Pergamon Environment International, Vol. 20, No. 3, pp. 363-368, 1994

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THE ROLE OF HUMIC SUBSTANCES IN THE ACIDIFICATION RESPONSE OF SOIL AND WATERmRESULTS OF THE HUMIC LAKE ACIDIFICATION EXPERIMENT (HUMEX)

Egil T. Gjessing Norwegian Institute for Water Research, 0411 Oslo, Norway

EI 9307-185 M (Received 12 July 1993; accepted 23 January 1994)

Major results of the Humic Lake Acidification Experiment (HUMEX) are summarized, based on 2 y of pretreatment and 2.5 y of posttreatment data. The major objectives of the HUMEX project are to quantify the role of acid deposition on the properties of humic substances (HS) and the role of humic substances (HS) in the acidification processes that occur in soil and water. The project involves artificial acidification of one half of a divided dystrophic lake and the corresponding catchment. A combination of sulphuric acid and ammonium nitrate has been applied via sprinkler systems, mounted on trees, during precipitation events since 1990. The treatment has resulted in small changes in water quality, including an increase in SO4, NO3, and H ÷ concentrations in the lake water and in the soil water of some of the upper soil horizons, and small changes in the nature of the HS. The results of biological studies show increased toxicity in fish, increase in the phytoplankton primary production, and disappearance of some of the dominating species of zooplankton. Epiphytic growth increased in the treated basin, whereas a group of macrophytes was reduced. Present knowledge of the relationships between chemical changes and biological response is not sufficient to explain the observed changes in biota.

INTRODUCTION

Twenty years of international research on the ef- fects of acid deposition, which started in the early 1970's, concluded that, in some areas, there have been serious effects on aquatic ecosystems. How- ever, many of the results have been inconsistent or even contradictionary. A part of this inconsistency can be attributed to the varying amount and function- al properties of humic substances (HS) in surface waters that receive runoff containing sulphur and nitrogen from atmospheric deposition.

In the Norwegian HUMEX project, a dystrophic lake-half (divided by a plastic curtain from the mid-

die of the natural outlet to the opposite shore) and its corresponding catchment, is treated with artificial polluted precipitation. The overall goal is to evaluate the role of HS on the acidification of surface waters and the effects of sulphur and nitrogen on the multi- functional properties of HS in soil and water. The HUMEX project is an interdisciplinary study with scientific input from more than a dozen institutes in Europe and North America. More than two dozen researchers from these institutions involved in the HUMEX project are chemists, biologists, and hydrologists. Changes and differences in both soil and water are studied.

363

364 E.T. Gjessing

The present paper describes the technical installa- tions and the experimental conditions of the HUMEX- project and summarizes the scientific input in the project and some of the major results after 2.5 y of treatment.

MATERIAL AND METHODS

Technical instaflations

In fall of 1988, a dystrophic lake, the Lake Skjer- vatjern, was divided into two halves by a plastic curtain. The lake was divided from the middle of the natural outlet to the opposite shore. Skjervatjern is located in an area of Norway which receives little polluted deposition. During the following 2 y, through September 1990, the water chemistry of the two lake halves was monitored and a number of scientists from Europe and North America studied the organic mat- ter and biota in the two water basins and their cor- responding catchments. Artificial acidification of one half (Basin A) was started in October 1990. This treatment of the 1.8 ha terrestral catchment is carried out by a series of sprinklers which are mouted on the top of the highest trees in the catchment. Two larger sprinklers were installed along the lake shore to pro- vide treatment directly to the 0.9 ha treatment side (Basin A) of the lake. The sprinkling system operates automatically, after being activated manually. The other lake- and catchment half (Basin B) serves as a control.

Application of pollutants

During spring, summer, and fall, artificial acid de- position is applied weekly (if there was any precipita- tion during the previous week). The total amount of artificial precipitation added to Basin A is about 10% of the natural precipi tat ion. The pH of the sprin- kling water is kept at 3.0-3.2. Ammonium ni t ra te is also added to the acidified precipitation (0.8 mg NH4NO3/mg SO42). The target loading of sulphur and nitrogen, sulphuric acid, and ammonium nitrate, respectively, is strongly comparable to the deposi- tion received by acidified areas of southernmost Nor- way.

Samples and data collection

From October 1988, when the lake was divided, water samples were collected weekly from the two outlets and analyzed for major ion chemistry. A num- ber of independent chemical and biological inves- tigations were initiated during the period of 1988 to 1993. These are being conducted by teams of scien- tific researchers from throughout Europe and North

America. Soil Lysimeters were installed in the two catchments and, during spring, summer, and fall, soil water samples are collected monthly. Samples for phyto- and zooplankton were collected at ap- proximately the same frequency. During a few weeks in the middle of the summer season, sampling is intensified and includes studies on the nature of the organic acids, l ipid-solubility of the HS, enzyme activities, insects and macro vegetation, soil and water interactions and sediments. Changes in the density and distribution of bentic algae are also under investigation.

There are no distinct inlets to the lake. It has been shown, however, that the water flow is not diffuse, but is transported to the lake from lake shore peat deposits, in discrete passageways (canuli). The canuli empty into the lake at well-defined points of entry, called hydrolic vents. Chemical and biologi- cal studies of these vents were conducted.

SUMMARY OF RESULTS AFTER 2.5 Y OF TREAT- MENT

Catchment~soil

The surrounding area of the lake consists of poorly drained, organic rich soil and sphagnum. There are no stream inlets into the lake, but numerous seeps have been located (Petersen et al. 1991). Imperme- able peats are underlain by a discontinuous strata of permeable sands. The sands are modified from the Quartzite bedrock. After one year of treatment, the peat deposits in Basin A were depleted of acidic functional groups and increased in phenolic sites compared to the control portion of the catchment (Collins et al. 1992).

Prior to the start of acidification, several ceramic- cup soil solution lysimeters were installed in both the experimental catchment and the control area. In general, changes in soil solution chemistry in Basin A were small or negligible in response to the acid addition. The most pronounced response to the addi- tion of artificial acid precipitation was found not, as exspected, in the poorly buffered mineral soils, but in the surface of the histosols, where the most acid soil water conditions ([H+]=35 I.tM) were found. In this uppermost horizon of the histosols, the addition of sulphuric acid for about one year (3.4 g/m 2) resulted in small decreases in dissolved organic carbon (DOC) and organic aluminium (A10) and an increase in the sulphate concentration (Vogt et al. 1992; Vogt et al. 1994).

Humic lake acidi f icat ion 365

16-

14-

12-

10-

I.teqv/L 8-

_

4-

H + SO 4 C1 NO 3 Ca Mg Na K LAI NH4-N

Fig. 1. Chemical changes due to t reatment in 30 months. Based on weekly samples taken from the outlet of the two lake halves (A = ex- perimental and B = control). The bars are: X-Y, where X = (A-B) after 30 months of treatment and Y = (A-B) before start of treatment.

Soil-water interface

A preliminary investigation of temperature varia- tion along the lake shore clearly showed that the waterflow into the lake is not defuse. According to Petersen and coworkers (1991), water enters the lake at discrete points of entry, through hydraulic "vents". The microbial respiration rate of coarse woody detrius was found to be significantly higher in the hydraulic vents, compared to the nonvent areas along the land/water interface. The respiration rate of detrius, collected at the vents, was about twice that of detrius collected elsewhere (Petersen et al. 1992).

During one-week periods in the summers of 1990 (pretreatment) and 1991 (posttreatment), pH, DOC, and octanol soluble carbon (OSC) were studied in two vents and two nonvents in each of the basins. The pH was lower and DOC was higher in the vents compared to the nonvents (Kullberg et al. 1992). Furthermore, the OSC appeares to have increased in the vents after the first six months of acidification.

Lake

General water chemistry From the time that the lake was divided in 1988,

water samples were collected weekly from the two

outlets and analyzed on major ion chemistry. In Fig. 1, the differences between the experimental halfAand the control half B, after 2.5 y of treatment, are com- pared with the corresponding differences during the 2.5-y period before the start of acidif icat ion. All major ions increased in the concentration in Basin A as a consequence of treatment (Fig. 1). Sulphate concentration increased by 14 Ixeq/L. All other changes were less than about 4 ~teq/L. The pH of Basin A water decreased by 0.06 pH units.

Acid-base characteristics One of the major objectives of the HUMEX project

is to assess the neutralization capacity of organic acids to the input of mineral acids. Clair and coworkers (1992) have developed a new method for the deter- mination of the organic anionic components in humic waters. This is based on the knowledge of pH, acid neutralization capacity (ANC), and content of DOC. Based on measurement of these variables, a titration curve can be produced, which estimates the change in organic anion concentration and its relationship to the exchange in ANC. Preliminary results suggest that, due to the low pH of Lake Skjervatjern (mean pH=4.6), organic acids will not provide substantial

366 E.T. Gjessing

additional buffering to resist pH decline from added mineral acids.

Interaction between HS and dissolved materials Some of the most important functional properties

of HS involve their interaction with nutrients and micro elements. It has been found that the content of organic sulphur is higher in water from acidified areas than in water from nonacidified areas (Gjessing 1990) and it has been indicated that these more S-rich HS are biologically more active (Gjessing et al. 1991).

Preliminary results suggest a small increase in organic S in the acidified basin. Based on laboratory experiments, it is suggested that the interaction be- tween HS and S is not taking place as a chemical reaction in the lake, but rather in the catchment, and probably as a biochemical reaction (Albrethsen 1993).

The interaction between HS and P and Fe has been studied by Shaw and coworkers (1992), using 55Fe and 32po43". Their experiments involved also addi- tion of r a d i o - l a b e l l e d iron and phospha te to discriminate molecular-size fractions of HS. The pre- liminary summation of the transformations of the added 55Fe and 32po43", observed at different pH values for the whole water sample, cannot be ex- plained by simple summation of the transformations observed for each of the individual fractions. This also suggests that these processes in nature may not be purely chemical , but rather biological and may take place in the catchment, rather than in the lake.

Nature of HS It is long known that the nature of HS changes

dramatical ly with pH (Gjessing 1971; Gjessing 1976). As shown above, the addition of acid to the experimental half of the HUMEX lake has resulted in only a small pH decrease. At the same time, as discussed below, there have been significant respon- ses of the biota, even though this cannot be ex- plained by the observed changes in inorganic water chemistry.

One of the two main goals of the HUMEX-project is to character ize potential changes in the nature of HS and relate these changes to the biological response. Malcolm and Gjessing (1993) used XAD to isolate HS from the HUMEX basins. Their results showed that this technique can account for more than 80% of the to ta l DOC. Analysis of isolates from water samples, taken before and one year after start of the treatment suggests:

(1) Increase in organic N; (2) Decrease in the hydrophobic/hydrophilic ratio; (3) Decrease in % carbon and carboxylic acidity. This is in agreement with Kortelainen et al. (1992), who stated: "There are slight signs that the con- centration of organic acids to overall lake acidity has increased since the acidification was initiated". Similar effects were found in the soil (Collins et al. 1992) and finally, (4) Increase in oxygen percentage and phenolic acidity. Collins and coworkers (1992) also suggested an increase in the number of phenolic sites.

Phytoplankton Since 1989, there has been intensive sampling

and analysis of phytoplankton during the growing seasons in both basins. In the control basin, the development and change in the main groups of phytoplankton throughout the growing season was almost identical from year to year. This was also the case between the two basins prior to start of treat- ment. In 1991 however, after six months of treatment, there was a marked change in the composition of the phytoplankton community in the treated half. Some species showed a decrease in response to the treat- ment whereas others increased.

In general, there was an increase in primary produc- tion in the experimental half in 1991, compared to the control. In 1992, there was still this pronounced difference in composition and distribution of phyto- plankton in Basin A (acidified) compared to Basin B (control), although, the same difference in primary production was not observed (Brettum 1994). The initial increase in primary production may be ex- plained by a reduced grazing from zooplankton (see below).

Periphyton During the first summer after the start of the ex-

perimental treatment, an extensive growth of filamen- tous green algae in Basin A compared to Basin B was observed. Therefore, studies on periphyton were started the following growing season. The growth of periphyton on artificial substrates, placed at 0.5 and 2 m depth in the two basins, was analyzed after four months of accumulation. The results showed a 50-100% higher dry weight and chlorophyll content (per surface unit) in Basin A compared to Basin B. The C/N ratio in freeze-dried material from Aand B, 10-11 and 15-19, respectively, suggests N-defficiency in B, but not in A. The additional N input in A, as part of the ex- perimental manipulation, may be a possible explana-

Humic lake acidification 367

tion for the higher growth of periphyton in the ex- perimental basin (Lindstr¢m 1994).

Macrophytes Transplantation experiments for the study of aquatic

macrophyte growth and vitality have been carried out since 1991. Macrophytes were planted in plastic pots filled with lake sediments and placed in the littoral zone in both basins. The submerged moss species Spagnum auriculatum showed a decreased growth in the acidified basin less than 1 y after start of treat- ment. This difference continued through the next season. For the other species, the response after 2 y of treatment was fairly similar in the two basins (Brandrud and Johansen 1994).

Zooplankton Zooplankton have been studied since June 1989.

The results in 1991, the first summer after start of the acidification, included a strong midsummer biomass peak, followed by a sudden collapse of the dominat- ing species (Holopedium gibberum) in the acidified half. As a consequence, the zooplankton community grazing also decreased (see above under phytoplankton). In the control basin, only small seasonal variations in the total biomass were observed (Hessen 1992).

Benthic macro invertebrates Although there is a general similarity in the taxonomic

structure of the benthic macro invertebrates popula- tion in the two sides of Lake Skjervatjern, there is a significant difference in the numbers of the various species (Hargeby et al. 1992). This may be explained by the morphometric differences between the two basins. Also, a difference was found in the mean size of a dominating species (Zygopteran, Coenagrion hastulatum) between years, suggesting an effect of climatic conditions on the time of reproduction. Studies of the mean size of these species may provide useful information for evaluating the potential stress effects of the treatment (Hargeby et al. 1994).

Fish Outflowing water from both the experimental and

control basin is routed to downstream aquarium sys- tems. During two periods in 1992, about two years after the start of the acidification, the toxicity of these two waters to brown trout has been tested. In spite of only small differences in chemical parameters, such as pH and aluminium, the water from the treated half was more toxic, based on mortality. Further- more, the concentration of chloride in blood plasma of the trout was, according to Lien (1994), significant-

ly lower in the trout exposed to water from Basin A (103 meq CI/L compared to 116 meq CI/L).

SUMMARY AND PRELIMINARY CONCLUSION

The acid rain research during the 1970s and 1980s focused almost exclusively on clear water from rivers and lakes. Even though the general con- clusion was that pollutants in precipitation seriously affected the aquatic environment, the interaction be- tween acid deposition and humic substances (HS) and organic acidity, was not well studied.

Because HS are present in all waters and affect all chemical and biological processes in soil and water, these substances should be evaluated in relation to the observed changes in soil and water due to acid rain.

The purpose of the HUMEX project is to study the effect of acid rain on the properties of HS, and the role of HS on the acidification processes in soil and water. The goals are to quantify these effects. The Humex project is based on an artificial acidification of a whole catchment.

The artificial acidification of half of the dystrophic lake Skjervatjern and its discharge basin has produced results that are of importance for recent and ongoing efforts throughout Europe and North America to as- sess and quantify acidification effects. Changes in the chemistry of soil and lake waters were small after 2.5 y of treatment. Biological responses, however, were more pronounced.

There has been a clear increase in primary produc- tion and a change in the distribution of the species of phytoplankton in the treated basin. At the same time, there appears to be a disappearance of some of the dominating species of zooplankton. This may imply reduced grazing, which may be one reason for the increase in phytoplankton. The growth of epiphytes increased in response to the treatment, as did the epiphytic chlorophyll content. Macrophytic growth, in contrast, was reduced.

Perhaps of greatest interest has been the observed increase in the toxicity of the experimental water to trout, which cannot be explained by present knowledge of biological effects related to inorganic chemistry.

Acknowledgment ~ This work was sponsored by The Norwegian Institute for Water Research, by the Commission of the European Communities (STEP-CT90-0112 and NV5V-CT92-0142) and by the Norwegian Research Council (NTNF). In addition, there were considerable contributions from a number of institutions in Canada, Europe, and U.S.A. The author emphasizes the technical assistance from Tor Holsen and Oddleiv Hjellum and all their help in collecting samples. The author also wishes to thank Dr. Tim Sullivan (Corvallis, Oregon, USA) for valuable advice and help in the preparation of the manuscript.

368 E.T. Gjessing

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