Towards a dystrophic lake: The history of Smolak Lake (northern Poland) on the basis of geochemical and biological data

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Towards a dystrophic lake: The history of Smolak Lake (northern Poland) on the basis of geochemical and biological data ⁎

Joanna Mirosław-Grabowska , Milena Obremska, Edyta Zawisza, Magdalena Radzikowska, Joanna Stańczak Institute of Geological Sciences Polish Academy of Sciences, INGPAN, Research Centre in Warsaw, Twarda 51/55 PL-00818, Poland

A R T I C LE I N FO

A B S T R A C T

Keywords: Dystrophic lake Lake evolution Multi-proxy analysis N Poland

This study presents a multi-proxy reconstruction of the evolution of Smolak Lake from harmonic to dystrophic conditions. The lake’s history and environmental conditions, such as zooplankton and vegetation compositions, trophic state, water temperature and water level fluctuations, are discussed. Smolak Lake is located in the southern part of the Masurian Lakeland, northern Poland. A 350 cm core comprising thick sediments (Sm profile) are mainly composed of homogenous organic gyttja and dy was taken from the lake and analysed for geochemical and biological (cladoceran and pollen) proxies. Radiocarbon data show that the sediments accumulated from the Late Glacial (Younger Dryas period) to recent times. Based on the results of pollen data, six local pollen assemblage zones were recognized. The subfossil cladoceran fauna in the sediments are represented by 25 species belonging to four families. Five zones of Cladocera development were distinguished. The Cladocera species indicate the initial oligotrophic status of the lake and its subsequent slight increase in trophic status. From ~9000 yr cal BP (at the beginning of the Atlantic period), Smolak Lake began a fast transition to a dystrophic state. At this time, the amount of total organic carbon (TOC) in the sediments significantly increased to above 50%, and deposits containing more than 97% water were transformed into dy. The sedimentation rate was extremely low, i.e., 0.08–0.2 mm/yr. Dystrophic conditions are also reflected in the Cladocera and pollen found in the core during this period: Cladocera composition was dominated by species very resistant to acid conditions (e.g., Alonella excisa); the abundance of green algae was at its lowest level and dominated by only one genus (Botryococcus); no hydrophytes were found; and rush vegetation disappeared. Currently, Smolak Lake is a shallow, humic lake characterized by brown water and the presence of floating mats. This study shows that it’s current status is a result of it’s small, relatively undisturbed catchment, which should be protected in order to maintain this unique dystrophic habitat.

1. Introduction

forests, peat mosses in the vicinity of water bodies, spreading floating mats on water surfaces, low phytoplankton biomass, low biodiversity and respiration rates higher than primary production (Brönmark and Hansson, 2005; Gąbka and Owsianny, 2006; Górniak et al., 1999; Hessen and Tranvik, 1998; Salonen et al., 1983; Wetzel, 2001). Paleolimnological reconstructions of the trophy of such lakes are very important as they allow the effects of climate variability and recent anthropogenic disturbances affecting the catchment characteristics, limnology and ecology of the lakes to be determined (Rantala et al., 2015). In the natural environment, nutrient availability in lakes is closely related to temperature (which affects both summer biological productivity and the length of the open-water period), and to enhanced weathering and nutrient loading from catchments (Luoto et al., 2012).

Dystrophic (humic) lakes are common in the boreal zone (e.g., Scandinavia, Russia, Canada) and rare in the Middle European lowlands. In Poland, they occur in the northern part of the lowlands, e.g., in Wigry National Park, Bory Tucholskie Forest and the Masurian Lakeland, where these lakes are surrounded by pine-spruce forests that make the landscape resemble the Scandinavian region (Drzymulska and Zieliński, 2013; Drzymulska et al., 2013; Gałka et al., 2014b; Zawiska et al., 2013; Zawisza et al., 2019). Dystrophic (humic) lakes are characterised by low pH (4.5–6.0), very low calcium content in the water and sediment, dy-like sediment, peat-covered catchment areas that are often overgrown with coniferous

⁎

Corresponding author. E-mail addresses: [email protected] (J. Mirosław-Grabowska), [email protected] (M. Obremska), [email protected] (E. Zawisza), [email protected] (M. Radzikowska), [email protected] (J. Stańczak). https://doi.org/10.1016/j.catena.2019.104262 Received 8 April 2019; Received in revised form 12 September 2019; Accepted 13 September 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Joanna Mirosław-Grabowska, et al., Catena, https://doi.org/10.1016/j.catena.2019.104262

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3. Methods

Therefore, dystrophic lakes may respond sensitively to both climatic fluctuations (Nevalainen et al., 2015; Karpińska-Kołaczek et al., 2016) and to anthropogenic pressures of both local and global scale (Gałka et al., 2014b; Rantala et al., 2015). Furthermore, sedimentation in dystrophic lakes depends in part on local hydrological regimes (absence of surface flow, groundwater level). Data derived from these lakes is therefore useful to reconstruct past climate and hydrological changes (Drzymulska et al., 2014). In this paper, we present the results of an interdisciplinary investigation of a small lake called Smolak Lake, located in the Masurian Lakeland, northern Poland. Smolak Lake is a fantastic object of study for tracking the natural development of this reservoir and the dystrophic processes. Using paleolimnology, this paper aims to reconstruct the history of this lake from its origin in the Late Glacial to its current dystrophic stage, focussing on the ecological and geochemical processes that occurred as dystrophic conditions developed. The main aim of this study was a determination of the history of the evolution of the Smolak Lake towards dystrophy. We realized it by following tasks: (1) to track trophic changes in Smolak Lake, (2) to examine how zooplankton biodiversity changed with changes in trophy, (3) to understand the processes leading to the dystrophic nature of this lake and (4) to reconstruct the evolution of the lake with regard to its geological and climatic background. In this study, we present results and interpretations of geochemical and biological (cladoceran and pollen) analyses of the Late Glacial and Holocene sediments of Smolak Lake. These data allowed us to determine the history of the lake and the transformation of its environmental conditions, such as zooplankton and vegetation development, trophic state, water temperature and water level fluctuation.

The lacustrine sediments of Smolak Lake were drilled in February 2014 using a Livingstone-type corer. The sampling point was located in the central part of the lake (water depth of 318 cm). The total length of the sediment profile (“profile Sm”) was 350 cm (depth: 320–670 cm below the lake surface – b.s.l.). Since Smolak Lake is located within the same fluvioglacial plain as the neighbouring Malinowe Lake (ca. 150 m between the lake basins – Fig. 1), questions arose during the study concerning the possibility that the two lakes were once adjoined. Therefore, four additional drill cores were obtained in August 2017 in the transitional area between Smolak and Malinowe Lakes (profiles 1–4, Fig. 1B). All sediment cores were sliced at intervals of 1–2 cm depending on the sediment lithology, and were visually assessed for changes in sedimentation characteristics. Sediments from Smolak Lake (profile “Sm”) were subjected to numerous multi-proxy analyses, including pollen, stable isotopes, chemical and subfossil Cladocera analyses. These analyses allowed us to also determine the history of the lake and the transformation of its environmental conditions, such as zooplankton and vegetation development, trophic state, water temperature and water level fluctuation. 3.1. Stable isotopes and organic geochemistry The stable isotope analyses of organic sediments from Smolak Lake (profile Sm) included analyses of the organic carbon and nitrogen contents and the carbon and nitrogen isotopes. Organic carbon and nitrogen contents were analysed using the elemental analyser Vario Micro Cube. The analyses were performed in the Laboratory for Isotope Dating and Environmental Studies at the Institute of Geological Sciences of the Polish Academy of Sciences in Warsaw, Poland. Analyses for carbon and nitrogen isotopes were performed on 91 samples of organic sediments from depths of 320–650 cm. The sediments were dried at 60 °C and ground. Carbon and nitrogen isotope compositions were analysed using a Flash Elemental Analyser 1112 and a Thermo MAT 253 mass spectrometer, which were calibrated based on an internal nicotinamide standard and reported as per mill (‰) deviations from atmospheric N2 (in the case of δ15N) and from the Vienna Pee Bee Belemnite (in the case of δ13C). The analytical errors (l SD) for the δ13C and δ15N measurements were 0.17‰ and 0.24‰, respectively.

2. Study site Smolak Lake (53°44′29″N; 21°31′28″E) is situated at an elevation of 118.4 m a.s.l. between Ruciane-Nida and Mikołajki in the southern part of the Masurian Lakeland, northern Poland (Fig. 1). It is a small, overgrown outflow reservoir (1.1 ha) with a depth of 3.2 m, situated within the “Krutynia Dolna” Reserve in the area of the Masurian Landscape Park. The lake is typical of dystrophic lakes in the region as it is characterized by low pH, low calcium content, very low conductivity of water, and dy sedimentation (see Table 1). The central part of the lake bottom is devoid of vegetation, with Stuckenia filiformis the dominant vegetation in the bank zone. Along the margin, floating mats differentiated into two zones are present. Closer to open water, the floating mats are composed mostly of Scheuchzeria palustris, Carex chordorrhiza and Carex vaginata. Closer to shore, Eriophorum gracile, Sphagnum, Vaccinium oxycoccos and Drosera rotundifolia occur (http:// parkikrajobrazowewarmiimazur.pl/mazurski/dolne_menu-ochrona_ przyrody-rezerwaty-krutynia_dolna.html). Smolak Lake and its catchment are located in a vast outwash plain formed and filled by fluvioglacial sands and gravels during the Pomeranian phase of the Vistulian Glaciation (Lisicki, 1994). In the west, the fluvioglacial plain is cut by the Krutynia River valley. Recently, this outwash plain has been occupied by lakes, including Malinowe Lake to the north and Jerzewko Lake to the east of Smolak Lake. Today, the catchment is covered with peat plains overgrown with coniferous forest. The study area is contemporarily characterized by a mean annual temperature of 6.9 °C and an annual precipitation of ca. 629 mm (with a mean of ca. 79 mm in July). The mean temperature of the warmest month (July) is 18.5 °C, and the mean temperature of the coldest month (January) is −6.4 °C. The vegetation period is one of the shortest in Poland and persists for 190–200 days (https://pl.climate-data.org/ europa/polska/warmian-masurian-voivodeship/ruciane-nida-49421/).

3.2. Pollen analysis Pollen analysis was performed on 93 samples at a depth interval of 1 cm from the bottom to 638 cm depth, and then every 4 cm from 638 cm depth to the top of the core. Palynological samples (1 cm3 of sediment per sample) were prepared and analysed using standard methods, according to Berglund and Ralska-Jasiewiczowa (1986). To estimate sporomorph concentrations in each sample, one tablet of Lycopodium was added (Stockmarr, 1971; Berglund and RalskaJasiewiczowa, 1986). For identification, keys (Faegri et al., 1989; Moore et al., 1991; Beug, 2004) and photographic reference collections were used. All sporomorphs were identified and counted until a minimum of 500 pollen grains of trees and bushes (arboreal pollen, AP) were obtained. The sum contains AP + NAP (non-arboreal pollen), except for the local aquatic and telmatic plants. The profile was split into zones based on pollen composition using CONISS (Grimm, 1992). The CONISS dendrogram and diagram illustrating pollen distributions were stratigraphically plotted by means of Tilia2 and Tilia-Graph (Grimm, 1992). During the palynological analysis, green algae coenobiae were also counted. Selected Chlorophyta were identified using the keys and taxonomic descriptions published in Komarek and Marven (1992), Jankovska and Komarek (2000), Komarek and Jankovska (2001), and Lenarczyk (2014). This group of NPP (non-pollen palynomorphs) was not included in the sum, but its presence was shown as 2

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Fig. 1. Location of Smolak Lake. (B) Detailed location of Smolak Lake profile (Sm profile) and additional drills: Sm – drilling point of lake sediment core; 1–2 drills documented the lake sediments (black dots); 3–4-drills without the lake sediments (white triangles). (C) Photo of Smolak Lake (Google Earth).

undiff).

Table 1 The physico-chemical parameters of water in Smolak Lake. Parameters

Smolak Lake (September 2017)

pH TH N TP Oxygen TOC

5.3 4 mg/L < 1.0 mg/L < 0.05 mg/L 7.05 mg/L 8.08 mg/L

3.3. Cladocera analysis Cladocera analysis was conducted on 94 samples from the depth interval 651–320 cm. The sediments were processed according the standard method (Frey, 1986; Korhola and Rautio, 2001). Each sample (1 cm3 of fresh sediment) was boiled in a 10% KOH solution for 20 min and stirred using a magnetic stirrer to remove organic matter. The residue was washed and sieved using a 40-μm sieve and diluted in 10 cm3 of distilled water. A tenth of a millilitre of solution was used for every microscope slide, and two to four slides were counted from each sample. The extracted remains were identified using a ZEISS microscope, with reference to Szeroczyńska and Sarmaja-Korjonen (2007). In each sample, all of the skeletal elements (head shield, shell, postabdomen, claw, ephippium) were counted. The results of the subfossil

percentage curves. The results are presented as the sum of species for the genus Pediastrum, (including P. boryanum v. longicorne, P. boryanum v. brevicorne, P. boryanum v. pseudoglabrum, P. boryanum v. boryanum, P., P. angulosum and P. duplex v. rugulosum) and the sum of species for Botryococcus (including B. neglectus, B. braunii, B. pila and Botryococcus 3

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ratio dropped to 11 and then fluctuated between 11 and 13. In the lowest sediments, accumulated below a depth to 647 cm, δ13C ranged from −27.4 to −26.9‰ (Fig. 2). Then (depth of 642 cm), the δ13C values abruptly dropped below −31‰ and then rapidly rose to ca. −19.5‰ (depth of 630 cm). After a decrease to −25.2‰ (depth of 606 cm), the δ13C values again increased to −19.2‰ (depth of 524 cm) and then systematically dropped. In the sediments accumulated at the depth of 484–430 cm, the δ13C values oscillated at around −23‰. Next (depth of 430–378 cm), the δ13C values abruptly dropped twice, to −27.4‰ (depth of 422 cm) and −28.4‰ (depth of 398 cm), and rose to −22.9‰ (depth of 410 cm) and −23.8‰ (depth of 378 cm). In the overlying sediments, the δ13C values decreased to −30.6‰ (depth of 352 cm) and then oscillated around −29‰. In the sediments accumulated below a depth to 647 cm, δ15N reached values of 2.3–3.2‰. Next, the δ15N values irregularly dropped to −1.8‰ (minimum of values – Fig. 2). At the depth of 606–508 cm, the δ15N values first increased to 0.1‰ (depth of 546 cm) and then irregularly dropped to −1.7‰. In the sediments accumulated at thedepth of 508–422 cm, the δ15N values rose to 0.4‰. Next (depth: 430–390 cm), a decrease in the δ15N values to − 0.5‰ (depth of 416 cm) was followed by an increase to 0.9‰. Then, the δ15N values irregularly decreased to −1.3‰ (depth of 374 cm) and rose to 0.7‰. In the overlying sediments (from depth of 340 cm), the δ15N values slightly decreased to −0.1‰ (Fig. 2).

Cladocera analyses are expressed as relative abundances. Cladocera zones (CAZ) were distinguished based on significant changes in the Cladocera relative abundance and species composition. 3.4. AMS radiocarbon dating Four samples of pollen extract were collected from the sediments of Smolak Lake for radiocarbon dating. Dating was performed by Beta Analytic radiocarbon dating, Miami, Florida, USA. Additionally, one sample of terrestrial plant macrofossils was analysed in the Poznań Radiocarbon Laboratory in Poznań. All of the conventional radiocarbon ages mentioned in the text were calibrated against the INTCAL13 calibration curve (Reimer et al., 2013). 4. Results 4.1. Core descriptions In profile Sm (taken from Smolak Lake), the lowest deposits (below a depth of 648 cm) consisted of dark grey fine sands. At depths of 6 28–648 cm, these sands were replaced by dark olive detritus gyttja. Above this, at a depth of 455–628 cm, dark brown homogenous gyttja with increasing water content occurred. The uppermost sediment (depth: 320–455 cm) consisted of dark brown dy. Two of these drill cores taken from the transitional area between Smolak and Malinowe Lakes (nos. 1 & 2, Fig. 1B) yielded a depth of 295–340 cm, and light grey, calcareous gyttja occurred. The lake sediments were covered by fluvioglacial yellow sands and gravels. In the other two drill cores from this area (nos. 3 & 4, Fig. 1B), the surface peat (thickness 30–60 cm) was directly situated on fluvioglacial sands and gravels. Profile Sm (Smolak Lake)

4.3. Pollen analysis Based on the results of the palynological analysis, six local pollen assemblage zones (LPAZ Sm-1 – Sm-6) were distinguished, reflecting the stages of vegetation history (Fig. 3A, Fig. 3B). LPAZ Sm-1 Juniperus-NAP (the lowest samples; depth: 651–626 cm) A high percentage of Juniperus (max. 29.2%) and NAP (max. 28.9%, including Poaceae) were noted. Artemisia reached a maximum value of 16.4% (Fig. 3A). The presence of pollen grains of Betula nana t., Helianthemum, Saxifraga, Dryas octopetala, and Rumex sp. was observed. The top border of this pollen zone was marked by a decline in Juniperus. LPAZ Sm-2 Betula-Pinus (depth: 626–520 cm) Juniperus, NAP, Artemisia and then Poaceae rapidly declined. A gradual increase in Betula (up to 69.9%) was observed. At the top of this pollen zone, Betula declined, and the Pinus percentage increased. Ulmus and Corylus appeared for the first time. Numerous pollen grains of aquatic plants (Nymphaea sp., Potamogeton sp., Myriophyllum spicatum and M. verticillatum) and green algae occurred (Fig. 3B). The top border of this pollen zone was marked by the growth of Corylus. LPAZ Sm-3 Pinus-Corylus (depth: 520–430 cm) Pinus reached its highest percentages (max. 68.2%). The share of Corylus pollen grains varied between 3 and 12.2%. The beginning of the Quercus, Fraxinus and Alnus curves were observed. Green algae populations significantly declined. Only single pollen grains of hydrophytes (Nuphar and Nymphaea) were noted. The top border of this pollen zone was marked by a rapid decline of Pinus. LPAZ Sm-4 Corylus-Ulmus (depth: 430–402 cm) A significant increase in the relative percentage abundance of Corylus pollen grains (max. 22.9%) was noted. The percentage of Pinus pollen grains declined to below 30% (min. 14.5%). The presence of continuous curves of deciduous trees (Tilia, Quercus, Fraxinus, Alnus) and the lowest share of NAP (below 1%) occurred in this zone. Pollen grains of aquatic plants and coenobiae of green algae (Pediastrum, Coelastrum, Scenedesmus and Tetraedron) disappeared. In this pollen zone, a small number of Botryococcus coenobiae was noted and next this number began to increase at the top border of this LPAZ. The top border of this pollen zone is marked by Quercus growth. LPAZ Sm-5 Corylus-Quercus (depth: 402–352 cm) A high percentage of Corylus (between 15.3 and 22.2%) and a decrease in Ulmus (below 5%) were noted in this zone. The percentage of Quercus pollen grains increased to above 10% (max. 11.4%). The

4.2. Stable isotopes and organic geochemistry In the studied sediments (profile Sm), the amounts of total organic carbon (TOC) and total nitrogen (TN) rose from 1.3 to 53% and from 0.1 to 5.6%, respectively (Fig. 2). The TOC/TN ratio fluctuated from 9 to 25. The carbon isotope ratio varied between −31.3 and −19.2‰, while the nitrogen isotope ratio varied between −1.8 and 3.2‰ (Fig. 2). The lowest sediments (below a depth of 647 cm) were characterized by the lowest content of organic carbon, ca. 1.3%, as well as by nitrogen below 0.1% (Fig. 2). At 646 cm both the contents of TOC and the TN increased, first to 15% and 1.3%, respectively (depth of 646 cm), and next to 32% and 2.5% (depth of 630 cm). At the depth of 628–602 cm, a further, irregular rise in the amount of organic carbon and nitrogen started. The TOC and TN increased to 50% and 4.8%, respectively (depth of 492 cm). After a slight decrease to 47% and 4%, the amount of TOC stabilized at 46–47% of TOC and ca. 4.3% of TN (depth of 430 cm). Then, further increases in the amount of organic carbon and nitrogen occurred, to 53% and 5.5%, respectively (depth of 420–412 cm), but they were not synchronic. In the overlying deposits, the TOC content varied from 50 to 53%, and the N content first dropped below 4% (depth of 352 cm) and then varied by approximately 4.3% (to depth of 348 cm). Next, the TOC values systematically decreased to ca. 48% (from depth of 352 cm), but the TN values decreased to 3.8%. In the lowest sediments, accumulated below a depth to 647 cm, the TOC/TN ratio was difficult to calculate due to the very low nitrogen content (below 0.1%), and could therefore be significantly overstated (Fig. 2). In sediments accumulated to a depth of 500 cm, the TOC/TN irregularly decreased from 13 to 10. After a slight increase to 13 (depth of 492 cm)), the TOC/TN ratio stabilized at 11 (to depth of 430 cm). Next, the TOC/TN ratio first increased to 13 (depth of 418 cm), then decreased to 9 (depth of 414–410 cm) and finally rose to 14 (depth of 398 cm). In the overlying deposits (depth of 394–374 cm), the TOC/TN 4

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Fig. 2. Results of geochemical and isotope analyses of the organic sediments from Smolak Lake. TOC – total organic carbon, TN –total nitrogen; Chronozones: SA – Subatlantic, SB – Subboreal, AT – Atlantic, BO – Boreal, PB – Preboreal, YD – Young Dryas.

Fig. 3A. Percentage diagram of selected trees and terrestrial plants from Smolak Lake succession. Sm-1 – Sm-6 LPAZ – local pollen assemblage zones; Chronozones: SA – Subatlantic, SB – Subboreal, AT – Atlantic, BO – Boreal, PB – Preboreal, YD – Young Dryas. 5

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Fig. 3B. Percentage diagram of telmatic, aquatic plants and green algae from Smolak Lake succession. Sm-1 – Sm-6 LPAZ– local pollen assemblage zones; Chronozones: SA – Subatlantic, SB – Subboreal, AT – Atlantic, BO – Boreal, PB – Preboreal, YD – Young Dryas. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Alonella nana (11%). CAZ II (depth: 626–538 cm) Fifteen Cladocera species were identified in zone CAZ II. The total number of Cladocera individuals significantly decreased and reached ca. 1300 individuals per 1 g of dry sediment. Planktonic species also significantly declined and were represented only by two taxa that occurred sporadically. The most dominant species were littoral Alona rectangula (33%) and Chydorus sphaericus (30%). CAZ III (depth: 538–427 cm) Seventeen Cladocera species were identified. The number of individuals was very low and did not exceed 700 individuals/g of dry sediment. During this time, only littoral species were present, and Alonella nana (24%), Chydorus sphaericus (14%) and Camptocercus rectirostris (12%) were the predominant taxa. CAZ IV (depth: 427–344 cm) The number of Cladocera individuals (average 2900/g of dry sediment) and the number of species (18) significantly increased. Planktonic species appeared one more time but in a very low abundance (up to 3%). Littoral taxa dominated, especially species associated with macrophytes: Alona affinis (22%), Acroperus harpae (8%) and the acidophilous Alonella excisa (30%). CAZ V (depth: 344–320 cm) The number of Cladocera individuals (2700/g of dry sediment) and the number of species (15) slightly decreased. Planktonic species occurred again (average 13%, max 35%): Bosmina (E.) coregoni and Bosmina (E.) longispina, Bosmina longirostris and Daphnia longispina – group. Littoral species were dominated by Alonella excisa (31%), Alona affinis (9%) and Alonella nana (12%).

continuous curves of Carpinus, Fagus and Picea started. A few pollen grains of Potamogeton sp. were observed. The number of Botryococcus coenobiae increased. The top border was marked by the decline of Corylus and Quercus. LPAZ Sm-6 (depth: 352–320 cm) The percentages of Corylus, Quercus, Ulmus, Tilia and Fraxinus deceased. The share of Carpinus pollen grains increased to above 5% (max. 8.8%). Pollen grains of cereals appeared. The presence of Botryococcus coenobiae was noted. 4.4. Cladocera analysis A total of 25 Cladocera species belonging to four families were identified in the sedimentary sequence from Smolak Lake (profile Sm). Planktonic species were represented by the families Bosminidae and Daphniidae, and littoral species, by Chydoridae and Sididae (Fig. 4). Littoral species represented the majority (an average 98%) of the Cladocera assemblages. Planktonic species were present in only two sections: in the lowest and in the topmost sediments (Fig. 4). The Cladocera species composition and structure allowed us to identify five Cladocera assemblage zones (CAZ) that summarized the main stages of Cladocera development in Smolak Lake (Fig. 4). CAZ I (the lowest samples, depth: 651–626 cm) Fourteen Cladocera species were identified in the oldest sediments of Smolak Lake. In the bottom part of profile Sm (651–647 cm), no Cladocera remains were recorded (Fig. 4). In the upper part of the sequence, Cladocera remains were recorded, and the number of individuals per 1 g of dry sediment was ca. 2000. Planktonic species were represented by 4 taxa from two families (Bosminidae and Daphniidae). The littoral species were dominated by Chydorus sphaericus (44%) and 6

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Fig. 4. Results of subfossil Cladocera analysis from Smolak Lake succession. Diagram of Cladocera species in relative abundance, Total sum of Cladocera (number of Cladocera individuals in 1 g of dry sediment), Plankton/Littoral – ratio of planktonic to littoral form. I-V CAZ – Cladocera Assemblages Zones, Cladocera zonation indicated by horizontal lines; Chronozones: SA – Subatlantic, SB – Subboreal, AT – Atlantic, BO – Boreal, PB – Preboreal, YD – Young Dryas.

4.5. Drill no. 1 – on the shore of Malinowe Lake In the sediment sequence from drill no. 1, a total of 12 Cladocera species belonging to two families were identified. In the oldest sediments (338–326 cm), no Cladocera remains were recorded. In the upper sediment sequences (324–302 cm), planktonic and littoral species were represented by only one family: Bosminidae (planktonic) and Chydoridae (littoral). Littoral species accounted for more than 90% of the total Cladocera relative abundance represented. In general, the total Cladocera amount was quite low and varied from a few hundred to 2500 individuals per 1 g of dry sediment. 4.6. Radiocarbon dating Five AMS 14C dates were obtained from profile Sm (Table 2). The calibrated ages of these samples are in stratigraphic order. Based on these data, the age of these sediments was determined (Fig. 5). The accumulation of sediments started ca. 12,000 yr cal BP. 5. Interpretation The palynology presented here enables the reconstruction of the vegetation in Smolak Lake and its surroundings, while the biostratigraphy - the establishment of the timing of environmental changes. The ecological preferences of Cladocera were used to reconstruct the changes in the water level and trophic status of Smolak Lake. The isotopic and geochemical data helped to describe the characteristics of the Table 2 AMS 14C dates from Smolak Lake succession. Depth (cm)

Laboratory number

AMS

329 399 481 551 590

Beta-451927 Beta-451928 Beta-451929 Beta-389257 Poz-64427

1080 4530 8920 9470 9630

14

C yr BP

± ± ± ± ±

30 30 30 30 50

cal. yr BP (68% range) 2σ

yr cal. BP

955–985 5125–5165 10130–10175 10679–10745 10850–11126

970 5150 10,160 10,722 10,977

Fig. 5. The age-depth model of the sediments from Smolak Lake.

7

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autochthonous organic deposits, and organic carbon and nitrogen in the sediments began to increase to above 20% and 2%, respectively (Fig. 2). These sediments were formed as dark olive detritus gyttja. The accumulation rate was 0.7 mm/yr (Fig. 6). The lower TOC/TN ratios (11–13) indicate an increased share of algae in the organic matter. δ13C rapidly increased (to −19.5‰), likely reflecting an increase in photosynthesis (Kołaczek et al., 2015). δ15N systematically dropped to −0.8‰, suggesting low water trophy (Brenner et al., 1999). At that time, zooplankton appeared (Fig. 4). In this initial state of the lake, the Cladocera species composition was dominated by species tolerant to cold waters, such as Chydorus sphaericus (44%), Alonella nana (11%), and Alona affinis (8%). This species composition suggests that the lake water was poor in nutrients, and was probably oligotrophic (Whiteside, 1970; Nevalainen et al., 2013; Zawisza et al., 2019). The dominance of littoral species indicates a low water level. However, the presence of planktonic species (Bosminidae and Daphniidae) suggests the existence of an open water zone (Fryer, 1985; Flössner, 2000; MirosławGrabowska and Zawisza, 2014). In the lake, green algae colonies appeared. The most numerous were Pediastrum and Botryococcus populations, but a low abundance of Tetraedron and Scenedesmus and a single coenobiae of Coelastrum reticulatum also appeared (Fig. 3B).

organic sediments and define the source of the organic matter. 5.1. The origin of Smolak Lake Smolak Lake is a postglacial lake originating in the final phase of the Vistulian glaciation (Lisicki, 1994). At the end of the Pleistocene, climate warming caused the deglaciation of the Masurian Lakeland area. First, the vast outwash plain was formed. Then, between melting dead ice blocks, longitudinal kettle holes situated transversely to the front of the disappearing ice sheet were formed. In these depressions, Malinowe and Smolak Lakes were created. Despite the close proximity of the lakes, they were not connected, as evidenced by the lack of lake deposits in the area between these lakes (additional drills nos. 3 & 4, Fig. 1B). The calcareous gyttja found in drills nos. 1 & 2 documented the initial, more extensive range of Malinowe Lake. Now, it is a very small reservoir surrounded by a fluvioglacial plain (Lisicki, 1994). During the Younger Dryas (YD), detrital deposits such as light grey, fine sands initially accumulated at the bottom of Smolak Lake. The oldest sediments contained the lowest content of organic carbon found anywhere in the core (ca. 1.3%) as well as nitrogen below (0.1%; Fig. 2), suggesting low primary production in the lake at this time. This is supported by low δ13C (between −27 and −31‰), which may reflect the dominance of C3 plants in the catchment (Meyers and LallierVergès, 1999), and by δ15N values above 2‰, which suggest low nitrogen availability in the surface waters (Talbot and Laerdal, 2000). The lack of aquatic macrophyte macrofossils and Cladocera remains in profile Sm during this period (Fig. 3B) confirms low primary production in the reservoir; it is likely that cold, nutrient-poor water was not conducive to macrophyte or zooplankton development. Because of the low productivity of the lake, organic matter from this time was predominantly of terrestrial origin (Björck et al., 1993). The pollen record suggests that the deposition of terrestrial organic matter was promoted by loose catchment vegetation cover dominated by juniper scrub, supplemented by herb steppe and shrub tundra with Betula nana (Fig. 3A). About 11,800 yr cal BP, a more typical lake sedimentation started. Organic matter delivery from land was partially replaced by

5.2. The harmonic development of Smolak Lake The transition from Late Glacial (Pleistocene) to Holocene (Younger Dryas/Preboreal) conditions took place ca. 11,500 yr cal BP. At the beginning of the Preboreal period, open landscapes prevailed around Smolak Lake, with grasses, herbs and, less frequently, juniper bushes (Fig. 3A). The wetland habitats in the region were dominated by willow scrub. Initially, loose birch forests appeared, and they soon became the dominant plant communities. At the end of the Preboreal period, poor habitats were covered by pine forests. In Smolak Lake itself, the onset of the Holocene was characterised by an increases in TOC (to 40%) and TN (> 3%) as warming led to increases in primary production and vegetation (Rantala et al., 2015; Karpińska-Kołaczek et al., 2016; Zawisza et al., 2019). During the

Fig. 6. Results of selected geochemical and biological data from Smolak Lake succession. Chronozones: SA – Subatlantic, SB – Subboreal, AT – Atlantic, BO – Boreal, PB – Preboreal, YD – Young Dryas. 8

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Preboreal period (PB), the accumulation rate first rose to 1.6 mm/yr (to ca. 10,700 yr cal BP) and then slightly dropped to ca. 1 mm/yr (Fig. 6). Organic carbon and nitrogen within these sediments continued to rise (ca. 45% and above 4%, respectively), but the TOC/TN ratio decreased (Fig. 2), suggesting an enhancement of primary productivity and the development of the algae population (Leng et al., 2005). Indeed, the rapid growth of two Chlorophyte genera: Tetraedron and Coelastrum reticulatum, was evident (Fig. 3B). In the organic matter, δ13C and δ15N both initially dropped (to ca. −25‰ and −1.8‰, respectively), and then increased (to −21‰ and −0.3‰, respectively) most likely reflecting the development of macrophytes in the lake (Ji et al., 2005). The pollen record shows that abundant aquatic plant communities with Nymphaea, Potamogeton, Myriophyllum spicatum and M. verticillatum developed, which quickly filled the small basin, used nutrients and limited light access. In the margin zone of the lake, the rushes appeared (Typha latifolia, Sparganium t.). The development of the plant communities likely caused a rapid decrease in the green algae population (Pediastrum and Botryococcus) and a disappearance of the Coelastrum reticulatum and Tetraedron colonies (Dembowska et al., 2018). The development of macrophyte plants in the lake is further reflected in the Cladocera record. There was a growth in the number of littoral taxa (14 taxa in YD, 17 taxa in PB; Fig. 4), and planktonic species almost completely disappeared, indicating a significant reduction or disappearance of the open water zone. There was also significant development of plant-associated species (e.g., Alona quadrangularis, Alona rectangula, Acroperus harpae, Pleuroxus uncinatus). These changes in Cladocera communities suggest that all parts of the lake were likely overgrown by macrophytes (Zawiska et al., 2013; Zawisza et al., 2016). The Cladocera species composition also suggests the low edaphic condition of Smolak Lake. At the end of this period, the Scenedesmus population absolutely dominated in the water. Around the lake, a peat bog with Cyperaceae, Filicales and Sphagnum developed. In the Boreal period (BO, ca. 10,400–9000 yr cal BP), pine, hazel and elm spread across the catchment as climatic conditions improved (Karpińska-Kołaczek et al., 2013). In the poor habitats, pine woods dominated. Areas with better soil conditions were occupied by hazel and elm (Zachowicz et al., 2004). Wet places were favourable for willow brush (Balwierz et al., 2004). The biological and geochemical data suggest that, after some initial changes in the early Boreal, this was a period of relative stability and harmonic conditions in Smolak Lake. At the start of this period, the TOC, TN, and TOC/TN ratio increased before stabilising at ca. 10,000 yr cal BP. A significant decrease in the green algae population was observed (it was the last time the coenobiae of Scenedesmus were present). At ca. 9500 yr cal BP, the sediments smoothly changed into dark brown dy. Towards the end of the Boreal period, Nymphaeaceae (Nymphaea, Nuphar) were communities, which were present at the start of the Boreal (Fig. 3B), probably disappeared. This event coincided with a low presence of green algae (there were only coenobiae of the Botryococcus and Pediastrum genera; Fig. 3B). Throughout the Boreal period, the zooplankton communities were characterized by the continued development of littoral species and the absence of planktonic species. At the time, the dominant taxa were Alonella nana, Camptocercus rectirostris, Alona rectangula and Chydorus sphaericus living in association with macrophytes (Fig. 4). Around Smolak Lake, peat bogs continued to develop, indicated by the presence of Sphagnum, Cyperaceae and Filicales in the pollen record.

in the conditions in Smolak Lake were reflected in all proxies. At about 9000 yr cal BP, the sediment accumulation rate dropped to an extreme low of 0.08 mm/yr (Fig. 6). An increase in TOC, TN and δ15N coinciding with a decrease in the C/N ratio and δ13C were recorded (Fig. 2). The increase in the amount of carbon and nitrogen reflects a further rise in the content of organic matter in the lake sediments. During the Atlantic period, the maximum amounts of organic carbon and nitrogen were observed. Recorded amounts of organic carbon and nitrogen reached their maxima. At about 7300–6700 yr cal BP, δ13C and δ15N increased (by ca. 4‰ and 2‰, respectively). These changes in the geochemical parameters are likely the result of lowering the water level. Alongside this water level lowering, the ecological proxies point to the lake transitioning to a dystrophic state. Pollen spectra show a lack of hydrophytes in the lake water and the absolute lowest presence of green algae (Fig. 3B, Fig. 6). During the Atlantic period, only one genus of algae, Botryococcus, appeared in the water. Most likely, in the littoral zone and in the immediate lake surroundings, changes in telmatic vegetation began. At the beginning of the Atlantic period, the rush vegetation disappeared (as suggested by the last record of the pollen grain Typha latifolia and by the almost-disappeared Cyperaceae). The lake transition to a dystrophic state is manifested in the Cladocera community by the dominance of littoral species, with the predominance of acid-tolerant species (Alonella excisa) and species occurring among vegetation (Alona affinis, Eurycercus lamellatus). These changes were also reflected by the increasing number of Cladocera individuals (to 2200 individuals/g of dry sediment; Fig. 6). It is highly probable that during the Atlantic period, an intensive process of lake overgrowth (probably by Sphagnum) started. At that time, a proper dystrophication process began, reflected by significant changes in the Cladocera and aquatic plant communities. An increasing number of Cladocera individuals, in which the majority consisted of acid-tolerant species, was observed in the beginning (transition period) of the dystrophication process in the other Polish lakes (Drzymulska et al., 2015; Zawisza et al., 2019). Such zooplankton reactions can be the result of increasing macrophyte cover and growing abundance of detritus in the lake. Also at this time, Scenedesmus definitively disappeared from Smolak Lake’s water. This genus shows a strong preference for an alkaline environment (Yang et al., 2018; Bakuei et al., 2015), and its disappearance is probably related to water acidification caused by the dystrophication process. In the Subboreal period (SB, ca. 5700–2500 yr cal BP), the forest composition changed once more, and new taxa of deciduous trees appeared (Carpinus, Fagus, Picea). The lake was surrounded by peat bogs with Sphagnum and Cyperaceae. The habitat was acid (the spores of Lycopodium annotinum were found in these sediments). The beginning of this period, organic matter in the lake sediments was still high (above 50% for TOC), and a drop in δ13C suggests the appearance of algae. A simultaneous increase in the TOC/TN ratio may reflect a supply of terrestrial organic matter. Sedimentation rate also very slightly increased to ca. 0.2 mm/yr at the beginning of the Subboreal. Later, at about 4400–3900 yr cal BP, the δ13C values further increased by ca. 4‰, probably indicating a higher share of plant detritus in the analysed deposits and a low water level. In the lake, some aquatic plants appeared again (but only the pollen grains of the genus Potamogeton were found), and the population of Botryococcus rapidly grew. The early part of this period was characterized by an increase in the number of Cladocera individuals. Similar to the Atlantic period, at this time, the Cladocera species composition in the Subboreal period was dominated by acid-tolerant and macrophyte-associated species (Alonidae, Acroperus harpae, Eurycercus lamellatus). Alonella excisa is considered one of the most acid-tolerant species, its rise to dominance (40%) during the Subboreal reflects the ongoing development of acid conditions in the lake. The forest composition in the Subatlantic period (SA) was similar to the Subboreal period, but the proportions of the main tree taxa changed. In particular, there was decrease in Corylus. At this time, human indicator plants such as Rumex a/a, Plantago lanceolata and

5.3. The dystrophic development of Smolak Lake In the Atlantic period (AT, ca. 9000–5700 yr cal BP), the vegetation surrounding Smolak Lake developed. The pollen record shows a lower abundance of Pinus, and suggests that most areas were occupied by mixed deciduous forest with hazel, elm, lime, oak and ash. In the moist habitats, willow was replaced by alder (Balwierz et al., 2004; Szczepanek et al., 2004). At the beginning of the Atlantic period, fundamental, rapid changes 9

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associated changes observed in zooplankton and aquatic plant compositions indicate progressive dystrophication. The littoral species of Cladocera were dominated by acid-tolerant Alonella excisa and species occurring among vegetation (Alona affinis, Camptocercus rectirostris). Similar changes in the Cladocera communities were observed in the other lakes in the region as they transformed from a harmonic state to a dystrophic state (Zawisza et al., 2019; Drzymulska et al., 2015). In those lakes, located ca. 100 km away, the transition into dystrophic conditions occurred at different times (e.g., Suchar IV – 7200 yr cal BP; Suchar II – 5800 yr cal BP). The ecological changes also were recorded by the lowest abundance of green algae and domination of only one genus, Botryococcus. All recognized taxa are typical for habitats with a low trophic status (Komarek and Marven, 1992). A very similar record was observed in Lake Suchar IV (Zawisza et al., 2019). Further environmental changes occurred when Smolak Lake became overgrown by floating mats of Sphagnum moss, and peat bogs with Sphagnum and Cyperaceae developed around the lake. In the lake, habitat acidification occurred, as indicated by the dominance of macrophyte-associated and acid-tolerant Cladocera species (Alonidae, Acroperus harpae, Eurycercus lamellatus, Camptocercus rectirostris). Alonella excisa replaced other species from the genus Alona (e.g., Alona quadrangularis, Alona rectangula). In particular, the Cladoceran species Chydorus sphaericus was significantly reduced. This species is considered one of the most tolerant species with respect to environmental conditions (Walseng et al., 2003; Belyaeva and Deneke, 2007), and is disappeared or decreased significantly in abundance when the process of dystrophication started in the other dystrophic lakes in the region, including Suchar IV and Suchar II (Drzymulska et al., 2015; Zawisza et al., 2019). This suggests that Chydorus sphaericus has low tolerance to dystrophic conditions, despite being widely observed in a wide spectrum of edaphic and thermal conditions. About 1500 yr cal BP, an open water zone again appeared. This environmental change was likely connected with (1) a limited floating mat area or (2) a higher water level. Higher water levels were recorded in other lakes in the region at this time, e.g. Lake Purwin (Gałka and Apolinarska, 2014) and Suchar Wielki (Drzymulska et al., 2014). The first traces of human activity were recorded in Smolak Lake sediments that accumulated during the Subatlantic period and could be connected to the Bronze Age. This signal was very weak (only single pollen grains of Plantago lanceolata) and occurred later than signals from other sites in NE Poland. Most pollen diagrams from this region show the first human indicators in the Neolithic, for example, at Lake Miłkowskie (Wacnik, 2009), Lake Salęt (Szal et al., 2014), and Gązwa (Gałka et al., 2014a). The human impact documented by pollen data at Smolak Lake remained weak, suggesting that the lake has never experienced significant pressures from human impact. Currently, the lake's surroundings are still protected from human influence by a nature reserve. Smolak Lake, which is characterized by brown water and the presence of floating mats, is a very good example of a dystrophic lake. The original name of the lake (Smolak) refers to the colour of the water being “pitch black”. Given the long history of dystrophic conditions in the lake, we recommend that protection of the lake and its catchment from human impact continue.

cereals appeared. From ca. 2500 yr cal BP, organic carbon and nitrogen remained high (ca. 48% and 3.8%, respectively) despite a slight decrease at the beginning of the period. δ13C and δ15N slightly decreased, with low values of δ13C (below −28‰) suggesting a share of freshwater algae in the organic matter (Leng et al., 2005). A very low sediment accumulation rate of 0.2 mm/yr is noted for this time (Fig. 6). Similar to Atlantic and Subboreal periods, only one genus of algae, Botryococcus, was present. About 1500 yr cal BP, the re-colonization of the lake by planktonic cladocerans from the families Bosminidae and Daphniidae occurred, suggesting the existence of an open water zone for the first time since the Late Glacial. The littoral zone remained inhabited by Cladocera species with a tolerance for reduced water pH living in association with aquatic vegetation (mainly Alonidae, Acroperus harpae). 6. Discussion The results obtained from multidisciplinary analyses of Smolak Lake sediments allowed us to describe the evolution of this lake from its origin in the Younger Dryas to the present. Our research demonstrates that the dystrophy process in Smolak Lake began about 9000 years cal BP, i.e. earlier than in other lakes in the region. Changes in the composition of phyto- and zooplankton as well as in the composition of organic matter due to dystrophy are similar to those in other lakes. As the transition into dystrophy occurred in different lakes at different times, it can be stated that it was independent of climate, but strongly associated with the small size of Smolak Lake and the small supply of mineral substances from its catchment. For the first three thousand years of its existence (ca. 12,000–9000 yr cal BP), Smolak Lake developed harmonically. Initially, the trophic state was oligotrophic, as indicated by the Cladocera species composition and the low number of Cladocera individuals. From about 11,800 yr cal BP, an open water zone with planktonic species, such as Bosminidae and Daphniidae, was established. At that time, a population of green algae developed (e.g., Scenedesmus, Tetraedron). During the next one thousand years (ca. 11,500–10,400 yr cal BP, Preboreal period), a rich community of aquatic plants (including Nymphaea and Potamogeton) developed, consuming nutrients and limiting light access to the lake. The expansion of macrophytes provoked the development of Cladocera species connected with aquatic plants, almost complete decline of planktonic species and a significant reduction of the open water zone. In the Boreal period a peat bog area developed around Smolak Lake, and the Cladocera communities were dominated by macrophyte-associated littoral taxa. Such changes are indicative of the lowering of the water level, as observed in other lakes in the area at that time, e.g.: Linówek Lake (Gałka et al., 2014b), Charzykowskie Lake (Mirosław-Grabowska and Zawisza, 2014), Romoty (MirosławGrabowska et al., 2015), Czarne Lake (Karpińska-Kołaczek et al., 2016) and the Skaliska Basin (Kołaczek et al., 2015). Despite the reduced water level, biological and geochemical data from Smolak Lake still indicate a harmonic state. Again, this reflected in similar studies of other early Holocene lakes in the region, e.g., Romoty (MirosławGrabowska et al., 2015), the Skaliska Basin (Kołaczek et al., 2015). About 9000 yr cal BP the rapid transition of Smolak Lake to a dystrophic state began (Fig. 6). At this time, the climate and vegetation surrounding the lake were relatively stable. It therefore seems unlikely that dystrophication was linked to climatic change. Rather, the process towards dystrophy was a natural phenomenon closely connected with Smolak Lake’s size, regime and catchment. The catchment of the lake was dominated by sandy fluvioglacial sediments. These sands were quickly activated (eroded) and the minerals and elements contained within them delivered to the lake. In addition, vegetation growing in the catchment (coniferous forests, peat bogs) supplied organic substances to the lake. These gelatinous, organic deposits identified as dy, were accumulated under dystrophic and acid conditions, when the sedimentation rate was extremely low (0.08–0.2 mm/yr; Fig. 6). The

7. Conclusions

• For the first three thousand years of its existence (ca. 12,000–9000 yr cal BP), Smolak Lake developed harmonically. The • dystrophy process in Smolak Lake began about 9000 years cal BP, i.e. earlier than in other lakes in the region. • Under dystrophic and acid conditions, when the sedimentation rate • 10

was extremely low (0.08–0.2 mm/yr), the gelatinous, organic deposits identified as dy (TOC > 50%) were accumulated. In the dystrophic conditions, macrophyte-associated and acid-tolerant Cladocera species (Alonidae, Acroperus harpae, Eurycercus

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• • •

lamellatus, Camptocercus rectirostris) dominated. Alonella excisa replaced other species from the genus Alona. During the Atlantic period, Smolak Lake became overgrown by floating mats of Sphagnum moss, and peat bogs with Sphagnum and Cyperaceae developed around the lake. About 1500 yr cal BP, an open water zone again appeared. Smolak Lake has never experienced significant pressures from human impact.

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