Introduction

Mountain regions are particularly sensitive to climate change. They are hotspots of biodiversity (Körner et al., 2016) and very valuable drinking water reservoirs (Beniston, 2006). The recent warming observed in the European Alps since the mid-1980s is roughly three times higher than the global average (Bogataj, 2007) and climate models predict even more severe changes in coming decades (IPCC, 1986–1991), Lange-Bertalot (1993), Lange-Bertalot et al. (1996), Krammer (1997), Lange-Bertalot & Krammer (2000–2003), Houk et al. (2010), Houk et al. (2014) and Hofmann et al. (2013), updated to recent taxonomic nomenclature using current publications and databases. Thin, needle-shaped Fragilaria nanana was mainly preserved in fragments. Therefore, the lengths of the fragments were measured, summed and divided by the medium length of the intact valves.

Changes in the ratio between benthic + epiphytic and planktic diatom species (B:P ratio) were used to track past changes in the depth of the littoral/profundal boundary. This boundary is determined by lake level and water transparency both affecting light availability for benthic growth (Wetzel, 2001; Hofmann et al., 2020). For the quantitative reconstruction of past lake levels of Oberer Soiernsee Hofmann et al. (2020) developed a diatom-inferred depth model based on the depth-distribution patterns of current diatom assemblages. The authors found that the strongest model is based on a weighted-averaging partial-least-square approach (WA-PLS) with two compounds that provides a robust reconstructive relationship between the distribution of diatoms and lake depth (r2 = 0.56; RMSEP = 2.55 m).

Past surface temperatures (Ø TSiVa) were inferred using the novel silicification value (SiVa) developed by Kuefner et al. (2020a). The diatom-temperature transfer function is based on a training set of 43 mountain lakes located in the Northern Calcareous Alps and refers to the negative correlation between the summer surface temperature and the average degree of silicification of the diatom assemblages preserved in surface sediments (Kuefner et al., 2020b).

$$\phi \;{\text{TSiVa}}\;{\text{(}}{}^ \circ {\text{C)}} = 25,956{-}4.7155\;*\;{\text{SiVa}}\quad ~\left( {{\text{RMSEP}} = 0.976} \right).$$

The ratio between Staurosirella pinnata (Ehrenberg) Williams and Round and Staurosira construens var. venter (Ehrenberg) Hamilton (P/V ratio) was used to indicate trends in lake temperatures. While S. construens var. venter is reflecting cooler water temperatures, S. pinnata is more competitive under warmer growing conditions (Cremer et al., 2001).

Past total phosphors (TP) concentrations were calculated by applying the TP inference model of Lotter et al. (1998) that is based on 68 small lakes (300–2350 m a.s.l.) located in the Swizz Alps. We chose this model as these lakes are also situated in catchments with calcareous lithology and 70% of the fossil diatoms of Oberer Soiernsee are part of the modern diatom training set.

Diatom concentrations were calculated by the number of microscope fields of view in relation to the amount of dry sediment.

$$diatoms/\mu l = \frac{{N~total}}{{Fv}}/Vcs*Vt*D$$

Ntotal is amount of counted diatom valves in total, Fv is analysed fields of view, Vcs is volume of one counting square ((volume of dropped diatom dilution * area of one counting square)/area of the cover glass), Vt is total volume of cleaned diatom solution, D is dilution of cleaned diatom material.

Pigment analysis

The sediments of C2 were analysed for sedimentary pigments following the method of Leavitt & Hodgson (2001). 0.3 ± 0.065 g freeze-dried subsamples were filled into 4 ml amber glasses under subdued green light, extracted using 99% acetone (Rotisolv HPLC, Carl Roth, Germany) and stored at − 20°C for 24 h. After gently shaking, 2 ml of the extract were filled into 2 ml Eppendorf tubes and centrifuged for 15 min at 2,000 rpm and − 9°C (Heraeus Fresco 17/21, Thermo Fisher Scientific, Schwerte, Germany). Subsequently, 1.0–1.5 ml of the supernatant was filtered through syringes equipped with 0.2 μm PTFE filters into 1.5 ml HPLC glasses. The samples were stored at − 80°C until analysis. Pigments were identified and quantified by reverse-phase, high-performance liquid chromatography (HPLC, MD-2015 plus, Jasko, Pfungstadt, Germany) and measured by 436 nm and 450 nm detectors, based on Wright et al. (1991). Pigment concentrations were related to dry weight and translated to μg g−1. Some pigments had to be identified by comparing peak time and spectra with data published in Wright et al. (1991) and then quantified by the peak area (mV min−1). Following Leavitt & Hodgson (2001) and Buchaca & Catalan (2008), we used chlorophyll a (chl a), its derivate pheophytin-a (phe-a), β-carotene and its degradation product ethyl 8'-beta apocarotenoate as marker pigments for total algal biomass and primary production. Diatoms were identified by the pigments fucoxanthin, diatoxanthin, diadinoxanthin and its degradation product diadinochrome. Chrysophyta were represented by violaxanthin, dinophyta by peridinin and diadinoxanthin/diadinochrome. The population dynamics of the chlorophyta were tracked by chlorophyll b and several carotenoids (lutein, neoxanthin, prasinoxanthin, zeaxanthin). Alloxanthin was selected to track abundances of cryptophyta. Pheophytin-a is a typical pigment for decaying communities (Buchaca & Catalan, 2008). Therefore, we used the chl a/phe-a ratio to estimate the degree of pigment preservation in the sediments (Tõnno et al., 2019). The ratio between alloxanthin and diatoxanthin was used to track past changes in water column transparency or lake-level fluctuations (Buchaca & Catalan, 2007).

Stable isotope analysis

Freeze-dried subsamples were sieved (250-μm mesh size) to eliminate organic macro-remains and only the fine fraction was used for isotopic analysis. Additionally, potential in-lake OM sources (phytoplankton, Chara sp., Potamogeton sp.) and OM sources from the catchment (soil, leaves, pine needles, herbages) were collected, oven-dried (40°C) and homogenised by sieving (soil) or grinding with a mortar. For robust results, we extended the OM dataset and involved catchment OM samples from 23 mountain lakes situated in the Northern Alps. For simultaneous organic carbon content (wt% TOC) and δ13Corg analyses, ~ 2.0 mg material was filled into silver capsules and decalcified with first 5% and thereafter 20% hydrochloric acid at 70°C on a heating plate. Droplets of the acid were successively added to the sediment in each capsule until no effervescence was observed anymore. After drying, the capsules were closed for subsequent isotope analysis (Mayr et al., 2017). To determine the total nitrogen content (wt% TN), δ15N, and total carbon (wt% TC), ~ 2.5 mg of bulk sediment was filled into tin capsules. Samples were combusted in an element analyser (NC 2500, Carlo Erba) coupled to a continuous-flow, isotope-ratio mass spectrometer (Delta Plus, Thermo-Finnigan, Germany) at Friedrich-Alexander-University of Erlangen. Isotope ratios are presented in δ-notation (δ15N, δ13Corg) as δ = (RS/RSt − 1), with RS and RSt as isotope ratios of the sample with the international standards VPDB (carbon) and AIR (nitrogen) and were quoted in ‰. An internal laboratory standard (peptone with δ15N = 4.93‰, δ13Corg =  − 23.80‰) and international standards (IAEA-N-1; IAEA-N-2; USGS41; IAEA-CH-7; Brand et al., 2014) were used for isotope calibration. Analytical precision (one standard deviation) was typically 0.1‰ for both δ15N and δ13Corg. Elemental contents of total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC) and total nitrogen (TN) were calculated from the ratios of peak area to sample weight using elemental standards (atropine, cyclohexanone-2.4-dinitrophenylhydrazone) for calibration and are shown as mass percentages (wt%). TOC/TN ratios are expressed as molar ratios.

Analysis of pore water pH value

Sediment pore water was sampled from core C3 directly in the field using Rhizon SMS 10 cm core solution samplers (2.5 mm diameter, 0.15 μm mean pore size; Eijkelkamp Soil and Water, Giesbeek, The Netherlands). A total of eight 3-mm-diameter holes were drilled through the tube wall of the closed core using a gimlet. Saturated Rhizon samplers were inserted horizontally into the sediment through the holes. 10 ml syringes (Eijkelkamp Soil and Water, Giesbeek, The Netherlands) were then luer-locked with the Rhizon samplers. The pistons of the syringes were fully withdrawn and held in position with a wooden retainer to generate a vacuum for pore water extraction. Pore water then passed from the sediment through the porous tubes and flexible hoses into the syringes. After 3 h, the Rhizons obtained a sufficient amount of pore water and the pH could be measured (Multi 3630 IDS, WTW, Weilheim, Germany).

Statistical analyses

Broken-stick models (Bennet, 1996) were calculated with the R package ‘rioja’ (Juggins, 2018) to detect the stratigraphic position of major changes of diatom assemblages, pigment compositions and stable isotope records. PAST (version 3.18) statistical software package (Hammer & Harper, 2006) was used for ecological analyses of life form and substrate preferences. Species abundances were Hellinger transformed (Legendre & Gallagher, 2001) before analysis. Square-root transformed diatom data was imported in a diatom-inferred depth model (Hofmann et al., 2020) and diatom-inferred TP inference model (Lotter et al., 1998) using R (version 3.4.3) and the package rioja (Juggins, 2018). Diversity was calculated by the Shannon Index using ln (natural logarithm) of percentage of each taxon. Down-core profiles were illustrated using C2 version 1.7.7. (Juggins, 2007), including only diatom species with relative abundances greater than 1% in at least two subsamples. As diatoms were very scarce in core depths greater than 22 cm (< 50 valves per sample), the subsamples C2_22 and C2_23 were not included in diatom statistical studies and stratigraphic illustration.

Results

Sedimentology, core chronology, sedimentation rate and correlation of the cores

The sediment of Oberer Soiernsee mainly consisted of dark grey, partly organic-rich clay and silty clay, interrupted by clearly visible light grey silt layers (Fig. 2).

Fig. 2
figure 2

CRS depth–age model of core C1 using activity of unsupported 210Pb (black dots) and 137Cs (white dots) (A). Photos of both cores and position of event layers (B). Correlation of C1 and C2 via δ13Corg records (C)

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Two 137Cs peaks occurred in the sediments of core C1. Assuming that the 137Cs peak at 1.5 cm corresponds to the Chernobyl disaster in ad 1986, the sedimentation rate has been very low during recent times (0.05 cm year−1). Since this does not correlate with our results of a stable isotope and pigment analysis that indicate enhanced in-lake productivity, we attributed this to the use of floristic foam to seal the core after sampling, as it destroys the topmost core section. A sedimentation rate of 0.087 cm year−1 could be calculated from the distance between the first and the second peak at 3.5 cm (1963). A very similar mean sedimentation rate was estimated using the CRS model (constant rate of supply) on 210Pb activity (0.084 cm year−1).

It is reasonable to interpret the light grey silt layers (C1: 11 cm, 13 cm, 16 cm and 20 cm) as flood layers resulting from fast deposition events such as debris flows and high surface runoffs, especially based on elevated TIC values. Hence, they were removed from the sedimentary record when develo** the age–depth model.

Core C2 was obtained two years after the master core. The core was complete and undisturbed (Fig. 2). The comparison of the δ13Corg records of both cores reveals a high inter-core consistency. Three event layers of C1 (11 cm, 13 cm and 16 cm) were found to correspond exactly in their thickness and sequence with event layers of C2 (17 cm, 19 cm and 22 cm). Another time marker is represented by the ~ 3 cm thick event layer at the top of C2, which is the result of a strong thunderstorm in August 2015 with extreme rainfall and mud flows. By correlating the δ13Corg records and event layers, the chronology of C2 was found to date back to ~ ad 1840.

Diatom analysis results

Diatom biostratigraphy

A total of 125 diatom taxa representing 39 genera were identified in 21 subsamples, 21 of these occurred in relative abundances > 2% in at least two samples (Fig. 3A). Benthic life forms dominated the diatom assemblages, especially fragilarioid species of the genera Pseudostaurosira, Staurosira and Staurosirella, which accounted for 41% of the total subfossil assemblages. Centric diatoms reached a total abundance of 6% and were mainly represented by Pantocsekiella comensis Grunow in van Heurck (3%), Cyclotella cf. woltereckii Hustedt (3%), and Cyclotella praetermissa Lund (1%). Overall, the most frequent periphytic taxa were S. pinnata (28%), Amphora pediculus (Kützing) Grunow (26%), S. construens var. venter (6%), Denticula tenuis Kützing (4%), Achnanthidium straubianum (Lange-Bertalot) Lange-Bertalot (3%), Achnanthidium minutissimum var. minutissimum (Kützing) Czarnecki (2%), F. nanana Lange-Bertalot (2%), Platessa conspicua (A. Mayer) Lange-Bertalot (2%), Pseudostaurosira elliptica (Schumann) Edlund, E. Morales, S.A. Spauld (2%), Pseudostaurosira trainorii E. Morales (2%), and Amphora inariensis (Krammer) Lange-Bertalot (2%). Species richness varied between 20 and 55, Shannon diversity between 1.8 and 2.8 (Fig. 3B). The total diatom concentrations in the sediment samples ranged from 5 × ·102 to 2 × ·104 diatoms μl−1 (Fig. 3B).

Fig. 3
figure 3

A Diatom stratigraphy of most common taxa with abundances > 2% in at least two samples. The four diatom zones (D-A to D-D) are separated by grey lines. B Variation in the ratios between benthic + epiphytic and planktic taxa (B/P ratio) and between Staurosirella pinnata and Staurosira construens var. venter (P/V ratio). Alteration of Shannon diversities and species richness. Reconstruction of lake temperature using the silicification value (Kuefner et al., 2020a, b), of lake depth by applying the model designed by Hofmann et al. (2020) and of TP using the model of Lotter et al. (1989). The four diatom zones (D-A to D-D) are separated by grey lines. The grey bar represents the two lowermost samples, where diatoms were too scare for including in statistical analyses

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The broken-stick model (Bennet, 1996) identified four different diatom assemblage zones (Fig. 3A, B).

Zone D-A (21–18 cm; ad 1856–1880)

Amphora pediculus and S. pinnata co-dominated the diatom assemblages of the deepest diatom zone. Fragments of Aulacoseira spp. occurred frequently in this zone.

Zone D-B (18–6 cm; ad 1880–2010)

The assemblages of this zone differ clearly from those of zone D-A, although A. pediculus and S. pinnata remained the dominant taxa. After a peak in ad 1960, S. construens var. venter declined in frequency. Pseudostaurosira elliptica disappeared around ad 1960, when P. trainorii clearly increased in its abundances.

Zone D-C (6–2 cm; ad 2010–2015)

Beside A. pediculus and S. pinnata, P. conspicua dominated the diatom assemblages. Centric diatoms and F. nanana started to increase distinctly in their abundances. The abundances of S. construens var. venter clearly decreased towards the end of the zone, while S. pinnata increased slightly.

Zone D-D (2–0 cm; ad 2015–2017)

Centric diatoms (31–65%) and F. nanana (8–22%) co-dominated the assemblages of this zone. Small P. comensis in particular is highly abundant (1–30%). Simultaneously, all periphytic taxa clearly declined. All small fragilarioid taxa were absent with the exception of S. pinnata.

The valve density (Fig. 3B) was very low in the deeper part of the core (zone D-A), but clearly increased after AD 1880. A distinct maximum was calculated in the transition between zones D-B and D-C (7.5–5.5 cm; ad 1986–2010). After this, the cell density declined visibly before it increased again in the younger sediments (2.5–0.5 cm; ad 2015–2017).

The ratio between benthic + epiphytic and planktic diatom species (B:P ratio; Fig. 3B) showed only small variations before ad 2010, but declined markedly in zone D-D. After the distinct dominance of benthic diatoms in the deepest sample, planktic taxa gained importance between ad 1870 and ad 1900, which is also reflected in the increasing species richness and diversity. Between ad 1940 and ad 2000, epipsammic fragilarioid taxa clearly dominated the periphytic (benthic + epiphytic) diatom assemblages, while centric, epilithic and epiphytic species occurred in low percentages, resulting in low species richness and Shannon diversities. Zone D-C is characterised by higher diversity indices and more diverse periphytic diatom assemblages. The B:P ratio decreased due to increasing abundances of centric diatoms. The distinct dominance of planktic species in zone D-D resulted in low B:P ratios and decreasing diversity indices.

The ratio between S. pinnata and S. construens var. venter (P:V ratio; Fig. 3B) generally increased towards recent times. Whereas the ratio varied around a median of 78 between 21.5 and 7.5 cm, it clearly increased after ~ ad 1980 to a median of 95.6, implying the declining influence of S. construens var. venter in zones D-C and D-D.

Diatom-based reconstruction of lake surface temperatures, lake levels, and TP concentrations

By calculating the lake surface temperatures using the silicification value (Kuefner et al., 2020a, b), there is clear evidence of a trend towards rising temperatures from 13.6°C (~ ad 1856) to 18.5°C (ad 2017) (Fig. 3B). The diatom assemblages of the deepest diatom zone (D-A) indicated the lowest mean surface temperature (13.9°C). Increasingly higher average surface temperatures were calculated for the diatom assemblages of the zones D-B (14.0°C), D-C (14.2°C), and D-D (17.2°C).

By applying the diatom-inferred depth model designed by Hofmann et al. (2020), the WA-PLS (RMSE = 2.43 m; r2 = 0.63) approach proved to be suitable. The reconstructed lake levels show a first maximum (12.5 ± 2.4 m) around ad 1868 and a second maximum (12.5 ± 2.4 m) around ad 1986 (Fig. 3B). Inferred depth values steadily increased between ad 1880 and ad 1986. Subsequently the value declined to a second minimum (9.7 ± 2.4 m) in 2015, before the increased again up to 11.7 ± 2.4 m in ad 2017 (Fig. 3B).

TP reconstructions (WA-PLS approach; RMSE = 0.27; r2 = 0.75) using the model of Lotter et al. (1998) revealed a maximum 29.9 ± 1.9 μg l−1 around ad 1860. TP concentrations fist declined, and then started to rise after ad 1890. After a second maximum (27 ± 1.9 μg l−1) around ad 1965 the values tended to decrease to 8.4 ± 1.9 μg l−1 around ad 2015. Within the event layer mean TP concentrations ~ 8.0 ± 1.9 μg l−1 were inferred. In the uppermost sample TP = 4.5 ± 1.9 μg l−1 was reconstructed (Fig. 3B).

Sedimentary pigment analyses

Five different phases with regard to pigment concentration and preservation could be distinguished (Fig. 4).

Fig. 4
figure 4

Sedimentary record of the marker pigments, of the pigment preservation (Chl a/Phe a) and of the ratio between alloxanthin and diatoxanthin (Allox/Diato). The five pigment zones (P-A to P-D) are separated by grey lines

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Phase P-A (22.5–16.5 cm; before ad 1880) was characterised by low chl a and β-carotene concentrations and low chl a/phe-a ratios, indicating low algal biomass and poor pigment preservation. Two clearly visible declines in all pigment concentrations at 16.5 cm and 18.5 cm are noticeable. The high concentrations of diadinochrome, a derivate of diadinoxanthin, suggest the dominance of periphytic diatoms in the outgoing nineteenth century.

Significantly higher concentrations of nearly all pigments and higher chl a/phe-a ratios were detected during phase P-B1 (15.5–9.5 cm; ad 1890–1960), suggesting an increasing total algae biomass and rising pigment preservation. However, a decrease in chl a and β-carotene concentrations are evident towards the end of phase P-B1.

Phase P-B2 (8.5–7.5 cm; ad 1970–1986) started with low preservation ratios and a depression of total algal biomass marker concentration. However, the concentrations of most pigments began to increase within this phase. After a maximum around ad 1970, violaxanthin (Chrysophyta) disappeared abruptly.

Since ~ ad 1990 (phase P-C; 6.5–5.5 cm), there has been a dramatic increase in total algal biomass and a steady, up to threefold rise in preservation ratios. Peridin (Dinophyta), alloxanthin (Cryptophyta) and the marker pigments of green algae (chlorophyll b, neoxanthin, prasinoxanthin and zeaxanthin) appeared in their highest concentrations.

Between ad 2015 and ad 2017 (4.5–0.5 cm; phase P-D), the preservation ratios increased markedly by a factor of 10. Simultaneously, the concentrations of all pigments decreased abruptly.

Stable isotopes analysis

Organic geochemistry and stable isotopes of the sediments

Four different phases could be separated in terms of organic geochemistry and stable isotope ratios (Fig. 5).

Fig. 5
figure 5

Stable isotope (δ13Corg, δ15N), geochemical (TN, TOC, TIC and TOC/TN data of sedimentary organic matter. The four phases (SI-A to SI-D) are separated by grey lines

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Phase SI-A (22.5–20.5 cm; before ~ ad 1868) began with high values of δ13Corg (− 25.5‰) and TIC (9.4 wt%), followed by a marked decrease towards the end of the zone. TOC (3.1 wt%), TN (0.3) and δ15N (1.9‰) exhibited low values at the base of the core and increased distinctly higher up. δ15N showed the highest values (2.6‰) and TIC (4.9 wt%) the lowest values of the entire record at 20.5 cm (~ ad 1860). The highest TOC/TN ratio of the entire record (10.2) occurred at 22.5 cm.

δ13Corg and TIC values initially rose during phase SI-B (19.5–8.5 cm; ad 1868–1980), accompanied by declining TOC and TN contents. Subsequently, between ad 1890 and ad 1950, hardly any changes were visible for the TN (median 0.4 wt %), δ13Corg (median − 26.6‰), TOC (median 3.8 wt%) and TIC (median 9.2 wt%) values. Only δ15N (median 1.5‰) exhibits greater fluctuations, especially before ad 1890. δ15N values began to decline after ~ ad 1960. From ad 1940 onwards, δ13Corg and TIC decreased steadily, while TOC and TN increased towards the end of the phase.

Phase SI–C (7.5–5.5 cm; ad 1980–2015) is characterised by several prominent changes: there was a marked drop in δ13Corg down to a minimum of − 30.8‰ at 5.5 cm and a concurrent increase in TOC and TN up to a maximum of 6.1 wt% and 0.9 wt%, respectively. The TOC/TN ratio also reached a distinct minimum at 5.5 cm (6.9).

During phase SI-D (4.5–0.5 cm; ad 2015–2017), δ13Corg shifted again to higher values (up to − 25.5‰ at 1.5 cm). TOC showed the lowest contents during the entire investigation period (2.5–2.6 wt%). TIC increased markedly to the highest values (10.4–11.6 wt%). TN had the lowest values (0.3 wt%) comparable to values at the base of the core.

Isotopic and geochemical signatures of OM sources and the origin of sedimentary OM

The isotopic signatures of modern OM (lake algae, herbaceous vegetation, terrestrial soils, trees and submerged macrophytes) collected in the lake and in its catchment are shown in Table 1. Based on modern isotopic fingerprints, algae appeared to be the main source of sedimentary OM (Fig. 6). Additionally, terrestrial soils contributed to sedimentary OM (Fig. 6). A proportion of sedimentary OM derived from macrophytes, herbaceous vegetation and trees, both coniferous and deciduous is very unlikely as their isotope signatures plot at a large distance to the sediment signatures (Fig. 6).

Table 1 Isotopic signatures of modern OM sources including macrophytes, lake algae and catchment soils and vegetation
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Fig. 6
figure 6

Isotopic and geochemical signatures of sediment record (red open circles) and potential sources of sedimentary OM. Red dots represent most recent samples (ad 2017–2017) (A–C). δ15N versus δ13Corg based on Finlay & Kendall (2007) to evaluate main geochemical in-lake processes controlling δ13Corg and δ15N of aquatic OM (D)

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Linkages between carbon and nitrogen cycles

Findlay & Kendall (2007) summarised the main geochemical processes controlling δ15N and δ13Corg of aquatic OM under the premise of a predominant algal origin of OM (Fig. 6). The TOC/TN ratios in the sediments of Oberer Soiernsee revealed that algae are the major sedimentary OM component, as pure algal matter typically ranges between 7 and 9 (Meyers & Teranes, 2001). Accordingly, photosynthesis and eventually methane oxidation are the main factors that regulated changes in δ13Corg records in Oberer Soiernsee (Fig. 6). The variations in δ15N are smaller than those in δ13Corg record and may point to denitrification. Before the ad 1860s (red dots), an increasing δ15N (positive shift of 0.8‰) and simultaneously decreasing δ13Corg (negative shift of 2.6‰) indicated a combination of incipient denitrification and methane oxidation processes (Fig. 6). Between 1870 and 1980 (green dots), a marked shift towards 1.4‰ lower δ15N values and 1.4‰ higher δ13Corg values suggests elevated primary production. After ad 1970 the records of δ15N and δ13Corg are decoupled. Towards 1980, decreasing δ13Corg indicates that respiration or, more likely, anoxic biochemical processes became increasingly important. Between ad 1980 and ad 2015, a marked negative shift (3.1‰) towards significantly lower δ13Corg values indicates enhanced methane oxidation processes (orange dots). After ad 2015 the shift towards higher δ13Corg values indicate enhanced photosynthesis and possibly nitrification as important processes (blue dots).

In situ pore water pH analysis (C3)

The pH value at the sediment–water interface was 8.6 in October 2018. Down-core, the values declined continuously from 7.8 (2 cm) to 7.4 (20 cm) (Table 2).

Table 2 pH values of the sediment–water interface and of the sediment pore water in specific depths
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Discussion

The combination of sedimentological, biological, biochemical and geochemical proxies, supported by historical information, proved as a powerful tool to reconstruct the ecological history of Oberer Soiernsee and to disentangle the effects of different environmental impacts on the lake evolution since ~ ad 1840. Diatom, pigment and δ13C records provided valuable information on the effects of catchment-related changes on lake-internal biogeochemical processes. Chl a and δ13C show very similar trends, facilitating the reconstruction of lake trophic history reliably. Diatom-inferred TP concentrations, however, show partly distinct differences. Juggins (2013) warns against the uncritical use of quantitative reconstructions, as biotic communities respond sensitively and complexly to multiple environmental gradients. Lotter et al. (1998) conceded that, besides TP a series of other causal factors such as mean summer temperatures, mixing regimes, the availability of light and other nutrients (nitrogen, silica and carbon) and interspecific competition control the composition of the diatom assemblages. The application of transfer functions requires a negligible influence of these secondary environmental variables; furthermore, the effects of temporal change in the underlying causal relationships must be considered (Juggins, 2013).

However, several facets of global environmental changes have affected remote mountain lakes since the industrial revolution (Catalan et al., 2013). The effects these global changes on the lake ecology of Oberer Soiernsee were revealed by the δ15N record (atmospheric deposition–fertilisation) and by shifts in diatom species composition (changes of water temperature, thermal stratification and light availability).

Depth–age model

The applicability of the CRS model to date the sediments of Oberer Soiernsee may be debatable as the lake was exposed to a series of both catchment-mediated and lake-internal changes resulting in an increase of algae biomass (Figs. 3, 4). However, after discarding the event layers (Fig. 2), our age–depth model correlates well with historical records including land use, hut construction, overnight stays (Müller, 1922; Cabin book of the small private cabin; Yearbooks of the Alpine Club Section Hochland 1920–2017).

Ecological history of Oberer Soiernsee

Period I (ad 1840–ad 1880): Little Ice Age, alpine pasturing, hut construction

The high frequency of paleoflood layers in the deeper part of the core (Fig. 1) are indicators for heavy rainfall events (Giguet-Covex et al., 2012; Wilhelm et al., 2012, 2013) and suggest the influence of the Little Ice Age (LIA), a period of low temperatures, high wind speeds and high precipitation that occurred from the sixteenth to the mid-nineteenth century, particularly in the Northern Hemisphere (Bracht-Flyr & Fritz, 2016; Ilyashuk et al., 2018). TOC/TN ratios > 10 indicate enhanced allochthonous OM sources (Meyers & Teranes, 2001) in particular soils (Fig. 5), due to the enhanced erosion induced by higher precipitation rates in the LIA.

The high flood frequency and the harsh conditions including low temperatures and short growing seasons led to low diatom and pigment concentrations before ~ ad 1850 (Fig. 3A, 4). Post-depositional dissolution of the siliceous diatom valves can be excluded as both heavily (e.g. Diploneis spp., Gyrosigma spp.) and slightly (e.g. Staurosira microstriata) silicified taxa were sporadically found. Furthermore, pore water analyses revealed pH 7.4 (Table 2), suggesting favourable preservation conditions as diatom dissolution especially occurs at pH > 9.0 (Lewin, 1961; Barker et al., 1994).

The large-celled, heavily silicified Aulacoseira species, occurring especially before ~ ad 1880 also indicate more turbulent conditions in context with the LIA, as this species requires turbulent mixing to remain in the photic zone and gain access to nutrient resources (Karst-Riddoch et al., 2005; Reynolds, 2006; Saros & Anderson, 2014; Weckström et al., 2016; Rühland et al., 2018).

The light availability for benthic growth depends on both water depth and light penetration. Hence, diatom-inferred high lake levels before ~ ad 1880 (Fig. 3B) more likely indicate the high-turbid conditions with low water transparency that often prevailed during the windy periods towards the end of the LIA (Mann, 2002a). The high ratios between alloxanthin and diatoxanthin also indicate limited light availability for benthic algae (Fig. 4). In contrast, high B/P ratios suggest extensive periphytic diatom growth, especially of small epipsammic fragilarioids and epipelic Amphora spp. (Figs. 3A, B), which are both indicative for the deeper littoral zone of Oberer Soiernsee today (Hofmann et al., 2020). The small epipsammic species are known to live firmly attached on sand grains, to be resistant to abrasion or damage due to a harsh and turbulent environment, and to even tolerate short terms of darkness and brief anaerobic conditions (Moss, 1977; Hofmann et al., 2020). Epipelic taxa have a competitive advantage, as they are able to avoid permanent burial due to their mobility (Moss, 1977; Burkholder, 1996).

Diatom-inferred TP reconstruction fits well with the δ13Corg trend. TP ~ 30 ± 1.9 μg l−1 revealed mesotrophic conditions around ad 1860 (Fig. 3) also reflected by rising pigment concentrations despite low preservation indices (Fig. 4). We assume that the high erosion rates related to the LIA may be responsible for the elevated TP concentrations, as the calcareous rocks in the catchment may contain considerable amounts of phosphorus-containing minerals (Valeton, 1988). The high TIC values in the deepest part of the core (ad 1840–1860) support this assumption (Fig. 5).

The decline in δ13Corg may have been triggered by the gradual 13C depletion of atmospheric CO2 since the industrial revolution. McCaroll & Loader (2004) found that the combustion of fossil fuels resulted in 1.5‰ more negative δ13Corg values of atmospheric CO2 as fossil fuels are relatively depleted in 13C. The shift to 2.61‰ lower δ13Corg values (Fig. 5) is, however, too large to be explained solely by a fossil fuel combustion effect. Enhanced external input by an increased flood frequency during the LIA and additionally alpine pasturing and forestry (Müller, 1922) seems to be a more plausible cause for 13C depletion of sedimentary OM as these activities are known to provide additional nutrient load to the lake and promote lake eutrophication (Schwoerbel & Brendelberger, 2013). Enhanced primary productivity resulted in hypolimnetic anoxic conditions that promote chemoautotrophic organisms and methanotrophic bacteria with 13C-depleted biomass (Braig et al., 2013). These microbes produce 13C-depleted CO2 and thus change the DIC reservoir of a lake (Hollander & Smith, 2001). The resulting 13C-depleted photoautotrophic biomass and the 13C-depleted microbial biomass culminate in the negative trends in the δ13Corg values of sedimentary OM (Fig. 5).

However, with progressing eutrophication since ~ ad 1870 the δ13Corg values become more positive again (Fig. 5). Diatom-inferred TP (20.8 ± 1.9 μg l−1) also reflects enhanced phosphorus input triggered by the massive deforestation related to the construction of the two mountain huts in ad 1866 (Müller, 1922). The limitation of carbon availability in the course of the rise in algae biomass results in decreasing isotopic discrimination during photosynthesis, culminating in more 13C-enriched OM (Fogel & Cifuentes, 1993), and the rising flux of 13C-enriched phytoplankton biomass started to overprint the microbially influenced isotopic signals after ~ ad 1870.

Plotting δ15N versus δ13Corg (Fig. 6) reveals a combination of methane oxidation processes and incipient denitrification in the deeper part of the core (Findlay & Kendall, 2007). We assume that denitrification processes in context with the enhanced allochthonous input due to the high precipitation rates may have stimulated the weak rise in δ15N values. This trend was additionally promoted by livestock, deforestation, and, after 1866, by the dry toilet, as human and animal waste is enriched in 15N (Kendall, 1998).

Period II (ad 1880–ad 1980): rising temperatures, increasing eutrophication and atmospheric N fertilisation

Rising temperatures towards the end of the LIA led to increasing photosynthesis rates (Hall & Smol, 2010) documented by rising diatom densities and pigment concentrations after ~ ad 1880 (Figs. 3B, 4). Within the benthic diatom assemblages, some structural shifts occurred especially within the small fragilarioids (Fig. 3A). Staurosira microstriata, a taxon indicating low mean summer water temperatures (Schmidt et al., 2004), became absent after ~ ad 1880. Pseudostaurosira elliptica a species favouring low NO3 concentrations (Morales, 2011) abruptly disappeared after ~ ad 1960. The clear shift in the ratio between S. construens var. venter and S. pinnata around ad 1970 may be indicative for gradually rising water temperatures, as S. construens var. venter was described as being more competitive under cooler conditions, while S. pinnata is more frequent during warmer periods (Cremer et al., 2001; Joynt & Wolfe, 2001; Bouchard et al. 2004). However, in line with Spaulding et al. (2015), we rather suppose the combined effect of rising temperatures and increasing nitrogen loadings may have triggered changes within the diatom assemblages.

Low alloxanthin/diatoxanthin and high B/P ratios indicate favourable light conditions for benthic production (Figs. 3B, 4). However, diatom-inferred lake-level increased since ~ ad 1880 inferred from decreasing frequency of epilithic species, such as A. minutissimum and D. tenuis. Hofmann et al. (2020) found that the distribution of the benthic diatoms in Oberer Soiernsee mainly depends on substrate availability. Epilithic species primarily occurred in the upper littoral zone (0–7 m), while epipsammic and epipelic diatom prefer the deeper littoral (7–11 m). We assume that the diatom-inferred lake-level changes are rather the consequence of enhanced availability of fine-grained substrate reflecting climate-mediated changes of the catchment. TOC/TN values revealed enhanced proportion of terrestrial sources on the sedimentary OM (Fig. 5). Between 1890 and the end of World War II, the small cabin situated at the northern shore of Oberer Soiernsee was rarely visited (Müller, 1922; cabin book; Alpine Yearbooks), as very well reflected by low diatom-inferred TP levels (mean 3.3 ± 1.9 μg l−1). Diatom-inferred TP started to increase since ~ ad 1950 (Fig. 3B). Concurrently declining δ13Corg indicates that respiration or, more likely, anoxic biochemical processes releasing 13C-depleted methane into the water column became increasingly important (Figs. 5, 6). The mesotrophic conditions around ad 1960 (diatom-inferred TP = 27 ± 1.9 μg l−1) may be due to the sharply increasing numbers of overnight stays in the small cabin since ~ 1960. Around the world the phosphorus loadings into water bodies markedly increased in the 1960s due to the development of phosphate detergents (Vollenweider, 1968). We suppose that the sharply rising TP is the result of the discharge of the cabin’s wastewater.

The short-term increase in δ15N (Fig. 5) may also be related to the rising numbers in overnight guests and day-trippers resulting in enhanced nitrate-N loadings from the nearby dry toilet, as human excrements are enriched in 15N (Kendall, 1998). A further positive correlation between δ15N and δ13Corg would be expected if variations in these parameters continued to be caused mainly by alterations of the algae productivity (Gu et al., 1996; Finlay & Kendall, 2007). However, the decoupling of both records after ~ ad 1970 (Fig. 6) suggests the increasing influence of atmospheric nitrogen deposition from distant sources (fossil fuel combustion, industrial fertiliser and livestock) which is known to be a major external source of nitrogen in remote mountainous regions, especially since the 1950s (Catalan et al., 2013). Atmospheric nitrogen produced by agriculture and industry is 15N depleted relative to preindustrial sources (Hastings et al., 2009). The clear and continuous depletion in sedimentary δ15N after ~ ad 1970 (Fig. 5) likely reflects the increasing influence of atmospheric nitrogen loading, a trend that was widely observed in lake sediments of the Northern Hemisphere (Holtgrieve et al., 2011).

Period III (ad 1980–ad 2010): rapid climate warming reinforced eutrophication

This period is characterised by the dramatic increase in both diatom productivity and pigment concentrations indicating that climate warming sharply reinforced the eutrophication processes impacting Oberer Soiernsee. Algal responses were accelerated when lakes surpassed a climate-mediated ecological threshold associated with the establishment of a stronger thermal stratification (Smol et al., 2005).

Since 1980 a particularly pronounced warming (~ 0.5°C per decade) has been observed in the European Alps (Gobiet et al., 2014). Also, at the Zugspitze, 30 km west of Oberer Soiernsee, a significant increase in annual mean temperatures was documented (data provided by DWD, Schneefernerhaus). In the sediments of Oberer Soiernsee a shift within the paleopigment composition around ad 1980 indicate the climate-induced stabilisation of the pelagic habitat. Violaxanthin, a proxy for cooler conditions (de Jong et al., 2013) suddenly disappeared and the indicators for a stronger thermal stabilisation (alloxanthin, peridinin and all green algae marker pigments) concurrently increased after ~ ad 1980 (Fig. 4).

However, besides the rising air temperatures, changes in light transmission and heat transfer may also have contributed to the stabilisation of the thermal stratification in Oberer Soiernsee. Climate-mediated changes in catchment vegetation cover, increased soil development and treeline migration (Vinebrooke and Leavitt, 1998; Sommaruga et al., 1999) may have led to an enhanced terrestrial input of light-aborting particles such as suspended inorganic and organic material and dissolved organic matter (DOC; Saros & Anderson, 2014) resulting in a enhanced heat transfer to the lake.

Around ad 2000 the rapid increase in algal biomass (Figs. 3, 4) indicates that an ecological threshold related to post-1980 warming has been surpassed. The intertwined stresses of rising temperatures and longstanding nutrient inputs from mountain hut tourism, atmospheric nitrogen fertilisation and climate-related catchment changes culminate in a marked increase in paleopigment (up to 14-fold) and diatom (4-fold) concentrations. The success of labile pigments (fucoxanthin, neoxanthin, peridinin) and rising chl a/phe-a ratios point to reduced pigment degradation, which is also indicative for enhanced primary productivity as light and oxygen availability was reduced and dead OM was buried more rapidly. Therefore, the favourable preservation conditions possibly overestimate the rapid increase in pigment concentrations. Furthermore, it should be noted that the concentrations of sedimentary pigments are generally much higher in the topmost sediment layers than in the deeper part of the sediment core, because the degradation processes are still in progress (Tõnno et al., 2019).

The marked negative shift (3.1‰) towards significantly lower δ13Corg values after ~ ad 1980 (Fig. 5) indicates that enhanced lake productivity triggered methane oxidation processes. Additionally, methanogenesis and subsequent oxidation of the 13C-depleted methane could occur also under oxic conditions in freshwater lakes (Bogard et al., 2014; Thottathil et al., 2018). The increase in total algae biomass suggests a progressive oxygen depletion at the bottom of the lake since ~ ad 1980, reinforced by the stronger thermal stability.

The continuously decreasing δ15N values, however, did not reflect the denitrification processes. We assume that the continuous deposition of atmospheric nitrogen overrides the in-lake N-isotope dynamics, a process also described by Lehmann et al. (2004). As atmospheric nitrogen deposition in Europe declined after the peak in 1980 (Engardt et al., 2017) a reversal trend in δ15N towards more positive values could have been expected after ~ ad 1980. However, δ15N continues to decline (Fig. 5). Continuous atmospheric measurements since 1995 revealed no negative trend in the concentrations of atmospheric nitrogen oxides at the northern foothills of the Alps (DWD, Hohenpeißenberg). Measuring nitrogen deposition in a two-year monitoring of sites in the Northern Alps located ~ 20 km west of Oberer Soiernsee, Kirchner et al. (2014) observed that the region is even nowadays affected by marked deposition rates of nitrogen due to long-range atmospheric transport.

The broken-stick model revealed no significant diatom assemblage changes between ad 1980 and ad 2010. However, the ratio of benthic/epiphytic versus planktic frustules (B:P ratio) slightly decreased in this period (Fig. 3B), while the ratio between alloxanthin and diatoxanthin markedly increased and diatom-inferred lake levels reached a maximum (12.5 m; Fig. 4). We assume, that the diatom-inferred lake-level rise is rather the consequence of reduced light penetration due to phytoplankton shading limiting the light availability for benthic diatom species.

Period IV (ad 2010–ad 2017): rapid warming and an extreme precipitation event

Rapid warming appeared to be the main driver for surpassing an important ecological threshold resulting in the recent success of planktic diatom species. In the Northern Alps the increase in air temperatures was again markedly accelerated around 2010 (data provided by DWD, Schneefernerhaus), resulting in significantly rising surface water temperatures (Dokulil, 2013), entailing a shorter ice-cover duration (Thompson et al., 2005; Weckström et al., 2014), longer growing seasons (Smol, 1988), the alteration of the balance between thermal stratification and turbulent mixing (Luoto & Nevalainen, 2013; Butcher et al., 2015), the development of more diverse littoral habitats (Lotter et al., 2010), and changes regarding the availability of light and nutrient resources (Douglas & Smol, 2010).

The stabilisation of the pelagic habitat and the establishment of macrophytes as substrate for epiphytic taxa (e.g. Encyonopsis spp.) appeared to represent a tip** point in the ecosystem of lake Oberer Soiernsee indicated by a marked shift towards more complex, species rich diatom assemblages and distinctly rising abundances of planktic species (Fig. 3A, B). Especially small Cyclotella/Pantocsekiella species and F. nanana occurred more frequently after ~ ad 2010 and their abundances abruptly increased after ~ ad 2015. Diatom-inferred temperature reconstructions by using the novel silicification value (Kuefner et al., 2020a, b) revealed an abrupt warming of lake surface temperatures after ~ ad 2015 (Fig. 3B). However, the direct effect of rising air temperatures is not likely alone the driver of diatom species shifts. Stratification patterns and mixing regimes were also regulated by the amount of light adsorbing particles. The marked increase in phytoplankton biomass and the extremely large amount of inorganic particles and soils released from the catchment after the extreme precipitation event in August 2015 reinforced the heat transfer into the lake by the enhanced adsorption of light energy. The complex interactions between the mechanism controlling the stratification patterns and the resulting effect of changing thermal regimes on the availability of light and nutrients greatly influence growth, structure and competition of both planktic and periphytic algae communities (Winder & Sommer, 2012; Saros & Anderson, 2014; Malik et al., 2017).

Sharply declining B:P ratios and increasing diatom-inferred lake levels after ~ ad 2015 (Fig. 3B) indicated favourable conditions for planktic diatoms. Cyclotella/Pantocsekiella spp. and F. nanana have competitive advantages during times of stronger stratification due to their low sinking rates, excellent light-harvesting skills and efficient nutrient uptake (small centrics) and their ability to reduce sinking velocity by forming ribbon-like colonies (F. nanana). Numerous diatom paleo-records from sites across a wide geographic range highlighted the recent success of small, fast-growing cyclotelloid species with enhanced water column stability and reduced vertical mixing (e.g. Rühland et al., 2003, 2008, 2015; Douglas et al., 2004; Rühland and Smol, 2005; Karst-Riddoch et al., 2005; Smol et al., 2005; Kuefner & Hofmann et al., 2020). However, changes in nutrient concentrations also may have triggered shifts in the abundances of Cyclotella/Pantocsekiella spp., as this species prefer oligo- to moderately mesotrophic waters. Pantocsekiella comensis, a taxon that is known to be a characteristic species in oligotrophic lakes (Marchetto & Bettinetti, 1995, Marchetto et al., 2004; Reynolds, 2006; Werner & Smol, 2006; Berthon et al., 2014; Saros & Anderson, 2014) occurred in low abundances (0.5–0.6%) between ad 1880 and ad 1900, reappeared around ad 2015 (1%) and subsequently sharply increased up to 30%. Contemporary hydrochemical analyses revealed decreasing TP concentrations from 14.4 to 4.5 μg l−1 and increasing nitrate-N concentrations from 0.27 to 0.36 mg l−1 after the extreme precipitation event in August 2015 (Hofmann et al., 2020; Ossyssek et al., 2020). Analysing the distribution of P. comensis across oligotrophic mountain lakes in the northern European Alps, Ossyssek et al. (2020) additionally found that this taxon was positively correlated with the nitrate-N concentrations providing an indicator for rising water temperatures under elevated nitrate concentrations. Beside atmospheric fertilisation, catchment soils are the main external nitrogen resource in remote lakes (Saros & Anderson, 2014). Based on the TOC/TN ratios and the fingerprints of modern OM sources terrestrial soils were found to influence the composition of the sedimentary OM deposited after ad 2015 (Fig. 6). Rising TOC/TN ratios after ad 2015 also suggest enhanced autochthonous sources of sedimentary OM (Fig. 5).

The marked collapse of the diatom cell densities and pigment concentrations (Figs. 3, 4) is related to the extensive catchment denudation triggered by the massive thunderstorm in August 2015. Highly turbid conditions existed for several weeks (Hofmann et al., 2020) and a 4–5 cm thick flood layer was deposited. During the cold period of LIA an increased frequency of flood events been observed (Fig. 1). The recent global warming, however, was found to increase the intensity of heavy rainfall events resulting in a significantly increasing activity of landslides and debris flows in the Northern Calcareous Alps (Dietrich & Krautblatter, 2016).

The ad 2015 flood event resulted in the reversal of the declining δ13Corg trend. We presume that the flood layer sealed the lake bottom and inhibited the resolution of TP. Contemporary hydrochemical analyses revealed steadily decreasing TP concentrations from 14.4 μg l−1 (2015) to 11.8 μg l−1 (2016) to 4.5 μg l−1 (2017). Diatom-inferred TP concentrations show a similar trend from 8.4 ± 1.9 μg l−1 (2015) to 4.5 ± 1.9 μg l−1 (2017). Further investigations are needed to determine whether this is only a short-term phenomenon or whether the flood layer has the potential for a long-term influence over the lake’s nutrient state.

Conclusions

The results of our study show that lake responses to climatic and human influences are complex, multidimensional, and often indirectly mediated through watershed processes.

The sedimentological, biological and biogeochemical information stored in its sediments revealed the eutrophication history of Oberer Soiernsee since the end of the Little Ice Age (ad 1840–ad 2017). By combining the multiple proxies, we were able to disentangle external disturbances and lake-internal processes and to unravel the complex responses of lake algae to local human activities, long-distance atmospheric deposition and climate warming.

We could demonstrate that changes in catchment-lake interaction linked to climate warming are mirrored in the sediment archives, in particular by changes in the frequency and intensity of extreme rainfall events and in the release of nutrients and DOC into the lake.

Our findings confirm the benefit of combined analyses of δ13Corg and δ15N values as well as TOC and TN contents of sedimentary OM to reveal and explain changing trophic conditions. Comparing with isotopic values of modern OM, we were able to determine the origin of sedimentary OM as mainly autochthonous (algae), partly influenced by terrestrial soils. Diatom productivity and species composition and the concentrations of chlorophyll a and β-carotene were found to share the analytical capability to reconstruct lake eutrophication.

Anthropogenic climate change and the linked chain of causal factors were found to be the main drivers influencing algal dynamics and species composition. The diatom record provided an important tool to reveal climate warming signals and alterations regarding water column transparency, TP- and nitrate-N concentrations. The timing of the establishment of thermal stratification could be determined via the success of centric diatoms and via the marker pigments of Dinophyta and Cryptophyta.

Our study highlights the threat multiplier character of climate change on mountain lakes influenced by local human activities, resulting in amplified responses of algal communities. Based on the findings of this study, we assume that climatically induced ecological thresholds have already been surpassed in most of the Alpine lakes. In view of the present and future challenges in effectively managing water resources our findings underline the urgency of a sharp reduction of local and atmospheric nutrient input to maintain valuable ecosystem services under future climate scenarios.