Decadal-scale variations in total organic carbon (TOC) concentration in lake water since AD 1200 in two small lakes in southern Sweden were reconstructed based on visible–near-infrared spectroscopy (VNIRS) of their recent sediment successions. In order to assess the impacts of local land-use changes, regional variations in sulfur, and nitrogen deposition and climate variations on the inferred changes in TOC concentration, the same sediment records were subjected to multi-proxy palaeolimnological analyses. Changes in lake-water pH were inferred from diatom analysis, whereas pollen-based land-use reconstructions (Landscape Reconstruction Algorithm) together with geochemical records provided information on catchment-scale environmental changes, and comparisons were made with available records of climate and population density. Our long-term reconstructions reveal that inferred lake-water TOC concentrations were generally high prior to AD 1900, with additional variability coupled mainly to changes in forest cover and agricultural land-use intensity. The last century showed significant changes, and unusually low TOC concentrations were inferred at AD 1930–1990, followed by a recent increase, largely consistent with monitoring data. Variations in sulfur emissions, with an increase in the early 1900s to a peak around AD 1980 and a subsequent decrease, were identified as an important driver of these dynamics at both sites, while processes related to the introduction of modern forestry and recent increases in precipitation and temperature may have contributed, but the effects differed between the sites. The increase in lake-water TOC concentration from around AD 1980 may therefore reflect a recovery process. Given that the effects of sulfur deposition now subside and that the recovery of lake-water TOC concentrations has reached pre-industrial levels, other forcing mechanisms related to land management and climate change may become the main drivers of TOC concentration changes in boreal lake waters in the future.
Several studies have demonstrated increases in dissolved organic carbon (DOC) concentrations and colour in surface waters across large parts of Europe and North America over the last three decades (Stoddard et al., 2003; Hongve et al., 2004; Evans et al., 2005; Worrall and Burt, 2007; Erlandsson et al., 2008; Arvola et al., 2010). These trends have raised concerns about drinking water quality, as contaminants and toxic compounds may be associated with DOC (Ledesma et al., 2012). This may lead to increased demands for chemical pre-treatment in drinking water plants. Increased DOC export to surface waters may also have major consequences for aquatic ecosystems (Karlsson et al., 2009) and recreational values, as well as the role of lakes as carbon sources to the atmosphere (Cole et al., 2007).
A number of hypotheses have been put forward as explanations of the recent increase in DOC concentration. Several studies have proposed a link to declining atmospheric acid deposition (Evans et al., 2006; Vourenmaa et al., 2006; Monteith et al., 2007), while others have coupled enhanced leaching of DOC from soils to changes in climate (Freeman et al., 2001; Hongve et al., 2004; Worrall and Burt, 2007; Haaland et al., 2010) or nitrogen deposition (Findlay, 2005). Local-scale land-use and land management practices have also been demonstrated to influence DOC concentrations (Corell et al., 2001; Mattsson et al., 2005; Armstrong et al., 2010; Yallop et al., 2011). The lack of agreement on the mechanisms controlling DOC and colour variations in lake water during recent decades may partly reflect that many studies have been performed on catchment areas with heterogeneous types of land use, making it difficult to distinguish between co-existing forcing factors. Moreover, most studies have been based on monitoring data covering only a few decades, and have therefore failed to place the recent DOC trends in the perspective of the pronounced dynamics of anthropogenic atmospheric sulfur emissions that have occurred during the last century. Correspondingly, long-term changes in vegetation, land use and climate have also not been considered.
One way of gaining an increased understanding of this important
environmental problem is to obtain long-term records of past changes in
total organic carbon (TOC) concentration in lake water by using inference
models derived from visible–near-infrared spectroscopy (VNIRS) of lake
sediments (Rosén, 2005; Cunningham et al., 2011; Rosén et al.,
2011). Following methodological development, this palaeolimnological
approach has recently gained increased attention as a trustworthy proxy for
ambient variations in lake-water DOC concentrations, building on the fact
that the dominant fraction (> 95 %) of TOC in Scandinavian lake
waters consists of DOC, usually defined as organic matter not retained by a
filter of 0.45
Here we present a detailed multi-proxy study based on well-dated sediment successions from two small nearby lakes in southern Sweden spanning the last approximately 800 years. One of them (Åbodasjön) is oligotrophic mesohumic with a mosaic landscape in its catchment area and with a long history of anthropogenic disturbance. The other lake (Lindhultsgöl) is oligotrophic polyhumic with a catchment area dominated by forest and wetlands, and is historically less influenced by anthropogenic disturbance (Bragée et al., 2013; Fredh et al., 2013). We applied a combination of palaeolimnological methods to the sediment sequences, including reconstruction of lake-water TOC concentration based on VNIRS (Rosén, 2005), diatom analysis to determine water pH, and pollen analysis and the Landscape Reconstruction Algorithm approach for reconstruction of catchment land-cover change (Sugita, 2007a, b). The aim of this study is to identify the major forcing mechanisms behind observed increases in TOC concentration in lakes of the upland area of southern Sweden during recent decades by comparing the impacts of changes in land use, sulfur and nitrogen deposition, and climate to long-term trends in lake-water TOC concentration since AD 1200. Particular focus is placed on the effects of differences in catchment characteristics and the degree of land-use intensity between the two study lakes. Ultimately, our findings may contribute to an enhanced understanding of lake-water TOC dynamics generally, on timescales beyond monitoring series, and to prediction of the future development of lake-water quality in boreal environments.
Morphometric and hydrological characteristics of the two study lakes, sampled in July 2007 (von Einem and Granéli, 2010).
The two study lakes, Åbodasjön and Lindhultsgöl, are situated 6 km apart, about 30 km north-west of Växjö in the province of
Småland, southern Sweden (Fig. 1). The crystalline bedrock is dominated
by granite and gneiss (Wikman, 2000) and covered by sandy till of various
thicknesses and scattered peat deposits (Daniel, 2009). The area is part of
the boreo-nemoral zone characterized by mixed coniferous and deciduous
forest (Sjörs, 1963; Gustafsson, 1996). The climate is generally
maritime with a mean annual temperature of 6.4
Åbodasjön (Table 1, Fig. 1) is an oligotrophic mesohumic lake fed by two inlet streams, situated in the south and north-east, and with an outlet in the south-west. The village of Åboda (40 residents in 2004) is situated west of the lake, and the area around the lake margin is semi-open with mainly deciduous trees, grassland and cropland. The vegetation cover within the catchment area is dominated by managed coniferous woodland, wetlands, and patches of grassland and cropland.
Lindhultsgöl (Table 1, Fig. 1) is an oligotrophic polyhumic lake with no visible inlet streams. At least two artificial ditches drain into the lake from nearby wetlands and woodland, and there is an outlet consisting of an artificial ditch in the south. The catchment area is covered by managed coniferous forest and wetlands with shrubs and scattered pine trees.
In early spring 2008 sequences of surface sediments were obtained from
Åbodasjön and Lindhultsgöl at water depths of 8.6
and 5.2 m, respectively, using a gravity corer and a 1 m long Russian peat corer.
Correlations between core segments and surface sediments were based on
mineral magnetic properties and X-ray fluorescence (XRF) measurements of
element compositions. The uppermost 1 m parts of the sequences were
subsampled into 0.5 cm contiguous sections for stratigraphic analyses.
Age–depth models were based on
Location of study sites.
Past changes of TOC concentration in the lake waters were reconstructed
using a calibration model based on visible–near-infrared spectroscopy
(VNIRS) of surface sediments from 140 Swedish lakes covering a TOC gradient
from 0.7 to 24 mg L
Past changes in lake-water pH were reconstructed based on diatom assemblages
in the sediment records. Diatom samples were prepared following standard
methods (Battarbee et al., 2001). Following oxidization of freeze-dried
sediment samples (0.01 g) with 15 % H
Changes in pH were inferred from sedimentary assemblages (Di-pH) using a
transfer function set, the online combined pH training set in the European
Diatom Database (
The carbon–nitrogen (C
Enhanced catchment erosion may be reflected by elevated concentrations of
lithogenic elements in the sediment profile (Engstrom and Wright, 1984).
Concentrations of phosphorus (P), zirconium (Zr) and titanium (Ti) in the
sediments were measured by X-ray fluorescence (XRF) analysis (Boyle, 2000)
followed by calculation of elemental Zr
Changes in land use were quantified using LRA (Sugita, 2007a; b) based on pollen counts of dominant taxa in the sediment records from the two study sites and an additional lake (needed for the LRA calculation). A minimum of 1000 pollen grains of modelled arboreal and non-arboreal taxa were counted for contiguous 0.5 cm samples (1–10 samples) covering 20-year time spans.
The LRA allows the estimation of changes in the spatial coverage of 26 target taxa at regional and local scales. The pollen data, the LRA approach with its associated parameters, and the reconstructions of land use were described in detail by Fredh et al. (2013, 2014) and Mazier et al. (2014). In this paper, we focus on local land-use dynamics at 20-year intervals since AD 1200 at a spatial scale (modelled area) identified by Mazier et al. (2014) as a radius of 1740 m around Åbodasjön and 1440 m around Lindhultsgöl. The inferred covers of individual taxa are grouped into five different categories of land use according to Mazier et al. (2014): coniferous woodland, deciduous woodland, grassland, cropland and wetland. Although the LRA approach provides no information on the spatial distribution of the types of land use within the modelled areas – larger than the actual catchment areas – we assume that the changes in land use within the modelled areas broadly reflect catchment-scale vegetation changes.
To explore the impact of various potential driving forces on the lake
environment as reflected in the sediment record, we carried out canonical
ordinations. The palaeolimnological parameters (VNIRS-inferred TOC
concentration, sediment TOC
For the entire period after AD 1200, 20-year time slices were used for the
analysis, using land cover as forcing. A mean value for each sedimentary
variable was calculated over each 20-year time slice. A few time slices
lacked measurements of P content and Zr
Ordinations were carried out using CANOCO v4.51. For all analyses, preliminary detrended canonical correspondence analysis showed the response data set had a gradient length < 1 standard deviation units, implying that linear based ordination techniques such as redundance analysis (RDA) were most suitable for these data sets (ter Braak and Smilauer, 1998).
Land-cover percentages were square-root-transformed, while the limnological parameters (which are measured in different units) were centred and standardized. Time was used as a co-variable to remove co-varying effects between, for example, changes in land use and atmospheric deposition. Manual forward selection was used to explore the explanatory power of the different forcing variables, and Monte Carlo tests with 999 unrestricted permutations were run to check their statistical significance in order to select the best explanatory variables for further analysis. The selected variables were checked for collinearity by inspecting their variance inflation factors, which were in all cases < 10, which indicates that the selected parameters are not too closely correlated (Oksanen, 2011).
Åbodasjön (Fig. 2): the inferred TOC reconstruction shows maximum inferred
values of 14 mg L
The diatom-inferred pH varies between 6.2 and 6.7, with a sample-specific
standard error between 0.32 and 0.45. (Fig. 2 and Supplement).
Periods of slightly elevated pH were recorded at AD 1350–1500 and AD 1700–1780, while lower values were recorded at AD 1520–1670 and after AD 1970. The diatom concentration increases to a peak around AD 1400, followed
by a decrease to relatively stable values and a second decrease after AD 1850. The planktonic diatom taxa vary between 40 and 70 % of the diatom
assemblage, and slightly elevated P
Sediment total organic carbon content (TOC
P concentration decreases gradually from the beginning of the sequence
interrupted by a shift to higher values at ca. AD 1440 and thereafter
followed by continuously decreasing concentrations. The onset of the 1900s
is characterized by an increase in P concentration peaking shortly after AD 1950.
The Zr
Records of VNIRS-inferred lake-water total organic carbon (TOC)
concentration, diatom-inferred pH (Di-pH) (horizontal error bars represent
Records of VNIRS-inferred lake-water total organic carbon (TOC)
concentration and pollen-based land use since AD 1900 from Åbodasjön
and Lindhultsgöl plotted together with atmospheric sulfur (sulfate
SO
The LRA-inferred woodland (coniferous and deciduous) cover around Åbodasjön varies between 33 and 80 % since AD 1200. The cover of grassland and cropland together is 40–50 % at AD 1240–1400, followed by a decrease to a minimum of 15 % at AD 1520–1540, when deciduous and coniferous woodland reaches a peak in cover. After around AD 1540 grassland and cropland cover increases and reaches maxima of ca. 60 and 12 %, respectively, between AD 1820 and 1900. During the 1900s coniferous woodland, dominated by spruce, increases from 10 to 30 %. Coniferous and deciduous woodland covers ca. 60 % of the lake catchment today.
RDA was used to describe the major gradients in the limnological data set and
relate these patterns to the land-use variables during the last 800 years.
Total woodland cover was identified as the most significant land-cover
factor, explaining a statistically significant 13 %. Other land-cover
variables that were significant when analysed on their own were spruce and
cropland cover, each explaining 12 %; wetland and coniferous woodland, 10 %; and deciduous woodland, 6 %. When total woodland cover was
included in the RDA analysis, deciduous woodland cover could still explain
an additional 6 % of the variation, while no other land-cover parameters
were statistically significant at the
The ordination results are presented as a so-called triplot (Fig. 4a)
showing the RDA scores for both the palaeolimnological response variables
and the selected forcing variables, as well as the trajectory of down-core
sample scores over time, along the first and second RDA axes. The figure
indicates that the VNIRS-inferred TOC concentration along with sediment
TOC
For the period after AD 1880, five significant drivers were retained on the
basis of forward selection. NH
Lindhultsgöl (Fig. 2): the VNIRS-inferred TOC concentration exhibits high and stable values (21–22 mg L
Diatom-inferred pH varies between 5.0 and 6.8, with sample-specific standard
errors between 0.31 and 0.47. The highest value was recorded following an
increase around AD 1250 to above 6 between AD 1300 and 1450. The period
between AD 1500 and 1800 shows rather stable values around 5.8. In the
1900s, pH decreases to a minimum of 5.0 around AD 1960, followed by a slight
increase until AD 2008. The pH reconstruction for Lindhultsgöl was
influenced by a few dominating diatom taxa. The high values inferred in the
lower parts were associated with the high abundance of the alkaliphilous (pH
> 7) diatom taxon
Relatively stable values were recorded for TOC
P concentration decreases gradually from the beginning of the sequence
interrupted by a shift to higher values in the 1400s and thereafter followed
by continuous decreasing concentrations. The onset of AD 1900 is
characterized by an increase in P concentration peaking shortly after AD 1950. The Zr
The woodland (coniferous and deciduous) cover around Lindhulsgöl varies between 44 and 70 % during the last 800 years. In contrast to Åbodasjön, wetlands cover more than 20 % during most of the period and decreases to less than 10 % after AD 1960. Grassland and cropland varies between 20 and 30 % at AD 1200–1580, followed by an increase to ca. 40 %. During the 1900s, coniferous woodland increases, and this land-use category covers ca. 50 % of the lake catchment today.
In the RDA analysis for the last 800 years, the forward selection for this site showed that spruce cover, the main explanatory variable, explains 20 % of the variance. After its inclusion in the RDA model, two other variables were found significant – cropland and wetland covers, explaining 7 % respectively 4 % of additional variance.
Scores for samples (black circles), palaeolimnological parameters
(blue arrows) and driving forces (green arrows) on the first and second axes
of the redundancy analyses for
A triplot showing the first and second RDA axes (Fig. 4c) indicates that
TOC
For the period after AD 1880, the RDA analysis indicates that cereal cover
was the most important driver, alone explaining 28 % of the variation in
the palaeolimnological data. The stepwise forward selection showed that
three further variables could contribute significantly to the explanatory
power of the model, i.e. S deposition (which could explain an additional
10 % of the variation if included together with cereal cover),
NH
In Åbodasjön, the highest lake-water TOC concentration was inferred
at the beginning of the record, around AD 1200, and decreased during the
following century, while human impact increased (Fig. 2). From AD 1260 the
pollen record indicates an agricultural expansion with increased extent of
croplands, meadows and pastures in the catchment (Fig. 2; Fredh et al.,
2014). This expansion probably resulted in increased erosion and input
of coarse lithogenic material, as indicated by elevated Zr
At Lindhultsgöl, increasing anthropogenic impact was recorded during the
1200s from enhanced Zr
From ca. AD 1350 there was a reduction of human-induced catchment disturbance
at Åbodasjön, as indicated by a decline in cropland and grassland
cover (Fig. 2), and coniferous woodland, in particular spruce, increased
substantially around AD 1400. This agricultural regression was followed by
decreasing catchment erosion and stabilization of TOC concentrations in the
lake water, an event that may be related to the Black Death pandemic, which
struck Sweden in AD 1350. This was followed by several outbreaks throughout
the 1400s, and as much as 60–70 % of the farms in the region were
abandoned (Lagerås, 2007; Myrdal, 2012). At ca. AD 1450 there was a shift
to lower lake-water TOC concentrations, accompanied by decreasing Zr
From ca. AD 1450 to 1800 TOC concentrations in Åbodasjön were
relatively stable, with only minor variations, despite major changes in land
use. Following the increase at ca. AD 1350, coniferous woodland reached
maximum cover of ca. 50 % around AD 1550, followed by a decrease related to
the onset of a second agricultural expansion in the region (Lagerås,
2007). The pollen records from both lakes showed a gradual increase in
cropland, meadows and pasture, more pronounced at Åbodasjön,
together with enhanced erosion as reflected by increasing C
A substantial increase in lake-water TOC concentration was inferred at
Åbodasjön from ca. AD 1800, peaking at AD 1860–1900, simultaneously
with a substantial increase in population density (Fig. 2). The increase in
rural population led to increased demands for land for crop cultivation,
meadows and grazing, and areas previously regarded as less suitable for
agriculture were cleared and drained (Myrdal, 1997). The pollen record shows
a dominance of open-land taxa, and the open-land cover, predominantly
grassland, reached a maximum of ca. 60 %. The RDA plot (Fig. 4a) also
reflects that both total and deciduous woodland cover reached minimum values
around this time. These changes were accompanied by maximum C
At Lindhultsgöl broadly similar trends in C
Records of VNIRS-inferred lake-water total organic carbon (TOC) concentration from Åbodasjön (upper graph) and Lindhultsgöl (lower graph) in the perspective of possible regional and catchment-scale forcings of TOC changes. Regional forcings include sulfur deposition, precipitation and temperature (Fig. 3). Local forcings include site-specific liming history, regional trends in ditching (Hånell, 2009) and changes in land use inferred from pollen data (Fig. 2) and historical accounts (agrarian intensity and modern forestry). Horizontal lines represent periods of activity, thick lines represent periods of increase or high intensity, and dashed lines represent periods of decrease or low intensity. Arrows indicate ongoing processes. The star marks a major drainage effort undertaken at the inlet of Åbodasjön in AD 1922. The vertical dashed lines represent AD 1900. Note the different scale for the period AD 1900–2010.
Around AD 1900 pronounced decreases in TOC concentrations were recorded in both of the study lakes (Figs. 2, 3). At Åbodasjön the decrease was slightly more gradual, reaching minimum values in the 1980s, while the inferred values at Lindhultsgöl declined rapidly to a sequence minimum around AD 1940 (Fig. 3). At AD 1980–1990 increasing trends were initiated at both lakes. These inferred variations in TOC concentration during the last century are in general inversely correlated with historically documented trends in sulfur deposition regionally in southern Sweden (Fig. 3), and the RDA data also indicate that sulfur deposition is among the significant drivers of limnological changes in both lakes since AD 1880. Sulphur deposition started to increase at the onset of industrialization at the end of the 1800s, which led to acidification of soils and surface waters across large parts of Europe (Rohde et al., 1995). Thereafter, sulfur deposition increased significantly in the 1940s, peaking at AD 1980–1995 (Schöpp et al., 2003), followed during recent decades by progressively decreasing deposition and widespread recovery from acidification through decreasing sulfate concentrations in lakes and streams throughout Europe and North America (Evans et al., 2001, Skjelkvåle et al., 2003). The timing of this recovery is largely consistent with the increasing TOC concentrations in our two study lakes (Fig. 3) as well as with a study of TOC trends in Swedish rivers (Erlandsson et al., 2010). The deposition of nitrogen oxides, which also contribute to acidification, also showed a dramatic increase during the 1900s, with deposition peaking slightly later than for sulfur deposition, around AD 1990 (Fig. 3). In addition to contributing to acidification, deposition of nitrogen, both in the form of nitrogen oxides and ammonia, may contribute to eutrophication, and therefore can have an impact on limnic ecosystems. It has also been suggested that the response of soil microbial activity to nitrogen deposition may affect the export of humic matter to freshwaters (Findlay, 2005). Our analysis indicates that nitrogen deposition was among the most significant drivers of change in the palaeolimnological record over the last century together with sulfur deposition.
Increases in lake-water DOC concentration have been linked to increased
solubility of soil organic matter in response to declining acid deposition
(Evans et al., 2006; Monteith et al., 2007), and, conversely, elevated
sulfur deposition usually results in reduced transport of soil organic
matter. In our lakes, declining VNIRS-inferred TOC concentrations were
accompanied by decreasing C
Despite the general negative correlation between sulfur deposition and
inferred TOC concentration at our study sites, major changes in land use
during the last century may also have had important effects on DOC export to
the lakes. The onset of industrialization in the late 1800s led to
urbanization and the documented decrease in rural population. Traditional
types of land use were abandoned, in particular meadows and pastures, which
were typically converted into spruce plantations and cultivated fields
(Antonsson and Jansson, 2011). This development is clearly reflected in our
pollen records as concomitant decreases in grassland and increases in
coniferous woodland cover in the 1900s (Fig. 3). This land-use change is
most pronounced at Lindhultsgöl, where grassland, cropland and wetland
cover are reduced at the expense of woodland, and the RDA indicates a
significant effect of especially the cereal cover reduction at this site. A
significant reduction of the supply of terrestrial organic matter, as
indicated by decreasing C
The increase in VNIRS-inferred TOC concentration at both lakes around AD 1990 is most likely linked to the recovery from acidification. The low sample resolution in the uppermost parts of the diatom records precludes detailed evaluation of recent changes in pH in response to decreased sulfur deposition, although the slight increase in the uppermost part of the record from Lindhultsgöl indicates a recent recovery. However, pH is not a straightforward measure of recovery from acidification (Skjelkvåle et al., 2003; SanClements et al., 2012), and the inconsistent responses in our records may be explained by the contemporary increase in lake-water TOC concentration as organic acids usually have an acidifying effect (Evans et al., 2001). Soil conditions are important for the solubility of organic matter, and the high proportion of coniferous woodland at both lakes and wetlands at Lindhultsgöl, typically associated with organic-rich soils, may have induced increased leaching of DOC in response to decreasing sulfur deposition during recent decades (Evans et al., 2012). Site-specific catchment soil properties may therefore be important for explaining the observed increases in TOC concentration in our study lakes after AD 1990 compared to other lakes in the region that show unchanged or even decreasing trends in DOC concentration (von Einem and Granéli, 2010). In addition, wetland areas in the catchments of both lakes have been treated by liming on a yearly basis to mitigate acidification, starting in AD 1984 at Åbodasjön and in AD 1993 at Lindhultsgöl, which may have contributed to the effects of declining sulfur deposition by accelerated leaching of DOC to the lakes (cf. Hindar et al., 1996).
In addition to changes in sulfur deposition and land management practices, climate may affect DOC concentration of lake waters through a variety of processes, including temperature-driven soil organic productivity and decomposition as well as precipitation-driven water table fluctuations and transport of organic carbon from terrestrial soils (e.g. Sobek et al., 2007). Increases in precipitation and temperature have been brought forward as potential causes of observed increases in DOC concentration in lake waters during the last three decades in several studies (Freeman et al., 2001; Hongve et al., 2004; Sarkkola et al., 2009). Future climate predictions for northern Europe include higher seasonal amounts and intensity of precipitation, as well as increasing mean annual air temperatures (Alcamo et al., 2002), which may result in continued increases in DOC export to lake waters (Larsen et al., 2010). Available meteorological data from Växjö (Fig. 1), reaching back to AD 1860, show an increase in annual precipitation from ca. AD 1980 and an increase in mean annual temperature from ca. AD 1990 (Fig. 3). Hence, climate change may have contributed to the observed and reconstructed increases in lake-water TOC concentration over recent decades, and the RDA data indicate that, at least at Åbodasjön, both precipitation and temperature have had an impact on the lake over the last century, while these effects seem to be less important at Lindhultsgöl. A possible explanation may be the larger catchment of Åbodasjön, making it more sensitive to changes in run-off, erosion and transport of terrestrial organic matter. The large proportion of wetland around Lindhultsgöl may also have a dampening effect on increased precipitation. At Lindhultsgöl, changes in land use have played a more important role at the centennial timescale. Changes in sulfur deposition during the last century have been a main driver for limnological change at both sites, despite their different land use and catchment characteristics (Figs. 3, 4), which supports the interpretation that this is a key factor behind the regional changes observed in lake-water TOC concentrations. This demonstrates the importance of applying a long-term perspective on lake-water DOC dynamics in order to differentiate between causal relationships.
Our reconstructions indicate that TOC concentrations in the lakes were
generally high during the past eight centuries, reaching similar or higher
concentrations than those observed during recent decades. Commonly, there is
a correlation between water colour (usually measured as absorbance at ca.
420–436 nm or using the platinum scale) and DOC concentration in lake
waters. However, colour is strongly influenced by the composition of DOC,
and a recent study has demonstrated that declining acidification in southern
Sweden has led to increased leaching from soils of mobile, hydrophobic and
aromatic DOC that contains relatively large and strongly coloured molecular
compounds (Ekström et al., 2011). Moreover, iron has a strong influence
on water colour, and elevated iron concentrations have been observed with
the recent brownification in the UK (Neal et al., 2008) as well as in Sweden
(Huser et al., 2011; Kritzberg and Ekström, 2012). Therefore, the
VNIRS-inferred changes in TOC concentration in our two study lakes may not
necessarily reflect changes in colour, although monitoring data from
Åbodasjön indicate that this was indeed the case during recent
decades (County Administrative Board of Kronoberg, unpublished data),
consistent with increases in water colour observed in several other lakes in
the study region. This is supported by high abundances of the diatom
In contrast, the elevated TOC concentrations recorded in Åbodasjön
during the late 1800s were most likely not associated with a corresponding
increase in water colour, as indicated by unchanged diatom
planktonic : benthic (P
The early agricultural expansion in the 1200s resulted in a change in the
diatom community towards elevated P
At Lindhultsgöl, minimum P
Based on our results we can conclude that the increases in TOC concentration
and water colour in our study lakes during the past three decades have been
driven mainly by declining atmospheric sulfur deposition. This suggests a
recovery from the phase of maximum sulfur emissions, which resulted in
exceptionally low TOC concentrations in the lakes at ca. AD 1930–90. The RDA
data obtained from the palaeolimnological records over the period since AD 1880 also indicate a recovery. At both sites, the temporal development of
the RDA scores during this period (Fig. 4b and d) show that the youngest
samples fall near the oldest, indicating a return to pre-industrial
conditions, following a time of highly anomalous conditions. At
Åbodasjön, there was first a period of low inferred lake-water TOC
concentration and high pH in the 1930s–1960s, followed by decreasing pH
but high Zr
Our long-term records demonstrate that the TOC concentrations of the study lakes were strongly influenced by changes in agricultural practices, general land-use pressure, and associated variations in forest cover during the last 800 years (Fig. 5). The historical differences in the extent of agricultural activity at the sites establish that site-specific catchment characteristics and land-use dynamics are of great importance for lake-water DOC variations. The recently initiated increase in TOC concentration in the lakes may continue in the near future depending on the quantity of organic carbon stored in catchment soils due to suppression of DOC leaching during the acidification episode. However, the recovery of lake-water TOC concentrations has now reached levels that are comparable to the situation before the onset of 20th century acidification, which may lead to a levelling-off of the increasing trend. Given the reduction of atmospheric sulfur emissions during recent decades, it is likely that previously suppressed or masked effects of changes in land management and climate during the last century will become progressively more important drivers of lake-water DOC concentrations in the future.
This work was funded by the Swedish Research Council Formas (grant to W. Granéli). The authors are grateful to Shinya Sugita for input on the quantitative vegetation reconstructions, and Sofia Holmgren, Linda Randsalu-Wendrup and Christian Bigler for helpful support with diatom preparation and categorization. We are very grateful to all members of the NordForsk network LANDCLIM (coordinated by M. J. Gaillard, Linnaeus University, Sweden) for useful and inspiring discussions during the numerous workshops (2009–2011). We acknowledge the Swedish Meteorological Institute (SMHI) for precipitation and temperature data, and the Swedish Environmental Research Institute and MAGIC for sulfur and nitrogen deposition data. Constructive comments by the reviewers improved the final presentation. Edited by: C. P. Slomp