Layout 1 INTRODUCTION The increasing trophic status or eutrophication of water bodies is a modern worldwide problem. Nowadays, rapid increases of lake trophic levels can be easily ob- served in a large number of lake ecosystems (Schmidt et al., 2000; Richerson et al., 2008). The natural dynamics of many lake ecosystems, especially those located in urban and agricultural areas, have been highly modified by human activity. The effects of human related nutrient supply have been widely investigated and discussed (Joye, 2006; Smith and Schindler, 2009; Gałka et al., 2014). It is common opinion that major human impacts on lake trophic processes began in the Middle Age or in the era of the Industrial Revolution (starting from the 19th century). However, paleolimnological studies have demonstrated that ancient cultures with a lesser techno- logical development, such as the Neolithic or even older Mesolithic human groups, were also able to substantially impact lake ecosystems (Goransson, 1986; Latałowa, 1994, 2003). In addition, changes in lake trophic level have also resulted from natural processes (mostly related to climate changes), and from their interactions with an- thropogenic influence (Zawisza and Szeroczyńska, 2007; Nevalainen et al., 2013; Grabowska et al., 2015). As a re- sult, the assessment of the relative influence of human and natural impacts on lakes in past times and at present is critical for establishing viable management plans for these ecosystems. In this sense, paleolimnological studies have substantially contributed to understanding the long term dynamics of eutrophication processes, e.g. in discriminat- ing the origin of nutrients (human or natural). Of special interest is the assessment of the time span required by im- pacts and processes to act upon water bodies and produce Advances in Oceanography and Limnology, 2016; 7(2): 163-176 ARTICLE DOI: 10.4081/aiol.2016.6234 This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). Subfossil Cladocera and pollen as indicators of natural and anthropogenic trophic changes of Lake Jelonek (Tuchola Forest, N Poland) during the Holocene Edyta Zawisza,1* Anna Filbrandt-Czaja,2 Alexander Correa-Metrio3 1Institute of Geological Sciences, Polish Academy of Sciences, Research Centre in Warsaw, ING PAN, Twarda 51/55, PL-00818 Warsaw, Poland; 2Chair of Geobotany and Landscape Planning, Faculty of Biology and Environmental Protection, Nicolaus Copernicus University in Toruń, Lwowska 1, Toruń PL-87100, Poland; 3Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 D.F., Mexico *Corresponding author: ezawisza@twarda.pan.pl ABSTRACT Lake Jelonek is a small lake located in central northern Poland, in the Tuchola Forest. The sediments of the lake represent a natural archive that offers insights into the natural history of the region from the Late Glacial to present. In winter 2002, a 1330 cm long sediment core was recovered from the deepest part of lake. Using a multiproxy approach (cladocerans, pollen and basic geochemistry), we reconstructed trophic status changes through the last ~15,000 years. Special attention was devoted to the evaluation of nutrient con- tributions to the lake from natural and anthropogenic sources. The Cladocera analyses yielded a total of 29 species belonging to five fa- milies (Bosminidae, Daphniidae, Leptodoridae, Chydoridae, Sididae), with planktonic species representing more than 60% of Cladocera relative abundance throughout the core. The pollen results suggested four periods of increased human activity, so-called settlement pha- ses. The first traces of human activity in the basin of Lake Jelonek appeared in the Atlantic period and were related with Mesolithic and Neolithic settlements. The second (Bronze Age) and the third (Iron Age) settlement phases are well marked by the paleolimnological proxies studied. This time period clearly manifested on the lake waters as an increasing trophy level probably caused by human-associated discharges of nutrients to the lake. After the third settlement phase cladoceran data indicated a significant decrease in the lake trophic level and the pollen data showed a recovery of forest cover. The fourth period of human economic activity during the early Middle Age was characterized by deforestation associated with land reclamation for grazing and cultivation of cereals, and the subsequent nutrient enrichment of lake waters. According to our results, the biological development of Lake Jelonek was determined by climate changes from Late Glacial up to the Atlantic period. Contrastingly, the most important driver for the lake development though the middle and late Holocene has been human activity within the lake catchment. The recovery of the lake trophic level during the last century, which is primarily related to extensive afforestation of the lake catchment, highlighted the importance of land use in the conservation of healthy water bodies Key words: Trophic changes; subfossil Cladocera; pollen analysis; Tuchola Forest; paleolimnological multi-proxy analysis; Holocene. Received: August 2016. Accepted: November 2016. Non co mmerc ial us e o nly 164 E. Zawisza et al. significant changes in their trophic status. It is well-known that lake-catchment systems respond physically, chemi- cally and biologically to natural climatic changes as well as to environmental changes caused by human activities (O’Sullivan, 1983; Ralska-Jasiewiczowa et al., 2003). Lake ecosystems most rapidly and clearly respond to nat- ural or anthropogenic nutrient supply (Smol, 2008). One of the best and most powerful biological proxies which can be studied in lake sediments for the reconstruc- tion of lake trophic evolution, both related to climate and humans, is represented by Cladocera remains (Zawisza and Szeroczyńska, 2007; Chen et al., 2010). Cladocera, as a key component of zooplankton, represent a biotic component of the lake ecosystem and provide important information on the lake environment (Lampert and Sommer, 2007). On the other hand, pollens represent one of the best and most important sources of information about changes in terres- trial vegetation and land use. The interpretation of pollen data offers the potential for reconstructing human impact on the lake catchment in prehistoric times (Behre, 1986; Birks et al., 1988). Thus, the combination of subfossil Cladocera studies and palynological analyses can enhance the reliability of reconstruction of past water trophic level and land-use within the lake catchment. In this study, we present the reconstruction of changes in trophic status of Lake Jelonek (N Poland) from its ori- gin until modern times, with the objective of assessing the relative contribution of natural and anthropogenic sources of nutrients on the basis of subfossil Cladocera and pollen analysis. The long sediments record of Lake Jelonek, which covers a time span of ~14,000 years, offered a good opportunity to understand the relative contribution of cli- mate and human activity to the development and dynam- ics of aquatic ecosystems in this region. From the Late Glacial to the early Holocene the record provided insights into the dynamics of a system driven mainly by natural forces. Contrastingly, the study of the regional and local conditions from the mid Holocene to modern times pro- vided scenarios for different human influences on the landscape. Thus, this time period mostly contributed to understanding lake dynamics under contrasting techno- logical systems of land use. Pollen indicator taxa provided information on the main modes of human land use of the lake catchment, while basic geochemistry and Cladocera provided insights into the impact of human activities on the trophic status of the lake. METHODS Study site Lake Jelonek is located in central northern Poland, in the south-eastern part of Tuchola Forest (Fig. 1 a,b), at an altitude of 91 m asl The lake is of glacial origin and lo- cated in a NW-SE oriented glacial channel on the ice line of the Last Glacial Maximum close to the Wda River (Błaszkiewicz, 2006). Lake Jelonek is a small shallow water body, with an area of 20.9 ha and an average depth of 4.5 m (zmax 13.8 m, Fig. 1c). The length of the shoreline is 2250 m and the shoreline development ratio is small (1.39). The water pH is circa neutral (~7.2), and the con- ductivity is around 300 µS cm–1. The modern trophic sta- tus of Lake Jelonek has been determined as mesotrophic (Jańczak, 1997). The Tuchola Forest region is character- ized by a climate from the group D – continental/mi- crothermal climates (McKnight and Darrel, 2000; Przybylak et al., 2010; Woś, 2010). Mean annual air tem- perature and precipitation are 7.6°C and 558 mm, respec- tively (Filbrandt-Czaja, 2009). The winter lasts around 70 days, with a mean temperature of -3.2°C. The duration of ice-cover is approximately 3 months, from the beginning of January until the end of March, with a growing season of approximately 210 days per year. The modern vegeta- tion is dominated by the monoculture of pine, arising from a one-way human activity. Nowadays, area the natural for- est ecosystems are almost absent in the Tuchola Forest (Boiński, 1992). Sampling and analyses In winter 2002, a 1330 cm long sediment core was re- covered from Lake Jelonek using a Livingstone-type corer. The sampling point (53°46.1’ N, 18°23.5’ E) was located in the deepest part of the lake (water depth ~13.8 m, Fig. 1c). After coring, the sediment was packed and transported to the laboratory for a lithological description, and a subsampling procedure, which was carried out at 5 to 10 cm intervals. Subsamples were analysed for subfos- sil Cladocera, pollen, and basic geochemistry. Fifteen points along the core were sampled for the ra- diocarbon dating of the sedimentary sequence. Twelve ter- restrial plant macrofossils samples were 14C dated by AMS (Accelerator Mass Spectrometry), whereas three of bulk sediment samples were analyzed by 14C conventional meth- ods (Stuiver and Polach, 1977). All radiocarbon analyses were carried out at GADAM Absolute Dating Methods Centre, Gliwice, Poland (Tab. 1). Ages were calibrated ac- cording to the IntCal 13 curve (Reimer et al., 2013). Se- lected pairs of depths and calibrated ages were used to fit a Bayesian age-depth model using Bacon (Blaauw and Christen, 2011). Carbonate and organic matter content were measured in 76 sediment subsamples (10 cm3), which were firstly dried at 105°C and homogenized in an agate mill, and then subjected to a two-step procedure (Bengtsson and Enella, 1986). In the first reaction, organic matter was oxidized at 550°C to carbon dioxide and ash. In the sec- ond reaction, carbon dioxide was evolved from carbonate at 925°C, leaving oxide. The carbonate content was cal- culated using the relationship Carb =1.36 LOI925, where Non co mmerc ial us e o nly Millenial-scale trophic changes of Lake Jelonek 165 Fig. 1. Location of Lake Jelonek. a) Location in Europe. b) Location of Lake Jelonek (asterisk) in the Tuchola Forest area, boundaries of Tuchola Forest area according to Kowalewski (2002). c) Bathymetric map of Lake Jelonek with the coring point marked at the lake deepest point. Non co mmerc ial us e o nly 166 E. Zawisza et al. LOI925=CO2 evolved from the sample, as defined by Heiri et al. (2001). The subfossil Cladocera analysis was conducted on 143 sediment subsamples (1 cm3), which were processed ac- cording to the standard method proposed by Frey (1986). Samples were boiled in a 10% solution of KOH, and later left for half an hour in a magnetic stirrer to eliminate or- ganic matter. The residue was washed with distilled water and sieved through a 35 μm mesh. The final residue was dissolved in 10 mL–1 distilled water. For every microscope slide, 0.1 mL of final solution was used and the identifica- tion of remains was made at magnifications of 100x, 200x, and 400x, using an OLYMPUS BX40 transmitted-light mi- croscope. Two to four slides (minimum of 200 remains) from each sample were counted. All cladoceran remains were counted (head shields, shells, postabdomens, ephip- pia), and complete individuals were added up from body parts. Cladocera identification was based on Szeroczyńska and Sarmaja-Korjonen (2007) and Flössner (2000). Palynological analyses were conducted on 137 sedi- ment subsamples (1 cm3) accounting at a resolution of 5- 10 cm. Samples were treated to eliminate non-pollen components of the sediment according to standard proto- cols (Berglund and Ralska-Jasiewiczowa, 1986), and ace- tolysis was conducted to remove cellulose. Two Lycopodium tablets were added to each sample prior to acetolysis in order to allow the calculation of absolute concentrations of sporomorphs (Stockmarr, 1971). Treated samples were stained with fuchsine and immersed in glycerine, and sporomorphs were counted under a mi- croscope Zeiss at 340 and 680 magnifications. At least 1000 pollen grains were counted from each sample, with total pollen counts being lower only in bottom samples, because of very low pollen amounts in the deepest core section. Identification of pollen grains and spores was based on Erdtman et al. (1961), Faegri and Iversen (1989), Reille (1992), and Beug (2004). Regional pollen assem- blages reported as markers of chronostratigraphic units for the studied time period were used to refine the chronology after Filbrandt-Czaja (2009). All arboreal (AP) and herbaceous (NAP) taxa were in- cluded in the pollen sum that served as the basis to calcu- late pollen percentages. Simplified pollen diagrams were constructed using the C2 software (Juggins, 2007) to sum- marize the changes of selected taxa, mainly trees and herbaceous plants considered as indictors of human im- pact (Behre, 1981). The identified taxa were subdivided into six major ecological groups (after Berglund 1969): i) trees preferring damp soils, e.g. Alnus; ii) highly compet- itive and shade-tolerant trees; iii) trees immigrating during late Holocene, e.g. Fagus, Carpinus, Picea; iv) scarcely competitive and light-demanding trees, often belonging to the primary stage of Holocene forest successions; v) apophytes, i.e. shrubs and herbs favoured by man; vi) an- thropochors, i.e. herbs and graminoids introduced by man. Core depth are indicated in all stratigraphic plots as cm below the lake surface (bls), in order to allow com- parison with previous paleolimnological studied on Lake Jelonek (Filbrant-Czaja 2009; Błaszkiewicz, 2006). RESULTS Sediment lithology and geochemistry The sediments of Lake Jelonek were mainly com- posed of detritus and calcareous gyttja (Fig. 2). Below 2705 cm bls, the sediment consisted of light beige sand. Between 2705 and 2695 cm bls, the sediment was domi- nated by sandy silt (ca. 20% CaCO3 content, Fig. 2), topped by a 5 cm-thick layer of sand (2695-2690 cm). From 2690 to 2580 cm bls, the sediment was composed of light grey silt with an increasing carbonate content reaching up to 40%. From 2580 to 2500 cm bls, sediment was characterized by dark olive-beige silt detritus gyttja (organic matter ca. 50%, CaCO3 >10%). From 2500 to 2400 cm bls, light grey detritus gyttja with a very low con- tent of CaCO3 >5% was identified. Between 2400 and 2160 cm bls calcareous-detritus gyttja with occasional laminations occurred. Dark detritus gyttja with a high con- tent of organic matter and low CaCO3 dominated from 2160 to 1800 cm bls. From 1800 to 1530 cm bls, sedi- ments were composed of dark calcareous-detritus gyttja with occasional lamination. From 1530 to 1480 cm bls, the sediment was dark detritus gyttja, whereas the upper- most part of the sedimentary deposit (1480 to1380 cm bls) consisted of dark brown-olive, calcareous detritus gyttja, with high CaCO3 content (on average ca. 45%). Chronology The age-depth model was constructed using 12 radio- carbon dates (Tab. 1) and 5 ages defined by the appear- ance of biostratigraphic markers of the Central Europe chronostratigraphy (Mangerud et al., 1974; Walanus and Nalepka, 2010). Three pollen-based dates were rejected due to stratigraphic inconsistencies. Thus, a total of 17 depth-age data pairs were used to fit the Bayesian age- depth model (Fig. 3), yielding a basal age of ~14,000 cal- ibrated years before present (cal yr BP) for the whole sedimentary sequence. Subfossil Cladocera A total of 29 Cladocera species belonging to five fam- ilies were identified in the sedimentary sequence of Lake Jelonek. Planktonic species were represented by the fam- ilies Bosminidae, Daphniidae, and Leptodoridae, as well as by littoral species of Chydoridae and Sididae. Plank- tonic species accounted to more than 60% of total Clado- Non co mmerc ial us e o nly Millenial-scale trophic changes of Lake Jelonek 167 cera relative abundance, thus representing the major com- ponent of the Cladocera assemblages during the last 14,000 years (Fig. 4). Six Cladocera Assemblages Zones (CAZ) were empirically distinguished based on the species composition and changes in the relative abun- dance of the species (Fig. 4). CAZ I, before 12900 cal yr BP (2705 – 2640 cm bls) The initial phase of Lake Jelonek development was characterized by low Cladocera abundance and low species richness. Eight Cladocera species were identified, with a maximum concentration of 800 specimens per cm3 of sediment (Fig. 4). Pelagic species, such as Bosmina longirostris (O.F. Müller, 1757) and Daphnia longispina group (O.F. Müller, 1785), were dominant (with a relative abundance over 80%). Littoral species tolerant to cold wa- ters, i.e. Alona affinis (Leydig, 1860), Chydorus sphaeri- cus (O.F. Müller, 1785), Acroperus harpae (Baird, 1835), Alonella nana (Baird, 1843), were also present. CAZ II 12900 - 9600 cal yr BP (2640-2540 cm bls) This phase was characterized by a sudden increase in Cladocera abundance. A total of 20 species were identified, consisting mainly in planktonic taxa, i.e. Bosmina lon- girostris, Bosmina (E.) coregoni (Baird, 1857), Daphnia pulex group (Leydig, 1860), and Leptodora kindti (Focke, Fig. 2. Schematic lithology and depth profiles of organic matter, carbonate and mineral fraction content in the sediments from Lake Jelonek; bls, below lake surface. Non co mmerc ial us e o nly 168 E. Zawisza et al. 1844), which accounted for over than 60% of total clado- ceran remains (Fig. 4). Noteworthy is the presence of species associated to warm waters, such as Camptocercus rectirostris (Schoedler, 1862), and Pleuroxus trigonellus (O.F. Müller, 1776), and of species living in association with aquatic plants and having higher edaphic require- ments, such as Chydorus sphaericus and species from the genera Alona (Baird, 1843) and Alonella (Sars, 1862). CAZ III 9600 - 6200 cal yr BP (2540 - 2420 cm bls) Cladocera species richness increased through this phase to up to 23. Pelagic Bosmina longirostris, Bosmina (E.) coregoni, and Bosmina (E.) longispina dominated the cladoceran community. The latter species showed its first appearance and reached its maximum abundance through the entire sediment record within this zone. A total of 17 littoral species were identified in CAZ III, the majority being reported as occurring in association with aquatic plants, such as Alona and Alonella spp. as well as Acrope- rus harpae, Pleuroxus sp. and Sida crystallina (Straus, 1820). Three species showed their first appearance within this phase, namely Alonella excisa (Fischer, 1854), Ley- digia leydigi (Schoedler, 1858), and Graptoleberis testu- dinaria (Fischer, 1854) (Fig. 4). CAZ IV 6200 - 3900 cal yr BP (2420 - 2350 cm bls) This period was characterized by a major change in the structure of the cladoceran community. Only one pelagic species, i.e. Bosmina longirostris, persisted with an average abundance of ca. 38%. Littoral species domi- nated for the first time through the history of the lake, es- pecially those favoured by higher trophic status of the water. The most abundant of these species were Alona rectangula (Sars, 1862) (35%) Acroperus harpae (10%), Alonella excisa (9%), and Pleuroxus trigonellus (8%). CAZ V 3900 - 1500 cal yr BP (2350 – 1730 cm bls) The flourishment of both pelagic and littoral clado- ceran species characterized this period. The upper and lower boundaries of this phase were marked by an in- crease in the abundance of Alona rectangula and Chy- dorus sphaericus. These two species together with Bosmina longirostris accounted for over 75% of all clado- cerans (Fig. 4). The early stages of CAZ V was marked by the disappearance of pelagic species associated to lower trophic status waters, i.e. Bosmina (E.) longispina and Bosmina (E.) coregoni, and by the decline of Alona rectangula and Chydorus sphaericus. In the second half of this phase, the abundance of planktonic species in- creased again: Bosmina (E.) coregoni and Bosmina (E.) longispina reached 17% and 11%, respectively, of the whole cladoceran remains. CAZ VI 1500 cal yr BP – present (1730 - 1380 cm bls) This phase was characterized by a clear decline in the abundance of Cladocera, namely from an average of 27000 ind. cm–3 in phase V down to 7000 ind. cm–3 in phase VI. Pelagic species (Bosminidae) dominated and Tab. 1. 14C dates from Lake Jelonek sediments. Sample name Depth Type of Laboratory number 14C yr BP cal yr BP (cm bls) dating Jelonek-1-1450* 1450 AMS GdA-823 172±30 340-302 (17.4%) 276-185 (50.9%) Jelonek-2-1550* 1550 AMS GdA-824 417±30 560-484 (86.7%) Jelonek-3-1660* 1660 AMS GdA-825 1382±35 1405-1311 (95.2%) Jelonek-4-2060* 2060 AMS GdA-826 1742±35 1782-1606 (95.4%) Jelonek-5-2120° 2120 AMS GdA-827 1647±35 1496-1596 (68%) Jelonek-6-2260° 2260 AMS GdA-828 2222±35 2181-2307 (68%) Jelonek-7-2350* 2350 AMS GdA-829 3462±35 3883-3691 (95.4%) Jelonek-8-2380* 2380 AMS GdA-830 4222±40 4909-4839 (38.5%) 4895-4676 (56.9%) Jelonek-10-2480* 2480 conv. Gd-30100 6702±180 7922-7322 (93.3%) Jelonek-11-2540* 2540 conv. Gd-30098 7852±120 9048-8578 (95.4%) Jelonek-12-2560° 2560 conv. Gd-15878 8602±140 9499-9857 (68%) Jelonek-13-2580* 2580 AMS GdA-832 10252±50 12219-10814 (94.1%) Jelonek-14-2600* 2600 AMS GdA-833 10882±60 12948-12737 (95.4%) Jelonek-15-2660* 2660 AMS GdA-834 10832±60 12872-12713 (95.4%) Jelonek-17-2705* 2705 AMS GdA-836 13192±70 13221-15693 (95.4%) bls, below lake surface; *samples included in the age-depth model; °samples excluded from the age-depth model because of stratigraphical inconsistencies; AMS, Acceleration Mass Spectrometry; conv., conventional 14C dating. Non co mmerc ial us e o nly Millenial-scale trophic changes of Lake Jelonek 169 accounted for about 60% of total cladoceran abundance. The contribution of Alona rectangula and Chydorus sphaericus significantly declined, accounting only for 8% and 6% of the total abundance, respectively (Fig. 4). Pollen Palynological taxa that are considered as indicators of water fertility, e.g. Nymphaea (L.), Nuphar (Sm.) or Pota- mogeton (L.) occurred at small percentages (<2%) throughout the entire stratigraphic profile, and therefore could not be used to reconstruct the trophic evolution of Lake Jelonek (Filbrandt-Czaja, 2009). Based on the analy- sis of pollen taxa which are considered as indicators of human activity four so-called human colonization phases were distinguished (Fig. 5). Pollen showed first evidence of human activity in the basin of Lake Jelonek during the Atlantic period, which identifies the first settlement phase (ca. 7.8 - 3.85 cal kyr BP between 2470 and 2350 cm bls). The phase covered the period of Mesolithic and Neolithic economic recovery and is characterized by a decline of elm (by 6%), oak (by 5%) and ash (by 2%), along with an increased abundance of pine (Fig. 5). The presence in this core section spores of fern (Pteridium aquilinum (L.) Kuhn), a fern species occurring on soils containing ash (Tinner et al., 2000), confirms the presence of human settlements in the region. At the end of this phase, the first pollen grains of ribwort plantain (Plantago lanceolata (L.)) were identified. How- ever, indicators of anthropic pressure occurred with a rel- atively low percentage in this core section. The beginning of the second settlement phase (3.6 - 2.9 cal kyr BP, 2300-2140 cm bls) coincided with the Bronze Age. This period was represented in the pollen di- agram (Fig. 5) by deforestation, in particular a decline of hornbeam (Carpinus betulus (L.)) as well as oak (Quercus (L.)), elm (Ulmus (L.)), linden (Tilia (L.)) and hazel (Corylus avellana (L.)). Reduction in the number of mesophilous deciduous trees was probably connected with the objective of settlers to acquire fertile lands over- grown with forests. This period was also characterized by an increased content of mugwort pollen (Artemisia (L.)), a ruderal plant which indicates presence of crops as it was a common weed of primitive cereal crops (Behre, 1981). The increase in the content of heather (Calluna vulgaris (L.) Hull) pollen along with the presence of common ju- Fig. 3. Age-depth model based on the 14C dating of Lake Jelonek sediments, showing sediment accumulation from Late Glacial to current times. Stars indicate 14C date (see Tab. 1), and arrows point the chronostratigraphical (pollen) borders according to Mangerud et al. (1974) and Walanus and Nalepka (2010). Non co mmerc ial us e o nly 170 E. Zawisza et al. niper (Juniperus communis (L.)) and broadleaf/hoary plantain (Plantago (L.) major/media) pollen indicate the presence of canopy gaps caused by cattle grazing, as well as an increase in the area of dry pastures. The end of the second settlement phase was characterized by a substan- tial increase in the abundances of hornbeam (Fig. 5). The third settlement stage encompasses the Iron Age (ca. 2.8 - 1.7 cal kyr BP, 2110-1790 cm bls) and was dis- tinguished by a significant increase of palynological an- thropogenic indicators and by the decline of deciduous trees such as hornbeam, linden, ash and elm (Fig. 5). The most important feature of this period was the increasing abundance of cereal pollen, mainly rye (Secale cereale (L.)), and sorrel pollen (Rumex acetosa (L.)/acetosella (L.)). The upper border of this phase is marked by inter- ruption or significant descent of percentages of all herba- ceous taxa (Fig. 5). The fourth and last settlement stage (1.3 cal kyr BP up to the present, 1660-1380 cm bls) began in the early Middle Age and was characterized by the increase of herbaceous taxa and the decline of deciduous tree species pollen (Fig. 5). A number of palynological taxa occurred, which Fig. 4. Relative abundance, total Cladocera sum (as individuals cm–3 of fresh sediment), and planktonic/littoral ratio of sediment subfossil Cladocera of Lake Jelonek from Late Glacial to current times; bls, below lake surface. Fig. 5. Simplified, pollen diagram from Lake Jelonek. The represented taxa are subdivided into six ecological groups according to Berglund (1969): i) trees on damp soils; ii) highly competitive and shade-tolerant trees; iii) trees immigrating during late Holocene; v) scarcely competitive and light-demanding trees; v) shrubs and herbs favoured by man (apophytes); vi) herbs and graminoids introduced by man (anthropochors). AP, arboreal pollen; NAP, non-arboreal and herbs pollen; bls, below lake surface. Non co mmerc ial us e o nly Millenial-scale trophic changes of Lake Jelonek 171 are considered as indicators of grazed forests, meadows and pastures, such as Poaceae, Rumex sp., Plantago lanceeo- lata, Ranunculaceae, Calluna vulgaris, Campanulaceae. Pollen grains of cultivated plants, e.g. Cerealia type, Secale cereale, wheat (Triticum (L.)) occurred in large numbers. DISCUSSION Based on the results of geochemical analyses, subfos- sil Cladocera remains and pollens the trophic conditions in Lake Jelonek were reconstructed from the Late Glacial to modern times. Changes in the composition of Clado- cera species, and in particular of taxa indicating higher lake trophic conditions (e.g. Bosmina longirostris, Chy- dorus sphaericus, Alona rectangula), enabled the assess- ment of the trophic history of Lake Jelonek. Changes in the afforestation rate (inferred by increasing or decreasing percentages of arboreal pollens), together with the pres- ence of pollen of synanthropic and cultivated plants, helped to determine the type of economy (breeding, cul- tivation) and its impact on the vegetation cover of the lake catchment (Berglund, 1969; Aaby, 1986). Lacustrine sedimentation in Lake Jelonek began dur- ing the Late Glacial. The pollen spectrum that character- ized this time period (CAZ I, before 12900 cal yr BP) indicates that the plant communities resembled the vege- tation of the arctic tundra where heliophytes and plants tolerant to low temperature and unfavourable edaphic conditions are dominant (Filbrandt-Czaja, 2009). The low Cladocera species richness (only eight species) indicated a relatively shallow lake characterized by cold and nutri- ent-poor waters. The Cladocera species composition and the dominance of Bosmina longirostris at the base of the record suggested that species typically living in the open water zone were more aggregated to the lake shores. Large areas of the lake were free of plant coverage, and nutrients were available only within the shoreline zone. The content of mineral matter deposited within CAZ I amounted up to 95% of wet sediments, likely as a result of high supply of clastic material from the vegetation free shores or melting dead-ice blocks. Species richness and abundance of cladocerans in- creased at the end of Late Glacial, indicating that climate and environmental conditions during the second half of this period were more suitable for zooplankton develop- ment. The more favourable climate conditions were con- firmed by the rising abundances of pine pollen, although pollen of numerous herbaceous taxa was still present in large numbers. High relative abundances of Cyperaceae indicate well-developed vegetation in the littoral zone. The higher abundances of pelagic species (i.e. Bosmina (E.) coregoni) indicated a likely deepening of the lake dur- ing the stage of progressive climate warming. Average an- nual air temperature showed a clear increase by the early Holocene (Ralska-Jasiewiczowa et al., 1998), which in turn induced a substantial transformation of the vegetation within the catchment of Lake Jelonek. The first stage of succession was the growth of birch and birch-pine forests. The warming was also reflected in the succession of Cladocera species, which showed a sudden and clear in- crease in species diversity and abundance. Through the second half of CAZ II (11400 - 9600 cal yr BP), in addi- tion to planktonic species (family Bosminidae), pelagic species Leptodora kindti and Daphnia pulex group ap- peared (Szeroczyńska, 1993; Korhola, 1999; Amsink et al., 2005), and accounted together to ca. 60% of the clado- ceran assemblage. A substantial increase of planktonic species suggested high water levels during the Preboreal period. An increase of species that live in association with aquatic plants was also observed, in particular of species belonging to the family Alonidae, and species associated with warmer and clearer waters, such as Pleuroxus trigonellus and Camptocercus rectrirostris (Korhola, 1999; Amsink et al., 2005; Szeroczyńska and Zawisza, 2011). The presence of these species indicates the expan- sion of the littoral zone and the development of littoral veg- etation. The content of organic matter in the sediment showed a gradual increase since the beginning of the Holocene, and reached ~50% at the end of the Preboreal period. However, despite the prevalence of more favourable climatic and edaphic conditions during the early Holocene, the subfossil Cladocera indicate no significant changes in lake trophic status, which probably remained at the oligo-mesotrophic level (Fig. 6). This persistence of low nutrient levels in the water might have been possibly related to dilution by the rising of water level. The development of forests, which initiated during the Preboreal period, continued through the Boreal one, and was mainly characterise by increasing pine accompanied by mesophilous trees species, namely Corylus avellana, Alnus, and Ulmus. This period was also characterized by an increased frequency and abundance of Cladocera species (at the beginning of CAZ III, 9600-8400 cal yr BP). Bosmina (E.) longispina, a species preferring well- oxygenated waters with low trophic status, occurred in the pelagic zone and reached its maximum abundance through this zone. The presence of both pelagic and lit- toral taxa indicates that the lake was fairly deep and its littoral zone was well developed. High values of organic matter (over 50%) deposited in the sediments during this period confirmed the richness of life in the littoral zone and an oligo-mesotrophic status (Fig. 6). During the Holocene Climate Optimum the vegeta- tion around Lake Jelonek was characterized by mixed deciduous forests dominated by Ulmus, Quercus, Tilia and Fraxinus. The presence of pollen grains of ivy (Hed- era helix (L.)) and mistletoe (Viscum (L.)) clearly indi- Non co mmerc ial us e o nly 172 E. Zawisza et al. cates mild winters and warm summers (Iversen 1944, Troels-Smith 1960). The Atlantic period (CAZ III and CAZ IV, 8400-5800 cal yr BP) manifested in the Clado- cera community by the dominance of species with higher edaphic requirements (Bosmina longirostris), and of species that usually occur in association with aquatic plants (e.g. Pleuroxus trigonellus and Acroperus harpae, both belonging to the family Chydoridae). Such a clado- ceran assemblage indicates a probably increase in lake trophic conditions up to mesotrophic levels (Fig. 6). It is possible that the increase in the trophic status of Lake Jelonek was the result of the first stage of human settle- ment, which was discriminated on the basis of palyno- logical evidences at the end of the Atlantic period. In fact, this period corresponds to the Mesolithic settle- ment, which, however, was characterized by little groups of settlers that generally had little impact on the existing water bodies (Filbrandt-Czaja, 2009). The economic ac- tivity of Mesolithic tribes of hunters mainly focused on forested areas where they cleared the undergrowth with fire, aiming of chasing out the game and intensify the growth of herbaceous vegetation that served as food for deer (Latałowa, 1994, 2003; Filbrandt-Czaja, 2009). It seems likely, therefore, that the increased trophic status of Lake Jelonek during the Atlantic period was mainly related to natural processes associated with favourable climate conditions (Zawisza and Szeroczyńska, 2007) rather than to human activity. Since the beginning of the Subboreal period, the Neolithic human impact was much more evident in the sedimentary record of Lake Jelonek, even though transformation of the natural environment by Neolithic groups of settlers still remained small, as the main subsistence strategy was still based on hunting and gathering (Latałowa, 2003, 2007; Filbrandt-Czaja, 2009). Nevertheless, human activity within the lake catchment was reflected by the succession of subfossil Cladocera, by a higher abundance of species that prefer nutrient rich water (e.g. Alona rectangula), and by the disappearance of pelagic species that prefer waters with a lower trophic level (Bosmina (E.) longispina and Bosmina (E.) coregoni). During this period, the lake was probably in mesotrophic conditions (Fig. 6). Fig. 6. Comparison of relative abundances of Cladocera species indicator of lake trophic status (i.e. Bosmina longirostris, Alona rec- tangula, Chydorus sphaericus) and total Cladocera abundance sum with settlement phases identified by pollen analyses. Trophic re- construction based on subfossil Cladocera analysis. CAZ, Cladocera Assemblages Zones; bls, below lake surface. Non co mmerc ial us e o nly Millenial-scale trophic changes of Lake Jelonek 173 Between 3850-3600 cal yr BP , Cladocera species pre- ferring low trophic conditions, such as Bosmina (E.) core- goni and Bosmina (E.) longispina (Hofmann, 1977; Korhola, 1999; Brancelj et al., 2009; Zawisza et al., 2016) further decreased, while Alona rectangula declined. Such changes in the zooplankton community indicate a reces- sion in the trophic status of Lake Jelonek back to oligo- mesotrophic levels, which was most likely related to a gap in the human colonization process (between settlement phase 1 and 2), which was inferred based on the pollen evidence (Fig. 6). Through the middle and late Subboreal period, there were substantial changes in the cladocerans succession (CAZ V). Whereas Alona rectangula and Chydorus sphaericus increased during this period (reaching the highest values in the profile), Bosmina (E.) coregoni sig- nificantly decreased, and Bosmina (E.) longispina disap- peared from the record. Such species composition suggests that the lake was affected by an eutrophication process at the time, probably caused by an increased sup- ply of nutrients into the water. The increasing curve of species considered indicators of increased trophic status (Chydorus sphaericus, Alona rectangula) was perfectly synchronous with the second and third settlement stages. i.e. during the Bronze and Iron Age, respectively, which both were determined based on palynological analysis (Fig. 6). The occurrence of grasses (Poaceae), Plantago lanceolata, Plantago major/media, Rumex sp. suggests that livestock played an important role in the human econ- omy. Most likely they were grazed in thinned forests where large areas were covered with heather, cow wheat (Melampyrum (L.)), and bracken. These activities played a major role during the Bronze and Iron Age (Filbrandt- Czaja, 2009), and continued during the Roman influence (Wielowiejski, 1981). During this time humans likely set- tled close or around Lake Jelonek, so that cattle breeding and other human activities resulted in an increased supply of nutrients to the lake, which increased the lake produc- tivity up to eutrophic conditions through CAZ V. The in- crease in the organic matter content in the sediments layers of Lake Jelonek deposited during this period offers additional support to the interpretation of increased lake trophic status. It is important to mention that significant changes in pollen spectra were recorded toward the end of the sec- ond settlement, especially consisting in an increasing abundances of hornbeam trees. This was high probably related to the colonization of abandoned fields and graz- ing lands by this arboreal taxon, which suggests that set- tlers abandoned the environs of Lake Jelonek. The recovery of the forest cover marked a discontinuity in the colonization, which lasted for around 150 years between phase 2 and 3. The second half of the Subatlantic period, correspon- ding to the Migration Period at the beginning of CAZ VI, was characterized by a significant decrease in the abun- dance of Cladocera species. In particular, the frequency of species indicating high trophic status declined (i.e. Alona rectangula and Chydorus sphaericus), whereas the contribution of pelagic species Bosmina (E.) longispina and Bosmina (E.) coregoni increased. Such a species as- semblage indicate a reduction in the trophic status of Lake Jelonek down to mesotrophic conditions, which persisted till present day. The improvement of the lake water quality was probably associated with human populations aban- doning the Lake Jelonek basin. This process began about 1600 years ago in the period referred to as “pre-modern human migration” (Guy, 2007; Kobyliński, 2015). Signif- icant regression of colonization between IV-VI century AD and the related forest regeneration, were reflected by the recovery of forest species and by a major decline of all herbaceous plants. Indicators of human occupation dropped to a minimum, which outline an encroachment of broadleaf forest dominated by hornbeam on areas aban- doned by man. The fourth settlement phase (Fig. 6) began during the early Middle Age and continued to present day. The pollen sequence from Lake Jelonek indicates that human activity through the early Middle Age was accompanied by land deforestation, probably associated with land reclamation for grazing and cereal cultivation. The high content of ce- real pollen clearly indicates an increased intensity of col- onization compared to the previous period, as well as the dominant role of cereal crops in the human diet. However, according to archaeological research, the region of Lake Jelonek was sparsely populated during the Middle Age (Wawrzeniuk, 2005). Probably, temporal human settle- ments characterized this time period (Woźny, 2005), which may imply a significant decrease in the amount of biogenic substances delivered to the lake. Indeed, a con- siderable decline of Alona rectangula and Chydorus sphaericus, both indicators of high trophic status, offer further support for the inference of small human popula- tions. Historical data suggests that the impact of human colonization in the vicinity of Lake Jelonek gradually in- creased since the 15th century (Chudziak, 2000). Maps from the late 18th century (Schrötter maps) show that at that time the lake was surrounded to the north and north- east by arable fields and meadows, as well as by pine forests to the south and west. The land-use change af- fected the structure of Cladocera zooplankton, which con- sisted in the decrease of species usually associated with oligotrophic clear waters, such as Bosmina (Eubosmina) and Alona affinis. The most extensive deforestation in the region of Lake Jelonek occurred in the first half of the 19th century (Ślaski, 1951), i.e. in the period associated with the up- permost sediments layer (CAZ VI). Abundance of species Non co mmerc ial us e o nly 174 E. Zawisza et al. that usually occur in nutrient rich waters (Bosmina lon- girostris and Alona rectangula) increased at that time. Since the early 20th century the lake has been surrounded again by a dense forest (Broda, 2000). Afforestation rep- resent the major land use change since then, and has re- sulted in a considerable isolation of the lake from the impact of human economic activity. After the lands around Lake Jelonek were abandoned by the local popu- lation, the nutrient input to the lake decreased consider- ably. This allowed the beginning of a process of so-called self-purification of the lake, which finally returned to present mesotrophic conditions. CONCLUSIONS The environmental and ecological development of Lake Jelonek through the Late Glacial and the early Holocene appeared to be mainly driven by climate changes, whereas the impact human activities became prevailing over climate related factors through the mid and late Holocene. Except during its initial formative phase, Lake Jelonek permanently was a relatively deep water body, while its trophic status oscillated several times between oligo- and eutrophic level. Oligotrophic conditions prevailed from the Late Glacial to the early Holocene, while eutrophic conditions dominated during the settlement stages of the Bronze and Iron Age. During the remaining periods the lake mesotrophic conditions prevailed. Given the lake size and depth and the popula- tion density near the shores, it can be concluded that the increase in the trophic status during the Subboreal and Subatlantic period was primarily the results of human eco- nomic activity. The last century has been characterized by the extensive recovery of the forest cover, which has sub- stantially contributed to the recovery of a mesotrophic lake status. The results presented here provide new important in- formation on effects of climatic changes on the Tuchola Forest area, and on the edaphic condition of its lakes dur- ing Late Glacial and Holocene. The data presented, espe- cially those on subfossil Cladocera analysis, integrated other paleolimnological studies, thus contributing at re- ducing the existing knowledge gap regarding causes and mechanisms of water eutrophication during the last mil- lennia. The present study also stressed the close relation- ship that exists between lake water quality and lake catchment processes. Changes in forest cover connected with human activity revealed to have played the major role in driving lake trophic changes, which was not out- lined for the area of Tuchola Forest before. The results of the present study can contribute to a better understanding of effects of climate change at anthropogenic influence on long term trophic fluctuation of lakes in Northern Poland and Central Europe. 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