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Bacterial activity in sediments of lakes receiving acid precipitation
Environmental Pollution (Series A) 36 (1984) 195-205
Bacterial Activity in Sediments of Lakes Receiving Acid Precipitation S. S. R a o , A. A. J u r k o v i c & J. O. N r i a g u National Water Research Institute, Canada Centre for Inland Waters, Box 5050, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6, Canada
A BS TRA C T Bacteriological dataJor water and sediment coresJrom some oJ the lakes receiving acid precipitation near Sudbury, Ontario, show a strong relationship between lake acidification and bacteria, pH values below about 5"5 appear to be critical for respiring and aerobic heterotrophic bacteria. Total bacteria in the waters of these lakes existed in the 10 6 ml- l range while the viable or activepopulations were approximately two orders oJ magnitude lower. In the sediments, a relationship was demonstrated between pH, bacterial populations, sediment microbial activity, and total organic matter. In the acid-stressed Silver Lake (pH 3"8) the sediment respiration was 10-20 % oJ that in the non-acidstressed McFarlane Lake (pH 7.2). Associated with this, a strong correlation was demonstrated between pH and total organic matter in surficial sediments.
INTRODUCTION The effects of acidity on microbial activities in aquatic ecosystems have generated a lot of interest recently (Leivestad et al., 1976; Alexander, 1978; Traaen, 1980; Pagel, 1981 ; Fjerdingstad & Nilssen, 1982; Kelly et al., 1982). In general, the field surveys show that bacterial populations and activity such as respiration were relatively lower in acidified lakes than in non-acidic lakes (Babich & Stotzky, 1978). Following such observations, laboratory studies were made to ascertain the effects of acid stress on bacterial isolates from either acidic lakes (Rao et al., 1982; Baker 195 Environ. Pollut. Ser. A. 0143-1471/84/$03"00© ElsevierApplied Science PublishersLtd, England, 1984. Printed in Great Britain
196
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$ Fig. 1. Microbiology sampling sites in lakes receiving acid precipitation near Sudbury, Ontario. 1, Windy Lake; 2, Hannah Lake; 6, Silver Lake; 7, Lohi Lake; 9, Wavy Lake; 10, McFarlane Lake; I1, Ramsey Lake; 19, Richard Lake. et al., 1982) or obtained from artificially acidified waters (Baath et al.,
1979). The data from the microcosm basically confirm that low pH inhibited microbial activity. In this paper, we present bacteriological data from water samples collected during 1981/82 from a series of eight lakes having different sensitivity to acid precipitation (Fig. 1). The lakes studied show a wide range in pH; Silver Lake (pH 4.0), Hannah Lake (pH 6.5), Lohi Lake (pH 4.8), Wavy Lake (pH 5.4), Ramsey Lake (pH 6.8), Richard Lake (pH 6.8), Windy Lake (pH 6.3), and McFarlane Lake (pH 7.6). In addition, microbiological data collected from sediment cores from an acidic lake and a non-acid lake are also presented in order to ascertain the effects of acid stress on the indigenous bacterial population and its activity. Few previous studies have related the bacterial activity to the insitu pH of the sediments. MATERIALS A N D METHODS
Study site The lakes examined are located within a radius of 30 km from the smelters in Sudbury, Ontario (Fig. 1). These Precambrian shield lakes encompass
Bacterial aetit'ity in lakes receiling acid precipitation
197
a wide diversity of physical and chemical characteristics and show varying degrees of stress from acid rain (Nriagu et al., 1982). Acidity of the lakes can be traced back to acid rain. The details of geological settings and the general limnoiogy and chemistry of lakes in the region have been outlined by Conroy et al. (1978). Sample collection Surface water samples (1 litre) from a series of eight lakes (Fig. 1) were collected in sterile containers. These samples were packed in ice and transported within 48 h to our laboratory for further processing. This procedure did not affect the levels of bacterial populations (Rao & Dutka, i 983). The study was carried out from May until October 1982. Sediment cores (50 × 6 cm) were collected by means of a lightweight coring device (Williams & Pashly, 1979) during August 1982 from the acid-stressed Silver Lake (pH 3.9) and the non-acid-stressed McFarlane Lake (pH 7.2). Only cores that came up with their sediment-water interfaces intact and undeformed were subsectioned and processed within 1 h after retrieval. Each core was subsectioned at 1-2 cm intervals up to 40 cm. The samples were collected in sterile plastic bags and refrigerated immediately. The samples were processed within 48 h in the shore laboratory. The pH measurements were made using a portable pH meter, with the electrodes inserted directly into the mud. Organic content of the sediments was measured by combustion according to standard procedure (American Public Health Association, 1980).
Bacteriological procedure Counting of bacteria was performed using epifluorescence microscopy after staining with acridine orange. The staining of filters and solutions used follow Zimmerman et al. (1978). Total and actively respiring bacterial populations were estimated microscopically on all water samples using the combined acridine orange INT-formazan reduction technique (Zimmerman et al., 1978). Reduction of tetrazolium dye in bacterial cells involves respiration and in actively respiring bacteria deposits of accumulated reduced INT-formazan, seen as optically dense dark-red intracellular spots, allow light microscope observation (Zimmerman et al., 1978). Total bacteria were estimated on the same slide using UV light. Numbers of bacteria
198
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were determined in 20 fields, each field with 25 rectangular fields of an occular. Generally, this resulted in a total count of 30-300 bacteria. Aerobic heterotrophic bacteria were measured in all samples using the spread plate procedure and a low-nutrient medium. Incubation of the inoculated plates was for seven days at 20°C (Dutka, 1978).
Sediment respiration Oxygen consumption by the various sediment fractions from two lakes (Silver Lake and McFarlane Lake) was measured at 20°C using a Gilson differential respirometer. Each standard Warburg flask contained 3 g of sediment (wet weight), 2 ml of filtered sterilised lake water, which served as diluent, and 0.2 ml of 20 ~o K O H in the centre well for carbon dioxide absorption. A preincubation period of 15 min was used in all experiments. Because oxygen uptake is a function of the amount of sediment added, the sediment was weighed to an accuracy of _+ 10mg with a sensitive toploading balance. No exogenous substrate was added. The Warburg flasks were shaken at a rate of l l0 strokes per minute to ensure that oxygen diffusion rate was not a limiting factor. KCN was used to stop microbiological activity. By calculating the difference between total oxygen uptake and oxygen uptake obtained by KCN poisoning, microbial oxygen uptake was determined. Results presented here pertain to microbial oxygen uptake.
RESULTS A N D DISCUSSIONS Figure 2 is a scattergram showing the representative relationship between bacterial populations and pH in waters of the eight lakes studied. The pH values of these lakes did not change significantly during the study period May-October. Total bacterial densities were in the l06 ml- 1 range in all eight lakes examined. Acid stress had no apparent effect on total bacteria since lakes having a pH of 7.8 had total bacterial populations somewhat similar to lakes with pH 3.8. These data clearly demonstrate that certain physiological groups of bacteria in the water column are insensitive to changes in pH. Respiring bacteria and aerobic heterotrophs in contrast show a definite response to pH changes. Generally, maximum respiring bacteria recorded in these lakes were in the l05 ml-1 range while aerobic heterotrophs recorded
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were in the 104 ml - 1 range. The pH values below about 5-5 may be critical for these populations. As far as we know, a critical pH value for aerobic heterotrophs and respiring bacteria in the water column has not been reported previously. The next phase of the study was to ascertain the critical pH for inhibition of the activity of heterotrophs in the sediments. This is shown in Fig. 3, which compares the pH profiles with changes in aerobic heterotroph populations in the sediment cores from the acid-stressed Silver Lake and the non-acid-stressed McFarlane Lake. It may be seen that in the top 5-cm layers of the sediment core from Silver Lake with pH of 3"8-5.0, the aerobic heterotrophic bacterial populations existed in the 103-104 ml- 1 range. However, their densities between 5 and 40 cm, where the recorded pH was ~ 5.5, were relatively larger (105-106 ml-- 1). On the other hand, the profile of these bacterial populations in the core from non-acid McFarlane Lake is quite different; the populations decreased from about l0 Tml-1 in the surficial sediments with pH of about 7.1 to about 106 ml - 1 in the deeper layers, where the pH is relatively constant at about 6.5 (Fig. 3). These data again suggest that pH below about 5.5 appears to be critical for bacterial populations in the sediments. Thus the top 5-cm of the sediment core of Silver Lake contained bacteria in the pH 3
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10+ ml-1 range where pH was below 5-0. However, in the deeper layers where pH of the sediment was found to be approximately 5.5, the bacterial populations increased by nearly two orders of magnitude. Figure 4 compares the bacterial oxygen uptake in the sediment cores of Silver Lake and McFarlane Lake. It is apparent from the figure that there is a strong relationship between acid stress conditions and sediment oxygen uptake. In McFarlane Lake, as the sediment pH decreased, a corresponding decrease in the rate and extent of sediment oxygen utilisation was noted. Such a distinct relationship was not obvious in the acid-stressed Silver Lake. Maximum oxygen uptake at the end of 3 h incubation in the top 5-cm layer of the acid-stressed Silver Lake did not exceed 10--12pig -~ sediment (dry weight). However, for the same incubation period, in the top 5-cm layer of McFarlane Lake, oxygen uptake was nearly 10 times more (approximately 125/~1 g- ~ dry sediment being the average value for the top 4cm). Similar observations on the effect of reduced pH on the growth and activity of heterotrophic sediment micro-organisms have been reported by Baker et al. (1982, 1983). Even 200 z
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though the physico-chemical nature of the sediment influences oxygen demand, our study indicates that microbial oxygen utilisation is the most likely cause. It should be noted that the rates of oxygen consumption generally decrease down the cores from the two contrasting lakes (Fig. 4). It has been shown that the sediments of the two lakes are heavily contaminated with trace metals from the smelter and that the highest concentrations of the pollutant metals are found in the surficial (0-1 cm layer) sediments (Nriagu et al., 1982). The inverse relationship between intensity of metal pollution and rate of sediment oxygen uptake suggests that the change in pH exercises a dominating influence on the activity of heterotrophic bacteria in these sediments. This finding was unexpected. One of the effects of lake acidification is reported to be the increased accumulation of organic matter in the lake sediments (e.g. see Grahn & Hultberg, 1974; Grahn et al., 1974; Baker et al., 1982; Fjerdingstad & Nilssen, 1982). Figure 5 compares the concentration of organic matter in the sediments with the pH of the overlying water for the eight lakes studied. The strong relationship found confirms the results of the previous studies. The build-up of organic matter has been attributed to the inhibition of microbial decomposition processes and/or to a shift from bacterial to less-efficient fungi mineralisation (Grahn et al., 1974; Baker et al., 1982; Fjerdingstad & N ilssen, 1982). The increased storage
203
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of organic matter in the sediments is tied in to the recycling of nutrients and hence can affect the behaviour of the whole lake ecosystem. This particular facet of lake acidification remains to be investigated in detail. The various observations on the effects of acid precipitation on the microbial population and its activity in lake sediments are summarised in a conceptual model (Fig. 6). It is surmised that lake acidification will reduce the bacterial activity and hence increase the organic matter content of the sediments. These parameters can, therefore, be used to trace the historical changes in the response of lakes to acid precipitation.
REFERENCES Alexander, M. (1978). Effects of acidity on microorganisms and microbial processes in soil. In Effects of acid precipitation on terrestrial ecosystem, ed. by T. C. Hutchinson and M. Haves, 363-74. New York, London, Plenum (published in co-ordination with NATO Scientific Affairs Division). American Public Health Association (1980). Standard methods for the examination oJ water, wastewater and sediments, 15th edn. Washington, DC, APHA-AWWA-WPCF.
204
S. S. Rao, A. A. Jurkovic, J. O. Nriagu
Baath, E., Lundgren & Soderstron, B. (1979). Effects of artificial acid rain on microbial activity and biomass. Bull. environ. Contam. & Toxicol., 23, 737-40. Babich, H. & Stotzky, G. (1978). Influence of pH on inhibition of bacteria, fungi and coliphages by bisulfite and sulfite. Environ. Res., 15, 405-17. Baker, M. D., Inniss, W. E., Mayfield, C. I. & Wong, P. T. S. (1982). Effects of pH in the growth rate and activity of heterotrophic sediment microorganisms. Chemosphere, 1I, 973-83. Baker, M. D., lnniss, W. E., Mayfield, C. I. & Wong, P. T. S. (1983). Effect of acidification, metals and metalloids on sediment microorganisms. Water Res., 17, 925- 30. Conroy, N. I., Hawly, K. & Keller, K. (1978). Extensive monitoring oj lakes in the greater Sudbury area. Technical report. Ontario Ministry of the Environment. Dutka, B. J. (1978). Methods jor microbiological analysis oJ water, waste water and sediments. Burlington, Ontario. IWD. Fjerdingstad, E. & Nilssen, J. P. (1982). Bacteriological and hydrological studies on acidic lakes in Southern Norway. Arch. Hydrobiol., 4, 443-83. Grahn, O. H. & Hultberg, H. (1974). Effect oJ acid!fication on the ecos)'stem of oligotrophie lakes-- integrated changes in species composition and dynamics. Gothenburg, Meddeland, Institutet f6r Vatten-och Luftvardsforskning. Grahn, O. H., Hultberg, H. & Landner, L. (1974). Oligotrophication--a selfaccelerating process in lake subjected to excessive supply of acid substances. Ambio, 3, 93-4. Kelly, C. A., Rudd, J. W. M., Cook, R. B. & Schindler, D. W. (1982). The potential importance of bacterial processes in regulating rate of lake acidification. Limnol. Oceanogr., 27, 868-82. Leivestad, H., Hendrey, G., Muniz, I. P. & Shekvik, E. (1976). Effects of acid precipitation on fresh water organisms. In Summary report on research results Jrom the phase I (1972- 75) oJ the research project on aeid!fl'cation -Effects on jorest and/ish, ed. by F.N. Braekke. In Impact oj acid precipitation onJorest andjreshwater ecosystems in Norway, 87-111. SNSF Project F.R 6/76, Oslo. Nriagu, J. O., Wong, H. K. T. & Coker, R. D. (1982). Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbury, Ontario. Environ. Sci. & Technol., 16, 551-60. Pagel, J. E. ( 1981). A review oj microbiological processes relevant to the effects oj acid precipitation. Technical report. Ontario Ministry of the Environment, June 1981. Rao, S. S. & Dutka, B. J. (1983). Influence of acid precipitation on bacterial populations in lakes. Hydrobiologia, 98, 153-7. Rao, S. S., Bhaskar, L. & Jurkovic, A. A. (1982). Microbiological studies oJ some watershed receiving acid precipitation in Canada. NWRI Report No. 37AMD-5-82, Burlington, Ontario. Traaen, T. (1980). Effects of acidity on decomposition of organic matter in aquatic environments. |n Proc. Int. Conj. Ecol. Impact Acid. Precpim--
Bacterial activity m lakes receiving acid precipitation
205
Norway (i 1-14 March 1980), ed. by D. Droblos and A. Tollan, 340-1. Oslo. Williams, J. D. H. & Pashly, A. E. (1979). Lightweight-corer designed for sampling very soft sediments. J. Fish. Res. Bd Can., 36, 241-6. Zimmerman, R., Iturriaga, R. & Brick, J. B. (1978). Simultaneous determination of the total number of aquatic bacteria and the number thereof involved in respiration. Era'iron. Microbiol., 36, 926-35.
Bacterial Activity in Sediments of Lakes Receiving Acid Precipitation S. S. R a o , A. A. J u r k o v i c & J. O. N r i a g u National Water Research Institute, Canada Centre for Inland Waters, Box 5050, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6, Canada
A BS TRA C T Bacteriological dataJor water and sediment coresJrom some oJ the lakes receiving acid precipitation near Sudbury, Ontario, show a strong relationship between lake acidification and bacteria, pH values below about 5"5 appear to be critical for respiring and aerobic heterotrophic bacteria. Total bacteria in the waters of these lakes existed in the 10 6 ml- l range while the viable or activepopulations were approximately two orders oJ magnitude lower. In the sediments, a relationship was demonstrated between pH, bacterial populations, sediment microbial activity, and total organic matter. In the acid-stressed Silver Lake (pH 3"8) the sediment respiration was 10-20 % oJ that in the non-acidstressed McFarlane Lake (pH 7.2). Associated with this, a strong correlation was demonstrated between pH and total organic matter in surficial sediments.
INTRODUCTION The effects of acidity on microbial activities in aquatic ecosystems have generated a lot of interest recently (Leivestad et al., 1976; Alexander, 1978; Traaen, 1980; Pagel, 1981 ; Fjerdingstad & Nilssen, 1982; Kelly et al., 1982). In general, the field surveys show that bacterial populations and activity such as respiration were relatively lower in acidified lakes than in non-acidic lakes (Babich & Stotzky, 1978). Following such observations, laboratory studies were made to ascertain the effects of acid stress on bacterial isolates from either acidic lakes (Rao et al., 1982; Baker 195 Environ. Pollut. Ser. A. 0143-1471/84/$03"00© ElsevierApplied Science PublishersLtd, England, 1984. Printed in Great Britain
196
S. S. Rao, A. A. Jurkoric, J. O. Nriagu
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$ Fig. 1. Microbiology sampling sites in lakes receiving acid precipitation near Sudbury, Ontario. 1, Windy Lake; 2, Hannah Lake; 6, Silver Lake; 7, Lohi Lake; 9, Wavy Lake; 10, McFarlane Lake; I1, Ramsey Lake; 19, Richard Lake. et al., 1982) or obtained from artificially acidified waters (Baath et al.,
1979). The data from the microcosm basically confirm that low pH inhibited microbial activity. In this paper, we present bacteriological data from water samples collected during 1981/82 from a series of eight lakes having different sensitivity to acid precipitation (Fig. 1). The lakes studied show a wide range in pH; Silver Lake (pH 4.0), Hannah Lake (pH 6.5), Lohi Lake (pH 4.8), Wavy Lake (pH 5.4), Ramsey Lake (pH 6.8), Richard Lake (pH 6.8), Windy Lake (pH 6.3), and McFarlane Lake (pH 7.6). In addition, microbiological data collected from sediment cores from an acidic lake and a non-acid lake are also presented in order to ascertain the effects of acid stress on the indigenous bacterial population and its activity. Few previous studies have related the bacterial activity to the insitu pH of the sediments. MATERIALS A N D METHODS
Study site The lakes examined are located within a radius of 30 km from the smelters in Sudbury, Ontario (Fig. 1). These Precambrian shield lakes encompass
Bacterial aetit'ity in lakes receiling acid precipitation
197
a wide diversity of physical and chemical characteristics and show varying degrees of stress from acid rain (Nriagu et al., 1982). Acidity of the lakes can be traced back to acid rain. The details of geological settings and the general limnoiogy and chemistry of lakes in the region have been outlined by Conroy et al. (1978). Sample collection Surface water samples (1 litre) from a series of eight lakes (Fig. 1) were collected in sterile containers. These samples were packed in ice and transported within 48 h to our laboratory for further processing. This procedure did not affect the levels of bacterial populations (Rao & Dutka, i 983). The study was carried out from May until October 1982. Sediment cores (50 × 6 cm) were collected by means of a lightweight coring device (Williams & Pashly, 1979) during August 1982 from the acid-stressed Silver Lake (pH 3.9) and the non-acid-stressed McFarlane Lake (pH 7.2). Only cores that came up with their sediment-water interfaces intact and undeformed were subsectioned and processed within 1 h after retrieval. Each core was subsectioned at 1-2 cm intervals up to 40 cm. The samples were collected in sterile plastic bags and refrigerated immediately. The samples were processed within 48 h in the shore laboratory. The pH measurements were made using a portable pH meter, with the electrodes inserted directly into the mud. Organic content of the sediments was measured by combustion according to standard procedure (American Public Health Association, 1980).
Bacteriological procedure Counting of bacteria was performed using epifluorescence microscopy after staining with acridine orange. The staining of filters and solutions used follow Zimmerman et al. (1978). Total and actively respiring bacterial populations were estimated microscopically on all water samples using the combined acridine orange INT-formazan reduction technique (Zimmerman et al., 1978). Reduction of tetrazolium dye in bacterial cells involves respiration and in actively respiring bacteria deposits of accumulated reduced INT-formazan, seen as optically dense dark-red intracellular spots, allow light microscope observation (Zimmerman et al., 1978). Total bacteria were estimated on the same slide using UV light. Numbers of bacteria
198
S. S. Rao, A. A. Jurkocic, J. O. Nriagu
were determined in 20 fields, each field with 25 rectangular fields of an occular. Generally, this resulted in a total count of 30-300 bacteria. Aerobic heterotrophic bacteria were measured in all samples using the spread plate procedure and a low-nutrient medium. Incubation of the inoculated plates was for seven days at 20°C (Dutka, 1978).
Sediment respiration Oxygen consumption by the various sediment fractions from two lakes (Silver Lake and McFarlane Lake) was measured at 20°C using a Gilson differential respirometer. Each standard Warburg flask contained 3 g of sediment (wet weight), 2 ml of filtered sterilised lake water, which served as diluent, and 0.2 ml of 20 ~o K O H in the centre well for carbon dioxide absorption. A preincubation period of 15 min was used in all experiments. Because oxygen uptake is a function of the amount of sediment added, the sediment was weighed to an accuracy of _+ 10mg with a sensitive toploading balance. No exogenous substrate was added. The Warburg flasks were shaken at a rate of l l0 strokes per minute to ensure that oxygen diffusion rate was not a limiting factor. KCN was used to stop microbiological activity. By calculating the difference between total oxygen uptake and oxygen uptake obtained by KCN poisoning, microbial oxygen uptake was determined. Results presented here pertain to microbial oxygen uptake.
RESULTS A N D DISCUSSIONS Figure 2 is a scattergram showing the representative relationship between bacterial populations and pH in waters of the eight lakes studied. The pH values of these lakes did not change significantly during the study period May-October. Total bacterial densities were in the l06 ml- 1 range in all eight lakes examined. Acid stress had no apparent effect on total bacteria since lakes having a pH of 7.8 had total bacterial populations somewhat similar to lakes with pH 3.8. These data clearly demonstrate that certain physiological groups of bacteria in the water column are insensitive to changes in pH. Respiring bacteria and aerobic heterotrophs in contrast show a definite response to pH changes. Generally, maximum respiring bacteria recorded in these lakes were in the l05 ml-1 range while aerobic heterotrophs recorded
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were in the 104 ml - 1 range. The pH values below about 5-5 may be critical for these populations. As far as we know, a critical pH value for aerobic heterotrophs and respiring bacteria in the water column has not been reported previously. The next phase of the study was to ascertain the critical pH for inhibition of the activity of heterotrophs in the sediments. This is shown in Fig. 3, which compares the pH profiles with changes in aerobic heterotroph populations in the sediment cores from the acid-stressed Silver Lake and the non-acid-stressed McFarlane Lake. It may be seen that in the top 5-cm layers of the sediment core from Silver Lake with pH of 3"8-5.0, the aerobic heterotrophic bacterial populations existed in the 103-104 ml- 1 range. However, their densities between 5 and 40 cm, where the recorded pH was ~ 5.5, were relatively larger (105-106 ml-- 1). On the other hand, the profile of these bacterial populations in the core from non-acid McFarlane Lake is quite different; the populations decreased from about l0 Tml-1 in the surficial sediments with pH of about 7.1 to about 106 ml - 1 in the deeper layers, where the pH is relatively constant at about 6.5 (Fig. 3). These data again suggest that pH below about 5.5 appears to be critical for bacterial populations in the sediments. Thus the top 5-cm of the sediment core of Silver Lake contained bacteria in the pH 3
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10+ ml-1 range where pH was below 5-0. However, in the deeper layers where pH of the sediment was found to be approximately 5.5, the bacterial populations increased by nearly two orders of magnitude. Figure 4 compares the bacterial oxygen uptake in the sediment cores of Silver Lake and McFarlane Lake. It is apparent from the figure that there is a strong relationship between acid stress conditions and sediment oxygen uptake. In McFarlane Lake, as the sediment pH decreased, a corresponding decrease in the rate and extent of sediment oxygen utilisation was noted. Such a distinct relationship was not obvious in the acid-stressed Silver Lake. Maximum oxygen uptake at the end of 3 h incubation in the top 5-cm layer of the acid-stressed Silver Lake did not exceed 10--12pig -~ sediment (dry weight). However, for the same incubation period, in the top 5-cm layer of McFarlane Lake, oxygen uptake was nearly 10 times more (approximately 125/~1 g- ~ dry sediment being the average value for the top 4cm). Similar observations on the effect of reduced pH on the growth and activity of heterotrophic sediment micro-organisms have been reported by Baker et al. (1982, 1983). Even 200 z
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though the physico-chemical nature of the sediment influences oxygen demand, our study indicates that microbial oxygen utilisation is the most likely cause. It should be noted that the rates of oxygen consumption generally decrease down the cores from the two contrasting lakes (Fig. 4). It has been shown that the sediments of the two lakes are heavily contaminated with trace metals from the smelter and that the highest concentrations of the pollutant metals are found in the surficial (0-1 cm layer) sediments (Nriagu et al., 1982). The inverse relationship between intensity of metal pollution and rate of sediment oxygen uptake suggests that the change in pH exercises a dominating influence on the activity of heterotrophic bacteria in these sediments. This finding was unexpected. One of the effects of lake acidification is reported to be the increased accumulation of organic matter in the lake sediments (e.g. see Grahn & Hultberg, 1974; Grahn et al., 1974; Baker et al., 1982; Fjerdingstad & Nilssen, 1982). Figure 5 compares the concentration of organic matter in the sediments with the pH of the overlying water for the eight lakes studied. The strong relationship found confirms the results of the previous studies. The build-up of organic matter has been attributed to the inhibition of microbial decomposition processes and/or to a shift from bacterial to less-efficient fungi mineralisation (Grahn et al., 1974; Baker et al., 1982; Fjerdingstad & N ilssen, 1982). The increased storage
203
Bacterial acticity in lakes receit'ing acid precipitation
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of organic matter in the sediments is tied in to the recycling of nutrients and hence can affect the behaviour of the whole lake ecosystem. This particular facet of lake acidification remains to be investigated in detail. The various observations on the effects of acid precipitation on the microbial population and its activity in lake sediments are summarised in a conceptual model (Fig. 6). It is surmised that lake acidification will reduce the bacterial activity and hence increase the organic matter content of the sediments. These parameters can, therefore, be used to trace the historical changes in the response of lakes to acid precipitation.
REFERENCES Alexander, M. (1978). Effects of acidity on microorganisms and microbial processes in soil. In Effects of acid precipitation on terrestrial ecosystem, ed. by T. C. Hutchinson and M. Haves, 363-74. New York, London, Plenum (published in co-ordination with NATO Scientific Affairs Division). American Public Health Association (1980). Standard methods for the examination oJ water, wastewater and sediments, 15th edn. Washington, DC, APHA-AWWA-WPCF.
204
S. S. Rao, A. A. Jurkovic, J. O. Nriagu
Baath, E., Lundgren & Soderstron, B. (1979). Effects of artificial acid rain on microbial activity and biomass. Bull. environ. Contam. & Toxicol., 23, 737-40. Babich, H. & Stotzky, G. (1978). Influence of pH on inhibition of bacteria, fungi and coliphages by bisulfite and sulfite. Environ. Res., 15, 405-17. Baker, M. D., Inniss, W. E., Mayfield, C. I. & Wong, P. T. S. (1982). Effects of pH in the growth rate and activity of heterotrophic sediment microorganisms. Chemosphere, 1I, 973-83. Baker, M. D., lnniss, W. E., Mayfield, C. I. & Wong, P. T. S. (1983). Effect of acidification, metals and metalloids on sediment microorganisms. Water Res., 17, 925- 30. Conroy, N. I., Hawly, K. & Keller, K. (1978). Extensive monitoring oj lakes in the greater Sudbury area. Technical report. Ontario Ministry of the Environment. Dutka, B. J. (1978). Methods jor microbiological analysis oJ water, waste water and sediments. Burlington, Ontario. IWD. Fjerdingstad, E. & Nilssen, J. P. (1982). Bacteriological and hydrological studies on acidic lakes in Southern Norway. Arch. Hydrobiol., 4, 443-83. Grahn, O. H. & Hultberg, H. (1974). Effect oJ acid!fication on the ecos)'stem of oligotrophie lakes-- integrated changes in species composition and dynamics. Gothenburg, Meddeland, Institutet f6r Vatten-och Luftvardsforskning. Grahn, O. H., Hultberg, H. & Landner, L. (1974). Oligotrophication--a selfaccelerating process in lake subjected to excessive supply of acid substances. Ambio, 3, 93-4. Kelly, C. A., Rudd, J. W. M., Cook, R. B. & Schindler, D. W. (1982). The potential importance of bacterial processes in regulating rate of lake acidification. Limnol. Oceanogr., 27, 868-82. Leivestad, H., Hendrey, G., Muniz, I. P. & Shekvik, E. (1976). Effects of acid precipitation on fresh water organisms. In Summary report on research results Jrom the phase I (1972- 75) oJ the research project on aeid!fl'cation -Effects on jorest and/ish, ed. by F.N. Braekke. In Impact oj acid precipitation onJorest andjreshwater ecosystems in Norway, 87-111. SNSF Project F.R 6/76, Oslo. Nriagu, J. O., Wong, H. K. T. & Coker, R. D. (1982). Deposition and chemistry of pollutant metals in lakes around the smelters at Sudbury, Ontario. Environ. Sci. & Technol., 16, 551-60. Pagel, J. E. ( 1981). A review oj microbiological processes relevant to the effects oj acid precipitation. Technical report. Ontario Ministry of the Environment, June 1981. Rao, S. S. & Dutka, B. J. (1983). Influence of acid precipitation on bacterial populations in lakes. Hydrobiologia, 98, 153-7. Rao, S. S., Bhaskar, L. & Jurkovic, A. A. (1982). Microbiological studies oJ some watershed receiving acid precipitation in Canada. NWRI Report No. 37AMD-5-82, Burlington, Ontario. Traaen, T. (1980). Effects of acidity on decomposition of organic matter in aquatic environments. |n Proc. Int. Conj. Ecol. Impact Acid. Precpim--
Bacterial activity m lakes receiving acid precipitation
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Norway (i 1-14 March 1980), ed. by D. Droblos and A. Tollan, 340-1. Oslo. Williams, J. D. H. & Pashly, A. E. (1979). Lightweight-corer designed for sampling very soft sediments. J. Fish. Res. Bd Can., 36, 241-6. Zimmerman, R., Iturriaga, R. & Brick, J. B. (1978). Simultaneous determination of the total number of aquatic bacteria and the number thereof involved in respiration. Era'iron. Microbiol., 36, 926-35.