Dynamics of particulate trace metals in the lakes of Kejimkujik National Park, Nova Scotia, Canada

The Science of the Total Environment, 87/88 (1989) 315-328 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

315

DYNAMICS OF PARTICULATE TRACE METALS IN THE LAKES OF K E J I M K U J I K NATIONAL PARK, NOVA SCOTIA, CANADA

JEROME O. NRIAGU and HENRY K.T. WONG

National Water Research Institute, Box 5050, Burlington, Ontario LTR 4,46 (Canada)

ABSTRACT A study of the settling characteristics and the accumulation rates in sediments suggests that a large fraction of the organic carbon and trace metals in the stmpended particulates in Kejimkujik Lake has been imported from the lake's watershed. The mean seston settling velocity has been estimated to be about 0.1m day -1 in Kijimkujik Lake and 1-2m day -1 in Mountain Lake. For the particulate organic carbon, it has been estimated that the mean flux through the water column is 167mg m -2 day -1, the mineralization rate is 16rag m -2 day -1, and the accumulation rate in sediments is 9rag m -2 day -1. It is shown that the movement of the.recalcitrant organic material is the key feature controlling the fate and behavior of trace metals in these lakes.

INTRODUCTION

Suspended particulates (or seston) play a major role in the cycling and removal of contaminants from the water column (Buff]e, 1987). The adsorption of trace metals onto the seston is believed to be of particular importance in determining their fate in natural waters (Sigg, 1987; Xue et al., 1988). Although sediment traps have been used extensively in studying the role of seston in the cycling of trace metals in the ocean (Jickells et al., 1984; Fischer et al., 1986; Paulson et al., 1988), their use in the study of particulate trace metal dynamics in lakes has been very limited. This paper deals with the chemical properties and depositional fluxes of seston and associated trace metals in the water and sediments of three lakes in Kejimkujik National Park, Nova Scotia. As the lakes are located in a remote area, most of the trace metal pollution in these lakes is apparently delivered via the atmosphere. The study is aimed at understanding the processes of metal immobilization from the water column. The impact of acid rain on the trace metal cycle in lakes is also of particular concern. STUDY AREA, MATERIAL AND METHODS

Kejimkujik National Park is located in central-west Nova Scotia (latitude 44° 20' north and longitude 65° 15' west), about 200 km southwest of Halifax and 450kin northeast of Boston. The park covers about 380kin ~ and contains 0048-9697/89/$03.50

© 1989 Elsevier Science Publishers B.V.

316 n u m e r o u s i n t e r c o n n e c t e d lakes in a gently-rolling landscape. M a n y lakes are b o r d e r e d by bogs and a b o u t 90% of the p a r k is c o v e r e d by a m i x t u r e of c o n i f e r o u s and h a r d w o o d trees. K e j i m k u j i k , the largest lake in the park, has b e e n u n d e r i n t e n s i v e s t u d y as a p a r t of the c a l i b r a t e d lake c a t c h m e n t p r o g r a m w h i c h s t a r t e d in 1978 (Kerekes, 1975). K e j i m k u j i k L a k e (Kej h e n c e f o r t h ) is an acidic (mean pH 4.9), h i g h l y colored (50-80 H a z e n units) body of w a t e r with a surface a r e a of a b o u t 26kin 2, a m a x i m u m d e p t h of 19.2 m and a m e a n d e p t h of a b o u t 4.4 m. T h e d r a i n a g e basin is a b o u t 28 times the surface a r e a of the lake and the w a t e r is r e n e w e d a b o u t 5.5 times per y e a r (Kerekes, 1975). G e o r g e L a k e is a small, shallow body of w a t e r r e c e i v i n g the outflow from Kej; b o t h lakes are d r a i n e d by the M e r s e y River. G e o r g e L a k e has a surface a r e a of a b o u t 78 ha, a m a x i m u m d e p t h of 8.5 m, a m e a n d e p t h of 4.4 m, and a c a t c h m e n t basin a b o u t 880 times t h e size of the lake's surface a r e a (Kerekes, 1975). W i t h a w a t e r r e n e w a l r a t e of a b o u t 320 times per year, this lake m a y be r e g a r d e d as a wide " r i v e r " . M o u n t a i n L a k e is a c l e a r h e a d w a t e r lake with a c a t c h m e n t basin approxim a t e l y 6 times the lake's surface a r e a of 136 ha. It has a m a x i m u m d e p t h of 14.3 m, a m e a n d e p t h of 4.3 m, and w a t e r r e n e w a l time of 1.2 times per y e a r (Kerekes, 1975). O t h e r physical and chemical c h a r a c t e r i s t i c s of the t h r e e lakes u n d e r s t u d y are given in T a b l e 1. S e d i m e n t t r a p s were deployed in the deepest basins of K e j i m k u j i k and M o u n t a i n L a k e s at depths of 3 m below the surface and 3 m above the s e d i m e n t w a t e r interface. One t r a p only could be installed at the mid-column of w a t e r in the basin of G e o r g e Lake. T h e traps in the t h r e e lakes were r e f u r b i s h e d e v e r y

TABLE 1 Chemical and limnological data for Kejimkujik area lakes a Kejimkujik Lake

Area (ha) Max. depth (m) Mean depth (m) Water renewal rate (times year- z) Color (hazen units) DOC (mg C 1-1) Conductivity ~umho cm- 1) pH (annual mean) Ca (mg 1-i) Mg (mg 1-1) Na (mg 1-1) C1 (mg 1-1) SO4 (mg 1-1)

2630 19 4.4

George Lake

78 8 2.4

Mountain Lake

140 14 4.3

5.5 50-80 12

300 60-150 12

1.2 10-15 2.9

30 4.9 0.83 0.47 2.7 4.5 3.9

30 4.9 0.83 0.47 2.7 4.5 3.9

23 5.4 0.78 0.45 2.7 2.2

"J.J. Kerekes, personal communication (1986).

317 3-4 weeks from April to November. Since Kejimkujik Lake was readily accessible during the winter months (ice conditions made the retrieval work unsafe in November and March, however), sediment traps were installed and retrieved by drilling holes through the ice cover. We thus have a continuous record of the seston flux in Kej from January to December, 1985. The particles intercepted by the traps were freeze-dried, digested with aqua regia and the concentrations of Pb, Zn, Cu, Ni, and Fe in the leachate determined by means of atomic absorption spectrometry (Nriagu et al., 1982; Wong et al., 1984; Wong and Nriagu, 1985). Fluxes were derived from the concentrations of trace metals and the dry weight of particulates intercepted per unit time. Sediment cores at the site of the trap moorings were also obtained (by divers), subsectioned and freeze-dried. Trace metal concentrations in aliquots of the sediment samples were determined following the techniques used for the analysis of the seston. The age of each core section was determined by lead-210 geochronology (Nriagu et al., 1982; Wong et al., 1984). RESULTSAND DISCUSSION During the period of this study, the particulate organic carbon (POC) concentrations in Kej ranged from 9 to 21% and averaged 14 + 2% (Table 2). Seasonal variations in the POC distribution are very subtle, with the lowest values observed in the winter months. Assuming that the average composition of planktonic biomass is C,2eN16H~Oe0(PO4)(SH) (Vollenweider, 1985), the POC would be related to particulate organic matter (POM) by the expression: POM = 2 × POC From such a relationship, it is estimated that organic matter accounts for 28% of the seston in this lake. The low sestonic organic matter content of Kej is unlike that reported in lakes in the Algonquin Provincial Park (Ontario) and in the Sudbury region where the suspended particulates typically contain 40-60% organic matter (Nriagu et al., 1982; Wong and Nriagu, 1985). It is believed that the seston in Kej is dominated by inorganic particulates (see below). By contrast, POM constitutes about 50% of the seston in Mountain Lake (Wong and Nriagu, unpublished results). There is a close similarity in the seasonal variations of the seston flux and the settling rates for A1 and Fe in Kej (Fig. 1). Such a relationship underscores the fact that inorganic solids constitute a major fraction of the seston in this lake. In Kej there is a tendency for A1, Fe and seston fluxes to increase from January to maximum values in late summer and to decrease in fall/winter (Fig. 1 and Table 2). Since the water in this lake is renewed every 66 days (Kerekes, 1975), the observed seasonal trend may be attributed to the increased supply of suspended inorganic solids from the watershed during the summer months. There is no discernable seasonal pattern in the A1, Fe and seston fluxes in Mountain Lake (Fig. 2), a surprising feature considering the higher organic matter concentration in the trapped material.

318 TABLE 2

Organic carbon and nitrogen contents of seston in Kejimkujik Lake

Jan-Feb

Feb-Apr

Apr-May

May-June

June~luly

July-Aug

Aug-Sept

Sept-Oct

Annual average

A1 a A2 B1 A1 A2 B1 B2 A1 A2 B1 B2 A1 A2 B1 A1 B1 B2 A1 A2 B1 A1 B1 B2 A1 A2 B1 B2

%C

%N

C/N

10.1 9.2 15.1 12.8 12.9 13.7 14.1 14.1 14.5 14.9 15.1 14.4 14.9 19.7 12.8 13.2 13.5 15.2 14.6 16.1 15.1 12.5 16.4 14.8 12.9 15.3 14.6 14

0.583 0.744 1.16 1.05 0.945 1.11 1.09 1.2 1.21 1.21 1.35 1.2 1.14 1.84 1.12 1.17 1.22 1.38 1.6 1.59 1.38 0.94 1.48 1.59 1.13 1.33 1.15 1.22

17 12 13 12 14 12 13 12 12 12 11 12 13 12 11 11 11 11 9 13 11 13 11 9 12 12 12 11

a A and B are stations in different depositional basins.

The distribution of particulate organic nitrogen (PON) also shows subtle temporal variations (Table 2). The concentrations in Kej ranged from 0.58 to 1.8% and averaged about 1.2 _+ 0.27%. The average POC/PON ratio of 11 for Kej is much higher than the values of 5-10 that have been reported for seston in the lakes of Ontario (Nriagu et al., 1982; Wong and Nriagu, 1985) or the ratio of 6.8 typical of biomass in aquatic ecosystems (Vollenweider, 1985). The high POC/PON ratio suggests that a large fraction of the organic matter associated with the seston is derived allochthonously. This is not surprising considering the large input of detrital material due to the fact that the catchment basin is about 28 times the size of the lake's surface area. Furthermore, the drainage basin is bordered by several marshes, bogs and wetlands, which are well-known

319

(s=~o,,) -100

5OO0

KEJIMKUJIK LAKE

4t18

(rag rn2Er I ) 4000"

-X-143OOO-

,(Fe)

so

#10 •

2000"

(,,1)

AI

-X-6-

R3

1000 ~4-

-X'0

J

#0.7 -

,(~)

3000 -

0.3

2OOO

0.2

(Zn)

-lt 0,3

0.1

IOOO

~O.1 ~-0

J~l

F~B MAR APR MAY

,,~N

JUL AUG

~P

O~

NOV

DEC

o

Fig. 1. Seasonal variations (1985) in the settling rates of seston and particulate A1, Fe, and trace metals in Kejimkujik Lake, Nova Scotia.

sources of allochthonous organic carbon. It is not clear, however, whether the organic matter is brought into the lake in particulate form or as dissolved organic compounds which are subsequently adsorbed by particulates in the lake water. The latterseems more plausible in view of the fact that the average dissolved organic carbon (DOC) concentration in the lake water is 11g m -s (Beauchamp, 1983). Average primary productivity during the ice-free season has been estimated to be about 55 mg C m - 2 day- 1(Beauchamp, 1983). By comparison, the measured POC flux during this study was 195mg C m -2 day -1. The fact that the POC flux is much higher than the actual plsnktonic production in the lake is another piece of evidence for extensive importation of organic matter from the surrounding watershed. In this connection it should also be noted that the POC

320 10

50

MOUNTAIN LAKE

(#g ~d "1)

40

30

(~)

(AO

So++ Fe 20

10

+~

o~ 0

0

1000

O9OO O8O0

0TOO

-

,•

(Pb)

(Zn)

0.08

0.20

0.06

0.15

0.04

0.10

0.02

0.05

Seston

O6O0 (Seston)

0500-

zn

04000300-

~

Pb

0200oi00 0

J~N FEB IvY" AI~R MAY JI~IN JLIL AUG S~Ip OCT I~ DEC

0

Fig. 2. Fluxes of seston a n d particulate Fe, A1 and trace metals in M o u n t a i n Lake d u r i n g the s u m m e r of 1985.

concentration in Kej was only 2.0 mg 1-1, compared with the DOC concentration of 11 mg 1-1 (Blouin, 1985). The net settling velocity, Q, for the seston through the water column can be derived from the relationship: Q = (seston flux, mg m -2 day-1)/(conc, of seston, mg m -a) From the POC concentration (2 mg 1-1), and since the POC constitutes only 14% of the suspended particulates (see Table 2), the standing crop (concentration) of seston in the water column has been estimated to be 14mg 1-1. Substituting the average particulate flux during the ice-free period (Table 2)

321

results in the mean sestonic settling velocity of 0.10 m day-~ for surface waters and 0.13m day -1 for the bottom waters. These settling velocities may be compared with the 1.2-1.8m day -~ for detrital organic carbon in Lake St George, Ontario (Burns and Rosa, 1980). Stabel (1987) found that the mean sinking velocities of seston in Lake Constance varied from 2.6m day -1 in the euphotic zone to 7.5m day -1 in the aphotic zone. The settling velocities in Lake Zurich of 10-15m day -~ (Weilenmann, 1986) are much higher than those in our lakes. Intuitively, one would expect, on the basis of Stokes relationship, that the mean size of the seston in Kej should be smaller than those of the other lakes. This may not be the case, however. Kejimkujik Lake is not only shallow but also contains about 120 islands (Beauchamp, 1983). Extensive resuspension of sediments, which would maintain the standing crop of seston at the high concentration of 14 mg l-1, should be expected. Thus, the low net settling velocity most likely reflects the processes responsible for the high reservoir of suspended particles in the water column. This feature of seston is quite important in understanding the cycling of trace metals in Kej and other shallow lakes. The average residence time, T, for seston in the water column is given by: T = (thickness of water layer, m)/(settling velocity, m day -1) For the average depth of 4.4m and settling velocity of 0.1m day -1, T is estimated to be 44 days in Kej. By remaining for a long time in the water column, the particles increase their effectiveness in scavenging the metals in the water. Furthermore, the water flushing time for the lake is only 66 days (Kerekes, 1975), suggesting that the seston and associated trace metals can be transported out of the lake rather than deposited in the sediments. Indeed, the long-term rate of accumulation of sediments in Kej of 90rag m -2 day -1 represents a mere 8% of the average flux of seston to the surface traps. In other words, only a small fraction of the seston and associated trace metals in the water column is retained in the sediments with the rest being exported out of the lake. The organic carbon content of the Kej sediments is 10% (Beauchamp, 1983), from which the accumulation rate of organic carbon is estimated to be 9 mg m- 2 day -~. As a first approximation, the mineralization rate of organic carbon in the water column can be assumed to be the difference between the mean production rate of 25 mg 111-2 day -~ (Beauchamp, 1983) and the accumulation rate of organic carbon in the sediments. The calculated mineralization rate of 1 6 r a g m -2 day -1 is small compared with the POC flux of 167rag m -2 day -~. It would thus appear that the organic carbon in the seston, as well as the associated trace metals in Kej, (a) are mostly imported into the lake, (b) may be extensively recycled but are not being significantly biodegraded in the water column, and (c) are exported to a large extent via the Mersey River. Although the lake functions as a large collection basin for the large quantity of organic compounds released from its watershed, the Kej sediments actually represent an inefficient sink for pollutant trace metals loaded into the lake basin (see below).

322

The levels of trace metals in the seston (Tables 3-5) are in reasonable agreement with the concentrations reported in Lake Windermere, United Kingdom (Hamilton-Taylor et al., 1984), Lake Constance (Stabel, 1987), and Lake Lugano, Italy (Premazzi and Marengo, 1982). The calculated fluxes of particulate Pb, Zn, Cu and Ni are also shown in Tables 3 (Kej) and 4 (Mountain Lake). In general, the fluxes into the bottom traps are marginally higher than those into the surface traps. The enhanced seston capture in the deep waters

TABLE 3

Particulate metal and seston fluxes in Kejimkujik Lake ( m g m -2 day -1)

Jan-Feb

Feb-Apr

Apr-May

May-June June~uly

July-Aug Aug-Sept

Sept-Oct

Oct-Nov

Mean annual

A1 a A2 B1 B2 A1 B1 B2 A1 A2 B1 B2 B1 B2 A1 A2 B1 B2 A1 A1 A2 B1 B2 A1 A2 B1 B2 A1 A2 B1 B2 A B

Pb

Zn

< 0.01 0.01 < 0.01 0.01 0.01 0.08 0.09 0.27 0.37 0.24 0.31 0.24 0.27 0.29 0.29 0.33 0.34 0.29

0.01 < 0.01 < 0.01 0.02 0.07 0.07 0.07 0.11 0.21 0.16 0.13 0.27 0.35 0.22 0.22 0.18 0.21 0.23

0.52 0.41 0.54 0.24 0.34 0.44 0.43 0.54 0.23 0.25 0.28 0.34 0.36

0.26 0.23 0.17 0.16 0.29 0.16 0.21 0.16 0.13 0.13 0.15 0.19 0.22

flux aA and B are stations in different depositional basins.

Cu

Ni

0.115 0.095 0.081 0.138

0.044 0.068 0.089 0.099

0.082 0.049 0.075 0.073 0.114 0.062 0.077 0.045 0.039 0.071 0.098 0.071 0.091

0.116 0.122 0.157

Seston 60 140 30 70 890 840 900 900 1280 790 860 1340 1840 1290 1610 1390 1610 1460 2045 2497 1208 1970 1970 2594 1442 2024 1680 1776 1390 1660 1190 1560

323 TABLE 4 Particulate metal and seston fluxes in Mountain Lake (mgm -s day-x)

Apr-May May-June

June-July July-Aug

Aug-Sept

Sept-Oct

Oct-Nov

Mean annual flux

AI" A2 B2 A1 A2 B1 B2 A2 B1 B2 A1 A2 B1 B2 A1 A2 B1 B2 A1 A2 B1 B2 A1 A2 B1 B2 A B

Pb

Zn

0.02 0.022 0.013

0.11 0.14 0.05

0.007 0.007 0.016 0.033 0.031 0.026 0.044

0.04 0.04 0.14 0.11 0.08 0.12 0.13 0.21

0.021 0.013 0.017 0.024 0.032 0.032 0.03 0,032 0.012 0.021 0.029 0.032 0.025 0.022

0.08 0.08 0.05 0.09 0.13 0.13 0.13 0.11 0.12 0.13 0.15 0.09 0.14 0.16

Cu

Ni

0.022 0.021 0.018

0.044 0.031 0.018

0.018 0.019 0.022 0.053 0.023 0.021 0.034

0.031

0.053 0.052 0.057 0.074 0.141 0.244 0.046 0.064

0.034 0.041 0.023 0.041

0.032 0.036

Seston 158 398 398 398 387 441 517 301 387 301 269 215 269 248 280 581 258 344 527 667 603 657 667 883 700 721 421 493

~A and B are stations in different depositional basins.

c a n be a t t r i b u t e d to the r e s u s p e n s i o n o f particles from the sediments ( N r i a g u et al., 1982; C h a r l t o n , 1983; R o s a et al., 1983). The low s e s t o n pool d u r i n g t h e w i n t e r period is also reflected in t h e c a l c u l a t e d p a r t i c u l a t e m e t a l fluxes (Table 3). D u r i n g t h e ice cover, t h e m e a n fluxes of p a r t i c u l a t e Pb a n d Zn in s u r f a c e t r a p s d e p l o y e d in Kej were f o u n d to be 0.05 a n d 0.04rag m -2 d a y -1 , respectively, c o m p a r e d w i t h 0.32 (Pb) a n d 0.18 (Zn) m g m - 2 d a y - 1 for t h e ice-free period (Table 3). O n a n a n n u a l basis, t h e m e a n s e t t l i n g r a t e s o f p a r t i c u l a t e Pb a n d Zn from t h e s u r f a c e w a t e r s were 0.34 a n d 0.19mg m -2 day -I. The flux of particulate C u during the ice-free period was 0.07mg m -2 day -I in surface waters and 0.09rag m -2 day -I in bottom waters

324

TABLE 5 Particulate metal and seston fluxes in George Lake (mg m-2 d a y - l )

June-July July-Aug Aug~ept Sept-Oct Oct-Nov Mean annual flux

Aa B A B A B A B A B A and B

Pb

Zn

0.29 0.33 0.29 0.4

0.18 0.23 0.24

0.41 0.24 0.44 0.54 0.28 0.36

0.23 0.16 0.16 0.16 0.15 0.19

Cu

Ni

Seston

0.058

0.068

0.06

0.091

0.049 0.073 0.062 0.045 0.07 0.06

0.12

1722 1453 829 872 1948 1582 2400 2540 1765 1625 1674

0.093

aA and B are stations in different depositional basins.

(Table 3). In winter months, the ice cover (i) cuts off the atmospheric supply of trace metals, (ii) arrests the tributary input of suspended material, and (iii) reduces the wind shear and hence the turbulence in the lake. The sharp difference between particulate metal fluxes during the winter and summer months thus reflects the over-riding influence of atmospheric inputs and particle resuspension on the behavior of trace metals in these lakes. The observed fluxes of the seston and particulate metals in Kej and George Lakes are similar (Tables 3 and 5). As noted previously, Kej drains into George Lake, and the similarity in seston and particulate metal concentrations in the two lakes lends further credence to the suggestion that only a small fraction of the suspended particulates in Kej is retained in its sediments. It is not surprising that a close relationship exists between the seston flux and the settling rates of Pb and Zn in the two lakes (Figs 1 and 2). Also, the weak relationship between the particulate trace metal fluxes and those of A1 and Fe (Figs 1 and 2) points to the significant role of inorganic particles in the dynamics of trace metals in these lakes. The flux of seston in Mountain Lake is much lower than the rates in Kej and George Lakes, and the observed settling rates for metals are also different (compare Tables 3, 4, and 5). Mountain Lake is a clear, headwater lake in which most of the trace metals are derived from the atmosphere and the organic carbon is generated mostly within the lake itself (see above). There is no obvious relationship between the settling rates of trace metals and fluxes of A1 and Fe in this lake. The fluxes of particulate metals in Kejimkujik National Park are generally lower than the settling rates of 0.48, 0.98, and 0.78 mg m -2 day-1 for Pb, Zn and Cu reported in Windermere Lake (Hamilton-Taylor et al., 1984). The rates, however, are higher than the 0.01 (Cu and Ni), 0.04 (Pb) and 0.05 (Zn) mg m -2 day -1 for lakes in Ontario's Algonquin Provincial Park (Wong and Nriagu, 1985). Kejimkujik Park is downwind and hence receives large doses of

325

pollutant metals released by the industrial complexes in central North America. The values in Nova Scotia lakes are small compared with the highest settling rates in Canada found in the lakes near the smelters at Sudbury (Ontario) where values of > 900, > 100, > 200, and > 25 mg m- 2day- 1for Ni, Cu, Zn, and Pb have been recorded (Wong and Nriagd, 1985). In the deep lakes, such as the Great Lakes, the fluxes of seston and particulate metals typically decrease exponentially with distance away from the sediment-water interface (Rosa et al., 1983; Charlton, 1983). For the lakes in Nova Scotia under study, there is only a slight difference in the particulate fluxes at the surface and bottom traps (Tables 3--5). The lack of stratification in settling rates can be attributed to the shallow depths of the lakes, which result in any resuspended particles being redistributed throughout the water column. The profiles of Pb and Zn in sediments of Kej and Mountain Lakes are shown in Figs 3 and 4. Typically, one finds that the accumulation rates increase Concentration 0 0

10 I

20 I

30 I

40 I

LEAD IN LAKE SEDIMENT

5 jf°~' Z

/.

50 I

(/.~g) 60 I

70 I

~J

o

/

80 J

90 I

100

.i]

!

15.

!

.... I ....

20-

E_

I

25-

Mountain I Lake |

!

! 30-

35

!

l

+

Fig. 3. Historical changes in lead concentrations in sediments of Kejimkujik and Mountain Lakes.

326

towards the sediment-water interface, reflecting an apparent increase in the intensity of pollutant metal inputs into these lakes. The decrease in Pb concentrations in the most recent sediments of Kej may be attributed to the reduced emission of lead associated with the phase-out of leaded gasoline in North America. The absence of a corresponding feature in Mountain Lake sediments seems anomalous and may be attributed to the slow sedimentation rate and higher pH of this lake. Mountain Lake shows a four-fold enrichment of Zn in the most recent sediments (Fig. 4), implying that the Kejimkujik National Park is importing large quantities of this element via the atmosphere. By contrast, the Zn concentrations in the Kej sediments decline towards the sediment-water interface and this phenomenon is unlike anything that has been reported previously in the literature. Such a profile suggests that the Zn reservoir in sediments is being removed by the combination of low acidity and high concentration of complexing organic ligands in the water. The Kej thus represents one of the few Concentration (~/o) 0

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 ] i ] i L i i i i i i i i i I \ +

o

~*

\o *~o

5-

/

o"~+

f E 15-

I

ZINC IN LAKE SEDIMENT

"%.

.../...(approx. .#9 100 years)

10"

/

+~

"\.<.. ( approx."~"...~,

~,,.,~./Lake Mountain

100 year,~if """

!

o

20-

o

/

p

÷

\

/

o

Fig. 4. Zinc profiles in sediments of Kejimkujik and Mountain Lakes.

327

documented cases where extensive mobilization of toxic metals from sediments can be attributed to natural processes of lake acidification. The sediment accumulation rates have been determined, by lead-210 dating, to be 33 g m -2 year -I for Kej and 13 g m -2 year -I for Mountain Lake. From the concentrations in surficial sediments (see Figs 3 and 4), the current rates of Pb accumulation have been estimated to be 2.6 and 1.0mg m -2 year -I, respectively, in Kej and Mountain Lakes. For Zn, the sediment retention rates have been estimated to be 0.92mg m -2 year -I in Kej and 0.45mg m -2 year -I in Mountain Lake. These metal accumulation rates in sediments are generally < 10% of the observed fluxes in surface waters (Tables 3 and 4). In other words, the sediments represent a sink for only a small fraction of the particulate metals being cycled through the water column. Thus, the data from the sediments support the suggestion that the trace metals associated with the seston do not accumulate in the Kej sediments. REFERENCES Beauchamp, S.,1983.Planktonic Primary Production in Three Acid StressedLakes in Nova Scotia. MSc thesis,Dalhousie University, Halifax, Nova Scotia. Blouin, A.C., 1985.Comparative Patterns of Plankton Communities Under DifferentRegimes of pH in Nova Scotia Lakes. PhD thesis,Department of Biology, Dalhousie University,Halifax,Nova Scotia. Buff]e,J., 1987. Complexation Reactions in Aquatic Systems: An Analytical Approach. Halsted Press, New York, 692 pp. Burns, N.M. and F. Rosa, 1980. In situ measurement of settling velocity of organic carbon particles and 10 species of phytoplankton. Limnol. Oceanogr., 25(5): 855-864. Charlton, M.N., 1983. Downflux of sediment, organic matter and phosphorous in the Niagara River area of Lake Ontario. J. Great Lakes Res., 9(2): 201-211. Fischer, K., J. Dymond and M. Lyle, 1986. The benthic cycle of copper: Evidence from sediment trap experiments in the eastern tropical North Pacific Ocean. Geochim. Cosmochim. Acta, 50: 1535-1543. Hamilton-Taylor, J., M. Willis and C,S. Reynolds, 1984. Depositional fluxes of metals and phytoplankton in Windermere as measured by sediment traps. Linmol. Oceanogr., 29(4): 695-710. Jickells, T.D., W.G. Deuser and A.H. Knap, 1984. The sedimentation rate of trace elements in the Sargasso Sea measured by sediment trap. Deep-sea Res., 31(10): 1169-1178. Kerekes, J., 1975. Limnological conditions in thirty lakes, Kejimkujik Park, N.S. In: J. Kerekes (Ed.), Aquatic Resources Inventory, Part 6. Parks Canada, Ottawa, Ont. Nriagu, J.O., H.K.T. Wong and R.D. Coker, 1982. Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbury, Ontario. Environ. Sci. Technol., 16: 551-560. Paulson, A.J., R.A. Feely, H.C. Curl, Jr., E.A. Crecelius and T. Geiselman, 1988. The impact of scavenging on trace metal budgets in Puget Sound. Geochim. Cosmochim. Acta, 52: 1765-1779. Premazzi, G. and G. Marengo, 1982. Sedimentation rates in a Swiss-Italian lake measured with sediment traps. Hydrobiologia, 92: 603-610. Rosa, F., J.O. Nriagu and H.K.T. Wong, 1983. Particulate flux at the bottom of Lake Ontario. Chemosphere, 12(9/10): 1345-1354. Sigg, L., 1987. Surface chemical aspects of the distribution and fate of metal ions in lakes. In: W. Stumm (Ed.), Aquatic Surface Chemistry. Chemical Processes at the Particle-Water Interface. Wiley-Interscience, New York, 435 pp. Stabel, H.H., 1987. Mechanism controlling the sedimentation sequence of various elements in Prealpine Lake. In: W. Stumm (Ed.), Chemical Processes in Lakes. Wiley-Interscience, New York, pp. 143-188.

328 Vollenweider, R.A., 1985. Elemental and biochemical composition of plankton biomass; some comments and explanations. Arch. Hydrobiol., 105: 11-29. Weilenmann, U., 1986. The Role of Coagulation for the Removal of Particles by Sedimentation in Lakes. Thesis, ETH Zurich, No. 8018, 162 pp. Wong, H.K.T. and J.O. Nriagu, 1985. Particulate metal dynamics in lakes near the smelters at Sudbury, Ontario, Canada. In: D.D. Hemphill (Ed.), Proc. Trace Substances and Environmental Health - - XVIII. University of Missouri, Columbia, MO, pp. 416-426. Wong, H.K.T., J.O. Nriagu and R.D. Coker, 1984. Atmospheric Input of heavy metals chronicled in lake sediments of the Algonquin Provincial Park, Ontario, Canada. Chem. Geol., (44) 187-201. Xue, H.B., W. Stumm and L. Sigg, 1988. The binding of heavy metals to algal surfaces. Water Res., 22(7): 917-926.