Analysis of Chironomid Remains from Lake Sediments in Paleoecological Reconstruction

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The results of methodological studies of paleoecological reconstruction of environmental characteristics are considered. The studies are based on the analysis of chironomid remains from the sediments of lake ecosystems. The investigation is focused
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   0097-8078/04/3102- © 2004 MAIK “Nauka   /Interperiodica”0203  Water Resources, Vol. 31, No. 2, 2004, pp. 203–214. Translated from Vodnye Resursy, Vol. 31, No. 2, 2004, pp. 223–235.Original Russian Text Copyright © 2004 by E. Il’yashuk, B. Il’yashuk.  INTRODUCTIONStudies of long-term changes in ecosystems in peri-ods of anthropogenic biospheric transformation are of great importance. As a consequence, paleoecologicalstudies have become an important component of com-prehensive monitoring programs. The implementationof this approach is often based on the use of physical,chemical, and biological data found in sediments of lake ecosystems to reconstruct long-term environmen-tal changes. Interpretation of such “archived” data is agood, and sometime the only, way to acquire long-termdata-set [105]. Remains of various groups of hydro-bionts are used as elements of biological data containedin the sediments of lake ecosystems [10]. In recentyears, chironomid remains have become popular in thereconstruction of environmental changes [114].Chironomids (Diptera: Chironomidae) form a fam-ily of dipterous insects containing a large number of species (>5000) [12]. Their larvae can be found in alllake ecosystems throughout the planet. The abundanceof chironomids ensures their paramount role in thefunctioning of aquatic ecosystems [1, 12]. The exuviaethat form during larval molting accumulate in the sedi-ments of lake ecosystems, where all their parts decay,except for the heavily chitinized head. Larva bodies, inthe case of death, also decay. The head capsules of chi-ronomids are well preserved in sediments. Usually, thisallows them to be taxonomically determined with reli-ability close to that for intact specimens of present-dayfauna [57]. The head capsules are abundant even in rel-atively small bottom-deposit samples [57], thus makingit possible to reconstruct the chironomid fauna. Addi-tionally, data on the age of bottom-deposit beds,obtained through radioisotopic analysis, allows the rec-ognition of past changes in chironomid assemblages.Knowledge of the biology of individual taxa and theirecological optimums with respect to various environ-mental characteristics enables the analysis of the struc-ture of chironomid assemblages to be applied to envi-ronmental reconstruction.The earliest findings of chironomid remains have beendated to the Jurassic period of the Mesozoic [6, 8, 108];however, they become abundant only in the sediments of lakes of the late or post-glacial periods of the last glacia-tion [57]. Therefore, fossil assemblages more recent thanlate and post-glacial periods can be regarded as represen-tatively reflecting the chironomid fauna that existed inlake ecosystems in the respective periods.The first results of chironomid studies describingabrupt changes in the chironomid fauna of a lake due topost-glacial climate changes in the territory of Den-mark were published more than 60 years ago [14].However, analysis of chironomid remains was rarelyused in paleoecological studies for a long time [114],and it is only in the past three decades that the numberof studies involving the examination of chironomidassemblages has increased dramatically.The first studies of fossil-chironomid assemblagesin our country were performed in the middle of the20th century in lakes in the Urals [11] and northernKazakhstan [9]. Unfortunately, Russian paleoecolo-gists later paid too little attention to the application of the analysis of chironomid remains in their studies.Only N.S. Kalugina performed a number of studies[6–8, 70], which, however, were mostly of a paleonto-logical character. At the same time, in recent decades,this method has been comprehensively developed inscience worldwide.Nowadays, paleoecological studies of chironomidremains are of two main types. The first includes thedevelopment of qualitative and quantitative calibrationmodels for the reconstruction of various environmentalcharacteristics based on the structure of chironomidassemblages. For this purpose, chironomid remains  Analysis of Chironomid Remains from Lake Sediments in Paleoecological Reconstruction  E. A. Il’yashuk and B. P. Il’yashuk   Institute of North Industrial Ecology Problems, Kola Scientific Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, 184200 Russia  Received April 10, 2002  Abstract  —The results of methodological studies of paleoecological reconstruction of environmental charac-teristics are considered. The studies are based on the analysis of chironomid remains from the sediments of lakeecosystems. The investigation is focused on the methodical basis of analysis and the perspectives and range of its application to long-term environmental changes of natural and anthropogenic srcin. It is shown that thedevelopment of this method in future studies will require a balanced combination of neo- and paleoecologicalapproaches.  WATER QUALITY AND PROTECTION:ENVIRONMENTAL ASPECTS   204  WATER RESOURCES   Vol. 31   No. 2   2004  E. IL’YASHUK, B. IL’YASHUK  taken from the surface (0–1 cm) layers of sediments ina group of lakes (commonly ~ 50 lakes) in the samegeographic region were analyzed and correlated withenvironmental characteristics. The results were used toconstruct a transfer function between the biologicaldata and environmental characteristics, which was usedas a basis for reconstruction models of these character-istics based on the structure of fossil assemblages.The second type includes the implementation of thestratigraphic analysis of chironomid remains containedin bottom-deposit cores taken from lake ecosystems.The age of the sediments in the cores to be examined isdetermined before the analysis by a radioisotopemethod (for example, by 210  Pb, 137  Cs, or 14  C). After-wards, data on the environmental preferences of indi-vidual chironomid taxa and/or the previously con-structed models are used to reconstruct the environ-mental-parameter changes.METHODOLOGICAL PRINCIPLES OF ANALYSISThe analysis of chironomid remains consists in asuccessive implementation of a data-set of procedures.The cores of sediments or samples from surface layersof sediments, intended for the analysis of chironomidremains, are commonly taken from the deepest part of the water body, where there is little if any wind-inducedmixing of sediments, the rate of sediment accumulationis the highest, and accumulation of the remains of hydrobionts inhabiting different zones and biotopes of the water body takes place. After sampling, the bottom-deposit cores are divided into 0.5–1.0-cm layers for anal-ysis. The head capsules of chironomid larvae are takenfrom the samples under a reflected-light microscope.Analysis can be performed with sediments mixed withwater without any preparation [116]. In this case, thetime spent for the inspection of 1 ml of sediments islarge and averages 14 h [124]. However, this approachexcludes the loss of important morphological struc-tures of head capsules (mandibles, premandibles,antennas, and labra) used to identify chironomid taxaand, thus, allows errors in paleoecological interpreta-tion to be avoided [101, 116]. In order to reduce thevolume of samples to be inspected and, hence, thetime required for their treatment, sediments are pre-treated with a hot KOH solution for convertingorganic matter flakes into a disperse state and with HFand HCl solutions, when sediments are rich in sili-cates. Next, the samples are sieved through a mesh of <100 µ  m, since the use of a large mesh will cause theloss of fine head capsules and the underestimation of small-sized species can have a serious adverse effecton the results of reconstruction [125]. Such proce-dures reduce the volume of samples to be analyzedfour-to-fivefold [50, 120, 121, 125, 130].The concentration of chironomid head capsules mayvary from zero to several thousands per 1 cm   3  of sedi-ments [118], depending on the rate of sediment accu-mulation and the hydrogeological conditions in thewater body. In most lakes, a concentration of >100 cap-sules per 1 cm   3  of sediments is considered ordinary[113]. In mountain lakes, as well as in most lacustrinebeds of sediments that formed in periods with cold cli-mate, such as Young Drias, the concentration can beless than 50 capsules per 1 cm   3 of sediments [77].Since the extraction of head capsules from sedi-ments during analysis is always made by hand and thetime and labor required to find and extract them arelarge, the question of the minimum number of headcapsules to be taken from each sample has receivedmuch attention in many studies. In the opinion of someresearchers [17, 28, 60, 87], 100–200 capsules need tobe analyzed, whereas others estimate the required num-ber at 40–50 [55, 77, 88, 94, 127, 128]. Later [49, 73],examples of models used to reconstruct paleotempera-tures based on chironomid remains allowed drawingthe conclusion that 50 capsules from each sample arethe minimum necessary for the subsequent inclusion of the results of analyses into the model. In cases in whichcalibration data-set included the results of the treatmentof samples containing < 50 head capsules, the changesin the reconstructed temperatures became wider and thetemperatures were under- or overestimated [49]. Anincrease in the number of analyzed capsules of up to100 from each sample through increasing the size of thesample treated commonly causes an increase in thenumber of taxa recognized in the sample due, as a rule,to rare species [92]. Commonly, these species are notincluded in the data-set for the construction of quanti-tative models, but they can introduce additional infor-mation and improve the quality of the interpretation of results [73, 92].Once extracted from sediment samples, the headcapsules are mounted for subsequent taxonomic identi-fication. A wide spectrum of taxonomic literature onthe morphology of chironomid larvae and a data-set of special keys developed by now for the determination of fossil chironomid remains [51, 96, 115] are used in thiswork.In the analysis of results, the fossil chironomidassemblages should not be regarded as something iden-tical to the structure of their communities that existed inthis part of the water body [121]. There are several rea-sons for doing so.First, the hydrobiological samples for studying thepresent-day chironomids are commonly taken within ashort period of time and they are rarely taken in winter,whereas the process of chironomid remains accumula-tion in sediments, reflecting both summer and winterfauna, takes place throughout the year. Thus, the struc-ture of fossil chironomid assemblages is a reflection of the state of the water body integrated over time [18].Second, according to notions used in taphonomy(science about processes that control the formation of paleontological sequences), the assemblage of remainsof profundal taxa in sediments of deepest zones of lake ecosystems represents necrocenosis [50, 51]—the   WATER RESOURCES   Vol. 31   No. 2   2004  ANALYSIS OF CHIRONOMID REMAINS205  assemblage of remains of organism that have inhabitedthe site examined. However, the paleoassemblagesfrom this zone represent thanatocenosis [2, 50, 51] oran assemblage of elements of profundal necrocenosismixed with secondary components of littoral faunaintroduced into the area from outside. Since the thana-tocenosis is subject to the influence of other tapho-nomic processes taking place in sediments, such asdiagenesis, a more exact term—  taphocenosis  —can beused [2]. The redeposition of littoral taxa in the profun-dal zone is an important factor, which affects the com-position and abundance of fossil assemblages [71],although the migration of remains from the littoral intoprofundal zone in large lakes is insignificant [41]. Insmall lakes, fossil assemblages from the profundal zonequite adequately reflect the fauna of the entire waterbody [121].Third, the chironomids differ in the duration of theirlife cycle. Therefore, bi- and multivoltine species pro-duce a larger number of head capsules per year thanunivoltine species [122]. Moreover, the difference inthe duration of the life cycle and the number of headcapsules produced can be observed in abundances of the same species living in different conditions.Fourth, chironomids in their development undergofour larval stages. Theoretically, neglecting the possibledeath of larvae, the srcinal abundance of chironomidcommunity can be assessed by dividing the number of found head capsules by four. However, the chitinousprocuticle of the larva head capsule at the first and sec-ond stage completely dissolve before molting [66]. Andeven if larvae at this stage die for some reason, theirweakly sclerotized head capsules rarely persist in sedi-ments because they are rapidly destroyed there as aresult of bacterial activity or mechanical impact duringredeposition [15, 50, 107].Fifth, for a number of natural reasons, reflecting thedecrease in the proportion of adult stages in the abun-dance, the head capsules at the third larval stage con-tained in sediments theoretically should be morenumerous than those at the fourth stage. However, itwas shown in [32] that the ratio of the number of headcapsules of the third stage to that of the fourth stagedecreases downward along the sediment column,because the third-stage capsules have thinner chitin andpersist worse than those of the fourth stage. From this itfollows that although the number of larval stages of chi-ronomids is constant, the concentration of head cap-sules of these larvae in sediments can differ from thesize of the producing abundance [32].Sixth, the taphonomic processes that affect the per-sistence of remains in sediments also affect the taxo-nomic structure of chironomid paleoassemblages. Forexample, the head capsules of larvae of subfamilyTanypidinae persist somewhat worse than the head cap-sules of other subfamilies [113].Thus, specific features of the life cycle of chirono-mids and the taphonomic processes that affect the spa-tial redistribution of their remains in lake ecosystemsand preservation in sediments, control the distinctionbetween the structure of paleoassemblages of theseaquatic insect and the structure of their communitiesthat existed in the period examined. However, in theopinion of some researchers [103–105, 122], paleoas-semblages reflect the situation that existed in the hydro-ecosystem in the period in question much better thanthe results obtained from a set of conventional hydrobi-ological samples discrete in both space and time.Another advantage of this approach in studying long-term changes in ecosystems in the period of anthropo-genic biosphere transformations is the possibility toapply this approach to recognize the tendencies in eco-system changes within a long time period in the past,which is virtually impossible to do using conventionalhydrobiological studies based on observational data-setembracing as short as several decades.The paleoecological reconstruction of the environ-mental conditions based on chironomid remains analysisbecomes an efficient method, which is used in studyinglong-term local changes in ecosystems (eutrophication,acidification, salinity changes, toxic pollution of lakeecosystems) and global problems, such as airborne con-tamination and climate changes.EUTROPHICATION OF LAKE ECOSYSTEMSThe paleoecological method for the reconstructionof long-term changes in hydroecosystems based on chi-ronomid remains analysis is gaining in popularity inbiomonitoring programs in lake ecosystems subject toanthropogenic eutrophication. This method was widelyused in studying the consequences of eutrophication inlakes of temperate latitudes in North America andEurope. As shown in [19, 72, 85, 86, 126, 129, 130,134, 135], the development of eutrophication in lakeecosystems brings about oxygen deficiency in bottomwater layers, transforms trophic nets, and increases therate of sedimentation. The result is a decrease in theconcentration of chironomid remains in sediments of lakes; disappearance of stenobiont taxa, such as Het-erotrissocladius, Micropsectra, Paracladopelma; and anincrease in the share of eurybiont taxa of Chironomusand Procladius.It was found [39, 84, 102] that the rise in the trophicstatus of a lake is accompanied by an increase in theshare of littoral and sublittoral taxa (Tanytarsus, Ortho-cladius, Cricotopus, Polypedium, Dicrotendipes, Glyp-totendipes) and their role in the ecosystem of the lakeas a whole. In I.R. Walker’s opinion [114], this may bedue primarily to the formation of oxygen deficiency inbottom water layers in deep-water zones of lakesaccompanied by the development of eutrophicationprocesses, which cause gradual suppression of chirono-mid larvae in such zones. This accounts for the increasein the portion of head capsules of littoral and sublittoraltaxa found in sediments. Changes in the species com-position are also observed among littoral chironomid   206  WATER RESOURCES   Vol. 31   No. 2   2004  E. IL’YASHUK, B. IL’YASHUK  taxa, since an increase in the nutrient flux during waterbody eutrophication causes silting of the littoral zoneand reduces water transparency, which results in a dra-matic drop in the proportion of submersed macro-phytes, associated with the habitats of many littoraltaxa, in the near-shore plant cenoses [39].Results of studies of the chironomid remains distribu-tion in surface layers of sediments in stratified lakes inOntario Province [76, 92, 94] were used to examine thedependence of chironomid distribution on the anoxiafactor (AF), which integrates the duration and depth of oxygen deficiency in hypolimnion. The value of AF wasdefined as the number of days in summer when the con-centration of dissolved oxygen in bottom water layerswas <1 mg/l. A close correlation (  r   = 0.77,  p < 0.05) wasfound to exist between the distribution of chironomidsand AF [94]. Thus, Protanypus, Heterotrissocladius, andMicropsectra, which prefer the conditions with high con-centrations of dissolved oxygen, featured the lowest val-ues of optimums in terms of AF (6.3, 7.4, and 8.3, respec-tively), whereas Chironomus, which occurs mostly inmesotrophic and eutrophic lakes, had the largest opti-mums with respect to AF (19.2). A high optimum withrespect to AF (18.4) was also recorded for Procladius.It was shown also [92] that the shares of Heterotrissocla-dius and Protanypus exceeding 15 and 2.5%, respec-tively, is indicative of a high oxygen concentration (>8mg/l) in hypolimnion in late summer.The results of these studies were used to developquantitative models for the reconstruction of durationand the degree of oxygen deficiency in lake hypolim-nion [76, 92, 94]. These models were used for studyinglong-term changes in the trophic status and oxygenregime of lakes in Ontario [75, 93]. Reconstruction of changes in oxygen concentration in the bottom waterlayers based on chironomid remains was also used inthe studies of eutrophication processes in lakes of Swe-den [20], Finland [85, 86], Denmark [19], Great Britain[46], and the United States [39].A study of 54 shallow lakes in Denmark [18] wasfocused on the dependence of chironomid paleoassem-blages from the surface layers of sediments on thetrophic conditions in the lake assessed from chlorophylla concentration for lakes from meso- to hypertrophic.Chironomid assemblages were shown [18] to be closelycorrelated with chlorophyll a concentration (  r   = 0.85,   p  < 0.01) and Secchi depth (  r = –0.84,  p  < 0.01). Thissuggests the feasibility of reconstruction of the trophicstatus of a lake based on chironomid remains analysis,as was shown in [16] by the reconstruction of changesin chlorophyll a concentration during the past four cen-turies in Lake Esrum, the largest lake in Denmark.Studies [78] in 68 stratified and not stratified lakesin the Alps with total phosphorus (P   tot  ) concentrationsvarying from 6 to 520 µ  g/l showed the chironomidremains from surface layers of sediments to have a rel-atively weak correlation (  r    2  = 0.68) with P   tot  concentra-tion in water, whereas assemblages of diatom algae aremore closely correlated with this variable (  r    2  = 0.79).Studies of chironomid remains from surface layers of sediments in 44 lakes in Great Britain [27] showed thatthe concentration of P   tot  in water could be among themost significant variables affecting chironomid distri-bution. The obtained relationships later served as abasis of a model used to reconstruct changes in P   tot  con-centration in a lake in Great Britain during the past170 years [27]. The results of this reconstruction werefound to be very close to those obtained using a modelbased on the dependence between P   tot  in water and thestructure of diatom assemblages in sediments. How-ever, the error in P   tot  calculation in the first case wassomewhat higher than in the second, when diatomassemblages were used [27]. The authors of [27] con-sider the difference in the accuracy of reconstructionquite explicable, since they reflect the difference in thebiology of the aquatic group involved. Diatom algae(primary producers of organic matter in the ecosystem)directly depend on the concentration of P in water,because this element can serve as a limiting factor intheir development. Contrary to that, chironomid larvae(secondary producers) are indirectly dependent on Pconcentration via oxygen content of water and theavailability of food, including diatom algae.An important benefit of the method is the possibleapplication of stratigraphic analysis of chironomidremains to the reconstruction of the trophic status of awater body for the period before the beginning of itsanthropogenic eutrophication, which allows the assess-ment of possible results of various programs for therehabilitation of the water body. For example, if the lakewas mesotrophic before the anthropogenic pollution, thereduction of nutrient load to the srcinal level will notcreate oligotrophic conditions in this lake [99, 135].POLLUTION OF LAKE ECOSYSTEMS BY METALS AND TOXIC COMPOUNDSIn addition to eutrophication of lake ecosystems, animportant problem is water pollution with metals andtoxic compounds contained in industrial wastewaterand airborne anthropogenic pollutants. Some aspects of this problem were discussed in [4, 5, 72, 126, 130],where changes in lakes due to industrial pollution arereconstructed using stratigraphic analysis of chirono-mid remains. However, such studies of long-term trans-formation of freshlake ecosystems caused by theiranthropogenic pollution by metals and toxic com-pounds have received and still receive very little atten-tion. At the same time, the potentialities of biostrati-graphic analyses based on various aquatic groups,including chironomids, in this field are very wide.Thus, detailed analysis of changes in a subarcticecosystem that have been polluted by wastewaters frommetallurgical works for more than 60 years [4, 5, 65]allowed two successive stages to be recognized in thechanges in chironomid assemblages. The first stage (thestage of early warning) exhibited an abrupt increase in   WATER RESOURCES   Vol. 31   No. 2   2004  ANALYSIS OF CHIRONOMID REMAINS207  chironomid abundance and the number of species,maximum species diversity, and beginning of changesin the dominating species. At the second stage, whichstarted with the completion of changes in the dominatingspecies, a dramatic drop was recorded in the total chi-ronomid abundance, the number of species, and speciesdiversity; head capsules of Chironomus with morpholog-ical deformations of mentum appeared in the samples.The observed changes caused the change from mon-odominant chironomid assemblages to oligodominantassemblages. At the second stage, based on the presenceor absence of the high occurrence of head capsules of Chironomus with inherited morphological deformations,such as mentum asymmetry, two successive stages wererecognized: the phase of stress on the physiological leveland the phase of stress on the genetic level [4, 5, 65].Morphological deformations in chironomids werefirst described in 1970 [97] in neoecological studies.A decade later they were first considered in paleoeco-logical studies [130]. Deformations of different struc-tures of chironomid head capsules were studied indetail in recent years [36, 43, 45, 63, 131–133]. Thedeformations were shown to start forming at moltingstage of chironomid larvae as a result of disturbance inthe endocrine regulation of formation of new structures[69, 112]. These phenomena were most thoroughlystudied in larvae of Chironomus genus. The results of studies [67, 82, 83] suggest that the high occurrence of deformations in larvae can reflect sublethal effects of pollutant impact on these larvae and serve as an indica-tor of sediments toxicity. A positive correlation wasfound to exist between the frequency of deformationsin Chironomus larvae and the pollution degree of sedi-ments [5, 38, 65, 68, 83, 110, 111]. Since the morpho-logical deformations in chironomids are the conse-quences of biochemical and physiological disturbancesrecorded in individual organisms and caused by stressfactors, an important aim of the studies in this directionis the assessment of their use as an indicator of the pol-lutant toxicity and the extent of environmental pollu-tion. However, there still exist a number of problems,the solution of which can be facilitated by the applica-tion of the paleoecological approach, which allows theexamination of cause-and-effect relations betweentoxic pollution and morphological deformations in chi-ronomids within long time periods.ACIDIFICATION OF LAKE ECOSYSTEMSOne more problem that became important in late20th century was due to the pollution of lake ecosys-tems with acidic substances and acidification of manylakes, primarily in North Europe and North America.Sufficiently long observation data-set are required toadequately study this problem. Therefore, the results of paleoecological reconstruction of changes in the lakes,including those based on chironomid remains, can con-tribute significantly to the understanding of processesassociated with acidification of these hydroecosys-tems [106].A number of studies of fossil chironomid assem-blages were conducted to reconstruct changes inbenthos and transformation of lake ecosystem as awhole in the beginning and in the course of lake acidi-fication [3, 5, 13, 23, 24, 34, 64, 95, 101, 109]. Theresults of all these studies show the processes of lakeacidification to proceed differently depending on themorphometric characteristic of each lake, its hydro-chemical regime, and hydrochemical characteristics of its catchment area. Therefore, the difference betweenthe models used to reconstruct the state of lakesdepends on the specific features of the lakes.In most studies [3, 5, 37, 44, 48, 64, 109, 117], theeffect of acidification of a water body on chironomidcommunity is found to include a reduction in taxa of Chi-ronomini and Tanitarsini and an increase in the share of Orthocladiniae and acid-tolerant taxa of Heterotrissocla-dius marcidus (Walker), Sergentia coracina (Zetterstedt),Procladius, Psectrcladius, Zalutchia, Tanytarsus, a dropin the share of acid-sensitive Microspectra and Stictochi-ronomus. According to data of [24, 48], an increase inwater pH is accompanied by a decrease in the total abun-dance, the number of taxa of chironomids, and the spe-cies diversity of chironomid assemblages, whereas otherstudies [3, 13, 44, 64] suggest the opposite tendency.The results of paleoecological studies in Kola Pen-insula lakes subject to acidification [5] also revealed nodistinct changes in the total amount of chironomid taxadue to water acidification. The species diversity andhomogeneity of chironomid assemblages in deep lakesincreased with growing water acidification; whereas inshallow lakes, where extensive development of acido-philic species of water mosses reduced the heterogeneityof chironomid habitats, these characteristics decreased.Changes in the role of individual taxa in the Kola lakessubject to acidification [5] have resulted in the replace-ment of oligodominant chironomid assemblages bypolydominant, whereas the tendency in shallow lakes isquite inverse—the predominance of a single taxonincreased dramatically.Generalizing the results obtained in studies of thetransformation of freshwater ecosystems caused bywater acidification, we should note that the knowledgeof pH levels preferred by chironomid larvae is stillinadequate. Additional studies are needed to establishthe optimums and tolerance ranges for many taxa anddevelop quantitative models for the reconstruction of lake water pH based on analysis of chironomidremains.CHANGES IN WATER SALINITYWater salinity is considered as an important abioticfactor for aquatic organisms, and its changes are a goodindicator of climate changes and other radical transfor-mations in the geological past [33, 114]. Chironomid
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