# Holocene ecosystem and temperature development inferred from invertebrate remains in Zminje Jezero (Dinaric Alps, Montenegro)

Noé R. M. M. Schmidhauser, Walter Finsinger, Eleonora Cagliero, Oliver Heiri

► To cite this version:

Noé R. M. M. Schmidhauser, Walter Finsinger, Eleonora Cagliero, Oliver Heiri. Holocene ecosystem and temperature development inferred from invertebrate remains in Zminje Jezero (Dinaric Alps, Montenegro). Journal of Paleolimnology, 2024, 72, pp.343-361. 10.1007/s10933-024-00334-y. hal-04668776

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# Holocene ecosystem and temperature development inferred from invertebrate remains in Zminje Jezero (Dinaric Alps, Montenegro)

Noé R. M. M. Schmidhauser · Walter Finsinger · Eleonora Cagliero · Oliver Heiri

Received: 11 April 2024 / Accepted: 18 June 2024
© The Author(s) 2024

**Abstract** Lake Zminje Jezero (1535 m a.s.l.) in Montenegro was studied for chironomid and other aquatic invertebrate remains in a sediment sequence dating back to 12,000 calibrated $^{14}$C years before present (cal yr BP), providing, to our knowledge, the first lake-sediment record studied for chironomids and other associated chitinous aquatic invertebrate remains in the Dinaric Alps. Changes in chironomid and other invertebrate remains along the record make

it possible to constrain changes of relevant environmental variables for aquatic invertebrates in the lake including temperature, oxygen availability, trophic status and water depth. The results suggest moderate changes in trophic conditions with chironomid assemblages indicating meso- to eutrophic conditions during the analysed interval. Invertebrate assemblages were typical for lakes with relatively high oxygen availability with a minor trend during the middle to late Holocene to conditions observed in lakes that are more hypoxic. A change in temperature is suggested in the earlier part of the record when the share of chironomid taxa adapted to warmer conditions increased, at the Younger Dryas to Holocene transition, whereas later no particularly pronounced shifts in temperature-sensitive taxa were observed. July air temperatures were estimated based on chironomid assemblages using a transfer function based on calibration data consisting of 117 lakes sampled in the Swiss Alps and northern Switzerland. The results suggest that temperatures rapidly increased by 5 °C at the onset of the Holocene (11,500 cal yr BP) leading to a relatively warm early to mid-Holocene and thereafter slightly decreased during the late Holocene. Reconstructed temperatures are discussed together with a previously published pollen record for our study site and are broadly consistent with other chironomid-based temperature reconstructions from Central, Eastern and Southern Europe. From ca. 3000 cal yr BP sedimentation rates increased and from ca. 500 cal yr BP onwards pollen data suggest

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10933-024-00334-y.

N. R. M. M. Schmidhauser (✉) · O. Heiri
Geoecology, Department of Environmental Sciences, University of Basel, 4056 Basel, Switzerland
e-mail: noe.schmidhauser@unibe.ch

N. R. M. M. Schmidhauser
Institute of Geography, University of Bern, Hallerstrasse 12, 3012 Bern, Switzerland

N. R. M. M. Schmidhauser
Oeschger Centre for Climate Change Research, University of Bern, 3012 Bern, Switzerland

W. Finsinger · E. Cagliero
ISEM, CNRS, IRD, University of Montpellier, Montpellier, France

E. Cagliero
Department of Land, Environment, Agriculture and Forestry (TeSAF), University of Padova, Viale dell'Università, 16, 35020 Legnaro, PD, Italy
that the vegetation and lake catchment were affected
by human activities, possibly influencing chironomid
and other invertebrate assemblages in the lake and
thereby also reconstructed temperatures. Overall, our
results show that combining analyses of chironomid
and other invertebrate assemblages can provide valu-
able insights into long-term environmental changes
and can provide temperature reconstructions for small
mountain lakes in the Balkans, such as Zminje Jezero.

**Keywords** Lake · Balkans · Sediment · Chironomids · Temperature reconstruction

**Introduction**

The Balkan Peninsula is characterized by diverse hab-
itat types and a long history of human presence. How-
ever, in comparison to other European regions, only
a limited number of palaeoecological and particularly
lake-sediment studies are available from some parts
of the Balkans such as the Dinaric Alps (Finsinger
et al. 2017). Lake-sediments are excellent natural
archives for reconstructing past environmental condi-
tions in and around lakes (Gregory-Eaves and Smol
2024). Remains of a wide range of organisms can be
preserved in lake-sediments such as chironomid head
capsules, diatom valves, cladoceran remains, man-
dibles from different macroinvertebrates or pollen,
other plant remains and fungal spores (Walker 1987;
Ursenbacher et al. 2020; Courtney-Mustaphi et al.
2024). These remains provide an avenue for recon-
structing changes in lacustrine community and eco-
system composition and, indirectly, for reconstructing
and constraining changes in environmental conditions
in lakes and their catchments.

Chironomids (non-biting midges) are a family of
true flies (Diptera). After hatching from the egg, chi-
ronomids live as larvae in the uppermost sediments
or on hard substrates in freshwater ecosystems or in
other habitats such as water-logged soils, peats and
dung (Walker 1987). When chironomid larvae die
or moult, the head capsules are well preserved in the
sediments (Walker 2001). Chironomids are abun-
dant, present nearly everywhere (ubiquitous), persis-
tent and have, as family, a wide range of tolerance to
environmental change (Brooks 2003; Campbell et al.
2017). These traits, combined with the narrow envi-
ronmental range that some species have (Lencioni

and Rossaro 2005), makes them ideal indicators to
reconstruct past environmental conditions. Because of
rapid generation times and the ability of the adults to
fly, changes in the assemblage as a response to envi-
ronmental change can be considered instantaneous
on multidecadal and longer time scales (Brooks et al.
2007). In small lakes, the distribution of chironomid
taxa has been shown to be related to temperature,
nutrients, pH, oxygen, salinity and other variables
(Walker 1987; Brooks et al. 2007), with temperature
often seen as a primary variable driving chironomid
assemblages on longer time scales (Heiri and Lotter
2005). Changes in trophic status, often resulting from
land-use activities, can also strongly influence the
composition of chironomid assemblages causing, for
example, increased nutrient availability and reduced
oxygen concentrations (Heiri et al. 2003). In this con-
text, chitinous remains of other aquatic invertebrate
groups can provide valuable supporting informa-
tion for interpreting chironomid records (Courtney-
Mustaphi et al. 2024). For example, Ursenbacher
et al. (2020) have shown that the abundance of chi-
ronomid remains relative to other invertebrates such
as Chaoboridae, Trichoptera, Ephemeroptera, and
oribatid mites can provide crucial information on
changes in deep-water oxygen conditions in lakes.
These remains are easily sorted from samples pre-
pared for chironomid analysis during processing and
mounting and can therefore be counted during analy-
sis of chironomid samples.

From Europe, a large number of chironomid
records are now available that cover the Lateglacial
(Heiri and Millet 2005; Samartin et al. 2017) and
the Holocene period (Ilyashuk et al. 2011; Tóth et al.
2015). These records have provided detailed insights
on past ecosystem dynamics and chironomid response
to climatic changes and early human impacts (Taylor
et al. 2013; Perret-Gentil et al. 2024). Furthermore,
several of these records have provided the basis for
estimating changes in summer temperatures based
on the close relationship between summer tempera-
ture and chironomid distribution (Heiri et al. 2011).
So far, most of the chironomid-based temperature
reconstructions in Europe have shown an increase in
temperature of 3–5 °C from the Younger Dryas (YD)
to Holocene transition (centred around 11,500 cal yr
BP), as well as a Holocene thermal maximum recon-
structed between ca. 10,000 and 5000 cal yr BP. This
is often followed by a slow decrease in temperature
from the mid- to late Holocene (Ilyashuk et al. 2011; Heiri et al. 2015; Tóth et al. 2015; Samartin et al. 2017). However, from Southern European mountain ranges only few such records are available (Tóth et al. 2015; Samartin et al. 2017; Jimenez-Moreno et al. 2023) and to our knowledge none from the Balkans.

Here we provide the first record of chironomid and other macroinvertebrate subfossils from a small mountain lake (Zminje Jezero) in the Dinaric Alps, a mountain range separating the continental Balkan Peninsula from the Adriatic Sea. The aims of this study were to:

(1) reconstruct changes in the chironomid and other macroinvertebrate assemblages over the past 12,300 years in and around Zminje Jezero;

(2) assess possible variations of in-lake factors (changes in trophic state, deep-water oxygen availability, and water depth) based on the chironomid and other invertebrate assemblages;

(3) explore the new chironomid record to assess its potential to develop a first Holocene temperature reconstruction for the western Balkan region; and

(4) compare the developed temperature reconstruction with other paleoenvironmental and palaeotemperature evidences from southern Central, Southern, and Southeastern Europe.

## Study site

Zminje Jezero is a lake of glacial origin situated in the central part of the Dinaric Alps (Durmitor Massif, Montenegro; Fig. 1), a mountain range that is made almost entirely from Mesozoic sedimentary rocks (Gachev and Mitkov 2019). The lake is located on limestone bedrock at 1535 m a.s.l. (43°09'21" N, 19°04'14" E). A small stream drains the water to a wider lake (Crno Jezero, 1460 m a.s.l.). Dense mixed forests dominated by *Picea abies* and *Abies alba* with lesser amounts of *Fagus sylvatica* and *Acer* sp. presently surround the lake (Cagliero et al. 2023). The lake is situated ca. 400 m below treeline (around 1950 m a.s.l.), which is formed by *Pinus mugo*, *Juniperus communis*, and *Picea abies* (Cagliero et al. 2023). Today, the lake is nearly 190 m long and 100 m wide with a surface area of around 1.2 ha and a maximum depth of 9.5 m. pH at the surface was 9.5, surface water conductivity 293 µS cm⁻¹ and

the lake had a Secchi depth of 1.5 m during fieldwork (September 2019). The lake shore is characterized by wetland vegetation, mainly *Molinia caerulea* and *Comarum palustre*. Climate is continental with a mean annual air temperature of 4.6 °C, mean July air temperature of 13.9 °C (Burić et al. 2014) and mean annual precipitation of ca. 1450 mm at Žabljak (1450 m a.s.l.; Djurović 2012), which is situated ca. 4 km from Zminje Jezero.

## Materials and methods

### Sampling

Surface sediments were sampled with an UWITEC gravity corer (6 cm diameter) in September 2017 and in September 2019. In September 2019, two parallel cores were taken with a Livingstone piston corer with 6 cm diameter and 1-m long barrels. The coring site is situated nearly in the middle and deepest part of the lake (9.2–9.3 m water depth). Core segments were split longitudinally in the laboratory and correlated to a continuous sequence based on lithological marker layers and XRF analyses as described in Cagliero et al. (2023). Several visually identified turbidite layers were considered to be instantaneous sedimentation events that do not represent long-term sedimentation and were omitted from the continuous composite sequence.

### Dating

The chronology of the sediment record (Fig. S1, Supplementary material 1) is constrained by 19¹⁴C dates from terrestrial plant macrofossils (Cagliero et al. 2023), the sediment surface (2019), the onset of the Holocene as apparent in the pollen assemblages (dated to 11,500±250 cal yr BP; Giesecke et al. 2014) and the *Ambrosia* pollen increase (1950±30 CE) (Table S1, Supplementary material 1). The first pollen appearance of the neophyte *Ambrosia* was dated around 1950 CE in the Southern Alps (Tinner et al. 1998; van der Knaap et al. 2000). This broadly coincides with results from the Dinaric Alps, where the mean sample ages of *Ambrosia’s* first pollen appearance have been dated to ca. 1930±10 CE (Cagliero et al. 2022) and ca. 1940±20 CE (Cagliero et al. 2023), thereby granting its use as a reliable, albeit biostratigraphic, age-control
**Fig. 1** a Location of Zminje Jezero and the different study sites with chironomid-, pollen- and MBT-CBT-based temperature reconstructions mentioned in the discussion. The study sites are: Hypkana (Hájková et al. 2016), Lake Brazi (Tóth et al. 2015), Lake Gemini and Verdarolo (Samartin et al. 2017), Schwarzsee ob Sölden (Ilyashuk et al. 2011), a stacking of different lakes in the Alpine region (Heiri et al. 2015), Lake Maliq (Bordon et al. 2009) and Lake Dojran (Thienemann et al. 2017). Map data from Esri, HERE, Garmin, FAO, NOAA, USGS, © OpenStreetMap contributors, and the GIS User Community and available under https://www.arcgis.com/home/item.html?id=91b43cb1be4b418eafd522348a618531. The map was created using ArcGIS® software by Esri. **b** Location of Zminje Jezero (red dot) in the Dinaric Alps, Montenegro. Map data from Esri, Maxar, Earthstar Geographics, and the GIS User Community and available under https://www.arcgis.com/home/item.html?id=10df2279f9684e4a9f6a7f08febac2a9. The map was created using ArcGIS® software by Esri.

<figure><img src="image_2.png" /></figure>

point. Radiocarbon ($^{14}$C) samples were analyzed at Isoptech Zrt. in Debrecen (Hungary) and at the Poznan Radiocarbon Laboratory (Poland) using Accelerator Mass Spectrometry (AMS). $^{14}$C ages were calibrated to calendar years BP using the IntCal20 dataset (Reimer et al. 2020) prior to modelling the age-depth relationship with RBacon v2.4.1 (Blaauw and Christen 2011) following Cagliero et al. (2023). Sample ages of this chronology differ slightly (<60 cal yr BP) from the one

by Cagliero et al. (2023), which included for the topmost 17.5 cm a chronology derived from short-lived radionuclides ($^{210}$Pb and $^{137}$Cs).

Chironomid, aquatic macroinvertebrate and macrophyte analyses

In total, 79 samples (2–8 cm$^3$ wet sediment) were taken every 8 cm for the analysis of chironomid and

<figure><img src="image_3.png" /></figure>
other invertebrate remains. Samples were washed through a 100 µm mesh sieve. Macroinvertebrate fossils (including chironomid head capsules) and selected macrophyte remains were then picked with fine forceps from the sieve residue in a Bogorov counting tray under a stereomicroscope (Leica, 30–50× magnification). Picked remains were transferred to a microscope slide and mounted in Euparal mounting medium.

Macroinvertebrate fossils were identified under a compound microscope (Leica, 100–400× magnification). Chironomids were identified to genus level or, whenever possible, to species morphotype level, while other macroinvertebrates and a few selected aquatic macrophyte remains were identified to a coarser taxonomic level. Half chironomid head capsules were counted as half individuals to provide a conservative estimation. For the other macroinvertebrate remains, individual body parts (frons, frontoclypeus, labrum, pair of mandibles) were used to estimate the minimum number of individuals. Literature used for identifying chironomids included Brooks et al. (2007), Schmid (1993), Andersen et al. (2013) and Rieradevall and Brooks (2001). To identify other invertebrate remains, we used the following literature: Vandekerkhove et al. (2004) for cladoceran ephippia, Francis (2001) for bryozoan statoblasts, Solhøy (2001) for oribatid mites, Gelorini et al. (2011) for Rhabdocoela and Courtney-Mustaphi et al. (2024) as well as the slide collection of mounted invertebrate specimens of the Geocology group (University of Basel) for Ephemeroptera, Plecoptera, *Sialis* (Sialidae), Trichoptera, Ceratopogonidae, Amphi poda, Coleoptera, Lepidoptera, Odonata, Sciaridae and Ostracoda. Charophyte oogonia were identified following Haas (1994), fern sporangia following Piasecki (2022) and testate amoebae following Mitchell et al. (2008). Chironomids were grouped into categories (terrestrial and stream, littoral, profundal) according to their preferred habitats following Brooks et al. (2007), Andersen et al. (2013), Janecek et al. (2017), Vallenduuk and Moller Pillot (2007), and Moller Pillot (2009, 2013). Invertebrate remains were classified into planktonic/benthic groups according to their preferred habitats following Thorp and Rogers (2014). Abundances of the main chitinous invertebrate remains are presented as percentages. Abundances of Rhabdocoela

cocoons, testate amoeba, charophyte oogonia, *Procladius* pupal thoracic horns and fern sporangia are presented as counts per sample.

## Numerical analyses

Sedimentation rate was first calculated at 0.5 cm resolution using the age-depth model. To smooth the estimates of sedimentation rate, we applied a running average filter at 8 cm resolution. Influx of chironomid head capsules was calculated by multiplying the smoothed sedimentation rates (cm year⁻¹) with chironomid concentrations (head capsules cm⁻³), as the smoothed sedimentation rates were considered more realistic than unsmoothed values.

For some samples, mostly between 22.5 and 323.5 cm, the recommended number of 40–45 chironomid head capsules (Heiri and Lotter 2001) was not reached. Thus, for temperature reconstruction and other numerical analyses, adjacent samples were combined to reach a minimum of 40 head capsules per sample. Zonation was based on stratigraphically constrained hierarchical cluster analysis (CONISS; Grimm 1987) using square-root transformed chironomid percentages and Euclidean distance. The number of significant zones was determined using the broken-stick model (Bennett 1996). To summarize overall variations in chironomid assemblages, a Detrended Correspondence Analysis (DCA) was conducted from square-root transformed chironomid percentages. To assess changes in deep-water oxygen conditions of Zminje Jezero, we conducted a Canonical Correspondence Analysis (CCA) based on a modern invertebrate dataset from 36 Swiss lakes (Ursenbacher et al. 2020). This dataset consisted of count data of aquatic invertebrate taxa such as aquatic insect remains, cladoceran ephippia, bryozoan statoblasts and oribatid mites (Acari) as well as associated environmental data. Sample scores of the first CCA axis of this analysis are closely correlated to deep-water oxygen concentrations. We passively added the Zminje Jezero invertebrate samples to the CCA of Ursenbacher et al. (2020) and considered variations of the passively added samples towards higher CCA axis 1 scores to represent changes in invertebrate composition towards oxygen-richer conditions. CONISS was calculated using the rioja v0.9–15.1 package (Juggins 2015) and ordinations using the vegan v2.6-2 package
(Oksanen et al. 2020) in the R environment (R Core Team 2021).

July air temperature estimates were developed from the chironomid record using a temperature-inference model (transfer function) based on the Swiss chironomid-temperature calibration dataset (Heiri and Lotter 2010). The dataset consists of modern (surface sample) chironomid data and associated mean July air temperature estimates from 117 lakes of which 15 were deleted as outliers as described in Heiri and Lotter (2010). The dataset covers a mean July air temperature range of 5–18.4 °C and was selected since it comes from a Central European mountain range (the Alps) that is biogeographically similar to the Dinaric Alps and includes many sites that lie on carbonate bedrock, as Zminje Jezero. Also, it includes the majority of chironomid taxa expected in late Pleistocene and Holocene lake-sediments from Western, Eastern, and Central Europe as well as in Southern European mountain lakes (Heiri and Lotter 2010). The calibration dataset was used at the taxonomic resolution described in Heiri et al. (2011) although some additional taxonomic amalgamation was necessary to achieve the same taxonomic resolution in the Zminje Jezero and training-set data. Categories merged included *Micropsectra contracta*-type and *Micropsectra insignilobus*-type; *Psectrocladius sordidellus*-type, *Psectrocladius barbatipes*-type, *Psectrocladius bisectus*-type and *Psectrocladius psilopterus*-type; *Cricotopus albiforceps*-type and *Cricotopus* indet.; and *Chironomus plumosus*-type and *Einfeldia pagana*-type. *Paratanytarsus* individuals without mandibles in the Zminje Jezero data were merged with *Paratanytarsus penicillatus*-type, since this was clearly the most abundant *Paratanytarsus* morphotype in the record. The reconstruction was made using weighted averaging partial least squares (WAPLS) regression (ter Braak and Juggins 1993) and square-root transformed percentages using the program C2 (Juggins 2007). The model with two WAPLS components was selected because it had a smaller root mean square error of prediction and higher r² than the model with one WAPLS component, if assessed using cross validation with 9999 bootstrapping cycles. Data for the Swiss calibration dataset were taken from Heiri et al. (2019).

## Results

Chironomids, macroinvertebrate assemblages and macrophytes

A total of 3485 chironomid head capsules were isolated from the sediments that encompass the past 12,300 years. A small fraction (n=88; c. 2.5% of the total) could not be identified to a detailed taxonomic level due to poor preservation. The 79 analyzed samples were combined to 41 samples for ecological and numerical analyses to ensure a minimum of 40 identified head capsules per sample.

Head capsules belonging to 66 different chironomid taxa were identified. The most frequent taxon was *Tanytarsus mendax*-type (Fig. 2). CONISS zonation identified four statistically significant zones (ZMN-1 to ZMN-4). Chironomid concentrations were highest (up to 100 head capsules cm⁻³) around 11,000–10,200 cal yr BP (zone ZMN-2, Fig. 2). Concentrations were also high in the youngest samples (up to 40 capsules cm⁻³) but did not exceed earlier values. From bottom to top, a reduction of the sub-family Tanypodinae and an increase of the subfamily Orthocladiinae is apparent. Also, an overall increase in taxa that prefer littoral habitats can be observed from 11,500 to 500 cal yr BP.

A total of 11,987 invertebrate individuals or macrophyte fragments (including chironomid head capsules) were identified and amalgamated to the same 41 samples as the chironomid head capsules. Fossil remains belonging to 26 different invertebrate groups were identified. The most predominant were Chironomidae followed by oribatid mites with *Plumata tella* also present at high abundances (Fig. 3). Being mostly driven by the concentration of Chironomidae, changes in overall abundance of invertebrate remains resembles the ones of chironomids, with higher concentrations in zones ZMN-2 and ZMN-3. Also, the relative importance of planktonic groups seems to decline along the record, with higher values in ZMN-1 to ZMN-3 and lowest values in ZMN-4. For easier comparison, changes in the invertebrate assemblages and macrophytes are described below, in the same zones (ZMN-1 to ZMN-4) as the chironomid-assemblage description.
<figure><img src="image_4.png" /></figure>

**Fig. 2** Simplified chironomid percentage diagram of Zminje Jezero, showing identified chironomid taxa that occurred in at least four samples. To the right, the abundances of chironomids by tribe or subfamily and by preferred habitat type (terrestrial and stream, littoral, profundal) are shown, along with

DCA axis 1 scores of the chironomid assemblages. The horizontal lines represent the statistically significant CONISS zone boundaries for the chironomid-assemblage zones (ZMN-1 to ZMN-4)

ZMN-1 (12,300–11,960 cal yr BP; 613.5–593 cm)

*Procladius, Chironomus anthracinus*-type and *Microtendipes pedellus*-type are present at high abundances. The proportions of these taxa decline when approaching the zone boundary to ZMN-2. *Pagastiella, Tanytarsus mendax*-type and *Tanytarsus lugens*-type are also present at lower abundances.

Macroinvertebrate assemblages are dominated by Chironomidae and *Daphnia*, with noticeable abundances of Trichoptera, Sialidae, and Amphipoda.

ZMN-2 (11,960–8370 cal yr BP; 593–416.5 cm)

*Tanytarsus mendax*-type and *Procladius* are present at high abundances throughout the zone. A range of taxa such as *Dicrotendipes*, *Einfeldia dissidens/natchitocheae*-type, *Cladotanytarsus mericus*-type, *Ablabesmyia*, *Tanytarsus pallidicornis*-type, *Tanytarsus glabrescens*-type, *Paratanytarsus*, *Chironomus plumosus/Einfeldia pagana*-type, and

*Pseprocladius sordidellus*-type appear in this zone and are present at noticeable abundances. *Tanytarsus lugens*-type observed in zone ZMN-1 is still present but at lower abundances.

Abundances of Chironomidae and *Daphnia* ephippia remain high and those of Trichoptera, Sialidae, and Amphipoda decline. Some taxonomic groups slightly increase such as Coleoptera, Ephemeroptera, Acari and *Plumatella*. Testate amoebae show a major peak between 11,300 and 10,800 cal yr BP.

ZMN-3 (8370–5150 cal yr BP; 416.5–323.5 cm)

*Tanytarsus mendax*-type and *Procladius* are still co-dominant and the abundance of *Microtendipes pedellus*-type increases slightly. Taxa which appeared in ZMN-2 remain present throughout this zone in noticeable abundances although *Tanytarsus lugens*-type only occurs occasionally. New taxa such as *Cladopelma*, *Tanytarsus lactescens*-type,
<figure><img src="image_5.png" /><figcaption>**Fig. 3** Simplified percentage diagram of invertebrate remains from Zminje Jezero (showing only taxa that occurred in at least three samples). Abundances of Rhabdocoela, testate amoebae, charophyte oogonia, *Procladius* pupal thoracic horns, fern sporangia and Ostracoda shells are shown as remains per sample. A classification of taxa according to the preferred habitat type</figcaption></figure>

*Limnophyes-Paralimnophyes* increase in abundance for the first time in this zone.

The proportion of Coleoptera, Ephemeroptera, Acari and *Plumatella* increases in this zone. Among the Cladocera, *Alona* reaches highest abundance whereas *Daphnia* decreases slowly, while the abundance of Rhabdocoela, testate amoebae and fern sporangia visibly increases.

ZMN-4 (5150 cal yr BP-today; 323.5–0 cm)

More taxa become co-dominant next to *Procladius* and *Tanytarsus mendax*-type. This is the case for *Paratanytarsus*, *Dicrotendipes*, *Tanytarsus lactescens*-type, *Psectrocladius sordidellus*-type, and *Chironomus plumosus/Einfeldia pagana*-type, which are all present at relatively high abundances. New taxa increase such as *Polypedilum*

(planktonic/benthic) and the first axis score of the CCA following Ursenbacher et al. (2020), representing changes in invertebrate assemblages indicative of variations in deepwater oxygen concentrations, are also shown. The horizontal lines represent the statistically significant CONISS zone boundaries for the chironomid-assemblage zones (ZMN-1 to ZMN-4)

*nubeculosum/nubifer*-type, *Glyptotendipes*, and *Heterotrissocladius marcidus*-type. From 1000 cal yr BP onwards, additional new taxa increase in abundance, such as *Corynoneura edwardsi*-type and *Orthocladius* together with taxa that were already present in zones ZMN-1 or ZMN-2, such as *Tanytarsus lugens*-type, *Chironomus anthracinus*-type or *Microtendipes pedellus*-type.

Acari and *Plumatella* are present at their maximum abundances in this zone. Coleoptera and Ephemeroptera have similar abundances as in ZMN-3. Amphipoda appear again at similar abundances as in ZMN-1 and *Daphnia* shows the lowest abundance, whereas Rhabdocoela, testate amoebae and fern sporangia proportions remain high. Charophyte oogonia show an increase in the second half of this zone.

<figure><img src="image_6.png" /></figure>
**Fig. 4** Records representing local environmental and ecosys-
tem change at Zminje Jezero, including sedimentation rates,
chironomid influx, axis 1 scores of a CCA of modern inverte-
brate data (Ursenbacher et al. 2020) with Zminje Jezero data
added passively, percentages of planktonic invertebrates and
profundal chironomids, and pollen percentages of selected
pollen types and pollen groups (from Cagliero et al. 2023).
Secondary indicators in the pollen data indicate the sum of
adventives and apophytes and cultural indicators, the sum of
Cerealia-type and *Plantago lanceolata*-type pollen (see Cagli-
ero et al. 2023 for details). Olea, Juglans and Castanea repre-
sent woody crops that were cultivated at lower altitudes along
the coast of the Adriatic Sea. The vertical black dashed lines
indicate the statistically significant CONISS zone boundaries
of the chironomid-assemblage zones (ZMN-1 to ZMN-4)

DCA, CCA, sedimentation rate and chironomid
influx

DCA axis 1 scores based on chironomid assemblages
show highest values in the lowest samples (Fig. 2).
The scores decrease rapidly in the early Holocene,
and then stay at relatively low values throughout the
early Holocene and zone ZMN-2. A minor decreas-
ing trend to even lower values is apparent for most
of the mid- to late Holocene (ZMN-3 to ZMN-4) but
scores increase again in the uppermost ca. 1000 years
(ZMN-4) to similar values as in the early Holocene,
suggesting a reversal in the trajectory of chironomid
assemblages. Overall, the DCA indicates that the
most distinct assemblage changes occurred at the YD
to Holocene transition and in the earliest Holocene.

In the CCA of the modern invertebrate dataset of
Ursenbacher et al. (2020), axis 1 correlates strongly
with deep-water oxygen availability in modern Swiss
lakes. The CCA axis 1 scores of the passively added
samples are highest (up to 3.5) before 12,000 cal yr
BP indicating the occurrence of highest deep-water
oxygen availability during the YD. Overall, assem-
blages from Zminje Jezero seem to remain indica-
tive of high O₂ concentrations throughout the entire
sequence, since their CCA axis 1 scores (> 1.2;
Figs. 3, 5) are in the upper half of the range of CCA
axis 1 scores (–1.2 to 3.1) from extant Swiss lakes
examined by Ursenbacher et al. (2020). Nonetheless,
we can observe a trend towards lower values during
the mid- to late Holocene (Zones ZMN-3 to ZMN-4).

Sedimentation rates in Zminje Jezero (Fig. 4)
remained moderately high from 12,300 to 10,400 cal
yr BP (higher than 0.05 cm year⁻¹). Between 10,000
and 3000 cal yr BP, sedimentation rates were low

and stable (below 0.05 cm year⁻¹). From 3000 cal yr
BP onward sedimentation rates rise progressively to
reach a maximum of 0.18 cm year⁻¹ at the sediment
surface.

<figure><img src="image_7.png" /></figure>
Chironomid influx values (Fig. 4) are highest between 11,500 and 10,000 cal yr BP (with a local maximum of 6 HC cm⁻² year⁻¹) and in the topmost section of the core (8 HC cm⁻² year⁻¹ in the topmost sample). Otherwise, values are low with a minimum between 1000 and 500 cal yr BP (down to 0.07 HC cm⁻² year⁻¹). Local maxima are apparent around 8250 cal yr BP and 1350 cal yr BP when influx values are slightly higher (up to 1.53 HC cm⁻² year⁻¹).

## Chironomid inferred July air temperature reconstruction

Reconstructed mean July air temperatures (Fig. 5) are lowest between 12,300 and 11,700 cal yr BP with values below 14 °C. Temperature then rises to reach a short-lived maximum near 18 °C around 10,800 cal yr BP. Afterwards, temperatures rapidly decrease and then remain relatively stable until 2000 cal yr BP with values between 14.6 and 16.2 °C except for a centennial-scale period with slightly lower temperatures down to 14.2 °C from 5000 to 4000 cal yr BP. From 2000 years to 1000 cal yr BP a progressive decrease in temperature is observed, with the lowest values of 13.2 °C at 1000 cal yr BP. Over the past 1000 years, temperatures show a slight increase to reach 14.8 °C in the uppermost sample.

## Discussion

Chironomid and macroinvertebrate assemblages and lake development

### The end of the Younger Dryas

Low concentrations of chironomid and other aquatic invertebrate remains characterize ZMN-1 (12,300–11,960 cal yr BP), possibly a consequence of low in-lake productivity affecting macroinvertebrate assemblages. High proportions of *Procladius* are observed, a generalist taxon that occurs under varying climatic and environmental conditions (Brooks et al. 2007; Heiri et al. 2011). A similar ecology characterizes *Tanytarsus mendax*-type, a morphotype which was found throughout the core and is characteristic for intermediate cool to warm lakes (Saether 1979; Brooks et al. 2007; Heiri et al. 2011). In ZMN-1 *Tanytarsus mendax*-type is found at lower abundances

<figure><img src="image_8.png" /></figure>

**Fig. 5** Comparison of the Zminje Jezero chironomid-based July air temperature reconstruction with other chironomid-based July air temperature reconstructions, the pollen-based temperature reconstruction of Lake Maliq (mean temperature of the warmest month) and the MBT-CBT-based temperature reconstruction of Lake Dojran (annual MAT). Uncertainties (eSEPs) in the chironomid-based reconstructions are indicated by the shaded area. All temperature reconstructions were corrected to the altitude of Zminje Jezero (1535 m a.s.l.) using an altitudinal lapse rate of 0.6 °C 100 m⁻¹. The location of the different sites is shown in Fig. 1
than in other zones, possibly due to cooler conditions
that favor other chironomid taxa.

The temperature in and around the lake was cool
to intermediate during the YD. This is shown by a
chironomid assemblage co-dominated by *Chironomus anthracinus*-type typical of cool to intermediate
temperatures in small lakes (Heiri et al. 2011) and by
*Microtendipes pedellus*-type that can also be found in
relatively cool conditions (Bolland et al. 2021). This
is also in accordance with the presence in smaller
numbers of *Tanytarsus lugens*-type, also primarily
living under cool conditions (Heiri et al. 2011). *Tanytarsus mendax*-type and *Procladius* are able to survive over a broad range of trophic conditions (Lotter
et al. 1997, 1998). However, other taxa having narrower
ecological requirements show that the lake was
probably oligotrophic to mesotrophic during the YD.
For example, *Chironomus anthracinus*-type is found
in oligo- to mesotrophic conditions (Saether 1979;
Brooks et al. 2007) and *Microtendipes pedellus*-type
mainly in oligo- to mesotrophic conditions (Brooks
et al. 2007; Moller Pillot 2009) but can also be found
in eutrophic lakes (Lotter et al. 1998). Also, *Tanytarsus lugens*-type, present at lower abundance, is typical
for oligotrophic conditions (Brooks et al. 2007).
Of the macroinvertebrate remains found, the presence
of Sialidae could indicate lower lake levels or a
pronounced contribution of remains from near-shore
areas, as these aquatic insects are usually considered
indicative of littoral conditions and muddy lake bottoms
(Lemdahl 2000). However, relatively high abundances
of chironomids that can survive in profundal
conditions and of planktonic invertebrate remains
(Fig. 4) suggest that the lake was not particularly
shallow during this period.

*The Younger Dryas/Holocene transition and the early Holocene*

In zone ZMN-2 (11,960-8370 cal yr BP) several
chironomids often found in relatively cool con-
ditions in small Central and Southern European
mountain lakes (*Chironomus anthracinus*-type,
*Microtendipes pedellus*-type; Heiri and Lotter 2010;
Samartin et al. 2017) diminish in abundance or dis-
appear whereas taxa with a wide temperature dis-
tribution remain at high abundances (*Procladius*)
or increase (*Tanytarsus mendax*-type). We observe

higher concentrations of chironomids which sug-
gests overall improved conditions for this family
or higher habitat diversity. High chironomid influx
values from 2 to 6 HC cm⁻² year⁻¹ can be seen in
this zone between 11,500 and 10,000 cal yr BP
(Fig. 4). This may be the result of an increase of
the chironomid abundance during this interval due
to changing environmental conditions. Many taxa
appear that were previously absent or only present
at low abundances. These include *Dicrotendipes*,
*Einfeldia* *dissidens/natchitocheae*-type, *Cladot-
anytarsus mancus*-type, *Ablabesmyia*, *Tanytarsus*
*pallidicornis*-type, *Tanytarsus glabrescens*-type,
*Paratanytarsus*, *Chironomus plumosus/Einfeldia*
*pagana*-type and *Psectrocladius sordidellus*-type.
*Cladotanytarsus mancus*-type, *Ablabesmyia*, *Tany-
tarsus glabrescens*-type and *Tanytarsus pallidi-
cornis*-type have a relatively wide distribution with
respect to temperature but tend to be more abundant
in warmer lakes (Heiri et al. 2011) and this would
agree with some warming compared to the zone
below. However, *Tanytarsus lugens*-type, often an
indicator of cold conditions, remains at low abun-
dances in some samples and *Psectrocladius sor-
didellus*-type, which has a very wide distribution
but is also able to live in colder conditions (Heiri
et al. 2011), increases in the upper part of this zone.
This may indicate some minor variation of tem-
perature. Most of the taxa listed above, which are
present in this zone at high abundances, are indica-
tors of mesotrophic or eutrophic conditions with the
exception of some species within *Paratanytarsus*
and *Tanytarsus lugens*-type which are indicators of
more oligotrophic conditions (Brooks et al. 2007).
This suggests more nutrient-rich conditions of the
lake compared to the previous zone.

Higher concentrations of aquatic invertebrate
remains (Fig. 3) suggest improved conditions for
most of these groups, although no abrupt change
can be seen at the transition from ZMN-1 to ZMN-
2. Instead, a slow increase in some macroinverte-
brates, or the disappearance of groups present at low
proportions in zone ZMN-1, such as Trichoptera,
Sialidae, and Amphipoda, is observed. Chironomids
that can colonize the profundal of lakes are present
at relatively high abundances, and planktonic inver-
tebrate remains are common (Fig. 4), suggesting a
lake of at least intermediate depth.
## The middle Holocene

At the transition to ZMN-3 (8370–5150 cal yr BP), taxa which appeared in ZMN-2 increase slowly or remain at similar abundances. Only *Tanytarsus lugens*-type, which previously persisted at low abundances disappears from the record. A few new taxa like *Cladopelma*, *Tanytarsus lactescens*-type and *Limnophyes-Paralimnophyes* increase or occur for the first time. *Cladopelma* and *Limnophyes-Paralimnophyes* are typical for intermediate to warm July air temperatures in Central European mountain lakes (Heiri et al. 2011). *Tanytarsus lactescens*-type, which first appears in this zone, indicates temperate climatic conditions (Heiri and Lotter 2010). The appearance of chironomid taxa typical for mesotrophic (*Cladopelma*) and eutrophic conditions (*Tanytarsus lactescens*-type) according to Brooks et al. (2007), show that the mesotrophic to eutrophic conditions found in zone ZMN-2 persisted in zone ZMN-3. However, the general decrease of taxa that can survive in profundal environments in favour of taxa restricted to the littoral of lakes (Fig. 2) could indicate a tendency to lower oxygen availability in the deepest part of the lake (Walker et al. 1993). Only chironomid taxa adapted to low oxygen availability, for example by optimizing or regulating the oxygen uptake, can survive in deeper water with seasonally reduced oxygen concentration. The genera *Chironomus*, *Ablabesmyia* and *Procladius* have these abilities (Brodersen et al. 2004) and this possibly explains why these taxa can be found in this zone in noticeable proportions. Habitats for deep-water chironomids, and oxygen concentrations in the lake, may also have been influenced by progressive infilling of the lake basin, which will have reduced the maximum depth in the lake center. However, the decrease of chironomid taxa that can colonize the lake profundal is largely attributed to the decrease of *Tanytarsus mendax*-type, a taxon that can also be very abundant in littoral conditions, and this decrease of profundal chironomid taxa must therefore be interpreted with caution.

In this section, the proportion of chironomid remains declines in favor of more Coleoptera and Ephemeroptera (Fig. 3). The increase of the latter order possibly indicates shallower waters (Luoto 2009). *Plumatella* rises in abundance. In small Swiss lakes in the Alpine region, high *Plumatella* values have been observed in relatively warm lakes with

a tendency to hypoxic conditions where the abundances of deep-water invertebrate groups such as Chironomidae are reduced (Ursenbacher et al. 2020). Alternatively, high abundances of bryozoans such as *Plumatella* may also represent more habitats for this taxon, such as aquatic macrophytes in near-shore areas (Francis 2001). In agreement with this, Rhabdo-coela oocytes, which are often considered as indicators of higher productivity and eutrophication (Haas 1996), show a noticeable increase in this zone. The lower axis 1 values of the CCA based on invertebrate remains (Fig. 4), indicative for lower bottom-water oxygen concentrations, also agree with less deep-water oxygen, which is also supported by the chironomid genera typical of eutrophic conditions such as *Chironomus*, *Ablabesmyia* and *Procladius* (Brodersen et al. 2004). Planktonic invertebrate remains are initially present at similar abundances as in earlier parts of the record, although they decrease towards the top of this zone (Fig. 4), a decrease which could also be explained by a shallowing of the lake due to infilling.

## The late Holocene

In ZMN-4 (5150 cal yr BP-today) taxa with wide distributions such as *Tanytarsus mendax*-type and *Procladius* decrease (Fig. 2). This suggests changing environmental conditions favorizing better adapted specialists. Taxa indicative of relatively warm climate conditions, which were present in zones ZMN-2 and ZMN-3 (e.g. *Dicrotendipes*, *Einfeldia dissidens/natchitocheae*-type, *Cladotanytarsus mancus*-type, *Ablabesmyia*, *Tanytarsus pallidicornis*-type), are still present. *Chironomus plumosus/Einfeldia pagana*-type, *Paratanytarsus*, *Tanytarsus lactescens*-type and *Psectrocladius sordidellus*-type increase in abundance. These taxa are mainly indicators of eutrophic conditions except for some species of *Paratanytarsus* that typically live in oligotrophic waters (Brooks et al. 2007) and *Psectrocladius sordidellus*-type, which occurs over a wide nutrient gradient (Lotter et al. 1997, 1998). Other taxa appear for the first time and increase, such as *Polypedilum nubeculosum/nubifer*-type, *Glyptoten-dipes*, and *Heterotrissocladius marcidus*-type. *Polypedilum nubeculosum/nubifer*-type and *Glyptoten-dipes* are present in lakes at relatively high July air temperatures (Heiri et al. 2011) and eutrophic conditions (Brooks et al. 2007). Taxa that can colonize
profundal habitats decrease even more whereas littoral taxa increase. This could again indicate a tendency to lower oxygen concentrations in the deepest part of the lake (Walker et al. 1993) or alternatively shallower in-lake conditions due to infilling of the lake basin. Interestingly, we also see a return of some taxa that were present in zones ZMN-1 and ZMN-2. *Tanytarsus lugens*-type, *Chironomus anthracinus*-type and *Microtendipes pedellus*-type increase again in the uppermost core section, possibly showing a recovery after a possibly human-induced eutrophication. Some of these taxa are also typical for cooler temperatures (Heiri et al. 2011). This change at the core top can also be observed in the increase in chironomid concentrations and in the proportion of the subfamily Orthocladiinae (Fig. 2).

The abundance of invertebrate remains varies little in ZMN-4 compared to ZMN-3 (Fig. 3). For example, Ephemeroptera are still present in high proportions and therefore indicate the presence of shallow waters (Luoto 2009). However, the proportion of invertebrates living in benthic habitats is highest in this zone (Fig. 3) suggesting that either the lake became shallower due to infilling, or littoral habitats increased relative to deep-water habitats. Rhabdocoela stay in high abundances possibly indicating high productivity and eutrophication as in zone ZMN-3 (Haas 1996). The relative abundance of *Plumatella* also increases, possibly indicating expanding habitats for this group, such as aquatic macrophytes. Axis 1 values of the CCA based on the aquatic invertebrate assemblages remain relatively low (Fig. 4), suggesting that the assemblages were typical for relatively low bottom oxygen concentrations in this zone (Ursenbacher et al. 2020). However, the values increase again in the topmost samples.

## Comparison with catchment vegetation and sedimentation rates

Chironomid-based temperature reconstructions rely on the assumption that chironomid assemblages are indicative for ambient temperature conditions and that the relationship between chironomid assemblages and temperature is not strongly distorted or biased by other drivers of ecosystem or community-composition change (Eggermont and Heiri 2012). Other mountain-lake records have shown that even early human activities such as pasturing can lead to

substantial changes in chironomid assemblages, and thus, also of temperature reconstructions, therefore affecting their reliability (Heiri and Lotter 2005). Here we draw on the detailed reconstruction of past vegetation changes around Zminje Jezero that was developed in a parallel study based on pollen, plant macrofossils, and stomata from the same sediment record (Cagliero et al. 2023) to compare the temperature reconstruction to changes in vegetation and discuss possible influences of past human activities on Zminje Jezero based on past vegetation changes and sedimentation rates.

High pollen percentages (and presence of needles and stomata; Cagliero et al. 2023) indicate that *Pinus* dominated the vegetation both regionally as well as locally around the lake between 12,300 and 11,500 cal yr BP. The occurrence of a *Pinus*-dominated tree cover is typical of locations below the tree line during the Lateglacial (thus, including the YD) in southern Central as well as in Southeastern Europe (Rey et al. 2017; Lang et al. 2023). Colder and drier climate is confirmed by high percentages of Poaceae, *Artemisia* and total upland herb pollen typical for steppe/tundra (Fig. 4). This is in agreement with our chironomid-inferred temperature reconstruction showing colder temperature until 11,500 cal yr BP.

Vegetation composition shifted around 11,500 cal yr BP, both locally (expansion of *Betula*) as well as regionally (expansion of deciduous and rather mesophilous trees: *Quercus*, *Ulmus*, and *Fraxinus*). The latter is typical of early Holocene pollen records from the region (Lang et al. 2023) and agrees with the early Holocene rise of the chironomid-inferred temperature values. Interestingly, sedimentation rates dropped around 10,000 cal yr BP (Fig. 4) in conjunction with the start of the expansions of *Picea abies* and *Abies alba* in the surroundings, suggesting that increased forest cover contributed to reduce slope instability in the catchment. Forest cover remained high thereafter, first with a *Picea abies* and *Abies alba* dominated forest until 6400 cal yr BP and later with a mixed *Picea-Abies-Fagus* dominated forest until present day (Cagliero et al. 2023). In keeping with the persistent local occurrence of a dense forest cover, sedimentation rates in the lake were mostly low during the past 10,000 years. The rare signs of human activities documented by a single *Secale cereale* pollen finding starting from 5800 cal yr BP suggest
that humans had very little influence on ecosystem
dynamics locally (Fig. 4).

However, human activity became more notice-
able from around 2000 cal yr BP onwards. Olea
europea, Juglans regia, and Castanea sativa pollen
abundances increased (Fig. 4) as these trees were
cultivated at lower elevation along the Adriatic
Sea coast. Moreover, cultural indicators (includ-
ing Plantago lanceolata-type and Cerealia-type
pollen) and secondary anthropogenic indicators
(including Plantago major/media-type, Urtica-type,
and Rumex acetosa) also increased starting from
2000 cal yr BP (Fig. 4). These insights in conjunc-
tion with first evidence of increasing sedimentation
rates from ca. 3000 cal yr BP onwards, together
with a more pronounced increase in sedimenta-
tion rates at ca. 2000 cal yr BP (Fig. 4), suggest
that human activities may have influenced the lake
ecosystem via increased catchment erosion. Indeed,
late Holocene increases in sedimentation rates have
been observed for other European mountain lakes
where pasturing occurred in the catchment since the
Bronze age (Thöle et al. 2016; Perret-Gentil et al.
2024). However, in the Zminje Jezero vegetation
record there is no indication of major local human
activities such as clearcutting or animal husbandry.
Plant macrofossils and stomata of trees (Picea,
Abies, and Fagus), which indicate the local pres-
ence of taxa (Ammann et al. 2014), were persis-
tently found since at least 9500 cal yr BP, and dung
fungi spores (Sporormiella) that are indicative for
the presence of herbivores (Davis 1987) are absent
for the Holocene. Conversely, the decrease of tree
pollen centered on 500 cal yr BP and a substantial
increase of pollen indicative of human activities
from 500 cal yr BP to present may attest to the first
settlements in the Durmitor Massif during the Mid-
dle Ages and associated land-use activities such as
exploitation of timber and pasturing (Cagliero et al.
2023). For our record this implies that variations in
the youngest interval of the chironomid-based tem-
perature reconstruction should be interpreted with
caution, as the increase in sedimentation rates from
ca. 3000 cal yr BP and particularly the changes in
vegetation from ca. 500 cal yr BP onwards may
have been associated with significant influences of
human activities on the lake and its environments.

Interpretation of the chironomid-based temperature
reconstruction

There is no local chironomid-temperature calibration
dataset available for the Western Balkans. Neverthe-
less, the application of the Swiss calibration dataset
and chironomid-temperature transfer function to
Zminje Jezero resulted in a chironomid-based July
air temperature record that features the main characteristics
expected for the temperature development
in this region, including relatively cool temperatures
during the YD, higher mid- than late Holocene temperatures
and a decrease in reconstructed temperature
towards the latest Holocene. Compared with other
chironomid-based Holocene temperature reconstructions
from the Swiss Alps, the Apennines and the
southern Carpathians (Heiri et al. 2015; Tóth et al.
2015; Samartin et al. 2017; Fig. 5), the main difference
of the Zminje Jezero record is the unexpectedly
warm early Holocene, with higher temperatures than
recorded in the mid-Holocene, the relatively high
temperatures that persist well into the late Holocene
after a first temperature decrease around 11,000 cal
yr BP, and the relatively late and stepwise decrease
in latest Holocene temperatures at ca. 2000 cal yr BP.
However, other chironomid-based reconstructions
from the eastern Alps (Ilyashuk et al. 2011) and the
northern Carpathians (Hájková et al. 2016; Fig. 5)
also feature higher early Holocene temperatures than
observed during the mid-Holocene, and the Zminje
Jezero record therefore falls within the variation of
early to mid-Holocene temperature development
observed in other southern Central, Southern and
Eastern European records. Similarly, the exact timing
and amplitude of the mid-to late Holocene temperature
decrease observed between chironomid records
differs to some extent, with some records showing a
gradual temperature decrease towards the late Holocene
(Heiri et al. 2003) whereas others feature more
stepwise changes (Heiri et al. 2015) or a combination
of gradual and stepwise temperature decreases (Tóth
et al. 2015). These differences may be associated
with differences in temperature development across
Europe, but also local hydrological and topographical
effects and the sensitivity with which different lake
ecosystems respond to gradual changes in temperature,
which may also result in stepwise changes once
certain ecosystem thresholds are passed.
The closest chironomid-based temperature reconstructions from the Mediterranean climate region are from two lakes in the northern Apennines (Italy), Gemini and Verdarolo (Samartin et al. 2017), situated close to each other at similar elevation (1349 and 1390 m a.s.l.) as Zminje Jezero (1535 m a.s.l.). Chironomid-based temperature reconstructions from both of these lakes show a mid-Holocene thermal maximum between 9000 and 5000 cal yr BP with temperatures 1–2 °C higher than during the past 2000 years (Fig. 5). This is in accordance with our record also showing 0.3–1.5 °C higher temperatures in this same time period compared to the pre-industrial temperature levels. The timing of the beginning of the temperature decline after 5000 cal yr BP in these records and in our record also matches well. In contrast, both of these records show increasing temperatures during the earliest Holocene but not maximum values, although this may represent differences in the geographical and climatological setting between these lakes and Zminje Jezero.

Few other fully quantitative temperature reconstructions are available from the Western Balkans region though none based on fossil invertebrate remains. The closest site with a temperature reconstruction of the entire Holocene is ancient Lake Maliq (Albania), around 300 km from Zminje Jezero, a lake that has dried up in the past century because it was drained for agriculture. The pollen-based temperature reconstruction for this site (Bordon et al. 2009; Fig. 5) also features a pronounced increase in the mean temperature of the warmest month (MTWA) at the YD to Holocene transition. However, this increase is much more pronounced (ca. 8 °C, Bordon et al. 2009) than in any other reconstruction from the region, including the one from Zminje Jezero (Fig. 5). The increase in chironomid-inferred temperatures at Zminje Jezero agrees better with the estimated temperature change at this transition from other chironomid records from southern Central and Southern Europe (ca. 3–5 °C; Ilyashuk et al. 2011; Tóth et al. 2015; Hájková et al. 2016; Samartin et al. 2017). Further, the pollen-based temperature record from Lake Maliq only shows a relatively low variability in summer temperatures and lower temperatures in the mid-Holocene compared to the late Holocene, a feature that disagrees with Holocene summer insolation changes and has also been described for some other pollen-based temperature

reconstructions from southern Europe (Samartin et al. 2017).

A further reconstruction of annual mean air temperature (MAT), based on methylation and cyclisation of branched tetraethers (MBT-CBT) indices is described by Thienemann et al. (2017) for Lake Dojran (North Macedonia/Greece), situated less than 400 km away from Zminje Jezero. Here the lowest temperature is observed at 11,500 cal yr BP during the YD which is in accordance with our findings. After this period, Thienemann et al. (2017) describe an increase of 3 °C during the early Holocene with a temperature maximum at 9500 cal yr BP. This maximum takes place somewhat later than in our reconstruction, where the relative thermal maximum, with reconstructed temperatures roughly 5 °C higher compared to the YD, takes place at 10,800 cal yr BP. However, overall the relative annual MAT variations for Lake Dojran follow a similar pattern as in our reconstruction during the Holocene (Thienemann et al. 2017; Fig. 5). Specifically, between 7500 and 5000 cal yr BP stable warm conditions are reconstructed as at Zminje Jezero. During the mid-Holocene/late Holocene transition, Thienemann et al. (2017) describe colder conditions that coincide with the dry and cold 4.2 kyr event. This event is apparently evident across the Mediterranean region and was followed at Lake Dojran by environmental instability at the start of the late Holocene as shown through distinct changes in some studied proxies (Francke et al. 2013). The temporal resolution of our record does not allow us to document such short-lived, abrupt temperature variations. However, the reconstructed temperatures from Zminje Jezero nevertheless follow a similar pattern as the annual MAT record from Lake Dojran during the last 1200 years, notably including a temperature drop of ca. 1 °C at 1000 cal yr BP followed by a minor temperature increase thereafter.

## Conclusions

The Zminje Jezero chironomid record provides evidence for a pronounced shift of chironomid-assemblage composition at the YD to Holocene transition, and more gradual changes within the Holocene. Chironomid and other aquatic invertebrate assemblages suggest meso- to eutrophic conditions over the entire analysed interval, with possibly slightly more
oligotrophic conditions during the YD. The comparison of the invertebrate assemblages with those presently observed in small Swiss lakes (CCA axis 1 scores in Fig. 3) indicates that they remained typical for lakes with relatively high oxygen availability over the entire record. However, the assemblage composition from ca. 8400 cal yr BP until today (ZMN-3 and ZMN-4) is more characteristic of lakes with slightly reduced hypolimnetic oxygen conditions, suggesting, together with the occurrence of some chironomid taxa able to tolerate low oxygen conditions, that the lake may have experienced occasional phases of hypoxia particularly in the past ca. 5200 years (ZMN-4). Variations in the abundance of Ephemeroptera, littoral chironomid taxa and benthic invertebrate remains, particularly towards the late Holocene (ZMN-4), suggest that the lake may have experienced decreases in water depth possibly due to infilling of the basin. However, chironomid assemblages clearly remain indicative for aquatic conditions during the entire record, suggesting that these water-depth changes were limited. Applying a chironomid-temperature transfer function from Central Europe to the record resulted in a July air temperature reconstruction which agrees, in general features, with other chironomid-based temperature reconstructions from southern Central, Eastern and Southern Europe. It is characterized by relatively low reconstructed temperatures during the YD, relatively high temperatures in the early and mid-Holocene, with a distinct maximum in the earliest Holocene, and slightly lower values in the late Holocene. It shows a close agreement with another quantitative temperature reconstruction from Greece/ North Macedonia (Lake Dojran) based on the MBT-CBT proxy, but also some differences with a pollen-based temperature reconstruction from the Balkan region (Lake Maliq), particularly regarding temperature stability during the Holocene. By showcasing the effectiveness of chironomid-based temperature reconstructions at Lake Zminje Jezero, we hope that this study will pave the way for future chironomid-based paleoclimate research in the Balkans.

**Acknowledgements** We thank Pierre Lapellegerie for help with sample preparation, identifications and statistical analyses and Colin Courtney-Mustaphi for assistance in identifying invertebrate and macrophyte remains. We are grateful to the Environment Protection Agency of Montenegro for granting permission to the fieldwork, and are thankful to D. D'Aprile, R. Motta, N. Marchi, Z. Lisztes-Szabó, M. Braun, M. Čurović,

and V. Spalevic for help during fieldwork. Finally, we thank Stan J. Schouten and Anais M.F.A Schmidhauser for the comments on the manuscript. We thank two anonymous reviewers for their comments.

**Author contributions** We use the formal Contributor Roles Taxonomy (cRediT) to acknowledge the work of this team. NS: Formal analysis, Investigation, Writing—Original Draft, Writing—Review & Editing, Visualization. OH: Conceptualization, Resources, Writing—Review & Editing, Supervision, Project administration, Funding acquisition. WF: Resources, Writing—Review & Editing, Supervision. EC: Writing—Review & Editing.

**Funding** Open access funding provided by University of Bern. N.S. was partly funded by SNF 200020_204220.

**Data availability** Novel data associated with this submission are included in the supplementary information files. Datasets published in Cagliero et al. 2023 (chronology, pollen, spores, stomata, plant macrofossils, charcoal and XRF records) are publicly available through the European Pollen Database (EPD) and can be accessed at the following https://doi.org/10.21233/9BC8-ZK56.

## Declarations

**Conflict of interest** Authors have no competing interests as defined by Springer or other conflicts of interest that could be perceived as having influenced the results and/or discussion in this paper.

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