Lake Ohrid is located at the border between FYROM (Former Yugoslavian
Republic of Macedonia) and Albania and formed during the latest phases of
Alpine orogenesis. It is the deepest, the largest and the oldest tectonic
lake in Europe. To better understand the paleoclimatic and paleoenvironmental
evolution of Lake Ohrid, deep drilling was carried out in 2013 within the
framework of the Scientific Collaboration on Past Speciation Conditions
(SCOPSCO) project that was funded by the International Continental Scientific
Drilling Program (ICDP). Preliminary results indicate that lacustrine
sedimentation of Lake Ohrid started between 1.2 and 1.9 Ma ago. Here we
present new pollen data (selected percentage and concentration taxa/groups)
of the uppermost
According to the age model, the studied sequence covers the last
The period corresponding to MIS11 (pollen assemblage zone OD-10,
428–368
Our palynological results support the notion that Lake Ohrid has been a refugium area for both temperate and montane trees during glacials. Closer comparisons with other long southern European and Near Eastern pollen records will be achieved through ongoing high-resolution studies.
The study of past climate change is pivotal to better understand current climate change (Tzedakis et al., 2009) and its impact on terrestrial ecosystems, particularly at the mid-latitudes, where human activities are concentrated. It is well established that the study of fossil pollen contained in sediments fundamentally contributes to the reconstruction of terrestrial palaeoenvironmental changes that occurred during the Quaternary, and constitutes the only quantitative proxy that can provide continuous and accurate representations of vegetation changes. This fact was already clear at the end of the 1960s when the pioneer pollen study of Wijmstra (1969) at Tenaghi Philippon (Greece) was published. The study of long lacustrine pollen records from southern Europe is particularly important, as at such latitudes, glaciations have not caused stratigraphic gaps in lacustrine systems, unlike northern European sequences (e.g. Zagwijn, 1992). The relationship of terrestrial vegetation with terrestrial, marine and ice core records is a further step in the understanding of global climate dynamics and lead–lag relations. A broader correspondence between the climate signals provided by terrestrial pollen records and marine oxygen isotope records has been observed (e.g. Tzedakis et al., 1997, 2001). Subsequent studies of both terrestrial (pollen) and marine (planktonic and benthic oxygen isotopes) proxies in marine cores from the Iberian margin confirmed the mostly in-phase relation of Mediterranean and North Atlantic climate variability during the Late Pleistocene (e.g. Sánchez Goñi et al., 1999; Tzedakis et al., 2004b). But the exact phase relations to marine systems, regional variations in vegetation response, and exact locations of refugia are still poorly known mostly due to the complications of obtaining records in key regions and with independent age control.
Southern Europe encompasses five lacustrine pollen records spanning more than
the last two glacial–interglacial cycles. They are the composite record of
Bouchet/Praclaux in southern France, spanning the last
Southern European long pollen records have caught the attention of many researchers, as these archives are arguably among the best available sources of information for past vegetation and climate changes (e.g. Tzedakis et al., 1997, 2001; Pross et al., 2015). Molecular genetic data revealed considerable divergence between populations of many arboreal species in southern refugial centres in Iberia, Italy, the Balkans and Greece. Arboreal refugia and migration paths, identified by both biogeographical, palaeobotanical and phylogeographical studies (Petit et al., 2005; Cheddadi et al., 2006; Magri et al., 2006; Liepelt et al., 2009; Médail and Diadema, 2009; Tzedakis, 2009; Tzedakis et al., 2013), sometimes confirmed the speculated locations (e.g. Bennett et al., 1991) and their link to modern biodiversity hotspots, but most mechanisms still have to be fully understood. From this perspective it is essential to compare the locations of refugia and those of regional hotspots of plant biodiversity.
Located in a strategic position between higher-latitude and lower-latitude
climate systems, Lake Ohrid is at the border between the Former Yugoslavian
Republic of Macedonia (FYROM) and Albania. As one of the biosphere reserves
of the United Nations Educational, Scientific, and Cultural Organization
(UNESCO), it is a transboundary World Heritage Site in the Balkans. It is
thought to be the oldest extant lake in Europe, with an uninterrupted
lacustrine sedimentation probably starting between 1.2 and 1.9
The SCOPSCO (Scientific Collaboration on Past Speciation Conditions in Lake
Ohrid) international science team carried out a deep drilling campaign in
spring 2013 in the framework of the International Continental Scientific
Drilling Program (ICDP). The aim of this initiative is an interdisciplinary
analysis of environmental and climate variability under different boundary
conditions throughout the Pleistocene. Initial results, based on the DEEP
borehole in the lake centre, show approximately 1.2
Specific objectives of this study are (1) to outline the flora and vegetation changes that occurred in the last half million years in the area surrounding Lake Ohrid, (2) to understand the glacial and interglacial vegetation dynamics, and (3) to correlate the vegetation changes with benthic and planktic marine isotope stratigraphy.
Considering the core length, in this paper we aim to provide a comprehensive overview of millennial-scale vegetation dynamics during glacial–interglacial stages at Lake Ohrid before analysing intervals at high resolution. The aim of this study is not in fact to discuss in detail the features of either interglacial or glacial periods. Existing high-resolution pollen studies focusing on different time intervals (e.g. Tzedakis et al., 2004b, 2009; Tzedakis, 2007; Fletcher et al., 2010; Margari et al., 2010; Moreno et al., 2015) offer a more detailed picture of ecosystem dynamics in the Mediterranean region. High-resolution studies using the exceptional Lake Orhid archive are in progress for selected intervals (e.g. MIS 5–6, MIS 11–12 and MIS 35–42).
Lake Ohrid (40
Map of Lake Ohrid modified from Panagiotopoulos (2013) and locations of terrestrial and marine records discussed in the text.
Lake Ohrid has a sub-elliptical shape: it is 30.3 km long and 15.6 km wide
and is located at an altitude of 693 m a.s.l. It has a water surface of
The river Crni Drim is the lake emissary and its outflow is artificially
controlled. Lake Ohrid is separated from Lake Prespa, which is situated at
849 m a.s.l. (
The bedrock around the lake mainly consists of low- to medium-grade metamorphosed Paleozoic sedimentary rocks and Triassic limestones intensely karstified along the eastern coast. The western shoreline is characterized by Jurassic ophiolites of the Mirdita zone. Cenozoic sediments including Pliocene and Quaternary deposits are mainly found southwest of the lake (Wagner et al., 2009; Hoffmann et al., 2012).
Climatic conditions are strongly influenced by the proximity to the Adriatic
Sea and the water bodies of lakes Ohrid and Prespa, which reduce the
temperature extremes due to the presence of high mountain chains (Wagner et
al., 2009; Hoffmann et al., 2012). An average precipitation for the Lake
Ohrid watershed of
Studies on regional flora and vegetation are rather scarce in the
international literature. The main source of information is from a detailed
survey carried out in Galičica National Park (Matevski et al., 2011).
Concerning the flora, the Mediterranean and Balkan elements dominate, but
several central European species are also widespread in the area. The
vegetation is organized into altitudinal belts, which develop from the lake
level (700
In riparian forests, the dominant species is
Lake Ohrid is well known for its rich local macrophytic flora, consisting of
more than 124 species. Four successive zones of vegetation characterize the
lake shores: the zone dominated by floating species such as
Details about core recovery, the core composite profile and sub-sampling are
provided by Wagner et al. (2014) and Francke et al. (2016). From the DEEP
site (ICDP site 5045-1) in the central part of Lake Ohrid
(41
The DEEP core chronology down to 247.8
Sample processing and pollen microscope analysis are the fruit of strict cooperative work by several investigators across many European laboratories. Prior to the pollen analysis, considerable time was invested in assessing and standardizing the treatment protocol and pollen identification issues. More specifically, (1) we joined previous lists of taxa that were derived from older studies in Lake Ohrid and the western Balkans and produced a final list that has been accepted by all the analysts; (2) we thoroughly elaborated on systematic issues like synonyms and different degrees of pollen determination, particularly focusing on the identification of problematic taxa; (3) we shared pollen pictures of key taxa (e.g. oak types) and of dubious ones; (4) we also performed analyses of samples from the same core depth in different laboratories. Samples were mostly distributed in batches of consecutive samples; and (5) finally, close checks were performed at the intervals where two different analysts' samples met in order to avoid any potential identification bias.
A total of 306 sediment samples at 64 cm intervals down to the depth of
197.55
For each sample, 1/1.5
Oak pollen has been divided into three types according to morphological
features following Smit (1973):
Lake Ohrid (FYROM), DEEP core. Pollen percentage diagram of selected taxa against depth scale. Lithology, tephra layers and tuning points adapted from Francke et al. (2016).
Lake Ohrid (FYROM), DEEP core. Pollen diagram of selected ecological
groups (%) and concentration curves against chronology (Francke et
al., 2016). Ecological groups: montane trees (
Comparison of selected proxies from Lake Ohrid with other records
spanning the last 500
We present data in two pollen diagrams: (i) a percentage pollen diagram (main taxa) based on the sediment depth scale and including lithostratigraphy and tie points used to assess chronology of the DEEP site sequence (Francke et al., 2016, Fig. 2); (ii) a pollen diagram showing the percentage sums of ecological groups and selected concentration curves drawn according to the age scale (Fig. 3).
In total, 296 samples (97 % of the total analysed) yielded low–medium to
high pollen concentrations allowing a detailed palynological analysis.
Samples with counts less than 80 terrestrial pollen grains were excluded from
the diagram. Mean pollen counts of 824 terrestrial pollen grains have been
achieved. The physiognomy of vegetation shows maximum variability: arboreal
pollen (AP) ranges from 19 to 99 % (Fig. 2). The total pollen
concentration of terrestrial taxa is quite variable, ranging from ca. 4000 to
ca. 910 000
The main vegetation features are summarized in Table 1. The pollen record was
subdivided into 13 main pollen assemblage superzones (PASZ, OD – named after
the Ohrid DEEP core) on the basis of changes in AP versus non-arboreal pollen
(NAP), changes in pollen concentration and major changes in single taxa. The
most abundant taxon is
Climate variability paces the pronounced intra-interglacial vegetational shifts inferred from the pollen record, while different patterns of ecological succession emerge during interglacials (Fig. 3).
Long-term vegetation dynamics correspond accurately to the glacial and interglacial periods, even if admittedly the established chronology for the Lake Ohrid DEEP record could be further improved with tuning to higher-resolution proxy data (see Zanchetta et al., 2015), with the detection of other tephra layers and the general improving of analyses obtained for the record.
In addition, most interstadials and some higher-order variability have been previously reported from south-eastern Europe, i.e. Ioannina (MIS6: Roucoux et al., 2011) and Tenaghi Philippon (MIS8: Fletcher et al., 2013). Ongoing high-resolution studies will help define dynamics of specific taxa, revealing extinctions and detecting possible new refuge areas.
A close look at the Lake Ohrid pollen record reveals distinct characteristics
for glacial and interglacial phases during the investigated past
500
A clear correspondence between the climate signals provided by our terrestrial pollen record and marine oxygen isotope records (Fig. 4) is apparent, even if the limits between pollen zones and marine isotope stages are often not identical (Figs. 2, 3).
Glacial periods (PASZ OD-12, 11, 9, 7, 5, 4, 2, Table 1) are generally
characterized by dominance of Poaceae,
In contrast, interglacial complexes (PASZ OD-13, 10, 8, 6, 3 and 1, Table 1)
are marked by expansions of woods dominated by
The pollen diagram shows that, in the past 285
Main vegetational features of Lake Ohrid DEEP core pollen assemblage
zones (OD-PASZ) and related chronological limits. The basis sum for AP and
NAP taxa does not include
Continued.
This general trend is visible in the reduction of montane trees present in
OD-10 and 12 (roughly corresponding to MIS11 and 13) and the expansion of
mesophilous and Mediterranean taxa in the present and penultimate
interglacials (Fig. 3). The pre-penultimate interglacial (OD-8,
333–288
OD-12 (488–459
If we do not consider pine, the passage to the following interglacial (OD-10,
428–366
Glacial conditions prevailed during zone OD-9, 366–333
The following interglacial OD-8, 333–288
OD-7, 288–245
OD-6 (245–190
A long glacial phase is represented in OD-5 (190–160
Forests of OD-3, 129–70
The last glacial period, i.e. MIS4-2, is represented in PASZ OD-2
(70–14
The present interglacial is characterized by the strong and prominent
expansion of the
In Fig. 4 alignment of the TOC, TIC, AP percentages and AP
The similarity between Lake Ohrid and Tenaghi Philippon curves is striking.
All the main changes in forest cover match, and they both correspond to
marine records too. There are some differences in the timing of the onset of
interglacial phases. DEEP core chronology benefited in fact from the presence
of several tephra layers (see Fig. 2, Leicher et al., 2015). The main
difference with Tenaghi Philippon is in the fact that arboreal taxa show a
continuous presence at Lake Ohrid, even during the glacials, while at Tenaghi
Philippon they often disappear to spread again during the interglacials,
often with a certain delay. This behaviour could anyway have been expected
considering the differences in water availability at the two sites. In
Greece, not only Tenaghi Philippon, but also the Kopais (Okuda et al., 2001)
areas, resulted in not being ideal refugia for mesophilous trees (Tzedakis et
al., 2004a). A quite different situation is found at Ioannina (western
Greece), a refugial site for temperate trees featuring sub-Mediterranean
climate and vegetation in the last
Besides a close correspondence to the Tenaghi Philippon AP % curve, Fig. 4 also shows a close correspondence between our pollen data and the Mediterranean benthic and planktic composite curves (Wang et al., 2010; Konijnendijk et al., 2015). Compared to the global isotope stack (Lisiecki and Raymo, 2005; Railsback et al., 2015), additional detail in the pollen diagram is clearly representative of regional Mediterranean conditions and of the influence of moisture availability on the expansion of plants. Both marine deep and surface water features show additional warm phases during interglacials that are also observed in the pollen data. For example, the tripartite forests during MIS7 are well reflected in the pollen data but likely overprinted by the effect of ice volume in the global benthic isotope stack. Completion of the downcore analysis of the DEEP core from Lake Ohrid will allow for a more accurate correlation of the entire sequence with the orbitally tuned Mediterranean isotope records, and provide a finer tuning of the present age model (Francke et al., 2016) to independently dated records in the Mediterranean region where available.
The 500
The richness of pollen diversity and continuity along this long-time series point to the particular climatic and environmental conditions that contributed to the high plant diversity encountered at Ohrid at present. This has deep roots in the past, as the lake has probably acted as a permanent water reservoir providing moisture to its surroundings even during dramatic dry or cold climatic phases. In fact trees never disappeared from the investigated area.
The main novelty of this pollen record from the Balkan Peninsula is
summarized by the following key findings.
The continuous record of glacial–interglacial vegetation successions
shows that refugial conditions occurred in the Lake Ohrid area. Tree
extinction, whose timing and patterns need accurate checks and refined
analyses, will be focused on in a dedicated study. A clear shift from relatively cool/humid interglacial conditions prior to
288 Similarities and dissimilarities with other southern European and Near
Eastern pollen records, even if already visible, will be better defined with
the improvement of analyses through ongoing high-resolution studies. A close correspondence of interglacial and glacial climate and vegetation
evolution to regional benthic and planktic isotope data is apparent. The
Ohrid pollen record integrates temperature data from the marine stratigraphy,
with a clear indication of humidity/dryness changes.
This article is the product of strict cooperative work among palynologists who all contributed to the Lake Ohrid pollen analysis and its interpretation. The manuscript was written by L. Sadori with substantial contribution of T. H. Donders, A. Koutsodendris and K. Panagiotopoulos. A. Masi (c.a.) was responsible for data management and refined diagrams drawn by T. H. Donders and A. Koutsodendris. All coauthors contributed to the writing of this paper.
The authors are indebted to the two referees Thomas Litt and Chronis Tzedakis for the constructive comments that were used to improve the paper. Chronis Tzedakis was also of great help in giving hints to better anchor the chronology of the record.
We also thank Zlatko Levkov for organizing the SCOPSCO meeting in Skopje (2015) that gave the opportunity to A. Koutsodendris, A. Masi, K. Panagiotopoulos, L. Sadori and T. H. Donders to discuss with Valdo Matevski, botanist expert in the vegetation of Lake Ohrid, several aspects of the pollen diagram. The help of Valdo Matevski was valuable in figuring out some possible arboreal dynamics of the past. A. Koutsodendris, A. Masi, K. Panagiotopoulos, L. Sadori and T. H. Donders would also like to express their gratitude to Renata Kysterevska, Slavcho Hristovski and Mitko Kostadinovski from the University of Ss. Cyril and Methodius in Skopje for botanical literature and herbarium consultations.
The SCOPSCO Lake Ohrid drilling campaign was funded by ICDP, the German Ministry of Higher Education and Research, the German Research Foundation (DFG), the University of Cologne, the British Geological Survey, the INGV and CNR of Italy, and the governments of the Republic of Macedonia (FYROM) and Albania. Logistic support was provided by the Hydrobiological Institute in Ohrid. Drilling was carried out by Drilling, Observation and Sampling of the Earth's Continental Crust's (DOSECC) and using the Deep Lake Drilling System (DLDS). Special thanks are due to Beau Marshall and the drilling team. Ali Skinner and Martin Melles provided immense help and advice during logistic preparation and the drilling operation. A. Koutsodendris received funding from the German Research Foundation (grant KO4960/1). Edited by: T. Wilke