In 2019 two parallel peat profiles with a distance of 50 cm were cored from the centre of Linje peatland with a Russian type corer (chamber dimension: 10cm×100cm), from a vegetation area characteristic of the ombrotrophic section of the site. The longer profile (Linje_2019 core; Szambelan et al., 2025), with a total length of 1230 cm, was chosen for analyses in this study and it was cut into 2.5 cm thick sections.

Radiocarbon dating was performed on selected plant macrofossils (Table 1) at the Poznań Radiocarbon Laboratory (Poland) using their standard pre-treatment protocols. The chronology of the peat profile is based on a Bayesian age-depth model generated from 12 out of 14 14C AMS dates (Table 1). The model was constructed using OxCal. 4.4.4 software (Bronk Ramsey, 1995, 2006), by applying the P_Sequence function with the following parameters: k0=0.75, log⁡10(k/k0)=1 and interpolation=0.5cm (Bronk Ramsey, 2008; Bronk Ramsey and Lee, 2013). The IntCal20 (Reimer et al., 2020) 14C atmospheric curve was used as the calibration set. Dates Poz-128079 and Poz-128073 were distinctly younger than the neighbouring dates. Preliminary model runs, both with the full set of dates included, and with only one problematic date excluded, showed that these two ages had very poor agreement with the model. Therefore, both were excluded from the final age-depth model. The sections of the profile in which peat properties point to potential changes in peat accumulation rate (ARpeat) were introduced to the model as boundaries (Boundary command). These were placed at depths: (i) 1230 cm – the base of the model, (ii) 1130 cm – transition from higher water tables towards Sphagnum peat, retrieved from low-resolution palynological data (data not published) and plant macrofossils (iii) 0 cm – top of the profile.

In the following sections of this article, μ (mean) values retrieved from the age-depth model were rounded to the nearest 50 years. The age was expressed as cal BP (before present).

Plant macrofossils were analysed in subsamples (volume: 2–27 cm3) with a 5–10 cm resolution from depths 2.5–1187.5 cm. The analysed volume depended on the available sediment. The section 1187.5–1230 cm was not analysed to keep these data consistent with testate amoebae as below 1187.5 cm testate amoebae were not present in significant numbers. Samples were sieved through a 125 µm mesh and disaggregated using tap water without chemical pre-treatment. The residue on the sieve was observed under a stereomicroscope (Olympus SZX9) and a light microscope (Nikon Eclipse 90i). Quadrat and Leaf Count protocols (Barber et al., 1994; Mauquoy et al., 2010) were used to estimate the peat components and Sphagnum species compositions in percentages. When total Sphagnum was ≤5% of the sample then the Sphagnum species were not identified. Seeds, bud scales, catkin scales etc. were counted as absolute numbers and calculated as concentrations for 5 cm3. During Sphagnum identification the branch leaves having most similarity to S. angustifolium, S. balticum and S. fallax were marked down as “Sphagnum recurvum complex” (Duffy et al., 2020). Stem leaves of all these species were found and counted separately. Other species that are difficult to identify in the fossil form include S. divinum/medium and S. rubellum/fuscum, thus they are combined in the macrofossil diagram. The presence of S. fuscum in Sphagnum section Acutifolia-dominated samples was confirmed with stem leaves and dark stem colour. Sphagnum identification followed (Laine et al., 2018) and a reference collection. Brown mosses and vascular plant remains were identified using several keys (Cappers et al., 2012; Grosse-Brauckmann, 1972; Katz et al., 1977; Mauquoy and van Geel, 2007; Tomlinson, 1985) and a reference collection. Vascular plant and bryophyte nomenclature follows GBIF Backbone Taxonomy (GBIF Secretariat, 2023).

In addition to botanical remains, the percentages of unidentified organic matter (UOM) and macrocharcoal (≥1mm) were estimated in the samples as a part of the Quadrat method.

Testate amoebae were analysed in ca. 5 cm3 subsamples with a 5 cm resolution from depths 0–1187.5 cm. Samples were washed under a 300 µm mesh following the method described by Booth et al. (2010). Testate amoebae were analysed under a light microscope with ×200 and ×400 magnifications until a sum of 150 tests per sample was reached (Payne and Mitchell, 2009). In a few layers where the concentration of individuals was lower, testate amoebae were counted until a sum of 100 or 50 was reached; these were still suitable for performing further statistical analyses (Payne and Mitchell, 2009). To achieve the highest taxonomic resolution, species identifications were supported by several taxonomic keys and monographs (Clarke, 2003; Mazei and Tsyganov, 2006; Meisterfeld, 2000, 2001; Ogden and Hedley, 1980), and online resources (Siemensma, 2024). The results were used to reconstruct palaeohydrological conditions at the site.

The stratigraphical diagrams were created in Tilia v. 3.0.1 (Grimm, 2011) or C2 v. 1.8.0. (Juggins, 2007) and edited in Adobe Illustrator (Adobe Inc., 2019). Subfossil bryophytes were used as a main proxy of this article, thus the changes on the peatland are based on the changes in bryophyte assemblages. Sphagnum leaf count data and the abundance of other mosses were extracted from the general plant macrofossil data, and the percentages were re-calculated considering the taxonomic proportions of bryophytes only. These bryophyte data were analysed in RStudio (R Core Team, 2024) to create a stratigraphic zonation. The vegdist function from the vegan package (Oksanen et al., 2001) was used to compute a Bray-Curtis dissimilarity matrix from the bryophyte data, then hierarchical clustering was performed on the dissimilarity matrix using the constrained incremental sum of squares (CONISS) from the rioja package (Grimm, 1987; Juggins, 2009). The broken-stick method was used to determine the number of meaningful clusters (Bennett, 1996). Initially 11 clusters were identified. However, cluster 8 fell below the reference line (Fig. A1), Thus, cluster 8 was merged with cluster 7, resulting in a final solution of 10 clusters. In addition, bryophytes were grouped according to their hydrological microhabitat preferences (Table A1). The percentages of wet microhabitat species were transformed to negative values following the example of Piilo et al. (2023), so that the summed taxon percentages yield a bryophyte-based moisture curve. The results of a testate amoebae analysis were used for the quantitative depth-to-water table (DWT) reconstruction, which was performed in RStudio (R Core Team, 2024) using the Pan-European training set (Amesbury et al., 2016). Testate amoebae and bryophyte species composition (total 48) were also visualised as a Non-Metric Multidimensional Scaling (NMDS) ordination plot across different samples (total 178) using RStudio and the metaMDS function in the vegan package. To calculate dissimilarity between sites, the Bray-Curtis distance metric was used. For illustrating disturbances to the peatland, the abundance of Sphagnum and the sum of abundances of Archerella flavum and Hyalosphenia papilio species were plotted. These testate amoebae species were selected as they were dominant in the Linje core and considered as indicators of wet and stable conditions on peatlands (Łuców et al., 2022; Marcisz et al., 2014a). Thus, the drop in their abundance would suggest a disturbance.

The age-depth model (Fig. 2) recorded a model agreement (Amodel) equal to 66.7 %, which is above the recommended minimum of 60 % (Bronk Ramsey, 2008). The profile spans the time period between ∼-69 cal BP (2019 CE) and ∼11710±139 (1σ error) cal BP. The mean 1σ error was ca. 65 cal years. For the topmost 100 cm we lack the chronological control to detect short-term differences in accumulation rates. This section should thus be interpreted with caution due to this limitation as it is based on one date and it does not reflect an exponential age-depth relationship as it should with well-developed acrotelm and catotelm layers (Marcisz et al., 2015).

Ten zones were identified based on statistically significant changes in the bryological assemblages (Figs. A1 and 3). These zones, along with general plant macrofossil dynamics and testate amoebae assemblages (Figs. 4 and 5), are discussed in detail below. Depth to water table (DWT) reconstructions are derived from testate amoebae analyses.

The Linje peatland macrofossil record (Fig. 4) is abundant in bryophytes, which constitute the largest proportion of peat-forming components. At the beginning of the study's timeline, the moss assemblage (Fig. 3) reflected high water levels, nutrient availability, and elevated pH. After ∼11200 cal BP, brown mosses were gradually replaced by Sphagnum species, coinciding with the first appearances of typical oligotrophic mire plants such as Andromeda polifolia, Oxycoccus sp., and Drosera spp. Around 10 350 cal BP, a decline in pH and nutrient levels is evident as species from the Sphagnum recurvum complex established dominance, which persisted until ∼8200 cal BP, when Sphagnum divinum/medium became the prevailing taxa. Approximately 400 years later (∼7500 cal BP), Sphagnum sect. Acutifolia began to dominate until ∼5500 cal BP, when the interplay between S. divinum/medium and Sphagnum recurvum complex species began. Sphagnum sect. Acutifolia experienced only a brief resurgence ∼600–450 cal BP before being replaced by Sphagnum subgen. Cuspidata species, which have remained dominant to the present.

From the plant macrofossil record it is evident that birch (Betula pubescens/pendula) was already growing near Linje peatland ∼11500 cal BP. Pinus sylvestris macrofossils were first found in the record at ∼11200 cal BP. Wetter periods are marked by increased abundances of Equisetum fluviatile, Carex spp. and Scheuchzeria palustris macrofossils, whereas drier intervals are sometimes characterized by a higher proportion of ericaceous remains.

Testate amoebae-based water table reconstruction indicates that over the past 11 300 years, the average depth to the water table was 7.6 cm, increasing to a maximum of 15 cm during the Middle Holocene (Fig. 5). Depths exceeding 16 cm occurred only in the past ∼200 years. Since the beginning of the record, the dominant testate amoebae species at the site have been A. flavum and H. papilio. After ∼7500 cal BP declines in the abundance of A. flavum and H. papilio closely correspond with decreases in Sphagnum abundance. The two proxies exhibit strong general agreement in hydrological interpretation, with the exception of the last 600 years.

In this study, “disturbance” refers to phases of hydrological instability. At Linje, these are marked by elevated proportions of unidentified organic material (UOM), reduced test counts, and declines in dominant testate amoebae taxa (A. flavum, H. papilio), as well as reductions in Sphagnum abundance once the peatland became Sphagnum-dominated after ∼9050 cal BP. Sudden peaks in UOM are interpreted as reflecting periods of increased peat decomposition, which are associated with peat surface drying in otherwise stable peatland systems (Loisel et al., 2017; Sim et al., 2021b). Disturbance episodes are also associated with higher abundances of testate amoebae species such as Galeripora discoides, Assulina muscorum, and Assulina seminulum, which are linked to either drier conditions (Assulina spp.) or fluctuating water tables (G. discoides) (Lamentowicz et al., 2008b). Despite multiple disturbance episodes, major testate amoebae species turnover is absent until a shift ∼200 years ago. Importantly, these disturbances were relatively short-lived, and peat accumulation resumed quickly, resulting in a stable and continuous age-depth model.

Peat formation in Linje mire began in the Early Holocene resulting from the short-term terrestrialization of a depression left by a dead ice block (Kloss, 2005; Kloss and Żurek, 2005). As the accumulation of peat started with brown mosses, there is a possibility that they initially grew on deposits overlying the buried ice block (Słowiński et al., 2015). Given that some dead ice blocks in northern Poland persisted until the Early Holocene if meltwater drainage was sufficient (Błaszkiewicz et al., 2015; Słowiński, 2010), it is possible that Linje depression was able to actively accumulate peat since the beginning of Holocene. In Linje, the sediments beneath the peat deposits consist of sand and gravel (Kloss and Żurek, 2005), suggesting that conditions for delayed melting may have been present. The dead ice should be marked by the presence of basal peat overlain by lacustrine sediments. These were not detected in the Linje profile. The layer of brown mosses suggests the development of a wet rich fen (with phases of short-term standing water) on the bottom of the site (Figs. 3 and 4). Therefore, this is not a typical situation as seen in other kettle-hole peatlands in Pomerania. Most kettle hole peatlands, in e.g. Tuchola Forest, possess brown moss peat under gyttja (Kowalewski, 2014; Lamentowicz, 2005). To sum up there are many pathways of so-called kettle-hole peatlands development (Brande et al., 1990; Succow and Joonsten, 2012; Timmermann, 1999, 2003, 2010; Timmermann and Succow, 2001) and there is a high uncertainty that dead ice played a role in the Linje basin formation. According to Żurek (2005) the dead ice block at Linje mire melted during the Allerød, as evidenced by basal brown moss peat dating. However, AMS 14C dates from our study (Fig. 2., Table 1), suggest that the onset of peat accumulation during the Allerød is questionable, as these previous dates (Kloss, 2005; Kloss and Żurek, 2005) were carried out from bulk peat so they might have been affected by reservoir effect i.e. revealing much older age, as brown mosses such as Drepanocladus sp. may also absorb old HCO3- anions (Madeja and Latowski, 2008).

The transition from rich fen to poor fen in the centre of the peatland began ∼ 10 900 cal BP, as E. vaginatum began to form a suitable base for Sphagnum mosses to establish (Hughes and Dumayne-Peaty, 2002). A prerequisite for this process was the lowering/fluctuations of the water table during zone 3. The vegetation between ∼10900 and ∼ 8350 cal BP is already suggesting a decrease in nutrients and increase in acidity. Peatland water table fluctuations in these early successional stages can most likely be attributed to autogenic peatland processes (Hughes et al., 2000).

The period between ∼ 9000–∼ 8000 cal BP is recognized as one of the periods of RCC and it is expressed as a cooling in the Northen Hemisphere (Mayewski et al., 2004). During this period pioneering oligotrophic Sphagnum communities became established in Linje, making the record more sensitive to climate fluctuations (Hughes and Barber, 2003). Between ∼8500 and ∼ 8350 cal BP samples from Linje show high decomposition, with a large proportion of UOM and an absence of Sphagnum. This suggests water table lowering, which aerated the acrotelm and accelerated decomposition. Concurrently, testate amoebae indicative of stable wet conditions show a decrease in abundance (Fig. 7c). This disturbance may be linked to the Early Holocene Thermal Maximum (Väliranta et al., 2015), that Zander et al. (2024a) have described as a period of maximum summer warmth in NE Poland between 8500 and 8100 cal BP (Fig. 7e). However, chironomid-based summer temperature reconstructions from Central Poland (170 km southeast of Linje) suggest slight cooling during this period (Kotrys et al., 2020) (Fig. 7d). Linje record does not resolve this apparent contradiction, owing to poor sample preservation during this period.

When Sphagnum recolonized after the disturbance (∼8200 cal BP), the species composition in Linje shifted from wetter microform species to those indicative of slightly drier conditions. This timing aligns with the well-documented 8.2 ka event, with strong evidence from multiple independent proxies across the Northern Hemisphere, including pollen-based reconstructions (Seppä et al., 2007; Veski et al., 2004), stable isotopes and geochemical analyses from lake sediments, including varves (Hammarlund, 2003; Fiłoc et al., 2017; Zander et al., 2024a), peat-based archives using testate amoebae and plant macrofossils (Gałka et al., 2014; Fletcher et al., 2024a), and stable peat isotopes (Daley et al., 2016). The hydrological response at the latitude of Linje and in Northern Europe is often associated with a drier climate during the 8.2 ka event (Magny et al., 2003, 2007), agreeing with the Linje record. At the same time the multi-proxy paleorecord of lake Suminko (NE Poland) shows a major lake-level rise between ∼ 8300 and ∼ 8100 cal BP (Pędziszewska et al., 2015). Overall, the period between ∼ 9000 and ∼ 8000 cal BP seems to be a time of hydrological instability in Linje peatland but the exact interpretation in relation to temperature, humidity or specific climate events remains uncertain because of the high degree of peat decomposition.

The Holocene Thermal Maximum (HTM) was a period of elevated temperatures in the Northern Hemisphere, with noticeable effects in Northern Europe, including high-altitude tree migration, glacier retreat, and increased evapotranspiration (Davis et al., 2003, and references therein; Fletcher et al., 2024b; Kaufman et al., 2020; Wanner et al., 2015). Temperature reconstructions from Lake Żabińskie indicate the HTM in Poland began and peaked ∼ 8500 cal BP, with stable warm conditions lasting until 4700 cal BP (Fig. 7e) (Zander et al., 2024a, b). However, in Linje, bryophyte and testate amoebae records indicate the driest conditions between ∼7600 and ∼ 6800 cal BP (Figs. 3–5, and 7a and b). Around 7600 cal BP, high macrocharcoal percentages suggest increased local fire activity, while dominant testate amoebae and Sphagnum mosses experienced a simultaneous dramatic decline between ∼ 7600 and 7350 cal BP (Figs. 4 and 7c).

Pollen records from northern Finland also indicate the warmest summer temperatures and lowest precipitation between ∼ 7900 and ∼ 5700 cal BP (Seppä and Birks, 2001). Similarly, evidence of increased dryness appears in reduced peat formation rates in southern Finland (Korhola, 1995) and lowered water tables in southern Sweden ∼ 7500–7300 cal BP (Digerfeldt, 1988). Plant macrofossil and pollen records from a Polish lake (∼320 km northeast of Linje) suggest a corresponding temperature peak ∼ ∼7750 cal BP (Gałka et al., 2014). Chironomid-based summer temperature reconstructions from Lake Żabieniec further confirm a temperature peak either ∼7760–7450 or ∼7450–7230 cal BP, depending on the method used (Kotrys et al., 2020), aligning with Linje's testate amoebae and bryophyte records (Fig. 7d).

The onset of drier conditions in Linje aligns with several paleoclimate records that indicate Holocene temperature peaks following the 8.2 ka cooling event (Fletcher et al., 2024b; Wanner et al., 2015). However, this dry phase does not correspond with multiple regional climate reconstructions. For example, the period 7600–7250 cal BP has been identified as a cold phase based on pollen and aquatic invertebrate data from lakes in northeastern Poland (Fiłoc et al., 2017). The time interval between ∼ 8000 and 6800 cal BP is described to be favourable for peat initiation in Walton moss in England (Hughes et al., 2000) and in southern Finland (Korhola, 1995), indicating increased effective precipitation. These inconsistencies highlight how atmospheric and oceanic circulation shifts during this period caused regional climate anomalies and variations in the timing of the HTM (Fletcher et al., 2024b). Cartapanis et al. (2022), further note that the Northern Hemisphere HTM, as inferred from terrestrial proxies, lasted ∼ 4000 years (8–4 ka), making precise synchronization across regions difficult. Additionally, it cannot be ruled out that the dry shift observed in Linje reflects a localized autogenic succession.

Drier conditions persisted in Linje until ∼5500 cal BP, when a sudden peak in UOM occurs, followed by a gradual wet shift indicated by the bryophytes (Figs. 3, 4, and 7b and c). This coincides with “Mid-Holocene Cooling”, characterized by glacier advances and North-Atlantic ice-rafting events in many regions between ∼ 6000 and ∼ 5000 cal BP (Fletcher et al., 2024b; Kobashi et al., 2017; Mayewski et al., 2004; Wanner et al., 2015). A strong cooling signal has been recorded in varved lake sediments in eastern Poland ∼ 5500–5200 cal BP (Pędziszewska et al., 2015). Lake Gościąż (central Poland) also shows a higher water table between ∼ 6000 and ∼ 5200 cal BP (Pazdur et al., 1995). Similarly, in Gązwa bog, a period of increased humidity begins ∼ 5750 cal BP (Gałka and Lamentowicz, 2014). Multiple paleorecords across Europe also indicate a shift to wetter conditions during this time (see Table 4 in Hughes et al., 2000). Additionally, a wet shift (∼ 5800–4800 cal BP) has been recognised from subfossil Sphagnum lipid biomarkers in Ireland (Jordan et al., 2017). However, not all records show a uniform pattern of increased moisture. In contrast, Stążki peatland experienced a period of hydrological instability and drier conditions ∼5400 cal BP (Gałka et al., 2013b) and ∼ 5600 cal BP, lake-level lowering is observed in lake Linówek (northeastern Poland) (Gałka et al., 2014).

Between ∼4200 and ∼ 4150 cal BP, Linje experienced a brief disturbance marked by hydrological instability (Fig. 7a–c). This event likely corresponds to the 4.2 ka climatic event, which is often underrepresented in peat (Roland et al., 2014) but displays broad temporal variability in other paleo-proxies (Geirsdóttir et al., 2019; McKay et al., 2024). The 4.2 ka event is generally expressed as cooling or drought in the Northern Hemisphere (Yan and Liu, 2019). Hydrological instability or drought has also been recorded in other Polish peatlands coinciding with the 4.2 ka event (Gałka et al., 2013b; Lamentowicz et al., 2019b). A lake water decrease that resulted in a terrestrialization was also recorded ∼4150 cal BP in Rąbień mire (Central Poland) (Słowiński et al., 2016).

Between ∼3500 and ∼ 2500 cal BP Sphagnum and testate amoebae assemblages indicate a period of rapid hydrological fluctuations in Linje, agreeing with a RCC interval identified by Mayewski et al. (2004). Following this disturbance of the 4.2 ka event, the Sphagnum composition in Linje shifted to low-lawn and hollow species. Wetter conditions prevailed until ∼3250 cal BP, followed by a drier phase ∼3050 cal BP, when A. flavum and H. papilio abundance drops drastically (Fig. 7c). Moisture levels increased again towards the end of this period but remained variable. These fluctuations likely reflect the complex climatic dynamics between the Bronze Age Cold Epoch, the Bronze Age Optimum (characterized by drought and disturbance ∼ 3050 cal BP), and the Iron Age Cold Epoch, or the 2.8 ka event (Gauld et al., 2024; Kobashi et al., 2017). The latter is a widely documented wet shift in Europe (Barber et al., 2004; Mauquoy et al., 2008), including in Polish peat archives: in Kusowskie Bagno ∼2700 cal BP (Lamentowicz et al., 2015) and in Głęboczek ∼2800–2600 cal BP (Lamentowicz et al., 2019b). However, this pattern contrasts with the findings of Słowiński et al. (2016), who suggest that the 2.8 ka event in Europe followed a wet-to-dry gradient from west to east, with some Polish sites experiencing drier conditions. The hydrological response in Linje follows the pattern observed in Western Europe.

Another notable disturbance appears ∼ 2000 cal BP in plant and testate amoebae records (Fig. 7c), coinciding with the Roman Warm Period (Gauld et al., 2024), which is reflected in two Northern Ireland peatlands as drought episodes over an extended interval (Swindles et al., 2010). Similarly, lake sediment records from Northern Europe suggest an increase in dry conditions ∼ 2000 cal BP (Seppä et al., 2009). Pollen records shows that human impact in the vicinity of Linje during ∼2000 cal BP was low (Marcisz et al., 2015). However, human impact to the forest surrounding Linje is clearly visible in the pollen record between ∼3000 and ∼2500 cal BP (Szambelan et al., 2025), thus it is difficult to disentangle whether the observed disturbance is climate-driven or caused by human activity.

The period 1200–1000 cal BP is marked as a global RCC interval (Mayewski et al., 2004). Marcisz et al. (2015) have recorded a strong wet shift coinciding with this period, however in this work conditions are wet according to all proxies, but no major shifts or events are visible (Fig. 7a–c).

A relatively brief (around 200 years) dry period occurred in Linje between ∼600 and ∼ 450 cal BP characterised by changes in the Sphagnum composition in this section of the record. This shift can be associated with the climate fluctuations of the Little Ice Age (LIA) (Gauld et al., 2024; Marcisz et al., 2015, 2020a; Mauquoy et al., 2002). From a nearby coring location in Linje, a rapid shift to dry conditions (water table drop from 0 to 25 cm) in the testate amoebae record was found especially ∼ 550 cal BP (1390–1425 CE) (Marcisz et al., 2015), agreeing with the dry shift found in mosses in this work. Contradictory wet indicators, such as green algae, were also reported by Marcisz et al. (2015) and attributed to reduced evapotranspiration due to decreased forest cover and rapid climatic oscillations, causing temporary snowmelt excess. The dry phase in Linje coincides with hydrological instability recorded in peatlands from northern Poland, such as Słowińskie Błota (∼600 cal BP) (Lamentowicz et al., 2009b), ∼ 170 km northwest from Linje, Stążki (∼850–450 cal BP) (Lamentowicz et al., 2008b), located ∼ 140 km north from Linje and in Kusowskie Bagno (∼710–400 cal BP), a peatland ∼130 km northwest from Linje. These sites display high water table fluctuations, growth of E. vaginatum, and the disappearance of S. fuscum. In contrast, Linje exhibited persistent S. fuscum and S. rubellum growth, suggesting uninterrupted peat accumulation. Peatlands in central Poland might have had a different response as a multiproxy study from Żabieniec kettle hole contrastingly demonstrates a rapid wet shift at ∼600 cal BP (Lamentowicz et al., 2009a). In Saxnäs Mosse bog in Southern Sweden, the period of ∼675–360 cal BP records a cooling signal in the palynological data while the shifts in moss composition might indicate either a dry shift or an eutrophication signal (van der Linden and van Geel, 2006).

The drier period in Linje is interrupted ∼ 450 cal BP (1510 CE) when Sphagnum mosses indicate a sudden switch to wetter conditions. This coincides with the beginning of a prominent cooling during the LIA in Europe. The wet shift ∼ 450 cal BP is also observed in Stążki bog (Lamentowicz et al., 2008b), Żabieniec kettle hole (Lamentowicz et al., 2009a), Kusowskie Bagno (Lamentowicz et al., 2015), Finland (Väliranta et al., 2007) and Estonia (Sillasoo et al., 2007). However, the last 1000 years of climate records in Northern European peat show many alterations between dry and wet conditions that often do not align and are likely caused by differences in oceanic and continental climate (Marcisz et al., 2020b; Väliranta et al., 2007). The lack of spatial and temporal coherency during the LIA is also pointed out by Neukom et al. (2019) and possible climatic signals could be obscured by increasing human impact since early Medieval times resulting in deforestations and increasing openness affecting the hydrology of peatlands (Lamentowicz et al., 2009b). The early 16th-century wet phase may also reflect factors unrelated to climate. As Marcisz et al. (2015) point out, ∼430 cal BP (coinciding with the onset of the wet phase identified in this study) was marked by high fire activity in the Linje region, likely linked to military activities during the Polish-Teutonic war, suggesting that anthropogenic impact on the peatland was increasing.

Testate amoebae and bryophytes show contradicting information in the top 30 cm peat layer, which started ∼ 250 cal BP (1700 CE): testate amoebae show rapidly decreasing water tables and dry conditions whereas the Sphagnum taxa indicate wet, even waterlogged conditions (Figs. 3–6, and 7a–c). During these shifts, human impact in the vicinity of Linje was increasing, culminating with drainage in the 19th century CE, resulting in destabilized hydrology (Marcisz et al., 2015). Marcisz et al. (2014b) has found that the current testate amoebae communities in Linje differ significantly in spring and summer across all microforms. Thus, it is possible that the drought-resistant testate amoebae species recorded in the top layer reflect summer droughts that intensified after the drainage whereas plant macrofossils reflect the seasonal inundation and post-drainage peat subsidence. Current observations confirm that Linje's surface reacts sensitively to meteorological fluctuations due to drainage (Słowińska et al., 2022). The recent drying trend at Linje is consistent with patterns documented across many European peatlands (Swindles et al., 2019). The increased presence of S. fallax on the peatland might indicate a change in peatland chemistry, as S. fallax is more tolerant of nutrient inputs compared to the other Sphagnum mosses and the expansion of this species is connected to increased N deposition to nutrient-poor peatlands (Gąbka and Lamentowicz, 2008; Limpens et al., 2003). This change could be further investigated using geochemical analyses.

Linje lies within a forest surrounded depression, that produces a distinctive microclimate compared to adjacent areas, with lower air temperatures and increased ground frost days (Słowińska et al., 2022). Although the forest cover surrounding the peatland has been dynamic (Marcisz et al., 2015; Szambelan et al., 2025), it is likely that during the formation of the peat layer in the depression, the site experienced a unique microclimate as well. Consequently, the peat profile's hydrological signals likely reflect both broader climate trends and local microclimatic conditions.

In addition to microclimate, peatland type should be considered while talking about climate sensitivity. Although we found in this study that the centre of Linje has been Sphagnum-dominated since ∼ 9050 cal BP, it should be considered that the peatland is not ombrotrophic as groundwater incursion influences its western side (Słowińska et al., 2010). This groundwater input cannot be overlooked, as it may have impacted the recorded hydrological shifts. However, other Sphagnum-dominated peatlands, beyond ombrotrophic bogs, have been recognized as valuable climate archives (Booth, 2010; Lamentowicz et al., 2008c). To know whether the shifts observed in Linje are climate-driven or products of internal peatland processes, it is essential to identify synchronous changes in other regional records (Hughes et al., 2000; Swindles et al., 2012), which is the case in Linje: after the peatland became Sphagnum-dominant, bryophyte species turnovers, the decreases in Sphagnum cover or dominant testate amoebae species closely coincide with major Holocene climatic events (Fig. 7).

One peat profile can provide detailed information on bog vegetation at a microform scale (Mauquoy and Yeloff, 2008). As observed by Słowińska et al. (2022), the microclimatic conditions within the microsites in Linje exhibit considerable variability. Ideally multiple cores from the same site could be used to reconstruct mesoscale ecohydrological changes through time (Barber et al., 1998; Mauquoy and Yeloff, 2008).

Among previous studies, the work by Marcisz et al. (2015), based on a nearby coring location within the same vegetation zone, is most directly comparable to this study. Covering the last 2000 years, their reconstruction of the upper 2 m of peat shows strong agreement with our results: showing a progression from wet to waterlogged conditions, followed by a brief dry phase, and culminating with hydrologically unstable conditions in the uppermost layer.

Earlier paleoecological studies at Linje offer valuable points of comparison, despite differences in methodology and data resolution. The stratigraphy outlined by Kloss and Żurek (2005) broadly agrees with the succession in the vegetation described in this work. From the same core, Kloss (2005) further described hydrological transformations in Linje based on a phytocoenotic moisture index, which presents a moisture gradient ranging from moderately wet to heavily waterlogged. This hydrological interpretation agrees with water table changes captured by testate amoebae in this study as it shows rather wet conditions throughout the Holocene. Moreover, a decrease in moisture ∼ 400 cal BP was also recorded in the work of Kloss (2005). However, a distinctly drier episode around the Middle Holocene is not recorded in this earlier paleohydrological work. The discrepancy could be attributed to the different resolution and methods employed for plant macrofossil analysis in Kloss's work (2005). Additionally, the analysed core in the work of Kloss (2005) is closer to the margin of the peatland (Kucharski and Kloss, 2005) located about 50 m southward from the coring site of this study.