J Paleolimnol (2009) 41:407–430 DOI 10.1007/s10933-008-9234-2

ORIGINAL PAPER

A 40,000-year record of environmental change from ancient Lake Ohrid (Albania and Macedonia)

Bernd Wagner Æ Andre´ F. Lotter Æ Norbert Nowaczyk Æ Jane M. Reed Æ Antje Schwalb Æ Roberto Sulpizio Æ Verushka Valsecchi Æ Martin Wessels Æ Giovanni Zanchetta

Received: 19 December 2007 / Accepted: 24 June 2008 / Published online: 11 July 2008 Ó Springer Science+Business Media B.V. 2008

Abstract Lake Ohrid is considered to be of Pliocene vegetation had changed to forest dominated by pine origin and is the oldest extant lake in Europe. A 1,075- and summer-green oak. Several of the proxies suggest cm-long sediment core was recovered from the south- the impact of abrupt climate oscillations such as the eastern part of the lake, from a water depth of 105 m. 8.2 or 4.0 ka event. The observed changes, however, The core was investigated using geophysical, granu- cannot be related clearly to a change in temperature or lometric, biogeochemical, diatom, ostracod, and humidity. Human impact started about 5,000 cal. year pollen analyses. Tephrochronology and AMS radio- BP and increased significantly during the past carbon dating of plant macrofossils reveals that the 2,400 years. Water column mixing conditions, inflow sediment sequence spans the past ca. 39,500 years and from subaquatic springs, and human impact are the features a hiatus between ca. 14,600 and 9,400 cal. most important parameters influencing internal lake year BP. The Pleistocene sequence indicates relatively processes, notably affecting the composition and stable and cold conditions, with steppe vegetation in characteristics of the sediments. the catchment, at least partial winter ice-cover of the lake, and oxygenated bottom waters at the coring site. Keywords Lake Ohrid Mediterranean The Holocene sequence indicates that the catchment Pleistocene Holocene Palaeolimnology

B. Wagner (&) A. Schwalb Institut fu¨r Geologie und Mineralogie, Zu¨lpicher Str. 49a, Institut fu¨r Umweltgeologie, TU Braunschweig, 50674 Ko¨ln, Germany Pockelsstrasse 3, 38106 Braunschweig, Germany e-mail: [email protected] R. Sulpizio A. F. Lotter V. Valsecchi CIRISIVU, c/o Dipartimento Geomineralogico, Laboratory of Palaeobotany and Palynology, Institute of via Orabona 4, 70125 Bari, Italy Environmental Biology, Universiteit Utrecht, Budapestlaan 4, 3584 CD Utrecht, The Netherlands M. Wessels Institut fu¨r Seenforschung, LUBW, Argenweg 50/1, N. Nowaczyk 88045 Langenargen, Germany Geoforschungszentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany G. Zanchetta Dipartimento di Scienze della Terra, University of Pisa, J. M. Reed via S. Maria 53, 56126 Pisa, Italy Department of Geography, University of Hull, Cottingham Road, Hull HU6 7RX, UK 123 408 J Paleolimnol (2009) 41:407–430

Introduction as proposed by Collier et al. (2000) may explain some of these discrepancies, but more research is In the Northern Hemisphere, most present-day lakes needed to elucidate spatial variability in climate and are of glacial origin and were formed during the late environmental history over time. Pleistocene or early Holocene. Lakes of tectonic Despite the age of Lake Ohrid and the extensive origin, however, can be much older, sometimes dating biological, biogeographical, and, more recently, lim- back to the Tertiary. Lake Ohrid, a transboundary lake nological studies, there has been very little between the Republics of Albania and Macedonia, is palaeolimnological research to date. A study on short located in a deep tectonic graben (e.g. Aliaj et al. gravity cores gives evidence for eutrophication during 2001) and is considered to be the oldest lake in Europe. the past decades (Matzinger et al. 2006a, 2007), and High levels of endemism with more than 200 endemic highlights significant heterogeneity in sediment accu- species described, mainly comprising invertebrates mulation rates across the basin, with relatively low and algae, but also some fish (e.g. Jerkovic 1972; Kenk sedimentation rates in the central part and increasing 1978; Decraemer and Coomans 1994; Michel 1994; sedimentation rates towards the littoral zone (Wagner Levkov et al. 2007) are an indication of its age. Based et al. 2008). The only longer-term study published to on biogeographical principles, Stankovic (1960) esti- date is a low-resolution analysis of an 8.85-m-long mated that the origin of Lake Ohrid dates back to the sediment core recovered in 1973 (Roelofs and Kilham Pliocene, some 3–5 million years ago. 1983). This core is estimated to span the past ca. Because of its geographic position and its age, 30,000 years. Proxies such as water content, organic

Lake Ohrid represents an important link between matter, CaCO3, and diatom assemblages show evi- climatic and environmental records from the Medi- dence for an increase in productivity at the terranean Sea and the adjacent continent. In the Pleistocene–Holocene boundary. However, the inter- eastern Mediterranean Sea, most records focus on the pretation was hampered by chronological uncertainty, Late Pleistocene and Holocene history (e.g. Geraga probably due to sediment slumping resulting from et al. 2005) and only a few cover several glacial– tectonically-induced mass movement, a common interglacial cycles (e.g. Schmiedl et al. 1998; Howell phenomenon in this region (e.g. Aliaj et al. 2004). et al. 1998). Similarly, most terrestrial records from In order to generate rigorous palaeoclimatic and the northern Mediterranean region are restricted to palaeoenvironmental data for the northern Mediterra- the Late Pleistocene and Holocene (e.g. Dene`fle et al. nean region, and to assess the potential of Lake Ohrid 2000; Ramrath et al. 2000; Schmidt et al. 2000; for longer-term palaeoenvironmental reconstruction, a Sadori and Narcisi 2001); longer continuous records sediment sequence from the lake is investigated in this covering more than the last glacial/interglacial cycle study using a multidisciplinary approach, which are relatively sparse (e.g. Wijmstra 1969; Ryves et al. includes chronological, geophysical, sedimentologi- 1996; Tzedakis et al. 1997; Brauer et al. 2000). In cal, biogeochemical, and biological proxies. addition, the Pleistocene and Holocene records from the northern Mediterranean region suggest significant spatial climate variability (cf. Tzedakis 2007). For Setting example, while some authors propose relatively cold and dry climatic conditions with a typical steppe Lake Ohrid (40°540–41°100 N, 20°380–20°480 E) is biome during the Last Glacial Maximum and low located at an altitude of 693 m a.s.l. It is about 30 km lake levels in some regions (e.g. Rossignol-Strick and long, 15 km wide, and covers an area of 360 km2 Planchais 1989; Wijmstra et al. 1990; Watts et al. (Fig. 1). Bathymetric measurements revealed that the 1996; Allen et al. 2000; Digerfeldt et al. 2000), lake has a simple, tub-shaped basin morphology with others suggest moister conditions for the same period, a maximum water depth of 286 m (e.g. Stankovic as for example indicated by significantly higher lake 1960). A complete overturn of the entire water levels or faunal remnants (e.g. Alessio et al. 1986; column occurs approximately once every 7 years, Giraudi 1989; Narcisi et al. 1992; Belis et al. 1999). whereas the upper up to 200 m of the water column is Enhanced seasonality during the glacial period, with mixed every winter (Stankovic and Hadzisce 1953; cool, dry summers and wet winters in the region, such Hadzisce 1966; Matzinger et al. 2007). The episodic 123 J Paleolimnol (2009) 41:407–430 409

15° 20°E (A) Crni Drim Sateska R. Koselska R.

Lake

Ohrid L a k

e

P

r

e

s

p a

Neapolitan volcanoes 40°

Mount Etna volcano

Mediterranean 0 100 200km 35° Sea N

Fig. 1 Map of the northern Mediterranean region showing the location of Lakes Ohrid and Prespa at the Macedonian/ Albanian border. Coring location Lz1120 is indicated by the white dot in the southeastern part of Lake Ohrid

50 m Lz1120 isolation of the bottom waters causes oxygen deple- 100 m 150 m tion and increases the total dissolved hypolimnetic 200 m phosphorus concentration. This dissolved phosphorus N Cerava R. 250 m T can be transferred to the epilimnion during complete 004 km overturn, where it enhances productivity. Today, (B) 100 m Lake Ohrid is oligotrophic (TP = 4.6 lgl-1; Matz- Lz1120 inger et al. 2007), which is reflected by a Secchi disc transparency of more than 9 m (Stankovic 1960; Ocevski and Allen 1977; Naumoski 2000; Matzinger et al. 2006a; Naumoski et al. 2007). Macrozooben- 120 m thos is common at the sediment surface, even in the deepest part of the lake (Stankovic 1960). 200 m The high water transparency also results from a high proportion ([50%) of inflow from karst aquifers SW NE and minor contributions from rivers and direct Fig. 2 (A) Lake Ohrid with bathymetry in 50 m contour precipitation (Matzinger et al. 2006a). River runoff intervals. The main inlets and outlets are indicated by the 3 -1 contributes ca. 20% to the total inflow of 37.9 m s arrows. St. Naum (N) and Tushemisht (T) spring areas to the and was even lower prior to 1962, when the River southeast of the lake are indicated by asterisks. (B) Seismic Sateska was diverted into the northern part of Lake profile (SW–NE) across the coring location Lz1120 Ohrid (Fig. 2). About 50% of aquifer input is thought to enter the lake as sublacustrine flow, and 50% as surface inflow, which is concentrated at the southeast- 2006a; Fig. 1). The outflow of Lake Ohrid is the river ern and northwestern edge of the lake. The karst Crni Drim in the northern part of the lake, which aquifers are charged by precipitation and by the 150 m accounts for 63% of the water loss, with the remaining higher Lake Prespa, located 20 km to the east and 37% accounted for by evaporation (Watzin et al. separated by a mountain ridge from Lake Ohrid 2002). The theoretical hydraulic water residence time (Stankovic 1960; Anovski et al. 1980; Matzinger et al. of about 70 years (Matzinger et al. 2006a) is partly a 123 410 J Paleolimnol (2009) 41:407–430 result of the small catchment area (2,600 km2) and the platform using a short gravity corer for undisturbed relatively dry Mediterranean climate. surface sediments and a 300-cm-long piston corer The local climate is influenced by the proximity to (both UWITEC Co.). Overlapping of the individual the Adriatic Sea, by the surrounding mountains, and 300-cm core segments allowed the recovery of a by the thermal capacity of Lake Ohrid itself (Watzin composite sediment sequence with a total length of et al. 2002). Average monthly temperatures range 1,075 cm. A silty and very compact horizon at this from ca. 26°C during summer to -1°C during winter. depth prevented further penetration of the corer. After Precipitation averages around 750 mm year-1 and is recovery, the single 300-cm-long sediment cores at a minimum during summer. Prevailing wind were cut into segments of up to 100 cm length, which directions are northerly or southerly, being topo- were stored in the dark at 4°C until opening and graphically controlled by the shape of the lake valley further processing in the laboratory. and the surrounding mountains. Lake Ohrid is situated in a tectonic graben formed Analytical work during the latest phases of Alpine orogeny, which affected the entire interior of Albania since the Core description and photographic documentation Pliocene (Aliaj et al. 2001). The lake is surrounded were carried out immediately after core segments by mountains reaching ca. 1,500 m a.s.l. to the west were split lengthwise in the laboratory. One of the (the ‘‘Mokra’’ mountain chain) and [1,750 m a.s.l. to core halves was used to measure magnetic suscepti- the east (the ‘‘Galic¸ica’’ mountain chain, Fig. 1). The bility (MS). MS was determined on U-channels with bedrock comprises a variety of lithologies from a fully automated core logger developed at the Paleozoic to Cenozoic age (Watzin et al. 2002). GeoForschungsZentrum Potsdam, Germany. This Palaeozoic metamorphic and magmatic rocks form system is equipped with a Bartington MS2E sensor. the country rock of the entire western Macedonian The susceptibility readings were carried out in Zone. Triassic carbonates and clastics are widely contiguous 1 mm steps, although the MS2E sensor exposed to the southeast and northwest of the lake has a spatial resolution of 3.7 mm, in order to have and are intensely rugged, broken, and karstified. redundant data to smooth without losing resolution. Cenozoic sediments including Pliocene and Quater- For monitoring of the sensor’s drift, a background nary deposits are exposed particularly southwest of reading in air was taken every 10 mm. The drift was the lake. Because Lake Ohrid is located in a highly then subtracted from the sediment measurements. active seismic zone with frequent earthquakes (Muc¸o After the MS measurements this core half was 1994, 1998; Sulstarova et al. 2000; Muc¸o et al. 2002; archived for future analyses. The other core half NEIC database, USGS), the lake sediments on the was contiguously subsampled at 2-cm intervals. subaquatic slopes are subject to mass wasting and These subsamples were then split into two aliquots, seismite formation (Wagner et al. 2008). one of which was kept fresh and the other freeze- dried and used to determine the water content by weight loss. Materials and methods For grain-size analyses, 2 g of fresh sediment was

treated with 10 ml 15% H2O2 to remove organic Core recovery material. Subsequently, 0.3 g of Na4P2O7 was added to prevent flocculation of the sample. After settling of The selection of the coring site was based on a the suspension, the clear supernatant water was shallow seismic survey carried out in summer 2003 removed, the sediment homogenized using a spatula, (Wagner et al. 2008). Due to limitations of the coring and transferred to a laser-diffractometer (Micromer- equipment, a coring location of less than 150 m water itics DigiSizer 5200). Half a minute of ultrasonic depth was selected in the southeastern part of the lake treatment and flow rates of 10 l min-1 resuspended (Lz1120), where the seismic profile indicated a the sediments prior to detection. The laser-diffrac- subaquatic rim with a water depth of 105 m and tometer uses a 1 MB CCD and calculates 160 grain- undisturbed and widely horizontally-bedded sedi- size classes with the average values of two runs. ments (Figs. 1 and 2). Coring was carried out from a Calculations of grain-size parameters and statistics 123 J Paleolimnol (2009) 41:407–430 411 were made using the program GRADISTAT (Blott noted by Levkov et al. (2007), the separation of and Pye 2001). Grain-size data were then processed Cyclotella fottii (diameter 40–90 lm) and C. hus- using full phi-intervals between \2 and 1,000 lm tedtii (20–30 lm) does not stand. Smaller forms with and visualized by the interpolation software the same morphology also occur and are separated as SURFER. C. af. fottii (C. hustedtii var. 1 in Roelofs and Ostracod valves were separated from fresh sedi- Kilham, 1983). Cyclotella ocellata probably repre- ment according to a modified version of Forester sents a species complex (Baier et al. 2004), and is (1988; see also Schwalb and Dean 1998) that split morphologically based on the number of ocelli. promotes disintegration of the sediment. Between 6 Results are displayed using Tilia, Tiliagraph and and 10 g of wet sediment was placed into a wide- TGView (Grimm 1991). Stratigraphic zone bound- mouth plastic bottle and was shaken with 250 ml aries were defined using CONISS (Grimm 1987)on deionized water and one teaspoon of sodium bicar- square-root transformed percentage data. Due to a bonate at a temperature of 90°C. To promote full combination of lack of modern analogues (endemic dispersal of the sediment, the sample sat for several taxa), poor gradient coverage of Mediterranean hours. The sample was then frozen, allowed to thaw karstic lakes (Reed 2004), and the wide nutrient and sat again for several hours. The disaggregated preferences of C. ocellata, quantitative diatom- sediment was then slowly sieved by hand through a inferred total phosphorus (TP) reconstruction (cf. 63-lm-mesh size sieve, rinsed with de-ionized water Battarbee et al. 2000) was unreliable, producing an and air dried. Samples from a total of 275 levels with estimate of 34.6 lgl-1 TP for the surface sediment different spatial resolution were processed for ostrac- compared to the measured water chemistry of ods. A total of 885 ostracod valves from 28 selected 4.6 lgl-1 TP (cf. Matzinger et al. 2007). The results levels were analyzed using a stereo-microscope. are not presented. Taxonomy is based on an unpublished list provided Freeze-dried samples were used for biogeochem- by T. Petkovski that compiles taxonomic information ical analyses. An aliquot of the freeze-dried sample from Holmes (1937), Klie (1939a, 1942), Mikulic was ground to \63 lm and homogenized. Total (1961), Petkovski (1960a, b), and Petkovski et al. carbon (TC), total nitrogen (TN), and total sulphur (2002). Only adult specimens were identified to (TS) contents were measured by combustion CNS species level. Juvenile candonids were lumped elemental analyzers (VARIO Co. and EuroVector together as Candona spp. Co.). Total organic carbon (TOC) contents were Samples for diatom analysis were taken at approx- measured with a Metalyt CS 1000S (ELTRA Co.) imately 16-cm intervals, except for the uppermost analyzer, after pre-treating the sediment with 10% three samples, which were taken at 8-cm intervals. HCl at 80°C in order to remove carbonates. Total Preparation of oven-dried, weighed samples of ca. inorganic carbon (TIC) was obtained from the 0.1 g followed standard techniques (Battarbee 1986), difference between TC and TOC and used to using hot 30% H2O2 to oxidise organic matter and calculate the carbonate content, assuming that TIC HCl to remove carbonates. At the final stage of originates completely from CaCO3. Biogenic silica preparation, known quantities of plastic microspheres (opal) content was measured according to the wet were added to each subsample for calculation of chemical method described by Mu¨ller and Schneider absolute diatom concentrations (Battarbee and Kneen (1993). 1982). NaphraxÒ was used as a mountant in slide Samples for pollen analysis were treated with preparation. Diatoms and microspheres were counted standard palynological methods (Bennett and Willis under oil immersion at 1,0009 magnification, using a 2001) and pollen, spores, and palynomorphs were Zeiss Axioscop 2 plus binocular microscope with determined under a light microscope at 4009 mag- phase contrast. Diatom preservation was variable, nification and identified with keys and pollen atlases ranging from uncountable (fragments) to well-pre- (Punt et al. 1976–2003; Moore et al. 1991; Reille served samples for which a count of 500 valves was 1992–1998). made. Diatom identification followed Krammer and Tephrochronology and radiocarbon dating were Lange-Bertalot (1986, 1988, 1991a, b), Hustedt used to establish the chronology of the sediment (1945), Jurilj (1954) and Levkov et al. (2007). As sequence. The three tephra layers encountered in the 123 412 J Paleolimnol (2009) 41:407–430 sediment core are described in detail by Wagner et al. the composition of fallout deposits of the Campanian (in press). Energy-dispersive-spectrometry (EDS) Ignimbrite (Wagner et al. in press). This tephra analyses of glass shards and glasses from pumice equates to the marine tephra layers Y-5 and C-13 in fragments were performed using an EDAX-DX the central and eastern Mediterranean Sea (Keller micro-analyser mounted on a Philips SEM 515 et al. 1978; Thunnell et al. 1979; Thon-That et al. (operating conditions: 20 kV acceleration voltage, 2001; Pyle et al. 2006), with the most reliable 100 s live time counting, 10-9 A˚ beam current, ZAF radioisotopic age being 39,280 ± 110 cal. year BP correction). Instrument calibration and performance (Table 1; De Vivo et al. 2001). The second tephra is described in Marianelli and Sbrana (1998). Radio- occurs between 897 and 896 cm depth and matches carbon dating of core Lz1120 was conducted by closely the composition of the Y-3 tephra of Keller accelerator mass spectrometry (AMS) at the Leibniz et al. (1978) dated at ca. 30,670 ± 230 cal. year BP Laboratory for Radiometric Dating and Isotope (Table 1; Sulpizio et al. 2003; Di Vito et al. in press; Research in Kiel, Germany, on five samples of Wagner et al. in press; Zanchetta et al. in press). In terrestrial plant macrofossils or charcoal. The radio- addition to the two ages derived from tephrochronol- carbon ages were calibrated into calendar years ogy in the lower part of unit I, one age at its top was before present (cal. year BP) using CALIB 5.0 obtained by AMS radiocarbon dating. Terrestrial (Stuiver and Reimer 1993; Reimer et al. 2004). The plant remains at 552 cm depth were dated to uncertainties were calculated on the probability 14,665 ± 450 cal. year BP (Table 2). All three ages distribution at the 2r level. indicate that unit I was deposited during the late Pleistocene, when glacial and periglacial conditions affected the region. The apparently stable sedimen- Results and discussion tation rate (ca. 0.2 mm year-1; Fig. 4) throughout unit I correlates well with the relatively uniform Lithology and chronology sedimentological characteristics and implies rather stable environmental conditions at this time. Correlation of the individual, overlapping core seg- The sharp transition to unit II is confined within a ments was based on visual identification of few centimetres and is characterized by distinct stratigraphic horizons and on correlations using MS coarser, dark grey sediments (Fig. 3). The stronger data. The length of the composite sediment core influence of inlets close to the coring location is Lz1120 is 1,075 cm. Two major lithological units can possible, but unlikely, due to the water depth of be recognised according to the macroscopic core 105 m and distance from the shoreline today. Alter- description, colour, grain-size measurements, water natively, the coarser grains could originate from a content, and MS. mass movement process, which might have led to Unit I (1,075–552 cm) comprises relatively uni- erosion of sediment, but also to accumulation of form, massive, grey, clastic sediments (Fig. 3), which coarser sediment. A significant erosive contact, are poorly sorted and mainly composed of medium to however, cannot be observed at the transition. coarse silt with minor proportions of fine silt and fine Therefore, coarsening is most likely to be due to sand, and occasional occurrences of coarse sand and sustained erosion or restricted accumulation of fine- gravel. Calcareous and plant fossils were not found, grained sediment by subaquatic currents, which may except for some terrestrial plant remains at the top of have existed and are still observed in the lake today the unit. The dominance of clastic material is con- (Matzinger et al. 2006a). A hiatus would explain the firmed by a low water content ranging mostly between abrupt lithological change between units I and II. 30% and 40%. The occasional occurrence of coarse Unit II (545–0 cm) is characterized by repeated sand and gravel implies that ice transport, probably ice colour changes from light grey to brownish-black floes, contributed in part to sediment input from medium to coarse silt, with relatively low amounts allochthonous sources. The MS is relatively high and of fine silt and fine sand (Fig. 3). Coarse sand and exhibits some minor fluctuations throughout unit I. gravel are absent, indicating that ice-rafted transport Two tephras were identified in unit I. The lower was negligible or absent. Organic material is finely tephra occurs at the core base and corresponds well to dispersed, and remains of ostracods and molluscs can 123 J Paleolimnol (2009) 41:407–430 413

core Lz1120 GSD MS H2 O TOC TN TS TOC/TN TOC/TS SiO23 CaCO (cm) (µm) (10 -6 SI) (%) (%) (%) (%) (%) (%) 500 64 8 2 0 1000 20 40 020.0 0.2 0.0 0.4 48 040 08080 0

100

200

300

400

500 1.4

600

700

800

900

1,000

Fig. 3 Core photographs, grain-size distribution (GSD), MS, tephras. The dashed, grey line separates unit I (bottom) from water content (H2O), and TOC, TN, TS contents, TOC/TN and unit II (top). The shading in the grain size contour plot TOC/TS ratios, biogenic opal (SiO2) and carbonate (CaCO3) visualizes the relative proportion of individual classes contents of core Lz1120 from Lake Ohrid. Arrows to the right (dark = 25–30%, light = 0–5%), and thus the modality of of the core photos indicate horizons from which macrofossils the distribution were taken for radiocarbon AMS dating. Asterisks indicate the

Table 1 Main characteristics of tephra layers recognised in ages, and inferred source region (after Wagner et al. in press, core Lz1120, including correlated depth (corr. depth), lithology and references therein) and composition of volcanic fragments, raw and calibrated Core section Corr. depth (cm) Cal. year BP Lithology Source region

Lz1120-5 310–315 3,370 ± 70 Dark-coloured glass fragments (5YR 2/2), poorly vesicular, Mount Etna aphiric, benmoreitic composition Lz1120-9 896–897 30,670 ± 230 Highly vesicular, aphiric, micropumice and colourless Campi Flegrei glass shards (5Y 8/1), trachytic composition Lz1120-9 1,070–1,075 39,280 ± 110 Light-coloured micropumice (5Y 7/2 to 5Y 5/2), highly to Campi Flegrei extremely vesicular, almost aphiric, dark-coloured glass shards (5RP 2/2) are abundant, trachytic composition Colour from GSA Rock Color Chart (1991) be observed macroscopically. The higher proportion clearly indicated throughout unit II and matches well of organic matter is also reflected in slightly higher with field observations of crustacea being abundant water content of generally [40%. Bioturbation is on surface sediments even from the deepest part of 123 414 J Paleolimnol (2009) 41:407–430

Table 2 Radiocarbon measurements of charcoal and plant remains collected at different correlated depths (corr. depth) along core Lz1120 Sample Core section Corr. depth Material C weight 14C age Calendar age (cm) (mg) (year BP) (cal. year BP)

KIA27394 Lz1120-5 398 Plant remains 1.2 4,490 ± 35 5,170 ± 130 KIA27393 Lz1120-4 454 Plant remains 1.4 6,145 ± 35 7,055 ± 110 KIA27395 Lz1120-6 462 Charcoal 1.6 6,095 ± 30 7,020 ± 135 KIA27396 Lz1120-6 500 Plant remains 0.6 6,930 ± 60 7,800 ± 130 KIA27423 Lz1120-6 552 Plant remains 0.6 12,570 ± 110 14,665 ± 450

age (cal. year BP) et al. in press), and its age is in agreement with the 0 10,000 20,000 30,000 0 * radiocarbon chronology. Age control of the sediment tephra surface is given by a sample obtained from 20 cm 14 C dating depth in a gravity core taken ca. 100 m to the east of 200 coring location Lz1120 in May 2007. Its age of MEV 150 ± 150 cal. year BP confirms that the sediment

400 surface in core Lz1120 probably represents the present. All age estimates indicate that unit II was hiatus deposited during the Holocene. Despite the distinct 600 changes in colour and MS, no significant variation in core depth (cm) sediment accumulation rates (ca. 0.6 mm year-1)is apparent during the Holocene. This, however, could 800 be due to the relatively low number of dated horizons. Y-3 The slight decrease in sediment accumulation rates downcore, as indicated by the age-depth model 1,000 Y-5 (Fig. 4), is probably the result of sediment compac- tion with increasing depth. Fig. 4 Age-depth model for Lake Ohrid sediments based on 14 Based on the age-depth model, and assuming a calibrated C ages derived from terrestrial plant remains and relatively constant decrease in the sediment accumu- charcoal, and radioisotopic ages of tephra layers. For radio- isotopic ages of the tephras, see Wagner et al. (in press) and lation rates downcore, the hiatus between units I and references therein II spans the period between ca. 14,600 and 9,400 cal. year BP and would correspond to a thickness of the lake. Grain-size distribution and MS show some ca. 130 cm (Fig. 4). marked fluctuations (Fig. 3), which correlate partly with colour changes. Biogeochemistry The lowermost of the four AMS radiocarbon samples from unit II originates from 500 cm depth The biogeochemical characteristics support the divi- and has an age of 7,800 ± 130 cal. year BP sion of core Lz1120 into a lower, glacial and an (Table 2). The ages of the two samples from 462 upper, Holocene unit. and 454 cm depth match well, at 7,020 ± 135 Glacial unit I is characterized by low and relatively and 7,055 ± 110 cal. year BP, respectively. The constant contents of TOC and TN, which represent youngest sample at 398 cm was dated to the amount of organic matter in the sediment. 5,170 ± 130 cal. year BP. In addition to these four Slightly enhanced contents of TOC, however, can radiocarbon ages, the chronology of unit II was be observed between the core base and 900 cm depth confined by the occurrence of a cryptotephra between (Fig. 3). TS, which at least partly derives from 315 and 310 cm depth. According to its composition, organic material, correlates well with the TOC this tephra was related to an eruption of Mount Etna content and also has a broad maximum in the lowest volcano dated at 3,370 ± 70 cal. year BP (Wagner part of the core. A second, very sharp peak of TS is 123 J Paleolimnol (2009) 41:407–430 415 inconsistent with the TOC content and occurs at the the karst springs, which accounts for ca. 25% of the top of unit I, at the location of the presumed hiatus. total inflow, ca. 40–50% of which derives from Lake This maximum may be due to early diagenetic Prespa (Matzinger et al. 2006b, 2007). The karst processes, such as the formation of pyrite in strongly spring water has a temperature range of 9–12°C and reducing conditions (e.g. Ha˚kanson and Jansson enters Lake Ohrid in autumn primarily at depths 1983), or enrichment of relatively heavy mineral between 15 and 40 m, where high proportions of particles, such as pyrite (q = 4.95–5.20) or marka- primary productivity take place (Matzinger et al. site (q = 4.8), originating from raised sediments 2006a, b). Interflow in summer is likely deeper, but during a mass movement process or being left from could still affect primary productivity. The influence the erosion of fine-grained sediments by subaquatic of nutrient input into Lake Ohrid by karst springs currents. The TOC/TN ratio in general reflects the depends strongly on the trophic state of Lake Prespa ratio of allochthonous versus autochthonous organic and its water level. Most of the phosphorus load matter. According to the Redfield ratio, TOC/TN supplied by karst springs is bio-available (Matzinger ratios of around 7 should be indicative of pure et al. 2006a, b), and minor changes in nutrient input autochthonous organic matter, and values [10 indi- can have significant effects on productivity in Lake cate enhanced allochthonous input (Meyers and Ohrid. Models have shown that phosphorus input into Ishiwatari 1995). The TOC/TN ratios in unit I vary Lake Ohrid is highest when the water level of Lake between 4 and 6, except for a minor peak in the Prespa is reduced by ca. 10–15 m, which is known to lowest part, consistent with the TOC peak. The low have occurred in historical times (Matzinger et al. values in glacial unit I suggest enhanced postsedi- 2006b). However, higher organic matter accumula- mentary decomposition of organic matter. This is tion in Lake Ohrid sediments could also be due to supported by notable CH4 release and gas expansion better preservation and less decomposition of organic of glacial sediment cores immediately after their matter as a consequence of reduced mixing of the recovery during the field campaign. The TOC/TS complete water column. Gradual decreases in TOC ratio, which provides information about changes of and TN contents can be observed between ca. 255 the salinity in the water column (Berner and and 150 cm, and, after a setback, between 145 and Raiswell 1984;Mu¨ller 2001; Cohen 2003) and 25 cm, where the values are similar to those of glacial changes in the redox conditions of the bottom waters unit I. TS correlates only weakly with TOC and TN (Wagner et al. 2004, 2006), is stable and low profiles in unit II, indicating that changes in the throughout unit I. This indicates that the redox oxygen content of the bottom waters have probably conditions have not changed significantly and that occurred during the Holocene. The TOC/TN ratio is, TS originates primarily from organic matter. One except for a minimum between ca. 255 and 150 cm exception occurs at the topmost part of this unit, higher during the Holocene than during glacial unit I where a significant decrease in the TOC/TS ratio and corresponds with TOC/TN ratios measured in implies increased deposition of minerogenic sulphur. sediment traps from the central part of the lake Biogenic opal is formed by diatoms and the siliceous (Matzinger et al. 2007). Maxima of TOC/TN sponges Ochridaspongia rotunda and O. interlitho- between ca. 8 and 10 in the Holocene unit still nis, which live in the vicinity of subaquatic springs indicate dominance by autochthonous organic matter, (Gilbert and Hadzisce 1984). The opal content but also suggest a higher proportion of allochthonous throughout unit I remains relatively stable, except organic matter and/or restricted decomposition. The for maximum values at the core base. Calcite values TOC/TS ratio correlates negatively with the TS are negligible in unit I. content and displays three broad and distinct peaks Unit II is characterized by generally higher, but in unit II. Low TOC/TS ratios probably result from significantly fluctuating organic matter content, as anoxic bottom water conditions that promote the indicated by TOC and TN values (Fig. 3). This formation of sulphides in the sediments. As noted, probably results from enhanced productivity in the complete overturn only occurs every ca. 7 years and lake. Productivity, particularly in the southeastern the hypolimnion is episodically oxygen depleted. part of the lake, is influenced by water inflow from Modelling has suggested that a rise in air temperature

123 416 J Paleolimnol (2009) 41:407–430 of 4°C would only have a minor impact on the period of complete overturn (Matzinger et al. 2006a, 2007). In contrast, atmospheric warming by 0.04°C year-1 would lead to significantly reduced mixing of the water column and to practically anoxic conditions below 110 m. Varying environmental conditions in the past were, therefore, probably related to alternat- ing phases of complete mixing and oxygenated bottom waters and phases of reduced mixing and hypolimnetic oxygen depletion. Alternatively, low TOC/TS ratios could be a result of enhanced mineralization of settled organic matter during peri- ods of increased productivity (Matzinger et al. 2006a, 2007). As indicated in the upper 200 cm of unit II, significant decreases in TOC concentrations do occur when the TOC/TS ratio increases and TS decreases. These correlations support the hypothesis that lake productivity may be reduced during extended phases of complete overturn. Oxic bottom waters hamper phosphorus release from the sediments, and the availability of phosphorus from the sediments seems to have a much higher influence on primary produc- tivity than nutrient input from karstic springs (Matzinger et al. 2006a, 2007). Additionally, low TOC concentrations could be promoted by enhanced Fig. 5 SEM photos of bulk sediment from the centre of Lake bacterial activity and decomposition of organic Ohrid indicating (a) diatoms and their fragments and (b) matter, such as is presumed to have dominated autigenous carbonates during the glacial period with complete overturn. The amount of biogenic opal in unit II is relatively low. Ostracod record The negative correlation with calcite implies that the relative amount of biogenic opal is an artifact Figure 6 shows the ostracod assemblages based on triggered by changes in the calcite contents. The valve counts per gram wet sediment. In the Pleisto- calcite, with maxima between 545 and ca. 250, 150 cene unit I only 6 out of 146 levels yielded poorly and ca. 60 cm depth, and close to the sediment preserved single valves or chitinous remains of surface, is mostly formed by precipitation during ostracods. In the Holocene unit II, ostracods were summer. This is indicated by SEM photos (Fig. 5), by present in all 129 processed samples. The 22 samples investigations of sediment traps from the central part that were selected for analysis of species assemblages of the lake (Matzinger et al. 2007), and by good contained between 11 and 140 valves per sample correlation with TOC contents. The calcite minima (1.6–19 valves g-1). Maximum abundance and spe- around 505 and 330 cm depth, and more distinctly cies diversity occurred in the uppermost 150 cm of between 250 and 150 cm and between 60 and 10 cm the core. depth imply a restriction of calcite precipitation due Identified ostracod specimens belong to ten spe- to lower productivity. Alternately, calcite dissolution cies including the benthic Candonids C. neglecta, could have occurred due to enhanced bacterial C. trapeziformis, C. hadzistei, C. holmesi, C. ovalis, decomposition of organic matter and a decrease in C. depressa, C. marginata, as well as C. sp.1; Lept- pH. In either case, the marked fluctuations in calcite ocythere sp., a species of marine origin, and the precipitation and/or preservation indicate that Lake nectonic ostracod Cypria sp. The occurrence of the Ohrid experienced significant environmental changes marine genus Leptocythere may possibly hint at a during the Holocene. marine origin of the lake. Its occurrence in freshwater 123 J Paleolimnol (2009) 41:407–430 417

core Lz1120

) P B r sp. a e y . spp. j sp. 1 l cm) sp. a sp. j ca ( ( th e p g e A D number of speciesnumber of valves Cypria CypriaCandona CandonaCandona neglecta trapeziformisCandonaCandona hadzisteiCandona holmesiCandona ovalisCandona depressaCandona marginataLeptocythere -55 0 0

100

1,000

200

300

5,000 400

500 9,000 15,000 600

20,000 700

25,000 800 no or only few ostracods preserved

30,000 900

35,000 1,000

4 481216 20 2224812 valves / g

Fig. 6 Ostracod record (in valves per gram) of core Lz1120 from Lake Ohrid. Note that remains were only present in six levels below 550 cm. Candona spp. comprises juvenile specimen of Candona species was discovered in Lake Ohrid and described by Klie supply that could have limited the presence of (1939b). Although ostracods from Lake Ohrid have ostracods at this site. Roelofs and Kilham (1983) been the target of a series of taxonomic investiga- report carbonate contents of ca. 2% in Pleistocene tions, especially because of their high degree of sediments of a core taken at a water depth of 210 m endemism (e.g. Stankovic 1932, 1960; Holmes 1937; in the north-central basin of Lake Ohrid. Also, Klie 1934, 1939a, b, 1942; Mikulic and Pljakic 1970; Petkovski (1960a) found living ostracods at water Griffiths and Frogley 2004), little is known about depths of up to 120 m. Thus, there is evidence for the their specific ecology. Collecting was mostly presence of ostracods in the deeper areas of the lake. restricted to the littoral regions and there is little Probably local inflows, local hydrology, hydrome- information on profundal taxa. Thus, the ecological chanics, or bacterial decomposition, leading to CO2 interpretation of this record is mostly restricted to production and a pH decrease in the bottom waters changes in diversity and abundance as a consequence promoted a dissolution of carbonates and ostracods of changes in productivity and preservation. during the Pleistocene at coring location Lz1120. The general variation in abundance of valves and Distinct fluctuations in ostracod abundances between species diversity tracks the carbonate content. The glacial and interglacial periods have also been low carbonate content of B1% in unit I of core observed by Frogley et al. (2001) in the 500-ka-old Lz1120 suggests that the scarcity of ostracod remains sequence from Lake Pamvotis, NW Greece, where in this unit may be a result of low carbonate they were related to relative anoxia. Resurgence of preservation rather than a deficiency in oxygen species was matched by productivity and a significant 123 418 J Paleolimnol (2009) 41:407–430 increase in precipitation versus evaporation (P:E). taxon Discostella stelligera and, at lower abundance, Ostracod records from Lake Malawi also display Cyclotella ocellata. These taxa are likely to occupy swings in ostracod abundance during the past 140 ky an epilimnetic habitat in the surface waters. The true and were related to insolation changes (A. Cohen, hypolimnetic endemic, C. fottii, is present at low unpublished data). This suggests that the fluctuating abundance. The presence of the putative benthic ostracod abundances may represent an expression of taxon, Amphora pediculus, is likely to derive from its climate change and climate forcing mechanisms. growth on the surface of phytoplankton, as in the glacial sequence of Eski Acıgo¨l crater lake, Turkey, Diatom record for example (Roberts et al. 2001). Small Fragilariales including the genera Pseudostaurosira, Staurosira, The summary diatom diagram (Fig. 7) shows that the and Staurosirella, which are typical of unstable cold most significant biostratigraphic boundary coincides glacial environments (e.g. Schmidt et al. 2004; with the lithostratigraphic boundary at 552–545 cm Wilson et al. 2008), and are some of the few genera depth. The two main zones, DI and DII, are divided to be able to withstand low light conditions below ice into subzones. The main factor dictating diatom in the littoral zone (e.g. Smol and Douglas 2007), are concentration is preservation quality rather than also present at low abundance. productivity, with low concentration where frustules Subzone DIb (1,039–958 cm; 37,550–33,750 cal. are poorly preserved. year BP) exhibits lower diatom concentrations (ca. Diatoms are well preserved in diatom zone DIa 8 9 103 valves g-1). There is a transition to (1,063–1,039 cm; ca. 38,700–37,550 cal. year BP), dominance by C. fottii, which has been found to be which has high concentrations (ca. 3 9 105 val- located at depths of up to 200 m, although it is most ves g-1) and is dominated by the small planktonic abundant within the photic zone (Allen and Ocevski

core Lz1120 Planktonic Fac. plank. Benthic

centres ens ensvar. var.venter binodis . 3 ocelli 4 ocelli >4 ata s ica gustumacum tii <20 . dubia. martyi rocephalaa oides lum

Age (cal. yearDepth BP) (cm)Discostella stelligera Cyclotella fottii Cyclotella af. fotCyclotella ocellataCyclotellaCyclotellaCyclotella ocellataStephanodiscus ocellataStephanodiscus ocellataDiscostella neoastraea sp. minutulusStaurosirellaPseudostaurosira pinnStaurosiraStaurosira brevistriataStaurosirella construStaurosirella construAmphora varPlaconeis var pediculuNaviculaNavicula balcan spp.Cocconeis rotundaNaviculaGomphonema diminutaEncyonema subrotundataPinnulariaEncyonopsis an Epithemiasilesi borealisCymbellaCymbella mic adnatCavinula spp.Gomphonema thumensisNavicula scutellplanktonic seminu spp fac plankbenthicConcentration ZONE CONISS -55 0 0 b

100 c 1,000

200 b

300 DII

a 5,000 400

500 9,000 15,000 600

20,000 700

25,000 c 800 DI

30,000 900

35,000 1,000 b a 4080 4080 40 40 20 20 40 20 20 2020 20 20 40 80 200 400 4 8 12 % v/g/1000 tot sum of squares

Fig. 7 Summary diatom diagram of core Lz1120 from Lake Ohrid, showing only taxa with an abundance [2% 123 J Paleolimnol (2009) 41:407–430 419

1976). The stimulation of hypolimnetic phytoplank- of major hydrological changes linked to the connec- ton may reflect increasing nutrient availability at tion with karstic water from Lake Prespa, for depth (P or Si). Alternatively, the reduction in example, is difficult to ascertain. epilimnetic plankton may be a response to reduced It is tempting and speculative to infer that the temperature; thermal characteristics of stratification, single sample dominated by C. ocellata at 511 cm and water temperature, have been shown to be the (ca. 8,360 cal. year BP) relates to the 8.2 ka event. most significant factor driving phytoplankton dynam- The reduction in hypolimnetic plankton and domi- ics in the modern lake (Allen and Ocevski 1976). nance of C. ocellata would point to a climatic Zone DIc (958–552 cm; 33,750 to 14,600 cal. year warming rather than the opposite. BP and spanning the Last Glacial Maximum) has Zone DIIb (224–153 cm; ca. 1,925–1,020 cal. very poor preservation and low diatom concentrations year BP) again exhibits poor preservation, but only (max. ca. 1,000 valves g-1). Fragments of the large, one sample is dominated by robust C. fottii. The heavily silicified C. fottii are present throughout dominance in other samples of relatively well- (being countable only in the upper half of the zone), preserved valves of the fragile taxa, D. stelligera with the occasional occurrence of more fragile and P. balcanica, for example, may suggest mixing, valves. Taking into account taphonomic bias, the which would match with enhanced decomposition of flora appears to be similar to Zone DIb, after organic matter as indicated in low TOC and carbon- dissolution of smaller taxa. ate contents. The discrepancy between low diatom Following the hiatus in core Lz1120, zone DIIa concentration and relatively high biogenic silica (545–224 cm; ca. 9,400–1,925 cal. year BP) exhibits content (Fig. 3) confirms that the rise in the latter is a major transition to a more diverse flora, with more an artefact of low carbonate abundance. Subz- fluctuating co-dominance by planktonic C. fottii and one DIIc (153–73 cm; ca. 1,925 to 330 cal. year BP), mesotrophic C. ocellata. The hypolimnetic mesotro- exhibits an increase in small Fragilariales and benthic phic taxa, Stephanodiscus neoastraea and (at \2%) taxa at the expense of plankton. The increase in Asterionella formosa, are present consistently at low Fragilariales, which are ‘pioneering species,’ sug- abundance and indicate a rise in productivity. gests limnological instability. The high relative C. ocellata dominates in oligotrophic to eutrophic abundance of benthic taxa, which is presumably not waters of varying depth in many karstic lakes of the related to a major reduction in lake level in a basin of region, including Shkodra, Albania (Rakaj et al. this size, may indicate enhanced subaquatic currents, 2000), Lake Mikri Prespa (Tryfon et al. 1994)and such that a higher proportion of taxa from the littoral Ioannina (Wilson et al. 2008), Greece. Compared to zone was transported to the coring site. the glacial sequence, zone IIa also exhibits an Subzone DIId (73–0 cm; ca. 330 cal. year BP– increase in the relative abundance of benthic present) marks a return to plankton dominance by naviculoid taxa such as Epithemia adnata and C. ocellata and D. stelligera. The low relative Placoneis balcanica. Roelofs and Kilham (1983) abundance of C. fottii could be a temperature interpreted this increase in diversity and productivity effect, or may be related to increased anthropogenic in general terms as a function of temperature impact and associated increases in nutrient avail- increase across the Pleistocene–Holocene boundary. ability over the past ca. 300 years. The shift Temperature-induced shifts in the growing season towards dominance by C. ocellata in the upper (including reduced ice-cover duration) are now well two samples may be the result of eutrophication documented with warming of Arctic lakes (Ruhland and/or climate warming (cf. Smol and Douglas et al. 2003;SmolandDouglas2007), and may 2007) over the past ca. 50 years. favour epilimnetic taxa, since hypolimnetic taxa are less likely to be affected. As noted above, recent Pollen record studies in Ohrid have also demonstrated that the major internal source of increases in within-lake Given the large size of the open water surface of Lake productivity in the modern lake system is through Ohrid, the relevant pollen catchment area (Sugita complete mixing, which may drive changes through- 2007) can be assumed to include thousands of square out the Holocene. The possible additional influence kilometers, thus integrating several Mediterranean 123 420 J Paleolimnol (2009) 41:407–430 vegetation belts. This fact needs to be considered as oak in the montane belt of the Balkans. when interpreting the pollen data and comparing Fluctuations in the ratio of arboreal (AP) to non- them to other sites with contrasting pollen catchments arboreal pollen (NAP) are mainly reflecting the from this region. relative amount of Pinus pollen. Despite the The pollen record of core Lz1120 (Fig. 8) relatively coarse sampling resolution of the pollen correlates well with the two lithological units. record, the AP/NAP fluctuations compare generally During unit I the pollen assemblages are dominated well with palynological results from Greece (e.g. by herb pollen indicative of steppe conditions such Tzedakis 2005). The phases of decreased AP at as Artemisia, Chenopodiaceae, Compositae, or Poaceae. ca. 38,000 cal. year BP, between ca. 31,000 and Apart from values of between 20 and 60% of Pinus, 28,000 cal. year BP, and at ca. 23,000 cal. year BP the tree and shrub pollen mainly derive from can be correlated tentatively with Heinrich events deciduous taxa such as Quercus, Abies, Hippophae¨, H4, H3, and H2, respectively. Due to the hiatus and Juniperus. This is in agreement with other between 14,600 and 9,400 cal. year BP, the late- palynological results from the Mediterranean region glacial and the early Holocene vegetation history is and the Balkans (e.g. Allen and Huntley 2000; not represented. Tzedakis et al. 2004) indicating full-glacial cold and Unit II includes the vegetation history of the past dry conditions with refugia of temperate trees such 9,400 years. This unit can be subdivided into two

e core Lz1120 p p e ty ty p e ty s p ta ) u y la s P o t o u B u a e n r id s c a a c o n cy a e e in e a ll y d t p l a s ie l. ia e s s lu y o e y ia a (cm) s a u g t s ia g s r a s m c s i e n cu a u s l a n le u o r ( th s b s m c s s i r s y l u a t a a ta m r ro e p e ru rb e a u ie rp e u tr ry g re n l a c n a b o g e re h e rt o in b a u ln s o a e la g le e e e m p A D T S H A P P A C QeA O C F C P Ju O S C Z A S -55 0 0

100

1,000

200

300

5,000 400

500 9,000 15,000 600

20,000 700

25,000 800

30,000 900

35,000 1,000

20 40 60 80 100 20 40 20 20 40 60 20 20 40 20 % AP + NAP

Fig. 8 Pollen record of core Lz1120 from Lake Ohrid 123 J Paleolimnol (2009) 41:407–430 421 major parts. The older part between 545 and 250 cm period between ca. 39,500 and 14,500 cal. year BP. (9,400–2,320 cal. year BP) is characterized by Overall, the grain-size distribution, the biogeochem- pollen assemblages dominated by pine and sum- ical analyses, and the records of diatoms, ostracods, mer-green oak, with Abies, Fagus, and Ostrya as and pollen indicate that Lake Ohrid experienced only important AP. A short-term increase in NAP at moderate environmental changes during this period. 510 cm sediment depth is concurrent with the 8.2 ka The occasional occurrence of coarse sand and cooling event identified in different environmental gravel can be related to ice transport and suggests that archives (e.g. Rohling and Pa¨like 2005). Due to the Lake Ohrid had significantly more ice cover between coarse resolution of the pollen diagram the short ca. 39,500 and 14,500 cal. year BP than today, when fluctuation is only present in one sample that is the lake remains ice-free even during winter. During characterized by increased values of pollen of the glacial period the lake had winter ice-cover at Artemisia. This would indicate a dry/cool oscillation least along the shore, with ice-rafted debris being at Lake Ohrid. Based on data from the Aegean Sea, released during spring melt. This is consistent with Kotthoff et al. (2008) interpret this phase of sapro- cold conditions during the last glacial. These condi- pel S1 interruption in the Mediterranean Sea as tions are well recorded in the fossil pollen reflecting cold and dry winter conditions. However, assemblages, which indicate a typical steppe biome a synchronous oscillation is recorded at Lago di comparable with that proposed by Rossignol-Strick Vico (Magri and Parra 2002) during a period of and Planchais (1989), Wijmstra et al. (1990), Watts generally wetter conditions in the Mediterranean et al. (1996), or Allen et al. (2000). The diatom data region (e.g. Zanchetta et al. 2007). are also consistent with an unstable, glacial environ- The occurrences of Cerealia pollen together with ment with low light penetration. Plantago lanceolata pollen point to agricultural activ- Given that Lake Ohrid is an oligomictic lake ity and human impact in the catchment of Lake Ohrid. today, with a complete overturn of the water column A peak in the Quercus pollen curve as well as the onset only every few years during long and cold winters of the Olea curve are dated to about 5,000 cal. year BP (Matzinger et al. 2006a), it is conceivable that the which is 600 years older than at Lake Maliq, some colder climate between ca. 39,500 and 14,500 cal. 50 km to the S of Lake Ohrid (Dene`fle et al. 2000). year BP must have promoted a complete overturn of This difference is most likely attributable to errors in the water column once or twice a year, making the the age-depth models of both records. lake monomictic or dimictic. Complete annual mixis The top 250 cm (past 2,320 cal. year BP) are would have led to oxic bottom waters, restricting the characterized by a significant increase in herb pollen amount of phosphorus release from the sediments indicative of a substantial opening of the landscape. (Psenner et al. 1984;Gu¨de and Gries 1998), and This is mainly the result of decreasing values of pine promoting decomposition of organic matter at the pollen with a simultaneous increase in grass pollen. sediment surface. Since phosphorus release from the The occurrence of increased values of Cerealia, sediments is considered to be one of the main triggers Secale, and P. lanceolata pollen as well as a major for primary productivity in lakes (e.g. Gonsiorczyk increase in the relative abundance of the coprophilous et al. 2001), as it can be observed also in Lake Ohrid fungal spores of Sporormiella points to a distinct today (Matzinger et al. 2006a), the presumed reduced increase in regional human impact during the past nutrient availability during the glacial period likely 2,500 cal. year BP, which is comparable with the added to the effects of low temperatures in restricting results from Lake Maliq (Dene`fle et al. 2000). primary productivity and carbonate precipitation. Enhanced decomposition of organic matter due to increased bacterial activity at the sediment surface

Interpretation led to release of CO2. The resulting lower pH in the pore water could have promoted, along with the oxic Pleistocene (ca. 39,500–14,500 cal. year BP) conditions in the bottom waters, the dissolution of carbonates (cf. Mu¨ller et al. 2006), such as indicated The Pleistocene part of core Lz1120 is represented by by the poorly preserved single valves or chitinous lithological unit I (1,075–552 cm) and covers the remains of ostracods in unit I. The significant 123 422 J Paleolimnol (2009) 41:407–430 dissolution of diatoms between ca. 33,750 and core Lz1120 is, however, not observed. This suggests 14,600 cal. year BP is in contrast to relatively high that the hiatus is more likely to be a result of biogenic opal concentrations, which may indicate that continuous erosion or prevention of accumulation of the amount of biogenic opal is predominantly formed fine-grained sediments by subaquatic currents. These by sponge spicules or that diatom valve dissolution currents are common in the lake today (Matzinger was not complete. Relatively high values in MS et al. 2006a) and could have been modified in terms throughout the Pleistocene part of the core imply of velocity and water depth during a significant lake- sufficient allochthonous minerogenic input, including level change. Such lake-level changes have been

SiO2, from the catchment. Hence, dissolution of the observed in several other records from the Mediter- diatom valves is probably promoted by complete ranean region (Digerfeldt et al. 2000; Schmidt et al. mixis of the water column and oxic bottom waters. 2000; Valero-Garce´s et al. 2004; Wick et al. 2003) Some marked changes in the Pleistocene record and may have also affected Lake Ohrid at the from Lake Ohrid apparently correspond with envi- transition from the relatively cold climatic conditions ronmental changes deduced from other records. The of the Pleistocene to the relatively warm and humid high abundance and good preservation of diatoms at conditions of the Holocene; currents may also have the base of core Lz1120 correlates with a broad peak been affected by changes in inflow from Lake Prespa, in biogenic SiO2 around 37,000 cal. year BP and is although a separate study would be necessary to indicative of slightly warmer temperatures, such as elucidate this. indicated in the record from the southern Aegean Sea (Geraga et al. 2005). The phases of decreased Holocene (ca. 9,400 cal. year BP to present) arboreal pollen at ca. 39,000 cal. year BP, between ca. 32,000 and 28,000 cal. year BP, and at ca. The Holocene part of core Lz1120 is represented by 23,000 cal. year BP can tentatively be correlated with lithological unit II (545–0 cm) and covers the period Heinrich events H4, H3, and H2, respectively. There between ca. 9,400 cal. year BP and the present. In is now general consensus that the Y-5 tephra marker contrast to the glacial sequence, the Holocene is is located at the base of the H4 event (e.g. Thon-That characterized by several distinct changes in grain-size et al. 2001; Fedele et al. 2003). The Y-3 tephra is distribution, biogeochemical proxies, and in the potentially a good marker for the H3 event though diatom, ostracod, and pollen data (Fig. 9). This probably located not at the very beginning (Zanchetta implies that mixing conditions, preservation of et al. in press). The effect of the fallout of these ashes organic matter, and primary productivity changed onto the vegetation in the catchment of Lake Ohrid significantly over time. Extreme fluctuations in remains unclear at this stage. The persistence of a carbonate content, forming up to 80% of the sediment relatively warm and humid climate in the eastern composition during the Holocene, are superimposed Mediterranean region between ca. 27,000 and upon all other geochemical and physical sediment 24,000 cal. year BP (Tzedakis 2007) is indicated in characteristics, including TOC and biogenic opal the pollen record; however, a pronounced cooling fluctuations and the variations in the MS. with temperatures 7–10°C below present and The higher organic matter content and the dom- enhanced aridity during the Last Glacial Maximum inance of carbonate originating from carbonate (23–19 ka), is only vaguely recorded in the very poor precipitation since ca. 9,400 cal. year BP suggest diatom preservation in Lake Ohrid. higher productivity and/or less decomposition of The hiatus at the transition between the Pleisto- organic matter in the lake compared to the glacial cene and the Holocene part of core Lz1120, covering period. Both higher productivity and decreased the period between ca. 14,600 and 9,400 cal. year BP, decomposition are likely results of higher tempera- can be accounted for in several ways. The frequent tures. This is supported by the relatively high occurrence of earthquakes in the region promotes concentrations of ostracods and diatoms, and by a mass movement processes in the lake and has a pollen assemblage dominated by arboreal pollen. particular impact on areas close to the steep slopes With respect to palaeotemperatures, our findings (Wagner et al. 2008). A distinct erosive contact match partly other reconstructions in the Mediterra- throughout the Pleistocene/Holocene transition in nean region, which indicate that relatively warm and 123 J Paleolimnol (2009) 41:407–430 423

core Lz1120

Age GSD MS TOC TS TOC/TN TOC/TS CaCO3 diat. conc. P/FP/B ostr. conc. AP/S/NAP -6 (cal. year BP) (µm) (10 SI) (%) (%) (%) (v/g/1000) (%) (v/g) (%) 500 32 2 0 1000 0 2 0 0.3 4 10 0 60 0 80080020 0 80 080 0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

Fig. 9 Geophysical, biogeochemical, and biological charac- planktonic, and benthic diatom species. AP/S/NAP is the teristics of the Holocene part of core Lz1120 from Lake Ohrid. proportion of arboreal, shrub, and non-arboreal pollen P/FP/B indicates the proportion of planktonic, facultative humid conditions prevailed during the first part of the most clearly as a minimum (ca. 40%) in carbonate Holocene (e.g. Ariztegui et al. 2000; Ramrath et al. concentration, but is also indicated in the MS, diatom 2000; Schmidt et al. 2000; Sadori and Narcisi 2001; and pollen data. According to the age-depth model Zanchetta et al. 2007; Wilson et al. 2008), probably (Fig. 4), the period of changed sediment characteris- with warm and dry summers and mild winters tics corresponds to the interval between ca. 8,500 and (Lawson et al. 2005). However, palynological tem- 8,000 cal. year BP, and coincides with the 8.2 ka perature and precipitation reconstructions are cooling event, given that the temporal resolution in ambiguous for the region (Davis et al. 2003; Tzeda- Lake Ohrid sediments is blurred by bioturbation. The kis 2007; Kotthoff et al. 2008) and changes in distinct decrease in carbonate concentration around palaeohumidity, such as proposed in the other 8,200 cal. year BP implies that cooling during this reconstructions, cannot be deduced with confidence period led to a decrease in primary productivity and from the Lake Ohrid record. Given the deep and distinct reduction of carbonate precipitation in the oligotrophic character of Lake Ohrid, minor lake lake. Alternatively, as indicated in the glacial part of level changes are of little importance for the proxies the core, the colder conditions could also have investigated. enhanced mixis of the water column and thus According to the biogeochemical data from core favoured decomposition of organic matter and car- Lz1120, which have the highest sample resolution of bonate dissolution. In contrast to the glacial period the various proxies investigated, several short-term however, when carbonate concentration and diatom environmental changes may be inferred, particularly abundance were low, diatom abundance between between ca. 9,400 and 6,000 cal. year BP. The most 8,500 and 8,000 cal. year BP displays a maximum prominent of these changes occurs only ca. 10 cm and is dominated by epilimnetic rather than hypo- below a preserved terrestrial plant macrofossil dated limnetic taxa, completely unlike the glacial flora. to 7,800 ± 130 cal. year BP (Table 2). It is recorded This suggests that cooling might not be the only 123 424 J Paleolimnol (2009) 41:407–430 reason for the sedimentary characteristics observed. ion concentrations and reduced mixis. Similarly, the An unambiguous distinction between temperature- distinct decrease in carbonate concentration between and precipitation-driven changes in Lake Ohrid is ca. 4,100 and 3,600 cal. year BP could be an difficult to define, particularly since the influence of indication of increased aridity as it is recorded Lake Prespa remains unclear at this stage. Matzinger between 4,200 and 3,500 cal. year BP in northern et al. (2006a) have shown that the spring water Italy (Drysdale et al. 2006), at Lake Van (Wick originating from Lake Prespa stratifies in autumn in et al. 2003), and at Lake Zeribar in Iran (Stevens Lake Ohrid at 15–20 m water depth in the euphotic et al. 2001). As discussed above, it remains difficult, zone. Lake level changes at Lake Prespa are signif- however, to distinguish between a temperature and a icant (Popovska and Bonacci 2007) and affect the precipitation signal at Lake Ohrid. Hence, the activity of springs, which contribute a significant distinct decrease in carbonate concentration along amount of calcium and bio-available phosphorus to with a maximum in diatom abundance around Lake Ohrid (Matzinger et al. 2006b). A lake-level 4,200 cal. year BP could result from a combination lowering of Lake Prespa by 10–15 m can increase the of both cooling and a drought, such as is proposed trophic state of the lake and thus lead to higher to have occurred around this time in the region phosphorus load via the springs feeding Lake Ohrid. (Bar-Matthews et al. 1999; Cullen et al. 2000; However, further lake-level lowering of Lake Prespa Weiss and Bradley 2001). would reduce the amount of water supply from Lake Enhanced mixis and oxygen saturation of the Prespa to Lake Ohrid and thus would decrease the bottom waters after 3,600 cal. year BP are indicated phosphorus input into Lake Ohrid. Hence, the by a decrease in the TS concentrations and an minimum in carbonate concentration in core increase in the TOC/TS ratio. In contrast to these Lz1120 around 8,200 cal. year BP could also result relatively gradually changing proxies, an abrupt from reduced phosphorus supply to Lake Ohrid, when decrease in carbonate concentration can be observed climate was more arid and the level of Lake Prespa at 2,400 cal. year BP, preceding a second zone of was significantly lower. This would match with other poor diatom preservation. An explanation for the observations indicating that the 8.2 ka cooling event decrease in carbonate concentration could be human- was characterized in the Mediterranean region not induced erosion such as indicated in the synchronous only by cooling but also by a short-term aridity in a decline of arboreal pollen. The erosion probably led period of generally warm and wet conditions (cf. to increased input of nutrients and organic matter Ariztegui et al. 2000; Wick et al. 2003; Kotthoff into Lake Ohrid. Decomposition of organic matter at et al. 2008). the sediment surface would have led to an increase in

Minor fluctuations in TOC and the diatom flora CO2 and a decrease in the bottom water pH. On the between ca. 6,000 and 2,400 cal. year BP indicate other hand, it is likely that the mixis of the water that environmental conditions probably became column, which is also reflected in the poor preser- more stable, despite an increasing human influence vation and low concentration of diatoms between as documented in the pollen record. The period 1,925 and 1,020 cal. year BP, restricted the release of between ca. 6,000 and 4,000 cal. year BP is phosphorus into the water column. This would have considered as a climatic optimum in eastern Ana- reduced productivity and carbonate precipitation, and tolia (Wick et al. 2003) and increasing aridity at this could have led, along with increased decomposition, time is recorded around the Adriatic Sea (Ramrath to the observed decrease in organic matter content. et al. 2000; Schmidt et al. 2000; Sadori and Narcisi TOC, TS, and carbonate concentration are set back at 2001). These climatic conditions are probably ca. 1,000 cal. year BP to values similar to those prior expressed by the more stable sedimentary charac- to distinct anthropogenic influence and decrease teristics as a result of less influence and less again markedly until ca. 100 cal. year BP. These fluctuation of spring water inflow into Lake Ohrid. significant changes in sediment characteristics of In the sediment record from Lake Ohrid, the gradual Lake Ohrid during the late Holocene are probably increase of TS to a maximum at 3,600 cal. year BP correlated with distinct changes in the level of Lake probably reflects increasing aridity in the region, Prespa. Historical settlements from the 11th and assuming that the increase of TS is a result of higher 12th century AD imply that the level of Lake Prespa 123 J Paleolimnol (2009) 41:407–430 425 was about 6 m below the present water level Holocene history of the lake and related changes in (Matzinger et al. 2006b). This period of lake-level the catchment area. lowstand would have led to increased input of Our results indicate that Lake Ohrid is a relatively phosphorus or less inflow of relatively cold water complex system and that the sedimentological char- into the epilimnion of Lake Ohrid in summer, and acteristics depend on a range of interacting processes. thus may have caused enhanced carbonate precipi- Mixis, spring activity, decomposition, and preserva- tation. A possible explanation for these major longer- tion of organic matter and the input of allochthonous term changes during the past ca. 2,400 cal. year BP mineralogical particles all have an influence on comes from the pollen record, which indicates sediment characteristics. Mass movements in Lake significant deforestation and enhanced agricultural Ohrid are known to be relatively common in areas activity and human impact in the catchment of Lake close to the lake shore. Thus it is no surprise that one Ohrid, marked by a decrease in Pinus and corre- hiatus is recorded in the sediment sequence recov- sponding increase in Cerealia pollen and Plantago ered. This hiatus covers a period of about 5,200 years lanceolata pollen. We cannot exclude the possibility at the Pleistocene/Holocene transition and is probably that the anthropogenic influence overrides the cli- related more to subaquatic current flow than to mass matic signal of the past millennia and hampers movement processes. Although bioturbation can be regional comparison. It seems that human impact in observed throughout the core, short-term events on a the catchment of Lake Ohrid was much higher than, centennial or even decadal scale, as well as long-term for example, is indicated in the record from Lago di environmental changes, are sensitively recorded. Pergusa in Southern Italy (Sadori and Narcisi 2001) These changes are chronologically constrained by or Lake Van in Turkey (Wick et al. 2003). With radiocarbon dating and tephrochronology. respect to climate changes, the observed changes in The Pleistocene environment between ca. 39,500 Lake Ohrid do not match the palaeo-rainfall patterns and 14,500 cal. year BP was characterized by a deduced from the speleothem record from Soreq steppe biome with relatively stable and cold condi- cave in Israel (Bar-Matthews and Ayalon 2004). tions. The occurrence of coarse sand and gravel in the However, two temperature changes across the north- sediments indicates that the lake was at least partially ern hemisphere, the Medieval Warm Period and the ice-covered during this period. Poor preservation of Little Ice Age, are tentatively indicated by biogeo- ostracods and diatoms and the biogeochemical char- chemical characteristics and in the pollen record acteristics of the sediments imply that the water from Lake Ohrid. It is likely that the vegetation column was well mixed. Slightly warmer conditions changes in the catchment also affected patterns of between ca. 39,500 and 35,000 cal. year BP are erosion and water retention time and in consequence indicated in the biogeochemical characteristics. the characteristics of spring water inflow. Minima in the percentage of arboreal pollen between Recent anthropogenic eutrophication, such as ca. 39,500 and 20,000 cal. year BP can be related suggested by Matzinger et al. (2007), may be indi- tentatively to Heinrich events H4–H2. cated by increased organic matter and an increase in The Holocene environment after ca. 9,400 cal. C. ocellata at the expense of D. stelligera, as well as year BP is characterized by distinct environmental a decrease in ostracod abundance, towards the surface changes. Most significant events during the early and sediment. However, these subtle changes are within middle Holocene are around 8,200 and 3,900 cal. the limits of Holocene variability, and sample year BP, however it is hard to define whether these resolution is not sufficiently high to determine the events are related with a short-term cooling or drying exact chronology. at Lake Ohrid. First human impact is recorded at ca. 5,000 cal. year BP and increases distinctly at 2,400 cal. year BP. Environmental changes during Conclusions the past 2,400 years are correlated with deforestation and erosion or with climate changes. The subtle The multi-disciplinary investigation of a sediment evidence for recent eutrophication of Lake Ohrid is core recovered from the southeastern part of Lake within the range of variability of earlier inferred Ohrid provides information about the Glacial and periods of enhanced productivity. 123 426 J Paleolimnol (2009) 41:407–430

Overall, and in spite of difficulties in disentangling of sapropel S1: inferences from Late Quaternary lacus- the complexity of forcing factors, Lake Ohrid clearly trine and marine sequences in the central Mediterranean region. Palaeogeogr Palaeoclimatol Palaeoecol 158:215– has high potential for long-term paleoenvironmental 240. doi:10.1016/S0031-0182(00)00051-1 reconstruction in the northern Mediterranean region. Baier J, Lu¨cke A, Negendank JFW, Schleser G-H, Zolitschka B The sequence investigated correlates broadly with (2004) Diatom and geochemical evidence of mid- to late other records from the region, but as is common in Holocene climatic changes at Lake Holzmaar, West-Eifel (Germany). Quat Intern 113:81–96. doi:10.1016/S1040- this topographically and climatically complex region, 6182(03)00081-8 also shows local peculiarities. One of the most Bar-Matthews M, Ayalon A (2004) Speleothems as palaeocli- obvious ways to improve the confidence of interpre- mate indicators, a case study from Soreq Cave located in the tation would be to carry out an additional, linked Eastern Mediterranean Region, Israel. In: Batterbee RW, Gasse F, Stickley CE (eds) Past climate variability through palaeolimnological study of Lake Prespa. Europe and Africa. Springer, Dordrecht, pp 363–392 Bar-Matthews M, Ayalon A, Kaufman A, Wasserburg G Acknowledgements The project is funded by the German (1999) The eastern Mediterranean palaeoclimate as a Research Foundation (grant WA2109/1). Thanks are also due reflection of regional events: Soreq Cave, Israel. Earth to the British Council and the German Academic Exchange Planet Sci Lett 166:85–95. doi:10.1016/S0012-821X(98) Service (DAAD) for travel grants to J.M.R. and A.S. within the 00275-1 Project Based Personnel Exchange Programme. We would like Battarbee RW (1986) Diatom analysis. In: Berglund BE (ed) to thank Goce Kostoski, Sasho Trajanoski, Zoran Spirkovski, Handbook of Holocene palaeoecology and palaeohydrol- Zoran Brdaroski, Mitat Sanxhaku and Emirjeta Adhami for ogy. Wiley, Chichester, pp 527–570 enormous logistic support during the field campaign. Trajan Battarbee RW, Kneen MJ (1982) The use of electronically Petkovski and Burkhard Scharf are thanked for help with counted microspheres in absolute diatom analysis. Limnol ostracod taxonomy. The coring location was selected based on Oceanogr 27:184–188 a shallow seismic survey by Gerhard Daut from University of Battarbee RW, Juggins S, Gasse F, Anderson NJ, Bennion H, Jena, Germany. Hendrik Vogel contributed with numerous Cameron N (2000) European diatom database (EDDI). An fruitful discussions. information system for palaeoenvironmental reconstruc- tion. 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