Monitoring compared with paleolimnology: implications ... - Springer Link

Environ Monit Assess (2008) 146:295–308 DOI 10.1007/s10661-007-0081-9

Monitoring compared with paleolimnology: implications for the definition of reference condition in limed lakes in Sweden Matilda Norberg & Christian Bigler & Ingemar Renberg

Received: 27 June 2007 / Accepted: 6 November 2007 / Published online: 5 December 2007 # Springer Science + Business Media B.V. 2007

Abstract Surface water acidification was identified as a major environmental problem in the 1960s. Consequently, a liming program was launched in Sweden in the 1970s. The primary purpose of liming is to restore conditions that existed prior to acidification. To reach this goal, as well as achieve ‘good status’ (i.e. low levels of distortion resulting from human activity) in European freshwaters until 2016 under the European Union Water Framework Directive, lake data are required to define reference conditions. Here, we compare data from chemical/biological monitoring of 12 limed lakes with results of paleolimnological investigations, to address questions of reference conditions, acidification, and restoration by liming. Using diatom-based lake-water pH inferences, we found clear evidence of acidification in only five of the 12 lakes, which had all originally been classified as acidified according to monitoring data. After liming, measured and diatom-inferred pH agree well in seven lakes. The sediment record of three of the five remaining lakes gave ambiguous results, presumably due to sediment M. Norberg (*) : C. Bigler : I. Renberg Department of Ecology and Environmental Science, Umeå University, SE-901 87 Umeå, Sweden e-mail: [email protected] C. Bigler e-mail: [email protected] I. Renberg e-mail: [email protected]

mixing or low sediment accumulation rates. It is difficult to determine whether liming restored the lakes to a good status, especially as some of the lakes were not acidified during the twentieth century. In addition to acid deposition, other factors, such as natural lake and catchment ontogeny or human impact through agricultural activity, influence lake acidity. This study shows that monitoring series are usually too short to define reference conditions for lakes, and that paleolimnological studies are useful to set appropriate goals for restoration and for evaluation of counter measures. Keywords Acidification . Lakes . Liming . European Union Water Framework Directive . Reference conditions . Diatoms . Sediments . Paleolimnology . Monitoring

Introduction Surface water acidification was identified as a major environmental problem in the late 1960s. Since then, thousands of lakes and watercourses in northern Europe and North America have been classified as acidified due to the deposition of airborne pollution. In 1979, the Convention on Long-Range Transboundary Air Pollution (LRTAP) was signed by 49 countries, and during the 1980s, several programs were established to reduce the emissions of acidifying compounds and to counteract the effects of acidification, usually by liming. Extensive monitoring pro-

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grams, covering both chemical and biological parameters, were established in Scandinavia (Moldan et al. 2001; Skjelkvåle et al. 2001), Canada (Jeffries et al. 2003) and the UK (Monteith and Evans 2005). International monitoring activities were initiated (Kvaeven et al. 2001) with the aim of evaluating the effects of acidification and the efficacy of remediation measures. A primary objective is to re-establish the chemical and biological conditions that prevailed prior to major anthropogenic impacts. According to the European Water Framework Directive (WFD), all waters in Europe should have reached a ‘good status’ by 2016 (European Union 2000). ‘Good status’ is defined as ‘low levels of distortion resulting from human activity, but deviating only slightly from those normally associated with the surface water body type under undisturbed conditions’ (European Union 2000). More commonly, this is referred to as reaching ‘reference conditions’. Monitoring programs can provide valuable information on the development of water bodies, but are usually restricted to the past few decades. To determine ‘reference conditions’, and be able to differentiate between natural temporal variability and the effects of acidification and/or remediation measures, it is necessary to augment monitoring programs with longer-term evaluation (Bennion et al. 2004; Simpson et al. 2005; Taylor et al. 2006). Sporadic historical data are sometimes available and can provide useful information about water body conditions prior to the monitoring programs. Models such as MAGIC (Moldan et al. 2003; Erlandsson et al. 2007) can provide a reasonable approximation of the conditions that prevailed before acidification. However, paleolimnological studies offer the only way of examining the biology and chemistry of a lake and its catchment in a decadal to millennial perspective (Anderson 1995; Renberg et al. 2001; Bennion and Battarbee, 2007). Paleolimnological studies have been conducted within some monitoring programs (Dixit et al. 1999; Monteith and Evans 2005), but have not been extensively applied to lake monitoring. In Sweden, a program for liming of surface waters commenced in 1976. Since then, 8,000 lakes and 12,000 km of watercourses have been limed to counteract the chemical and biological effects of acidification (Appelberg and Svenson 2001). Also, in Norway, liming has been used at several thousand sites (Hindar

Environ Monit Assess (2008) 146:295–308

et al. 1998; The Norwegian Pollution Control Authority 2006). The primary purpose of liming is to re-establish the conditions that are thought to have existed prior to acidification. In Sweden, the original restoration goals were pH >6 and alkalinity >100 μeq L−1 (Bernes 1991). Since 2002, these goals have been adjusted, and there are now three pH-categories (5.6, 6.0 and 6.3) based on biological indicators (Swedish Environmental Protection Agency 2002), which, to a larger extent, comply with the goal of reaching a good status as defined by the WFD. In 1989, the Swedish liming program was expanded to include monitoring of 13 limed lakes (ISELAW – Integrated Studies of the Effects of Liming Acidified Waters). The program includes observations of macrophytes, phytoplankton, zooplankton, benthic invertebrates, fish and water chemistry. The main focus of ISELAW is to study the long-term effects of liming; to determine if there are unwanted effects of liming and if liming restores the species composition and biodiversity that existed prior to acidification (Appelberg et al. 1995; Appelberg and Svenson 2001). In 1999, a paleolimnological study of 12 of the original 13 lakes was included within ISELAW. Lake sediments were analysed for pollen, fly-ash particles from fossil fuel burning, lead (stable isotopes and concentration) and diatoms in order to study long-term vegetation and agricultural history, deposition of atmospheric pollutants, and pH variation over time (Guhrén et al. 2007). This paper compares results of the monitoring program with paleolimnological analyses to address the following questions: Do monitoring and paleolimnological data agree on the magnitude of acidification during the twentieth century? Do monitoring and paleolimnological data agree on the effects of liming during recent decades? Did liming restore lakes to a good status, as defined by the Water Framework Directive?

Materials and methods Monitoring data The 12 lakes are mainly situated on granitic bedrock with poorly developed, shallow soils (Fig. 1, Table 1). Their catchments are dominated by forests of Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and birch (Betula pubescens). Since 1989, the lakes


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twentieth century have been collected via questionnaires. Since the start of ISELAW, water samples have been taken using a Ruttner water-sampler and fish have been caught by net. For sampling of benthic invertebrates, an Ekman sampler was used for profundal areas and a fine mesh net (mesh size mostly 0.6 mm, occasionally unknown) was used for the littoral zone. Zooplankton sampling has been undertaken using mainly fine mesh nets (mesh size usually 65–75 μm, occasionally unknown). Water-chemistry and biological data collected prior to 1989 tend to be isolated and sporadic, and the methods used were not always well documented or validated. The methods used to assess water chemistry, benthic invertebrates and zooplankton appear to vary over time, with inconsistencies between sampling undertaken during the ISELAW program and earlier sampling. More detailed descriptions of methodology are given in Appelberg et al. (1995); Appelberg and Svenson (2001); Persson (2001); Persson and Ekström (2001); Persson and Wilander (2002); Reizenstein (2002) and Östlund (2005). Sediment data

Fig. 1 Distribution of the 12 ISELAW lakes in Sweden. A Källsjön, B Bösjön, C Tryssjön, D Västra Skälsjön, E Lien, F Stensjön, G Ejgdesjön, H Långsjön, I Stora Härsjön, J Stengårdshultasjön, K Gyslättasjön and L Gyltigesjön

have been sampled once a year for benthic invertebrates and fish, while zooplankton are collected on a monthly basis from June until September. Water chemistry measurements are performed from April to October. Although, several water chemical parameters have been measured, only pH, the most extensively used parameter indicative of acidification, is discussed in this paper. pH is expressed here as an arithmetic annual mean and only pH measurements from the top 5 m of the water column have been used. Data on fish from the late nineteenth to the early

During the winters of 1999–2003, sediment sampling was conducted using a modified version of the freezecore sampler described by Renberg and Hansson (1993) and the Russian peat corer detailed by Aaby and Digerfeldt (1986). Diatoms were analyzed to infer past pH (DpH), using weighted averaging regression and calibration (Birks et al. 1990) as described in Guhrén et al. (2007). The diatom calibration data-sets used are the SWAP dataset with a boots-trapped rootmean-square error of prediction (RMSEP) of 0.32 pH units (Stevenson et al. 1991) for the six southern lakes, and the dataset of Korsman and Birks (1996) with a jack-knifed RMSEP of 0.36 pH-units for the six northern lakes. Diatom-inferred and measured pH are considered to differ if DpH and annual measured mean pH differ by more than the RMSEP given in the transfer functions. To some extent, similarities in the pattern of the data (i.e., tendencies) have also been considered. The sediment cores were dated using flyash particle records, referring to key features of the SCP record in Sweden (Wik and Renberg 1996), including AD 1850 (first SCP occurrence), AD 1950 (major increase), and AD 1970 (maximum peak of SCP) (for details see Guhrén et al. 2007). Between these chronological markers, our depth-age models

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Table 1 Location, limnological data and first year of liming for the 12 ISELAW lakes Lake name

Code

Lat. N

Long. E

Area (ha)

Max depth (m)

Mean depth (m)

Catchment (km2)

Retention time (year)

Källsjön Bösjön Tryssjön Västra Skälsjön Lien Stensjön Ejgdesjön Långsjön Stora Härsjön Stengårdshultasjön Gyslättasjön Gyltigesjön

A B C D E F G H I J K L

61° 61° 60° 59° 59° 59° 58° 58° 57° 57° 57° 56°

16° 44′ 14° 16′ 15° 05′ 15° 33′ 15° 31′ 18° 19′ 11° 28′ 14° 43′ 12° 19′ 13° 48′ 14° 29′ 13° 10′

24 114 30 41 149 39 86 67 257 489 32 40

17.4 17.0 19.6 18.7 29.2 20.6 28.6 17.8 42.0 26.8 9.8 20.0

7.1 4.2 7.2 7.4 7.8 9.1 7.0 4.2 14.1 7.1 2.8 9.1

16.4 7.3 12.0 1.0 44.9 7.8 3.6 6.1 22.7 78.6 2.8 172.0

0.3 1.3 0.5 6.0 0.7 2.0 2.0 1.5 2.4 0.8 1.0 0.03

38′ 19′ 26′ 56′ 48′ 10′ 53′ 50′ 42′ 33′ 06′ 45′

assume a constant rate of sedimentation. For Bösjön, only one dating point is available (1850 AD), and therefore a lake with a similar sedimentation rate (Källsjön) has been used as an approximation.

Results Paleolimnological and monitoring results are presented for each lake individually and are summarized in Fig. 2 and Table 2. The results are compiled from the monitoring reports of Persson (2001); Reizenstein (2002); Persson and Ekström (2001) and Persson and Wilander (2002), additional processed data from the ISELAW database (held by the Department of Environmental Assessment at the Swedish University of Agricultural Sciences), and paleoecological reports by Korsman et al. (2000); Gählman et al. (2001); Ek et al. (2001) and Guhrén et al. (2003, 2004, 2007). Previously, the monitoring data have been presented in detail only in Swedish-language project reports. The level of detail given here is that considered necessary to provide a reasonable representation of the extent and diversity of data. Källsjön From the early to mid-twentieth century, the DpH was 5.8. From 1976 until the start of liming in 1984, the measured pH ranged between 5.5 and 5.8, with two individually recorded values below 5. There are no

First liming (year) 1984 1983 1981 1975 1983 1978 1974 1987 1977 1981 1985 1982

indications of acidification in the paleolimnological data during the same time period. After liming, the recorded pH increased to 6.5, similar to the DpH in the most recent sediments. During springtime in the late 1960s, the population of trout (Salmo trutta) declined and dead pike (Esox lucius) were found in nets in tributaries. The pH and fish monitoring data indicate that acid episodes could have occurred in the lake in the spring during the 1970s. However, short-term acid episodes that can affect fish populations are very difficult to detect in paleolimnological data. There were no observed signs of acidification effects on zooplankton in the early 1980s. After liming there was a positive effect on the reproduction of perch (Perca fluviatilis) and the number of recorded zooplankton taxa increased, including a doubling of the number of acid-sensitive Rotatoria species.

Fig. 2 Measured pH as average pH-values for each year (unfilled circles) and diatom inferred pH (filled circles) for 12 lakes within the Swedish liming program. The vertical lines indicate the first year of liming in each lake. The biological data (fish, benthic invertebrates, zooplankton) are semi-qualitatively summarized for the three main periods (early, pre-liming and liming time). Abbreviations: N Normal fish population; A Affected fish population; R Recovered fish population; (R) Partly recovered fish population, AS Acid-sensitive species present, AS Acid-sensitive species absent, arrows Number of species is increasing, constant, and decreasing, respectively


299

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Table 2 Summary of the effects indicated by, and comparison between, the monitoring and paleolimnological data from the 12 ISELAW lakes Parameters

Källsjön

Bösjön

First measured pH (year) Acidification effect Measured pH Diatom inferred pH Fish Benthic invertebrates Zooplankton Liming effect Measured pH Diatom inferred pH Fish Benthic invertebrates Zooplankton Comparison measured pH/DpH Early twentieth century Late twentieth prior to liming Liming Similar tendency in pH

1976

Tryssjön

Västra Skälsjön

Lien Stensjön

Ejgde- Lång- Stora sjön sjön Härsjön

Stengårdshultasjön

Gyslätta- Gyltigesjön sjön

1968 1977

1943

1949 1947

1970

1975

1935

1935

1927

1970

Yes No Yes ?

Yes Yes Yes ?

Yes No ? ?

Yes No Yes Yes

Yes No Yes ?

Yes Yes Yes ?

Yes No Yes Yes

Yes Noa Yes ?

Yes Yes Yes Yes

Yes Yes Yes ?

Yes No Yes ?

Yes Yes Yes ?

No

?

?

Yes

?

No

?

?

?

?

No

?

Yes Yes Yes ?

Yes Yes Yes ?

Yes Yes Yes ?

Yes Yes Yes Yes

Yes Yes Yes ?

Yes Yes Yes Yes

Yes Yes Yes Yes

Yes Yes Yes ?

Yes Yes Yes Yes

Yes Yes Yes ?

Yes Yes No No

Yes Yes Yes ?

Yes

Yes

?

Yes

?

Yes

Yes

?

Yes

?

Yes

Yes

?

Yes

?

Yes

Yes

No

?

?

No

?

No

?

No

Yes

Yes

No

No

Yes

No

No

No

No

Yes

Yes

Yes No

No Yes

Yes ?

No No

Yes No

Yes Yes

Yes ?

Yes Yesa

No Yes

Yes Yes

No No

No Yes

Yes Effects of acidification/liming observed, or agreement between measured pH and diatom-inferred pH (DpH). No Effects of acidification/ liming not observed, or disagreement between measured pH and DpH. ? The data are too sparse for meaningful comparison. a

A decrease in pH occurs but it could not be established whether this was solely the result of modern acidification

Bösjön In the spring of 1968, the measured pH was 6.3, which is similar to the DpH (6.0–6.1) from the late nineteenth to early twentieth century. From the late 1970s until initial liming commenced in 1983, pH was recorded at 5.8–5.9 (n=3), which also corresponds to the lowest DpH. After liming, the measured pH increased to an average of 6.7, with individual values ranging between 6.0 and 7.2. The large interannual variations indicate unstable conditions. The DpH after liming is generally a half unit lower than the measured pH. Prior to liming, the fish population indicated some disturbance from acidification. The Eurasian minnow (Phoxinus phoxinus) was sparse, there were no arctic char (Salvenius alpinus) younger than 3 years and

there were no small trout. Within the zooplankton community, the number of species of acid-sensitive Rotatoria was low. After liming, the fish population recovered, and the number of zooplankton taxa increased. The zooplankton taxa found after liming correspond to those found in other lakes holding a char population (Persson and Ekström 2001). Tryssjön Only two measured pH-values, 4.4 and 5.8, respectively, are available from the late 1970s. The latter value correlates well with DpH of around 5.9 from the late nineteenth century until the start of liming in 1981. This indicates that acidification did not occur in the lake in the twentieth century. Tryssjön is a humic lake, as are three other lakes in the same area that


exhibit a similar fossil diatom flora to Tryssjön. Using paleolimnological methods, acidification could not be detected in any of these lakes. Rather, they were all naturally acidic (Renberg and Ek 1998). According to the monitoring data, the pH increased after liming to an average of 6.3 with individual values ranging between 5.5 and 6.8. This agrees well with the DpH which was 6.1 after liming. The only available biological information from the early twentieth century is a single observation of trout occurrence in the lake during the 1950s. In the 1990s, after liming, minnow populations increased and perch became established in the lake. Västra Skälsjön In 1943, a pH of 6.3 was recorded on one occasion in the lake. The DpH was found to decrease from 6.7 in the mid nineteenth century to 6.2–6.3 in the mid twentieth century, a value which corresponds well with the single measured pH-value. Between 1960 and the first liming in 1975, the measured pH was low, averaging 5.2, with individual values as low as 4.2. After liming, pH increased gradually until 1987, stabilizing at 6.9 (range 6.2–7.4). The lowest DpH in the twentieth century was 6.1, i.e., not as low as in the monitoring data. After liming, the DpH was more than half a unit lower than the measured pH. This discrepancy is likely caused by sediment mixing from extensive sampling of profundal benthic invertebrates and macrophytes in the same area where the sampling for paleolimnological analysis took place. A healthy fish population consisting of perch, trout and minnow was recorded in 1881 and 1906. In 1943, a survey of zooplankton and benthic invertebrates revealed acid-sensitive species. During the early and mid twentieth century, arctic char was introduced into the lake. From the late 1960s to the early 1970s, the catches of fish decreased: the minnow population was nearly extinct and arctic char and perch did not reproduce. Prior to liming, the number of recorded benthic invertebrates had decreased and no specimens of the previously present, acid-sensitive Pisidium were detected. Acid-sensitive species were also lacking within the zooplankton community. In 1976, Sphagnum moss covered an extensive part of the lake bottom, as is common in many acidified lakes (Grahn 1985; Renberg and Hultberg 1992). After liming in 1975, the fish population recovered and the catches

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increased, the number of benthic invertebrate taxa increased and Pisidium species were again recorded in the lake. By the beginning of the 1990s, the zooplankton community displayed a more acid-sensitive species composition and the number of taxa had increased. By 1996, the Sphagnum moss had disappeared completely from the lake bottom. Lien In 1949, an isolated pH value of 6.5 was recorded. From the mid 1960s until liming commenced in 1983, the pH averaged >6, with individual spring values of between 5.5 and 6. The DpH in the early twentieth century was slightly below 6.5, which corresponds well with the first few measured pH values. No indication of modern-day acidification can be detected in the paleolimnological data, however, there was a thick minerogenic layer originating from mining activities, which precluded diatom analysis for the period 1920 to 1960. After liming, the pH stabilized at around 6.6–6.8, with individual values as low as 6. This correlates well with the DpH, which increased to 6.8 after liming. In 1881, perch were caught in the lake, along with trout, burbot (Lota lota) and smelt (Osmerus eperlanus). By 1949, only perch, pike and burbot were caught. Several fish introductions were made during the twentieth century and, as a result, roach (Rutilus rutilus) and zander (Sander lucioperca) occur in the lake today. In the years prior to liming, the recruitment of roach may have been affected, whereas, after liming, indications of increased recruitment were found. Stensjön In the late 1940s, a few measurements indicated a pH of 5.8–6.2. The DpH during the late nineteenth and early twentieth centuries was between 6.5 and 6.7, which is somewhat higher than the measured pH. Prior to liming in 1978, the pH was around 5.6 with individual values as low as 4.8. The DpH decreased during the 1960s to 5.6, corresponding closely with the average measured pH for the same time period. After liming, the pH gradually increased to 6.1 and, by 1982, it had increased further to around 7, after which it gradually decreased to just above 6.5. The corresponding DpH is 6.6–7.1, i.e., it is within the same interval as the measured pH after liming.

302

In the 1920s, perch, pike, roach, bleak (Alburnus alburnus), ruffe (Gymnocephalus cernuus), rudd (Scardinius erythrophthalmus), vendace (Coregonus albula) and eel (Anguilla anguilla) were caught in the lake and acid-sensitive taxa such as Pisidium were found among the benthic invertebrates. In the 1930s, trout and vendace were stocked in the lake. Prior to liming in the 1960s to 1970s, rudd was extinct and roach and perch reproduction was affected. The number of zooplankton taxa identified during the 1970s, prior to liming, remained the same as in the late 1920s, including acid-sensitive species, although these were found in low numbers. After liming, roach and perch reproduction improved and the number of taxa increased among both benthic invertebrates and zooplankton. The rotifer Filinia longiseta, a species that occurs mainly in limed lakes (Persson and Ekström 2001), was also found.

Ejgdesjön From the mid nineteenth to the mid twentieth century, the DpH was 6.2–6.3. In the early 1970s, the pH was 4.7–5.7 (n=3). No recent acidification was detected in the DpH. This could be due to the very low sedimentaccumulation rate in the lake (Guhrén et al. 2007), which yields a low time resolution with the 0.5 cm thick samples. After liming in 1974, the pH increased to 6.9, but decreased to pH 5.3–5.5 (with individual values as low as 4.7) for a few years before it again increased to pH 7 (range 5.7–9). The DpH increased slightly to a maximum of 6.6 after liming. Considering the fluctuations in the measured pH and the low sediment accumulation rate, the measured pH and the DpH are reasonably similar after liming. At the end of the nineteenth century, perch, roach, eel and trout were caught in the lake and, in the mid twentieth century, trout was stocked. Prior to liming, roach had disappeared and perch and trout populations were severely diminished. The invertebrate community was also impoverished and acid-sensitive taxa were absent. After the lake was limed, the number of recorded zooplankton and benthic invertebrate taxa increased and the species composition changed. For example, acid-sensitive Asellus aquaticus was found. Between 1974 and 1987, the fish population was influenced by acidification, liming and re-acidification, resulting in large fluctuations in


the size and abundance of perch. Both perch and trout populations have improved in the 1990s. Långsjön From the mid nineteenth to the mid twentieth century, the DpH was approximately 6.3. There are few limnological data from the period prior to liming, with only five pH measurements from 1975–1985 ranging from 4.9 to 5.4. The DpH just prior to liming was about 6.1, almost 1 pH-unit above the measured pH for the same time period. This might be because the pH measurements in the lake were made in the spring during acid episodes that are not recorded in the DpH. The liming in 1987 increased the pH to an average of 6.4, with individual values ranging between 5.6 and 6.9. The DpH increased to 6.4, corresponding well to the average measured pH. Reizenstein (2002) suggests that, in 1986, the fish population was adversely affected by acidification. Today the lake contains a healthy fish population. Stora Härsjön The DpH in the mid nineteenth century was 7.4, declining to 7.1 in the first half of the twentieth century. A single measurement was made of pH 6.7 in 1935. This value does not agree well with the DpH for the first half of the twentieth century. In the 1950s, the DpH declined to 6.6 and, in the 1970s, the average measured pH was 4.9. After liming in 1977, the measured pH increased to an average of 6.6 and, since the year 2000, the average pH has exceeded 7. In the top sample of the sediment, a slight increase in DpH to 6.8 was detected. Both the acidification and the liming periods are diffuse in the paleolimnological record. Besides the very low sediment accumulation rate, another reason for the difference between monitoring and paleolimnological data could be that the lake is large (257 ha) and has several deep basins and islands. The paleolimnological study was carried out in a different basin than that used for the monitoring program to avoid sediment mixing caused by sampling of benthic invertebrates. The basin where the sediment was sampled for the paleolimnological study is relatively isolated from the rest of the lake. In 1887 and 1935, perch, pike, roach and eel were caught in the lake and, in the 1960s, vendace was introduced. During the 1950s and 1960s, the roach


population decreased and no roach was caught in the 1970s. The reproduction of vendace and perch was disturbed, resulting in reduced populations. In 1977, no acid-sensitive taxa were found within the benthic invertebrate community. After liming, the fish population recovered and the number of zooplankton and benthic invertebrate taxa increased, including several acid-sensitive taxa. Stengårdshultasjön The DpH in the lake was 6.8 in the second half of the nineteenth century. The first measured pH from 1935 was 5.8 and 6.8 on two separate occasions. During the 1970s, the average pH was 5.4 with several observed values below 5. The DpH declined during the early twentieth century and prior to liming the DpH was 6.4, 1 pH-unit above the average measured pH. Gählman et al. (2001) discussed this discrepancy and could not exclude the possibility that during the 1970s water chemistry samples may have been taken close to the outlet of the lake. The outlet is bordered by mires and takes in water from an old peat excavation site that could have resulted in a lower measured pH, thus yielding a pH value which is not representative of the lake as a whole. After liming, the pH increased to 6.5 and has since increased further to pH 7. Simultaneously, the DpH increased to around 6.9, which agrees well with the measured pH. In 1895, perch, pike, roach, bleak, minnow, burbot, eel and trout were caught in the lake. Several stockings of lake whitefish (Coregonus spp.) and pike occurred during the first half of the twentieth century. It was noted that bleak and minnow subsequently disappeared from the lake and the population of trout decreased during the twentieth century. In the 1980s, roach occurred in the lake but with poor reproductive success. Today, the fish population in the lake is not considered to be adversely affected by acidification. Gyslättasjön In the late 1920s, the measured pH was above 6. From the mid nineteenth century, the DpH was around 5.5 and, in the 1950s, it decreased to 5.3. After the 1950s, the DpH increased to 5.6–5.7. There is no indication in the sediment record of a recent acidification. From 1960 until liming in 1985, the average pH was 5.1 and individual values below 5

303

were recorded on several occasions. The lowest DpH correlates relatively well with the lowest average measured pH. In the first few years after liming, pH fluctuated, with an average below 6 and single values below 5. After a peak above 7 in 1989, the pH stabilized around 6.4. The lake is considered to be difficult to lime. The discrepancy between measured pH and DpH is large both in the early twentieth century and following liming. The weak stratigraphy of the spheroidal carbonaceous particles (Guhrén et al. 2007) indicates a large impact from sediment mixing in the lake, making it difficult to interpret the results from the paleolimnological study with confidence. Acid-sensitive benthic invertebrates were found in the lake in a survey in 1930 and acid-sensitive zooplankton taxa were identified in the early 1980s. Prior to liming, carp bream (Abramis brama) and roach were almost extinct and both species were consequently stocked in the late 1980s. Following liming, catches of fish have been small and the reproduction of roach is unstable. The fish population is still considered to be affected by acidification (Reizenstein 2002). Since liming, no acid-sensitive taxa have been found and there has been no recorded change in the number of benthic invertebrates; however, the number of zooplankton taxa has increased. Gyltigesjön Throughout the late nineteenth and early twentieth century, the DpH value was 6. In the early 1970s, the average pH was above 6, although isolated values less than 6, including a single measurement of 4.3, were recorded. In the spring of 1976 and 1977, the pH dropped to just above 5, with individually lower recorded values, but increased above 6 during the summer and autumn seasons. Similarly, the DpH decreased at this time, and, in the early 1980s, it was 5.5. Considering the large short-term variations in the measured pH during this time period, the DpH and measured values are rather similar. After liming, the pH increased and ranged between 6.5 and 7, while the DpH increased to around 6, somewhat lower than the average measured pH value. During the late nineteenth century, perch, pike, roach, carp bream, burbot, rudd, eel and trout were caught in the lake and both vendace and zander have

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been introduced. Burbot and trout disappeared from the lake, probably due to acidification, and the catches of fish decreased during the 1960s. The population of carp bream was reduced and the reproduction of vendace was negatively affected. After liming, the number of zooplankton taxa has increased, as has the carp bream population. Even though catches remain small, the lake is not considered to be affected by acidification today.

Discussion Do monitoring and paleolimnological data agree on the magnitude of acidification over the twentieth century? In 1989, the 12 lakes in this study were chosen to represent typical limed lakes in Sweden with respect to physical, chemical and biological factors, such as retention time, depth, area, water colour and fish populations. The selected lakes have better data from the pre-liming period than lakes in general. However, the data are still insufficient to describe the conditions of the lakes prior to extensive acid deposition in the twentieth century, and as a result, to fully assess if the lakes were acidified and if liming has restored the lakes to reference conditions. Furthermore, the paleolimnological study has shown that some of the lakes were not the most typical lakes; Lien was severely impacted until the late twentieth century by mining and Bösjön is affected by a lead mineralization in the catchment (Guhrén et al. 2007). The most comprehensive data from the monitoring program are the fish surveys (Fig. 2, Table 2). Based on these surveys, Reizenstein (2002) argues that all of the lakes except Tryssjön are affected by acidification, resulting in diminishing fish populations and catches from the early twentieth century until liming commenced. However, for five of the lakes no fish data are actually available from the early twentieth century (Källsjön, Bösjön, Tryssjön, Långsjön and Gyslättasjön). Furthermore, there have been extensive stockings of several fish species in most ISELAW lakes in the past, and it is possible that some of the lakes have not provided sustainable conditions for acid-sensitive fish species, i.e., the populations have disappeared gradually for natural reasons. Moreover, the early fish surveys are mainly based on questionnaires, for which


bias cannot be ruled out. Benthic invertebrate and zooplankton data also fail to provide a clear picture of the pre-liming period. The data are scattered and sometimes contradictory. For example, in Stensjön, the zooplankton community, contrary to the fish community, does not seem to be affected by acidification. Only in Västra Skälsjön and Ejgdesjön is the information on benthic invertebrates unambiguous with regard to acidification of the lakes, and in Västra Skälsjön the acidification is confirmed by the zooplankton community. Overall, the available fish, zooplankton and benthic invertebrate data do not give a clear picture of acidification of the lakes. All study lakes were classified as acidified according to the criteria for liming in the 1970s to 1980s (pH0.5 units), although the tendencies in pH are similar (Table 2). Low sediment accumulation rate or sediment mixing could explain these discrepancies (see above). The better agreement between the two methods following liming is probably because the monitoring data series improved after the ISELAW program started. Did liming restore lakes to a good status, as defined by the Water Framework Directive? The principal aim of liming is to restore the lakes to their pre-acidification condition which is in alignment with the WFD goal that all lakes should have a good status prior to 2016, including reaching an established

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reference condition. To be able to reach the WFD objective and to manage this process effectively, knowledge of previous aquatic conditions and the natural variability of the lakes and their catchments is essential. The assessment of monitoring data from the ISELAW lakes, as discussed in a previous section, clearly illustrates how difficult it is to obtain a consistent picture of the conditions of the lakes prior to extensive acid deposition. Extensive fish stockings from the mid nineteenth century onwards, together with several changes in the methods used within the monitoring program further compound the difficulties in interpreting the data. Moreover, a critical question is whether the nineteenth century is the most appropriate period to use as a reference point. Several studies have questioned this; especially in areas which have suffered long-term human impact (Renberg et al. 1993b; Andersen et al. 2004; Leira et al. 2006), and the importance of long-term perspectives in defining reference conditions has been stressed by Willis and Birks (2006). Several paleolimnological studies have already demonstrated extensive human impacts in the nineteenth century, through agriculture, mining and forestry activities, as well as from air pollution (Brännvall 2000; Ek 2000; Korsman 1993). Of particular interest in the context of acidification and liming is the historical impact of agriculture on the acidity of lakes. Renberg et al. (1993b) observed an alkalization period, caused by early agricultural landuse starting approximately 1,000–2,000 years ago, in acid-sensitive clear-water lakes in southwestern Sweden. In addition to human impacts, lakes also undergo a natural ontogeny (Engstrom et al. 2000), which should be considered when defining a reference condition and in environmental planning and management. Following these arguments, conclusions on whether liming has restored these lakes to a good status is difficult to make. The paleolimnological study indicates that seven of the ISELAW lakes have not been acidified in modern time, and therefore the liming cannot be considered as restoring these lakes to a good status. However, in at least two of these (Västra Skälsjön and Gyslättasjön) the paleolimnological record is diffuse and, in Västra Skälsjön, the monitoring data indicates recovery to a state found prior to acidification. In lakes where both monitoring and paleolimnological analysis have indicated a modern


acidification (Bösjön, Stensjön, Stora Härsjön, Stengårdshultasjön and Gyltigesjön), both monitoring and paleolimnological analyses suggest, at least to some extent, that conditions are returning to those that existed prior to acidification. However, in the case of Stensjön, agreement between DpH and monitored pH is good even though the diatom flora established after liming differs from that found previously in the lake’s history (Guhrén et al. 2007). Taking into consideration data quality, long-term human influences, lake ontogeny and other factors discussed here, it is not possible to define whether a good status has been reached in these lakes. A thorough discussion of the factors that should be considered when judging whether a lake meets the ‘good status’ criteria according to the WFD is clearly needed.

Conclusions Water quality management objectives, which are set to achieve a desired European WFD reference condition for a given water body, require historical data. If no data on previous biological and chemical conditions are available, it is difficult to define reasonable restoration goals. Management measures for the WFD may lead to the establishment of water conditions that have not prevailed before in the lake ecosystem. Based on monitoring data, all of the ISELAW lakes were classified as acidified by acid deposition in the twentieth century. However, the paleolimnological study suggests that several lakes were not. The purpose of liming is to restore the water to pre-acidification conditions, i.e., to good status as defined by the WFD. However, according to the paleolimnological study, the response to liming varies for different lakes, and from a long-term perspective, this goal has not been achieved for many lakes. This demonstrates the importance of long-term studies extending back several hundred years, which provide valuable information on past natural conditions and the degree of human impact on aquatic systems. The WFD suggests initiating extensive monitoring programs that include the evaluation of several chemical and biological parameters. To become really useful such monitoring programs must produce longterm data series that can be evaluated effectively (see also Lovett et al. 2007). In the past, monitoring programs have often been established and then


terminated after only a few years, or methods and parameters have been changed due to shifts in interests and concern about different environmental problems, leading to inconsistent and poorly comparable data series. One way of dealing with these problems may be to integrate paleolimnological methods into the monitoring process. By using paleolimnological methods it is possible to look back in time to deduce the conditions that prevailed tens, hundreds or thousands of years ago, without having to rely entirely on assumptions or sporadic historical data sources. Although not all lakes have sediment suitable for paleolimnological studies, paleolimnology can often contribute to our knowledge of previous conditions in a lake (Last and Smol 2001a, b; Smol et al. 2001a, b; Willis and Birks 2006). A clear strength of paleolimnology is that the study material – the sediment – is preserved in the lakes to address new questions and environmental problems as they appear. Acknowledgements This study was financed by the Swedish Environmental Protection Agency. We would like to thank Ove Emteryd and Birgitta Olson at SLU (Umeå), Jan-Erik Wallin, Anna Ek, Veronika Gählman, Tom Korsman, and Peter Rosén, Umeå University, for their contributions to the paleolimnological analysis. We would also like to thank Gunnar Persson for valuable discussions, and Karlyn Westover for improving the English.

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