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In this study, we present a record spanning the last 4000 years from a Baltic bog (Kusowskie Bagno) in northern Poland. Using numerous biotic and abiotic proxies, such as testate amoebae (depth to water table reconstructions), stable carbon isotopes ( 13 C), plant macrofossils (proxies for local vegetation and mire surface wetness), pollen and spores (proxies for regional vegetation and human impact), we reconstructed and identified the regional hydro-climatic signal of Kusowskie Bagno bog and compared it to other bog records around the Baltic Sea. Our aims were to: 1) combine the species traits of bryophytes and testate amoebae, and more common proxies (isotopes, plant micro-and macro-remains) to infer past peatland development, 2) compare the hydroclimatic signal of Kusowskie Bagno bog to existing records around the Baltic Sea. We found that Kusowskie Bagno bog was very wet during the last 4000 years, and even drainage and peat exploitation had not disturbed its hydrology in northern part in the last 200 years. Carbon isotopes and plant macrofossils were significantly related to specific traits of testate amoebae, which in turn reflected the water table changes over the last 4000 years. Kusowskie Bagno recorded at least the following wet shifts: AD 250, 550, 850, 1250 and 1700, while wet conditions occurred during the Migration period at ca AD 550. Furthermore, the testate amoeba-based quantitative wetness reconstruction in Kusowskie Bagno bog resembles the pattern observed in other sites around the Baltic, i.e., Estonia, Finland, Ireland, northern Britain and the 7500-year record from the Stążki bog, northern Poland. Our results provided statistically validated evidence that interactions between plant and microbe need to be more considered further to reconstruct past hydrological. This is the first study of past hydro-climatic changes in peatlands based upon a trait-based approach.

This high‐resolution, multiproxy, palaeoenvironmental study of the Słowińskie Błota raised bog in N Poland, 10 km from the Baltic Sea, covering the last 1200 years reveals different aspects of environmental change in a range of spatial scales from local to regional. Testate amoebae allowed quantitative reconstruction of the local water table using a transfer function based on a training set from N and W Poland. Special attention is paid to the testate amoeba Arcella discoides, which responds to rapid water‐table fluctuations more than to average surface wetness. Macrofossils supported by local pollen tracked the local vegetation dynamics caused by local human impact and disturbance, including nutrients. Regional pollen showed human‐induced landscape change outside the bog. Tree rings of Pinus sylvestris reflected the history of tree establishment and desiccation of the bog. Strong correlations between DCA axes 1 of regional pollen, of macrofossils and of testate amoebae, and a testate‐amoebae‐based water‐table reconstruction that excludes A. discoides, indicate that changes on all spatial scales are linked, which is explained by a strong hydrologic connection between bog and surroundings. The combination of proxies shows that groundwater levels were modified by both human impact and climate change.

We studied the vegetation, testate amoebae and abiotic variables (depth of the water table, pH, electrical conductivity, Ca and Mg concentrations of water extracted from mosses) along the bog to extremely rich fen gradient in sub-alpine peatlands of the Upper Engadine (Swiss Alps). Testate amoeba diversity was correlated to that of mosses but not of vascular plants. Diversity peaked in rich fen for testate amoebae and in extremely rich fen for mosses, while for testate amoebae and mosses it was lowest in bog but for vascular plants in extremely rich fen. Multiple factor and redundancy analyses (RDA) revealed a stronger correlation of testate amoebae than of vegetation to water table and hydrochemical variables and relatively strong correlation between testate amoeba and moss community data. In RDA, hydrochemical variables explained a higher proportion of the testate amoeba and moss data than water table depth. Abiotic variables explained a higher percentage of the species data for testate amoebae (30.3% or 19.5% for binary data) than for mosses (13.4%) and vascular plants (10%). These results show that (1) vascular plant, moss and testate amoeba communities respond differently to ecological gradients in peatlands and (2) testate amoebae are more strongly related than vascular plants to the abiotic factors at the mire surface. These differences are related to vertical trophic gradients and associated niche differentiation.

Northumbria University has developed Northumbria Research Link (NRL) to enable users to access the University's research output. Copyright © and moral rights for items on NRL are retained by the individual author(s) and/or other copyright owners. Single copies of full items can be reproduced, displayed or performed, and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided the authors, title and full bibliographic details are given, as well as a hyperlink and/or URL to the original metadata page. The content must not be changed in any way. Full items must not be sold commercially in any format or medium without formal permission of the copyright holder. The full policy is available online: http://nrl.northumbria.ac.uk/policies.html This document may differ from the final, published version of the research and has been made available online in accordance with publisher policies. To read and/or cite from the published version of the research, please visit the publisher's website (a subscription may be required.) In this paper we present results of high-resolution multi-proxy palaeoecological 28 studies of two parallel peat cores from the Baltic raised bog Mechacz Wielki in NE Poland. 29We aim to disentangle the effect of the regional climate from the autogenic processes of the 30 raised bog itself in driving the vegetation and hydrology dynamics. Based on partly 31 synchronous changes in Sphagnum communities in the two study cores we suggest that 32 extrinsic factors (climate) played an important role as a driver in mire development during the 33 bog stage (AD 500-2012). Using testate amoebae transfer function, we found that the bog 34 revealed exceptionally stable hydrological conditions during the last 2k with a relatively high 35 water table and lack of local fire events that allowed for rapid peat accumulation (2.75 36 mm/year) in the bog. Further, the strong correlation between pH and community-weighted 37 mean of testate amoeba traits suggests that other variables than water-table depth play a role 38 in driving microbial properties under stable hydrological conditions. There is a difference in 39 hydrological dynamics in bogs between NW and NE Poland until ca CE 1500, after which the 40 water table reconstructions possess more similarities. The most different case is Linje mire, 41 which most probably driven by land-use changes partly coinciding with the Little Ice Age. 42Our results show how various functional traits relate to different environmental variables in a 43 range of trophic and hydrological scenarios on long time scales. Moreover, our data suggest a 44 common regional climatic forcing in Mechacz Wielki, Gązwa and Kontolanrahka. Though it 45 may still be too early to attempt a regional summary of wetness change in the southern Baltic 46 region, this study is a next step to the long-term perspective of peatland palaeohydrology in 47Europe. 48 49 3

a b s t r a c tWe present a record of peatland development during the last 1000 years from Mauntschas mire in the eastern Swiss Alps (Upper Engadine valley; 1818 m a.s.l.) inferred from testate amoebae (pH and depth to the water table (DWT) reconstructions), stable oxygen isotopes in Sphagnum (d C correlates with DWT only during AD 1000e1570. Part of this apparent instability among the four time series might be attributed to shifts in the local mire conditions which potentially formed very different (non-analogue) habitats. Lack of analogues, caused, for example, by pre-industrial human impact, might have introduced artefacts in the reconstructions, since those habitats are not well represented in some proxy transfer functions. Human impact was probably the main factor for peatland development, distorting most of the climate signals.

45 46Transfer functions are widely used in palaeoecology to infer past environmental 47conditions from fossil remains of many groups of organisms. In contrast to 48 traditional training-set design with one observation per site, some training sets, 49 including those for peatland testate amoeba-hydrology transfer functions, have a 50 clustered structure with many observations from each site. Here we show that this 51 clustered design causes standard performance statistics to be overly optimistic. 52Model performance when applied to independent data sets is considerably weaker 53 than suggested by statistical cross-validation. We discuss the reasons for these 54 problems and describe leave-one-site-out cross-validation and the cluster bootstrap 55 as appropriate methods for clustered training sets. Using these methods we show 56 that the performance of most testate amoeba-hydrology transfer functions is worse 57 than previously assumed and reconstructions are more uncertain. One motivation for developing clustered training-sets is the presence within 74 each site of substantial environmental gradients, which may be large relative to the 75 differences between sites. This contrasts with the traditional one observation per 76 site training-set where typically the environmental variable (e.g. lake-pH) is assumed 77 to be spatially homogeneous at each site. Standard methods for assessing the 78 performance of transfer functions assume that the observations are independent 79 and are thus inappropriate for clustered data. Lack of independence between 80 observations, either because of spatial autocorrelation or a clustered design, will 81 cause performance statistics to be over-optimistic (Telford and Birks, 2005 (Fig. 1). 97To provide an independent estimate of transfer function performance, we 98 apply five transfer functions to all comparable independent datasets with 99 appropriate corrections for taxonomic and methodological differences (Appendix I). 100 Table 1 shows that most transfer functions perform worse than suggested by leave-101 one-out (LOO) cross-validation when applied to independent data. Methodological 102 explanations for the poor model performance can largely be excluded. Differences in 103 time-discrete water-table measurements cannot explain the differences in rank-104 order shown by Spearman's ρ. Any differences in sample preparation and analysis, or 105 residual taxonomic biases cannot explain poor performance where these are closely 106harmonised (e.g. Polish data). Performance is particularly poor for two datasets from 107 Typically, transfer function model performance is assessed by either leave-117 one-out (LOO) or bootstrap cross-validation. In LOO, one observation at a time is 118 omitted from the training-set of size n and the environmental value predicted using 119 the remaining n-1 observations. For clustered data, this can be extended to leave-120 one-site-out cross-validation (LOSO), where data from one site is omitted from the 121 training set, and data from the remaining m...

Summary1. Plant community composition is recognized more and more for playing an important role in ecosystem processes, such as C cycling. In particular, plant functional type (PFT) composition seems to have a key regulatory role, yet the underlying mechanisms in the interaction between PFTs and ecosystem processes are still to be identified. 2. Here, we assess the link between PFTs and dominant microbial consumers along a calcareous poor to extremely rich fen gradient in western Poland. We particularly focussed on dominant microbial consumers (testate amoebae), which can exert large effects on the functioning of peatlands. Using moving-window analyses and path-relation networks subjected to structural equation modelling (SEM), we investigated linkages among abiotic factors, PFTs and testate amoebae. 3. We show that along the poor to extremely rich fen gradient, the dependence of testate amoebae to PFTs is higher than their dependence to abiotic factors. We also found that the link between testate amoebae and PFTs differs between size assemblages of testate amoebae. While large testate amoeba species (i.e. high trophic level) were highly linked to Sphagnum mosses cover, small species (i.e. low trophic level) were linked to brown mosses. Distinction between shallow-rooted and deeprooted vascular plants also showed that shallow-rooted plants play a role on testate amoeba community structure at the 'poor' side of the gradient. 4. Our results further show a dominant role for calcium content and the structure of the bryophyte community on testate amoeba size assemblages at the poor to extremely rich fen scale, both for diversity and abundance of testate amoebae. 5. Synthesis. Variations in plant functional type composition drive niche-size-structure of testate amoebae along the (calcareous) poor to extremely rich fen gradient. Furthermore, strong relationships between moss types and testate amoeba size-structure suggest that mosses specifically influence testate amoeba development through autogenic effects. Therefore, moss cover composition is key to microbial consumers and may be the driving factor determining microbial network structure and associated ecosystem processes, such as carbon cycling. ¶ V. E. J. Jassey and Ł. Lamentowicz contributed equally to the data extraction, statistical analyses and preparation of the manuscript and are therefore considered to be co-first authors.