ELSEVIER Earth and Planetary Science Letters 131 (1995) 127-141

EPSL

First data on trace metal level and behaviour in two major Arctic river-estuarine systems (Ob and Yenisey) and in the adjacent

Kara Sea, Russia

Min-Han Dai, Jean-Marie Martin

Institut de Biogbochimie Marine, Unite’ Associie au CNRS No. 386, Unite’ de Recherche Marine IFREMER No. 6, Ecole Normale Sup&ewe, 1 rue Maurice Arnoux, 92120 Montrouge, France

Received 10 October 1994; accepted after revison 15 February 1995

Abstract

(Cd, Cu, Fe, Ni and Pb) were determined in the Ob and Yenisey river-estuary systems and in the adjacent Kara Sea in September 1993. The data show a natural low concentration level of ‘dissolved’ ( < 0.4 pm) trace metals (Cd, Cu, Fe, Ni and Pb) in the two rivers and in the Kara Sea as compared to world unpolluted rivers and the central Arctic Ocean, suggesting that the region studied is pristine with respect to trace metals. The pathway of trace metals transported from rivers to the ocean seems to be complicated, and largely influenced by biogeochemical processes taking place in the estuarine mixing zone. Colloidal material (lo4 Daltons-0.4 pm), in addition to its significant contribution to the so-called ‘dissolved’ fraction, has been shown to play a fundamental role in determining the behaviour of both conservative and non-conservative trace metals during estuarine mixing. Hence, colloids may control to a large extent the fate of ‘dissolved’ trace metals as well as their net input from the rivers to the Kara Sea.

1. Introduction

Although there is growing interest in the study of the Arctic environment owing to its signifi- cance in regulating global climate change [l-5], very few studies have been devoted to chemical oceanography in this environment [5-81. This is especially the case for Arctic estuaries and coastal seas, among which the Ob-Yenisey estuaries and the Kara Sea remain one of the most poorly understood Arctic coastal regions. Reliable trace metal studies have not yet been reported for these two estuarine systems, and this paper as- sesses for the first time the concentration level and behaviour of trace elements in this region.

In addition, there is an increasing awareness of the importance of colloids in understanding the geochemistry of trace elements in natural waters [9-181. The Rivers Ob and Yenisey are major Arctic rivers whose biogeochemistry remains to- tally unknown but which are regarded as ‘black rivers’ and suggested to be enriched with organic colloidal material [19]. Thus, they may represent good systems for studying the behaviour of natu- ral colloidal materials at the river-ocean inter- face. Indeed, it has been noted that large amounts of dissolved and colloidal humic substances are transported to the River Ob from surrounding forests, bogs and lakes during the spring flood, whereas suspended organic matter and aquatic

0012-821X/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0012-821X(95)00021-6

128 M.-H. Dai, J.-M. Martin/Earth and Planetary Science Letters 131 (1995) 127-141

organisms play a less significant role in the over- a11 balance of the organic matter 1201. For exam- pie, a preliminary study in a Siberian river (the River Lena) has shown that 57% of the ‘dis- solved’ organic C was associated with colloidal material [21]. In this context, we also examined the role of colloidal material in the trace metal behaviour across this land-ocean interface.

istry of trace metals in estuarine-coastal systems, highlighting the mechanism of trace metal be- haviour during estuarine mixing, which ultimately controls the riverine input to the ocean.

2. Study areas

The results presented here not only feed the very limited trace metal database for the Arctic shelf seas but also provide some direct evidence for the significance of colloids in the geochem-

The Kara Sea is the second largest shelf sea of the Arctic Ocean. It receives more than one third of the total freshwater discharge into the Arctic Basin. The River Ob is the largest river in terms

Annual average

Month

,Ycnisey)

Month

Fig. 1. Discharges from the Rivers Ob and Yenisey Rivers: Long-term averages and during the year of this investigation. (A) Water

discharges. (B) Particulate discharges.

M.-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141 129

130 M.-H. Dai. J.-M. Martin /Earth and Planetaw Science Letters 131 (1995) 127-341

of catchment area (2.99 x 10h km21 and length (5410 km), whereas the Yenisey has the greatest water discharge (1.98 X 10’ m-?/s), with a catch- ment area of 2.5 X lOh km2 and a length of 3844 km. The River Ob has a mean water discharge of 1.35 x lo4 m3/s, and in this respect is the third largest Russian river after the Rivers Yenisey and Lena. The water discharge from both rivers is seasonally variable (Fig. 11, and maximum dis- charge occurs consistently in July. The River Yenisey has a characteristic low turbidity whereas the River Ob has a fairly high concentration of suspended material. This is because the latter flows between banks composed of easily erodible rock, downstream of the confluence of the fast- flowing mountain streams Biya and Katun.

3. Sampling and analysis

Sampling was carried out in September 1993 along two transects across the salinity gradient of the Yenisey and Ob estuaries extending from the river end members to the Kara Sea (Fig. 2). Subsurface waters ( < 10 m> were sampled using an acid-cleaned Teflon pump fitted with PTFE tubing and connectors and were collected in a portable laminar-flow clean bench. Two real sur- face samples (Station 4416 and 4401) were taken from a small plastic boat with acid-cleaned 10 1 polypropylene bottles. Deeper samples (> 10 ml were collected with Teflon-coated Go-F10 bottles fixed on a clean plastic hydrowire. Two litre sam- ples were filtered immediately on board through Nuclepore filters (0.4 pm, 047 mm) under N, pressure. For the colloidal fraction, - 16 1 of sample were prefiltered with Nuclepore filters (0.4 pm, 0142 mm) and a Teflon filter holder in a closed pressurized system (N, gas). Both the filter holders and connectors were made of PTFE and were precleaned by soaking in acid. Then - 8-12 1 of this prefiltrate were further pro- cessed through a crossflow ultrafiltration (CFF) system with a lo4 Dalton polysulfate membrane (corresponding to a pore size of - 3 nm) to separate colloids from the truly dissolved frac- tion. Before sampling, the whole CFF system was carefully cleaned with several acid washes (see

1221 for details). Between samples the system was cIeaned by flushing with Milli-Q water, cycling with 0.1 N HNO, and then with Milli-Q water, and finally by - 1000 ml of sample to condition the system. The CFF system was completely closed to reduce any risk of contamination. The ultrafiltration was interrupted when an enrich- ment factor of about lo-20 was obtained in the colloid concentrates. The CFF experiments (which have been described in detail elsewhere [l&22]) were undertaken on board immediately after the prefihration. Both the prefiltration and ultrafil- tration operations were conducted in laminar-flow clean benches in a clean mobile container. Spe- cial attention was paid to minimizing contamina- tion risk by using ultraclean techniques during all stages of the sample collection, handling and processing.

The trace metal concentrations were measured in both the prefiltrate and ultrafiltrate (being acidified to pH = 2 on board with ultrapure HCI> by graphite-furnace atomic-absorption spec- trophotometry after extraction in a Class-100 clean room using a method modified from [23]. Relatively low recoveries were observed for Pb (- 80%), whereas 90-105% recoveries for other metals were obtained when compared to the NASS-4 standard seawater reference material (Canada). Colloidal metal concentrations were calculated as the difference between concentra- tions measured in the prefiltrate (< 0.4 pm, re- ferred to in the text as ‘dissolved’) and in the ultrafiltrate (< 10” Daltons, referred to as ‘truly dissolved’).

It should be noted that the colloidal metal concentration calculated as mentioned above might be questionable because of the lack of mass balance of the different fractions of metals from ultrafiltration. With this in mind we used freshwater from the River Seine in France to test for possible adsorption of metals onto the CFF membrane. Concentration variations during CFF recycling were determined in the ultrafiltrate as a function of time (up to 136 min). The results (see [18]) showed that the concentration variations were not significant, and therefore the adsorption of metals onto the membrane is probably not important on a timescale of minutes to 2 hours,

M.-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (199.5) 127-141 131

Salinity

0 10 20 30 40

Depth 0

(m) 20

40

60

80

100

120

140 I

-2 -1 0 1 2 3

Temperature (“C)

Fig. 3. Vertical distributions of temperature and salinity at Station 4397 in the mixing zone (maximum depth 140 m).

Table 1

whereas our ultrafiltration process usually lasts < 15 min. Another study has already calculated the mass balance from CFF processing (with a membrane similar to ours), and the recoveries have been shown to be acceptable (72-124%) [24]. However, further experiments on the mass balance for ultrafiltration are still necessary to confirm the colloidal trace metal concentration calculated by difference, which is the method used in this study.

4. Results and discussion

4.1. Important hydrological and hydrochemical fea- tures

The hydrology of the study area is very similar to that observed in the Lena estuary and the Laptev Sea [25]. There is a large difference between surface and bottom salinity as the result of strong stratification. The surface layer is made up of relatively warm river water discharged by the Rivers Yenisey and Ob and the bottom layer

Ranges of trace metal concentrations in the Rivers Ob and Yenisey and in the Kara Sea

Metal Fraction Ob river Yenisey river Bottom water, Kara sea Cd (PM) “Dissolved” 5.4-l 5 10.7-16.4 184.8-364.6

Truly dissolved 2.3-3 7 26 203 4

Colloidal 3.0-3.8 81 18.6

Cu (r-W “Dissolved” 29. I-38.0 21.5-29 5 2 6-4.2

fruly dissolved 9.1-l I 2 I5 0 I9

Colloidal 20.0-24 6 14.5 0.7 Fe (W “Dissolved” 429 7-654 2 251.0-317 1 2 9-5 2

Truly dissolved 36 O-44 6 90 I7

Colloidal 385 l-414.2 308. I 34

Ni (t&l) “Dissolved” 21 .O-23.7 8 8-9.4 4.5-5.9

Truly dissolved 10.5-12.2 34 4.4

Colloidal IO 4-10.6 60 01

Ph (PM) “Dissolved” 54 9-82.6 24.9-29 4 I I .4-35 6

Truly dissolved 26.3-35 3 I94 123

Colloidal 25 l-28 5 56 I3 2

Organic C (PM) “Dissolved” 614 4-828.9 333 8 76.7

Truly dissloved 356.1-492.2 212.4 64.4

Colloidal 258.3-336.6 121.3 12.3

Note that concentrations for each fraction might not refer to the same sample, and thus the truly dissolved concentration may be larger than the total dissolved concentrations

132 M.-H. Dai, J.-M. Martin/Earth and Planetary Science Letters 131 (1995) 127-141

Ob River

300 Cd

“Dissolved”

Truly dissc:lvetl A

40 1

t “Dissolved” (R’=O.911 cu

E a 30 Truly dissolvetl (112=O.X4)

2 ‘C e 20 C’olbitlnl (R’=O.XS)

9 z G 10

0

0 10 20 30 -IO

700

E 600

.= 500 .: ~III~ clissolvctl (P=n.w 400

z t; 300 aI $ 200

u 100

0 0 10 20 30 40

Salinity

Yenisey River

0 10 20 30

“Dissolvecl” (R’=O.X7) cu

i Truly clissdvccl (11*=0.78)

FC “Dissolvctl”

( Truly tlisiolvucl (l<‘=O.30) I

0 10 20 30 40

Salinity

Fig. 4. Trace metal fraction concentrations as a function of salinity in the Oh and Yenisey estuaries. The curve fitting was processed

with the microprogram Excel 5.0. Most of the correlation coefficients are > 0.7, with a confidence level of > 99.99%.

M.-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141 133

Ob River

30

A “DLsolved~‘(R2=0.RS) Ni

Truly dlssolvetl (1~2=0.S5)

0 10 20 30 .0’

100

0 10 20 30 -IO

900

Truly dirsolvcd (R’=O.PO)

0 10 20 30 -+o

Selinit)

Yenisey River

20 Ni

“Dissolved”

0 10 20 30 40

0 10 20 30 40

100

0 -1

0 IO 20 30 -IO

Sidinit>

A- “tlisuolvctl” frilctiou

X .-.- .-. .---.. truly dissolved fraction

O- cdll~idill fraction

Fig. 4 (continued).

134 M.-H. Dai. J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141

is cold Arctic seawater. Such a structure results in a good correlation between salinity and tempera- ture CR2 = 0.92). These two water masses are frequently separated by a well-delimited interme- diate mixing layer. A typical example obtained at Station 4397 is shown in Fig. 3.

The nitrate concentration is 2.1-3.2 PM in the River Ob and 0.08-0.19 PM in the River Yenisey, the value for the Yenisey being much lower than previously reported (5 PM for both rivers) [26]. The phosphate concentration ranges from 1.3 to 1.5 PM in the River Ob and from 0.07 to 0.22 PM in the River Yenisey, which is similar to previously reported values [26]. The low concen- tration of nitrate is probably due to consumption by phytoplankton, given that the River Yenisey is characterized by low turbidity (i.e., high trans- parency), a condition that is favourable for pri- mary productivity. Conversely, the River Ob has a rather high concentration of suspended sediment, which could result in a slight limitation on pri- mary production. Such nutrient cycling should influence trace metal concentration and be- haviour in estuaries. Phosphate requirements are clearly supplied by oceanic sources, judging from the PO:--salinity curve, which exhibits a marked increase on passing from the rivers to the Kara Sea. Accordingly, the nitrate/phosphate mole ra- tios are 1.6-2.5 in the River Ob and 0.4-2.7 in the River Yenisey, ratios that are lower than those in typical world rivers [15,27].

4.2. Trace metal concentration level

The ranges of trace metal concentrations in the truly dissolved, colloidal and ‘dissolved’ phases in the area studied are shown in Table 1.

The ‘dissolved’ Cd concentrations are 5-8 pM in the River Ob; those for the River Yenisey range from 11 to 16 pM. These values are the lowest reported so far for major world rivers, although they are similar to the values in a pris- tine mountain watershed in the Sierra Nevada, California (11 pM on average) [28]. The concen- trations of ‘dissolved’ Cd in the Kara shelf bot- tom waters (salinity S > 33%0) are ca. 0.1-0.4

nM, comparable with those reported for the cen- tral Arctic waters (0.3 nM) [29,38], the eastern Arctic Ocean (0.08 nM, the labile fraction) [30,31] and an Arctic Ocean ice island [32]. Colloidal Cd concentrations vary from 3 pM to 54 pM, which accounts for l-76% of the ‘dissolved’ Cd in the estuary. The maximum colloidal concentrations are in river waters (the River Ob (50-57%) and the River Yenisey (76%)).

‘Dissolved’ Pb concentrations are generally low in the study area, some samples being close to the detection limits of AAS even though they had been concentrated 200 times. Analytical uncer- tainties can consequently be large (up to 30%). The concentration range of ‘dissolved’ Pb is 55-83 pM in the River Ob, which is comparable to that in the River Lena (80 pM) [33]. In the River Yenisey the concentration of ‘dissolved’ Pb is 2-3 times lower (25-29 pM) than in the River Ob. Rahn [61 suggested 72 pM as a ‘typical’ Arctic river Pb concentration, a level similar to that in the River Ob. All these values are lower than the previously proposed world average river ‘dis- solved’ Pb concentration (150 PM) [34], but they are similar to the value measured in the above- mentioned mountain stream in California (43 pM) [28], and could be regarded as a natural Pb level for river waters. Colloidal Pb accounts for 42-52% of the ‘dissolved’ Pb in the River Ob and 22% in the River Yenisey, but these proportions de- crease seaward.

‘Dissolved’ Cu concentrations are 22-29 nM in the River Yenisey and 29-38 nM in the River Ob. These values are twice those in the River Lena [33] but are very close to measured values in the Amazon [35], the Mississippi [36] and the River Changjiang [37]. In the waters of the Kara Sea ‘dissolved’ Cu concentrations decline to 3-5 nM, corresponding to the central Arctic seawater concentration (5 nM) [30]. Both the colloidal Cu concentration and its proportion in the ‘dissolved’ fraction are higher in the Ob estuary than in the Yenisey estuary. It has been argued that colloidal Cu is mainly associated with organic colloidal material [22]. Indeed, colloidal organic carbon is much more enriched in the River Ob (337 PM) than in the River Yenisey (121 PM).

‘Dissolved’ Fe concentrations in both rivers

M.-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141 135

are very similar to the estimated world average [34]. At high salinities these concentrations vary from 2 to 7 nM, in accord with the concentrations reported for the Laptev Sea [33]. The lower limit is also very close to that in the central Arctic [40]. Colloidal Fe is clearly prevalent in the so-called ‘dissolved’ fraction in both estuaries, comprising 30-97% of the ‘dissolved’ Fe. In the rivers stud- ied almost all the dissolved Fe is associated with colloidal matter (97% in the River Yenisey and 89-92% in the River Ob). It has been argued that

many studies have overestimated the dissolved Fe concentration in rivers [39]. This study, as well as studies on Venice Lagoon [18], on several Japanese rivers 142-441 and on two small rivers in northwestern Spain (Dai et al., in prep.), confirms that the so-called ‘dissolved’ Fe is essentially as- sociated with colloidal material in river waters.

‘Dissolved’ Ni ranges from 8.8 to 9.4 nM in the River Yenisey, twice that in the River Lena [33], whereas in the River Ob ‘dissolved’ Ob varies between 21 and 24 nM, which is similar to values

60

50

g 40

G 3 30

._

2 20

s

10

0

16

II

- E

12

5 10 ._ 2 z 8

._ o 6

=

s 4

2

0

y = 0. l-lx + 1.1-l

R’ = 0.87 (Yenisey)

IO0 1%) 200 250 3l)O 330

Colloidal organic carbon (PM)

y = 0.03x + I.30

R’ = l~.S7 (Ob)

0 IO0 I50 200 250 .300 350

Colloidal orgilllie C;I&OII (PM)

Fig. 5. Linear relationship between organic C and trace metals in the colloidal fraction. (A) Cu. (B) Ni.

136 M.-H. Dai, J.-M. Martin/Earth and Planetary Science Letters 131 (1995) 127-141

reported for the Mississippi [36]. The concentra- tion decreases to 3-5 nM in the Kara Sea, as observed in Arctic Ocean waters [31,40,41]. Col- loidal Ni shows higher concentrations than in other coastal zones (e.g., Venice Lagoon [18] and the RhGne delta [22]), and makes up - 50% of the ‘dissolved’ Ni in the River Ob and - 60% in the River Yenisey.

4.3. Trace element behauiour in the mixing zones

Both ‘dissolved’ and colloidal Cu appear to be conservative in these estuaries (Fig. 4). Such be- haviour has already been observed in the Rhone river estuary [22] and has been hypothesized as

40

35

30

25

20

15

10

5

0

being due to the organic character of colloidal Cu, which prevents its flocculation. A satisfactory correlation between colloidal Cu and organic col- loidal carbon in the Ob and Yenisey estuaries (Fig. 5A) provides some direct evidence for such a mechanism. A very similar relationship has already been observed in the Rh6ne delta [22]. However, a critical question arises concerning the colloidal organic carbon-salinity relationship, which shows removal of colloidal organic carbon (Fig. 4): how can colloidal Cu remain conserva- tive while colloidal organic carbon is removed? It is possible that colloidal organic matter is com- posed of at least two subfractions, one a physico- chemical refractory (conservative) subfraction and

46 (Ob) A

i

R’ = 0.

0 0

Y= : 0.28x + 5.54 Rz = 0.77 (Y enisey)

0 20 40 60 80 100

25 r v = 0 34s + 0 08 B A R’ = 0 33 (Ob)

y = 0.14x + 0.90

R’ = 0.93 (Yenisey

0 I A I L / I

I I

0 20 40 60 80 100

Silicate (PM)

Fig. 6. Relationship between silicate and Cu in ‘dissolved’ and colloidal fractions in the Ob and Yenisey estuaries. (A) ‘Dissolved’ 03) Colloidal.

M.-H. Dai, J.-M. Martin/Earth and Planetary Science Letters 131 (199.5) 127-141 137

10

._ z

% 6

.w 0

=:

s 4

2

0

0 5 10 15 20 25

Colloidal Cu (nM)

Fig. 7. Linear relationship between Cu and Ni in the colloidal fraction.

another removed across the river-ocean inter- face. Moreover, as observed by [45], ‘dissolved’ Cu covaries closely with silicate, especially in the Yenisey estuary (Fig. 6A). An even more signifi- cant correlation between colloidal Cu and silicate (Fig. 6B) in the Y enisey estuary might imply that the colloidal particles are composed mostly of degradation products of diatoms that predomi-

‘5. 20

2 18

.- 0 16 ‘: 8 14

4,- 12

8 5. lo .- 3

8

2 6 .I D 4 3 ?

2

b 0

nate in the phytoplankton community in the River Yenisey [46].

Like Cu, ‘dissolved’ Ni exhibits similar conser- vative behaviour in the Ob estuary (Fig. 4). This resemblance may indicate that both elements are largely governed by colloidal organic material. The correlation between colloidal Cu and Ni supports this point (Fig. 7). In the Yenisey estu-

0 1 Nitraf2e (FM)

3 4

A Truly dissolved Ni o Colloidal Ni

Fig. 8. Nitrate versus truly dissolved and colloidal Ni in the Yenisey estuary.

138 M.-H. Dai, J.-M. Martin /Earih and Planetary Science Letters 131 (1995) 127-141

ary, however, some excess compared to the theo- retical dilution line is observed when the salinity increases from 0 to N 10%0. At that point the concentration of ‘dissolved’ Ni declines, a case similar to that in the River Lena 1331 and in many other estuaries [45]. Although desorption from particles could account for this excess, it is more likely to be related to a biological regeneration process as this additional Ni does not occur in the River Ob where biological activity is lower than in the River Yenisey. Additional support for this hypothesis is provided by the close relationship between nitrate and truly dissolved Ni (Fig. 81 but not colloidal Ni, which suggests that the addi-

tional Ni has the same origin (regenerated from organic matter) as nitrate whereas colloidal Ni is primarily associated with organic matter. Indeed, a reasonably good correlation between colloidal Ni and colloidal organic carbon occurs in both estuaries (Fig. 5B).

The distribution trends of ‘dissolved’ Cd with salinity in the two estuaries appear to be compli- cated and scattered. However, overall the con- centrations increase from the river end members to a salinity of 5 lO%o, which is probably related to the well-recognized desorption of particulate Cd 135,471, and then declines until a salinity of N 2.5%0. Finally, it increases again (Fig. 41, which

400

350

300

250

200

150

100

50

0

y = 201 57x ~7 49.0!

A k’ = 0 54 (Yenisey

450

- z

400

a 350

z 300

u a

250

2 200

eg 150

.z 100

= 50

0

Phosphate (pM)

I y = 36.68x + 89.80 ~ . hb R’ = 0 78 (Yenisey)

.*o B . . 0 . .

,-+

4 0 8 IO

Nitrate (pM)

1 A Ob 0 Yenkey 1

Fig. 9. ‘Dissolved’ Cd versus (A) phosphate and (B) nitrate.

M.-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141 139

is probably related to an additional benthic source, or, because the high-salinity samples were collected in bottom water layers, to desorption from resuspended sediment. However, the maxi- mum Cd concentration may also be associated with nitrate and phosphate regeneration, the Cd concentration correlating reasonably well with phosphate and nitrate concentrations (Fig. 9).

Fe is strongly removed in both the Ob and the Yenisey estuary (Fig. 4). As noted earlier, col- loidal Fe dominates in the ‘dissolved’ fraction in the upper parts of the estuaries and it definitely plays a central role in controlling the overall Fe behaviour during the mixing process. Such char- acteristic reactivity of colloidal Fe is confirmed by the overall conservative nature of truly dissolved Fe, indicating that the flocculation of colloidal material is responsible for the removal of the ‘dissolved’ Fe. However, the nature of this re- moved fraction of colloids is not clear, although it is probably related to humic substances, which are made up of macromolecules and removed in estuaries [48].

unpolluted rivers flowing into the Arctic Ocean. A similar situation has been observed in the River Lena [33]. The concentrations of trace met- als in the River Yenisey are overall lower than in the River Ob, which may be not only related to the lower weathering rate but also to the higher biological productivity in the River Yenisey com- pared to the Ob. Nevertheless, both rivers have comparable or even lower trace metal concentra- tions than most major world rivers.

Similarly, ‘dissolved’ Pb shows some removal but less significantly than for Fe, whereas truly dissolved Pb remains approximately conservative. Indeed, the proportion of colloidal Pb in the ‘ dissolved’ fraction decreases along the salinity gradient.

The relatively significant removal of Fe and Pb in the Ob estuary as compared to the Yenisey estuary is most likely related to the greater abun- dance of colloids in the River Ob than in the River Yenisey. Indeed, suspended macroparticu- late material @PM) is much more abundant in the River Ob (20.1-143 mg/l) than in the River Yenisey (5.4-5.7 mg/l). Although there is no information on the colloid mass concentration, a tightly linear relationship (R2 = 0.88) between colloidal organic carbon and SPM would support the idea that colloid abundance could be propor- tional to SPM concentrations.

As already reported ]18,22], a significant amount of the ‘dissolved’ trace metals is actually associated with colloidal material. In addition, the colloidal material appears to be the most reactive and heterogenous phase, which deter- mines to a large extent the behaviour of the trace metals (i.e., either conservative or non-conserva- tive behaviour). Honeyrnan and Santschi [9] sug- gested that colloid flocculation is responsible for the net scavenging of Th in the marine environ- ment. This may also be true for trace metals in estuaries where colloid aggregation leads to the removal of the so-called particle-reactive trace metals (e.g., Fe and Pb). So far there is no convincing evidence to allow us to specify whether this flocculated colloidal fraction is organic, inor- ganic or both (e.g., an organic coating on inor- ganic colloids), because colloidal organic carbon (CO0 is also seen to be flocculated in both the estuaries studied. In addition, organic-preferring metals such as Ni and Cu, which are usually conservative during estuarine mixing, are also controlled by the ‘conservativity’ of the subfrac- tion of organic colloids that is refractory to physicochemical variation and hence more stable across mixing zones. Regarding the non-conserva- tive behaviour of COC across the mixing zone, it is suggested that even COC could be a heteroge- nous mixture.

5. Conclusions

More complicated mechanisms such as defloc- culation may be playing an important role, as has been suggested for the Rh6ne delta [22]. Under certain circumstances colloid flocculation might not be observable on the timescale of estuarine mixing (e.g., the case of Mn in Venice Lagoon ml>.

With regard to trace metal concentrations both In summary, in addition to the qualitative dif- the Ob and the Yenisey can be regarded as ferentiation of the colloidal material from both

140 M-H. Dai, J.-M. Martin /Earth and Planetary Science Letters 131 (1995) 127-141

dissolved and macroparticulate substances [lo- 121, this study, like previous ones [l&22], confirms that colloids play a major role in the biogeochem- ical cycling of trace metals in aquatic environ- ments, in particular in estuaries.

Acknowledgements

This study was undertaken in the framework of the ‘Scientific Program on the Arctic and Siberian Aquatorium’ (SPASIBA) as part of the Russian JGOFS, which was partly funded by the Centre National de la Recherche Scientifique (France) (PICS 99). We thank the captain and the crew of the R.V. Dcmitriy Mendeleerl for their help during the expedition. A. Lisitzyn and V.V. Gordeev are gratefully acknowledged for organiz- ing the cruise. I. Sidorov provided the discharge and SPM data. We also acknowledge M. Co- query and P. Stunzhas for their permission to use the nutrient data. We appreciate W.W. Huang and M.H. CottC for their assistance during the analytical work and P. Statham for his critical comments. [ MK I

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