Relationships Between Land Use and Terrestrial Organic Matter Transfer to the Baltic Sea Over the Last 500 Years

Terrestrial organic matter (OM) plays a key role in coastal organic carbon burial. However, few studies focus on the relationship between land use in the watershed and the transport of terrestrial OM to coasts from a long‐term perspective. In this study, we compared terrestrial OM deposition between an inlet of the Baltic Sea and an upstream lake within the same watershed over the last 500 years, using lignin biomarkers in the sediments. In combination with pollen‐based quantitative land cover reconstruction, we assessed the impacts of semicentennial‐scale changes in land use on terrestrial OM export. The results indicated that the concentration, composition, and degradation state of the lignin‐derived OM differed substantially between the two sites. The lake received larger amounts of lignin‐derived OM during periods of intensified agriculture, but the coastal site did not. The composition of lignin in the coastal sediment did not directly reflect variations in vegetation cover in the watershed. The reason could be that the OM was settled in the upstream basins. Furthermore, the terrestrial OM that did reach the coastal sediments was modified through degradation during the transport, and only the refractory component was deposited at the coast in a relatively unaltered form.


Journal of Geophysical Research: Biogeosciences
YANG ET AL. 10.1029/2023JG007477 2 of 14 oxygen levels of bottom waters.The expansion of agricultural land often causes a significant increase in soil erosion leading to elevated deposition of terrestrial OM and other nutrients in both freshwater and coastal-marine basins (e.g., Köster et al., 2005;Li et al., 2008;Neumann et al., 2002;Routh et al., 2004;Yang et al., 2021).The increased nutrient loading leads to coastal eutrophication, which in turn can lead to bottom water hypoxia ([O 2 ] < 2 mg/l) (Diaz & Rosenberg, 2008).Enhanced primary productivity, largely due to human impact, has been recorded since the 20th century in the Baltic Sea (Savage et al., 2010).To assess the long-term ecological changes in response to human activities in the Baltic Sea, Ning et al. (2018) conducted a suite of analyses including diatom assemblages, total organic carbon (TOC) and nitrogen content, stable carbon and nitrogen isotopic ratios, and biogenic silica content on a sediment sequence from a coastal inlet (Gåsfjärden), and reconstructed the land use history based on the pollen analysis of a sediment core from an upstream lake (Lake Storsjön) ∼ 25 km NW to Gåsfjärden.Their results suggested that the intensified human impacts on the coast of the Baltic Sea can be traced back to at least the 1800s.However, due to the lack of monitoring programs beyond the last few decades, there is a lack of data available on the effect of land use on the OM transfer from land to sea over centennial time scales.In this study, we analyzed lignin phenols on the two sediment cores from Ning et al. (2018) and further reconstructed the land use history at the coast using previously unpublished pollen data from the Gåsfjärden sediment and modeling the vegetation cover using the Landscape Reconstruction Algorithm (Sugita, 2007).
Lignin phenols were used as biomarkers to investigate the terrestrial OM deposition in this study.Lignin is a macromolecule almost exclusively found in vascular plants (Ertel & Hedges, 1984;Hedges & Mann, 1979).It is highly resistant to microbial degradation and therefore still retains information of its origin after deposition in sediments (Goñi & Hedges, 1992;Hedges et al., 1982;Jex et al., 2014).Thus, lignin composition is widely used to trace the abundance, source, and degradation state of the terrestrial OM deposited in sediment archives (e.g., Chmiel et al., 2015;Cui et al., 2017;Hyodo et al., 2017;Tareq et al., 2011).The aims of this study were (a) to investigate the variation of terrestrial OM export in response to land-use changes in the coastal area of the Baltic Sea over the last 500 years when human disturbance on the landscape increased and (b) to explore the transport and burial of terrestrial organic carbon from a long-term perspective by analyzing the difference in the sedimentary OM between the coastal inlet and the upstream lake within the same watershed.

Site Description
Gåsfjärden (57°34′N, 16°34′E; Figure 1) is situated on the southeast coast of Sweden.It is a fjord-like inlet connected to the Baltic Sea through an approximately 500 m wide and less than 20 m deep channel.Numerous small islands are distributed between the inlet and the open sea, which contribute to a relatively restricted water exchange between the inlet and the open sea.Gåsfjärden has a surface area of 22 km 2 and an average water depth of 10 m.The hydrological catchment is around 1,500 km 2 .Lake Storsjön (57°42′N, 16°14′E; Figure 1) has a surface area of 2.1 km 2 and a mean water depth of 5 m.It drains into the river Botorpsströmmen, which is one of the river systems draining into Gåsfjärden.
Multiple sediment cores were retrieved from the inlet at a water depth of 31 m, using a twin-barrel Gemax corer (modified Gemini corer, diameter 9 cm, Ning et al., 2016Ning et al., , 2018)).Our study was conducted on core VG31F LU which was 55 cm long, and the sediment was brownish gray with laminations.A 4.5 m long sediment sequence was collected from Lake Storsjön using a Russian corer and a HON-Kajak corer (Ning et al., 2018).The sediment consisted of homogeneous brown gyttja.Our study focused on the top 30 cm of the sequence.The sediment cores from both sites were subsampled at 1 cm intervals.The analyses were done at a resolution of approximately 50 years.
The vegetation cover in the region is dominated by mixed coniferous forest, with smaller areas of agricultural land (Figure 1).Deciduous forests, dominated by oak are more abundant along the coast than further inland.Mining was active between the 1630s and 1920 (Solstad Gruva) on the shore of Gåsfjärden (Ning et al., 2018;Söderhielm & Sundblad, 1996).A sawmill industry in the village of Blankaholm, on the shores of Gåsfjärden, started in 1886 and expanded substantially during the 1910s and the 1920s to be the largest in southern Sweden during the 1920s (Ning et al., 2018).

Chronology
The age-depth models for the sediment sequences from Gåsfjärden and Lake Storsjön were published in Ning et al. (2016Ning et al. ( , 2018) ) respectively.Both were constructed based on a combination of 210 Pb and 137 Cs dating using the constant rate of supply model (CRS) for the top sediment and 14 C dating for the older part of the sediment sequences.Four terrestrial macrofossils from Storsjön sediment (Ning et al., 2018), seven mollusc shell samples and nine terrestrial plant fragments from Gåsfjärden sediment (Ning et al., 2016) were dated with radiocarbon.The ages are presented as the calendar year (CE).

TOC, TN Analysis
Freeze-dried and homogenized sediment samples were decalcified in Ag capsules by adding 300 μl of 1M HCl to 5-10 mg of sample.The samples were analyzed for the TOC and TN content using an elemental analyzer (COSTECH ECS4010) at the Department of Geology, Lund University.The atomic C/N ratios were calculated using the weight percentages of TOC and TN multiplied by 1.167 (Meyers & Teranes, 2001).

Lignin Analysis
The lignin phenols in the sediments were extracted using the CuO oxidation method developed by Hedges and Ertel (1982) and modified by Sun et al. (2015).Briefly, a freeze-dried sediment sample was treated with CuO and Fe(NH 4 ) 2 (SO 4 ) 2 6H 2 O in a 2 N NaOH solution under an argon atmosphere.The sample was heated at 155°C for 3 hours.After cooling, known amounts of ethyl vanillin and trans-cinnamic acid were added to the solution as internal standards.The aqueous solution was adjusted to pH 1 by adding 6 N HCl, and subsequently extracted with ethyl acetate.After evaporation under N 2 flow, the extract was dissolved in pyridine, derivatized with bis-(trimethylsilyl)-trifluoroacetamide-trimethylchlorosilane (BSTFA-TMCS) and analyzed on a gas chromatography-mass spectrometry (Shimadzu QP2010 GC-MS) fitted with a capillary column (DB-5, 30 m × 0.25 mm i.d., 0.25 μm film thickness).The 11 lignin phenols were quantified using trans-cinnamic acid as an internal standard and a four-point calibration curve.
A set of proxies based on lignin phenols was used in this study.The lignin phenols concentration (Λ 8 ) calculated as the sum of vanillyl (V), syringyl (S), cinnamyl (C) phenols normalized by 100 mg organic carbon can be used to assess terrestrial plant-derived OM contribution to the sediments.The ratio between p-hydroxyacetophenone and total p-hydroxyl phenols (PON/P) can be used to indicate the relative abundance of lignin-containing plants and lignin-free plants, as PON is derived from lignin while hydroxybenzaldehyde and p-hydroxybenzoic acid can be produced by lignin-free organisms, such as phytoplankton and bacteria (Hedges, Clark, & Come, 1988).The ratio between syringyl and vanillyl phenols (S/V) can be used to distinguish between angiosperms and gymnosperms, as syringyl phenols are not abundant in gymnosperms (Hedges & Mann, 1979).The ratio between cinnamyl and vanillyl (C/V) can be used to distinguish between woody and non-woody tissues, as cinnamyl phenols are primarily derived from non-woody plant tissues (Hedges & Mann, 1979).C phenols are more liable to degradation compared to the other three groups, which can lead to a decrease in the C/V ratio in highly degraded terrestrial OM (Hedges, Clark, & Come, 1988;Opsahl & Benner, 1995).Degradation leads to an increase in the acids compared to the aldehydes, so the acid to aldehyde ratio of vanillyl phenols (Ad/Al) v is a proxy for the degradation state of the terrestrial OM (Ertel & Hedges, 1984;Opsahl & Benner, 1995).The P phenols are more resistant to degradation than the other three groups, so the P/(S + V) can be used as another indicator of degradation (Dittmar & Lara, 2001).

Pollen Analysis and Regional Vegetation Cover Reconstruction
Sediments were prepared for pollen analysis, according to the standard acetolysis method of Berglund and Ralska-Jasiewiczowa (1986).At least 1,000 pollen grains were counted for each sample using a microscope at 400 times magnification.The pollen counts of each sample, together with relative pollen productivity estimates from southern Scandinavia (Broström et al., 2004;Fredh et al., 2012;Nielsen, 2004;Sugita et al., 1999), were subsequently used to estimate regional vegetation composition through the application of the REVEALS model (Sugita, 2007).Default model settings with a wind speed of 3 m/s and neutral atmospheric conditions were applied.REVEALS was originally developed to obtain regional vegetation estimates from pollen records from large lakes, but Azuara et al. (2019) demonstrated recently that it can also be applied to coastal sites.The REVEALS reconstruction from Storsjön was previously described by Ning et al. (2018), while the data from Gåsfjärden is presented here for the first time.

Bulk Elemental Analysis
The TOC content in the sediment of Lake Storsjön (Figure 2) varied between 11.8% and 14.0%, with an average value of 13.1%.The TOC was relatively constant before ca.1800, and decreased to a minimum in ca.1950, followed by a rapid increase in the late 20th century.The C/N ratio (Figure 2) ranged between 11.8 and 15.3, with an average value of 14.1.It remained relatively constant before ca.1900 except for a slight drop at ca. 1700, and increased to the maximum in ca.1950, followed by a rapid decrease throughout the late 20th century.
TOC content in the sediment of Gåsfjärden (Figure 3) varied between 5.6% and 7.9%, with an average value of 6.4%, showing a gentle decreasing trend between ca.1500 and 1900 and an increase between ca.1900 and 2010.The C/N ratio (Figure 3) ranged between 9.0 and 9.4, with an average value of 9.3.It exhibited a slight increase between ca.1550 and 1700, and remained approximately constant until ca.1950, followed by a decrease in the late 20th century.The TOC and C/N in Core VG31F LU used in this study were within the range of those in Core VG31D LU (4.8%-8.4%,6.9% on average for TOC; 8.9-10.2,9.2 on average for C/N) which was a parallel core published by Ning et al. (2018).

Lignin Phenols
The lignin phenols concentration (Λ 8 ) in the sediment of Lake Storsjön (Figure 2) remained relatively stable with an average value of 1.45 mg/100 mg-OC from the start of the record until ca.1800.This was followed by a decrease to 1.17 mg/100 mg-OC in ca.1900 and increased to 1.60 mg/100 mg-OC in ca.1950, and remained constant afterward.The sediment sequence from Gåsfjärden (Figure 3) had lower lignin phenols concentrations than the one from Lake Storsjön.The lignin phenols concentration in the sediment of Gåsfjärden was rather constant with an average value of 0.84 mg/100 mg-OC which was much lower than Lake Storsjön.
The PON/P ratio in the sediment of Lake Storsjön (Figure 2) ranged between 0.12 and 0.16 with an average of 0.14.It exhibited a peak at around 1950.The PON/P in the sediment of Gåsfjärden (Figure 3) was lower than in Storsjön, with values ranging between 0.04 and 0.06 with an average value of 0.05.
The S/V ratio in the sediment of Lake Storsjön (Figure 2) decreased from 0.4 to 0.3 from the start of the sediment sequence at ca. 1550 until ca.1900, and remained around 0.3 in the 20th century, followed by an increase to around 0.4 in the early 21st century.The C/V ratio of the sediments in Lake Storsjön (Figure 2) varied from 1.3 to 2.3, with an average of 1.7.Two peaks were observed in the late 17th century and around 1900, respectively.The S/V ratio in the sediment of Gåsfjärden (Figure 3) was relatively constant at around 0.4 between ca.1600 and Figure 2. Carbon/nitrogen ratios (C/N), total organic carbon, lignin data, and pollen-based land cover reconstruction from the sediment record of Lake Storsjön.Λ 8 : sum of vanillyl, syringyl, and cinnamyl phenols normalized to 100 mg organic carbon; PON/P: ratios of p-hydroxyacetophenone to total p-hydroxyl phenols; S/V: ratios of syringyl to vanillyl phenols; C/V: ratios of cinnamyl to vanillyl phenols; (Ad/Al) V : ratios of vanillic acid to vanillin; and P/(S + V): ratios of p-hydroxyl to the sum of vanillyl and syringyl phenols.The pollen-based land cover reconstruction is based on Ning et al. (2018).
Figure 3. Carbon/nitrogen ratios (C/N), total organic carbon, lignin data and land cover reconstruction based on pollen, all from the sediment record of Gåsfjärden.Λ 8 : sum of vanillyl, syringyl, and cinnamyl phenols normalized to 100 mg organic carbon; PON/P: ratios of p-hydroxyacetophenone to total p-hydroxyl phenols; S/V: ratios of syringyl to vanillyl phenols; C/V: ratios of cinnamyl to vanillyl phenols; (Ad/Al) V : ratios of vanillic acid to vanillin; and P/(S + V): ratios of p-hydroxyl to the sum of vanillyl and syringyl phenols.
1950 and increased in the 1950s.The C/V ratio in the sediment of Gåsfjärden (Figure 3) stayed constant at around 0.7 between ca.1600 and 1900 and increased to above 1 in the 20th century.
The (Ad/Al)v ratio in the sediment of Lake Storsjön (Figure 2) ranged between 0.7 and 1.1 with an average of 0.9.It exhibited an increasing trend before ca.1700 and was relatively high until ca.1950 followed by a drop in the early 21st century.The P/(S + V) ratio (Figure 2), ranged between 0.3 and 0.6 with an average of 0.5.Before ca.1750, the P/(S + V) remained relatively constant but showed large variation in the last two centuries.In the sediment of Gåsfjärden, the (Ad/Al)v ratio ranged between 0.5 and 0.7 with an average value of 0.6 (Figure 3), which was lower than Lake Storsjön sediment.The P/(S + V) ratio ranged between 0.4 and 0.7 with an average of 0.5 (Figure 3).

Pollen-Based Land-Cover Reconstruction
Twenty three pollen taxa, which accounted for more than 95% of the total pollen sum in each sediment sample of Gåsfjärden, were included in the REVEALS model.Four zones were established based on constrained cluster analysis (CONISS) (Grimm, 1987) of the reconstructed vegetation proportions.The diagram of estimated vegetation cover (Figure 4) reflected that Pinus and Picea were the major tree species with an average coverage of 55% throughout the whole study period.Open land, largely attributed to human activities, such as cultivation and grazing, covered an average of 33% of the region.Cropland, quantified as the cover of Rye (Secale) and other cereals (Cerealia-type), covered an average of 11% of the region.
During the period 1500-1600 (Zone I), over 80% of the region was covered by woodlands dominated by Picea (37%) and Pinus (29%).The deciduous trees covered 14% of the region.Cropland dominated by Cerealia-type has an average cover of 5%.In the transition to Zone II (1600-1800), Picea cover decreased by more than half to 17%, while the deciduous tree cover increased slightly to 16%.An increase in openness of the region was observed during Zone II, as the grassland cover increased from 11% to 33%, particularly driven by the increases from 7% to 15% in the Poaceae and 3% to 8% in Juniperus, a coniferous shrub occurring frequently on pastured land in southern Sweden.The cropland cover increased from 3% to a maximum of 13% with an average of 7% within this period.It was still dominated by Cerealia-type, but Secale was observed for the first time.
During the period 1800-1900 (Zone III), a change was seen in the coniferous woodland, with Pinus cover decreasing from 26% to 17%, while Picea cover showed a gentle increase.The deciduous tree cover decreased to 10.1029/2023JG007477 7 of 14 10% on average compared to 16% in the previous period.Open land increased to more than 50% at the expense of woodland.The cropland cover reached a maximum in this zone with an average of 24%, of which the Cerealiatype reached a maximum of 28% in the early 1800s.After 1900 (Zone IV), an expansion of woodland cover was observed.The coniferous cover increased to 67% on average, with increases in both Pinus and Picea, while the deciduous tree cover decreased slightly to 8%.The open-land cover dropped approximately by half, of which the cropland cover decreased to 12%.
When comparing with the land cover reconstruction from Storsjön (Ning et al., 2018; Figure 2), the overall trends and periods of change were similar, which is not surprising considering the pollen catchment of the two sites overlap, as seen in Figure 1.However, there were also differences, which were probably due to differences in both natural vegetation and land-use history (Berglund et al., 2002) between the upland and coast of the Småland region where the two sites are located.The upland area makes up a larger proportion of Storsjöns pollen catchment, and the coastal areas make up a larger fraction of the pollen catchment for the Gåsfjärden pollen record.The coastal area has slightly more clay and nutrient-rich soils compared to the uplands (Ning et al., 2018).While both reconstructions reflected a dominance of coniferous forest, with roughly equal amounts of Pinus and Picea, the coniferous tree cover was overall higher (65% on average) and more stable in the Storsjön record.Deciduous trees had a higher cover proportion in Gåsfjärden throughout the period, especially Quercus was more abundant, which can also be observed in the vegetation in the region today.Cropland cover was substantially larger in the Gåsfjärden record, especially in zones II and III, while grassland cover was more variable.

Variations in Terrestrial Organic Matter Input to Lake Storsjön in Relation to Human Impacts
In the sediment of Lake Storsjön, C/N had an average value of 14.5 before the 21st century, suggesting that terrestrial OM contributed largely to the organic deposition in the lake.The relatively high values of Λ 8 and C/N before 1600 reflect a relatively high input of terrestrial OM in the lake, which coincided with a short-term small-scale farming expansion with an increase in cropland cover indicated by the pollen-based vegetation estimates.This expansion coincides with a population increase in Småland that also led to increased agricultural production (Dalhström, 2006).The expansion of agriculture at the expense of forest cover almost always leads to enhanced soil erosion (Hooke, 2000;Montgomery, 2007), and the sediment accumulation rate was relatively high in this period (Figure S1 in Supporting Information S1).Furthermore, arable soils have lower organic carbon capacity than forest soils (Scharlemann et al., 2014).Thus, the elevated deposition of terrestrial OM in Lake Storsjön before 1600 was likely associated with farming expansion.In the 17th century, the extent of farmland in the region declined, and the forest re-established.This coincides with a decline in agricultural output and famine in parts of Sweden (Gadd, 2000), but it is difficult to conclusively tie the observed decline to this.Λ 8 in the sediment decreased during this period, reflecting a decline in the terrestrial OM delivery to the lake.
Between ca.1700 and 1900, cropland, pasture and meadows expanded as agricultural land use intensified in the region (Ning et al., 2018).This expansion was part of a wider pattern of population increase and expansion agriculture in Sweden, that led to a large increase in the area of arable land (Gadd, 2000).In eastern Småland, there is evidence that the ratio of arable land to pastures doubled between around 1650 and 1800 (Dalhström, 2006).As the study region is located in a marginal area for agriculture, it was sensitive to changes in population pressure, and the available land was utilized to support the increasing population (Lagerås, 2007;Nielsen & Odgaard, 2010).As evidenced by the high Λ 8 (Figure 2), the deposition of terrestrial OM in Lake Storsjön increased in the 18th century.Farmland further expanded in the 19th century (Ning et al., 2018), and Λ 8 decreased during the same period.Meanwhile, a decrease is also seen in the PON/P ratios, likely indicating an increase in the relative abundance of lignin-free OM deposition, which could originate from phytoplankton.The expansion of farming and grazing probably contributed to increased nutrient input to the lake system, which subsequently caused enhanced aquatic primary production.Thus, the decrease in Λ 8 was likely attributed to a higher amount of autochthonous input compared to allochthonous input, which was further confirmed by the drop in C/N ratios.
Agricultural activities reached their maximum in the early 20th century, as indicated by the vegetation cover estimates (Ning et al., 2018).The soil erosion further intensified as evidenced by the large increase in the sediment accumulation rate (Figure S1 in Supporting Information S1), and led to a higher amount of terrestrial OM export to Lake Storsjön indicated by the increase in the Λ 8 and C/N ratios.The dilution effect by increased minerogenic material input from the enhanced soil erosion contributed to a drop in TOC content.After ca.1950, the land use in large parts of the region was converted to managed coniferous forest reflected by the land cover estimates, and the area for cropland and grazing was reduced as a result of the introduction of modern agricultural practices and abandonment of poorer soils (Ning et al., 2018).However, as indicated by Λ 8 , the contribution of terrestrial OM was still higher than it was before the 18th century when the catchment experienced minor human impacts.A similar case has also been observed in the sediment of a lake located in southwestern Sweden (Yang et al., 2020(Yang et al., , 2021)).Many studies have reported an initial loss in soil OM after reforestation partly resulting from pre-planting disturbances, and the soil carbon stock level is lower in a managed conifer forest than in a comparable natural forest (Bastida et al., 2018 and references therein).The increase in TOC during the same period implies that the minerogenic deposition decreased as the soil erosion decreased due to reforestation.The large drop in the C/N ratio in the late 20th century was likely associated with the significant input of inorganic nitrogen from the usage of artificial fertilizer.The increase in nutrient input also contributed to enhanced aquatic primary production.Thus, the drop in PON/P ratios was probably due to the increase in plankton-derived OM deposition.
The S/V ratios of the Storsjön sediment decreased gradually from ca. 1600 to 1800, indicating a relative decrease in material derived from deciduous trees.The change in lignin phenols agrees well with the reconstructed regional vegetation cover that shows a general decline of deciduous tree cover.Correlations between S/V ratios of the sediments and the proportions of angiosperm vegetation have been observed in other studies (e.g., Hyodo et al., 2017;Kuliński et al., 2007;Teisserenc et al., 2010).The S/V ratios further declined after 1800, whilst the grassland cover increased at the expense of coniferous forest.The cover of Juniperus, which is a gymnosperm, increased in the 19th century.Thus, the low S/V ratios in the early 19th century could be related to the expansion of Juniperus in the region.
The C/V ratios had an average of 1.7 in the sediment of Storsjön.Such high C/V ratios are not common in sediments, soils or most fresh plant tissues (Goñi et al., 1998;Hedges & Mann, 1979;Moingt et al., 2016).However, the pollen of Picea is rich in p-coumaric acid and has C/V ratios ranging between 13 and 35 and p-coumaric acid/ ferulic acid (CAD/FAD) ratios above 1.5 (Hu et al., 1999;Ishiwatari et al., 2006;Keil et al., 1998).High C/V (>1) ratios have been observed in the soils under conifer-dominated forests and the sediments from the water basin surrounded by conifer-dominated forests (Hu et al., 1999;Ishiwatari et al., 2006).Besides, unusually high CAD/ FAD ratios (>2) are found in the sediments with relatively high contributions from pollen grains on the Washington coast, US (Keil et al., 1998).Figure 5 shows that the constantly high C/V ratios throughout the sediment profile are mostly attributed to the high concentration of C phenols ranging between 0.76 and 0.94 mg/100 mg-OC dominated by p-coumaric acid.The relatively high values of p-coumaric acid (0.68 mg/100 mg-OC on average) and CAD/FAD ratio (4.0 on average) in the 17th century coincided with a high concentration of Picea pollen (1,113 grain/mg-OC on average).The drop of p-coumaric acid concentration and CAD/FAD ratio between ca.1700 and 1900 generally followed the reduced Picea pollen concentration.Hence, one hypothesis could be that the concentration of C phenols is related to the deposition of coniferous (especially Picea) pollen.However, both C/V and CAD/FAD ratios peaked around 1900 when a drop in the Picea pollen concentration was recorded.The reason for the discrepancy could be that the number of pollen grains relative to the OC content did not necessarily reflect their absolute changes when the OC content varied largely.
High C/V ratios (>1) have also been observed in the inorganic horizons of forest soils and explained by the demethylation of lignin (Caron et al., 2008).The p-coumaric acid lacks the methyl group, so it is not affected by demethylation processes and can be well preserved in deep inorganic soils leading to high C/V ratios (Caron et al., 2008).A complementary process explaining the high C/V in the sediment of Lake Storsjön could be that the lake received material that had gone through an active demethylation process, with a relatively high content of p-coumaric acid and a relatively low content of V.The hypothesis is supported by the negative relation between the concentrations of V phenols and p-coumaric acid in the sediment.It was likely that the peak in C/V ratio in ca.1900, with the low coniferous pollen concentration, a slight increase in the p-coumaric acid concentration and a drop in the V phenols concentration (Figure 5), was related to the erosion of deeper inorganic soils bringing in demethylated lignin material.This was during a period of intensified agricultural activity, which potentially increased the erosion of deeper soil bringing material with high C/V ratios to the lake.
In the sediment sequence of Lake Storsjön, (Ad/Al)v and P/(S + V) ratios, the two common indicators of the degradation status of lignin, are not correlated, suggesting that the variations of P/(S + V) do not necessarily provide information about the OM degradation.The P/(S + V) ratios can be affected by the input of OM originating from phytoplankton, as p-hydroxybenzaldehyde and p-hydroxybenzoic acid can also originate from protein-rich organisms, such as plankton (Hedges, Clark, & Come, 1988).The (Ad/Al)v ratios in Lake Storsjön sediments ranging between 0.7 and 1.1 are considered to be from highly degraded material (Hedges, Blanchette, et al., 1988).The (Ad/Al)v ratios do not show an increasing downward trend in the sediment profile, suggesting little post-depositional degradation.It has been reported that fresh plant tissues have (Ad/Al)v ratios of 0.1-0.3(Opsahl & Benner, 1995), whilst soils under forests could have (Ad/Al)v ratios between 0.3 and 4.5 (Thevenot et al., 2010).Thus, it was likely that the terrestrial plant material got degraded before it was deposited in Lake Storsjön.The increasing trend in (Ad/Al)v ratios before 1700 and the relatively high values from 1700 onward probably indicated increased input of soil OM.The increased deposition of soil OM was likely associated with human disturbance in the forest through intensified agricultural activities in the catchment.

Variations in Terrestrial Organic Matter Input to Gåsfjärden in Relation to Human Impacts
The C/N ratios of the sediments of Gåsfjärden were less than 10 (Figure 3), indicating that the OM was dominated by aquatic production (Meyers, 1994).Ning et al. (2018) also show that marine-brackish organic carbon was the primary source of OM in sediments.Before ca.1700, a substantial expansion of pasture and meadows for grazing, and a decline of the forest were reflected by the pollen-based vegetation estimate.The cropland in the area significantly expanded on the expense of the forest after ca.1700, which was likely due to the large population and agricultural expansion in Sweden in the 1700s and 1800s (Gadd, 2000).However, not like the case in Storsjön, no significant change was observed in the terrestrial OM deposition during this period, reflected by the low and stable Λ 8 values.The catchment of Gåsfjärden is composed of a series of water basins and the landscape is rather flat (Figure 1), providing a relatively inefficient route for transporting the land-derived organic material to the sea.This morphological condition not only increases transport time leading to a higher rate of degradation (Catalán et al., 2016) but is also favorable for the deposition of materials during transport so that the OM could settle down in the freshwater system before reaching the sea.In addition, Ning et al. (2018) reported a continuous increase of δ 15 N values in the sediments from 1700.The high sedimentary δ 15 N was linked to elevated agricultural runoff and animal manure input, which typically display high δ 15 N (Ning et al., 2018;Teranes & Bernasconi, 2000).The increased nutrient availability can promote primary production.Therefore, the relatively low proportion of lignin-derived OM in the sediment could also be caused by the dilution effect of elevated aquatic OM deposition.
The S/V and C/V ratios of Gåsfjärden sediment did not show much variation except at the top and the bottom part of the record (Figure 3), suggesting the composition of lignin-derived OM deposition was rather stable during most of the study period.The slightly higher S/V in the 20th century could be associated with the establishment of the sawmill where the plant debris was exported to the Gåsfjärden.Between ca.1600 and 1900, agricultural land use was quite intensive in the region and covered more than 40% of the area as suggested by the land-cover reconstruction.The C/V ratios in Gåsfjärden had an average value of 0.7, which is within the range of values reported for pasture soils (Bélanger et al., 2017).The slight increase in C/V ratios after ca.1950 indicated an increasing input of the terrestrial OM from non-woody tissues, which might be associated with the disposal of non-woody tissues during logging for the sawmill.
In the sediments of Gåsfjärden, there is a weak positive relationship between the (Ad/Al)v and P/(S + V) ratios (r 2 = 0.28, p < 0.01), suggesting that the variations in both proxies may provide information about lignin degradation.Although both proxies have relatively high values at the bottom part of the sediment profile, the lack of a downward decreasing trend in Λ 8 and C/V ratios suggested the absence of downcore degradation.The relatively stable (Ad/Al)v, S/V and C/V ratios after ca.1700 reflected that the composition of the terrestrial OM deposited in the sediment was relatively consistent.

Comparison of Lignin Phenol Compositions Between Storsjön and Gåsfjärden
The average lignin phenol concentration in the sediments of Gåsfjärden (0.84 mg/100 mg-OC) was much lower than Storsjön (1.47 mg/100 mg-OC) (Figures 6a and 6b), but within the reported ranges of the concentrations in the sediments from the central Gotland basin (0.3 mg/100 mg-OC) and from rivers discharging into the Baltic Sea (1.3 mg/100 mg-OC, Miltner & Emeis, 2001).Most eroded soil that terrestrial OM tied to tends to be redeposited in land (Smith et al., 2001), and water use and river management increase sediment retention, in turn decreases sediment fluxes (Jenny et al., 2019).Also, both particulate and dissolved organic matters are likely removed through biotic and abiotic degradation, sedimentation in upstream basins, scavenging, and salinity-induced flocculation during the transport (Bauer et al., 2013 and references therein).Therefore, Gåsfjärden, as an inlet in the Baltic Sea, generally received lower amounts of terrestrial OM from the catchment than Storsjön.Additionally, the OM in the sediment of Gåsfjärden was dominated by aquatic production.Thus, the lower proportion of lignin in the total OM in the sediment of Gåsfjärden compared to Storsjön was also likely attributed to a relatively larger amount of aquatic OM input to the sediments.
The S/V and C/V ratios of the sediments from the two sites are quite different (Figure 6c).The average S/V ratio in the sediments of Gåsfjärden (0.4) was higher than Storsjön (0.3).This was likely due to the lower average coverage of conifers in the catchment around Gåsfjärden (55%) than Storsjön (64%).The C/V ratios in the sediments of Gåsfjärden were significantly lower than in Storsjön.The high C/V ratios in the sediments of Storsjön, as they have been discussed in the previous chapter, were probably site-specific.Hence, the difference in the C/V ratios between the two sites was not likely related to the difference in the vegetation cover between the catchments around the two sites, as the average coverages of grassland in the two catchments were quite similar.The higher S/V ratios in the 21st century at both sites reflected a higher proportion of lignin originating from angiosperms in the sediments.However, the vegetation in the watershed was dominated by conifers and no expansion of deciduous trees or grassland was observed in the pollen-based land-cover reconstruction.
The degrees of oxidative degradation of lignin indicated by (Ad/Al) V ratios were lower in the sediments of Gåsfjärden than in Storsjön (Figure 6b), suggesting that the terrestrial OM buried in Gåsfjärden was less degraded  Meyers and Ishiwatari (1993).(b) Relation between Λ 8 and (Ad/Al) V with the boundary according to Goñi et al. (1998).(c) Relation between C/V and S/V ratios with the boundaries according to Goñi et al. (1998).Λ 8 : sum of vanillyl, syringyl, and cinnamyl phenols normalized to 100 mg organic carbon; (Ad/Al) V : ratios of vanillic acid to vanillin; S/V: ratios of syringyl to vanillyl phenols; C/V: ratios of cinnamyl to vanillyl phenols; G w : woody gymnosperms; A nw : non-woody angiosperms; and G nw : non-woody gymnosperms.Lake Storsjön is in red and Gåsfjärden is in blue.than in Storsjön.This pattern implies a transformation of the OM composition during transportation to the coast.The terrestrial OM can be deposited and stored when passing through intermediate reservoirs along the inland drainage system.This "trapping" effect may stop the terrestrial OM from reaching the coast, and the terrestrial OM deposited in the coastal area is mostly coming from the nearshore environment with shorter transport time and therefore fresher material.Besides, biotic and abiotic degradation of the OM occurs in the inland drainage systems, so some of the particulate terrestrial OM exported from the continent may have been lost on the way to the coast (Hou et al., 2021).Our data would then indicate that the material susceptible to degradation was preferentially removed during transport, so the resistant OM was more abundant relative to the labile OM in the sediments of Gåsfjärden.
In addition to the differences in the values of the lignin proxies between the two sites, the differences in the variability of the proxies are also worth noting.The Λ 8 , S/V, C/V, and (Ad/Al) V in the sediments of Gåsfjärden displayed much less variability than in the sediment of Storsjön (Figure 6).From the pollen-based land-cover reconstructions, it was apparent that the vegetation was more variable around Gåsfjärden than around Storsjön during the last 500 years.Thus, the difference in variability cannot be explained by differences in vegetation.The lack of sensitivity to environmental changes could be attributed to the "buffered" sediments through the storage, remobilization, sortation, and alteration within the upstream basin (Walling & Fang, 2003).Subsequently, the fraction deposited in the sediments of the coastal marine basin is rather homogenized.

Conclusions
Our study shows that the terrestrial OM deposited in a coastal inlet of the Baltic and a lake upstream within the same watershed exhibit different characteristics in terms of concentration, composition, and degradation state despite the similar vegetation history in the watershed where the two sites are located.The composition of lignin-derived OM in the lake sediment varied in response to vegetation cover changes in the watershed.However, the composition of lignin in the coastal sediment is not sensitive to variations in vegetation cover on the watershed.Therefore, caution is needed when using lignin phenols in marine sediments as the indicator to reflect the vegetation composition in the catchment.Furthermore, during the period of intensified agricultural activities, a large increase in the deposition of terrestrial organic carbon was observed in the lake but not at the coast, probably because the OM from upstream was subject to storage in intermediate reservoirs or/and degraded during the long transport.Besides, the coast received less degraded terrestrial OM.The reason could be that the OM liable to degradation has been removed during transport to the sea.Our study highlights the potential of the combined use of lignin phenols and pollen-based quantitative land-cover reconstructions for the study of changes in terrestrial OM input to lakes in response to early anthropogenic disturbances.We would like to thank Conny Lenz and Leo de Jong for their help with fieldwork in Gåsfjärden, the captain and crew of R/V Ocean Surveyor for their help during sampling in Gåsfjärden, Anna Broström, Christine Åkesson, Anupam Ghosh, and Ants Aader for their help with fieldwork in Storsjön.The project was funded by FORMAS Strong Research Environment: Managing Multiple Stressors in the Baltic Sea (217-2010-126), the Crafoord Foundation, the Royal Physiographic Society in Lund, and China Scholarship Council (CSC).

Figure 1 .
Figure 1.Map of the study site with modern land use (Adapted from Ning et al. (2018)).

Figure 4 .
Figure 4. Pollen-based land cover reconstruction in percentage from Gåsfjärden based on the Landscape Reconstruction Algorithm-REVEAL model (Sugita, 2007).Cerealia-t includes pollen of Hordeum type and Triticum/Avena type.The vegetation zones were defined by constrained cluster analysis (CONISS).The percentages of reconstructed land cover are based on the sum of the 23 selected taxa.The gray curves are 10× exaggerations of the percentage values.

Figure 6 .
Figure 6.(a) Relation between Λ 8 and C/N ratio with the boundary according toMeyers and Ishiwatari (1993).(b) Relation between Λ 8 and (Ad/Al) V with the boundary according toGoñi et al. (1998).(c) Relation between C/V and S/V ratios with the boundaries according toGoñi et al. (1998).Λ 8 : sum of vanillyl, syringyl, and cinnamyl phenols normalized to 100 mg organic carbon; (Ad/Al) V : ratios of vanillic acid to vanillin; S/V: ratios of syringyl to vanillyl phenols; C/V: ratios of cinnamyl to vanillyl phenols; G w : woody gymnosperms; A nw : non-woody angiosperms; and G nw : non-woody gymnosperms.Lake Storsjön is in red and Gåsfjärden is in blue.