DIATOMS FROM SPHAGNUM PEAT BOGS IN THE SREDNA GORA MTS (BULGARIA) 65
GEOLOGICA CARPATHICA, 55, 1, BRATISLAVA, FEBRUARY 2004
6576
PALEOECOLOGY OF HOLOCENE DIATOMS FROM SPHAGNUM
PEAT BOGS IN THE CENTRAL SREDNA GORA MOUNTAINS
(BULGARIA)
ROSALINA STANCHEVA and DOBRINA TEMNISKOVA
Department of Botany, Faculty of Biology, St. Kliment Ohridski University of Sofia, D. Tsankov Blvd 8, 1164 Sofia, Bulgaria;
temn@biofac.uni-sofia.bg
(Manuscript received November 15, 2002; accepted in revised form June 23, 2003)
Abstract: The fossil record from three sphagnum peat bogs situated at altitudes of 14001550 m in the Central Sredna
Gora Mts (Bulgaria), consisting of 115 Diatomaceea species, 15 varieties and 2 forms, represents four diatom zones
covering the time span of the last 10 ky. The diatom flora in the peat bogs emerged 8 ky ago. The fluctuating develop-
ment of diatom communities was controlled by unstable water level, affected by local humidity and other climatic
changes.
Key words: Holocene, Bulgaria, paleoecology, peat bogs, diatoms, quantitative pollen analysis.
Introduction
The first data on diatoms from Holocene sediments in Bulgar-
ia have been so far reported for the Beloslav and Dourankou-
lak lakes on the Bulgarian Black Sea Coast (Temniskova &
Dojceva 1989; Temniskova & Atanassova 1996), the Sozopol
Bay (Ognjanova-Rumenova 1995; Ognjanova-Rumenova &
Zaprjanova 1998). The Holocene history of the development
of the two coastal lakes has marked three periods of saliniza-
tion of their waters. A marine transgression, which had affect-
ed the composition of the diatom flora in the two lakes, was
traced out at about 4000 BC for Dourankoulak and at about
4170±150 BC for Beloslav lake. Ognjanova-Rumenova
(2001a,b) studied the diatom flora of Quaternary sediments
from the subalpine lake Dulgoto in the Northern Pirin Mts.
Five diatom biozones have been differentiated and the local
paleoecologial conditions have been reconstructed. The first
studies of Holocene diatoms from the peat bogs relate to the
species composition of diatom flora from the Barierata strati-
graphic profile in the Central Sredna Gora Mts (Stancheva
2001a,b). It was taken from inundated meadows in the defor-
ested areas at 1000 m a.s.l., where Sphagnum spp. were miss-
ing in the vegetation cover. This makes a significant differ-
ence when compared to the sphagnum peat bogs situated at
higher altitudes in the region. Stancheva (2001c) published
only the taxonomical composition of the diatom flora from the
Shiligarka sphagnum peat bog.
For the Central Sredna Gora Mts, four forest succession
phases have been established in the 10,025
±
225 BP period,
and light has been thrown on the development of vegetation
and the main climatic changes until the formation of the con-
temporary beech forests (Petrov & Filipovitch 1987; Filipov-
itch 1992, 1996, 1998; Filipovitch et al. 1998; Stefanova &
Filipovitch 1997).
The purpose of the present study is to establish the succes-
sion of the diatom community in the sediments of the investi-
gated peat bogs and to reconstruct the paleoecological condi-
tions in the last 10,000 years by interpreting the available data
on autochthonous siliceous microfossils.
Study area
The Sredna Gora Mts lies to the south of the Balkan Range
and Rearbalkan valleys, with a predominantly West-East ori-
entation. Morphographically, the Sredna Gora Mts has a high-
ly variable relief comprising hill-and-valley, lower and higher
mountain stretches. Geologically and petrographically, most
of that region is formed of anticlinal folds, with cores of Early
Paleozoic (Hercynian) granites and pre-Paleozoic and Paleo-
zoic metamorphic rock complexes (gneisses, micaceous
schists, amphibole schists, etc.) (Georgiev 1991). The phys-
icogeographical region of the objects of study extends to the
southernmost part of the Temperate Continental and to the
northern part of the Transitory Continental Climatic Zone.
Winter is cold and comparatively dry, summer is warm, with
maximum precipitations in May and June (Mishev 1989).
The Central Sredna Gora Mts falls into the European Decid-
uous Forests Zone, the Balkan (Illyrian) Province, Sredna
Gora region. Beech and oak forests dominate, with an occa-
sional thin hornbeam forest and secondary communities of
Corylus, Carpinus orientalis, with artificial afforestations of
Pinus sylvestris in some places (Bondev 1991, 1997).
The investigated sphagnum peat bogs are located in the
Central Sredna Gora Mts (Fig. 1). Shiligarka and Bogdan-3
peat bogs lie above the upper tree line, at an altitude of 1500
1550 m, between peaks Bogdan (1604 m a.s.l.) and Malak
Bogdan (1532 m a.s.l.). Bogdan-3 peat bog is situated on a
southern slope of the mountain crest, while Shiligarka peat
bog is neighbouring a beech forest. Bogdan-6 peat bog lies in
the beech belt, at an altitude of 1400 m. The investigated peat
bogs vary in area from several square meters to 10,000
20,000 m
2
and have various slope gradients. The fontinal peat
bogs represent heterotrophotypic complexes of mosaically ar-
66 STANCHEVA and TEMNISKOVA
ranged eutrophic and oligotrophic plant communities, depend-
ing on terrain lowering (Filipovitch et al. 1998). The following
species prevail in the vegetation cover of the sphagnum peat
bogs: Eriophorum vaginatum L., E. latifolium Hoppe., Carex
flava L., C. leporina L., C. echinata Murr., Juncus effusus L., J.
thomassi Ten., Parnasia palustris L., Pinguicula vulgaris L.,
Drosera rotundifolia L., Orchis maculatus L., Epilobium
palustre L., Deschampsia caespitosa P.B., Potentilla erecta
(L.) Rausch., Galium palustre L., Myosotis scorpioides L., Eq-
uisetum hiemale L., E. palustre L., E. arvense L., Geum coc-
cineum S.S., Vaccinium vitis-idaea L., V. uliginosum L., Caltha
palustris L., Hieratium pilosella L., Epilobium angustifolium
L., Ajuga genevensis L., Athyrium filix-femina (L.) Roth. The
peat bogs in higher locations are surrounded by mountain
meadow communities, dominated by Nardus stricta L., Junipe-
rus communis L. and Bruckenthalia spiculifolia (Salisb.) Rchb.
Material and methods
The material studied was obtained from three cores retrieved from
each peat bog. The cores were of the following length: Bogdan-3
(150 cm), Bogdan-6 (80 cm) and Shiligarka (70 cm). The sampling
was done by means of a manual, Dachnowsky-type corer, with cam-
era diameter of 2 cm (Filipovitch et al. 1998). Lithological composi-
tion of the studied cores was detailed as follows:
Bogdan-3: 031 cm peat with plant remains; 3182 cm
dark, strongly humified peat, with five consecutive layers; 82
145 cm humified brown peat with plant remains; 145150 cm
coarse clay with sand.
Bogdan-6: 040 cm brown peat; 4070 cm black peat;
7075 cm peat with plant remains; 7580 cm coarse clay
with abundant of sand.
Shiligarka: 060 cm peat with plant remains; 6065 cm
peat with plant remains and fine sand; 6570 cm peat with phy-
to-remains and gravel (Stancheva 2001c).
Radiocarbon dating of the sediments from Bogdan-3, Bogdan-6
and Shiligarka cores was carried out at the
14
C und
3
H Laboratori-
um, Niedersächisches Landesamt für Bodenforschung in Han-
nover, Germany. The results of the dating are presented in Table 1
(acc. to Filipovitch et al. 1998).
A total of 156 samples for diatom analysis were extruded at 1
5 cm intervals, next to the samples for pollen analysis. Laboratory
processing followed the technique of Hasle & Fryxell (1970), as
modified by Ognjanova-Rumenova (1991). Microscopic slides for
LM were prepared according to Gleser et al. (1974), and for SEM
according to Loseva (1982), with a gold coating. Light microscopic
investigation was carried out by means of an Amplival light micro-
scope with oil immersion objective
×
100. Jeol JSM-T 300 and Jeol
JSM-5300 scanning electron microscope were used for the identifi-
cation and documentation of the problematic taxa. Quantitative
analysis followed the standard percentage counting technique of
Battarbee (1986), with 500 valves counted in each slide. For the dia-
tom taxa with known autecology, ecological spectra were deter-
mined in accordance with Abbott & Van Landingham (1972). They
were based on literary sources with comparable data by various au-
thors (Kolbe 1927; Hustedt 19371939; Lowe 1974; Krammer &
Lange-Bertalot 19861991; Denys 1991; Foged 1993; Häkansson
1993; Vos & De Wolf 1993; Van Dam et al. 1994; Van de Vijver et
al. 1998; etc.).
They featured water salinity after Hustedt (1957), active water re-
action after Hustedt (19371939), trophic status after Naumann
(1921, cit. acc. to Van Dam et al. 1994), specific habitats according
to the definition of Vos & De Wolf 1993, and geographical distribu-
tion. The percentage ratio of diatom frustules/chrysophycean stoma-
tocysts was determined after Smol (1985). This reflected the chang-
es in trophic and hydrological status of paleobasins.
In this paper pH reconstruction uses the categories of Hustedt
(19371939) and is based on Index B (Renberg & Hellberg 1982).
It was applied to two equations expressing the ratio between pH and
log Index B.
Index B based on data from lakes in Bohuslän (Renberg & Hell-
berg 1982) was:
pH = 6.400.85 log Index B (r
2
= 0.91; S.E. = ± 0.30).
Index B based on data from 33 lakes in Galloway (Flower 1986)
was:
pH = 6.300.865 log Index B (r
2
= 0.82; P < 0.001; S.E. = ± 0.36).
Ecological analysis and pH reconstruction were applied only to
samples with 500 counted diatom valves.
Diatoms were identified using the floras of Germain (1981),
Krammer & Lange-Bertalot (1986, 1988, 1991, 1991a) etc. For no-
menclature refer to Krammer & Lange-Bertalot (1986, 1988, 1991,
1991a) and Round et al. (1990).
Results
A total of 115 diatom species, 15 varieties and two forms be-
longing to 28 genera (see Appendix) were identified. Eight
taxa were identified to the generic level. One hundred and
Fig. 1. Location map of the studied peat bogs in the Central Sredna
Gora Mts.
Locality
Depth (cm) Lab.
¹
14
C Dates Age BP
Bogdan-3
100105
Hv 6722
7145±95
Bogdan-3
125135
Hv 6723
8115±100
Shiligarka
5560
Hv 21662
5285±180
Bogdan-6
3540
Hv 21663
5060±165
Bogdan-6
6570
Hv 21664
7835±445
Bogdan-6
7080
Hv 21665
10025±225
Table 1: Radiocarbon datings (Filipovitch et al. 1998).
DIATOMS FROM SPHAGNUM PEAT BOGS IN THE SREDNA GORA MTS (BULGARIA) 67
eight taxa were found in the sediments of the Shiligarka sam-
pling site, 76 in Bogdan-6 and only 41 in Bogdan-3 peat bogs.
The identified diatom flora consisted entirely of recent spe-
cies.
Diatom biostratigraphy
The stratigraphic distribution pattern and relative abundance
of the most frequently occurring diatoms and the values of the
diatom frustules/chrysophycean stomatocysts ratio in the three
cores are presented on Figs. 2, 3 and 4. The diagrams include
all diatom species that exceeded 2 % of the total numbers
found within each of the samples. Definition of the biostrati-
graphic diatom zones and their correlation are based on the to-
tal fossil record of diatoms (Salvador 1994; Nikolov &
Sapunov 2002). Changes in the diatom species composition
enabled us to distinguish four diatom assemblage zones, rep-
resenting different ecological conditions:
DAZ A (Bogdan-3: 140120 cm, Bogdan-6: 7065 cm,
corresponding to the beginning of PAZ SG 2 of Filipovitch et
al. (1998)
This zone was characterized by a low number of taxa and
low abundance of diatoms. The highest species diversity was
recorded in Pinnularia (6 species) and Eunotia (5 species).
Eunotia glacialis Meister, E. monodon Ehrenberg (Fig.
5.16,17), E. praerupta Ehrenberg, Pinnularia borealis Ehren-
berg (Fig. 6.5), P. lata (Bréb.) W. Smith (Fig. 6.6,7), and P.
maior (Kütz.) Rabenhorst were the most frequently occurring
species. The remaining taxa were represented by single speci-
mens. Fragilaria ulna (Nitz.) Lange-Bertalot and Navicula ra-
diosa Kützing occurred only in the sediments of Bogdan-6
peat bog on that level. Abundantly occurring chrysophycean
stomatocysts have also been observed in great numbers.
DAZ B (Bogdan-3: 12055 cm, Bogdan-6: 6540 cm, cor-
responding to the end of PAZ SG 2, PAZ SG 3a of Filipov-
itch et al. (1998)
In DAZ B diatom flora is represented by very low number
of taxa and the diatom record is incomplete. Sediment sam-
ples taken from 108100, 9590, 8580 cm (Bogdan-3) and
6461, 5552 cm (Bogdan-6) depth intervals were barren. In
other levels Eunotia glacialis, E. monodon, Gomphonema an-
gustatum (Kütz.) Rabenhorst, Meridion circulare var. con-
strictum (Ralfs) Van Heurck, Stauroneis anceps Ehrenberg,
and Pinnularia maior, Pinnularia sp. were identified. The
valves were poorly preserved and the large valves of Eunotia
monodon and Pinnularia sp. were fragmented. Representation
of Eunotia sp. and Navicula sp. were small and, owing to par-
tial dissolution, their determination was impossible. Rarely
occurring chrysophycean stomatocysts were identified in
some of the samples.
DAZ C (Bogdan-3: 5313 cm, Bogdan-6: 4018 cm, Shili-
garka: 6020 cm, corresponding to PAZ SG 3, and the be-
ginning of PAZ SG 4 of Filipovitch et al. (1998)
A species-rich diatom community has been identified in the
lowermost part of the sediments of Shiligarka peat bog, as
well as increased relative abundance of single taxa in the sedi-
ments of Bogdan-3 and Bogdan-6 peat bogs. Chrysophycean
stomatocysts considerably dominated over diatoms through-
Fig. 2. Diatom frequency diagram of the most common diatoms and percentage ratio of diatom frustules/chrysophycean stomatocysts in
the sediments of Bogdan-3 peat bog.
68 STANCHEVA and TEMNISKOVA
Fig. 3. Diatom frequency diagram of the most common diatoms and percentage ratio of diatom frustules/chrysophycean stomatocysts in
the sediments of Bogdan-6 peat bog.
(8.8 %) (Fig. 5.10), Hantzschia amphioxys (Ehr.) Grunow
(12.8 %), Pinnularia viridis (Nitz.) Ehrenberg (19 %), Pinnu-
laria sp. (15.6 %), P. streptoraphe Cleve (7.8 %) and P. lata
(7 %) showed high relative abundance in the sediments of
Shiligarka peat bog and reached their highest relative abun-
dance in this subzone. Aulacoseira granulata (Ehr.) Simonsen
and A. subarctica (O. Müller) Haworth, Fragilaria capucina
var. rumpens (Kütz.) Lange-Bertalot, F. virescens Ralfs, and
Melosira varians Agardh appeared for the first time. The low-
ermost part of the subzone is characterized by occurrence of
Fig. 4. Diatom frequency diagram of the most common diatoms and percentage ratio of diatom frustules/chrysophycean stomatocysts in
the sediments of Shiligarka peat bog.
out the entire zone. Changes in the diatom species composi-
tion allowed us to distinguish two subzones (DAZ C1 and
DAZ C2) within DAZ C.
DAZ C1 (Bogdan-3: 5335 cm, Bogdan-6: 4030 cm,
Shiligarka: 6052 cm, corresponding to PAZ SG 3, PAZ SG
3b of Filipovitch et al. (1998)
This subzone was characterized by a diatom flora rich in
taxa. Eunotia monodon (32.2 %), E. minor (Kütz.) Grunow
DIATOMS FROM SPHAGNUM PEAT BOGS IN THE SREDNA GORA MTS (BULGARIA) 69
Fig. 5. 1. Aulacoseira alpigena (Grunow) Krammer valve mantle, SEM. 2. Aulacoseira alpigena (Grunow) Krammer valve face, SEM.
3. Tabellaria flocculosa (Roth) Kützing septum, SEM. 4. Caloneis bacillum (Grunow) Cleve, SEM. 5. Achnanthes petersenii Hustedt, SEM.
6. Fragilaria construens f. venter (Ehrenberg) Hustedt valve exterior, SEM. 7. Fragilaria construens f. venter (Ehrenberg) Hustedt valve
interior, SEM. 8. Caloneis tenuis (Gregory) Krammer detail, SEM. 9. Caloneis tenuis (Gregory) Krammer, SEM. 10. Eunotia minor (Kütz-
ing) Grunow, SEM. 11. Eunotia incisa Gregory, SEM. 1215. Eunotia serra var. tetraodon (Ehrenberg) Nörpel, LM. 1617. Eunotia monodon
Ehrenberg, LM. Scale bars = 5
µ
m.
70 STANCHEVA and TEMNISKOVA
Fig. 6. 1. Pinnularia subcapitata Gregory valve exterior, SEM. 2. Pinnularia subcapitata Gregory valve interior, SEM. 3. Pinnularia
appendiculata (Agardh) Cleve, SEM. 4. Pinnularia microstauron (Ehrenberg) Cleve, SEM. 5. Pinnularia borealis Ehrenberg, LM. 6. Pinnu-
laria lata (Brébisson) Rabenhorst, LM. 7. Pinnularia lata (Brébisson) Rabenhorst, SEM. 8. Gomphonema clavatum Ehrenberg, SEM.
9. Neidium bisulcatum (Lagerstedt) Cleve, SEM. 10. Cymbella perpusilla Cleve, SEM. 11. Cymbella silesiaca Bleisch, SEM. 12. Cymbella
mesiana Cholnoky, SEM. 13. Nitzschia hantzschiana Rabenhorst, SEM. 14.16. Chrysophycean stomatocyst, LM. 17. Cymbella minuta
Hilse, SEM. Scale bars = 5
µ
m.
DIATOMS FROM SPHAGNUM PEAT BOGS IN THE SREDNA GORA MTS (BULGARIA) 71
Eunotia soleirolii (Kütz.) Rabenhorst. It represented by vege-
tative cells and valves possessing internal resting spores. To-
wards the end of the Subzone C1, relative abundance of domi-
nant species has gradually decreased and the number of taxa
was reduced. The diatom frustules/chrysophycean stomato-
cysts ratio ranged from 2 to 28 %.
DAZ C2 (Bogdan-3: 3513 cm, Bogdan-6: 3018 cm,
Shiligarka: 5220 cm, corresponding to the end of PAZ SG 3,
PAZ SG 3b, and the beginning of PAZ SG 4 of Filipovitch et
al. (1998)
Diatoms of this subzone are characterized by a low number
of taxa which were also recorded in the preceding subzone,
e.g. Eunotia steineckei Petersen, E. monodon, E. minor, Pin-
nularia lata, and P. viridis. They were represented by single
specimens, some with traces of fragmentation and dissolution
of the valves. Mass occurrence of chrysophycean stomato-
cysts dominating over diatoms was recorded in this subzone.
DAZ D (Bogdan-3: 130 cm, Bogdan-6: 180 cm, Shili-
garka: 200 cm, corresponding to PAZ SG 4 of Filipovitch
et al. (1998)
This zone was characterized by a diatom assemblage typical
for peat-bog habitats. In the sediments from all three peat bogs
studied Cymbella perpusilla Cleve (Fig. 6.10), Eunotia gla-
cialis, E. steineckei, Hantzschia amphioxys, Pinnularia mi-
crostauron (Ehr.) Cleve (Fig. 6.4), P. subcapitata var. hilsea-
na (Janisch) O. Müller, P. borealis and P. viridis dominated.
The rich and diverse diatom flora identified in this zone was
predominantly composed of different representatives of the
genera Eunotia, Pinnularia, Frustulia, and Stenopterobia, de-
scribed by Patrick & Reimer (1966) as closely specialized
species, suited to live in progressively developed peat bogs. A
group of acidobiontic taxa such as Eunotia paludosa Grunow,
Navicula subtilissima Cleve and Pinnularia subcapitata var.
hilseana attained higher frequencies in this zone. In the sedi-
ments of Bogdan-6 peat bog Orthoseira roeseana (Rab.)
OMeara and Stenopterobia delicatissima (Lewis) Brébisson
were most characteristic, while the north-alpine diatom spe-
cies Frustulia rhomboides var. crassinervia (Bréb.) Ross,
Achnanthes petersenii Hustedt (Fig. 5.5) and Caloneis lauta
Carter et Bailey-Watts were more frequent in Shiligarka peat
bog. Diatoms dominated in over chrysophycean stomatocysts.
Ecological analysis of the diatom flora
Only 8.4 % of the identified diatom taxa have unknown
ecology.
The ecological composition of the diatom flora from
Bogdan-3 was generally characteristic with dominance of oli-
gohalobous-indifferent (54 %), acidophilous (37 %), olig-
otrophic (33 %), epiphytic (40 %), and cosmopolite (78 %)
diatoms. In the sediments of Bogdan-6 the diatoms were rep-
resented by oligohalobous-indifferents (57 %), pH indiffer-
ents (36 %), oligotrophs (30 %), epiphytes (32 %), and cos-
mopolites (70 %). Oligohalobous-indifferent and halophobous
diatoms (49 % each), acidophilous (36 %), oligotrophic
(37 %), and cosmopolite (69 %) species prevailed in the dia-
tom flora of the Shiligarka peat bog.
The diatom flora in the peat bogs emerged 8000 BP ago
(DAZ A). Halophobous and oligohalobous-indifferent diatom
species prevailed. Acidophilous and oligotrophic species de-
veloped, followed by indifferents and oligo- to mesotrophic
species. All identified species were benthic: epiphytes or aero-
philous, with the exception of Fragilaria ulna.
In the Middle Holocene the development of diatom commu-
nity gradually abated (DAZ B, 7145
±
95 BP for Bogdan-3).
Sediments of strongly humified black peat with several al-
ternating layers have preserved single, fragmented valves,
usually chemically eroded in the smaller specimens. This
impeded the ecological analysis of the diatom flora of that
period.
Fig. 7. Relative abundance of the diatom ecological groups and diatom-inferred pH values in the sediments of Bogdan-3 peat bog. Legend:
A. Halobion diatom groups (hl oligohalobous-halophilous, ind oligohalobous-indifferent, hb oligohalobous-halophobous); B. pH dia-
tom groups (ab acidobiontic, acp acidophilous, ind indifferent, alk alkaliphilous); C. Trophic diatom groups (o oligotrophic, om
oligo-mesotrophic, m mesotrophic, me meso-eutrophic, eu eutrophic, oe oligo- to eutrophic); D. Habitat diatom groups (epl
euplanktonic, tpl tychoplanktonic, ep epiphytic, b benthic, a aerophilous); E. Geographic distribution (bor boreal, n-a north-
alpine, cos cosmopolitan); F. Diatom-inferred pH values ( - - index B (Flower 1986), index B (Renberg & Hellberg 1982)).
72 STANCHEVA and TEMNISKOVA
About 5285 BP (DAZ C) such favourable conditions in peat
bog Shiligarka encouraged the rapid formation of a species-
rich diatom community, with prevalence of oligohalobous
(halophobous and indifferent); acidophilous, oligotrophic and
oligo- to mesotrophic species (Fig. 9). The active water reac-
tion values obtained by application of Index B (Fig. 9E) for
the sediments from Shiligarka peat bog varied between 5.2
5.9 in the 6055 cm interval.
During the latest period of existence of the peat bogs (DAZ
D), the percentage shares of the different ecological groups of
diatoms in the peat bogs of Shiligarka and Bogdan-6 were
rather similar. In terms of salinity, only oligohalobous species
have been identified, with prevalence of halophobes and in-
differents over halophiles (04 %) (Figs. 8A, 9A). In terms of
active water reaction, acidophilous and indifferent species
showed more significant relative abundance. Acidobionts var-
ied between 6 % and 10 %, and alkaliphilous species were be-
tween 18 % and 30 % (Figs. 8B, 9B). In terms of trophic con-
ditions, the groups of oligo- and oligo- to mesotrophic species
were best represented, followed by oligo- to eutrophic and
eutrophic species. A minimum presence of meso- and meso-
eutrophic species was registered (Figs. 8C, 9C). According to
their specific habitats, benthic diatoms dominated, with al-
most equal percentage shares of epiphytes, aerophilous and
benthos (sensu lato). Euplanktonic species formed from 3 %
to 10 %, and tychoplanktonic species varied in the different
peat bogs as follows: 1322 % in Shiligarka, and from 3 % to
7 % in Bogdan-6 (Figs. 8D, 9D). The low proportion of the
euplanktonic group is quite legitimate for the acid lakes,
where contrary to benthos, diatom plankton is often poorly
developed (Battarbee et al. 1999). During that stage of devel-
opment of Shiligarka and Bogdan-6 peat bogs the active water
reaction in both sites varied between 5.65.9 (Figs. 8E, 9E).
In the Bogdan-3 peat bog active water reaction registered a re-
duction from 5.9 to 5.3 towards the surface (Fig. 7E). Another
important feature of the diatom community in Bogdan-3 peat
bog was the absence of halophilous species and the higher
percentage of acidobiontic (up to 16 %) and oligotrophic (up
to 52 %) diatom algae (Fig. 7). The low values of active water
reaction probably related to the processes of natural acidifica-
tion established in other water basins by the same methods
(Jones et al. 1989).
Discussion
Clayey sediments of the Early Holocene (10,025
±
225 BP
for Bogdan-6) did not contain siliceous microfossils (diatoms
and chrysophyceans). Apparently, the harsh environmental
conditions were unfavourable for the development of algal
flora in the early stage of peat-bog formation in Bogdan-3 and
Bogdan-6 core sites situated on the mountain crest. During
that period 10,0008000 BP (PAZ SG 1) the mountain slopes
were fully clad by grassy communities, while paleoclimatic
conditions were characterized by low temperatures, deficient
humidity and poor soils (Filipovitch et al. 1998).
At ca. 8000 BP (DAZ A), pioneer algal flora emerged in
Bogdan-3 (8115
±
100 BP) and Bogdan-6 (7835
±
445 BP) peat
bogs. The diatom assemblage was species poor and frequency
was low. The benthic character of the diatom community
shows that during that period water level was low. Water in
the peat bog had acid reaction and was poor in nutrients. Tem-
peratures were still low, which encouraged the development
of oligothermic species Pinnularia borealis and the oligo- to
mesothermic species Fragilaria ulna and Navicula radiosa
(Lowe 1974). Quantitative predominance of chrysophycean
stomatocysts over diatoms indicated that initially the condi-
tions were rather unfavourable for the development of algal
flora. Smol (1983, 1985) explained the great abundance of
chrysophycean stomatocysts in post-glacial sediments by the
oligotrophic conditions and late beginning of the vegetation
season. That period (PAZ SG 2) witnessed the afforestation of
Fig. 8. Relative abundance of the diatom ecological groups and diatom-inferred pH values in the sediments of Bogdan-6 peat bog. Legend:
A. Halobion diatom groups (hl oligohalobous-halophilous, ind oligohalobous-indifferent, hb oligohalobous-halophobous); B. pH dia-
tom groups (ab acidobiontic, acp acidophilous, ind indifferent, alk alkaliphilous); C. Trophic diatom groups (o oligotrophic,
om oligo-mesotrophic, m mesotrophic, me meso-eutrophic, eu eutrophic, oe oligo- to eutrophic); D. Habitat diatom groups
(epl euplanktonic, tpl tychoplanktonic, ep epiphytic, b benthic, a aerophilous); E. Geographic distribution (bor boreal, n-a
north-alpine, cos cosmopolitan); F. Diatom-inferred pH values ( - - index B (Flower 1986), index B (Renberg & Hellberg 1982)).
DIATOMS FROM SPHAGNUM PEAT BOGS IN THE SREDNA GORA MTS (BULGARIA) 73
The water in the peat bog had low salinity and nutrient con-
tent. The pH of the water showed distinct decrease from its
maximum value of 5.92 (at 52 cm) to the lowest value in the
entire history of the peat bog development of 5.28 (at 57 cm).
This resulted in another change on the same depth level: re-
duction and disappearance of the eutrophic and oligo- to
eutrophic taxa. The initially higher mineral content of water in
the peat bog has gradually decreased. This corresponded well
to the fact that Eunotia soleirolii developed intensively only
at the beginning of this level. Contrary to other representa-
tives of the genus identified in the sediments, that species was
indifferent with respect to pH. This invites the assumption
that initially there were totally water-covered patches in the
peat bog, with open water surfaces, where the euplanktonic spe-
cies Aulacoseira granulata and A. subarctica used to develop,
as well as the tychoplanktonic species Melosira varians, Fragi-
laria capucina and F. virescens. In the high-altitude mountains
of Southwest Bulgaria, Rila and Pririn, Tonkov & Bozilova
(1995) found out that after 5500 BP peat had increased in area
and accumulation in the fontinal peat bogs.
The same, but less pronounced trends have been observed
in the sediments from Bogdan-3 and Bogdan-6 sites. In addi-
tion to Aulacoseira granulata, another euplanktonic species
Aulacoseira islandica developed characteristically in the
sediments of Bogdan-6 peat bog. Generally for DAZ C1,
Shiligarka and Bogdan-3 marked the peak development of the
north-alpine acidophilous species, Pinnularia lata. Towards
the end of this subzone the peat bogs once again gradually
dried out. This phenomenon was much more strongly devel-
oped in Bogdan-3 and milder for Bogdan-6.
mountain slopes, dominated by Pinus and Betula, which sig-
nalled an improvement of climatic conditions and develop-
ment of soils (Filipovitch et al. 1998). The interval 9000
8000 BP witnessed one of the most dynamic processes in the
vegetation cover formation of low-altitude mountains in Bul-
garia (Tonkov & Bozilova 1995).
In the Middle Holocene (DAZ B), within the range of
7145 BP for Bogdan-3 and up to 5060 BP for Bogdan-6, dia-
tom remnants in peat-bog sediments were scanty and poorly
preserved. Straub (1993) explained analogical findings in
lacustrine sediments by sedimentological discontinuities or
changes in climatic conditions. Another possible explanation
could be sought in average annual temperature rises in the
80007000 BP period (Tonkov & Bozilova 1995), which re-
peatedly caused the peat bogs to dry out, as well as in the
greater access of oxygen to the peat and the latters stepped-
up humification. Poor preservation under acid condition
seems to be related to oxidation of the sediments (Round
1964). The climate during the Late Atlantic (60005000 BP)
was determined by the transport of air masses from the Atlan-
tic Ocean eastwards far inside Eurasia more than in the pre-
ceding and subsequent periods (Tonkov & Bozilova 1995).
This zone corresponded to PAZ SG 3a, which witnessed the
development of mixed deciduous forests of Ulmus, Tilia and
Quercus. The wide distribution of Ulmus and Tilia and con-
siderably poorer participation of Quercus promted the as-
sumption of a stronger expressed mesophilic character of
these woods (Filipovitch et al. 1998).
Shiligarka peat bog emerged about 5285
±
180 BP (DAZ
C1) as a result of intensive seepage of underground waters.
Fig. 9. Relative abundance of the diatom ecological groups and diatom-inferred pH values in the sediments of Shiligarka peat bog. Legend:
A. Halobion diatom groups (hl oligohalobous-halophilous, ind oligohalobous-indifferent, hb oligohalobous-halophobous); B. pH dia-
tom groups (ab acidobiontic, acp acidophilous, ind indifferent, alk alkaliphilous); C. Trophic diatom groups (o oligotrophic,
om oligo-mesotrophic, m mesotrophic, me meso-eutrophic, eu eutrophic, oe oligo- to eutrophic); D. Habitat diatom groups
(epl euplanktonic, tpl tychoplanktonic, ep epiphytic, b benthic, a aerophilous); E. Geographical distribution (bor boreal,
n-a north-alpine, cos cosmopolitan); F. Diatom-inferred pH values ( - - index B (Flower 1986), index B (Renberg & Hellberg 1982)).
74 STANCHEVA and TEMNISKOVA
Sediments from the three peat bogs of DAZ C2 showed few
diatom species and of low abundance. Chrysophycean stoma-
tocysts occurred abundantly and maintained a relative index
of 8599 % as compared to diatoms. A certain analogy with
DAZ B could be seen, but DAZ C2 did not dry out as strongly
and the siliceous microfossils were generally better preserved.
The latest stages of the investigated peat bogs are character-
ized by a specific diatom assemblage, typical of the peat bogs.
It was rich in various species of high abundance, inhabiting
nutrient-poor waters with increased content of humic sub-
stances and low active pH. Battarbee et al. (1999) pointed out
that such waters inhabited by diverse diatom flora are often
unproductive. The specific hydrological conditions in the peat
bogs were the reason for the diatom algae to thrive, owing to
which the latter considerably exceeded in percentage share the
chrysophycean stomatocysts.
The latest changes in vegetation (PAZ SG 4) were charac-
terized by the development of beech as the main tree species
on the mountain slopes, related to a slight drop in tempera-
tures (Filipovitch 1996; Filipovitch et al. 1998). About 4500
4000 cal. BP, beech began to play an important part in the for-
est cover of all Bulgarian mountains (Bozilova & Tonkov
2000). According to Huntley et al. (1989), climatic changes
were determinant for the distribution of Fagus in Europe and
North America. Palynological data has shown a rise in humid-
ity between 4000 and 2000 BP, which led to formation of
many peat bogs in the Sredna Gora Mts (Filipovitch 1996). A
combination of higher humidity and lower temperatures had a
favourable effect on diatom flora in the three mountain peat
bogs, which then reached its maximum species diversity.
Shiligarka peat bog offered the most favourable conditions for
the development of a rich diatom flora, followed by Bogdan-6.
This related to the higher water level in these peat bogs and
patches of open surface water in them. The favourable hydro-
logical regime of the Shiligarka peat bog was due to ample in-
flow of fontinal waters and its situation in a concave relief.
This created conditions for Aulacoseira alpigena (Grun.)
Krammer (Figs. 5.1,2) to reach considerable numbers only in
that peat bog. This species develops in waters with pH 6.2
6.7 and, apart from plankton habitats, it could occur in benthic
communities (Haworth 1988). Both peat bogs are located in
close proximity to the forest border. In comparison, Bogdan-3
site had a lower water level, with a lower active water reaction
and was poorer in biogenic elements. These local conditions
in the latter peat bog were unfavourable to diatoms, which
maintained low diversity there. Shortage of water in Bogdan-
3 peat bog was most probably due to the fact that it was con-
siderably distanced from the forest as a result of anthropogen-
ic impact in the last 2000 years in the investigated region
(Filipovitch et al. 1998).
Conclusion
Analysis of the fossil diatom community identified in the
sediments of the three investigated mountain peat bogs enable
us to distinguish four diatom assemblage zones. A pioneer di-
atom flora emerged about 8000 BP. The peat bogs dried up in
the Middle Holocene. About 5000 BP, humidity increased. A
diatom assemblage formed, limited once again by another,
slighter drought. At the end of the Holocene temperatures
dropped slightly, a richer and more diverse diatom flora
emerged in the sphagnum peat bogs. Their water level re-
mained inconstant and sensitive to climatic changes. Water
salinity in the peat bogs varied from 0.01 to 0.3 g/l and their
active water reaction was between 5.25.9. The water was
poor in biogenic substances and mild to low in temperature.
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76 STANCHEVA and TEMNISKOVA
Taxonomical composition of the diatom flora from the invesigated peat-bog sediments and ecological spectra of the diatoms. Legend: B-3 core Bogdan-3; B-6 core
Bogdan-6; Shil core Shiligarka; 1. Halobion diatom groups (hl oligohalobous-halophilous, ind oligohalobous-indifferent, hb oligohalobous-halophobous); 2.
pH diatom groups (ab acidobiontic, acp acidophilous, ind indifferent, alk alkaliphilous); 3. Trophic diatom groups (o oligotrophic, om oligo-mesotrophic,
m mesotrophic, me meso-eutrophic, eu eutrophic, oe oligo- to eutrophic); 4. Habitat diatom groups (epl euplanktonic, tpl tychoplanktonic, ep epi-
phytic, b benthic, a aerophilous); 5. Geographical distribution (bor boreal, n-a north-alpine, cos cosmopolitan).
ECOLOGICAL SPECTRA
CORES
TAXONOMICAL LIST
1
2
3
4
5 B-3 B-6 Shil
Achnanthes biasolettiana Grunow, 1880
ind alk m
ep bor
+
A. lanceolata (Bréb.) Grunow, 1880
ind alk eu
ep cos
+
+
A. minutissima Kützing, 1833
ind alk oe
ep cos
+
+
A. nodosa Cleve, 1900
o
n-a
+
A. petersenii Hustedt, 1937
ind ind
o
ep n-a
+
Aulacoseira alpigena (Grun.) Krammer,
1990
hb acp
o
tpl n-a
+
+
+
A. ambigua (Grun.) Simonsen, 1979
ind ind eu epl cos
+
A. distans (Ehr.) Simonsen, 1979
hb acp
o
tpl n-a
+
A. distans var. nivalis (W. Sm.) Haworth,
1988
hb acp
o
tpl n-a
+
A. granulata (Ehr.) Simonsen, 1979
ind alk eu epl cos
+
+
+
A. granulata var. angustissima (O. Müller)
Simonsen, 1979
ind alk eu epl cos
+
+
+
A. islandica (O. Müller) Simonsen, 1979
ind alk oe epl n-a
+
+
A. laevissima(Grun.) Krammer, 1990
+
A. muzzanensis (Mest.) Krammer, 1991
ind alk eu epl cos
+
A. pfaffiana (Reinsch) Krammer, 1990
hb acp
o
tpl cos
+
A. subarctica (O. Müller) Haworth, 1988
hb acp om epl n-a
+
+
A. tenuior (Grun.) Krammer, 1990
+
Brachysira brebissonii Ross, 1986
hb acp
o
b
cos
+
Caloneis bacillum (Grun.) Cleve, 1894
ind alk me
b
cos
+
C. lauta Carter & Bailey-Watts, 1981
hb
o
n-a
+
+
C. silicula (Ehr.) Cleve, 1894
ind alk me
b
cos
+
C. tenuis (Greg.) Krammer, 1985
ind ind m
a
cos
+
Cocconeis placentula var. euglypta (Ehr.)
Grunow, 1884
ind alk eu
ep cos
+
+
Cyclotella meneghiniana Kützing, 1844
ind alk eu
tpl cos
+
+
Cymbella elginensis Krammer, 1981
o
ep n-a
+
+
C. gracilis (Ehr.) Kützing, 1844
hb acp om ep n-a
+
+
+
C. helvetica Kützing, 1844
ind alk
m
epl bor
+
C. mesiana Cholnoky, 1955
ind alk
ep cos
+
C. minuta Hilse, 1862
ind ind
ep cos
+
+
C. norvegica var. lapponica Cleve-Euler,
1951
ind ind
o
n-a
+
C. perpusilla Cleve, 1895
hb acp
o
a
cos
+
+
+
C. silesiaca Bleisch, 1864
ind ind oe
ep cos
+
+
Cymbella sp.
+
Diatoma anceps (Ehr.) Kirchner, 1878
hb alk oe tpl n-a
+
D. mesodon (Ehr.) Kützing, 1844
hb alk
m
tpl n-a
+
Diploneis ovalis (Hilse) Cleve, 1891
ind alk
a
cos
+
+
Epithemia adnata (Kütz.) Brébisson, 1838 ind alk me ep cos
+
+
+
Eunotia arculus (Grun.) L.-B. et Nörpel,
1991
hb acp om
bor
+
E. exigua (Bréb.) Rabenhorst, 1864
hb
ab
oe
a
cos
+
E. fallax Cleve, 1895
hb acp
o
ep cos
+
E. fallax var. groenlandica (Grun.) L.-B. et
Nörpel, 1991
hb acp
o
a
cos
+
E. flexuosa (Bréb.) Kützing, 1849
hb acp om ep
+
+
E. glacialis Meister, 1912
hb acp om ep cos
+
+
+
E. incisa Gregory, 1854
hb acp
o
ep n-a
+
+
+
E. meisteri Hustedt, 1930
hb acp
o
a
cos
+
E. minor (Kütz.) Grunow, 1881
hb acp om
a
cos
+
+
+
E. monodon Ehrenberg, 1843
hb acp
o
ep cos
+
+
+
E. naegelii Migula, 1907
hb acp
o
ep cos
+
+
E. nymanniana Grunow, 1881
hb acp
o
n-a
+
E. paludosa Grunow, 1862
hb
ab
o
a
cos
+
+
+
E. praerupta Ehrenberg, 1843
hb acp
o
ep cos
+
+
E. praerupta var. bidens Grunow, 1880
hb acp
o
ep cos
+
E. serra var. tetraodon (Ehr.) Nörpel, 1991 hb acp
o
ep n-a
+
+
+
E. soleirolii (Kütz.) Rabenhorst, 1864
hb ind
o
ep cos
+
E. steineckei Petersen, 1950
hb
o
+
+
+
E. sudetica O. Müller, 1898
hb acp om ep cos
+
E. tenella (Grun.) Hustedt, 1913
hb acp
o
ep cos
+
Eunotia sp.
+
Fragilaria brevistriata Grunow, 1885
ind alk oe tpl cos
+
F. capucina var. rumpens (Kütz.) L.-B.,
1991
ind ind om ep cos
+
F. construens f. subsalina (Hust.) Hustedt,
1957
hl
alk me tpl cos
+
F. construens f. venter (Ehr.) Hustedt, 1957 ind alk me tpl cos
+
+
+
F. pinnata Ehrenberg, 1843
ind alk oe tpl cos
+
F. ulna (Nitzsch) L.-B., 1980
ind alk oe
ep cos
+
+
F. virescens Ralfs, 1843
hb ind om tpl cos
+
+
+
Frustulia rhomboides var. crassinervia
(Bréb.) Ross, 1947
hb acp
o
b
n-a
+
Gomphonema angustatum (Kütz.)
Rabenhorst, 1864
ind alk eu
ep bor
+
+
G. clavatum Ehrenberg, 1832
ind ind me ep cos
+
+
+
G. gracile Ehrenberg, 1838
ind ind
m
ep cos
+
+
+
G. parvulum (Kütz.) Kützing, 1849
ind ind me ep cos
+
Hantzschia amphioxys (Ehr.) Grunow, 1880 ind ind oe
a
cos
+
+
+
Luticola cohnii (Hilse) D.G. Mann, 1990
ind ind eu
b
cos
+
L. goeppertiana (Bleisch) D.G. Mann, 1990 ind ind eu
a
cos
+
L. mutica (Kütz.) D.G. Mann, 1990
hl
ind eu
a
cos
+
+
Melosira varians Agardh, 1827
ind alk eu
tpl cos
+
Meridion circulare var. constrictum (Ralfs)
Van Heurck, 1880
ind alk oe
ep cos
+
+
+
Navicula cocconeiformis Gregory ex
Greville, 1856
hb ind om
b
n-a
+
N. contenta Grunow, 1885
ind ind oe
a
cos
+
N. difficillima Hustedt, 1950
hb acp om
b
+
N. elginensis (Greg.) Ralfs, 1861
ind alk eu
b
cos
+
N. elginensis var. cuneata (Møll. ex Foged)
L.-B., 1985
ind alk eu
b
cos
+
+
+
N. hambergii Hustedt, 1924
acp
a
cos
+
N. lenzii Hustedt, 1936
ind alk
+
N. radiosa Kützing, 1844
ind ind me ep cos
+
+
N. soehrensis Krasske, 1923
hb acp
o
a
cos
+
N. subtilissima Cleve, 1891
hb
ab
o
b
bor
+
+
Navicula sp. 1
+
Navicula sp. 2
+
Neidium ampliatum (Ehr.) Krammer, 1985 ind ind om
b
bor
+
N. bisulcatum (Lagerst.) Cleve, 1894
ind ind
o
b
n-a
+
+
N. iridis (Ehr.) Cleve, 1894
hb ind
m
b
cos
+
Nitzschia amphibia Grunow, 1862
ind alk eu epl cos
+
N. hantzschiana Rabenhorst, 1860
hb ind
m
a
bor
+
N. paleaceae (Grun.) Grunow, 1881
ind alk eu
ep cos
+
N. perminuta (Grun.) M. Peragallo, 1903
ind alk om ep cos
+
+
+
N. terrestris (Pet.) Hustedt, 1934
ind ind
a
+
Nitzschia sp.
+
Orthoseira roeseana (Rab.) OMeara, 1875 ind alk
b
cos
+
Pinnularia acrosphaeria Rabenhorst, 1853 hb ind om
b
cos
+
P. appendiculata (Ag.) Cleve, 1895
hb acp om
a
n-a
+
+
+
P. borealis Ehrenberg, 1843
ind ind om
a
cos
+
+
+
P. borealis var. rectangularis Carlson, 1913 ind ind
cos
+
+
+
P. borealis var. undulata Hustedt, 1934
n-a
+
+
+
P. brevicostata Cleve, 1891
hb acp
o
b
bor
+
+
P. gibba Ehrenberg, 1841
ind acp oe
b
cos
+
+
P. gibba var. linearis Hustedt, 1930
hb acp oe
b
cos
+
P. hemiptera (Kütz.) Rabenhorst, 1853
ind ind
o
b
n-a
+
P. interrupta W. Smith, 1853
hb ind om
b
bor
+
P. lapponica Hustedt, 1942
+
P. lata (Bréb.) Rabenhorst, 1853
hb acp
o
b
n-a
+
+
+
P. macilenta (Ehr.) Ehrenberg, 1843
ind
cos
+
+
+
P. maior (Kütz.) Rabenhorst, 1853
ind acp me
b
cos
+
+
P. microstauron (Ehr.) Cleve, 1891
ind acp oe
b
cos
+
+
+
P. obscura Krasske, 1932
ind ind
o
a
n-a
+
+
P. pulchra Østrup, 1897
hb ind
o
b
bor
+
P. rupestris Hantzsch, 1861
hb ind
o
a
n-a
+
+
P. stomatophora (Grun.) Cleve, 1895
hb acp
o
a
cos
+
+
P. streptoraphe Cleve, 1891
hb acp om
b
cos
+
+
P. streptoraphe var. minor (Cl.) Cleve, 1895 hb acp
o
b
cos
+
P. subcapitata Gregory, 1856
ind acp om
b
cos
+
P. subcapitata var. hilseana (Janisch)
O. Müller, 1898
hb
ab
o
b
cos
+
+
+
P. viridis (Nitz.) Ehrenberg, 1843
ind ind oe
b
cos
+
+
+
Pinnularia sp. 1
+
+
Pinnularia sp. 2
+
Sellaphora laevissima (Kütz.) D.G. Mann,
1989
hb ind
m
b
cos
+
Stauroneis anceps Ehrenberg, 1843
ind ind me
b
cos
+
+
S. phoenicenteron (Nitz.) Ehrenberg, 1843
ind ind me
b
cos
+
+
Stephanodiscus medius Håkansson, 1986
alk eu
+
S. minutulus (Kütz.) Cleve et Möller, 1878 ind alk eu epl cos
+
+
Stephanodiscus sp.
+
+
Stenopterobia delicatissima (Hust.)
Krammer, 1987
hb acp
o
cos
+
Tabellaria flocculosa (Roth) Kützing 1844 hb acp m
tpl cos
+
+
Appendix