Holocene subfossil chironomid stratigraphy (Diptera: Chironomidae) in the sediment of Plešné Lake (the Bohemian Forest, Czech Republic): Palaeoenvironmental implications

Biologia, Bratislava, 61/Suppl. 20: S401—S411, 2006 S401

Holocene subfossil chironomid stratigraphy (Diptera:Chironomidae) in the sediment of Plešné Lake (the BohemianForest, Czech Republic): Palaeoenvironmental implications

Jolana Tátosová1, Josef Veselý2 & Evžen Stuchlík3

1Institute for Environmental Studies, Faculty of Science, Charles University in Prague, Benátská 2,CZ-12801 Prague, CzechRepublic; e-mail: [email protected] Geological Survey, Geologická 6, CZ-15200 Prague 5, Czech Republic3Hydrobiological Station, Institute for Environmental Studies, Charles University in Prague, P.O. Box 47, CZ-38801 Blatná,Czech Republic

Abstract: A faunal record of chironomid remains was analyzed in the upper 280 cm of a 543 cm long sediment core fromPlešné jezero (Plešné Lake), the Bohemian Forest (Šumava, Bohmerwald), Czech Republic. The chronology of the sedimentwas established by means of 5 AMS-dated plant macroremains. The resolution of individual 3-cm sediment layers is ∼115years and the analyzed upper 280 cm of the sediment core represent 10.4 cal. ka BP. As the results of DCA show, twomarked changes were recorded in the otherwise relatively stable Holocene chironomid composition: (1) at the beginningof the Holocene (ca. 10.4–10.1 cal. ka BP) only oligotrophic and cold-adapted taxa (Diamesa sp., M. insignilobus-type,H. grimshawi-type) were present in the chironomid assemblages, clearly reflecting a cool climate oscillation during thePreboreal period, and (2) during an event dated in the interval 1540–1771 AD, when most taxa vanished entirely and onlyZavrelimyia sp. and Procladius sp. were alternately present accompanied by Tanytarsus sp. Although, the age of this eventis in agreement with the dating of the Little Ice Age, the most probable reason for the elimination of many chironomid taxawas very low sums recorded in this part of the sediment, rather than cool conditions connected with the LIA. Variations inthe chironomid fauna after the Preboreal period were reflected mainly by changes in abundances of dominant taxa ratherthan by changes in species composition. These variations could be explained by: (1) climatic changes, namely temperatureand amount of rainfall resulting in oscillations in lake level, with changes in the occurrence of macrophytes in the littoraland (2) increasingly dense afforestation which led to a considerable input of organic material into the lake and a subsequentincrease in the trophic status of the lake water.

Key words: Chironomidae, glacial lake, climate changes, palaeoenvironmental reconstruction, palaeolimnology.

Introduction

Larvae of Chironomidae (Insecta: Diptera) colonize allfreshwater systems, from large lakes to the smallestponds, and often dominate the benthos. This familyis noted for its taxonomic richness, with nearly 10,000species distributed globally (Cranston, 1995), repre-senting more than 20% of all freshwater insects in riversand lakes. As a result of their short generation timesand the dispersal capacity of the winged adults, chi-ronomids respond rapidly to changes in a wide varietyof environmental variables (Walker, 2001). Transferfunctions have been developed for a range of environ-mental parameters including salinity (Heinrichs et al.,2001), dissolved oxygen (Quinlan & Smol, 2002) andnutrients (Brodersen & Lindegaard, 1999;Brookset al., 2001); however, over large climatic gradients, airtemperature is often the best explanatory variable forchironomid distribution (Lotter et al., 1997;Walker

et al., 1997; Brooks & Birks, 2000). Chironomid lar-vae possess chitinized head capsules that are resistantto decomposition. Consequently, fossilized chironomidhead capsules tend to be well preserved in lake sed-iment over thousands of years, and can generally beidentified to genus or, more rarely, to species-grouplevel. This offers the possibility of using the fossil chi-ronomid record to infer past environmental conditionsin lakes. Knowledge of modern chironomid ecology al-lows the use of chironomid subfossils for the recon-struction of palaeoproductivity (Brodersen & Linde-gaard, 1999), acidification (Brodin & Gransberg,1993; Schnell & Willassen, 1996), palaeosalinity(Walker et al., 1995) and, more recently, summer tem-perature variations (Brooks & Birks, 2001; Kor-hola et al., 2002; Heiri et al., 2003; Larocque &Hall, 2004; Heiri & Lotter, 2005).The preliminary results presented here form part

of a multi-disciplinary project investigating the biotic

S402 J. Tátosová et al.

Table 1. Radiocarbon dating of plant remains.

Laboratory code Depth (cm) CAR yr. BP Range 1) cal. ka BP Intercept with cal. ka BP 2)

NZA–9645 51–54 2005 ± 60 2.11–1.82 1.95NZA–9317 105–108 3637 ± 60 4.13–3.82 3.95NZA–13686 114–117 3949 ± 50 4.52–4.24 4.42NZA–11663 141–144 4733 ± 55 5.59–5.32 5.53NZA–9599 234–237 8264 ± 65 9.43–8.99 9.28

Explanations: CRA – Conventional Radiocarbon Age; NZA – Laboratory Code of the Rafter Radiocarbon Lab., New Zealand; 1) 95%confidence thresholds of ka BP; 2) REIMER et al. (2004).

and abiotic responses to climate changes that occurredduring the Late Glacial and Holocene in the BohemianForest (Šumava, Bohmerwald), Czech Republic. Similarstudies are available for many areas of Europe (Gan-douin & Franquet, 2002; Korhola et al., 2002;Porinchu & Cwynar, 2002; Lotter & Birks, 2003;Brooks & Birks, 2004; Larocque & Hall, 2004;Langdon et al., 2004; Dalton et al., 2005; Velleet al., 2005), but have never been done on lakes ofglacial origins in the Czech Republic. Similar studieswere published by Veselý et al. (1993), Veselý (1998,2000), Pražáková & Fott (1994) and Bitušík &Kubovčík (2000). However, all of these concentrateon changes during shorter periods than are presented inthis paper. The first integrated information on the mod-ern chironomid fauna in these lakes of the BohemianForest was given by Bitušík & Svitok (2006).The main aim of this paper is to describe the

Holocene chironomid stratigraphy of Plešné jezero(Plešné Lake) and to provide a preliminary Holocenereconstruction of key environmental conditions in thisarea.

Study site

Plešné Lake is a small lake of glacial origin (7.6 ha, max.depth 17 m) located at an altitude of 1087 m a.s.l. andsituated in a relatively remote and uninhabited part ofthe Bohemian Forest (48◦47′ N, 13◦52′ E) in C Europe.The bedrock of the catchment (0.67 km2) is formed bygranite, and is covered with a thin layer of lithosol, pod-zol and spododystric cambisol (JANSKÝ et al., 2005). Thecatchment is steep and forested, with Picea excelsa domi-nating. The average annual and July air temperatures are4.4 and 13.1◦C, respectively (Czech HydrometeorologicalInstitute), and the period of ice-cover averages 135 daysyr−1. The present average TP loading (11 µg L−1) suggestsmesotrophic conditions in the lake.

Plešné Lake was partially affected by atmospheric acid-ification in the early 1960s, and during the peak of acidifi-cation in 1980s the pH of lake water dropped below 4.7(VESELÝ & MAJER, 1996; KOPÁČEK et al., 1998). At thepresent time, the lake is in the process of chemical and bi-ological recovery (MAJER et al., 2003; VRBA et al., 2003;NEDBALOVÁ et al., 2006). At the end of 19th century, Salmotrutta L., 1758 was introduced to the lake (VESELÝ, 1994),

but did not survive the period of maximum acidification andat present the lake is fishless.

Methods

In 1991, a 543 cm long core was collected by R. Schmidt (In-stitute for Limnology of the Austrian Academy of Sciences,Mondsee, Austria) with a modified Kullenberg piston corerclose to the deepest part of the lake. The core was sub-sampled every 3 cm and samples were continuously storedat 4◦C. A faunal record of chironomid remains was analyzedin the upper 280 cm. For analyses, a known weight of wetsediment was deflocculated in 10% KOH at 60◦C for 20 minand then washed with distilled water onto sieves with 230µm and 86 µm mesh-size in order to facilitate the sortingand picking of head capsules. The material was transferredfrom the sieves into a plankton counting tray and all headcapsules were picked with either fine forceps or a needleusing a stereomicroscope at 25× magnification. After dehy-dration in isopropyl alcohol, head capsules were mounted onslides in Euparal�mounting medium.

Taxonomy mainly follows WIEDERHOLM (1983),SCHMID (1993), BITUŠÍK (2000) and RIERADEVALL &BROOKS (2001). Members of the tribe Tanytarsini were re-viewed using the description given by HEIRI et al. (2004).

Concentrations of organic carbon (C) in lyophilizedsub-samples were analyzed with a TOC 5000A analyzer.

ChronologyA total of seven terrestrial plant remains were isolated fromfive levels of the Holocene sediment and dated by the AMS-radiocarbon method in the Rafter Radiocarbon Laboratory,Lower Hutt, New Zealand in the year 2000. Errors of datingby 14C-methods are ≤ 200 years and the age of the corebottom was ∼14.5 ka BP. The resolution of individual 3-cmsediment layers is ∼115 years in the Holocene (Tab. 1; formore details see PRAŽÁKOVÁ et al., 2006).

Data analysisThe chironomid percentage diagrams were created usingTILIA version 2.0.2 (GRIMM, 2004) and the zones were de-fined using the CONISS (constrained incremental sum ofsquares cluster-analysis) program available in TILIA. In ad-dition, Detrended Correspondence Analysis (DCA) was ap-plied to the chironomid data. DCA was performed using theprogram CANOCO version 4.5A (Ter BRAAK & SMILAUER,2003) and square-root transformation of species data anddown-weighting of rare taxa were used.

Subfossil chironomids from Plešné Lake S403

Results

TaxonomyWe distinguished 35 different taxa, most of which wereidentified to genus level using mentum characteris-tics (Dicrotendipes, Microtendipes, Pagastiella, Pseu-dochironomus, Cladotanytarsus, Parakiefferiella, Za-lutschia, and Diamesa). However, when mandibles orpremandibles were not available and the mentum wasdamaged or worn down, larger taxonomic groups wereformed in the cases of Limnophyes/Paralimnophyes,Cricotopus/Orthocladius, and Chironomus/Einfeldia.For a more synoptic display in the graphs, thesegroups include also specimens assigned to the genera. Insome instances, taxa were identified to species-groups;Corynoneura scutellata-type was determined by the dis-tinct pattern on the head capsule surface, and a men-tum with 3 median teeth and well defined first lateralteeth; Heterotrissocladius marcidus and H. grimshawi-type were separated on the basis of the specific submen-tum color; Psectrocladius psilopterus-type was identi-fied based on a mentum with two median teeth withnipple-like projections that do not exceed the lateralteeth in length, and Psectrocladius sordidellus-type bya mentum with two wide apically pointed medianteeth exceeding the lateral teeth in length. Remainsof Micropsectra were determined as M. insignilobus-type when a short spur was present on the antennalpedestals, the postoccipital plate was well developed,and the mandible had three inner teeth, one apical andone outer tooth. When available, the bifid premandibleswere used to confirm the Micropsectra genus. Tanytar-sus lactescens-type included specimens with the combi-nation of mandibles with two inner teeth and a straightblunt spur on the antennal pedestals. Head capsules ofProcladius were identified mainly on the base of thecephalic setation, because the lingulae were often miss-ing. Unfortunately, no remains of Procladius pupae,which have better characteristics for species identifica-tion (Langton, 1991), were found. Nevertheless, headcapsules of Procladius separated from the oldest lay-ers of the sediment were up to double in size comparedto specimens found in the rest of the sediment core.This could point to two different species of Procladius.Orthocladiinae genus B had a very wide slightly splitmedian tooth on the mentum not exceeding the firstlateral teeth and lighter than the four lateral teeth. Wedid not find any remains of Chaoboridae in the sedi-ment.

Chironomid stratigraphyThe analyzed part (276 cm) of the sediment core fromPlešné Lake covers the period from 10.4 cal. ka PB tothe present. Chironomid data were analyzed using DCAin order to assess the compositional structure and taxaturnover throughout the core profile. DCA calculated agradient length of 2.8 standard deviation units and pro-duced two significant axes that explained 30% of the cu-

mulative variance in the chironomid data. The samplescores of these axes were plotted on a time scale, withscaling in the original standard deviation units. Thefirst DCA axis (Fig. 1) separated faunal assemblagesbefore ca. 10.2 ka BP from younger samples. On axis2, decreases in values of sample scores were recordedbetween 1.5 ka BP and the present, with a significantevent in the period 1540–1774 AD. The Holocene sedi-ment was divided into 6 chironomid zones in agreementwith the results of CONISS cluster–analysis (Fig. 2).

ZONE-1 (276–267 cm, 10.4–10.1 cal. ka BP)In this zone, taxonomic richness was low; nevertheless,the total sum of individuals per sample reached themaximum value of the entire sediment profile. The chi-ronomid community consisted mainly of cold-adaptedtaxa. The dominant chironomid taxon was Procladius(∼50%) followed by Heterotrissocladius grimshawi-type(∼30%) and Micropsectra insignilobus-type (∼20%).The occurrence of Diamesa sp. was recorded only inthis zone, though in very low densities. At the top ofthe zone, the proportion of Corynoneura scutellata-typeincreased and some other taxa such as Heterotrissocla-dius marcidus, Microtendipes sp. and Zavrelimyia sp.appeared sporadically. Members of Procladius recordedin this zone probably belong to a different species thanthe specimens in the upper part of the sediment core.

ZONE-2 (267–198 cm, 10.1–7.8 cal. ka BP)The cold-adapted taxon H. grimshawi-type from theprevious zone persisted in lower densities at the be-ginning and later gradually disappeared. Procladius sp.formed only ca. 10% of the chironomid communityin Zone 2. Corynoneura scutellata-type became dom-inant (∼40%) at the bottom of this zone. The sec-ond most numerous taxon was H. marcidus (∼15%).Towards the top of the zone, the abundance of C.scutellata-type decreased to ca. 30% and H. marcidusincreased to ca. 25%. Other important components ofthe chironomid fauna in this zone were Microtendipessp. and Zavrelimyia sp., which were present in thewhole zone at more or less constant abundances ofca. 15 and 10%, respectively. Also, other temperate orwarm-adapted taxa were found, with abundances lessthan 5% (Parakiefferiella sp., Dicrotendipes sp., Crico-topus/Orthocladius). The appearance of Psectrocladiussordidellus-type was recorded only in this zone. Thetotal number of individuals per sample remained rela-tively high.

ZONE-3 (198–156 cm, 7.8–6.1 cal. ka BP)Cold–adapted taxa, such as M. insignilobus-type andH. grimshawi-type, disappeared entirely. The chirono-mid fauna was still dominated by H. marcidus andC. scutellata-type, with Microtendipes sp., Zavrelimyiasp. and Procladius sp. present at lower abundances(∼10%). The ratio of dominant taxa changed across thiszone- abundances of C. scutellata-type gradually de-

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Fig.1.

Fig. 1. A comparison of chironomid results with selected sedimentological parameters. The chironomid data are summarized in thefirst two axes of DCA. Axes scales are given in standard deviation units. The sum of head capsules is expressed as the number ofindividuals per gram of a wet weight of the sediment.

creased as H. marcidus gradually increased. Some taxa(Phaenopsectra sp. or Limnophyes/Paralimnophyes)occurred irregularly, and reached maximum abun-dances of almost 10%. With relatively stable abun-dances lower than 5%, P. psilopterus-type, Cladotany-tarsus sp. Dicrotendipes sp., and Cricotopus/Orthocla-dius were present. ZONE–3 is characterized by amarked decline in the total sum of chironomids.

ZONE-4 (156–84 cm, 6.1–3.1 cal. ka BP)Although Orthocladiinae dominated across the entiresediment profile, an increased proportion of Chironom-inae was found in this zone as a result of higherabundances ofMicrotendipes sp. and Phaenopsectra sp.Head capsules of H. marcidus and C. scutellata-typestill formed the main part of the chironomid assem-blages; nevertheless, a significant decline was recordedfor C. scutellata-type at the top of this zone, as wellas a high variability in the abundance of H. mar-cidus. Also, abundances of Zavrelimyia sp., Procladiussp. and Parakiefferiella sp. varied across this zone andthese taxa almost disappeared at the zone top, whereasdistinct increases of Limnophyes/Paralimnophyes andAblabesmyia appeared at the same time. Other taxa,

such as Psectrocladius psilopterus-type, Cladotanytar-sus sp. and Cricotopus/Orthocladius were present ir-regularly and in very low abundances. Dicrotendipessp. completely vanished in the second half of thezone, whereas Chironomus/Einfeldia appeared in no-table numbers. The total sum of individuals per sam-ple remained in the same low range as at the top of theprevious zone.

ZONE-5 (84–42 cm, 3.1–1.5 cal. ka BP)The ratio of the most abundant taxa again changed;C. scutellata-type varied between 10 and 30%, whereasH. marcidus gradually decreased down to less than 5%at the zone top. Zavrelimyia sp. and Procladius sp.,also abundant species, persisted in relative stable abun-dances across this zone, as did Microtendipes sp. andPhaenopsectra sp. However, these taxa were present inlower numbers than in the previous zone. A peak ofca. 20% was recorded for Limnophyes/Paralimnophyesin the middle of the zone, but its occurrence was ir-regular elsewhere, as was the presence of Parakief-feriella sp., Cladotanytarsus sp. and Dicrotendipes sp.In the second half of the zone, Ablabesmyia sp., Tany-tarsus lactescens-type, Psectrocladius psilopterus-type

Subfossil chironomids from Plešné Lake S405

Fig.2.ChironomidstratigraphyofPlešnéLake.Selectedtaxaaredisplayedaspercentagesofthetotalchironomidspersampleandarearrangedinorderofthepeakintheir

abundancesfrombottom-lefttotop-rightacrossthediagram.Thezonesdefinemajorchangesinthechironomidassemblagecomposition.

S406 J. Tátosová et al.

and Cricotopus/Orthocladius appeared. The total sumof individuals per sample remained in the same lowrange as in the previous zone.

ZONE-6 (42–0 cm, 1.5 cal. ka BP to ∼1950 AD)This zone is characterized by an event that happenedbetween ca. 18 and 10 cm (1540–1771 AD), when mostof the taxa totally disappeared. Only three taxa per-sisted, with highly variable abundances. Zavrelimyia sp.dominated (more than 40%) at the beginning of thisevent and then was replaced by Procladius sp. Tany-tarsus lactescens-type reached two peaks here, the firstless distinct and coinciding with an increase in Zavre-limyia sp. and the second more pronounced and associ-ated with higher abundances of Procladius sp.Before this event, at the 10–18 cm depth, the chi-

ronomids were represented by two types of assemblages.At the beginning of Zone 6 the fauna consisted mainlyof Microtendipes sp., Limnophyes/Paralimnophyes, Za-vrelimyia sp. and Procladius sp., whereas C. scutellata-type, Phaenopsectra sp. and Parakiefferiella sp. wereless abundant. This assemblage was accompanied by agroup of low abundant (∼5%) taxa such as Cladotany-tarsus sp., Dicrotendipes sp., Cricotopus/Orthocladius,Pagastiella sp. and Zalutschia sp. (first occurrence).Members of Tanytarsus sp. were missing. Immediatelypreceding the event at 10–18 cm depth, the domi-nant taxa were C. scutellata-type, Phaenopsectra sp.,Parakiefferiella sp. , Limnophyes/Paralimnophyes, andTanytarsus sp. The members of the accompanyinggroup vanished.After the 10–18 cm depth event, C. scutellata-

type,Microtendipes sp., Zavrelimyia sp., Procladius sp.,Parakiefferiella sp. and Chironomus sp. were all presentat ca. 10%; the other taxa such as Phaenopsectra sp.,Limnophyes/Paralimnophyes, Dicrotendipes sp., Crico-topus/Orthocladius and Tanytarsus sp. followed withabundances of 5%. In addition, H. marcidus again ap-peared in this community.

Discussion

Plešné Lake is unique among lakes of the BohemianForest. As analyses of the recent chironomid faunashow (Bitušík & Svitok, 2006), lakes located atlower altitude are inhabited by a more diverse chi-ronomid fauna than lakes at higher elevations (LakaLake, Prášilské Lake). Although Plešné Lake is amongthe higher situated lakes, its chironomid species rich-ness is relatively high. One reason is likely the dif-ferent geology of the lake catchment. Plešné Lake lieson granitic bedrock, while other lakes are situated onmica schist. According to Kopáček et al. (2006), thegranitic bedrock results in a different soil composition,and consequently can lead to higher terrestrial phos-phorus input into the lake. Mesotrophic conditions inthe lake are probably the reason for the higher chi-ronomid diversity as well as the co-existence of the

genus Chironomus, which is adapted to the low oxy-gen concentrations, with other taxa generally prefer-ring colder, well oxygenated conditions such as Zavre-limyia melanura/barbatipes, Heterotrissocladius mar-cidus, Psectrocladius sordidellus (Bitušík & Svitok,2006). The recent chironomid fauna consists mainly ofmembers of the Orthocladiinae, and this group domi-nated across the whole Holocene sediment core as well(Fig. 2). This suggests that the trophic status of thelake could have been oligotrophic to mesotrophic, butprobably never became eutrophic in its history, as isalso evident from the analysis of cladoceran remains(Pražáková et al., 2006). The changes in the chi-ronomid assemblages throughout the core took placemainly through changes in abundance of the followingtaxa: Corynoneura scutellata-type, H. marcidus,Micro-tendipes sp., Procladius sp. and Zavrelimyia sp. Larvaeof Procladius live in lakes with a range of trophic andtemperature conditions (Wiederholm, 1983; Lotteret al., 1998); nevertheless, the very high abundance ofthis genus at the beginning of the Holocene (ZONE-1) (Fig. 2) suggests that these specimens were coldstenothermous species of the Procladius genus. Thegenus Diamesa are typically rheophilous taxa and theiroccurrence in lake sediments is usually linked to anincreased amount of precipitation in the lake catch-ment, when the bodies of these taxa could be trans-ported into the lake by tributary waters (Ruck etal., 1998). Cricotopus/Orthocladius, C. scutellata-typeand Dicrotendipes are littoral taxa usually associatedwith aquatic macrophytes, which can provide evidenceof lake level fluctuation (Brooks, 2000). Taxa suchas Diamesa, Heterotrissocladius grimshawi and Mi-cropsectra insignilobus-type are typical cold adaptedtaxa (Säwedal, 1982; Sæther, 1975, 1979). Inaddition, H. marcidus or Zavrelimyia spp. can indi-cate colder well-oxygenated conditions (Fittkau, 1962;Sæther, 1975; Lotter et al., 1997) as the anal-yses of the recent chironomid fauna from the Bo-hemia Forest lakes has confirmed (Bitušík & Svitok,2006).Ablabesmyia, Parakiefferiella, Microtendipes andChironomus are considered to be temperate or warmadapted taxa, and particularly Microtendipes and Chi-ronomus indicate an increased trophic status of lakewaters. On the other hand, H. grimshawi, Micropsectrainsignilobus, Pagastiella and H. marcidus reflect gen-erally oligotrophic conditions in lakes (Wiederholm,1983; Brundin, 1949). Some taxa of the genus Psec-trocladius become abundant in acid lakes (Brodin, &Gransberg, 1993), so their presence can be connectedto changes in lake-water pH. Since P. sordidellus-typepresently inhabits Plešné Lake, in which the pH of sur-face lake water can fall below 5 during the brief pe-riod of the spring snow/ice melt, this species can beconsidered to be an acid-tolerant taxon. On the otherhand, this taxon was also found to be abundant in manylakes in the Alps with pH of about 8 (Lotter et al.,1998).

Subfossil chironomids from Plešné Lake S407

An environmental interpretation of major changes inthe chironomid stratigraphyIn order to assess compositional structure and taxaturnover throughout the profile, we applied DCA to thechironomid data. DCA produced two significant axesfrom which the sample scores were plotted on a timescale, with scaling in the original standard deviationunits (Fig. 1). As the results of DCA show, the chi-ronomid composition of Plešné Lake was relative stableduring the Holocene; nevertheless, two notable changeswere found. Firstly, in the period between ca. 10.4 and10.1 cal. ka BP, the highest scores on the first axis oc-curred. Next during the interval from 1540–1771 AD,the sample scores of both axes significantly decreased.At the Pleistocene/Holocene transition, four cli-

mate cooling oscillations were documented in the sedi-ment of Plešné Lake (11.2, ∼10.5, 10.2–10.1 and 9.6 cal.ka BP, respectively), and were clearly identified by de-creases of organic carbon concentrations (Pražákováet al., 2006). ZONE-1 (10.4–10.1 cal. ka BP) representsa ca. 300 yr-long period between the 2nd and 3rd Prebo-real cooling oscillations. As is known from analyses ofthe recent chironomid fauna, both the species compo-sition and low diversity recorded in this zone resemblethe chironomid fauna in cold and nutrient poor high-mountain and sub-arctic lakes (Bretschko, 1974;Aagaard, 1986; Cameron et al., 1997; Rierade-vall & Prat, 1999; Tátosová & Stuchlík, 2006).

The presence of entirely cold adapted taxa, such as Di-amesa sp., M. insignilobus-type, H. grimshawi, and thegenerally very low species diversity in ZONE–1 reflectthe cool climate conditions of this period. The highrate of sedimentation (Fig. 1) recorded at the start ofthis zone was probably the result of an input of al-lochthonous material from the lake catchment havingopen vegetation. A subsequent drop of the sedimen-tation rate by ∼50% (Fig. 1) was most likely causedby the afforestation of the originally treeless watershedby birch and pine forest (Jankovská, 2006), when theroots of trees slowed physical erosion in the steep catch-ment of Plešné Lake. An increased supply of organicmaterial, caused by the expansion of vegetation aroundthe lake, could have allowed the appearance of Chirono-mus at the border of ZONE-1 and ZONE-2. Althoughthis genus is considered to prefer warm conditions, lar-vae of Chironomus anthracinus-type are often presentin lakes after deglaciation (Brooks & Birks, 2000;Heiri et al., 2003; Larocque & Hall, 2004). Thistaxon is probably better adapted than Chironomusplumosus-type to tolerate cooler conditions (Brooks,2000).The following ZONE-2 (10.1–7.8 cal. ka BP) cov-

ers the period from the Preboreal / Boreal transitionto the Holocene climatic optimum. The Boreal (9–8ka BP, Svobodová et al., 2002) is characterized byforest development (Corylus and mixed oak woods)(Jankovská, 2006) which caused a continuing low sed-iment accumulation rate over this period. At the be-

ginning of this zone there are two cooling oscillations,as is reflected in the decline in organic carbon con-centrations (Fig. 1). These oscillations also correspondwell with the persisting cold-adapted H. grimshawi. Aswas mentioned above, H. marcidus and Zavrelimyia areable to tolerate colder conditions in lakes of the Bo-hemian Forest (Bitušík & Svitok, 2006), so their rel-atively high abundances may not preclude a cool cli-mate, though not as cold as is supposed during theprevious period, in which these taxa were missing. Inaddition, a distinct increase in the abundances of C.scutellata-type and a relatively high proportion of Mi-crotendipes sp. in the chironomid community indicatewarmer conditions during this period. As is evidentfrom the finding of Isoetes in the Boreal sediment ofPlešné Lake (Jankovská, 2006), the development ofmacrophytes in the littoral part of the lake probablytook place at this time. This corresponds well with thepresence of Corynoneura scutellata-type, Dicrotendipesand Cricotopus/Orthocladius in this period. Increasinghumic acid inputs during this period are suggested bythe brief appearance of Psectrocladius sordidellus-typein this zone.Subsequent significant decreases in abundances of

C. scutellata-type andMicrotendipes, and a marked in-crease of H. marcidus in the period between 7.8–6.1cal. ka BP may indicate increasing lake water level(Brooks, 2000) and input of allochthonous material(Warwick, 1989) because of very humid environmentduring the Late Atlantic (8–6 ka BP, Svobodová etal., 2002). The following period of the middle Holoceneis characterized by an overall decrease in abundancesof the two previously dominant taxa C. scutellata-typeand H. marcidus and an increased proportion of Mi-crotendipes sp. and Phaenopsectra sp. in the chirono-mid community. Furthermore, all present taxa showedhigh fluctuations in their abundances. The structureof the chironomid assemblages reflect well the unsta-ble conditions of the Epiatlantic epoch (6–4.5 ka BP,Svobodová et al., 2002), in which wet and dry pe-riods alternated and the climate was generally verywarm. The subsequent disappearance of C. scutellata-type, Dicrotendipes and Cricotopus/Orthocladius atthe end of this period corresponds with a consider-able decline in macrophyte pollen found in this partof sediment. As the pollen analyses also showed, thepopulation of the quillwort Isoetes, the only macro-phyte documented during the Holocene, were not abun-dant throughout the Holocene history of Plešné Lake(Jankovská, 2006). At present, it occurs only in theshallow part of the lake near the inlets, down to depthof 0.5–1 m. Therefore, it is possible to suppose thata decline of the lake level by more than 1–2 m dur-ing the very dry episode at the end of the Subboreal(4.5–3 ka BP, Svobodová et al., 2002) could haveled to the elimination of macrophytes as well as as-sociated chironomids. This could also have been thecause of the higher abundances of the semi-terrestrial

S408 J. Tátosová et al.

species Limnophyes/Paralimnophyes (Moog, 1995) inthe lake sediment. The increased proportion of Chirono-mini tribes in this period corresponds well to warm andhigher trophic lake water conditions.The period between 3.1–1.5 cal. ka BP is followed

by a gradual replacement of H. marcidus with Pro-cladius sp. in the profundal of the lake. One possibleexplanation could be that the persisting higher inputof nutrients and declining lake depth (even if only byabout 2 m) resulting from sedimentation, could haveled to anoxic or low oxygen conditions in the profun-dal part of Plešné Lake. These conditions would prob-ably have been unfavorable for larvae of H. marcidus,a taxon living in oligotrophic lakes. The continuous oc-currence of pollen of Plantago lanceolata and Rumexacetosella, indicating grazing, shows that the develop-ment of cultural fields and settlements may have in-creased the trophic status of the lake water during thisperiod (Jankovská, 2006). Also, the presence of otherwarm adapted, mesotrophic-preferring taxa, such asDi-crotendipes, Cladotanytarsus or Ablabesmyia could con-firm the mesotrophic status of Plešné Lake during thisperiod. On the other hand, the reason for the decreasedabundances of Microtendipes sp. or Phaenopsectra sp.compared to the previous zone is uncertain.The structure of the chironomid assemblage in the

uppermost part of the sediment core showed distinctchanges. The most marked changes took place in theperiod between 1540 and 1771 AD (18–10 cm) whenmost taxa entirely vanished and only Zavrelimyia sp.and Procladius sp. were alternately present, accompa-nied by Tanytarsus sp. This event is in agreement withthe dating of the Little Ice Age, and could be the re-sult of low oxygen concentrations at the lake bottomcaused by longer winter ice cover (Lindegaard, 1995;Heiri & Lotter, 2003). However, these sediment lay-ers also coincide with an episode of very low chironomidabundances (Figs 1, 2), which makes the fossil chirono-mid record less reliable. Moreover, samples with a lownumber of specimens usually tend to have lower speciesdiversity. It is likely, therefore, that this decrease in chi-ronomid abundances is at least partly related to the lowcount sums rather than to cool summer temperatures.On the other hand, the structure of chironomid as-

semblages recorded before the above-mentioned periodwas much more varied (layers 33–27 cm, 938–1212 AD).The well developed littoral community with the dom-inant taxon Microtendipes sp. accompanied by Dicro-tendipes sp., Corynoneura scutellata-type and Cricoto-pus/Orthocladius associated with macrophytes, infers awarmer climate such as has been described in the Mid-dle Ages. Also, the occurrence of Chironomus/Einfeldiain the profundal zone of Plešné Lake confirms thewarmer conditions in the lake and probably low oxygenconcentrations at the bottom. The most likely reasonfor this increase in lake water trophy was the heavy hu-man impact on the forest structure, and agriculture andlandscape exploitation of the lake surroundings, as has

been documented in pollen analyses by Jankovská,(2006). The very high sedimentation rate in the layers10 cm and up (ca 18th century) indicates high physicalerosion resulting from deforestation of the lake catch-ment.Plešné Lake, as well as many lakes in C and N Eu-

rope, was affected by atmospheric acidification at the1970s and 1980s of the 20th century (Fott et al., 1987,1994; Kopáček et al., 1998). Although the resolutionof the sediment core is relatively low for detailed analy-sis of the effects of acidification, a distinct change in thechironomid community was recorded in the upper layerof 3–0 cm (1956–1990 AD). Most of the taxa presentin the previous layer disappeared and the chironomidfauna has generally remained poor. Again, the prob-lem of a low chironomid concentration and accompa-nying lower species diversity in this part of sedimentrecords could explain this trend, but nevertheless acid-ification is likely to be at least partially responsible.The strong effects of acidification on the biological com-munity are well apparent from analyses of cladoceranremains (Pražáková et al., 2006).

Conclusions

The chironomid composition throughout the core wasrelatively stable during the existence of Plešné Lake,mainly reflecting crucial changes in the Holocene cli-mate. As shown by the DCA analyses, the species com-position of early Holocene sediment samples was dis-tinctly different from younger samples; the presenceof only oligotrophic and cold-adapted taxa in this as-semblage clearly reflects a cool climate oscillation ofthe Preboreal period. The variations in the chirono-mid fauna after the Preboreal period were mainly rep-resented by changes in abundances of dominant taxarather than by changes in species composition. Oscilla-tions in the lake level and changes in the occurrenceof macrophytes in the littoral, both connected withclimate changes, namely temperature and amount ofrainfall, provide possible explanations for variances inthe chironomid fauna. Another reason for chironomidchanges was most likely the development of vegetationin the lake catchment, resulting in a distinct declinein erosion and sedimentation at the Preboreal/Borealtransition. Later, the increasingly dense afforestationled to a considerable input of organic material into thelake and a subsequent increase in the trophic status ofthe lake water, which is evidenced by a significantlyhigher proportion of Chironomini in the chironomidfauna over the period of the Middle Holocene. Thesemesotrophic conditions have persisted to the present,as is evident from the upper sediment layers whereHeterotrissocladius marcidus was replaced by Chirono-mus/Einfeldia in the profundal community. This grad-ual progression of the chironomid fauna was interruptedby an event in the period 1540–1771 AD when the ma-jority of taxa entirely disappeared. This could be re-

Subfossil chironomids from Plešné Lake S409

lated to the cool conditions connected with Little IceAge dated in this period, though it is also possibly dueto the very low count sums recorded in this part of thesediment.

Acknowledgements

We would like to express our special thank to R. SCHMIDT,R. PSENNER and R. NIEDERREITER for sediment cor-ing and also J. KOPÁČEK for providing needful data onchemical analyses of the sediment. We thank to prof. P.BITUŠÍK for discussion on taxonomic problems and anony-mous reviewers for helpful and suggestive comments on themanuscript. Thanks also go to D.W. HARDEKOPF for lin-guistic correction of the manuscript. This study was sup-ported by the Enviroment Project of European Commis-sion EURO-LIMPACS (GOCE-CT-2003-505540) and partlyGrant Agency of Czech Republic project No. 206/03/1583(Nutrient cycling in the nitrogen-saturated mountain forestecosystem: history, present, and future of water, soil, andNorway spruce forest status).

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Received July 20, 2006Accepted November 15, 2006