# Lichen, moss and peat control of C, nutrient and trace metal regime in lakes of permafrost peatlands

Liudmila Shirokova, Artem Chupakov, Irina Ivanova, Olga Moreva, Svetlana

Zabelina, Nikita Shutskiy, Sergey Loiko, Oleg Pokrovsky

► To cite this version:

Liudmila Shirokova, Artem Chupakov, Irina Ivanova, Olga Moreva, Svetlana Zabelina, et al. Lichen, moss and peat control of C, nutrient and trace metal regime in lakes of permafrost peatlands. Science of the Total Environment, 2021, 782, pp.146737. 10.1016/j.scitotenv.2021.146737. hal-04842414

HAL Id: hal-04842414

https://ut3-toulouseinp.hal.science/hal-04842414v1

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Lichen, moss and peat control of C, nutrient and trace metal regime
in lakes of permafrost peatlands

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Liudmila S. Shirokova¹,², Artem V. Chupakov², Irina S. Ivanova³, Olga Y. Moreva²,

Svetlana A. Zabelina², Nikita A. Shutskiy⁴, Sergey V. Loiko⁵, Oleg S. Pokrovsky¹*

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¹ GET (Géosciences Environnement Toulouse) UMR 5563 CNRS; University of Toulouse,
14 Avenue Edouard Belin, 31400 Toulouse, France

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² Institute of Ecological Problems of the North, Federal Center of Arctic Research, 23

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Nab. Severnoi Dviny, Arkhangelsk, Russia

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³ Tomsk branch of the Trofimuk Institute of Petroleum Geology and Geophysics, SB RAS,
Tomsk, Akademichesky 4, 634055 Tomsk, Russia

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⁴ Lomonosov Northern (Arctic) Federal University, 17, Nab. Northern Dvina, 163002,

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Arkhangelsk, Russia

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⁵ BIO-GEO-CLIM Laboratory, Tomsk State University, 35 Lenina, Tomsk, Russia

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*Corresponding author. Email: oleg.pokrovsky@get.omp.eu

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Keywords: Organic carbon, CO₂, thermokarst, mesocosm, enclosure, vegetation, thaw
pond, micronutrient

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Submitted to *Science Total Environment*, after revision March 2021.

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33 **Abstract**

Permafrost thaw in continental lowlands produces large number of thermokarst (thaw) lakes, which act as a major regulator of carbon (C) storage in sediments and C emission in the atmosphere. Here we studied thaw lakes of the NE European permafrost peatlands - shallow water bodies located within frozen peat bogs and receiving the majority of their water input from lateral (surface) runoff. We also conducted mesocosm experiments via interacting lake waters with frozen peat and dominant ground vegetation - lichen and moss. There was a systematic decrease in concentrations of dissolved C, CO2, nutrients and metals with an increase in lake size, corresponding to temporal evolution of the water body and thermokarst development. We hypothesized that ground vegetation and frozen peat provide the majority of C, nutrients and inorganic solutes in the water column of these lakes, and that microbial processing of terrestrial organic matter controls the pattern of CO2 and nutrient concentrations in thermokarst lakes. Substrate mass-normalized C, nutrient (N, P, K), major and trace metal release was maximal in moss mesocosms. After first 16 h of reaction, the pCO2 increased ten-fold in mesocosms with moss and lichen; this increase was much less pronounced in experiments with permafrost peat. Overall, moss and lichen were the dominant factors controlling the enrichment of the lake water in organic C, nutrients, and trace metals and rising the CO2 concentration. The global significance of obtained results is that the changes in ground vegetation, rather than mere frozen peat thawing, may exert the primary control on C, major and trace element balance in aquatic ecosystems of tundra peatlands under climate warming scenario.

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57 **1. Introduction**

The surface waters of permafrost peatlands are known to exhibit strong CO2 oversaturation with respect to atmosphere and thus, yield sizeable CO2 emissions
60 (Laurion et al., 2010; Lundin et al., 2013; Roiha et al., 2015; Sepulveda-Jauregui et al., 61 2015; Vonk et al., 2015; Elder et al., 2018; Karlsson et al., 2021). These waters also 62 contain high Dissolved Organic Carbon (DOC) concentrations, rendering them important 63 reservoirs of C and related elements in Northern Hemisphere. Today, C biogeochemical 64 cycles in C-rich peatland waters from the permafrost zone are characterized in Canadian 65 Plane lakes (Negandhi et al., 2013; Bouchard et al., 2014; Matveev et al., 2016, 2019; 66 Arsenault et al., 2018; Wauthy et al., 2018), small bogs of northern Sweden (Roehm et al., 67 2009; Hodgkins et al., 2014, 2016), and most extensively, in western Siberian lowland 68 (WSL, Frey and Smith, 2005; Frey et al., 2007; Pokrovsky et al., 2011, 2013; Shirokova 69 et al., 2013, 2020; Manasypov et al., 2014, 2015; Loiko et al., 2017; Serikova et al., 70 2019).

71 Among various surface waters of permafrost landscapes, recently formed 72 thermokarst lakes are most interesting because they emit sizeable amount of CO2 and 73 CH4 to the atmosphere (Walter et al., 2008; Walter-Anthony et al., 2012, 2014; Hogdkins 74 et al., 2014; Ewing et al., 2015; Mann et al., 2015; Natali et al., 2015; Roiha et al., 2015; 75 Schuur et al., 2015; Wik et al., 2016; Serikova et al., 2019). Further, in case of flooding 76 or drainage, these lakes may deliver substantial amount of dissolved nutrients and metals 77 to adjacent hydrological network (Kokelj et al., 2009; Pokrovsky et al., 2011, 2016a, b). 78 The main interest to these shallow surface waters is in their location at the thawing 79 permafrost front, typically in the discontinuous and isolated permafrost zone, where the 80 soil temperature often fluctuates within the highly ‘fragile’ temperature range (0 to -2°C) 81 of the permafrost (Romanovsky et al., 2010). As a result, under on-going climate 82 warming and permafrost thaw, these waters may i) spread their surfaces much faster than 83 those in the region with mineral substrates and ii) become highly vulnerable to water 84 heating and heterotrophic respiration of dissolved and particulate organic matter (OM) 85 into CO2.
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The area coverage of thaw ponds and small depressions in permafrost-affected peatlands of Northern Eurasia is not fully quantified but may range from 5 to 20% of the territory which includes Western Siberia Lowland (WSL), Northern Siberian Lowland, Kolyma and Indigirka Lowland, of overall area > 2 million km² (Polishchuk et al., 2020; Zabelina et al., 2021). In Europe, the permafrost peatlands widely occur in the NE part of it, within the Bolshezemelskaya Tundra (200,000 km²), although biogeochemical parameters of related thermokarst lakes are quite poorly known.

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The main source of DOM in thaw ponds of frozen peatlands is leaching of C from allochthonous reservoirs of solid OM, such as peat and surrounding vegetation (mosses, lichens). The magnitude of this process is not yet quantified, and the relative importance of three main organic 'solid' substrates - moss, lichen and peat - in providing DOC, CO2 and nutrients to these thaw waters is not known. This, in turn, does not allow predicting the response of these aquatic ecosystems to the change of environmental conditions such as vegetation regime and increasing the active layer thickness (ALT) and the precipitation. The latter will likely bring about a surface drainage through litter layer rather than through underlaying mineral soils. To assess the capacity of ground vegetation and peat soil to modify the chemical and gaseous regime of thaw ponds, mere observations of natural environments are not sufficient and ecosystem (mesocosm-level) manipulations are needed.

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Indeed, the ecosystem manipulation or mesocosm experiments allow understanding the system behavior under various external impacts (i.e., Hering et al., 2015; de Rozari et al., 2016; Conroy et al., 2016). Although the mesocosm experiments were extensively used for boreal, permafrost- and peat-bearing aquatic landscapes (Bridgham et al., 1999; Judd and Kling, 2002; Blodau and Moore, 2003; Blodau et al., 2004; Basiliko et al., 2006; Balcarczyk et al., 2009; Green et al., 2014; Richardson et al., 2015; Nydahl et al., 2019), only one study (Manasypov et al., 2017) dealt with thermokarst lakes of permafrost peatlands. However, in their study, the pCO2 and
113 dissolved nutrient patterns were not assessed, and two important substrates contributing
to the lake water hydrochemical composition – moss and frozen peat – were not tested.
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115 In this work, we performed mesocosm experiments on isolated water volumes of
116 thermokarst thaw ponds and we characterized the change of chemical and gaseous (CO2)
117 parameters of pond water in response to the addition of peat, moss and lichen. We
118 hypothesized that peat will be an important source of chromophoric DOM (CDOM) and
119 nutrients, whereas a non-aromatic, bioavailable DOM and nutrients may be released from
120 moss and lichen. To test this hypothesis, we quantified the relative capacity of various
121 organic substrates to change the chemical composition (DOC, nutrients, metals, dissolved
122 CO2) of pond water over the time scale (10 to 20 days) comparable to the water residence
123 time in small thaw ponds. Further, we characterized the hydrochemical regime of small
124 thermokarst water bodies of poorly studied Bolshezemelskaya Tundra of NE Europe, and
125 we quantified the C, nutrient, and trace element regime concentration change in natural
126 ponds and lakes depending on their surface area (degree of maturity). We aimed at
127 comparing the pattern of dissolved CO2, DOC, nutrients and metals in natural sequence
128 of thermokarst lake evolution to that in the mesocosm manipulation experiments in order
129 to obtain quantitative constraints on future changes of thermokarst lake water chemical
130 composition in response to vegetation change and frozen peat thaw. Based on obtained
131 pattern of dissolved carbon, pCO2 and elements, we provided first-order predictions of
132 thermokarst lake water hydrochemistry change under various climate warming scenario.
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## 2. Study site, experiments and analyses

135 2.1. Physio-geographical context of thaw ponds and thermokarst lakes of the
136 Bolshezemelskaya Tundra

137 The BolsheZemelskaya Tundra (BZT) peatland is underlayed by permafrost,
138 discontinuous on the eastern part and sporadic to isolated on the western part. The BZT is
139 a hilly moraine lowland located between rivers Pechora and Usa (from the west and
south) and the Polar Ural and Pai-Khoi ridge from the east. The dominant altitudes are between 100 and 150 m, created by hills and moraine ridges, composed of sands and silt with boulders. Between the moraines and ridges there are many lakes, mostly of thermokarst origin. The dominant soils are Histosol of peat bogs and podzol-gleys in the south. The mean annual temperature is -3.1°C and the mean annual precipitation is 503 mm. The dominant vegetation of the tundra zone is mosses, lichens and dwarf shrubs. The peat (1.0-1.5 m thick) overlays Quaternary glacial deposits over Cretaceous sands, clays and silts. The dominant soil catenae are flat-mound bogs along the watershed divides, which comprise the Hemic Cryic Histosols on mounds and Fibric Gelic Histosols on troughs (the soil nomenclature is according to IUSS, 2014). The average thickness of the active layer ranges from 145±20 cm within the depressions to 41±5 cm at the peat mounds.

The thermokarst lakes are shallow (0.5 to 1.5 m) closed basins without distinct inlet and outlets, with watershed surface area smaller or comparable to the lake water area. They receive the surface inflow via lateral drainage of surrounding soil litter and ground vegetation and shallow subsoil water flow over the permafrost table, which is so-called suprapermafrost flow (Manasypov et al., 2015, 2017; Raudina et al., 2018). The early stage of lake formation includes soil subsidences, lichen and moss submergence, and peat abrasion (Audry et al., 2011; Kirpotin et al., 2011; Shirokova et al., 2013). It follows by lateral spreading of shallow water bodies, when the ratio of the lake circumference to the lake volume diminishes which leads to progressive decrease of allochthonous (terrestrial) organic matter input to the lake.

The leaching of surrounding organic substrates occurs via (1) rain and melt snow that provide a lateral surface input occurring over moss and lichen cover (Manasypov et al., 2015), (2) direct interaction of the lake water with wave- and thermokarst-abraded peat at the lake shore (Pokrovsky et al., 2011; Shirokova et al., 2013), and finally, (3) suprapermafrost water interaction with frozen peat layer during water travel from
167 elevated points of relief to the lake, over the permafrost table (Raudina et al., 2018).
168 Taking into account these three main processes supplying the C and inorganic solutes to
169 the thaw lakes, in the present study, we modelled the effect of three main organic
170 substrates – moss, lichen and frozen peat – interacting with thermokarst lake waters at
171 controlled mesocosm conditions.

172 To assess natural variability of lake solutes as a function of lake size and degree of
173 maturity, between July 14 and July 26 of 2016 and 2017, we sampled 29 stagnant isolated
174 surface waters located within upland frozen peat bog, including depressions (permafrost
175 subsidence), thaw ponds and thermokarst lakes (Fig. 1 A). Depressions in moss and
176 lichen cover are typically filled by water from thawing of ground ice and ranged in size
177 from 0.5 to 2 m² with an average depth between 0.1 and 0.3 m. A series of thaw ponds
178 (10 to 1000 m² size, 0.3 to 0.5 m depth), and thermokarst lakes (10³ to 10⁶ m², 0.5 to 1.5
179 m depth) represented several stages of thermokarst water body maturation, from youngest
180 small size ponds to large mature thermokarst lakes (Zabelina et al., 2021). These water
181 bodies are similar to the sequence of thermokarst lakes and ponds described in Northern
182 Europe (Matthews et al., 1997), Alaska (Hinkel et al., 2003) and western Siberia (Audry
183 et al., 2011; Kirpotin et al., 2011).

184 The surface waters were collected from the shore (depressions) or the PVC boat
185 (thaw ponds and thermokarst lakes). The water samples were filtered on-site using sterile
186 single use Sartorius filters (acetate cellulose, 0.45 µm) for analyses of cations and anions.
187 Unfiltered water samples for nutrient analyses were placed into 250-mL Milli-Q pre-
188 cleaned PVC bottles and kept refrigerated until arrival to the laboratory, within 2-3 h after
189 collection. Conceptual approach of field observations combined with mesocosm
190 experiments in this study is illustrated in Fig. 1 B.

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192 **2.2. Mesocosm experiments**
Closed-system mesocosms experiments were performed with thermokarst lake water (pH ~ 5.5) using three organic substrates that are likely to control the hydrochemistry of thermokarst lakes during ecosystem maturation. We selected a typical (6760 m², depth of 1.0±0.5 m) thermokarst lake with moderate DOC (15 mg/L) concentration. The lake was located in the palsa peat bog of Bolshezemelskaya Tundra, 40 km east of the Narian-Mar town (63°46′59″ N, 75°39′08″ E).

Peat from frozen layer of permafrost (50-70 cm from the surface which is 10 cm below the maximal active layer depth), moss and lichen constituting ca. 30% and 70% of peatland surface vegetation, were collected in the 100 m vicinity of thermokarst lake and dried at 40°C using Osmofilm® bags during 5 days. The oligotrophic peat was 35% decomposed and contained *Sphagnum angustifolium* (20%), cotton-grass *Eriophorum* (25%), herbs *Scheuchzeria palustris* (15%), bog-sedge (5%), lichens (5%), dwarf shrubs (5%) and some amount of wood debris (pine) and green mosses. The water was collected on 15 July 2017 from the surface (0-20 cm) of the lake and immediately pre-filtered through 100 µm nylon net to remove zooplankton, insects and large organic debris into 50-L PVC tanks. Duplicate tanks with lichen *Cladonia stellaris* (5.9 gdry/L), moss *Sphagnum palustris* (9.2 gdry/L) and peat (20 gdry/L) addition were closed with a 100 µm nylon net and placed directly in the lake ensuring their vertical orientation and no overflow of the lake water (Fig. 1 A). Additional aeration of incubated water was achieved through periodic opening of the tank and gentle stirring of the content with pre-cleaned plastic shovel. Control experiments included the same tanks filled with 100 µm-filtered lake water without addition of organic or substrate. The wind-induced wave movement in the lake provided sufficient agitation of the interior of the tank. The experiment lasted 16 days. The CO₂ analyses were run each 2-3 h during first day. The sampling for dissolved major and trace elements was performed after 1, 2, 4, 6, 9, 12 and 16 day of exposure in order to assess both short-term and long-term effect of added substrate on lake water hydrochemistry.
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## 2.3. Chemical analyses

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The water temperature, pH, dissolved oxygen, and electric conductivity in natural waters and mesocosm experiments were measured directly in the lake water column or inside the tank using Hanna and WTW portable instruments. Dissolved CO₂ concentration was measured using portable submersible Vaissala Carbocap® GM70 Hand-held carbon dioxide meter with GMP222 probes (accuracy 1.5%). Filtered waters of natural thaw ponds and lakes and of mesocosm samplings were processed using the analytical approaches employed by the GET Laboratory (Toulouse) to analyze low total dissolved solid (TDS), DOM-rich waters from boreal and permafrost-bearing settings (Pokrovsky et al., 2011, 2016 a, b; Shirokova et al., 2013). The major anion concentrations (Cl⁻ and SO₄²⁻) were analyzed by ion chromatography (Dionex 2000i), with an uncertainty of 2%. The DOC and Dissolved Inorganic Carbon (DIC) were determined by a Shimadzu TOC-VSCN Analyzer with an uncertainty of 3% and a detection limit of 0.1 mg/L. Specific UV absorbance (SUVA) was measured as absorbance at 254 nm normalized for DOC concentration in 1 mg⁻¹ m⁻¹). The major and trace elements were measured by quadrupole ICP-MS (7500ce, Agilent Technologies). Indium and rhenium were used as internal standards. The international geostandard SLRS-5 (Riverine Water Reference Material for Trace Metals, certified by the National Research Council of Canada) was used to check the validity and reproducibility of analyses. Good agreement existed between our replicated measurements of SLRS-5 and the certified values (relative difference < 15%).

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The nutrient analyses were based on colorimetric assays (Koroleff, 1983a, b). Total dissolved organic nitrogen (DON) was measured from the difference between the total dissolved nitrogen (persulfate oxidation) and the total dissolved inorganic nitrogen (DIN, or the sum of NH₄⁺, NO₂⁻ and NO₃⁻). Uncertainties of DON and DIN analyses were between 10 and 20 % and detections limits were between 10 and 50 µg/l. Si concentration
247 was measured by spectrophotometry with molybdate blue with an uncertainty of ± 5%
248 and a detection limit of 2 µg/L.

249 The total bacterial number and dominant cell size morphology were quantified
250 using DAPI fluorescence method (Porter and Feig, 1980). In addition, we performed
251 oligotrophic and eutrophic bacteria (OB and EB, respectively) count in the course of
252 mesocosm experiments, following the standard methodology used in biodegradation
253 experiments of peat waters (i.e., Stutter et al., 2013) as also described previously
254 (Shirokova et al., 2013, 2019; Chupakova et al., 2018).

255 **2.4. Quantification of element release from organic substrates**

256 In order to calculate the dynamics of element concentration and the amount of
257 element released in the mesocosms, we normalized the difference between element
258 concentration in the experiment and the control (lake water without added substrate) to
259 the concentration of dry substrate in the mesocosm. This allowed tracing the net amount
260 of element (i, mol L-1) released from the substrate (peat, moss or lichen) to the lake water
261 in the course of mesocosm experiment (Δi, mmol kg<sub>dry substrate</sub>-1). We estimated the
262 maximal rate (V<sub>max</sub>, mmol kg<sub>dry substrate</sub>-1 h-1) of element release from substrate as a derivative of
263 average maximal change of concentration over the first 24 h. We also quantified the mean
264 rate of element release (V<sub>mean</sub>) over the full duration of experiments, and the maximal
265 effect of concentration increase determined as the average of 2-3 neighboring
266 concentrations, typically after 100 h of reaction. The initial concentration used to trace
267 these dependences was the one measured 5-10 minutes following the addition of organic
268 substrate. The reason for this choice is that some elements could be present as loosely
269 bound on the surface of organic matter or within the remaining water of humid substrates,
270 notably moss and peat, and quickly released to the aqueous solution, immediately after
271 substrate addition to the reactors. The negative value of V<sub>max</sub> rate was indicative of
272 element concentration decrease in the course of experiments; in such case, the rates were
273 not quantified.
## 3. Results

### 3.1. Evolution of element concentration in the natural sequence of lake growth and maturation reflected in the thermokarst lake size

The evolution of major hydrochemical parameters in the course of lake maturation (increase in size) was characterized via a Mann Kendall test of probability at 90 and 95% level of significance (Fig. S1 of the Supplement). It is illustrated as a plot of concentration versus thaw pond and lake area for pH, pCO₂, DOC and DIC (Fig. 2 A-D). There was a systematic increase in pH (ca. 3 units) with lake size increase from 0.1 to 10⁵ m², whereas pCO₂ and DOC concentration decreased about ten times with lake growth and maturation, when the lake size increased over 6 orders of magnitude. The lowest pH values and the highest concentrations of DOC and pCO₂ were observed at the very beginning of thermokarst activity - in small soil depressions, subsidences, water puddles and thaw ponds (< 1-10 m²). The evolution of nutrient (N, P, Si), major cation (Ca) and metal (Fe, Al) concentrations over the lake maturation was less pronounced (Fig. 3). The change of pH value and element (Ci) concentration with lake area (S<sub>lake</sub>) can be described by a linear function:

$$[C_i] = A \times \log S_{\text{lake}} + B \quad (1)$$

with parameters A and B of the regression listed in Table S1. Considering the trend of element concentration with lake size increase, two main families of solutes were distinguished (Fig. S1). The value of Specific Conductivity (S.C.) and concentrations of eutrophic and oligotrophic bacteria, CO₂, DOC, nutrients (NO₂, NO₃, Si, Fe), and some trace elements (Al, Co, Cd, Pb, Sb, Cs, Tl) decreased with lake size; these components formed the first group of elements. The second group of components demonstrated significant (p < 0.05) increase in concentration with lake size: O₂, Li, Ca, K, Y, REE, As, Hf, U and the pH value. The other elements did not show any significant (p > 0.05, r <
0.3) correlation with lake size and the probability of their concentration increase or
decrease was below 90% (Fig. S1).

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303 **3.2. Mesocosmes of thermokarst lakes from peat palsa plateau**

The addition of organic substrates to the lake water caused progressive change of concentration of most dissolved components. The maximal element yield, mean and maximal rates of DOC, nutrient, bacteria, major component (Table 1) and trace element (Table S2) release were normalized to dry weight of organic substrate and averaged across replicates. These allowed to assess the degree of substrate effect on lake water enrichment (or impoverishment) by each element. For convenience, the element concentration evolution in the mesocosms and in the control enclosures are depicted for several representative major and trace components in Fig. S2 of the Supplement and specified by groups below.

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314 **3.2.1. Major parameters of lake water (pH, pCO2, carbon and bacteria).**

The pH of the lake water strongly decreased by ca. 1.5 units, immediately after
addition of all three organic substrates (Fig. 4 A). The peat addition produced the lowest
pH (ca. 3.75) whilst the moss and lichen addition yielded similar pH drop at the
beginning of experiments; the pH then remained stable for the full duration of exposure.
Specific conductivity steadily increased during experiments with mosses but did not
evolve in other treatments (Fig. 4 B). The DIC was an order of magnitude higher in
experiments with moss and lichen compared to peat and lake water control incubation
(Table 1). The oxygen regime was similar (80-100% saturation) between lake water
control and peat addition, but the O₂ level strongly decreased (ca., by a factor of 5 to 10)
after first 50-100 h of experiments with moss and lichen thus producing net negative ΔO₂
values (Fig. 4 D).
The pCO₂ pattern in experimental mesocosms demonstrated a 10-fold increase over the first 200 hours of lake water reaction with moss and lichen (Fig. 4 C; Fig. S2 B). The degree of pCO₂ increase relative to the control followed the order moss > lichen >> peat. However, when normalized to the amount of biomass used in mesocosm incubation, the moss, lichen, and peat released 19, 34 and 4.7 mmol CO₂ kg⁻¹, respectively. The DOC concentration was the highest in experiments with moss addition, rising 8 times, from 20 mg/L to 160 mg/L over 2 weeks of reaction (Fig. 4 E; S2 C). Lichen and peat addition produced similar, 2-3 fold increase in DOC over first 284 h of reaction. The mass-normalized DOC yield at the end of incubation was equal to 1500, 580, and 89 mmol DOC kg⁻¹ for moss, lichen and peat, respectively (Table 1).

The UV absorbency and SUVA₂₅₄ were drastically different among three substrates and evolved different to that of DOC (Fig. 4 F; S2 D). Peat and moss addition produced a 2-fold increase in absorbency whereas lichens did not affect the release of CDOM. The SUVA decreased after 5 days of reaction with moss and lichen but remained essentially constant in experiments with peat addition, when slightly positive rate increase was observed.

The concentration of eutrophic culturable bacteria steadily increased in moss and lichen mesocosms, but decreased after initial rise at 80-100 h in experiments with peat additions (Fig. S3 A). The number of oligotrophic bacteria significantly increased with time for all substrates (Fig. S3 B) and was ca. 1 to 2 orders of magnitude higher than that in lake water control run. The mesocosms with moss and lichen addition exhibited the highest concentration of cultivable bacteria. Consistent with cultivable (OB and EB) bacterial numbers, the total bacterial (TB) number measured by DAPI was the highest in moss and lichen treatments ((20-40)×10⁶ cell mL⁻¹ without systematic evolution), which was sizably lower in peat treatment ((1-2)×10⁶ cell mL⁻¹). The latter was comparable with TB in the lake water control (5-6)×10⁶ cell mL⁻¹.
353 **3.2.3. Nutrients (Si, P, N)**

354 Dissolved Si exhibited the minimal changes in the course of experiments with
355 stable concentrations achieved over the first day of reaction (Fig. 5 A, S2 H). The
356 biomass-normalized Si release was equal to 1.4, 0.97 and 0.26 mmol kg⁻¹ for moss, lichen
357 and peat treatments (Table 1). Total dissolved P was dominated (> 90%) by PO₄ whose
358 yield was orders of magnitude higher in moss and lichen mesocosms compared to peat
359 addition (Fig. 5 B). Total dissolved N was dominated by NO₃ which was generally stable
360 from the very beginning of experiment and did not systematically evolved over 380 h of
361 exposure (Fig. 5 C); mosses and lichens released significant amount of nitrate, unlike
362 peat. Addition of moss yielded strong increase in NH₄⁺ after 4 days of reaction however
363 the agreement between replicates was quite low (Fig. 5 D).

364

365 **3.2.3. Major and trace elements**

366 The final metal yield achieved after ~300 h of reaction for each substrate listed in
367 **Table S2** allowed distinguishing two main group of elements, depending on the moss or
368 peat control on their release in the mesocosms. Examples of major and trace elements are
369 shown in **Fig. S4**. Strong enrichment of the lake water in Li, Na, Mg, K, Fe, Cu, As, Rb,
370 Cs, Ga, Nb and Tl upon the moss addition persisted over full duration of experiments and
371 was significant (p < 0.0001) compared to peat and lichen treatments. Therefore, these
372 elements may serve as indicators of the dominance of moss leaching on thermokarst lake
373 water's chemistry. In contrast, high concentrations of lithogenic Be, Al, V, Ni, Y, heavy
374 REEs, Zr and Hf were observed in mesocosms with peat addition; these elements may be
375 indicative of the influence of peat on thermokarst lake water composition. Addition of
376 lichens enriched aqueous solution in Pb but removed Fe, Al, Ni, Ba, Y, Ga and Zr, as
377 supported by mean and maximal rates of element concentration increase after addition of
378 substrates (see **Table 1** and **Table S2**).

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380 **4. Discussion**

381 **4.1. Lake size control on C, nutrient and metal concentration in natural settings**

382 We observed gradual decrease of S.C., pCO₂, DOC, some nutrients (Si, NO₃), and
383 trace metals (Fe, Al Zn, Co, Cd, Pb, Cs) from small thaw depressions to thaw ponds and
384 lakes. This is consistent with evolution of water hydrochemistry during thaw pond and
385 thermokarst lake growth and maturation in other frozen peatlands of the world, such as
386 WSL (Pokrovsky et al., 2011; Shirokova et al., 2013; Manasypov et al., 2014) and palsa
387 peat bog of Canada (Peura et al., 2020). A decrease in concentrations of C, nutrients and
388 metals with lake size may be caused by several parallel mechanisms operating at the lake
389 shore/lake watershed and within the lake water column as depicted in **Fig. 6**. The
390 leaching of DOC and inorganic solutes occurs from ground vegetation (mainly moss and
391 lichen) and during peat abrasion at the lake shore (Shirokova et al., 2017; Payandi-
392 Rolland et al., 2020a). The delivery of these substrates decreases with an increase in the
393 ratio of lake circumference and lake watershed area to the lake volume (Manasypov et al.,
394 2015). Non-negligible amount of C can be delivered via diffusion from lake sediments
395 into the water column (Audry et al., 2011; Shirokova et al., 2020). An increase in the lake
396 volume and associated decrease of solute flux lead to mere dilution of dissolved C,
397 nutrient and metal concentration upon growth of thermokarst lake depth and area. This is
398 reflected by a linear response of DOC concentration to the logarithm of lake water areas
399 (**Fig. 2 C**). In addition to surface vegetation and upper (unfrozen) peat horizons, thawing
400 of ground ice during summer period (Connoly et al., 2020; Lim et al., 2021) may provide
401 sizeable input of DOC and related elements. It can be expected that the role of moss and
402 lichen cover is especially pronounced for small water bodies whereas the suprapermafrost
403 flow-delivered peat leaching products are more important for large lakes. The mineral
404 horizons can be leached by subsoil (shallow underground) waters, especially in large
405 lakes which exhibit deeper drainage basins and higher hydrological connectivity with
406 subsurface reservoirs (Ala-aho et al., 2018).
The mechanisms that are capable removing the solutes via DOM and nutrient processing within the lake water column include heterotrophic bacterial activity (Comte et al., 2015; Deshpande et al., 2016; Roiha et al., 2015, 2016), photodegradation (although of limited importance, Laurion et al., 2021), and photosynthesis (Tank et al., 2009; Rautio et al., 2011; Przytulska et al., 2016; Bogard et al., 2019; Wauthy and Rautio, 2020). Heterotrophic bacterial and photodegradation of DOM lead to coagulation of Fe, Mn and trace metals and their transfer to lake sediments in the form of particulate OC and metal hydroxides as it is known from laboratory (Oleinikova et al., 2017a, b) and field experiments (Shirokova et al., 2019; Payandi-Rolland et al., 2020b). The second mechanism is nutrient uptake by macrophytes, periphyton, and plankton, whose photosynthesis brings about a pH rise and a pCO₂ decrease in large thermokarst lakes compared to small thaw ponds and depressions as it is known from field observations in thermokarst lakes of the Mackenzie Delta (Squires et al., 2009; Tank et al., 2009). Note here that the pH increase in the course of lake maturation (Fig. 2 A) reflects, from the one hand, the phytoplankton and periphyton photosynthetic activity (Shirokova et al., 2015). From the other hand, lake size increase leads to increasing connectivity with shallow ground water and subsurface waters (Manasypov et al., 2015; 2020). These waters have higher pH due to their reaction with soil mineral horizons and thus can decrease the overall acidity of large thermokarst lakes.

426

427 **4.2. Behavior of C, nutrient and trace metal in thermokarst lakes after moss, lichen and peat addition in mesocosm experiments.**

The pattern of aqueous CO₂, DOC, nutrient and metal concentration in the presence of moss, lichen and peat substrates in the mesocosm experiments can serve as an experimental model of thermokarst water body evolution in the natural sequence of peat thawing and lake lateral spreading. The highest pCO₂ values observed at the very beginning of moss and lichen reaction with lake water (Fig. 4 C, S2 B, Table 1) are
consistent with elevated CO₂ measured in permafrost thaw subsidence with submerged mosses in this study (i.e., several thousand ppm, x 10 oversaturated with atmosphere, **Fig. 2 B**). High pCO₂ in small waterbodies and depressions was also reported in western Siberian (Polishchuk et al., 2018) and NE European (Zabelina et al., 2021) permafrost peatlands. Sizable CO₂ production by submerged moss and lichen may occur due to both respiration of live bryophytes, fungi and algae and heterotrophic (bacterial) degradation of DOC that was leached from moss and lichen biomass. The response of CO₂ increase to moss and lichen addition (1-2 days, **Fig. 4 C**), was much faster than the DOC increase (≥ 5 days, **Fig. 4 E**). This is also reflected in higher values of Vmean for CO₂ compared to DOC in the presence of both moss and lichen (**Table 1**). Such a difference suggests that the respiration of live lichen and moss rather than degradation of biomass is the main driving factor of rapid CO₂ rise. The O₂ concentration strongly decreased after first day of reaction with moss and lichen (**Fig. 4 D; S2 A**). This indicates that the heterotrophic degradation consuming O₂ from lake water was the primary cause of partially anoxic conditions in these experiments. It is worth noting that for moss and lichen, the rates of initial (0-24 h) O₂ decrease (7±1; 2.7±0.8 mmol kg⁻¹) and CO₂ increase (6±2; 4±1 mmol kg⁻¹) were comparable (**Fig. 4C**, insert). Furthermore, the heterotrophic degradation control on O₂ level in mesocosms is consistent with higher bacterial number measured in moss and lichen experiment compared to peat experiment (section 3.2.1). Therefore, results of the present study suggest that the plant-derived “fresh” DOM can be a driven factor of CO₂ rise, unlike soil refractory DOM. This is consistent with recent mesocosm experiments in a meso-eutrophic lake of the boreal zone (Nydahl et al., 2019). In these experiments, with an addition of colored allochthonous (soil) DOC, there was no relationship between bacterial activities and pCO₂.

The sphagnum moss was the most efficient substrate in terms of DOC and CO₂ production. The reason of higher reactivity of moss compared to lichen is not clear; the higher degradability of moss may be caused by its essentially wet status in natural
conditions thus allowing to initiate fast and active heterotrophic bacterial activity, necessary for an enhanced CO₂ and DOC production. In contrast, in dry lichens and frozen peat, the bacteria were rather dormant and thus not capable to efficiently degrade particulate organic matter at the time scale of mesocosm experiments. Interestingly, that at the end of experiment, corresponding to stable (steady) concentration of aqueous components, the mass-normalized effects of moss exceeded that of lichen and peat by a factor of 2 to 5 (see **Table 1** and **Table S2**). In particular, the moss mesocosm yielded the highest level of nutrients, notably PO₄³⁻, NO₃⁻, NH₄⁺, K, Fe, Mn, Zn. These nutrients may be potentially limiting for aquatic plankton and periphyton of surface waters from frozen peatlands. Thus, high ammonium concentrations in surface waters derived from decomposition of vegetation are well known for wetland ecosystems (Camargo and Alvaro, 2006). We therefore suggest that, at the beginning of thermokarst development, corresponding to early stages of soil subsidences, the degradation and leaching of mosses rather than that of lichens and peat determine the evolution of C, nutrient and metal concentration in thaw waters of peatlands. In particular, high DOC and pCO₂ values at the beginning of thermokarst development, observed in the present study of NE European peatlands (Fig. 2 B, C; S2 B, C) and supported by other observations (Zabelina et al., 2021 and references therein) are most likely linked to moss, and in a lesser degree, to lichen degradation. Note that the presence of dense *Sphagnum* moss was reported to be the main driver of variation in DOM composition in thermokarst water bodies of the North European permafrost peatlands (Hodgkins et al., 2016).

The finding that frozen peat releases quite low amount of nutrients compared to dominant terrestrial vegetation may have a global significance. It means that in case of permafrost thaw and tundra landscape modification, the primary factor of DOC concentration and CO₂ emission flux change will be the ground vegetation regime (colonization, flooding, drying), rather than thaw of frozen peat layer and reacting of thawed peat with surface waters. Moreover, given that peat addition is not capable
increasing pCO₂ much higher above that of the control, it is possible that progressive
thawing of frozen peat and its reaction with surface fluids might not produce drastic
change in CO₂ emission pattern from inland waters of frozen peatlands. In contrast, the
regime of ground vegetation flooding and decay in these wetlands might significantly
increase CO₂ concentration and relevant emission fluxes at the time scale of several days
of reaction. The reason for this is that the response of aquatic ecosystem chemical
composition and CO₂ emission to the submerging of mosses is on the order of first hours
to first days, which is comparable with the water residence time in local depressions and
ephemeral puddles and ponds. This conclusion is at odds with current paradigm that
thawing of permafrost will greatly enhance C fluxes in aquatic system, microbial activity,
and CO₂ emission rate (Voigt et al., 2019, Hayes et al., 2014). In case of permafrost
peatlands of continental lowlands, the reactivity of frozen peat in aquatic systems will
have a subordinate impact on C, nutrient and metal regime in surface waters, compared to
lateral expansion or retreat of moss cover and flooding of moss and lichens by surface
waters. At the same time, the relative importance of ground vegetation in element
delivery to the thermokarst lakes is likely to vary depending on season. Thus, in a recent
study of selenium (Se) biogeochemistry in thermokarst lakes of western Siberia
(Pokrovsky et al., 2018), we demonstrated that, in addition to direct atmospheric fallout,
Se input to the lakes occurs via surface flux over moss and lichen in June (spring) and via
supra-permafrost flow in summer and autumn. These mechanisms are fully consistent
with those revealed in the present study using mesocosm approach. In particular in
spring, Se, together with our trace metals of atmospheric origin, can be mobilized directly
from melted snow and indirectly from snow water interacting with plant litter and
moss/lichen biomass because the underlying peat is frozen.

The two groups of trace element revealed during addition of various substances to
the lake water (section 3.2.3) can be indicative either of moss (alkalis and alkaline-earth
metals, Fe, As, Cu, Ga, Nb, Tl) or peat (Be, V; Ni, Al and trivalent and tetravalent
hydrolysates) control on thermokarst lake water chemistry. It is not possible to ascertain specific processes (leaching, adsorption, heterotrophic degradation) controlling these element behavior in the mesocosm experiments without assessing the role of organic and organo-mineral colloids that are likely to control element speciation in surface waters of permafrost peatlands. These mechanisms may include the destabilization of Fe-organic complexes, liberation of ionic Fe (III), its coagulation in the form of Fe oxyhydroxides, and coprecipitation of TE with Fe hydroxides, as it happens in WSL rivers and lakes (Pokrovsky et al., 2016b) and observed in experimental studies of organo-ferric colloids degradation by microbial activity (Oleinikova et al., 2017b). A strong link of trivalent and tetravalent hydrolysates to Fe oxy(hydr)oxides in the form of organo-ferric colloids was already evidenced in the water column and porewater sediments of thermokarst lakes from permafrost peatlands (Pokrovsky et al., 2011; Audry et al., 2011; Shirokova et al., 2013). The elements exhibiting a concomitant release with DOC are likely to be present as strongly-bound organic species. Typically, these are divalent transition metals (Me²⁺). Their organo-colloidal status is maintained in a wide range of pH and DOC concentrations in thaw ponds and thermokarst lakes of western Siberia (Pokrovsky et al., 2011, 2013, 2016a; Shirokova et al., 2013).

532

### 4.3. The role of permafrost thaw and vegetation shift in hydrochemistry of thermokarst lakes subjected to climate warming

Thermokarst development in permafrost peatlands involves lateral spreading of the lake without significant deepening of its basin (Polishchuk et al., 2017; Manasypov et al., 2020). This leads to a decrease of the ratio water volume : border line (perimeter). This was first reported for Arctic tundra by Britton (1957) and later evidenced in western Siberia (Kirpotin et al., 2009, 2011; Audry et al., 2011; Manasypov et al., 2020). Such a decrease yields a concomitant decrease in DOC, conductivity (major cations), Fe, Al, and trace element of the lake water with an increase in the lake size (Pokrovsky et al., 2011,
542 2013; Shirokova et al., 2013; Manasypov et al., 2014, 2015). Based on results of natural
543 observation in NE Europe and mesocosm experiments, we interpret this trend as due to
544 the dominance of moss and lichen degradation at the beginning of thermokarst lake
545 formation, when small depressions in the moss/lichen cover are filled by water either
546 from thawing of soil ice or of atmospheric origin (stage 1 in Fig. 6). The accumulation of
547 organic and inorganic solutes in the newly-formed depressions triggers DOM, nutrient
548 and metal processing in the water column. This occurs via heterotrophic plankton
549 mineralization and sedimentation to the bottom under the dominance of allochthonous
550 input from vegetation leaching (stage 2 in Fig. 6). At later stages of lake development, the
551 peat becomes the main supplier of C, nutrient and metal to the water column via
552 suprapermafrost inflow as well as wave abrasion of peat at the lake shore. However,
553 relatively low reactivity of peat (i.e., Payandi-Rolland et al., 2020a and this study) does
554 not allow sufficient accumulation of organic and inorganic components in the lake water
555 column. Instead, mere dilution by atmospheric precipitation and enhanced processing of
556 autochthonous DOM and nutrients (such as photo- and biodegradation and uptake by
557 plankton and periphyton) lead to overall decrease of C, nutrient and metal concentration
558 in large lakes compared to small thaw ponds (stage 3, Fig. 6).

559 In this study, we hypothesized that peat is an important source of chromophoric
560 DOM (CDOM) and nutrients, whereas a non-aromatic DOM easily available by bacteria
561 may be released from mosses and lichens. As a result, the pattern of CO2 concentration is
562 determined by a combination of nutrients and DOC delivery from solid source to aqueous
563 phase. This pattern was found to be drastically different between bryophytes (mosses),
564 symbiotic organisms (lichens) and frozen peat. This finding therefore confirms our
565 hypothesis. We further anticipated that progressive change of the ratio of lake water to
566 surrounding organic substrates in the course of lake evolution will produce a systematic
567 change in lake water chemical composition: from moss and lichen degradation control at
568 the beginning of thaw pond formation, at the stage of permafrost subsidence/depression,
569 to coastal peat abrasion and dissolution control at the later stages of lake growth and maturation. At the current state of knowledge, this hypothesis cannot be unambiguously verified, as it requires an estimation of vegetation cover at the small thaw ponds and thermokarst lakes watershed, which is possible only with high-resolution GIS approaches.
570
571
572
573

According to results of this study, short-term (1-10 years) consequences of climate warming in permafrost peatlands will be mostly pronounced through the change of ground vegetation regime. On a longer perspective (50-100 years), the increase in the active layer depth (currently 1 cm y⁻¹, Johansson et al., 2011) will involve much larger amount of peat in water transport within the lake watersheds. This will inevitably increase the relative role of peat in element delivery from the land to the lentic waters of permafrost peatlands. Furthermore, a sizable amount of highly labile C and related elements can be mobilized from currently frozen peat ice into the hydrological network via the suprapermafrost flow (Lim et al., 2021).
574
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581
582

Overall, an increase in the active layer thickness, lake water temperature and the duration of unfrozen period due to climate warming should enhance carbon and element leaching from moss, lichen and peat to the thermokarst lakes. However, compared to wave-induced peat abrasion at the lake border, the moss and lichen colonization, submerging and degradation during permafrost thaw and subsidence formation are more important drivers of thermokarst lake water's chemical composition evolution under the on-going environmental changes. Note that this conclusion supports recent findings on very young age of C emitted from permafrost waters which reveals that inland water carbon emissions from permafrost landscapes may be more sensitive to changes in contemporary carbon turnover than the release of pre-aged carbon from thawing permafrost (Dean et al., 2020).
583
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593

594

595 **5. Concluding remarks**
Thawing of frozen peatlands produces depressions, thaw ponds and thermokarst lakes encountered in the NE European discontinuous permafrost zone (Bolshezemelskaya Tundra). There was a systematic decrease in concentrations of dissolved OC, CO2, nutrient and metal with an increase in the water body size. In the course of lake maturation (over six order of magnitude increase in the surface area), the pH increased by ca. 3 units while the pCO2 and DOC concentrations decreased ten-fold. Total P, Ca, Mg increased their concentrations by a factor of 2 to 3, whereas Si, Fe, Al, NO3 decreased concentrations with lake size increase. We interpret these changes as due to a combination of external factors (delivery of solutes from lake watershed) and DOM, nutrient and metal processing in the water column.

Mesocosm experiments on thermokarst lake water in the presence of frozen peat, moss and lichen demonstrated that moss and lichen leaching and degradation, rather than peat abrasion, act as a main driver of thermokarst lake water's evolution. The mass-normalized release of DOC, macro nutrients (Si, N, P), major elements (K, Na, Ca, Mg), micronutrients and trace metals (Fe, Mn, Cu, As, Rb, Cs, Tl) was controlled essentially by moss and, in a lesser degree, lichen with almost negligible impact of peat. When comparing to natural sequence of lakes and ponds chemical composition, these mesocosm experiments allowed to characterize the water chemistry during the first step of thaw pond formation, when the moss leaching determines the hydrochemical features of surface water. Upon progressive increase in the lake size, the lichen and moss leaching at the watershed surface become less important than the delivery of peat leaching products by the suprapermafrost flow. This happens due to an increase in the ratio of lake circumference to the lake volume which decreased the delivery of external terrestrial (allochthonous) material. In contrast, when lake water volume increases, the relative role of autochthonous processes of DOM, nutrient and metal transformation, such as photo- and bio-degradation, and phytoplankton/periphyton activity also increase. This leads to
622 gradual removal of DOC, nutrients and metals due to C emission to atmosphere and element deposition to the lake bottom.
623

624 Because moss leaching is likely to be most important process controlling hydrochemistry of dissolved elements and CO₂ emission/burial pattern in thermokarst water bodies, the response of the moss coverage regime to permafrost thaw and climate warming in NE European frozen peatlands will chiefly define the DOC and nutrient cycling in lentic waters and CO₂ emission fluxes. Given that the vegetation biomass leaching reactions are very fast, on the order of first hours to days, the hydrological pattern of moss and lichen flooding rather than biogeochemical reactions within the peat soil profile will be controlling short-term response of peat palsa landscapes to permafrost thaw and temperature rise. This contrasts a dominant view that thawing of frozen peat provides the main contribution to the negative feedback of environmental response to the climate change. At least on a short-term scale, the peat thaw may be a subordinate factor of biogeochemical C fluxes compared to the change of wetting and flooding regime of the lake shoreline and moss persistence/growth on the tundra surface.

637

638 Acknowledgements:

639 We acknowledge the main support from a RFBR grants 19-55-15002, and 20-05-00729_a and RSF grant No 18-77-10045 for data analysis. Partial support from Belmont Forum project VULCARE is also acknowledged.

642

643

644

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<figure><img src="image_2.png" /><figcaption>**Figure 1 A:** The map of the study site with position of sampled lakes and photos of mesocosm run at the lake shore.</figcaption></figure>
<figure><img src="image_3.png" /></figure>

1021 **Figure 1 B:** The work flow chart of the field observations, mesocosm experiments and analyses performed in this study.
1022
<figure><img src="image_4.png" /><figcaption>**Figure 2.** pH (A), pCO2 (B), DOC (C) and DIC (D) concentration in depressions, thaw ponds and thermokarst lakes as a function of logarithm of lake surface area (log S). Parameters of linear relationships (Eqn. 1) are listed in Table S1 of the Supplement.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Total dissolved N (A), P (B), Si (C), Ca (D), Fe (E) and Al (F) concentrations in depressions, thaw ponds and thermokarst lakes as a function of logarithm of lake surface area (log S). Parameters of linear relationships are listed in Table S1.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 3**, continued. Phosphate, nitrate, ammonia, Mg, K and Mn concentrations in thermokarst water bodies as a function of logarithm of lake surface area (log S).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** The pH value (A), and the difference between experiment (moss, lichen or peat) and control (lake water without substrate) in Specific Conductivity (B), O₂ (C), pCO₂ (D), DOC (E) concentrations and SUVA₂₅₄ (F) in mesocosms as a function of exposure time. Here and in other figures below, symbols represent the mean; the error bars correspond to the S.E. of two independent duplicate experiments. Solid lines represent a polynomial fit to the data that was used to calculate the maximal rates and concentration yields.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** Evolution of the difference in Si (A), PO₄ (B), NO₃ (C), and NH₄ (D)</figcaption></figure>

concentrations between the mesocosm (moss, lichen or peat) and control in the course of experiments. See Fig. 4 caption for detailed legend.

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<figure><img src="image_9.png" /></figure>

1090

1091 Legend:

1092 1 - Peat of active (unfrozen) layer

1093 2 - Frozen peat with ice lenses

1094 3 - Unfrozen mineral horizons

1095 4 - Frozen mineral horizons

1096 5 - Lake sediments

1097 6 - Flux of DOC and nutrients that are leached from plant litter

1098 7 - Flux of DOC and nutrients that are leached from peat (both active and permafrost layer)

1100

1101

1102 **Figure 6.** A cartoon of thaw pond and thermokarst lake evolution and the change of sources of DOC and nutrients in the course of evolution, from surface moss and lichen to frozen peat. A three main stage sequence of thermokarst lake formation and development includes:

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(1) soil depression filled by water which interacted with submerged moss and lichens thus leaching DOM and solutes to the lake;

(2) beginning of in-lake processing of DOM via heterotrophic plankton
mineralization and sedimentation to the bottom, occurring under the dominance of
allochthonous input from vegetation leaching;

(3) stage of large mature lake, influenced by nutrients and DOM input from
thawed and frozen peat leaching and nutrients and CO2 uptake by phytoplankton,
under on-going DOM, nutrient and metal processing via photo- and
biodegradation in the water column.

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<table><caption>**Table 1.** The mean and maximal release rates and maximal concentration yield of major components normalized to dry weight of substrate in mesocosm experiments. Significant (p < 0.05) results are labelled with asterisk (*).</caption><thead><tr><th>Index</th><th>Substr.</th><th>R<sup>2</sup></th><th>F-test<br>(p=0.05)</th><th>υ<sub>mean</sub>±SE, X·hr<sup>-1</sup></th><th>υ<sub>max</sub>±SE, X·hr<sup>-1</sup></th><th>ΔC<sub>max</sub>±SE</th><th>T<sub>ΔCmax</sub></th></tr></thead><tbody><tr><td rowspan="3">EB,<br>CFU·10<sup>6</sup>·kg<sup>-1</sup></td><td>moss</td><td>0.64</td><td>7.2*</td><td>0.05±0.02</td><td>0.1±0.4</td><td>21±7</td><td>379</td></tr><tr><td>peat</td><td>0.74</td><td>11.3*</td><td>0.022±0.001</td><td>0.036±0.004</td><td>2.1±0.1</td><td>97</td></tr><tr><td>lichen</td><td>0.88</td><td>29.2*</td><td>0.00074±0.00001</td><td>0.33±0.03</td><td>58±3</td><td>379</td></tr><tr><td rowspan="3">OB,<br>CFU·10<sup>6</sup>·kg<sup>-1</sup></td><td>moss</td><td>0.92</td><td>48.3*</td><td>0.044±0.002</td><td>0.08±0.02</td><td>17.5±0.8</td><td>379</td></tr><tr><td>peat</td><td>0.49</td><td>3.8*</td><td>0.012±0.005</td><td>0.05±0.02</td><td>5±1</td><td>216</td></tr><tr><td>lichen</td><td>0.92</td><td>45.0*</td><td>0.11±0.02</td><td>0.23±0.09</td><td>42±6</td><td>379</td></tr><tr><td rowspan="3">O<sub>2</sub>, mmol·kg<sup>-1</sup><br>(long)</td><td>moss</td><td>0.68</td><td>8.62*</td><td>-0.128±0.007</td><td>-0.98±0.09</td><td>-22.9±0.7</td><td>379</td></tr><tr><td>peat</td><td>0.52</td><td>4.3*</td><td>-0.05±0.01</td><td>-0.24±0.04</td><td>-4±1</td><td>97</td></tr><tr><td>lichen</td><td>0.68</td><td>8.62*</td><td>-0.182±0.008</td><td>-1.45±0.02</td><td>-37</td><td>97</td></tr><tr><td rowspan="3">CO<sub>2</sub>, mmol·kg<sup>-1</sup><br>(short, for peat<br>long)</td><td>moss</td><td>0.78</td><td>14.4*</td><td>6±2</td><td>6±2</td><td>19±5</td><td>3</td></tr><tr><td>peat</td><td>0.10</td><td>0.44</td><td>0.0026±0.0007</td><td>-0.112±0.0064</td><td>4.7±0.7</td><td>139</td></tr><tr><td>lichen</td><td>0.95</td><td>80.5*</td><td>4.23±0.04</td><td>6.0±0.8</td><td>33.8±0.4</td><td>8</td></tr><tr><td rowspan="3">DIC, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.31</td><td>1.78</td><td>0.14±0.03</td><td>0.7±0.2</td><td>14±3</td><td>97</td></tr><tr><td>peat</td><td>0.38</td><td>2.42</td><td>0.006±0.006</td><td>0.01±0.02</td><td>1±1</td><td>216</td></tr><tr><td>lichen</td><td>0.90</td><td>37.7*</td><td>0.323±0.007</td><td>0.54±0.06</td><td>31.4±0.7</td><td>97</td></tr><tr><td rowspan="3">DOC,<br>mmol·kg<sup>-1</sup></td><td>moss</td><td>0.52</td><td>4.39*</td><td>5±4</td><td>8±13</td><td>1484±1034</td><td>284</td></tr><tr><td>peat</td><td>0.78</td><td>14.0*</td><td>0.52±0.05</td><td>1.5±0.1</td><td>89±2</td><td>284</td></tr><tr><td>lichen</td><td>0.50</td><td>4.01*</td><td>2±1</td><td>4±5</td><td>577±184</td><td>284</td></tr><tr><td rowspan="3">NH<sub>4</sub><sup>+</sup>,<br>mmol(N)·kg<sup>-1</sup></td><td>moss</td><td>0.34</td><td>2.08</td><td>0.02±0.03</td><td>0.04±0.12</td><td>5±6</td><td>216</td></tr><tr><td>peat</td><td>0.36</td><td>2.25</td><td>-0.0002±0.0001</td><td>0.0006±0.0005</td><td>-0.04±0.02</td><td>284</td></tr><tr><td>lichen</td><td>0.11</td><td>0.49</td><td>0.0002±0.0002</td><td>0.0002±0.0010</td><td>0.03±0.08</td><td>97</td></tr><tr><td rowspan="3">NO<sub>3</sub><sup>-</sup>,<br>mmol(N)·kg<sup>-1</sup></td><td>moss</td><td>0.81</td><td>8.58*</td><td>0.05±0.03</td><td>0.08±0.06</td><td>3.1±0.9</td><td>43</td></tr><tr><td>peat</td><td>0.85</td><td>11.1*</td><td>0.030±0.002</td><td>0.050±0.008</td><td>1.53±0.02</td><td>43</td></tr><tr><td>lichen</td><td>0.80</td><td>8.18*</td><td>0.01±0.02</td><td>0.02±0.04</td><td>1.6±0.5</td><td>379</td></tr><tr><td rowspan="3">NO<sub>2</sub><sup>-</sup>,<br>μmol(N)·kg<sup>-1</sup></td><td>moss</td><td>0.70</td><td>9.44*</td><td>0.4±0.6</td><td>1±2</td><td>15±10</td><td>43</td></tr><tr><td>peat</td><td>0.81</td><td>17.5*</td><td>0.05±0.02</td><td>0.10±0.06</td><td>9.8±0.5</td><td>379</td></tr><tr><td>lichen</td><td>0.09</td><td>0.4</td><td>0.04±0.06</td><td>0.1±0.2</td><td>-21±2</td><td>16</td></tr><tr><td rowspan="3">N<sub>tot</sub>,<br>mmol(N)·kg<sup>-1</sup></td><td>moss</td><td>0.76</td><td>6.3*</td><td>0.014±0.009</td><td>0.04±0.03</td><td>8±2</td><td>284</td></tr><tr><td>peat</td><td>0.88</td><td>14.3*</td><td>0.006±0.003</td><td>0.04±0.01</td><td>3.4±0.5</td><td>379</td></tr><tr><td>lichen</td><td>0.75</td><td>6.09*</td><td>0.03±0.01</td><td>0.09±0.08</td><td>12±4</td><td>379</td></tr><tr><td rowspan="3">PO<sub>4</sub><sup>3-</sup>,<br>μmol(P)·kg<sup>-1</sup></td><td>moss</td><td>0.38</td><td>2.46</td><td>4±6</td><td>7±42</td><td>1262±1790</td><td>284</td></tr><tr><td>peat</td><td>0.04</td><td>0.16</td><td>-0.016±0.007</td><td>0.01±0.14</td><td>0.7±0.4</td><td>139</td></tr><tr><td>lichen</td><td>0.43</td><td>3.03</td><td>5±4</td><td>11±7</td><td>1234±812</td><td>216</td></tr><tr><td rowspan="3">P<sub>tot</sub>,<br>μmol(P)·kg<sup>-1</sup></td><td>moss</td><td>0.47</td><td>1.79</td><td>0.4±0.2</td><td>0.7±0.3</td><td>1647±2146</td><td>379</td></tr><tr><td>peat</td><td>0.19</td><td>0.45</td><td>-0.02±0.20</td><td>0.27±0.05</td><td>54±32</td><td>0</td></tr><tr><td>lichen</td><td>0.54</td><td>2.32</td><td>5±4</td><td>7±5</td><td>1695±1068</td><td>284</td></tr><tr><td rowspan="2">Si, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.20</td><td>0.99</td><td>0.003±0.005</td><td>0.005±0.007</td><td>1.4±0.6</td><td>216</td></tr><tr><td>peat</td><td>0.75</td><td>11.8*</td><td>0.0007±0.0003</td><td>0.0024±0.0009</td><td>0.26±0.07</td><td>379</td></tr></tbody></table>
<table><tbody><tr><td></td><td>lichen</td><td>0.76</td><td>12.76*</td><td>0.0023±0.0004</td><td>0.0035±0.0009</td><td>0.97±0.08</td><td>379</td></tr><tr><td rowspan="3">Na, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.80</td><td>15.8*</td><td>0.08±0.04</td><td>0.11±0.1</td><td>32±16</td><td>379</td></tr><tr><td>peat</td><td>0.69</td><td>8.98*</td><td>0.0005±0.0002</td><td>0.0036±0.0005</td><td>-0.035±0.005</td><td>379</td></tr><tr><td>lichen</td><td>0.63</td><td>6.67*</td><td>0.03±0.02</td><td>0.06±0.02</td><td>9±1</td><td>284</td></tr><tr><td rowspan="3">K, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.62</td><td>6.53*</td><td>0.2±0.2</td><td>0.3±0.4</td><td>70±49</td><td>379</td></tr><tr><td>peat</td><td>0.13</td><td>0.6</td><td>0.00002±0.00020</td><td>-0.0014±0.0004</td><td>-0.03±0.05</td><td>379</td></tr><tr><td>lichen</td><td>0.34</td><td>2.05</td><td>0.185±0.005</td><td>0.37±0.02</td><td>22.5±0.1</td><td>97</td></tr><tr><td rowspan="3">Ca, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.24</td><td>1.27</td><td>0.01±0.01</td><td>0.01±0.03</td><td>3±3</td><td>284</td></tr><tr><td>peat</td><td>0.68</td><td>8.55*</td><td>0.008±0.002</td><td>0.019±0.008</td><td>0.7±0.2</td><td>379</td></tr><tr><td>lichen</td><td>0.85</td><td>22.5*</td><td>-0.015±0.001</td><td>-0.03±0.01</td><td>-2.21±0.01</td><td>139</td></tr><tr><td rowspan="3">Mg, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.34</td><td>2.04</td><td>0.02±0.02</td><td>0.02±0.05</td><td>5±7</td><td>379</td></tr><tr><td>peat</td><td>0.81</td><td>17.2*</td><td>0.0005±0.0001</td><td>0.002±0.001</td><td>-0.45±0.02</td><td>379</td></tr><tr><td>lichen</td><td>0.82</td><td>18.2*</td><td>-0.025±0.003</td><td>-0.10±0.02</td><td>-1.6±0.1</td><td>97</td></tr><tr><td rowspan="3">Al, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.48</td><td>3.69*</td><td>0.0014±0.0005</td><td>0.003±0.003</td><td>0.14±0.03</td><td>97</td></tr><tr><td>peat</td><td>0.91</td><td>39.3*</td><td>0.00041±0.00003</td><td>0.0018±0.0003</td><td>0.149±0.007</td><td>379</td></tr><tr><td>lichen</td><td>0.99</td><td>263*</td><td>-0.0014±0.0002</td><td>-0.0028±0.0007</td><td>-0.223±0.006</td><td>379</td></tr><tr><td rowspan="3">Fe, mmol·kg<sup>-1</sup></td><td>moss</td><td>0.54</td><td>4.63*</td><td>0.004±0.002</td><td>0.005±0.006</td><td>1.4±0.4</td><td>284</td></tr><tr><td>peat</td><td>0.91</td><td>42.3*</td><td>0.00040±0.00004</td><td>0.0007±0.0004</td><td>0.086±0.009</td><td>379</td></tr><tr><td>lichen</td><td>0.85</td><td>22.2*</td><td>-0.0054±0.0001</td><td>-0.0167±0.0005</td><td>-0.959±0.007</td><td>97</td></tr></tbody></table>

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