Emironment International, Vol. 22, No. 5, pp. 591-604,1996 Copy&&t 01996 El&icr Science Ltd Printed in the USA. Ail rightt rcscmd

0160-4120/96 $15.00+.00

Pergamon

PIISO160-4120(96)00055-4

THE HUMIC LAKE ACIDIFICATION EXPERIMENT (HUMEX): MAIN PHYSICO-CHEMICAL RESULTS AFTER FIVE YEARS OF ARTIFICIAL ACIDIFICATION

Espen Lydersen and Eirik Fjeld Norwegian Institute for Water Research, 0411, Oslo, Norway

Egil T. Gjessing Agder College, Department of Chemistry, N-4604, Kristiansand, Norway

EI 9604-l 45 M (Received 26 April 1996; accepted 4 June 1996)

The HUMEX-project is a whole catchment manipulation experiment where the effects of the addition of H,SO, and NH,NO, to a humic-rich lake, Lake Skjervatjem, and its catchment were studied. The lake was physically divided into an experimental lake (Basin A) and a control lake (Basin B). Two yeqs after the division, Basin A and its catchment were artificially acidified. Hydrological data, meteorology, precipitation, and runoff chemistry collected during a 2-y pre- acidification period and during 5 y of acidification were evaluated. Randomized intervention analysis (RIA) was used to evaluate statistical significant differences between runoff chemistry from the two basins before and after the acidification. RIA showed significantly higher concentrations of SO,*-, H+, NH.,+, NO,, Al”+, Ca”, Mg*+, total reactive Al (RAL), and labile Al (LAL) in Basin A after treatment compared with the control basin. After the treatment, the acid neutralizing capacity (ANC) in Basin A was significantly lower than in Basin B. However, the average ANC is substantially higher in the control basin after acidification compared with the two years before acidification, while unchanged in the manipulated catchment. The main reason for this is the long lasting effect of Na leakage after seasalt-episodes. No significant changes were observed regarding the amount of total organic carbon (TOC), water color, or UV-absorbency after the treatment, but the anion deficiency (A-) was significantly lower in the treated basin. This indicates that the organic acids are more protonated in the treated basin compared with the control basin. After a cold winter in 1993/94, an extreme NH,+ increase was observed in runoff water from Basin A. This increase was accompanied by increases in water color and UV-absorbency, but without any increase in TOC.

INTRODUCTION

Humic substances (HS) may play an important role for the chemistry of soil and surface waters, because they catalyze weathering processes (Wilson 1986; Lundstriim 1990) and thereby cation leaching from soils (Reuss 1980; Krug and Isaacson 1984). Under ambient conditions, HS are present as negatively charged or- ganic macromolecules, but the degree of negatively charged sites is highly pM dependent (Gjessing 1976). Thus, HS also plays an important role in the acid/base chemistry of surface waters (Driscoll et al. 1989).

Results obtained by Krug and Isaacson (1984) also indicate that HS buffers against strong acid additions by a number of mechanisms other than exchange of base- cations and aluminium for H’. Complex changes in both the organic and inorganic composition have been sug- gested. Metal complexion of HS is dependent on not only the metals present, but the distribution of charge on the macromolecules (Wershaw 1986). The chemical and biological nature of the HS might be affected by sul- phuric acid acidification (Gjessing et al. 1991). All

591

592 E. Lydersen et al.

these factors are essential for the understanding of both the metal complexation properties and the biological availability of HS.

The main intention with the humic lake acidification experiment (HUMEX-project) was to study the impacts of artificial addition of sulphuric acid (H,SO,) and ammonium-nitrate (NH,NO,) on organic carbon in soil and surface water, both qualitatively and quantitatively. The portion of the acidity in water due to strong acids in the precipitation, and that portion due to naturally oc- curring organic acids was to be assessed. Further, the impacts of acidification on a whole dystrophic lake eco- system were studied. To obtain this, Lake Skjervatjern in Western Norway was divided into two lake halves (Basin A and B) by a plastic curtain in October 1988. The water chemical effects of the division were fol- lowed until October 1990, when Basin A and its catch- ment were treated by artificial rain. The average concen- trations of H,SO, and NH,NO, in rain water were nearly 100 ueq/L and 50 ueq/L, respectively, almost identical to the concentrations present in rain water in the far more acidified Southern Norway. More details of the HUMEX-project are given by Gjessing (1992, 1994). In the HUMEX-project, scientists from various parts of the world are being given the opportunity both to design and conduct studies on ecological and other aspects of acid precipitation. Because of the availability of a control (unacidified part of the lake), the results provide a high level of scientific validity.

This paper evaluates the physico-chemical data from the monitoring programme at Skjervatjern during the period October 1988-October 1995; two years of data before treatment, and five years of data from the treat- ment period.

MATERIAL AND METHODS

Runoff water from Basin A and B have been sampled almost weekly from October 1988. Discharge from A and B has been continuously logged since January 1991, while sun radiation, air and soil temperature as well as precipitation have been continuously logged since May 1991. Due to breakdowns during thunder, the loggers, especially the discharge logger, have been out of work during several periods. Because of problems related to discharge logging, runoff data is not being presented in this paper.

From January 1995, weekly samples of precipitation were analyzed regarding the major chemical com- pounds, e.g., pH, Ca2’, Mg”, Na: K,T NH, t SO, ‘;Cl ; NO,, and POd3-. In general, POd3- is not evaluated as a major chemical compound in wet-deposition, but the

POd3- contribution in precipitation was assessed because there is minor information about the atmospheric contri- bution of this essential nutrient element. However, the concentrations were generally below the detection limit of the method (2: 1 ug P/L) and are therefore not pre- sented in this article.

The monthly additions of H2S04 and NH,NO, to Ba- sin A and its catchment are presented in Fig. 1.

Physico-chemical analysis

The pH, Ca2’, MC, Na+, K’, NH,+, SO,“; Cl; NO,; and POd3- in precipitation were all analyzed by standard methods at the Norwegian Institute for Air Research (NILU). The same parameters, as well as total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC), total fluoride, Fe, and Mn, were analyzed in runoff water by the Norwegian Institute for Water Research (NIVA) following standard procedures. In addition, W-absorbency and color of runoff water were measured on Millipore filtered water (0.45 pm cutoff). W was measured at 254 nm, while color was measured at 410 nm, calibrated with a solution of potassium- hexachloroplatinum and cobalt chloride. Thus, the values presented are directly comparable to the tradi- tional color term in mg Pt/L. Aluminum was analyzed by the Pyrro-Catechol Violet method (PCV-method, Norwegian Standard, NS 4747), on both total and cation-exchanged samples. The cation-exchange proce- dure is described by Driscoll (1984). The total PCV- reactive Al is defined as RAL, while the Al present in the eluate is defined as non-labile Al (ILAL). The dif- ference between RAL and ILAL is the labile Al-fraction (LAL), primarily representing the low molecular weight inorganic Al-species. To estimate the average charge of Al @Al”‘), the ALCHEMI-Version 4.0 was used, and both the inorganic and organic constants applied are the same as originally present in the program (Schecher and Driscoll 1987; 1988). Total carbon and total nitrogen was also measured on filters after filtering the water through a glass fiber filter with cutoff = 0.6-0.7 pm, and analyzed by an HCN-analyzer after complete com- bustion of the solid material.

While monitoring of precipitation chemistry started 1 January 1995, monitoring of major runoff chemistry was performed from the beginning of the project. Ana- lyses on color and W-absorbency started in June 1989, on total phosphorus and phosphate in July 199 1, on filtered nitrogen and carbon in April 1993, and on iron, manganese, and fluoride in November 1994. Both NIVA and NILU laboratories are accredited according to the EN-45000 standards.

The HUMEX experiment after 5 y of acidification 593

0.5 8- -Sulphur .-..--Nitrogen -Sulphur - - - 7- Nitrogen

0.4

“f 2 0.3 P_ -5 z

iO.2 b4 wJ3 -

an - 0.1 M2 1 -

Statistical analysis

Randomized intervention analysis @IA) was used to detect changes in the manipulated Basin A relative to the control Basin B. This method is well suited for statistical assessment of differences before and after a manipulation (Carpenter et al. 1989; Carpenter 1993). RIA requires paired time series of data from both before and after manipulation (acidification), and is not affected by non-normal errors in the data. Monte Carlo simulation indicated that, even when serial auto- correlation was substantial, the true P value (i.e., from non-autocorrelated data) was < 0.05 when the P value from autocorrelated data was 0.01 (Carpenter et al. 1989). RIA derived from the ‘before-after-control- impact’ experimental design of Stewart-Oaten et al. (1986).

RIA begins with a series of parallel observations of experimental and reference ecosystems, paired in time, spanning periods before and after a manipulation. A time series of interecosystem differences is then cal- culated, and from these are calculated mean values from the premanipulation and postmanipulation differences, D(PRE) and D(POST), respectively. The absolute valueofthedifferencebetween~(PRE)and~(POST) is the test statistic. Its distribution is estimated by random permutations of the sequence of interecosystem differ- ences.

Definition of terms

In the charge balance (CB) of surface water (peq/L):

CB = ([Ca*‘] + [Mg*‘] + lNa+] + [K+] + [H+] + [NH,+]

+ [ZAl”+]) - ([SO,*-] + [Cl-] + [NO,-] + [HCO,-] + [A-])

[A-] is an expression of the amount of dissolved organic anions. Assuming all ions incorporated in the charge balance are being measured, the amount of A- (peq/L) can be estimated, so that charge balance is obtained. The concentration of ZAl”+ in the expression is the sum of positively charged Al-ions, which can be estimated by the ALCHEMI-speciation program (Schecher and Driscoll 1987; 1988).

Of the strong acid anions, Cl is the most mobile, usually following water through the ecosystem from precipitation to runoff, i.e., the influx of chloride is equal to the efflux of chloride. In general, the major source of Cl- is from neutral seasalts, entering the terrestrial ecosystems both as wet- and dry-deposition. The equivalent relationship between Cl and the major base cation of seawater, Na, is therefore either a good estimate of the non-marine contributions of Na, or a good estimate of seasalt effects due to different mobility of Cl- and Na+ through the catchment. The ‘non-marine concentrations ofNa’ are marked with asterisks, and the equivalent correction equation is:

[Na+]* = [Na+] - 0.856

RESULTS AND DISCUSSION

w-1 (1)

The physical and morphological data of the two lake halves of Lake Skjervatjern are presented by Gjessing (1992).

Hydrology and meteorology

The amount of precipitation at Skjervatjern has been continuously logged since May 199 1. Most of the pre- cipitation normally falls during September-March, which is typical for this region. In 1992,1993, 1994, and 1995, annual amounts of precipitation were 2 170 mm,

594 E. Lydersen et al.

Table 1. Annual precipitation, weekly maximum and minium amounts of precipitation, annual weighted concentration averages and weekly maximum and minimum concentrations of chemical compounds in wet deposition at Skjervatjern and Nausta from 1 January 1995

to 3 1 December 1995.

Parameter unit mean Skjervatjern

max min mean Nausta

max min

Precipitation mm 1793 * 180 0 2510 * 235 0 H+ peqL 12.3 218.8 2.1 12.3 114.8 0.5 so,2- peqk 19.7 323.1 3.1 18.5 121.3 0.6 Cl- peqL 80.9 403.9 1.7 69.5 511.1 2.5 NO, peqL 8.0 245.7 0.0 8.1 87.9 0.0 Ca*+ ueq& 5.5 34.4 1.0 4.0 19.5 0.5 Mg2 ueqL 16.6 86.4 1.6 14.1 93.8 0.8 Na+ ueqk 71.2 379.3 1.7 59.8 440.6 3.0 K’ peqk 2.1 13.0 0.3 1.7 11.5 0.3 NH,+ peqL 5.8 207.9 0.0 9.1 81.4 0.0 Xations Peqk 113.5 608.4 22.6 101.0 570.8 21.9 EAnions ueqA 108.6 611.1 12.2 96.2 564.8 16.2

ZCation-BAnions peqlL * Annual amount of precipitation.

4.9 42.3 -20.9 5.0 112.2 -20.2

100

0

z z % % 8 % z % % ‘“f&$$‘$$

? 8 s _,Ezzar-, z B 8

a co 6 z n

Fig. 2. Weekly precipitation (mm), and weighted concentrations of SO,*- and Cl- in precipitation at the local weather station at Skjervatjern since start of monitoring 1 January 1995.

1890 mm, 1867 mm, and 1793 mm, respectively. Both strong acid episodes as well as seasalt episodes Several scientists working at the HUMEX-site used were recorded during the first year (1995) of preci- chemical precipitation data from a nearby weather pitation chemistry monitoring (Fig. 2). From 6 March to station (Nausta, located 18 km northwest of Lake 13 March, the pH in the precipitation was 3.66 at Skjer- Skjervatjem) to estimate wet-deposition inputs at Skjer- vatjem, but the precipitation during this period was low, vatjem. From 1 January 1995, the precipitation chem- i.e., 1.2 mm. Four high seasalt episodes were recorded istry at Skjervatjem was monitored. The chemical data during the first year of monitoring, in early February, from both weather stations during 1995 are presented in late February, early April, and late September, with Cl- Table 1, and demonstrate relatively large differences concentration of 242 ueq/L, 367 peg/L, 404 peg/L, between the two stations regarding both concentrations and 364 peg/L, respectively. During the same periods, and fluxes of chemical compounds, as well as water the amounts of precipitation were 110 mm, 37 mm, input. 21.5 mm, and 66.4 mm. Thus, the acidic precipitation

The HUMEX experiment after 5 y of acidification 59s

Fig. 3. Monthly mean air and soil temperature (“C) during the monitoring period at Skjervatjem.

in March had minor acidification impacts on the eco- system because of low water input. The seasalt episodes were much more important events as long as high sea- salt concentrations occurred during precipitation periods. The wind strength and wind direction were not monitored, but these factors are also important for the seasalt inputs.

The most interesting climatic event recorded so far was the long lasting cold period during the winter 1993/94 (Fig. 3). During this period, mean air tem- perature was <O”C from November-March, and lowest in February, with a mean temperature of -9.2”C. During the same winter, soil temperature was < 0°C from 6 November 1993 to 26 April 1994. Soil temperatures < 0°C have never been recorded. The effects of this cold winter on the water chemistry at Lake Skjervatjern are presented later in this article.

In addition to the natural inputs of chemical compounds (Table l), Basin A of Lake Skjervatjern and its catchment artificially received an average of 1.4 g S rn-~-’ (as H,SO,) and 1 .l g N m-*y-l (as NH,NO,) from October 1990 to July 1995. The monthly and cumulative amounts of these additives are presented in Fig. 1. In concentrations, this means additives of about 100 ueq/L of H,SO, and about 50 ueq/L of NH,NO,. The additions were made by dissolving these two chemicals in lake water from a nearby lake, Lake Aasvatn. The artificial rain was equivalent to 10% of ambient precipitation. The lake water was analyzed, and the 10% contributions of chemical compounds from this lake water are: H+ (0.2 ueq/L), Ca*+ (3.5 ueq/L), MC (2.1 ueq/L), N$ (7.6 ueq/L), K+ (0.6 ueqJL), NH,+ (0.07 ueq/L), SO:- (3.1 peq/L), Cl‘ (8.7 peg/L), and NO,- (0.4 ueq/L).

Runoff chemistry

As shown in Figs. 4-6, the division of the lake caused only minor water chemical differences between Basins A and B. Only a weak tendency towards somewhat higher concentrations of chemical compounds in Basin B was observed, most likely because of a higher catch- ment/lake ratio in this system (Gjessing 1992).

After the start of treatment, there were minor differ- ences between Basin A and B regarding chemical parameters that were reasonably unaffected by the mani- pulation, e.g., the seasalt derived ions Na+ and Cl . Regarding ions expected to respond to the manipulation, H, Ca*‘, NH,‘, Al”+, SO,*; and NO,; generally higher concentrations were present in the manipulated Basin A, compared with the control Basin B, both regarding mean and maximum values, while ANC was lower in the manipulated Basin A compared with the control basin.

Several ions exhibited seasonal variations. The pH was normally lowest during late spring and early sum- mer, with high peaks normally related to high runoff according to heavy rainfalls during autumn and winter, and during early snowmelt. The lowest pH-values were observed during episodes of high seasalt inputs. During a hurricane in January 1993, concentrations of Na+ and Cl- of 339 ueq/L and 423 ueq/L were recorded at the nearby weather station, Nausta (Skjelkvaale 1993). Ac- cordingly, the pH in runoff from Basin A and B was 4.35 and 4.25, the lowest values measured so far. During the same event, the lowest ANC ever recorded was present in runoff from Basin B, -6 1.6 ueq/L, and a low ANC was also recorded in Basin A, -40.6 yeq/L. The lowest ANC recorded in Basin A was in January 1992 (-57.8 ueq/L). The highest concentration of labile Al (LAL, i.e., the acute toxic Al-fraction) was recorded

596 E. Lydersen et al.

75 T Basin A ---Basin B

Acidification (A)

80 Basin A --- Basin B

35 - Basin A ---Basin B

30 --

a25 -- Acidiiication (A)

40 T Basin A - Basin B

3o t Acidification (A)

- ti 20

$ 10

=o

b-10

-20

-30 z % J J 5 q G 5 $ 3 6 6 6 8 8 g g 8

80 Basin A - Basin B

- 60 e

Acidification (A)

$40 a

A; 20

rA 0

-20

Basin A - Basin B

h 30 G $20

i1:

20 Basin A-Basin B

Fig. 4. Weekly concentrations of H’, SO,*-, NH,+, and NO,- (meqiL) in runoff from Basin A and B, and the differences between lake A and B of Lake Skjervatjem fi-om October 1988 to October 1995.

The HUMEX experiment after 5 y of acidification 597

60 Basin A ---Basin B

40 Acidification

5

(A)

20

50 T Basin A ---Basin B

Acidification (A)

I 75 T Basin A ---Basin B

40

T Basin A - Basin B

Acidification (A)

Basin A - Basin B

Acidification (A)

20 T Basin A - Basin B

Acidification (A) O o I

100 Basin A - Basin B

Acidification (A’)

I 0

Basin A - Basin B

Acidification (A)

Fig. 5. Weekly ANC, Ca’+, LAL (labile or inorganic monomeric Al), and TP in runoff from Basin A and B, and the differences between Basin A and B of Lake Skjervatjem from October 1988 to October 1995.

Ott-

88

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Ott-8

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2

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4

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95

0 i?

-1 m E z g m

UV

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8 2 cn

0

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L

b 0 0 O

tt-88

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or (

mg

Pt/L

)

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08:s

8g

Ott-

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-I

TOC

(m

g C

/L)

0 P

m

h,

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TOC

(m

g C

/L)

The HUMEX experiment after 5 y of acidification 599

during the seasalt episodes in January 1993, i.e., 162 and 160 pg Al/L in Basin A and B, respectively. Low pH-values were also recorded during the distinctive snowmelt during spring 1994, after the cold winter. During the rapid hydrologic peak that spring, the TOC concentration fell from 8.2 to 2.3 mg C/L in Basin A, and from 7.7 to 2.3 mg C/L in Basin B from 6 March to 12 March, the initial period of snowmelt. Since the ground temperature was still < 0°C at that time, ionic- poor melt water must be the main reason for this dilution or decrease in TOC. The pH was 4.44 in runoff from both Basins A and B, both 6 March and 12 March.

Sulfate was at its lowest once during the winter, or during snowmelt, while the highest concentrations nor- mally occurred during summer. High summer concen- trations are most likely related to low flow and corres- ponding oxidation of sulfide compounds stored in the bogs. However, there are no good relationships between sulfate and H+ in either Basin A or B.

The concentrations of Ca2+ in runoff were normally at their lowest during springmelt, and their highest once during summer. However, as for Hf, the seasonal trend for Ca2’ is not distinctive. Their fluctuations are pro- bably more related to hydrologic status, i.e., high-flow or low-flow, than time of the year.

The NO, and NH,+ were normally lowest during the growth season during spring and summer, and highest during springmelt or during winter related to high runoffs. The high concentration of NO,-, and particu- larly of NH,+ from May 1994 might be a direct effect of the strong winter of 1993/94 (the climate data were presented earlier). Reduced biological activity during the extreme cold winter, and minor runoff, may have caused a significant accumulation of those compounds, primarily within the bog areas. The effect of tem- perature on nitrate concentration in surface water was described by Fowler et al. (199 1) and Proctor (1993). Proctor (1994) also showed that bogs are strong sinks for NH,+. That the NO, peak arose a couple of months earlier than NH4+ after the cold winter, might be due to a higher mobility of this ion. NH,+, on the other hand, should be tied up by organic anions, to a large degree by complexation reactions, and thereby leave the bogs to- gether with dissolved organic carbon. This might be one explanation for the significant correlation between water color and NH,+ during the same period. The increase in UV-absorbency, together with increase in color and NH,+, may also indicate that during the cold winter with ground temperature < 0°C from early November to late April, the biological degradation of organic matter is substantially affected. Accordingly, more reduced dis-

solved organic material should be present after a cold winter. This might be the reason for the high W- absorbency and color in relation to dissolved organic matter observed several months after springmelt 1994. Another possibility for the qualitative change in TOC and concentration of NH,+ is the runoff pattern from the catchment. This has not been controlled. If major amounts of water are entering the lake from other soil/ bog depths as a consequence of a precipitation- poor cold winter, followed by a warm summer (Fig. 3), more water feeds the lake from deeper soil/bog layers. The NH,+ relationship to aqueous organic parameters is dis- cussed in more detail later. After the same cold winter, the concentration of T-P was unusually high from the middle of June to early December, highest (19 pg P/L) in September 1994. High concentrations of NH,+ were recorded almost a year from May 1994 to May 1995. The special climate during the winter of 1993/94, and the special water chemistry that followed, clearly demonstrate the importance of long lasting experiments to distinguish between changes caused by natural variations and changes directly related to the mani- pulation itself. Several publications from the HUMEX- project presented in this special volume of Environment International (22: 5; 1996) discuss this special year and its consequences for both aquatic chemistry and bio- logy.

Labile Al, Fe, and organic parameters such as TOC, W-absorbency, and color exhibited the most distinctive seasonal trends during the period of monitoring. TOC, W-absorbency, and color were always at their highest during autumn, probably as a direct consequence of degradation of organic matter produced during the latest growth season, while the lowest concentrations were present during late winter. This also seems to be the typical pattern for Fe (Figs. 7 and 9) and, to a certain degree, for Mn. Regarding inorganic Al or labile Al (LAL, i.e., the primary acute toxic Al-fraction), the highest concentrations occurred during summer when the concentration of organic compounds were at average and pH was relatively low.

RIA-ANALYSIS

RIA was used to test if significant differences (at 99% level) were established between the two lake halves after 5 y of acidification of lake half A and its catch- ment.

Based on this statistical analysis, significantly higher concentrations of H, NH,+, NO,, SOJ2-, Al “+, Ca 2+, Mg2+, RAL (total reactive Al), LAL, and organic-N were present in Basin A in relation to Basin B as a

600 E. Lydersen et al.

Table 2. Mean values in Basin A (artificially acidified) and B-for two years before and five years after start of acidification (almost weekly sampling), the relative actual differences (D,, = Dprr - D &, the 95% and 99% values based on RIA. If the P95% or P99% values < absolute value of Dactual, there is a 95% or 99% probability that the changes due to the manipulation (acidification of Basin A) are real.

N, = 248, N,, = 63.

Acidified lake (A)

A Pre

A post

Control lake (B)

B Pre B PO*

D actual P95% P99%

H+

NH4+

NO,-

so,Z-

Al”+

Ca”

Mg2+

ANC

Na*

RAL

LAL

ILAL

26 f 4

1.1 f 0.6

0.6 f 0.8

27 f 4

3.6 f 1.0

10 f 2

25 f 7

-13 f 16

-5.1 f 12

65 f 20

29* 10

36 f 16

29 f 8

4.7 f 6.1

3.2 f 2.8

43 f 13

4.6 f 1.6

13 f 5

28 f 12

-11 f 16

5.7 f 14

82 f 29

22* 18

60 f 24

28 f 5

1.2 f 0.5

0.8 f 1.1

28 f 5

4.4 ??0.9

10 f 2

25 f 5

-12 f 17

-3.1 f 13

78 f 22

34 f 9

44* 19

26 f 7

1.1 f 0.7

0.7 f 0.9

27 f 7

4.5 f 1.6

11 f 4

25* 11

-0.1 f 16

6.8 f 15

87 f 31

17 f 17

70 f 25

4.77 1.75 2.28 TRUE

3.65 1.53 2.00 TRUE

2.69 0.68 0.85 TRUE

18.0 3.27 4.17 TRUE

0.84 0.29 0.38 TRUE

2.30 0.89 1.22 TRUE

2.97 1.38 1.74 TRUE

-10.2 3.25 4.06 TRUE

0.99 1.74 2.17 FALSE

8.64 4.57 6.12 TRUE

10.1 2.84 3.47 TRUE

-1.52 3.97 5.60 FALSE

TOC mg C/L 5.6 f 2.2 6.1 f 1.9 6.3 f 2.4 6.8 f 2.2 -0.003 0.30 0.40 FALSE

uv OD25.4 0.22 f 0.08 0.26 f 0.10 0.26 f 0.10 0.30 f 0.10 -0.012 0.016 0.021 FALSE

Color OD,,, 50 f 19 61 f 27 57 f 24 68 f 24 -1.06 4.79 6.11 FALSE

A‘ ueqn 17 f 16 22 f 23 21 f 16 30 f 14 -4.60 2.63 3.41 TRUE

Org-N ug N/L 198 f 67 250 f 118 191 f 70 177 f 63 65.5 24.4 32.7 TRUE

250 T-Basin A --.---Basin B 8 I-Basin A .-..-.Basin B

Fig. 7. The concentration of Fe and Mn in runoff from Basin A and Basin B during the monitoring period, i.e., December 1994-October 1995.

The HIJMEX experiment after 5 y of acidification 601

16 Basin A A Before o After

16 T Basin B A Before o After

I I I , 1 I

0 50 100 150 200 Color (mg PtIL)

16 A Before D After T

Basin B A Before 0 After

I I I I

50 100 150 200 Color (mg Ptn)

0.80

!

Basin A A Before o After

0.60 z ” 0.40

.$ .qO 0 08

Q

00 0

co.20

0.00 le / I I

0 50 100 150 200 Color (mg PtL)

A Before o After

0

100

0 0 50 100 150 200

Color (mg PtiL)

I I I ! I

50 100 150 200 Color (mg PtL)

I I I I

50 100 150 200 Color (mg PtK)

0.80 T Basin B A Before o After

SO.60 4

0.00 I m -A . I I I

I I I

0 50 100 150 200 Color (mg PtfL)

b Before o After

0 50 100 150 200 Color (mg Pt/L)

Fig. 8. The relationship between color and TOC, UV and NH,+ in Basin A and Basin B of Lake Slcjervatjem during October 1988 to June 1995. Before: Before acidification of Basin A and its catchment (October 1988 to October 1990). Afier: After acidification of Basin A

(October 1990 to June 1999). Before: Basin A, TOC = O.O78*color + 1.35 (3 = 0.71); UV: O.O037*color + 0.032 (? = 0.88); NH, = O.O08*color + 14.0 (i! = 0.007). Basin B, TOC = O.O85*color + 1.19 (r*= 0.84); UV: O.O038*color + 0.039 (9 = 0.88); NH, = O.O64*color + 11.6 (? = 0.07). After: Basin A, TOC = O.O54*color + 2.83 (? = 0.60); DOC = O.O37*color + 3.00 (? = 0.54); UV: O.O035*color + 0.044 (2 = 0.90); NH, = 2.10*color - 61.9 (? = 0.44). Basin B, TOC = O.O83*color + 1.19 (8 = 0.83); DOC = O.O76*color + 1.13 (8 = 0.75); UV:

O.O041*color + 0.026 (3 = 0.75); NH, = O.O35*color + 13.0 (3 = 0.01).

602 E. Lydersen et al.

result of 5 y with artificial acidification (Table 2). Cor- respondingly, a significantly lower ANC was also estab- lished in Basin A compared with Basin B. On average, ANC was about 10 ueq/L lower in Basin A compared with Basin B. However, the average ANC in Basin A during the 2 y before treatment (ANC = - 13 f 16 ueq/L) was almost identical with the average ANC for the 5 y of treatment (ANC = -11 f 16 ueq/L). During the same periods, the ANC in Basin B increased from - 12 f 17 ueq/L as an average for the 2 y before treatment, to only 0 f 16 ueq/L as an average for the 5 y of acidi- fication. This means that an increasing ANC would have occurred in Basin A without any treatment. The main reason for the increasing ANC in Basin B, and the almost unchanged ANC in Basin A, was the extreme seasalt episodes present during the last years. Because of the much higher mobility of the seasalt anion, Cl-, compared with the major counter-ion Na+, seasalt episodes initially cause short-term acidification as Na+ temporarily undergoes cation-exchange with IF’ and Al, with Cl‘ as the predominant mobile anion. However, the secondary effect is a more long lasting ANC improve- ment as Na+ gradually leaks out of the cat&rent again due to the reverse cation exchange processes. As shown in Table 2, changes in Na* explain the increase in ANC almost totally in Basin B. Since the RIA-analysis showed no significant differences in Na* between Basin A and Basin B during the corresponding periods, the ANC in both basins would have been about 10 ueq/L lower without these long lasting seasalt effects.

Regarding the major organic compounds, TOC, color, and UV-absorbency, no significant differences between Basin A and B were found by the RIA-method. The only significant change related to organic compounds was the lower anion deficiency (A-) in Basin A com- pared with Basin B, indicating that the organic acids are more protonated in the treated basin compared with the control basin as a result of the artificial acidification. The opposite tendency is now observed in the long-term monitoring sites in Norway (Skjelkvaale 1994), where an increase in A- and a reduction in base cation concen- trations are the most significant response to the declin- ing emissions of sulphuric acids in precipitation. These observations together clearly demonstrate the pH-buf- fering properties of weak organic acids, as deproton- ation/protonation reactions occur dependent on the con- centrations of strong acid inputs.

The water color

Because the extreme increase in color in 1994/95 occurred simultaneously with the NH,+ increase, this relationship needs to be evaluated in more detail. Water

color, related to mg Pt/L (OD: 410 nm), is a normally used index of dissolved organic matter (Thurman 1983). This is due to the strong existing correlation between brown organic color, which is derived chiefly from peat and marsh detritus, and the amount of dissolved organic carbon in surface waters (Juday and Birge 1933).

In both Basin A and Basin B of Lake Skjervatjern, the best correlation exists between color and UV-absor- bency (Fig. 8). This is expected since both analyses are performed spectrophotometrically at 4 10 nm (color) and 254 nm (UV) on filtered samples. There are also signifi- cant correlations between color and both TOC and dissolved organic carbon (DOC), and between color and iron (Fig. 9). It is well known that Fe in surface water is present as Fe-organic complexes. In Basin B and before acidification of Basin A and its catchment, no correla- tions were found between color and NH,+. After the addition of NH,NO, together with H,SO,, a significant correlation (? = 0.44) was found in Basin A (Fig. 8). During the period when NH,+ really started to increase (May 1994) and to October 1995, the correlation was far more significant, i.e., ? = 0.84 (Fig. 10). This close relationship might have several explanations: 1) NH,+ might, to a certain degree, undergo complexa- tion with organic anions which may lead to formation of color, and even the complexed NH,+ will be measured as free NH,+ ions in the analyses. 2) Both the color and NH,+ analyses are spectrophoto- metric methods, with color analyzed at 4 10 nm, NH,+ at 630 nm. Thus, an increase in NH,+ may, to a certain degree, interfere with color. So far, such studies have not been conducted, but should be done. 3) It is known from the literature that some aquatic fungus (e.g., Aurobasidiumpullulans) has the ability to decompose large amounts of organic matter and exude yellow colored substances in freshwaters (Day and Felbeck 1974). Strains of similar fungi are common soil organisms. Analysis of this exudate found character- istics consistent with previous research on color in freshwater (Midwood and Felbeck 1968) and in HS (Felbeck 1971). Thus, an increase in NH,+ in Basin A and its catchment might have stimulated such organisms to exude more yellow colored organic complexes. How- ever, since no terrestrial or aquatic studies on fungi have been conducted at Skjervatjern, this is just a theoretical possibility.

During the period when the NH,+ concentration was high in Basin A (Fig. 4), i.e., from late April 1994 until June 1995, the significant relationship between color and W (Fig. 10) increased from fi = 0.96 to 3 = 0.98 by adding NH,‘+ as the secondary explanatory variable,

The HUMEX experiment after 5 y of acidification 603

200 T Basin A 0 200 T Basin B 0

0

0 i v I I 0 I I / I

0 50 100 150 0 50 100 150 Color (mg Pt/L) Color (mg Pt/L)

Fig. 9. The relationship between Fe and color in Basin A and B during the period of monitoring, i.e., December 1994-October 1995. Basin A: Fe = 1.53*color - 18.5 (12 = 0.40); Basin B: Fe = 1.5O*color 19.0 (3 = 0.59).

0.80

1

500 -

$0.60 ,400 -- 0

’ 0.00 I I I I I 1 I I 0 50 100 150 200 0 50 100 150 200

Color (mg Pt/L) Color (mg Pt/L)

Fig. 10. The relationship beween color and UV and NH,+ in Basin A of Lake Skjervatjem from May 1994 to October 1995. UV = O.O029*color + 0.089 (? = 0.96); NH, = 3.56*color - 119.2 (12 = 0.84).

with the following regression equation: Color = 242.4 W + 0.069 NH,+ - 8.74. However, based on the linear regression between color(y) and NH,+ (x) from the start of treatment until now, i.e., y = 0.21x + 47.3, NH,+ will not interfere or exhibit correlation with color at NH,+ < = 50 pgN/L.

Aknowledgment-This work was sponsored by the Norwegian Institute for Water Research, by the Environment Research Program of the European Commission (EVSV-CT92-0142) and by the Norwegian Research Council. The authors also wish to acknowledge the important work carried out by our local observers, Tor Holsen and Oddleiv Hjellum, who have collected water samples over many years, even during storms, intense cold, and darkness.

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