Seasonal Dynamics of Chironomids in the Profundal Zone of a Mountain Lake (Ľadové Pleso, the Tatra Mountains, Slovakia)

Biologia, Bratislava, 61/Suppl. 18: S203—S212, 2006 Section Zoology DOI: 10.2478/s11756-006-0132-7

Seasonal dynamics of chironomids in the profundal zone of a mountain lake (Ľadové pleso, the Tatra Mountains, Slovakia)

Jolana Tátosová 1 & Evžen Stuchlík2

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

Abstract: The profundal community of Ľadové pleso (an oligotrophic high mountain seepage lake at an altitude of 2,057 m with a max. depth of 18 m and an ice-cover period from October – July) was studied from December 2000 – October 2001. Chironomidae, the most signiﬁcant part of the studied community, are represented by four taxa and dominated by Micropsectra radialis Goetghebuer, 1939 and Pseudodiamesa nivosa (Goethgebuer, 1928). These two species showed a 1-year life cycle. The total densities of chironomids varied from 0 to 5,927 ind. m−2; no chironomids, or very low densities, were found during the winter/spring period, probably due to low oxygen concentrations in the medial part of the lake. These low oxygen concentrations probably caused the relocation of larvae from the medial part of the sedimentary area at the same time. Key words: Non-biting midges, Chironomidae, life history, distribution, migration, environmental parameters, Slovakia.

Introduction conditions of the mountain climate (Armitage et al., 1995). Ľadové pleso was chosen as the key lake in the High mountain glacial lakes represent a very special en- Tatra Mts for the Fifth Framework Program of Euro- vironment for water organisms because of their low av- pean Union: project EMERGE, which made possible erage annual temperature, oligotrophic character and systematic investigations of biota life cycles and sea- the minor impact of human activities. These special sonal variations in lake water chemistry. properties aroused interest in lakes in the High Tatra This paper summarizes results of the first complete Mountains (Mts), although the accessibility of lakes was round-year study of chironomids in the profundal zone difficult, which especially complicated the investigation of an oligotrophic high mountain Tatra lake. The main of the profundal sediments. The first investigation of aim of the presented study is to describe the population profudnal fauna was carried out in the 1930s by Hrabě dynamics of chironomids in Ľadové pleso in relation to and Zavřel. In contrast to lowland lakes or ponds, the environmental factors and phytoplankton production. fauna of the deepest part of high mountain lakes was very poor and was usually formed only by oligochaetes Study site and the larvae of chironomids (Hrabě, 1939, 1942; Za- vřel, 1937). ◦ ◦ Later, the study of chironomids was con- Ľadové pleso (49 18 41 N, 20 16 29 E) is located in the nected with research of trophic status changes in some Veľká Studená dolina valley on the southern slope of the Tatra lakes (Ertlová, 1964), and since the 1980s the High Tatra Mts at 2,057 m a.s.l. The lake area is 1.72 ha, chironomid fauna has been studied mainly with an em- catchment area 12.3 ha, and maximum depth 18 m. Granite phasis on the process of acidification (Ertlová, 1987; dominates in the catchment, and bare rocks cover 85% of Tátosová, 2002; Bitušík et al., 2006). The sampling its area (KOPÁČEK et al., 2006). The lake has no visible of chironomid larvae is often an important part of sys- inflow or outflow, and the lake water level oscillates in-depth tematic limnological research because of their very sen- by more than 5 m during the year because of its seepage character (TUREK, 2002; KŘEČEK et al., 2006). Majority of sitive reaction to the amount and quality of available the lake bottom consists of rocks, and fine-grained sediment food, as well as temperature, concentration of dissolved is localized in the deepest part of the lake (Fig. 1). There oxygen, and pH (Sæther, 1979; Raddum & Sæther, arenofishinthelake. 1981). Not only their abundances or taxonomic compo- Ľadové pleso is situated at high elevation, which influ- sition, but also their life history, can reflect inclement ences the duration of ice-cover and average annual tempera-

c 2006 Institute of Zoology, Slovak Academy of Sciences S204 J. Tátosová & E. Stuchlík

Fig. 1. Bathymetry of Ľadové pleso. The stars indicate the coring sites and the arrows show the shift of the sample sites during the investigated period. The circles ⊗ indicate the positions of the emergence traps. The triangle marks the place of the sedimentary traps location. ture. Despite its location and generally oligotrophic charac- based on size groups formed from the capsule width and ter, this lake is one of the most productive lakes in the High length measurements (Tab. 1). Tatra Mts, probably due to its seepage character (FOTT Six emergence traps were installed above different lake et al., 1987). This lake remained non-acidified during the depths (Fig. 1). Traps with fixing solution could not be used peak of acidification in this area (STUCHLÍK et al., 1985; in Ľadové pleso because of concurrent analyses of organic FOTT et al., 1994; KOPÁČEK et al., 2000); nevertheless, pollutants in the lake water. The “live” emergence traps a temporary and partial acidification of the upper part of used instead require daily control that was not possible at the water column (to a depth of ∼5 m) has been repeatedly this site, therefore the time of the trap exposition varied recorded at the end of the snow/ ice melting period, when and the results are not suitable for the inference of chirono- pH dropped below 6 in this part of the lake water volume mid biomass production. These installed traps were used (DARGOCKÁ et al., 1997; KNESLOVÁ et al., 1997; TUREK, with the aim to obtain chironomid imagoes for more reli- 2002). able identification. Vertical stratification of physical and chemical parameters (temperature, pH and dissolved oxygen) was measured Methods in situ by a Hydrolab H2O multi-parameter probe and data logger Surveyor 3, (Hydrolab, USA) in 2 week intervals. Ver- Three sampling stations were chosen in the profundal zone tical samples for analyses of chlorophyll-a and total volume at depths varying from 15 to 18 m. Sites A and C were of seston were taken 9 times from September 2000 to Octo- situated at the edges of the sedimentary area, site B in the ber 2001, and during the winter period surface and bottom middle of this area (Fig. 1). This location of sample sites was samples were also taken on the following dates: 15 March, chosen so that the spatial distribution of the chironomid lar- 6 April, 11 May and 20 June. The water samples for de- vae would be recorded. The sample sites were moved slightly termination of chlorophyll-a were filtered through What- in a clockwise direction at each sampling in order not to man GF/C glass fiber filters, and after hot extraction in take samples from the same places and to obtain samples a 5 : 1 mixture of acetone : methanol (PECHAR, 1987) from the whole sedimentary area. Sediment was obtained analyzed fluorometrically on a Turner TD-700 (Turner, by a Kajak corer with a sampling area of 28 cm2. Four core USA). For more details of the procedure see FOTT et al. samples were taken at each site, seven times in the period (1999). Samples were analyzed for total volume of seston 3 −1 from December 2000 to October 2001. In total, 84 samples (TVP3.3-16800,mm L ) by filtration through a 40 µm were taken and processed; each sample was sieved through mesh and determination with a Coulter Counter model ZB a 100 µm polypropylene mesh in the shape of a plankton with a tube of 70 µmaperturesize(DARGOCKÁ et al., 1997). net (DAVIS, 1984) and stored in 4% formalin. Animals were The amount of particulate matter accumulated at the sorted by hand in the laboratory, and head capsules were lake bottom was taken using a sediment trap, which was photographed and measured using LUCIA software (Olym- suspended at a depth of 13 m (Fig. 1). The trap was formed pus C&S). They were then divided into four instar groups by four 50 cm long tubes with a diameter of 6 cm. Durations Seasonal dynamics of chironomids in Ľadové pleso S205

Table 1. Measured parameters of larval head capsules of M. radialis and P. nivosa.

Width (µm) Length (µm)

n Mean Min Max SE n Mean Min Max SE

M. radialis Instar: 1 5 79.26 64.28 96.06 11.83 6 83.36 74.74 93.25 7.50 2 50 131.75 104.65 154.48 9.80 49 134.43 108.32 150.73 11.71 3 84 208.27 163.15 240.94 15.17 87 214.10 174.48 242.03 13.56 4 122 318.51 251.64 376.91 21.20 124 327.51 260.85 388.94 24.20

P. nivosa Instar: 1 1 181.66 181.66 181.66 1 182.68 182.68 182.68 2 6 282.40 261.48 301.08 16.67 6 306.19 293.73 329.58 12.86 3 8 492.33 429.24 551.15 44.32 9 526.36 451.03 589.73 45.39 4 14 765.56 668.86 859.74 48.63 14 903.04 760.38 1100.42 82.78

Key: n – number of measurements; Max – maximum, Min – minimum, SE – standard error.

Table 2. Time intervals of the sedimentary traps exposure. 0

Start of End of Trap Duration of -3 an exposure an exposure depth an exposure (days) -6 8.12.2000 14.2.2001 13 68 15.2.2001 23.5.2001 13 97 -9 24.5.2001 30.6.2001 13 37 3.7.2001 2.8.2001 13 30 -12 3.8.2001 29.8.2001 13 26 -15 31.8.2001 27.9.2001 13 27 30.9.2001 26.10.2001 13 26 -18 O N D J F M A M J J A S O

of exposure are summarized in Table 2. TPV was analyzed 0 from this material by the method described above. -3

Results -6

-9

Physical parameters and food supply of Ľadové pleso -12 Ľadové pleso is a dimictic lake with a long period of winter ice cover and a short period of summer strat- -15 ification (Fig. 2). The study period began during the -18 autumn circulation (about 24.10.2000), that lasted 14 O N D J F M A M J J A S O days. Winter stratification followed with a duration of Fig. 2. Contour diagrams of the temperature (upper panel, ◦C) 245 days; a stable ice cover was created in early Decem- and the concentrations of dissolved oxygen (lower panel, mg L−1) ber and lasted 214 days, with a maximal thickness of in Ľadové pleso during the years 2000–2001. Source: Hydrolab. 270 cm in the spring. Ice melting started in the littoral part of the lake in the middle of May, and the final disappearance of ice from the lake surface took place at the tom, these values were reached during the autumn and beginning of July. The following spring circulation pro- spring circulations and summer stratification, but the ceeded for 13 days and then the summer stratification concentrations were much lower during the winter strat- developed at the beginning of August (47 days dura- ification: 0.11–6 mg L−1,withthemaximuminJanuary tion); the maximum summer surface temperature was (Fig. 2). 13.6 ◦C in the lake littoral. In the middle of September The annual value of pH varied mostly from 6.6 to the homometry (3.9 ◦C) was already recorded. The tem- 7.0 in the whole water column. The minimal pH of 5.4– perature profiles of Tatra lakes were studied in detail 5.8 was measured in interval from the middle of May to by Šporka et al. (2006). the beginning of June and reached down to the depth The amount of dissolved oxygen did not decrease of 4 m. This episodic acidification of the upper layers below 10 mg L−1 to the 12 m depth during the study pe- was caused by melting of the winter snow/ ice cover. riod and the maximum concentration was 14.5 mg L−1 A maximum value of 8 was first recorded at the depth at a depth of 10 m in December. Closer to the bot- of 10–12 m in December, and a second more prolonged S206 J. Tátosová & E. Stuchlík

IX X XI XII I II III IV V VI VII VIII IX X XI IX X XI XII I II III IV V VI VII VIII IX X XI 20 7000 3,0 7000 Chlorophyll-a (0 m) TPV (0 m) Chlorophyll-a (5 m) TPV (5 m) 6000 6000 Chlorophyll-a (8 m) 2,5 TPV (8 m) Chlorophyll-a (bottom) TPV (bottom) ] ] ] 15 -2 -2 -1 Chironomidae 5000 Chironomidae 5000

] 2,0

-1

4000 . L 4000 3 10 1,5 3000 3000

TPV [ mmTPV [ 1,0 2000 2000 Chlorophyll- a [ µg. L Chironomidae [ ind.m 5 m ind. [ Chironomidae

0,5 1000 1000

0 0 0,0 0 IX X XI XII I II III IV V VI VII VIII IX X XI IX X XI XII I II III IV V VI VII VIII IX X XI Date Date

Fig. 3. Seasonal and vertical variability of the concentrations of chlorophyll-a and total volume of particles (TVP) in relation to seasonal dynamics of chironomid density. Horizontal black and gray bars denote durations of the compact ice cover (black) and melting period (gray). maximum was found at the same depth in the middle 60 of August. TPV 13 m We used the concentration of chlorophyll-a and to- 50 tal volume of seston (TVP) for an expression of the 40 amount of available food in the lake. Concentrations of chlorophyll-a in the water column generally fluctuated 30 between 0 and 5.5 µgL−1, although an extreme peak µ −1 of 18.6 gL was found in December (Fig. 3.). A sec- 20 ond much lower peak was recorded in early July and in TPV [mm-3.m-2.day-1] TPV early August in the deeper layers of the water column. 10 The lowest values of 0–1.8 µgL−1 were observed during the period of winter stratification. For more details 0 see Nedbalová et al. (2006). The amount of seston ex- pressed as the total volume of particles (TVP) oscillated 3 −1 X XI XII I II III IV V VI VII VIII IX X XI XII between 0.2–1.2 mm L in the whole water profile of Date Ľadové pleso, but the same December extremely high peak of 2.6 mm3 L−1 was recoded at the depth of 8 m Fig. 4. Variations of the amount of TPV accumulated in sediment and a second lower one just under the water surface at trap at depth 13 m during the single part of the observed season. For more details about time intervals of the exposure see Table 2. the end of June. The lowest amount of particles was found during the winter ice cover period (Fig. 3). A sedimentary rate of TPVcalculatedfromthe amount of a material captured in the sediment trap Chironomid fauna is displayed in Fig. 4. In spite of the December peak In total, four chironomid taxa were identified in the recorded at the 8 m depth, no particles were accumu- quantitative samples. Micropsectra radialis Goetghe- lated in the depth of 13 m over the period December– buer, 1939 dominated the whole year, whereas larvae February, and in addition, the rest of the winter season of Pseudodiamesa nivosa (Goetghebuer, 1928) were less was followed by a very low accumulation of TPV (0.2 abundant overall and were absent in the April and mm3 m−2 day−1) (Fig. 4). A small increase of TPV May samples. Larvae of Procladius (Holotanypus)sp. sedimentation (6 mm3 m−2 day−1) was first recorded were observed in very low densities of 89 ind. m−2 at at the end of winter stratification and the highest val- the beginning of August and at the end of September ues were reached during the spring circulation and 2001. Heterotrissocladius marcidus (Walker, 1856) was the summer stratification (July – early September) (55 recorded only once in December 2000, with a density of and 41 mm3 m−2 day−1, respectively). During the au- 531 ind. m−2 (Fig. 5). tumn overturn, the amount of accumulated material de- We obtained 14 chironomid adults, 8 pupae and 12 creased by half values. pupal exuviae of M. radialis and 1 pupal exuvia of P. nivosa from emergence traps (Tab. 3). The average density during the sampling period Seasonal dynamics of chironomids in Ľadové pleso S207

7000 Micropsetra radialis 7000 6000 Pseudodiamesa nivosa Sampling site A Procladius sp. 6000 Sampling site B Heterotrissocladius marcidus Sampling site C 5000 5000 2

- 4000

2 4000 -

3000 ind. m 3000 ind m

2000 2000

1000 1000

0 0 XI XII I II III IV V VI VII VIII IX X XI XI XII I II III IV V VI VII VIII IX X XI

Date Date

Fig. 5. Changes in the species composition of chironomids in Ľadové pleso during the investigated year (left) and spatial distribution of larvae in the sedimentary area (right). Sites A and C were situated at the edge of the sedimentary area, site B in the middle of this area.

Table 3. Catches of emergence traps during the summer period.

Date of trap exposure (starting – ﬁnal day)

No. of traps Depth (m) 1.–2.VII. 2.–3.VIII. 4.–13.VIII. 13.–29.VIII. 19.–29.IX.

1100M. radialis (2PE,1P,1M,2F) 0 0 0 2170M. radialis (2PE,1P,2M,1F) 0 M. radialis (3 PE, 4 F) 0 3180P. nivosa (1 PE), M. radialis (1 P) 0 M. radialis (2 PE) 0 4 ∼12 0 M. radialis (3 P, 1 M) 0 0 0 590 M. radialis (1 PE) 0 M. radialis (2 PE, 1 M) 0 690M. radialis (2 P, 2 F) 0 0 0

Key: PE – pupal exuviae; P – pupae; M – male; F – female. Depth – lake depth above that the traps were installed. was 1,470 ind. m−2. At the beginning of the win- species M. radialis was present. This species reached ter stratification in December 2000 the second high- an average density of 1,243 ind. m−2 and was the most est amount of chironomid larvae was collected (2,477 abundant chironomid species in the lake. A total of 270 ind. m−2); however, no larvae or very low densities of individuals of M. radialis were measured and used for 30–60 ind. m−2 occurred during the rest of the winter the analysis of larval instars. Instar analysis (Fig. 6) period (Fig. 3). The abundance increased during the suggests that there is one generation per year with spring circulation (650 ind. m−2), doubled during sum- emergence in August (Tab. 3). According to this hy- mer stratification, and reached a maximum of 5,927 ind. pothesis, eggs from adults emerging in August proba- m−2 at the beginning of autumn circulation. bly hatched over September and reached the 3rd and The spatial distribution of larvae also varied dur- 4th instars before winter, as evidenced by the presence ing the year; in the time of autumn circulations in De- of 3rd and 4th instars in December 2000. Growth contin- cember 2000 and October 2001 chironomid larvae were ued during the winter and spring, since only 4th instar concentrated in the central part of the sediment area, larvae were found in April and May. The presence of 1st, whereas during the summer stratification in August 2nd and 3rd instars at the end of August and in Septem- and September higher densities were found in marginal ber 2001 supports the hypothesis of August emergence parts of this area (Fig. 5). Several larvae were even ob- for this species. served in the sediment traps at a depth of 13 m in April. We observed also swimming larvae of M. radialis near the water surface under the ice in April – May. Chironomid life history They appeared a few minutes after we removed snow Life dynamics could be inferred for the two most abun- cover from the sampling site and stayed there for ap- dant taxa: Micropsectra radialis and Pseudodiamesa proximately one hour. nivosa (Fig. 6.). The younger instars of larvae did not The second most numerous species (an average allow a thorough determination of Micropsectra to the density of 126 ind. m−2) for which we inferred life species level, but identification of emerged male and fe- dynamics is Pseudodiamesa nivosa. We sampled and male adults and pupal exuviae suggest that only the measured only 30 individuals in total, therefore the re- S208 J. Tátosová & E. Stuchlík

Micropsectra radialis the beginning of December, when a 30 cm thick layer of clear ice was created, which allowed the development Ice Ice Ice of phytoplankton in the lake. The maximum concentra- 2.- 3.VIII cover break free 13.- 29.VIII tion of chlorophyll-a at this time was at a depth of 8 m, possibly due to the high intensity of solar radiation. 4 This increase of the phytoplankton amount was respon- 3 sible for a December high peak of TPV at the same depth. The second peak of chlorophyll-a concentration

Instars 2

1 in the summer was much lower. This observation may be explained by diﬀerent species composition of phyto-

X XI XII I II III IV V VI VII VIIIb VIIIe IX plankton and diﬀerent speciﬁc chlorophyll-a content in % the phytoplankton cells as a reaction to actual under- water light conditions (Nedbalová et al., (2006). The other higher value of TPV found in the surface sam- Pseudodiamesa nivosa ple at the end of June was connected with the melting Ice Ice Ice of the ice-cover, when a high amount of allochthonous cover break free material from the ice and the snow entered the lake.

2.- 3.VIII During sedimentation, this material is continuously de- 4 composed, which is probably the reason for the lower

3 amount of TPV in deeper parts of the lake at the same time. Instars 2 Analyses of chlorophyll-a and TPV in vertical sam- 1 ples mainly provide current information on particulate matter in the water column. Conversely, data from the X XI XII I II III IV V VI VII VIIIb IX VIIIe sediment traps gives us much more information on the % long-term food supply for benthic animals, because the short-term increases of TPV recorded in the water col- Fig. 6. Instar analyses of Micropsectra radialis (upper panel) and Pseudodiamesa nivosa (lower panel). Horizontal black and white umn can be followed by a longer period of very low sedi- bars denote different generations. Black squares with arrows show mentation, and on a long-term scale the food supply can observed emergences (see Tab. 3), white square denotes supposed be low overall. This is one possible reason why a very time of emergence. Sampling months are underlined. (b) – the low amount of particles accumulated during the winter beginning of month, (e) – the end of month. season in spite of the December peak of TPV recorded at 8 m. The increase of available food for chironomids is connected with the increased input of allochthonous sults of instar analysis provide only a rough estimate material into the ice-free lake and its transport to the of life history due to the low numbers of individuals. bottom due to spring circulation, and with the devel- Emergence probably took place after the ice break in opment of phytoplankton during the summer season. July, since 1st instar larvae were observed at the turn of July/August and 2nd and 3rd instar larvae at the end of Chironomid fauna August (Fig. 6). Larvae reached the 4th instar proba- The occurrence of Micropsectra radialis,whichcom- bly before winter as evidenced by presence of only these posed the major part of the profundal fauna in this larvae in December 2000 and September 2001. lake, is always restricted to cold oligotrophic lakes, Only a few specimens of Procladius sp. and Het- where larvae inhabit both the littoral and profundal erotrissocladius marcidus were found in the profundal zones (Sawedal¨ , 1982); therefore, the dominance of zone of Ľadové pleso, and this low number of individu- this species is not unexpected. The second most abun- als did not allow us to infer their life cycle in this lake. dant chironomid species Pseudodiamesa nivosa is also considered to be an oligostenothermic species (Serra- Discussion Tosio, 1973) and is typical for ultraoligotrophic and oligotrophic lakes (Sæther, 1979). In general, the lar- Food supply vae of Procladius often inhabit standing waters and The winter peak of chlorophyll-a concentration found in they are also common in Tatra lakes (Hrabě, 1939, Ľadové pleso is not unusual in high mountain lakes. In 1942; Gowin & Zavřel, 1944). Although the larvae the High Tatra Mts, the phytoplankton and concentra- of this species did not allow the determination to the tions of chlorophyll-a were studied in three alpine lakes species level, Bitušík (2004) identified the pupal exu- by Fott et al. (1999). They found high chlorophyll-a viae of just two species P. choreus (Meigen, 1804) and concentrations during the ice-cover period as a result P. tatrensis (Gowin, 1944) in Tatra lakes. It can be of sufficient solar radiation penetrating the snowless ice assumed that larvae found in Ľadové pleso are Pro- cover. We observed these conditions at Ľadové pleso at cladius tatrensis, which was described by Gowin & Seasonal dynamics of chironomids in Ľadové pleso S209

Zavřel (1944), who found pupae and imagoes only Seasonal variations from Tatra lakes situated above the tree line. The last The variability of chironomid abundance was consider- recorded species, Heterotrissocladius marcidus,isthe able during the study period (from 0 to 5,927 ind. m−2), least cold stenothermic member of this genus, but it with the lowest densities recorded within the period is still restricted to relatively cold waters (Sæther, of the winter stratification (November – the beginning 1975) and together with Procladius are the most com- of July), when the concentration of dissolved oxygen mon taxa in the High Tatra Mts (Zavřel, 1937), even as well as the supply of available food were very low though its larvae occur in very low densities (Hrabě, (in spite of the high concentrations of chlorophyll-a at 1939). Paleolimnological studies of Tatra lakes support the early winter stratification in December). These win- that H. marcidus is a stable but not numerous compo- ter minima of both parameters are in close relation- nent of the chironomid fauna (Stuchlík et al., 2002, ship. As mentioned above, the high concentrations of Šporka et al., 2002; Kubovčík et al., 2003). chlorophyll-a recorded in December (18.6 µgL−1)are Chironomid taxa known from the profundal part commoninmountainlakes(Fott et al., 1999) and of Ľadové pleso are also found in other high mountain they occur when compact ice cover without snow is lakes in Europe. For example, the dominant species in formed, which transmits enough light. The sedimenta- Ľadové pleso Micropsectra radialis was the only species tion and subsequent decomposition of high amounts of found in the profundal part of high mountain Lago di phytoplankton can then cause a decline in the oxygen Latte Lake in the Alps (Cameron et al., 1997). M. radi- content at the bottom of oligotrophic lakes. This low alis together with Heterotrissocladius marcidus,formed winter oxygen concentration probably caused the mi- the profundal chironomid assemblage in Lake Redo in gration of larvae from the sediment to the upper layer the Pyrenees, and together with Corynoneura arctica of the water column, as evidenced by the observation were the only species in the deepest part of Lake La of swimming larvae, and relocation of larvae from the Caldera (at 3,050 m a.s.l.) in the Sierra Nevada Mts. sedimentary area to the upper part of the lake bottom. (SE Spain) (Rieradevall & Prat, 1999). Similar chi- Combined, these effects are probably the reason such ronomid compositions are found in deep high mountain low winter densities of chironomids were found in this lakes above the tree line in Austria (Bretschko, 1974). lake. This migrational behavior of chironomid larvae Not only a similar species composition but also low di- is one of many adaptations to low oxygen conditions versity in general is known for high mountain and sub- (Heinis & Crommentuijn, 1992). Such behavior of arctic lakes. Low numbers of chironomid taxa as were chironomids in a lowland Spanish lake has been pub- recorded in Ľadové pleso were also found in the profun- lished by Prat & Rieradevall (1995), but it has not dal of several north Norwegian lakes (Aagaard, 1986); been described in a mountain lake before. for example, in the similarly deep lakes (about 20 m) At the end of the winter stratification the increased Austerdalsvatn and Haukvatn, Heterotrissocladius sub- amount of the seston coming from the melting ice cover pilosus was the only species recorded in the profun- entered the lake and the dissolved oxygen concentra- dal part. Only Procladius sagittalis was collected in the tions at the lake bottom increased during the follow- Pyrenean Aguilo Lake (Cameron et al., 1997). Both ing spring circulation. Chironomid fauna responded to the species composition and low diversity reflect the these events with a slight increase in their abundance specific nature of the altitude and latitude of extremely from 59 to 649 ind. m−2 at the beginning of August. located lakes. This rise of abundance probably occurred due to re- The abundance of chironomid larvae usually does versed migration of chironomids to the sedimentary not reach very high values in high mountain lakes area as evidenced by the presence of only overwintering mainly due to the low productivity of these lakes. The 4th instars of the dominant species Micropsectra radi- average chironomid abundance of 1,470 ind. m−2 in alis. Also, the spatial chironomid distribution showed the profundal zone of Ľadové pleso is similar to that higher abundances in the marginal part of the sedimen- recorded by Brundin (1956) in the arctic ultraolig- tary area then in the central part during this time pe- otrophic Lake Kattejaure in northern Sweden, and Lin- riod. To confirm this migrational hypothesis, a detailed degaard & Mæhl (1992) found the same density of study would be necessary with the possibility to take 1,400 ind. m−2 in the profundal zone of arctic Lake samples from the sublittoral part of the lake bottom, 95 in South Greenland. In the profundal part of the which is composed of large boulders. mentioned north Norwegian lakes, the total number of The stable higher concentrations of chlorophyll-a, chironomids didn’t exceed 1,000 ind. m−2 (Aagaard, TPV and dissolved oxygen and good temperature con- 1986). Similarly, high mountain lakes in other parts of ditions following the period of summer stratification Central Europe have shown chironomid densities of this established suitable conditions for the development of magnitude. For example Steinbock¨ (1955) recorded the chironomid fauna that reached the maximum abun- from 300 to 2,800 ind. m−2 in eight Austrian moun- dance (5,927 ind. m−2) at the end of September, when tain lakes situated from 2,000 to 2,800 m a.s.l., and the chironomid populations were composed of individ- Bretschko (1974) found 2,100 ind. m−2 of chirono- uals of the new generation. mids in the Vorderer Finstertaler See (2,237 m a.s.l.). Also, the presence of single species changed dur- S210 J. Tátosová & E. Stuchlík ing the investigated season. As mentioned above, larvae gion need more than one year to complete their devel- of M. radialis are typical dwellers of cold oligotrophic opment (Armitage et al., 1995). For instance, Welch lakes, where they inhabit both the littoral and profun- (1976) found a long life cycle of 3 years for Heterotris- dal zones (Sawedal¨ , 1982). Therefore, their stable oc- socladius oliveri in the high arctic Lake Char. In sub- currence in the profundal part of Ľadové pleso during arctic Lake Thingvallavatn in Greenland, Lindegaard the whole year was expected. The species Pseudodi- (1992) assumed a 2-year life cycle for Chironomus is- amesa nivosa is also a typical inhabitant of ultraoligo- landicus,whose4th instar larvae of both younger and trophic and oligotrophic lakes, has been often found in older generations were distinguished by their average the littoral part of Tatra lakes (Hrabě, 1939, 1942; larval weight. Even though we didn’t weigh collected Ertlová, 1987); we also recorded them in abundance animals to confirm or disprove either the bivoltine or in the littoral of Ľadové pleso. The absence of these semivoltine life cycle of Micropsectra, that only 4th in- larvae in the profundal zone during winter was presum- star larvae of M. radialis were found in the ice-cover ably a result of the reaction to worsened oxygen con- period and that there was only the one August period ditions as well as to insufficient food supply. P. nivosa of the M. radialis emergence suggest that there was only as a predator usually preys on smaller chironomids and one generation of this species. In addition, the ice free other small organisms. As the littoral part of lakes is period of Ľadové pleso lasts 5–6 months, which means typically colonized by invertebrates more than the deep a relatively long growth season for chironomids every profundal zone, it is possible to assume that larvae of year. P. nivosa migrated from the profundal zone to the lit- Because of the disappearance of Pseudodiamesa toral during the winter period, where they stayed until nivosa larvae from the profundal part of the lake we the time of their emergence in July. This could also be have no information about their winter development. one reason why we found so few specimens of the 1st We assume that P. nivosa migrated from the profun- instar in the profundal at the beginning of August. dal zone to the littoral due to worsening life conditions, Even though the larvae of Procladius are common where they probably stayed until the time of their emer- in Tatra lakes (Zavřel, 1937), very low densities and gence in July. We found a few specimens of the 1st in- only the sporadic presence of this species were recorded star in the profundal at the beginning of August, which in Ľadové pleso. There is one possible explanation: as suggests that a majority of the population lived in the Brooks & Birks (2001) published, the temperature littoral after hatching. As was found by Lindegaard optimum of this species is about 11 ◦C, which is a tem- (1992), the growth of the littoral population of this perature that was measured in Ľadové pleso only over species can be very fast after hatching. This fast growth a very short time in the summer and only in the lit- rate in the littoral zone and relatively long ice-free pe- toral part of the lake. Ľadové pleso probably lies on the riod can support the univoltine life cycle of this species; border of the distribution area of this species, and its however, a more detailed study is necessary to confirm densities are affected by inter-annual air temperature this hypothesis. variations. Acknowledgements Life histories We inferred life cycles for the two most abundant chi- We wish to thank P. BITUŠÍK for identifying the Microp- ronomid taxa in Ľadové pleso. Preliminary results of sectra species and for revision of the identified chironomid instar analyses suggest that there is one generation taxa. We also wish to thank our colleagues for technical as- per year, with emergence of P. nivosa in the July sistance during field work and D. HARDEKOPF for linguistic and M. radialis in the August. According to published correction of the manuscript. This study was enabled by the data, this hypothesis could be correct. Moore (1979) FP 5 EC project EMERGE (EVK1-CT 1999-00032, address: found that chironomids in the subarctic Great Slave www.mountain-lakes.org). Lake were all univoltine, and the same was reported by Wiederholm et al. (1977) from a shallow subarctic References lake in northern Sweden. Also, chironomids from other high mountain lakes in C Europe are univoltine, as ob- AAGAARD, K. 1986. 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Received September 2, 2005 Accepted May 9, 2006