Early Land Use and Centennial Scale Changes in Lake-Water Organic Carbon Prior to Contemporary Monitoring

Early land use and centennial scale changes in lake-water organic carbon prior to contemporary monitoring

Carsten Meyer-Jacoba,1, Julie Tolua, Christian Biglera, Handong Yangb, and Richard Bindlera

aDepartment of Ecology and Environmental Science, Umeå University, 901 87 Umeå, Sweden; and bEnvironmental Change Research Centre, University College London, London WC1E 6BT, United Kingdom

Edited by Edward A. Boyle, Massachusetts Institute of Technology, Cambridge, MA, and approved April 23, 2015 (received for review January 23, 2015) Organic carbon concentrations have increased in surface waters carbon loss by stimulating microbial and bacterial activities (15). across parts of Europe and North America during the past decades, Other studies suggest that declining rates of acid deposition may but the main drivers causing this phenomenon are still debated. be the key driver behind the observed OC increase in surface A lack of observations beyond the last few decades inhibits a waters. Decreasing sulfur deposition rates allow catchment soils better mechanistic understanding of this process and thus a reliable to recover from acidification, where the resulting decrease in acidity prediction of future changes. Here we present past lake-water and/or ionic strength of the soil solution may increase the solu- organic carbon trends inferred from sediment records across central bility of soil organic matter (OM) and consequently OC leaching Sweden that allow us to assess the observed increase on a centen- (4, 16, 17). Some studies have linked the OC rise to recent changes nial to millennial time scale. Our data show the recent increase in in land-use practices, such as the drainage and burning of peat- lake-water carbon but also that this increase was preceded by a lands (18, 19), or to changes in land cover (20). Other proposed landscape-wide, long-term decrease beginning already A.D. 1450– explanations are the increase in atmospheric carbon dioxide levels 1600. Geochemical and biological proxies reveal that these dynamics (21) and the elevated atmospheric deposition of nitrogen (22, 23), coincided with an intensification of human catchment disturbance both of which support increased plant productivity. that decreased over the past century. Catchment disturbance was All proposed hypotheses are based on the recent OC trends driven by the expansion and later cessation of widespread sum- observed in monitoring programs, which reflect the development mer forest grazing and farming across central Scandinavia. Our in surface waters only over the past few decades. However, the findings demonstrate that early land use strongly affected past lack of long-term monitoring data—over centuries or even mil- organic carbon dynamics and suggest that the influence of histor- lennia—leaves us with an ambiguous understanding of the past ical landscape utilization on contemporary changes in lake-water trajectory of OC concentrations. For example, many of the cli- carbon levels has thus far been underestimated. We propose that mate-related hypotheses imply some change in ecosystem and past changes in land use are also a strong contributing factor in soil functioning, which in turn leads to new and possibly un- ongoing organic carbon trends in other regions that underwent precedented OC levels in surface waters. In contrast, if the de- similar comprehensive changes due to early cultivation and graz- cline in acid deposition were the main trigger for the recent OC ing over centuries to millennia. dynamics, then the observed increase would largely represent a recovery process that returns surface-water OC to more natural, lake-water quality | carbon cycling | land use | Holocene | paleoecology preindustrial concentrations. These two trajectories—unprecedented increase or recovery to past levels—imply substantially different ver the past three decades, monitoring programs have re- Ocorded a widespread increase of organic carbon (OC) con- Significance centrations in surface waters in parts of Europe and North – America (1 4). OC in lakes and rivers plays a major role in the Monitoring programs have recorded increases in organic car- global carbon cycle by transporting carbon from terrestrial to bon concentrations in northern lakes, which have important freshwater and marine environments (5), determining drinking implications for water quality and ecosystem functioning. water quality and associated treatment costs (6), and affecting Current hypotheses interpret this trend in light of recent en- aquatic ecosystem functioning. In aquatic ecosystems, OC influ- vironmental changes such as acidification and climate but do ences energy mobilization, light conditions (7), water acidity (8), not include an examination of long-term changes and their as well as the transport of metals and pollutants (9). The wide- causes. We inferred past trends from sediment archives across spread occurrence of this increase in surface water OC, also re- central Sweden, allowing us to assess recent changes on a ferred to as browning or brownification due to an associated millennial scale. Our data demonstrate that a long-term de- increase in color, suggests that regional rather than local factors cline beginning already in the 15th century preceded the re- are the drivers behind this phenomenon, but at present the un- cent organic carbon increase. This was a response to spatially derlying mechanisms are still controversial. extensive human–landscape interactions that included forest Several hypotheses have been proposed to explain this recent grazing and mire exploitation, which were common across OC increase, from climate change to changes in anthropogenic Europe and altered carbon cycling between terrestrial and SCIENCES forcing such as declining atmospheric acid deposition or alterations

aquatic ecosystems. ENVIRONMENTAL in landscape utilization. A number of climate-sensitive mechanisms have been suggested to cause an increase in OC export from Author contributions: C.M.-J. and R.B. designed research; C.M.-J., J.T., C.B., H.Y., and R.B. the terrestrial to the aquatic environment; for example, increased performed research; C.M.-J., J.T., C.B., and H.Y. analyzed data; and C.M.-J. and R.B. wrote temperatures enhance decomposition rates in organic-rich soils the paper. (10) and promote vegetation cover (11). Changes in the amount The authors declare no conflict of interest. and timing of precipitation potentially lead to alterations of hy- This article is a PNAS Direct Submission.

drological flow paths and intensities, controlling OC leaching 1To whom correspondence should be addressed. Email: [email protected]. SCIENCE – from soils (12 14). An increased frequency of severe droughts This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. SUSTAINABILITY followed by rewetting was also proposed to increase terrestrial 1073/pnas.1501505112/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1501505112 PNAS | May 26, 2015 | vol. 112 | no. 21 | 6579–6584 Downloaded by guest on September 29, 2021 reflected by the sediment’s VNIR spectrum. To infer past lake- water TOC, we applied a calibration model between spectra of lake surface sediments, the most recently accumulated material, and corresponding TOC concentrations in the water column (32, 33) (Materials and Methods and Fig. S1). The four study lakes—Långsjön, Lång-Älgsjön, Fickeln, and Gipsjön—are located in the spruce and pine-dominated boreal forest of central Scandinavia (Fig. 1 and Table S1). This landscape has been noticeably used by humans over many centuries, notably through a widespread transhumant system of summer forest grazing and farming [fäbod or säter in Swedish, sæter in Norwegian; similar to shielings in the United Kingdom or Fig. 1. Overview maps of Scandinavia (Left) and south-central Sweden with Almwirtschaft (German) and alpage (French) in the Alps], since registered historical summer forest farming and grazing sites (red circles) at latest the 15–16th centuries, with earlier traces in the Iron Age (Swedish National Heritage Board database) and locations of the study lakes (34). In conjunction with natural processes, human landscape Långsjön, Lång-Älgsjön, Fickeln, and Gipsjön (Right). utilization formed a diverse forest structure and vegetation composition. During the 20th century, this forest type has been altered by modern forestry, resulting in a younger, denser, and less future scenarios: either an advancing increase in OC concentra- diverse forest (35). All four study lakes are closely connected to tions strongly coupled to anthropogenic forcing or a stabilization mires, leading to humic waters with TOC concentrations above − at naturally higher, prehuman impact levels. Thus, it is important 10 mg·L 1. Gipsjön and Långsjön are part of the freshwater to understand the long-term trajectory of past OC dynamics to monitoring program of the Swedish Agency for Marine and identify the key drivers behind the recent OC increase. A better Water Management, and both have exhibited increasing TOC mechanistic understanding of this process will improve our ability concentrations since the beginning of their monitoring in 1987 to predict future OC changes in surface waters as well as their (Fig. 2, Insets). In boreal Northern Europe, the dissolved OC environmental implications. To assess the likelihood and likely (DOC) fraction (∼97%) greatly dominates the TOC pool (36, 37), intensity of future OC changes, it is a prerequisite to establish the and therefore, the TOC concentrations presented here can be natural, predisturbance baseline conditions and how surface-water considered as virtually equivalent to DOC. OC has evolved in response to climate change and superimposed Our aim was to assess whether past OC dynamics are lake- human impacts over tens to possibly thousands of years. specific or if there are similarities across landscapes, which would Lake-sediment studies have shown that contemporary eco- suggest a common driver behind these dynamics. TOC moni- system responses to environmental problems such as surface- toring data exist for two of our study lakes since the late 1980s water acidification, eutrophication, and climate change have been and were used to validate the reconstructed TOC values. We influenced in part by human activities such as land use over sev- further conducted a multiproxy analysis of one of the studied eral hundred to even several thousand years. Land use, including cultivation as well as low-intensity grazing, has been demonstrated to have affected not only forest cover and ecology in much of Europe—for example, upland woodland areas in Scotland (24)— but also lake trophic status in, for example, Ireland and Denmark already during the Neolithic and Bronze Ages (25, 26). In south- west Sweden, land use led in some areas to a change in lake-water pH by causing an alkalization of acid-sensitive lakes during the Iron Age and into the late 19th century (27) and possibly also changes in lake-water OC levels (28). We hypothesize therefore that the impact of early land use, which entailed an extensive, if not intensive, utilization of the landscape, on OC levels in surface waters has been underestimated and has instead played an important role in long-term OC dynamics. One of the important strengths of using sediments as an ar- chive is the ability to put recent environmental changes into a longer term perspective. The few decades of overlap that now exist between monitoring and sediment records allow us to make quantitative comparisons between recent trends measured in monitoring programs and the long-term patterns preserved in sediments. Here, we present reconstructions of past lake-water total OC (TOC) concentrations in four lakes across the central Swedish boreal landscape. These reconstructions are based on visible-near-infrared (VNIR) spectroscopic measurements of sediment records from the study lakes. VNIR spectroscopy is sensitive to changes in OM quality and is widely used for quality control in industrial processes, and also in environmental and ecological studies to determine, for example, plant and animal tissue composition or different soil constituents (29, 30). Surface waters receive different inputs of autochthonous and allochthonous Fig. 2. Lake-water TOC reconstructions for short sediment cores from the OM depending on climate and catchment characteristics such study lakes Långsjön, Lång-Älgsjön, Fickeln, and Gipsjön over the past 1,500 y. as vegetation, hydrological flow patterns, or nutrient supply (31). Insets show the agreement between sediment-inferred (open circles) and These qualitative and quantitative differences in OM ultimately observed (blue) lake-water TOC concentrations for the two monitoring lakes leave their fingerprint in the lake’s sediment and are consequently Långsjön and Gipsjön.

6580 | www.pnas.org/cgi/doi/10.1073/pnas.1501505112 Meyer-Jacob et al. Downloaded by guest on September 29, 2021 lake’s entire sediment sequence extending ∼10,500 y back in time covers only the past c. 400 y, which precludes reconstruction of to exemplify the long-term Holocene lake-water TOC develop- TOC dynamics before A.D. 1600. ment with the aim of elucidating potential driving factors behind The lake-water TOC reconstruction for Lång-Älgsjön over the the reconstructed TOC changes. These analyses include organic past 10,500 y since deglaciation shows that TOC levels rose from −1 and inorganic geochemistry as well as biological indicators 3to17mg·L within the first c. 1,000 y after lake formation. After this sharp increase, TOC concentrations remained rela- (Materials and Methods). − tively stable around ∼19 mg·L 1 for almost 9,000 y (Fig. 3A), Results and Discussion before the onset of the distinct decline from c. A.D. 1450. During Lake-Water OC Dynamics During the Past. In the two monitoring most of the Holocene, TOC levels fluctuated between 17 and −1 lakes, Långsjön and Gipsjön, sediment-inferred lake-water TOC 22 mg·L . concentrations agree well with the observed TOC levels and re- Driving Factors Behind Past Lake-Water OC Dynamics. cent trends, corroborating the possibility to correctly reconstruct – past lake-water TOC dynamics using sediment archives and Early lake ontogeny (8400 7400 B.C.) and natural long-term stability (7400 B.C.–A.D. 1450). Rapid changes in the landscape following de- VNIR spectroscopy (Fig. 2, Insets). Our lake-water TOC re- glaciation of central Sweden around 8500 ± 500 B.C. (Swedish constructions show a consistent trajectory at the four sites over Geological Survey database, maps2.sgu.se/kartgenerator/maporder_ the past 400 y indicating two distinct phases: (i) a long-term de- sv.html) drove the initial lake-water TOC dynamics at Lång- crease since at latest A.D. 1600, culminating in the lowest con- Älgsjön. Within the first 1,000 y after lake formation, lake-water centrations during the early to mid-20th century, and (ii) a recent TOC concentrations rapidly increased, reflecting catchment sta- – increase of TOC concentrations starting circa (c.) A.D. 1940 bilization through development of soils and vegetation. Counts of 1970 (Fig. 2). In the four lakes, TOC concentrations decreased Sphagnum −1 −1 leaf-like appendages of in the sediment record indicate from values in the range of 12–18 mg·L to 8–10 mg·L in the that the surrounding peatlands developed already during the first early/mid-20th century and have subsequently increased to levels centuries after lake formation (Fig. S2C). The build-up of soil −1 of 10–16 mg·L inferred for the top surface sediment. In contrast OM, including peat, led to an increased export of organic sub- to the other three sites, the TOC decrease in Gipsjön was less stances from the catchment to the lake, as indicated by the si- pronounced and of lower magnitude. Before the onset of these multaneous increase in sediment TOC (from 0.9% to 15%) and dynamics, stable TOC levels prevailed in three of the four lakes total lignin (from 0.1 × 106 to 1.0 × 106;Fig.3B and C), which for c. 1,000 y. For Långsjön, the recovered sediment sequence originates predominately from terrestrial vascular plants. In SCIENCES ENVIRONMENTAL

Fig. 3. Comparison of (A) the VNIR-inferred lake-water TOC development to geochemical and biological proxy data analyzed in the sediment record of Lång- Älgsjön for the past 10,500 y; (B) sediment TOC; (C) total lignin, indicating changes in the input of terrestrial-derived OM; (D) bSi; (E) K/Al ratio, indicating changes in the weathering degree of the sediment and consequently changes in catchment erosion; (F) diatom-inferred pH reconstruction; (G) grouped SCIENCE

diatom assemblage composition; and pollen counts of (H) apophytes (pollen from native plants favored by human disturbance) and (I) anthropochores (pollen SUSTAINABILITY from cultivated plants).

Meyer-Jacob et al. PNAS | May 26, 2015 | vol. 112 | no. 21 | 6581 Downloaded by guest on September 29, 2021 addition to the enhanced allochthonous input of OM, an the rural society and increasing demands for winter fodder (34). increased in-lake production likely contributed to elevated These seasonal settlements were established at some distance lake-water TOC levels and higher carbon sequestration rates. from permanent settlements in areas where resources were less Enhanced in-lake production is indicated by increasing concen- exploited and fodder not limited. Activities at these sites in- trations of biogenic silica (bSi; derived mainly from diatoms) cluded the grazing of the livestock in the forest, production and in the sediment (from 0.4% to 19%; Fig. 3D). Rapidly declining processing of milk, haymaking on mires for fodder, and occa- K/Al ratios from 0.6 to 0.2 further illustrate the catchment sta- sionally also cultivation of cereals (e.g., barley and rye) (40). To bilization (Fig. 3E). The K/Al ratio provides information about improve forest grazing, the canopy cover was opened and un- the weathering degree of the sediment because labile K is pre- wanted field-layer vegetation such as dwarf shrubs and mosses ferentially leached from aluminosilicates compared with in- was grazed, removed, or burned, leading to an altered, more soluble Al during chemical weathering (38). As a consequence open forest composition (42). of declining erosion rates, chemical weathering in soils had a The increase in apophytes and anthropochores in the sedi- stronger relative imprint on the minerogenic matter deposited at ments of Lång-Älgsjön from A.D. 1450 reflects the increasing the lake bottom. The diatom-based pH reconstruction shows that impact of summer grazing in the area. Pollen studies in the re- the lake progressively acidified after formation (Fig. 3F), which is gion show a similar timing for the expansion of this type of consistent with an increase in lake-water TOC and decline in landscape utilization around A.D. 1300–1500 (41, 42), and his- base cation export from the catchment (39). torical documents mention summer farms in the direct vicinity of After the initial catchment development and stabilization, lake- Lång-Älgsjön for the first time around A.D. 1650 (Swedish Na- water TOC concentrations in Lång-Älgsjön reached a stable tional Heritage Board database, www.fmis.raa.se/cocoon/fornsok/ state of natural variability, which prevailed for the following 8,850 y. search.html). The increased presence of humans is further sup- During most of this period—that is, until A.D. 200—none of the ported by a higher abundance of larger charcoal particles (>25 μm) analyzed biological and geochemical proxies show substantial in the sediment record (Fig. S4), reflecting more frequent variation (Fig. 3). From c. A.D. 200, sediment TOC increased to local fires associated with selective forest clearance by slash concentrations above 35% and bSi dropped to ∼14%, indicating a and burn. In addition to the structural changes of the forest, the strong decline in diatom production that also coincided with a grazingintheforestandonmiresdisturbedtheecosystem diatom community shift from benthic to tychoplanktonic species around the summer settlements. We suggest that trampling by dominance (Fig. 3 B, D,andG and Fig. S3). These changes were livestock (sheep, goats, and cattle) physically disturbed vege- most likely a result of an expansion of the mires bordering Lång- tation and soils in the catchment and exposed mineral soil Älgsjön. The lake’s present catchment has extensive areas of horizons. This physical disturbance contributed to an enhanced wetland, and floating Sphagnum mats cover most of the shore- transport of higher vascular plant debris and less intensively line. An advancing peatland development with Sphagnum mats weathered minerogenic matter to the lake, which is reflected in growing farther into the lake would have reduced the habitat for the sediment record by increases in total lignin and K/Al ratios, benthic production, which had dominated the diatom assem- respectively. Simultaneously with the decrease in lake-water blage until c. A.D. 200. We suggest that because peatlands were TOC, total lignin increased from ∼2.7 × 106 to ∼3.6 × 106 and already a dominant control on lake-water TOC, these natural the K/Al ratios from <0.1 to ∼0.2 (Fig. 3 C and E). shoreline dynamics did not have a measureable effect on the Three important effects of summer grazing and farming on lake-water TOC quantity and quality in Lång-Älgsjön. OC dynamics in the boreal forest are the following: First, opening Anthropogenic disturbance (A.D. 1450–1950). During the last 550 y, of the forest canopy, haymaking on mires, and grazing reduce the lake-water TOC dynamics in Lång-Älgsjön appear to be strongly aboveground biomass and litter production (42, 44). This conse- coupled to dynamics in human landscape utilization. Coinciding quently decreases the labile soil OC pool, which codetermines − with the lake-water TOC decline from 18 to 8 mg·L 1 starting c. OC concentrations in surface waters (45). Second, the regular A.D. 1450, pollen counts show both an increase in apophytes removal of biomass lowers the depth to the water table on mires from 2.6% to 6.6% and a continuous occurrence of anthro- (42, 46), reducing the possible OC generation in the soil solution pochores (cultivated plants; e.g., cereals) that increase from 0.1% upon rehydration (47). Third, the exposure of fresh, less weath- to 0.8% (Fig. 3 H and I). Apophytes are native plants indicative of ered mineral-soil horizons, associated with soil disturbance by the a more open landscape, which are favored by human landscape livestock, facilitates an enhanced adsorption of DOC to mineral disturbances such as forest grazing and cultivation. There is evi- surfaces (48). Together, these processes would lead to a de- dence of limited land use already during c. 500 B.C.–A.D. 1450, creased export of OC from the terrestrial to the aquatic system such as smaller increases in apophyte pollen and isolated occur- and thus to a decline in OC concentrations in surface waters. The rences of anthropochore pollen in Lång-Älgsjön, which are seen few existing studies aimed at estimating the impact of modern in other regional pollen records (40, 41). During this earlier pe- grazing on OC dynamics show no significant changes in the soil − riod, there is also a slight decline, ∼3mg·L 1, in inferred lake- solution or discharge due to grazing (44, 46, 49). However, there water TOC relative to mid-Holocene values, but given the iso- are no analog studies that reflect the changes associated with the lated occurrence for anthropogenic indicators and the possible comprehensive shift from a natural, undisturbed to a low-intensive, transport of pollen from outside the catchment, we do not in- human-altered boreal landscape. Existing studies investigated terpret this decline as such. The pronounced decrease in lake- only recent decennial effects of grazing in peatlands but not long- water TOC from A.D. 1450 is supported by changes in the di- term effects over centuries in the boreal forest as a whole and atom community toward a dominance of benthic diatom species were conducted in areas that have previously been altered by (Fig. 3G), indicating improved light conditions (7). humans over millennia. Clear evidence of land use in the form of forest grazing and Inventories of historical sites for summer forest farming and cultivation exists for at least 1,000 y in central Sweden, with grazing emphasize the spatially extensive significance of this low- traces further back into the Iron Age. This evidence occurs mainly impact type of landscape utilization (Fig. 1), which would explain in the form of disturbance indicators, resulting from the gradual the common past OC trajectory across central boreal Sweden. opening and alteration of the forest vegetation (40–43). How- Within a 3-km distance to Lång-Älgsjön, five summer enclosures ever, the region underwent more fundamental changes in agri- alone are documented, where each enclosure included several to cultural practices after the medieval agrarian crisis. During the tens of summer cabins and sheds for livestock. Långsjön, where − 15–16th centuries, a more widespread system of summer forest the lake-water TOC declines from 15 to 10 mg·L 1 during A.D. grazing and farming expanded, which was driven by changes in 1600 to the mid-20th century, is similarly located in an area that has

6582 | www.pnas.org/cgi/doi/10.1073/pnas.1501505112 Meyer-Jacob et al. Downloaded by guest on September 29, 2021 an extensive registry of historical summer farms in the lake’sclose during a cold phase c. 1500 B.C. of equal magnitude to the LIA proximity. Even though no historical summer settlements are reg- (51) (SI Materials and Methods and Fig. S6). Regional climate istered near Gipsjön, local place names reflect past land use, and reconstructions indicate that the scale of climate change during historical documents show that the lake is located along a pathway the LIA was not unprecedented (50), whereas the decline in − linking the town of Malung to three summer farms. Compared with lake-water TOC (∼10 mg·L 1) over the past 500 y had no − the other lakes, the lake-water TOC decrease in Gipsjön (2 mg·L 1) precedent following the early Holocene landscape development. is somewhat lower. No data about historical summer settlements Cooling during the LIA did play an indirect role by encour- exist for the county in which the third lake, Fickeln, is located. aging the expansion of summer forest grazing and farming. However, pollen counts of apophytes and cereals in the lake’s Historians note that farms in marginal growing areas switched sediment indicate also that past OC dynamics in Fickeln, with a from cultivation to pasturage and that colder winters facilitated − decline from 14 to 8 mg·L 1, were strongly coupled to forest grazing the transport of fodder from the summer forest farms back to the and also cereal cultivation (Fig. S5). villages to feed the increasing number of overwintering livestock Recovery (A.D. 1950 to present). Since the mid-20th century, lake- (34). Nevertheless, the recent increase in lake-water TOC in water TOC concentrations increased continuously in Lång- Gipsjön, which exceeds the TOC decline since A.D. 1600 and Älgsjön, as well as in the other study lakes. This increase in Lång- reaches TOC levels higher than what occurred during the preceding Älgsjön is accompanied by decreasing total lignin values, less 1,500 y (Fig. 2), indicates that land use, too, is not the only driver of abundant pollen of apophytes and anthropochores, as well as the recent OC increase in central boreal Sweden. fewer charcoal particles in the lake’s sediment, indicating re- As shown for surface-water eutrophication and acidification in duced catchment disturbance and human presence (Fig. 3 and Europe (25–27), our data demonstrate that early land use also Fig. S4). In central boreal Sweden, summer farming and grazing played, and still plays, an important role in long-term OC dy- started to become less important, decreasing gradually in the namics and contemporary changes in surface-water TOC. Based 19th century and more rapidly from the 1890s due to new on our findings in central Sweden and an earlier study of one changes in agricultural practice following the industrial revolu- lake in southwestern Sweden (28), we suggest that past changes tion (34). Historical documents describe the first abandonments in land use are also a contributing factor in ongoing OC trends in of summer farms in the vicinity of Lång-Älgsjön at the end of the other regions that underwent similar comprehensive changes due 19th century, but some sites remained active until the 1940s to long-term human landscape utilization. Even though the type (Swedish National Heritage Board database, www.fmis.raa.se/ of early land use described here may be specific for forested cocoon/fornsok/search.html). Together with the development of upland and mountainous areas in Europe, our findings exemplify modern forestry, the cessation of forest farming and grazing allowed the potential magnitude of past human land-use activities on OC a forest regrowth that has led to a denser forest with an increased dynamics in surface waters. Globally, humans have extensively stand volume (35). These changes in forest composition and struc- altered the natural vegetation cover over centuries (North ture reversed the effects of summer farming and grazing on the OC America, following European settlement) to millennia (Europe, dynamics in the boreal forest and may thus contribute to the recent Asia) and with it the long-term terrestrial carbon storage (52). In increase in OC concentrations in surface waters in the study area. other regions with observed changes in OC levels, other drivers such as the recovery from acidification in southern Scandinavia Recent Lake-Water OC Dynamics and Implications for Future Change. and the United Kingdom (4) may dominate current OC dy- Our results indicate that changes in lake-water TOC in central namics. However, also these regions have long land-use histories boreal Sweden are strongly coupled to historical patterns in that may need to be considered when assessing recent changes in landscape utilization that included widespread but low-intensive OC concentrations and determining appropriate reference con- summer forest grazing as well as small-scale farming. The fact ditions for water management. that not only does the reconstructed long-term TOC decrease coincide with the expansion of this type of land use but also that Materials and Methods the recent TOC increase coincides with its cessation strongly We recovered sediment cores that span several centuries to millennia from suggests that the observed recent increase is to a large extent a Långsjön, Fickeln, and Gipsjön and the last ∼10,500 y in the case of Lång- response to changes in human landscape utilization. Based on Älgsjön. Corresponding age–depth models were constrained by radiocarbon our findings, we cannot isolate the additional and likely interacting measurements. We applied 210Pb-dating to improve the age control for the roles of other hypothesized drivers for recent TOC dynamics. Acid near-surface sediment from Långsjön and Gipsjön, which allowed a com- deposition during the 20th century seems to have not significantly parison between our sediment-inferred trends and lake-water monitoring affected Lång-Älgsjön. The diatom-based pH reconstruction re- data. For Lång-Älgsjön, we used spheroidal carbonaceous fly ash particles veals that after the natural postglacial acidification of the lake (SCPs) and a correlation between significant changes in Pb isotope signa- from 6.5 to ∼4.9, the pH remained consistently acidic thereafter, tures and established historical patterns of Pb pollution from varve-dated – with a negligible (0.1 pH units) decline c. A.D. 1950 (Fig. 3F). No sediment records to further constrain the age depth model (SI Materials and Methods, Table S2, and Figs. S2 and S7). Lake-water TOC reconstructions are considerable pH changes occurred concurrently with the recon- based on a partial least squares regression (PLSR) model between VNIR structed lake-water TOC variations during the past centuries. spectra of surface sediment samples and corresponding TOC concentrations An explicit separation between climate and land-use effects on in the water column. We updated a previously published model (32, 33), lake-water TOC is complicated by the temporal overlap between which consists of surface sediments from 140 lakes distributed throughout the expansion of summer farms and climate cooling during the Sweden, including nemoral, boreal, and subarctic sites. The model covers a − Little Ice Age (LIA), when temperatures were ∼1 °C cooler than lake-water TOC gradient from 0.7 to 22 mg·L 1, comprises 7 PLSR compo- SCIENCES at present (50). The climate deterioration during the LIA may nents, and exhibits a cross-validated R2 of 0.65 and a root mean square error

−1 ENVIRONMENTAL have contributed to the TOC decline during the period A.D. of cross-validation (RMSECV) of 3.0 mg·L . 1450–1950; however, because of the limited variation in TOC We used standard methods in paleolimnology and soil science in the during the preceding 8,850 y despite other periods with signifi- multiproxy study of the sediment record from Lång-Älgsjön. Organic and cant changes in climate, we suggest that climate was not the main inorganic geochemical sediment properties were determined by means of elemental analysis for sediment TOC, pyrolysis-gas chromatography/mass driver. For example, no pronounced trends in lake-water TOC – spectrometry for total lignin (53), Fourier transform infrared spectroscopy occurred during the Holocene Thermal Maximum (HTM; 6000 for bSi (54), and wavelength dispersive X-ray fluorescence spectroscopy for

2800 B.C.), when annual temperatures were 2–2.5 °C warmer in total Al and K (55). Pollen and diatom analyses followed standard pro- SCIENCE

Northern Europe than at present, nor during the millennia fol- cedures, and diatom-inferred pH reconstructions are based on data from the SUSTAINABILITY lowing the HTM, when temperatures declined by >1 °C, or Surface Water Acidification Program. More specific details about the

Meyer-Jacob et al. PNAS | May 26, 2015 | vol. 112 | no. 21 | 6583 Downloaded by guest on September 29, 2021 methods, analytical procedures, instrument setups, and relevant references We also thank Ingemar Renberg, Anna Ek, Jan-Erik Wallin, and others in the are provided in SI Materials and Methods. paleolimnology group who have contributed to the analyses over time. We thank the Swedish Research Council, the university-supported research ACKNOWLEDGMENTS. We thank the anonymous reviewers for helpful environment, The Environment’s Chemistry, and the “Svenska Sällskapet comments and suggestions to clarify some sections of the original manuscript. för Antropologi och Geografi” for financial support.

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