due to the
increasing role of ground water input (Gordeev and
Sidorov, 1993).
2. Methods
Total organic matter in Lena River was determined on unfiltered river waters by means of dichromate and permanganate
oxidations
in an acidic
medium (Semenov, 1977). TOC was calculated by
the dichromate
oxidation
and the ratio between
dichromate and permanganate
oxidations (Skopintsev and Goncharova, 1988).
The samples for the determination
of dissolved
organic carbon (DOC) and particulate organic carbon
(POC) have been collected in the framework of the
international program SPASIBA, in September 1989
and 1991 in the coastal zone of Lena River and the
southeastern part of the Laptev Sea (Fig. I). Surface
water samples were collected with Teflon pumping
and Niskin sampling bottles; bottom water samples
were collected with GO-FL0 and Niskin sampling
bottles and transferred to glass bottles. All samples
were filtered under reduced pressure, with an all-glass
filter holder (Milli~re)
on 47-mm pre-weighed glass
fibre filters (Whatman GF/F,
0.7 pm), precombusted overnight at 450°C. After filtration, filters
were washed with distilled water to eliminate the
remaining salt, and dried for 24 h at 50°C. The dry
weight of suspension collected was used to calculate
the total suspended matter and the filters were analysed for POC. Four aliquots of the filtrate were
collected into lo-ml glass tubes and poisoned with
mercury chloride (HgC12) to avoid any bacterial
development and stored until DOC analyses.
POC is measured by dry combustion of the filters
in a LECO CS 125 carbon analyser. After being
dried and weighed the filters were folded into crucibles and impregnated with 2 N HCI in order to
destroy carbonates.
They were dried at 60°C to
eliminate the inorganic carbon and most of the remaining acid and water. The analysis was performed
by combustion
in an induction furnace and CO,
G. Cauwet, I. Sidoror/Marine
formed was quantitatively
measured by infrared absorption.
DOC analysis was previously described (Cauwet,
1984). The sample is acidified to pH 3 with HCl, and
bubbled with nitrogen to eliminate the inorganic
213
Chemistry 53 (19961211-227
carbon. Then it is pumped from an automatic sampler, mixed with a potassium persulphate solution
buffered with sodium borax, and UV-irradiated in a
quartz coil. Under these conditions the oxidation of
organic matter is achieved and the CO, is swept by
b
Kotelny
island
WJ
A20
A21
ns
l ne
Are
A22
Al8
A23
n 34
A17
LAPTEV
SEA
DMITRIY
LAPTE”
A24
STRAIT
l
AIS
l
ml
Al5
l2
YANSKY
PAY
Al4
Fig. 1. Location of sampling
( w ), September 199 1.
stations: Tiksi Hydrometeorological
Survey (1989-1991)
(0);
SPASIBA
1 (A 1, September
k
1989; SPASIBA 2
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 bubbling 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-dispersive 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
IO
3
jE
Q
Fig. 2. Seasonal variations of water discharge and TSM (A).
dissolved oxygen and carbon dioxide (B), TOC and SiO, concentrations (C) in lower reaches of the Lena River.
pure air (after acidification) and determined with an
infrared detector. The samples collected in September 1991 were analysed for DOC by a new HTCO
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 (NovemberMay), 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
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
annual TOC export is discharged during winter (Table 1).
During winter, climatological
conditions prevent
almost any biological activity and physical weathering 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
matter discharge represents 30-50%
of the total
dissolved solid. After the high water period, TOC
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
result the annual Lena TOC flux is 5.3 X lo6 t a-’
and the average concentration of TOC is 850 FM.
During the year the Lena River water has a deficit
G. Cauwet, I. Sidoroc /Marine
!S
130
I
135
T
140
I
145
1
Chemist?
125
53 (I 996) 2 I l-227
13%
130
140
1
14s
I
7%
78
77
77
7%
T%
75
1%
14
74
73
?3
72
T2
71
ll
71
15
I25
7%
130
i
135
.
I
140
b
14%
I
c
.
.
Ii
325
133
130
I
78
i4a
I
1
7%
l
l
77
.
.
.
.
.
*
7%
0
\@ \
h
75
\\=o
l
.
-.iKi
Fig. 3. ~~~~ri~~(ion of salinity (g kg-
t45
74
73
’ ) in the Laptev Sea in September 199 I : surface (a), 5.0 m fb), 10.0m (cl and bottom cdl.
G. Cauwrt, 1. Sidoroc~/Marine
216
in oxygen (Fig. 2). The oxygen concentration
is
minimal during the winter (April-May),
about 220
PM, which corresponds to 50-55% saturation. In the
Chemistry53
(19961 211-227
same time maximum carbon dioxide concentrations
(270-320
p_M) are observed. After the high water
period, oxygen concentrations
reach 380-440
FM
4
:
b
04
0
.
2
.
.
4
.
6
.
.
.
.
14
16
.
16
4
20
04
0
n
+.
I
.
2
.
4
.
6
6
I
10
I
.
12
.
14
.
.
16
16
16
16
1
20
Chkrinily (g kg-‘)
6
.
1.1
. .
I
l .
.n -
q
q
.
n
3
0
.
04
0
0.04
.
0
2
.
4
Fig. 4. Nutrient variations
(ml.
.
6
.
.
.
(p.M) and chlorinity
14
(g kg
.
16
.
16
J
20
04
0
.
2
.
2
.
4
.
4
6
.
6
.
.
.
C:btin;(gk;;
.
3
.
14
I
14
.
16
.
.
16
’ ) in swnmer time in the southeastern part of the Laptev Sea: surface ( 0) and bottom
G. Cauwet, I. Sidorov/Marine
(90-95% saturation). During the summer-autumn
period the carbon dioxide concentration does not
exceed 70 p,M.
O.do
2
4
6
0
10
12
Chbrinity (g kg’
0.
0
2
4
6
8
10
Fig. 5. Nutrient variations &M)
16
18
0
217
53 (1996) 211-227
Table 1 also shows the seasonal variation of all
dissolved forms of nutrients in the lower reaches of
the Lena River. During the flood time the content of
11
0
2
4
6
8
10
12
14
16
18
14
16
16
Chkrinky (Q kg’)
)
12
Chbdnity (g kg”
t=j.
14
Chemist?
14
16
18
)
and chiorinity (g kg-
0
2
4
6
8
10
12
Chbrinity (g kg.‘)
’f
in winter time in the southeastern part of the Laptev Sea: surface ( q ) and bottom
G. Cauwet, I. Sidoror/Marine Chemistry 53 (19961211-227
218
Table 1
Average concentration
of nutrients in the lower reaches
Lena River and fluxes to the Laptev Sea
Head of the delta
1
2
NO,z
SiO
632.1
NH,
2.1
DIN
4.2
DON
48
TDN
52
PO,
0.3
DOP
0.7
TDP
1.0
C/N
22
C/N 1500
N/P
69
Mouth of the delta
3
Concentration, pM
TOC 1050
620
4
300
722.9 108
16
2.9
2.9
5.8
19
31
5.7
37
25
0.8
0.1
1.2
0.1
2.0
0.3
20
53
520 3000
26
57
Flux, Mt a- ’
TOC 3930
1300
SiOz 1200
760
NO,
9.6
7.0
NH,
9.6
7.0
DIN
19
14
DON 209
76
TDN 228
90
PO,
2.5
4.2
DOP
7.2
6.7
TDP
9.7
11
of the
1
850
980
703.6
431.4
2.1
2.9
5.7
4.3
39
39
45
43
0.4
0.2
0.9
0.5
1.3
0.7
22
25
950 2000
43
78
2
3
700
310
830
584.3 103
16
523.6
2.9
2.9
2.9
7.2
19
6.5
30
6.4
34
37
25
40
0.7
0. I
0.4
1.3
0.1
0.8
2.0
0.3
1.2
32
48
25
540 3100
1040
23
64
43
130 5360
3680
1480
230 2190
800
610
7.8
24
6.3
11
1.4
18
13
7.0
9.2
42
19
18
2.8 288
168
74
12
330
187
92
0.1
6.8
1.6
4.1
0.1
14
5.3
7.0
0.2
21
6.9
1I
1, flood; 2, summer and autumn:
fluxes).
4
140 5300
230
1640
7.8
25
1.4
21
9.2
46
3.2 245
12
291
0.1
5.8
0.2
13
0.3
18
3, winter; 4, average
(sum for
dissolved forms of nutrients (except ammonium) are
decreasing within the Lena delta (15-40%).
The
ammonium concentration in this time is increased by
30-40%. In the summer-autumn
period the concentration of nitrate in the Lena delta increased by
40-60%, but the ammonium concentration
remains
almost unchanged.
The main part of total dissolved nitrogen (TDN)
in the Lena River is the dissolved organic nitrogen
(DON), which is about 90% of TDN as a yearly
average. A maximum of DON is discharged during
the flood time, which is related to the supply of
superficial waters, enriched with terrestrial organics,
into the river. The dissolved organic phosphorus
(DOP) is also prevailing over the inorganic form of
this element, contributing to 70% of the mean annual
total concentration.
Maximum
concentrations
of
phosphates and DOP are observed during the summer-autumn
period, whereas in the winter time the
concentration of both forms reaches a minimum.
The relationship between TOC, DON and DOP is
given by the C/N ratio (average 221, C/P ratio
(950) and N/P ratio (431, what is very similar to the
average world’s rivers ratios (Meybeck, 1982). Minimum values of these ratios were observed in the
summer-autumn
period. In the delta, C/N and C/P
ratios were higher. Maximum C/N and C/P ratios
were observed in winter period and N/P ratio in
flood time.
Seasonal variations
of concentration
and discharge of dissolved silica (SiO,) in the lower reaches
of the Lena River are shown in Table 1 and Fig. 2.
During the flood time, melting ice waters decrease
the concentration
of SiO, by 25-30%. In the summer-autumn
period the SiO, concentration also decreases by H-20%.
The biggest branches of the Lena delta are situated in a way that the major mass of the water (more
than 90%) entering the sea moves towards east and
northeast (Fig. 3), in accordance with that, the main
influence of Lena River waters is observed in the
eastern part of the Laptev Sea.
Fig. 4 shows the distribution of dissolved nutrients in summer time in the southeastern part of the
Laptev Sea. For all nutrients, except TDN and ammonia, higher concentrations were observed in nearbottom waters than in surface water masses. The
stratification existing in the coastal zone of the Laptev
Sea during the year prevents the mixing of surface
and near-bottom water masses and preserves the high
concentration of nutrients in the near-bottom waters.
The same situation occurs in the winter period (Fig.
51.
3.2. Particulate
organic carbon
The particulate matter carried by the river can be
divided into four parts: detrital inorganic matter,
non-algal organic matter, phytoplanktonic
organic
material and autochtonous
calcite particles (in the
Lena River basin, carbonate weathering is prevailing
over the silicate process (Gordeev and Sidorov,
1993)). The phytoplanktonic
material is characterized by the low ratio POC/total-pigments,
where
G. Cauwet,I. Sidoror/Marine Chemistry53 (1996) 211-227
total pigments
are the sum (chlorophyll-u
+
phaeopigments)
based on Lorenzen equations or ratio POC/chlorophyll-a,
based on the SCOR-UNESCO equations.
In September 1989 and 199 1, POC concentrations
in lower reaches of the Lena River were in the range
0.86-1.43
mg I-‘, representing
3.1-4.3% of total
suspended matter. The chlorophyll-u
concentrations
were between 3 and 6 pg 1-l (Heiskanen and Keck,
19961, while the ratio POC/chlorophyll-a
in river
waters was in the range 100-200, showing insignificant influence of the primary productivity.
Fig. 6 shows the variation of TSM and POC in
surface water on a river-sea transept in southeastern
part of the Laptev Sea. The relation between TSM
and the organic content of particles in rivers was
described by Meybeck (1982). To the higher turbidity corresponds the lower carbon content. More re-
25,
(1.0
219
cently, the same relation was established in turbid
Chinese estuaries like the Yangtze and Hoanghe
(Yellow)
Rivers
(Cauwet,
1989; Cauwet
and
Mackenzie, 1993) and the Rhone estuary (Cauwet et
al., 19901, giving a more general sense to this relation. For the Lena estuary, the POC content was
plotted against suspended matter, in surface (area 1
and 2) and near-bottom samples (Fig. 7). In bottom
samples (area 31, the carbon values are generally
lower than in surface waters. The individual group of
points (area 2) characterizes the relation between
TSM and POC for water in the central part of the
Yanskiy Bay and in northern part of the Buor-Khaya
Bay.
Fig. 8 shows the distribution
of POC in the
surface layer in the Laptev Sea during the September
199 1 survey. A maximum POC content (in percent
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
relatively high chlorophyll-u
concentration (1.3- 1.7
p.g l- ’ > and low POC/chlorophyll-a
ratios (140180). 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 .O33.0%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
Fig. 6. Variation of suspended matter and particulate
carbon in surface water on a river-sea transect.
organic
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
flood time, approaching
1000 pM or more. With
such concentrations,
and taking in account the high
G. Cauwet, 1. Sidoror/Marine
Chemistv
53 (1996) 211-227
4-
3-
I
1
I
0.4
0.5
II111
0.6
0.8
l.0
I
I
1
I
2
3
4
5
Load
Fig. 7. Variation of POC (%‘c)and suspended
125
78
130
135
I
I-
140
I
I
(mg
matter (mg 1-l ): surface
145
,,,,I
7
8
9
10
20
30
l_
4050
I-1)
(I), bottom (2) and Buor-Khaya
125
I
I
/
6
130
I
1
135
I
Bay surface water (3).
140
145
I
I
a
77
l
.
.
76
-0.1-
75
74
I
I
IO
i74n
‘I
73
72
I
71
125
,1”t130
Fig. 8. Distribution
135
140
145
125
7
130
135
140
of POC (%)(a) and (mg 1-l) (b) m surface layer of the Laptev Sea: SPASIBA 2, September
145
I99
I.
G. Cauwet, I. Sidoroc/Marine
water discharge in flood periods (around 70000
m3/s) the DOC flux in these periods is in the range
of 0.8 tons of carbon per second (about 69000
tons/day).
In the period considered (September),
where the discharge was 17 000 m3/s, it is about 7
times less but still considerable.
This high carbon
input must have a great influence on the whole
coastal zone.
Looking at the distribution of DOC in the delta
and in the Laptev Sea in surface and bottom waters
(Fig. 9) shows clearly the influence of river water in
the surface layer. DOC concentrations remain important, from 600 to 300 pM, with a wide extension
towards the open sea. In bottom waters, where the
marine character (salinity) is more pronounced, concentrations are lower but still high compared to other
marine environments.
In marine waters (salinity >
Table 2
Results from cruise SPASIBA-1,
Station
September
Chemist?
53 (19961211-227
221
30%0) on the edge of the continental slope, the DOC
concentration remains around 200 p,M which is two
times more than in most of the coastal sea waters.
The prevailing impression is that the whole Laptev
Sea, and possibly the Arctic Ocean, keep a stock of
carbon from the input of the Lena River (and probably from the Ob and Ienissei in the western Arctic
seas).
One important question is if this DOC input from
the river is transferred to the sea without transformation or undergoes some exchange with the particles
or some partial degradation during the transfer. Plotting the DOC content against salinity (Fig. 10) shows
a very linear relationship, suggesting a pure dilution
process. A slight dispersion of data is observed in the
middle part, which can be attributed to variability of
sampling rather than to a biological
or physical
1989
Total depth, m
Sample depth, m
Load, mg/l
POC. mg/l
POC, %
DOC, (LM
Salinity, g/kg
24
0.5
12.0
0.5
0.5
6.5
3.0
13.0
1.0
11.0
4.0
14.0
4.0
14.0
5.0
13.0
5.0
13.0
5.0
13.0
5.0
12.5
4.0
12.0
5.0
3.0
5.0
4.0
3.0
12.0
3.0
21.4
39.7
15.7
5.4
11.0
4.2
11.0
7.3
4.9
1.7
3.1
1.8
1.8
1.1
1.1
1.1
1.3
2.0
1.5
5.2
0.5
0.8
9.0
1.5
3.0
4.6
4.9
4.4
17.2
5.4
0.93
1.43
0.57
0.60
0.95
0.58
0.39
0.39
0.27
0.18
0.17
0.17
0.13
0.19
0.20
0.13
0.22
0.15
0.11
0.99
0.09
0.14
0.33
0.22
0.36
0.30
0.42
0.39
0.6 1
0.59
4.35
3.60
3.61
11.10
8.60
13.82
3.53
5.31
5.49
10.95
5.44
9.92
7.24
16.98
18.32
11.43
16.08
7.23
7.14
19.14
18.84
17.08
3.69
14.69
11.68
6.35
8.5 1
8.89
3.52
10.67
608
495
478
616
493
592
401
478
418
376
353
379
348
362
351
349
266
293
267
308
306
444
329
417
480
512
501
633
388
588
0.06
0.06
0.06
1.71
13.77
2.31
23.84
9.70
18.44
17.93
25.50
19.00
28.60
19.65
28.95
20.77
28.61
24.03
29.58
22.92
23.66
17.00
23.38
14.50
9.49
8.90
4.15
2.36
21.74
3.78
2
9
14
17
15
14
16
23
17
25
19
25
20
42
21
16
22
25
23
12
24
25
26
27
28
18
12
22
1.5
15
30
7
G. C&wet, I. Sidonx
222
/Marine
process. The environmental
conditions
during the
two cruises (river flow, production, turbidity etc.>
were so similar that we can assume that we found
the same situation and we may compare the results.
This is obvious for DOC, the plots from both cruises
being almost superposed. We must note that data
Table 3
Results from cruise SPASIBA-2
Station
L-O 1
L-09
L-15
L-16
L-23
L-25
L-21
20
21
22
23
24
25
26
21
28
29
30
32
33
34
35
36
31
38
Sample depth, m
0.5
3.0
10.0
18.0
0.5
2.5
3.5
3.0
2.0
2.0
1.0
10.0
3.5
10.0
3.0
14.0
6.0
2.5
9.0
2.5
7.5
10.0
5.0
20.0
6.5
7.0
2.5
25.0
4.0
30.0
4.0
35.0
3.0
5.0
20.0
6.0
5.0
30
6.0
20.0
3.5
9.0
5.0
September
Chemistry 53 f 1996) 21 I-227
obtained with two different methods are very comparable after a careful estimation of blanks, destroying
definitely the idea that UV-persulphate
and HTCO
methods give very different results. If the conservativity of DOC is obvious along the salinity gradient,
we must notice that in the riverine part (from river to
1991
Load, mg/l
POC, mg/l
POC, o/c
DOC, FM
DIC, FM
Sal., g/kg
21.7
28.8
30.9
32.0
10.4
9.1
2.9
18.5
30.5
9.1
2.0
2.9
2.5
0.9
2.1
3.1
0.4
3.0
0.8
1.1
2.9
3.9
0.4
3.1
5.8
3.4
0.6
2.5
0.3
1.o
0.6
11.8
1.5
3.6
10.8
1.2
0.6
0.89
1.00
1.14
1.20
0.61
0.53
0.39
0.57
1.14
0.52
0.36
0.19
0.32
0.11
0.35
0.16
0.09
0.33
0.11
0.2 1
0.17
0.10
0.07
0.09
0.20
0.14
0.13
0.09
0.10
0.06
0.10
0.11
0.30
0.18
0.32
0.2 1
0.10
0.24
0.24
0.47
0.28
0.30
639
661
608
628
615
513
587
555
555
515
593
451
574
354
415
247
312
468
367
313
307
290
301
203
300
292
428
262
441
233
446
201
497
399
218
460
379
167
276
278
518
443
413
541
523
537
528
669
669
789
567
132
820
741
1788
853
1718
1083
2123
1375
1115
1682
1235
1814
1972
1385
2169
1619
1483
1302
1880
1182
1995
1698
2193
1028
1518
2171
1101
1712
2228
2108
2107
958
1469
1814
0.10
0.10
0.10
0.10
0.10
0.10
3.13
0.26
0.82
2.06
3.21
18.55
4.48
19.15
7.73
28.29
16.40
11.18
20.40
13.40
20.98
21.03
16.67
33.21
21.05
18.70
13.19
25.40
I 1.36
3 I .49
20.59
32.63
8.18
15.84
29.88
9.18
2.3
4.9
4.7
3.1
5.6
3.9
3.5
3.7
3.1
5.9
5.8
13.2
3.1
3.7
5.1
18.0
6.6
12.6
12.5
11.2
4.3
20.8
11.0
12.7
20.2
5.7
2.7
19.8
3.0
3.5
4.3
20.6
3.7
19.7
6.1
11.1
0.9
20.0
5.1
3.0
17.4
16.3
5.5
10.7
4.9
10.1
8.7
5.4
223
G. Cauuet, I. Sidorot, / Marine Chemistc 53 (1996121 l-227
76
75
74
72
Fig. 9. Distribution
of DOC (FM) in surface (a) and bottom layers(b)
the end of the delta, salinity = 0) some lower values
appear, like if during this transport some DOC is
removed. It is more evident for SPASIBA 2, the
most riverine samples having been collected more
700,
of the Laptev Sea: SPASIBA 2. September
1991.
upper in the river than for SPASIBA 1. To try to
verify if there is an aggregation mechanism, we
made an ultrafiltration on a few samples in the river
and in the deltaic environment. The first interesting
result was the existence of a large fraction of col-
1
0
SPASIBA 1
- SPASIBA 2
100-l
0
5
10
I
15
20
25
SO
35
Salinity
Fig. 10. Variation of DOC (FM) and salinity (g kg-’ ); SPASIBA
1 and SPASIBA 2.
d
Lb1
L:16
LJC9
Lb
L&i
Ll15
StEltiOllS
Fig. I 1. Ratios of colloidal organic and inorganic
dissolved concentrations atong a river section.
carbon on total
G. Cauwet, I. Sidoror/Marine
224
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
I__
A*
-1
*
I.
“,
0
5
.
lb
8 -_ ; : .
15
!%I
DC
DOC
I
is
n .
30
i5
Salinity
Fig. 12. Variation
SPASIBA 2.
of DOC and DIC (FM) and salinity (g kg-’
),
loidal carbon (between 0.7 and 0.01 km), which can
represent of the so-called “dissolved”
fraction (Fig.
11). According to the morphology of the area, the
succession of the stations chosen does not represent
a straight transept; this can be seen with the CIC/DIC
ratio which is “globally”
constant but within large
limits. Anyway, the COC/DOC
ratio is clearly decreasing from more than 50% to about 25%. This
high colloid content and its decrease could explain
the deficit observed
in the upper part of the
DOG/salinity
curve.
Dissolved inorganic carbon (DIG) is low in the
river water (about 500 ~.LM) and increases towards
the sea. It also exhibits a very conservative
behaviour (Fig. 12a), with a good correlation coefficient CR2 = 0.985, n = 31). Uptake by primary pro-
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,
phosphate, organic nitrogen and phosphorus,
but
three times richer in silica and nitrate.
According to Rosswall (1976), the high pH soils
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
similar. Maximum nitrite concentrations
occur during the flood time, and the ammonium varies little
over the year.
To explain the increase of ammonium concentration during the flood time, it should be noted, that
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,
the ammonium
concentration
in Lena delta is increased. These results are in connection
with the
existence
of phytoplanktonic
and zooplanktonic
species.
According to the data of the Tiksi Hydrometeorological Survey, in the Lena delta and coastal waters
of the Laptev Sea more than 100 species of bacillariophyta were identified, from which more than 60
species are diatoms, 20 are green algae, 1.5 are
blue-green
algae, and 6 species are flagellates. In
summer period, more than 90% of the total amount
and 95% of the phytoplankton
biomass was constituted by diatoms. The dominant fresh water species
are Melosira granulata, Asterionella formosa and
Diatoma elongatum, and seawater species are Thalassiosira baltica, Achnanthestaeniata,
Chaetoceros
G. Camvet. I. Sidomw/
Marine Chemistp
wighamii and Nitzshia ,frigida. Within zooplankton,
the dominant groups were Daphniae and Copepods.
However, it is necessary to remark, that the Lena
River water is characterized by a very small total
planktonic biomass (Table 4).
The coastal water masses with salinity l.O-3.0%
are characterized
in summer time by the intense
short term bloom of bacillariophyta and a significant
decrease of all nutrients, except ammonia, was observed in these water masses. In the mixing zone
with salinity more than 3.0%, river diatoms disappear but marine diatoms do not appear. Autotrophic
plankton was present not as seaweeds but as symbiotic infusoria mesodimium.
The POC concentrations
depend primarily on the
amount of suspended matter (SM), and then on the
origin and age of the particulate material. If we plot
the variation of turbidity and particulate organic
carbon, in surface water, from Station 1 and Ll (in
the Lena River) to more marine environments
(Stations 22 and 29), we can observe that total and
organic loads (mg 1-I) are decreasing at the same
rate, while the organic content of suspensions
is
considerably increasing (Fig. 6). These results suggest that an important fraction of suspended matter is
rapidly sinking in the estuary and that particulate
organic matter is involved in this phenomenon
as
much as inorganic particles. Nevertheless,
the increase in organic carbon percentage corresponds to
an increased colonization by marine organisms with
an increasing production found in brackish waters
and coastal marine waters.
Buor-Khaya waters were formed early in bloom
time (June-July),
as a result of mixing the Lena
flood water and Laptev Sea water and are characterized by higher turbidity and POC concentrations.
The POC pattern in the coastal waters of the Laptev
Sea is complex due to its three different origins:
riverine plankton, living and detrital, which is rich in
Table 4
Abundance
of phytoplankton
Period
and zooplankton
21 l-227
225
pigments, terrestrial POC detritus mostly carried by
the river during the flood time. and marine planktonic POC (POC up to 20%).
The total carbon brought by the river. in particulate, colloidal or dissolved form, represents an important discharge. especially during the flood period.
One of the questions is what the fate is of this carbon
pool, if it is consumed more or less rapidly or if it
accumulates in the coastal zone. Because of the low
temperatures registered most of the year, we did not
expect a very intense microbial activity, with the
exception of the summer period when fresh organic
matter is produced by the primary productivity and
the surface temperature higher than the rest of the
year. The nutrient distribution in surface and bottom
waters. in summer and winter (Figs. 4 and 5) gives
us some information about the recycling of nitrogen,
phosphorus and, consequently, carbon. In winter. we
can observe that nitrate in surface water is decreasing from the river (12 p,M) to the sea, showing some
uptake and a production of ammonium. On the contrary, close to the bottom NH, decreases while NO,
increases. reaching concentrations higher than in surface water which is indicative of nitrification. A very
important increase of phosphate also occurs in bottom waters, while it is constant in surface samples.
In summer, nitrates decrease very rapidly at low
salinity in surface waters and remain about constant
in the estuarine zone. In the bottom, on the contrary.
the nitrate regeneration is intense, the concentration
increasing from I.5 to 5 PM. Ammonium
is produced in surface as well as in bottom levels, representing the general biological activity. It seems that
in winter. there is a slow mineralization
process
going on despite of the temperature in bottom waters, regenerating phosphate and nitrate. In summer,
kinetics are faster, and uptake of nitrate is higher
than regeneration in surface water, the deficit being
compensated by that produced in bottom water. In
in waters of the Lena delta
Zooplankton
Phytoplankton
Amount.
Flood
Summer, autumn
Winter
53 ilY!Xi
1000 cells/l
400- 1000
1000-4000
40-200
Biomass, mg/l
0.6-I .4
1.o-4.0
0.1-0.3
Amount, rib/l
0.46-0.70
0.50-0.76
0.07-0.23
Biomass, kg/l
1.2-21.1
I7.0-55.1
0.05-9. I
226
G. Cauwet. I. Sidorm~/Marine
terms of budget, total nitrogen is decreasing in the
low salinity range but about constant in the Laptev
Sea. The system is then reaching a dynamic equilibrium. All these observations
are not proving that
riverine organic matter is consumed, not even partly.
The POC content of bottom sediment is very variable, due to dynamics and differential sedimentation.
Because of this, it cannot be easily compared to that
of river suspensions.
We have seen that DOC is
almost conservative along the salinity gradient. The
relatively high DOC concentrations
encountered on
the shelf breakdown (about 200 ~_LM)suggest that
DOC is accumulating in coastal water on a long term
basis, like a stock of accumulated carbon input from
the Lena. Though we do not have any direct evidence, it seems consistent to think that most of the
marine production is recycled, in surface waters in
summer or in bottom waters in winter, and that only
a small fraction of the terrestrial organic matter
undergoes degradation on a yearly basis. A more
precise budget would need further studies directed to
this problem, with a tentative appreciation of carbon
accumulated in Laptev Sea. a better estimation of the
total primary production and more studies on bacterial degradation (Saliot et al.. 1996).
Acknowledgements
This work was performed in the frame of the
French Russian cooperation program SPASIBA, supported by CNRS (GDR ICO and PICS-99). We
thank Prof. Savostin, Director of the Institute of
Oceanology for the invitation to participate in his
expedition in the Laptev Sea. The authors are indebted to the captains and crews of Russian research
vessels Mezen, Okeanolog, Olikhon and Alexandr
Smimiskiy and the members of the Laboratory of
Regional Geodynamics
(LARGE), for their helpful
participation.
I. Sidorov was supported for nine
months by the University of Perpignan with a fellowship from the French Ministry of Research.
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