Quaternary Science Reviews, Vol. 6, pp. 29-40,1987. 0277-3791/87 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright © 1987 Pergamon Journals Ltd.

CHIRONOMIDAE (DIPTERA) IN PALEOECOLOGY

Ian R. Walker Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6

A comprehensive review of chironomid paleoecology is provided, with a discussion of relevant aspects of chironomid biology. The systematics, ecology, morphology, and taphonomy of chironomids are specifically addressed, as is the application of chironomid remains in investigations of lake ontogeny, anthropogenic eutrophication and acidification, paleosalinity, and paleoclimate.

INTRODUCTION

Although the attributes of fossil Chironomidae (non- biting midges) as paleoecological indicators are rather well known within the limnological community, their value is poorly known among paleoecologists in general. According to Frey (1964), the earliest report of the chitinous sedimentary remains of Chironomidae may be attributed to Ekman (1915). Numerous sub- sequent reports (e.g. Lundbeck, 1926; Gross, 1937; Brehm et al., 1948) have since been compiled by Frey (1964), who considers the earliest attempt to interpret such remains in terms of past conditions to be that of Gams (1927). A decade thereafter, Andersen (1938) noted the response of midge communities to Danish late-Pleistocene climatic variations. Although Ander- sen's (1938) research 'seemed prophetic of things to come' (Frey, 1964), the technique continued to develop slowly.

In the half century to follow, fossil remains of chironomids have been used to trace the paleo- productivity of lake systems (e.g. Deevey, 1942; Stahl, 1959; Bryce, 1962), to assess anthropogenic eutrophi- cation (e.g. Carter, 1977; Warwick, 1980; Wiederholm, 1979; Wiederholm and Eriksson, 1979) and acidifi- cation (Henrikson et al., 1982), and to monitor the impact of salinity fluctuations (Paterson and Walker, 1974; Clair and Paterson, 1976) and climatic variations (Walker and Mathewes, in press) upon aquatic com- munities. In the present paper, I review the present knowledge of chironomids in paleoecology, as well as relevant aspects of the organisms' biology and ecology. My objective is to convey information regarding the value of chironomids in paleoecological-studies to the many Quaternary scientists who are, as yet, unfamiliar with these insects. Chironomid paleoecology has sig- nificantly advanced since the earlier and less com- prehensive reviews provided by Frey (1964, 1976), Stahl (1969), and Hofmann (1979a).

BIOLOGY

The Chironomidae constitute a family of true flies (Diptera) which are prominent as larvae in the bottom

communities of virtually all freshwater habitats. Upon hatching from an egg (Fig. 1), the first instar larva begins a period of growth which eventually necessitates the shedding of the exoskeletal integument. Upon replacing this integument, the larva then continues growth in its second instar. Two further episodes of ecdysis (replacement of the exoskeletal wall) and growth follow, defining the third and fourth larval instars. Although bearing a well-developed, strongly- sclerotized head capsule, the elongate soft-bodied larvae otherwise resemble maggots. As mature fourth instar larvae, they vary from 1 to 30 mm in length (Oliver and Roussel, 1983).

Lacustrine chironomid larvae inhabit the uppermost sediments of both littoral and profundal (deep-water) environments, cling or burrow into aquatic plants, tunnel in moist wood, or parasitize other invertebrates (Oliver and Roussei, 1983). Similar habitats are occupied in streams, and chironomid taxa are known to inhabit littoral marine environments, water-logged soils, peats, and dung (Oliver and Roussel, 1983). They are among the most ubiquitous of insects.

The first instar larva of lacustrine midges are gener- ally planktonic, allowing for dispersal of larvae to suitable substrates. Larvae from habitats other than standing water, and the later instars of lacustrine midges are largely sedentary. The first instar is typically brief, with each later instar usually of progressively longer duration. Although the first instar larvae may derive some nourishment from the egg yolk, larvae feed on a variety of materials including algae, organic detritus, macrophytes, and other invertebrates (Oliver, 1971). Predation occurs among free-living Tanypodinae and in genera of the Harnischia complex (Chirono- minae), whereas most other chironomidS combine algae and detritus to form their diet.

During the latter part of the fourth larval instar, the thoracic segments expand, achieving a pre-pupal con- dition. Following ecdysis, the chironomid enters the pupal stage in which morphology is reorganized to that of the adult insect. Apart from the Tanypodinae, almost all pupae are sedentary. The pupal stage is brief, persisting a few days at most (Oliver, 1971).

At maturity, the pupa moves to the water surface,

29

30 I R . Walker

r

1st instar larva

th instar

FIG. 1. Chironomid life cycle. (Adapted from Borror et al . , 1976.)

allowing emergence of the winged adult. Although mosquito-like, adult chironomids seldom bear a pro- boscis, and do not share the biting habit of mosquitoes. Persisting for a few weeks only, the adult stage permits dispersal and reproduction.

The duration of the midge life cycle varies greatly among taxa, and with habitat. Long life cycles typify arctic taxa, occasionally extending to several years (Oliver, 1968; Butler, 1982; Hershey, 1985). In warmer climates several generations are common in a single s e a s o n ,

CLASSIFICATION

Ten subfamilies constitute the Chironomidae. One subfamily, the Telmatogetoninae, are all but com- pletely restricted to marine environments. In Hawaii this family also inhabits freshwater (Cranston, 1983). Three subfamilies are very rare, known only from a small segment of the globe. The Chilenomyiinae, represented by a single species, is known only from southern Chile (Brundin, 1983a). The Buchonomyiinae include two species native to Europe and Asia (S~ether, 1983). The Aphroteniinae, including four genera, are known only from South America, South Africa, and Australia (Brundin, 1983b). Such a distribution suggests that this primitive group originated before the Mesozoic disintegration of Gondwanaland (Brundin, 1965). An extinct Aphroteniinae tribe, the Electro- teniini, records this subfamilies' former, more wide- spread Cretaceous distribution, extending into Siberia (Kalugina, 1980). Chironomidae are also known from Triassic deposits (Ashe, 1983).

Three subfamilies, the Orthocladiinae, Tany- podinae, and Chironominae, constitute the great majority of taxa encountered in lake sediments. The remaining subfamilies, Podonominae, Diamesinae, and Prodiamesinae, are predominantly cold-stenothermous

taxa inhabiting temperate and montane to polar and alpine climatic regions (Oliver, 1971).

Considerable confusion of chironomid systematics arose from two early publications of Meigen (1800, 1803). According to Oliver and Roussel (1983), Meigen proposed different names for the same insects in each publication. Thus, the earlier names, including Ten- dipes and Pelopia were replaced by Chironomus and Tanypus respectively. The latter names were widely adopted before rediscovery of the earlier publication. With this rediscovery, a second system of nomenclature entered common use, but the International Com- mission on Zoological Nomenclature ruled in favour of the system established by Meigen's (1803) second publication (Oliver, 1971). Consequently, Chir- onomus, and the derived tribe, subfamily, and family designations are correct, whereas the respective names derived from Tendipes, including the family name Tendipediae, are not. Similarly, Tanypus and Tany- podinae are preferred to Pelopia and Pelopiinae. These synonyms are frequently encountered in recent literature.

Despite these difficulties, and others (Stahl, 1959; Ashe, 1983), the systematic treatment of chironomids is achieving stability. A recent volume (Wiederholm, 1983) has reviewed the systematic position of Holarctic Chironomidae, and is destined to remain the standard for future investigations. Ashe (1983) has prepared another monumental publication which has catalogued each of the world's chironomid genera. Future research should rely principally upon these volumes for a solid taxonomic base. This should facilitate improved com- munication among ecologists, paleoecologists, physi- ologists, and other persons investigating chironomid biology.

Although the taxonomic keys of Wiederholm (1983) rarely permit identification of fossil larval head cap- sules, reference to the illustrations and diagnoses of

Chironomidae (Diptera) in Paleoecology 31

each genus usually do permit classification. Fortunate- ly, most of the characters employed by systematists are borne on the head capsule and consequently may be available to the paleoecologist. Hofmann (1971a) provides keys and illustrations which should permit identification of head capsules encountered in lake sediments. Reference to other treatments (e.g. Oliver et al., 1978; Simpson and Bode, 1980; Oliver and Roussel, 1983) may occasionally be necessary. In any instance, recent systematic literature must be con- sulted. Simpson (1982) has conveniently catalogued North American systematic publications.

HEAD CAPSULE MORPHOLOGY

The chitinous head capsules of larval Chironomidae are abundant and usually well preserved in freshwater sediments. The structure of the head capsule varies greatly, however the greatest part of this variation can be illustrated with reference to the subfamilies Chir- onominae, Orthocladiinae, and Tanypodinae (Fig. 2). In this paper, only those structures most important for subfossil diagnosis will be discussed. The terminology employed for describing anatomical features varies among publications. Here, I adopt the standard nomenclature established by Sa~ther (1980), although I will often present synonomous terms in parentheses following the preferred term.

a ) / ) ~ ~ ~ 7~n:ir ~':nna ' / * . ' ~ ' ~ . ~ . se gmant

." • ~ d ~ r s ~ e n t a l

" ligula

The head capsules of Tanypodinae (Fig. 2a) are generally weakly pigmented and somewhat elongate. The retracted first segment of each antenna is often visible within fossil head capsules near the anterior lateral margin. The proportions of the head capsule and first antennal segment can be important characters for subfossil diagnosis. A fork-shaped ligula is situated in a median anterior ventral position. The ligula may become separated from head capsules during burial or processing of fossil material. Variations in the shape, number and colour of ligula teeth also aid identifi- cation. The dorsomental teeth (paralabial teeth), lack- ing in the tribe Pentaneurini, are, otherwise, situated laterally adjacent to the ligula and will usually remain with the head capsule. In well preserved material, one or both mandibles may be retained at the antero-lateral margin.

The principal features facilitating identification of fossil Chironominae (Fig. 2b) and Orthocladiinae (Fig. 2c) are the ventromental (paralabial) plates and the teeth of the mentum (hypostomial or labial plate). The mentum is situated in an anterior median ventral position. The pair of ventromental plates are situated laterally, adjacent to the mentum on either side. In the Chironominae, the ventromental plates are very large and usually conspicuously striated. Although fan- shaped in the tribe Chironomini, laterally elongated, strap-shaped plates occur on the Pseudochironomini and many members of another Chironominae tribe, the Tanytarsini.

In the Orthocladiinae, the ventromental plates are always much less conspicuous, and never striated. In many instances, these plates are greatly reduced, vestigial structures. In all Chironominae and Ortho- cladiinae the shape, number, and arrangement of the mental teeth and characteristics of the ventromental plates are critical diagnostic features. Most other structures are normally lost from the head capsule.

b)

e)

- - - - mandibles

i i': en,a, plates

~' _. \----~ventromental ~ plates

FIG. 2. Prominent head capsule features (a) Tanypodinae, (b) Chironominae, (c) Orthocladiinae.

TAPHONOMY

Head capsules are produced by each midge, during each larval instar. Thus, a chironomid surviving to the fourth larval instar will have produced four head capsules. A chironomid expiring prior to attaining its fourth instar will produce fewer head capsules. The head capsules subsequently preserved could include both those shed with exuviae during ecdysis, and the remains of dead larvae.

There exist, however, numerous reasons why the abundance of fossil midge remains might not reflect the abundance of the living larvae from which they were derived. Quality of preservation may vary among taxa, among instars, and between head capsules derived from expired insects and those shed as exuviae. As Iovino (1975: p. 41) has stated, 'If remains result only from expired individuals, age distribution of the re- mains would vary with age specific mortality.' Hof- mann (1971b) has observed that because pupae of some chironomids, for example Chironomus, retain the

32 I.R. Walker

fourth instar's exuviae, this instar's head capsule may be blown onto shore following emergence of the adult. Bivoltine and multivoltine taxa produce more head capsules per season than univoltine taxa of the same abundance. Redistribution of head capsules might influence the concentration and composition of fossil assemblages. Unfortunately, little study has yet been devoted to these problems.

Iovino (1975) has investigated the composition of Chironomus attenuatus exuviae by means of the chito- san-iodine test (Campbell, 1929). Iovino (1975) deter- mined that first and second instars usually dissolve the procuticle completely prior to ecdysis, leaving only a thin, non-chitinous, epicuticular exuviae. In contrast, third instar exuviae usually include chitin, and fourth instars were chitinous almost without exception. Thus, exuviae of early instars are much less durable than both those of later instars and remains of expired larvae. This, in part, would explain the great under-represen- tation of early instar head capsules in lake sediments (Iovino, 1975). Such remains may also be lost through the use of coarse sieves during processing (Walker and Paterson, 1985). Similar tests for chitin should be repeated for taxa other than Chironomus to assess whether Iovino's (1975) results apply generally.

To avoid counting individual larvae twice, Carter (1977) and D6vai and Moldovfin (1983) chose to base their analyses on fourth instar remains only. Iovino (1975) presents data which questions the ease with which instars may be separated. Also, if fourth instar remains are blown ashore with the pupal exuviae of some taxa, fourth instar head capsules may be under- represented in sediments (Hofmann, 1971b).

Both lovino (1975) and Walker et al. (1984) have compared the composition of chironomid life and death assemblages. In both instances, a good correlation was evident. Walker et al. (1984) do provide evidence suggesting that Procladius may be under-represented in fossil material. Bryce (1962) and Roback (197(1) have also suggested that Tanypodinae head capsules may be poorly preserved.

Iovino (1975) demonstrates that some offshore dis- placement of head capsules occurs, particularly in shallow lakes and from the littoral to the sublittoral of stratified lakes. Wiederholm (1979) and Brodin (1982) found large numbers of littoral chironomid remains in profundal sediments of Lakes Washington and Vfixj6s- j6n respectively. Chironomids characteristic of streams have also been recorded in lake profiles (Warwick, 1980; Walker and Mathewes, in press).

D6vai and Moldovfin (1983) report that head capsule preservation was not as good in the high energy environment of a shallow Hungarian lake as would be expected in deeper waters. They also suggest that because chironomids may burrow to 15 cm depth in the sediment, some extant taxa may not be well rep- resented in sediments collected within 5 to 10 cm of the mud surface. Despite the recent eutrophication of Lake Balaton, they report maximum Chironomus concen- trations 15 cm below the sediment surface

I have noted that head capsules of planktonic first instar larvae dominate sediments beneath a saline, meromictic basin of White Lake, British Columbia. Unfortunately, first instar head capsules differ greatly from those subsequently produced. Thus, identification of first instars is rarely possible. The dearth of head capsules in meromictic basins, also noted by Crisman (pers. comm. 1984), suggests that little offshore dis- placement of head capsules occurs in such lakes.

METHODOLOGY

Meticulous preparation, sorting, and identification of midge remains is requisite for analysis of past com- munities. As experienced chironomid paleoecologists will attest, these procedures are extremely tedious. A carefully cleaned sample is greatly appreciated when the sorting operation begins.

Small lakes in forested watersheds usually contain sediments with a high organic content. My experience suggests that 1 or 2 ml of wet sediment from such lakes provides 50 to 100 head capsules, sufficient for analysis (Walker and Paterson, 1983; Walker and Mathewes, in press). Inorganic sediments, such as those frequently encountered in late glacial deposits, may yield much lower concentrations.

However, head capsule concentrations vary greatly among sites. Deevey (1942) has reported maximum concentrations of 68 head capsules/ml at Linsley Pond, but according to Frey (1964), Deevey (1955a) later encountered 1700/ml at Pyramid Valley, New Zealand. The highest concentration yet reported, nearly 8000/ ml, was tallied for Eight Lake, Alaska (Livingstone e¢ al., 1958). Stahl (1959) reported concentrations of 6 to 65/ml. Sediments of the Sch6hsee yield concentrations ranging from 0 to 260/ml (Hofmann, 1971b). Warwick (1980), for the Bay of Quinte, Lake Ontario, describes concentrations varying from 4.6/ml at the sediment surface to 124/ml at the 1.14 m depth. The range observed in Warwick's (1980) study may result from several factors including compression of the deeper sediments, recent increased sedimentation rates, and changes in head capsule production.

Samples are usually deflocculated in 5 to 10% KOH prior to analysis. In calcareous sediments, an acid wash (t0% HCI) may also be necessary. Warwick (1980) emphasizes the delicate nature of midge remains and the need for mild treatments. He recommends the use of 8% KOH at 60°C for 30 minutes. Apparently, chitinous structures may be bleached and deformed by high temperatures (Warwick, 1980). It is important to remember that although chitinous structures are very resistant, exuviae of early instars, if present, may contain little or no chitin (Iovino, 1975). Also, harsh treatment may disarticulate head capsule features.

Following the above chemical treatments, head capsules are generally separated from finer debris by means of sieving. Methodology varies, but a 100 ~m or finer sieve will retain most head capsules and is recommended (Walker and Paterson, 1985). Sub-

Chironomidae (Diptera) in Paleoecology 33

sequently, the residue is back-washed from the sieve into a container and stored wet until the material is sorted. Unless sorted shortly after sieving; preservation in 99% ethanol (Warwick, 1980) is advisable.

Sorting of wet residues may be accomplished in a petri dish, watch glass, or Stender dish at magnifi- cations of 25 to 50 x (Warwick, 1980). To limit the possibility of overlooking remains, I prefer to sort head capsules from sediments at 40 to 50 x in a Bogorov counting tray (Gannon, 1971). The particular Bogorov tray which I employ includes 7 parallel grooves cut in a perspex plate. The bottom width of each groove, 4.5 mm, corresponds to the field of view, at 50 x, of a Wild M5 stereomicroscope. As remains are located, each is transferred with forceps to coverslips for mounting and identification.

Researchers should note that head capsules of some taxa, especially the Orthocladiinae, readily split into two equal, identifiable halves. Thus, it is important to count these fragments as 1/2 head capsule. Head capsules bearing more than one half of the mentum may conveniently be counted as whole head capsules, and head capsules consisting of less than one half of the mentum may be ignored. This splitting of the head capsules creates difficulties in the application of con- ventional statistics. If the halves are sedimented in- dependently, perhaps each half should be treated as if representing an 'individual' for statistical purposes. If the head capsules split within the sediment, or during processing, and the head capsules remain in close proximity, they would be better treated as halves.

Presentation of results varies among authors. Saw- toothed figures similar to those of pollen diagrams are most common, yet histograms convey the results equally well. In any circumstance, figures convey information more readily than tables of data. Com- puter programs for plotting pollen diagrams are easily adapted for use with chironomid data.

More critical is the decision to present data as either percentages, counts, or influx estimates. Percentages are limited chiefly because changes in the relative abundance of one taxon may result either from an actual change in its influx or a change in the influx of one or more other taxa.

Count data are difficult for the reader to assess since the counts at adjacent levels may not readily be compared unless sedimentation rates remain constant. Readers are required to perform the mental gymnastics necessary to standardize the data. Interpretation of count data suffers from variations in sedimentation rates and sample volume. Consequently, percentage results are usually more convenient.

Ideally, influx rates for each taxon should convey the most information. In practice, influx data are more informative only if accurate measures of sedimentation rate are available. Influx calculations frequently assume a constant sedimentation rate over a broad sampling interval, yet it is likely that sedimentation rates vary considerably in the short term. In calculating sedimentation rates for surface cores, investigators

must carefully consider the effect of compaction on apparent rates. Sediment focussing may have important effects (Davis et al., 1984), especially during the early history of a lake. Periods of rapid natural or anthro- pogenic environmental change may induce abrupt changes in sedimentation. Periodic catastrophic events such as forest fires and debris slides in the catchment may induce brief but rapid episodes of sediment deposition. Inaccurate estimates of sedimentation rates cause influx data to reflect these inaccuracies rather than the varying abundance of chironomid taxa.

It is also possible, particularly in small lakes, that as the lake shallows the changing sedimentary environ- ment may alter the influx of chironomid remains without a change in a taxon's actual abundance. A core might record a transition from a profundal stage to a littoral environment. If chironomid remains tend to become concentrated in the sublittoral, then peak concentrations in the middle of a core could reflect this artifact (Walker and Mathewes, in press).

Authors should cautiously consider how the data are to be displayed, and how the changes can most accurately and honestly be portrayed. Perhaps, in future, another form of presentation will be attempted, providing rates of chironomid production. A simple cubic relationship could be calculated between the width of a taxon's head capsule and biomass of the living larva. More perplexing is the task of quantifying each of the taphonomic processes regulating which head capsules are deposited at a site, and which would subsequently be preserved.

APPLICATIONS

Lake Ontogenetic Studies As indicated with my introduction, midge remains

have been employed by paleoecologists with a variety of goals. The earliest application (Gams, 1927) was for investigating the natural ontogenetic processes which influence lake ontogeny via nutrient supply. Such a goal may appear esoteric to many Quaternary scien- tists, but is a theme central to much limnological theory, and to paleolimnology as a science.

Early limnologists sought to classify lakes by a variety of means. Prominent among early investigators were Einar Naumann and August Thienemann. Borrowing terms which Weber (1907) had coined for nutrient supply to bogs, Naumann (1919) categorized lakes according to their phytoplankton productivity, providing the basis for our present lake trophic classi- fication. Naumann (1919) described two basic lake types, the highly productive or 'eutrophic' Baltic lakes and the unproductive or 'oligotrophic' north European lakes. Hansen (1962) also credits Naumann (1917, 1918, 1920) with introducing the 'dystrophic', humic or brown-water lake as a sub-type of the north European lakes.

Thienemann (1918, 1921) derived similar conclusions through his attempts to classify lakes on the basis of dominant components in their benthic fauna. Thiene-

34 I.R. Walker

mann (1921), accepting Naumann's (1919) terminol- ogy, described oligotrophic Tanytarsus lakes, eutrophic Chironornus lakes, and humic lakes in which both Chironomus and Corethra (now Chaoborus, Chao- boridae) were prominent.

Brinkhurst (1974) provides an excellent review of subsequent attempts to refine Thienemann's typology. The scheme of Brundin (1949, 1956, 1958) describes several classes of temperature stratified lakes: arctic Heterotrissocladius subpilosus lakes (ultraoligo- trophic), subarctic and high boreal Tanytarsus-Hetero- trissocladius lakes, boreal and montane Tanytarsus lugens lakes (oligotrophic), Stictochironomus rosen- scholdii-Sergentia coracina lakes (a transitional 'meso- trophic' type between oligotrophy and eutrophy), Chironomus anthracinus and C. plumosus lakes (eutrophic), and C. tenuistylus (dystrophic) lakes. Furthermore, Brundin (1951) argued that oxygen microstratification at the mud-water interface was a major determinant of the profundal (deep-water) bottom fauna. Larger chironomid taxa, commonly associated with more productive lakes, could better cope with a micro-layer of O2-depleted water at the mud-water interface. Also, such larvae (e.g. Chiron- omus) frequently possess hemoglobin. Because lakes with higher productivity generally display greater profundal oxygen deficiencies, a correlation exists among lake productivity, oxygen deficit, and benthic fauna.

As Rodhe (1969) has emphasized, trophic categories are abstract entities with overlapping ranges in be- tween. A continuum of lakes among all of those described probably exists. This is apparent in S~ether's (1975, 1979) analysis of benthic lake typology. He describes 15 trophic categories, ranging from ultra- oligotrophy to extreme eutrophy, in addition to the mesohumic and polyhumic types. S~ether's (1975) analysis extends European benthic lake typology to North America. He also describes atrophic range for each of many chironomid taxa. Warwick (1975) and S~ether (1979) suggest that food may be more critical than oxygen microstratification in determining the benthic fauna of lakes.

Naumann (1919) had speculated that lakes should gradually become less productive as a consequence of constant leaching of catchment soils. Many limnol- ogists, however, have subsequently perceived eutrophication, a gradual increase in lake productivity, as the dominant, if not universal process. Whiteside (1983) traces this perception tO 'The oft redrawn figure showing eutrophication proceeding with community succession (Lindeman, 1942) and the early work of Deevey (1955[b]) . . . ' . Deevey (1955b) had empha- sized that the gradual infilling of lakes, by reducing the hypolimnetic volume, could generate an O2-poor hypo- limnion. Regeneration of phosphorous, from the sediments beneath the hypolimnion could then produce a real increase in lake productivity. This process Deevey (1955b) dubbed 'morphometric eutrophi- cation'.

Whiteside (1983) emphasizes the 'mythical' nature of a universal trend towards eutrophy. Eutrophication had unfortunately become the deus ex machina invoked by paleolimnologists to explain observed changes in sedimentary sequences.

Several chironomid pateoecologists attempted to trace the natural ontogenetic development of lakes. According to Frey (1964), Gams (1927) was able to demonstrate that Eutanytarsus, abundant in inter- stadial sediments of Lunzer Obersee, was later re- placed by Bezzia (Ceratopogonidae) and Chironomus. Similarly, Deevey (1942) describes evidence that an early Tanytarsus fauna at Linsley Pond was first succeeded by Endochironomus and Glyptotendipes (his description and illustration of 'Glyptotendipes" are more likely to be that of Dicrotendipes), and sub- sequently by Chironomus. Frey (1955) reported Eutanytarsus as initially abundant in L~ingsee, Austria, but with Chaoborus (Chaoboridae) arriving later. Each of these investigations could be interpreted as suggest- ing a natural tendency to eutrophication.

However, Livingstone et al. (1958) suggest, on the basis of large concentrations of chironomid remains (principally Corynocera (as 'Dryadotonytarsus') and Tanytarsus) and other microfossils, that Eight Lake in arctic Alaska may have experienced an early eutrophic stage, becoming less productive as the lake tended to dystrophy. At Myers Lake, Indiana (Stahl, 1959), an early Sergentia-dominated fauna declined as Chaoboruz' increased. Stahl (1959) argues that even in its early stages this lake may have experienced 'moderate severe oxygen depletion', and that subsequent changes arose from a reduction in hypolimnetic volume rather than an increase in productivity.

Bryce (1962) also presents contrary results, indicat- ing an early dominance by Chironomus at his Malham Tarn Moss site. Bryce (1962) argues that marl de- position may have reversed the ontogenetic process, causing the site to become more oligotrophic. Stahl (1969) finds this conclusion unsubstantiated. Stahl's (1969) remarks also question the reliability of detailed Russian investigations (Lastochkin, 1949; Konstan- tinov, 1951) with tenuous systematic analysis, recovery, and interpretation techniques.

In southern Finland, Alhonen and Haavisto (1969) have noted an early eutrophic stage subsequent to a lake's isolation from the sea. Hofmann (1971b, 1979b) indicates that the eutrophic north German Chironornus lakes were formerly oligotrophic Tanytarsus lugens lakes. Lawrenz (1975) suggests that Green Lake, Michigan, has always remained oligotrophic, although the fauna did respond to a variety of factors including changes in sediment type, water level, and climate. Chironomid succession in a dystrophic, bog lake perhaps responded to natural increases in lake acidity more than to Holocene trophic variations (Walker and Paterson, 1983; Walker et al., 1985).

The above results suggest that broad generalizations regarding lake ontogenetic patterns are unwarranted. The initial oligotrophic condition suggested in early

Chironomidae (Diptera) in Paleoecology 35

lake sediments often relates to cold climatic conditions prevailing at that time. Many lakes appear to achieve 'trophic equilibrium' (Hutchinson and Wollack, 1940) during the Holocene. In addition, Hofmann (1971b: p. 55; 1980) notes that Thienemann's (1915) Tanytarsus lakes were originally characterized by a misidentified species, later placed in Lauterbornia, and now recog- nized to belong to Micropsectra. Since few paleoecol- ogists have been able to distinguish among several Tanytarsini genera (including Micropsectra), the genus Tanytarsus has been employed in a broad sense incorporating taxa not characteristic of profundal oligotrophic environments. Similarly, although Chiron- omus anthracinus and C. plumosus are important indicators of eutrophy, some Chironomus species may be abundant in dystrophic, or even oligotrophic situ- ations.

Stahl (1969) has noted that chironomid paleoeco- logical investigation sites had included unstratified lakes. The system of benthic lake typology conceived by Thienemann (1915, 1918, 1921) and Brundin (1956) was intended only for stratified lake environments. Nevertheless, Warwick (1975) and S~ether (1979) have argued that food may be more important than 02 microstratification in determining the benthic fauna. If true, benthic lake typology can be applied to shallow polymictic lakes.

Anthropogenic Eutrophication One of the major environmental issues to concern

limnologists has been the problem of anthropogenic eutrophication. Increased nutrient loading from urban and agricultural land-use has dramatically increased the productivity of some lakes, to the point where tremen- dous algal blooms foul shorelines and produce anaero- bic conditions within the profundal environment. In addition to the induced autotrophic production, allochthonous inputs of sewage and other organic wastes compound the de-oxygenation problem. One might argue that these lakes are not 'dead' (as has been suggested for many such lakes); rather they are too full of life!

Since chironomid communities respond dramatically to such trophic alterations, their remains provide a detailed record of eutrophication events. Thus, it is possible to determine man's impact upon lake pro- cesses.

Goulden (1964) noted a recent shift in the midge fauna of Esthwaite Water, following thousands of years of stable conditions. The rapid increase in Chironomus abundance suggested that the lake had experienced a period of eutrophication beginning with Norse immi- gration and later deforestation. Similarly, Carter (1977) described a shift from a Tanytarsus to a Chironomus- dominated fauna. This change spanned the last 100 to 150 years with more rapid change following the Second World War. Wiederholm (1979) records less dramatic, but similar changes in the fauna of Lake Washington.

Swedish investigations have identified a period of gradual eutrophication in Lake M/il/iren preceding

rapid eutrophication between 1940 and 1950 (Wieder- holm and Eriksson, 1979). Similarly, Brodin (1982) outlines a shift to extreme eutrophy in Lake V[ixj6sj6n beginning in the early 1800s and accelerating in the present century.

Certainly the most detailed paleoecological investi- gation of anthropogenic eutrophication is that described by Warwick (1975, 1980). Warwick's (1980) analyses depict 2800 years of human impact upon the Bay of Quinte, Lake Ontario, beginning with abor- iginal land-use. He suggests that a slight increase in lake productivity, ca. 500 B.C. to 300 A.D., 'probably is attributable to the developing Hopewell culture.' (Warwick, 1980: p. 78). These aboriginal people practiced extensive agriculture. A return to a more oligotrophic condition characterizes the subsequent Algonkian and Iroquois periods. A slight increase in productivity later began with French contact, but ended with extensive logging by subsequent British colonists in the early 1800s. Finally, rapid industrial expansion and population growth, particularly during this cen- tury, have produced the present eutrophic situation.

Warwick's (1980) analysis depicts the great potential of subfossil Chironomidae as indicators of human impact upon aquatic systems. Although the technique has been applied principally to determining the impact of modern man, it may prove valuable in an archaeo- logical context for determining the environmental consequences of early land-use.

Acidification Studies Recent concern with regard to acidic precipitation

and its impact upon aquatic communities has raised the possibility of using chironomids to monitor these impacts as well. Several recent studies document chironomid communities inhabiting waters of different acidities (Roff and Kwiatkowski, 1977; Wiederhoim and Eriksson, 1977; Mossberg and Nyberg, 1979; Raddum and Sa~ther, 1981; Clair, 1982; Dermott, 1985; Walker et al., 1985). Chironomidae are well rep- resented even in strongly-acidic waters (pH 3.5 to 5.5). Chironomus, Phaenopsectra, Psectrocladius, and Zalut- schia tend to be present in greater relative abundance at low pH. As Wiederholm and Eriksson (1977) observed, the favoured taxa tend to be large-bodied, having smaller surface areas relative to volume. Con- sequently, it may be easier for large larvae to maintain their internal pH balance. JernelOv et al. (1981) also note that the increased buffering capacity afforded by hemoglobin may favour hemoglobin-bearing larvae.

As yet paleolimnological studies documenting the impact of acidification upon chironomid populations are few. Henrikson et al. (1982) have examined recent anthropogenic impacts upon two Swedish lakes. Clair (1982) has attempted a similar investigation. Walker and Paterson (1983) have examined chironomid succession in a naturally acidic system.

Henrikson et al. (1982) investigated the recent sediment of two heavily-acidified (pH < 5), oligo- trophic lakes in southwestern Sweden. The 15 cm long

36 I.R. Walker

cores were inferred to represent sedimentation begin- ning early in the present century. The total abundance of Chironomidae is reported to have since declined. Phaenopsectra and Psectrocladius exhibit an increase on a relative basis. However, the Tanytarsini appear to have been adversely affected. Curiously, Henrikson et al. (1982) report Chironomus and Dicrotendipes as the present dominants in Lake G~rdsj6n, yet neither are recorded from the surface sediments. Perhaps this is a very recent change. The upper sediment sample was 1 to 2 cm below the surface, possibly representing sediments 10 to 20 years before the sampling data.

Henrikson et al. (1982) cite several factors as possible influences upon the chironomid community, including pH, metal mobilization, increased oxygen deficit, decreased decomposition, changes in the algal flora, and altered predatory-prey interactions. They also note that the midges which appear more abundant in acidified lakes are taxa reported to be more common in fish diets. Thus acidification, by eliminating fish, could favour these taxa.

The investigation of bog lake succession at Wood's Pond in Atlantic Canada (Walker and Paterson, 1983) spans the postglacial. A late-glacial Tanytarsus-Hetero- trissocladius community is succeeded by a Tanytarsus fauna in which Lauterborniella and Stempellinella occur. Lauterborniella and Stempellinella are reported to be sand-case building taxa (Coffman, 1978; Ferring- ton, 1985) characteristic of clear-water, oligotrophic, circum-neutral to weakly-acidic lakes (Beck, 1977; Raddum and S~ether, 1981; Warwick, 1980). The eventual disappearance of these taxa may be linked to slight increases in water acidity and humic content, or to a disappearance of sand substrates as peatlands enclosed the lake margin. Subsequently, Chironomus and Monopsectrocladius, taxa common in small, strongly-acidic bog lakes and peat pools (Walker et al., 1985) increase markedly. Although the record had originally been interpreted as suggesting acidification 2 to 3 ka BP, recent diatom analyses suggest that a relatively high pH (>5.5) was maintained until at or about the time of European settlement (Walker and Paterson, unpubl, data). This is when Chironomus increased most abruptly. The high dissolved organic content of the water indicates that these acids con- tribute much of the present acidity. However, the coincident pH drop and land-use changes suggest that man may have influenced recent events in this lake. Perhaps logging and the subsequent natural regener- ation of catchment forests favoured expansion of the encircling bog.

Salinity The feasibility of using chironomids as indicators of

the ionic content of saline waters is largely unexplored. Two investigations linking midge distributions to salinity differences among lakes are worthy of note.

The midge fauna of saline lakes in central British Columbia can be divided into three distinct associ- ations, apparently related to salinity. Cannings and

Scudder (1978) consider an association of Cricotopus abanus and Procladius bellus as indicative of low salinity (mean conductivity = 40 p~S/cm 25°C). Glypto- tendipes barbipes and Einfeldia pagana dominate in highly productive lakes of moderate salinity (480 to 2770 ~S/cm). At high salinities (4000 to 12,000 ~S/cm) with somewhat lower productivity, Tanytarsus gracilentus and Cryptotendipes ariel prevail.

Conducting a similar study in western Victoria, Australia, Timms (1983) provides data suggesting two groupings. Procladius spp. and Chironomus duple,,: dominated in low salinity lakes (1 to 13 g/l). Tanytarsus barbitarsus characterized lakes of higher salinity (13 to 200 g/l). Timms (pers. comm. 1984) has indicated his intention to conduct similar work for the saline lakes in Saskatchewan, Canada.

According to Stahl (1969), Konstantinov (1951) interpreted chironomid stratigraphy at two sites in Kazakhstan as indicating salinity variations, although it was unclear how this was ascertained. Subsequently, only two chironomid paleoecological studies have been conducted in relation to salinity. Clair and Paterson (1976) record a salt-water intrusion, and the sub- sequent rapid recolonization by midges of a marsh lake in Atlantic Canada. Paterson and Walker (1974) examined salinity variations in an Australian saline lake.

The investigation by Paterson and Walker (1974) illustrates that subfossil midges have great potential as salinity indicators. An early freshwater community including Chironomus duplex was replaced by Tany- tarsus barbitarsus, indicating increasing salinity. Later Procladius paludicola immigrated to the lake. A brief return of C. duplex, peak numbers of P. paludicola, and lower sediment conductivity later suggest a fresh- water interval. The disappearance of C. duplex and high sediment conductivities imply a subsequent return to the higher salinities.

Paleoclimate The relation of chironomids to climate is also little

investigated, but is implicit to early benthic lake classifications. Thienemann's (1918) Chironomus lakes were low elevation Baltic sites, but his Tanytarsus lakes were described as sub-Alpine. Similarly, Brundin described the ultraoligotrophic Heterotrissocladius lakes as principally arctic~ whereas Tanytarsus lugens lakes were common in temperate climates. Further- more, Brundin (1958: p. 14) states, 'In a lake type system of the world the ultraoligotrophic lake indicates one extreme of a climatically based type series, where the ultraeutrophic equatorial lowland lake forms the other extreme'.

Numerous climatic effects, direct and indirect, might influence the chironomid fauna. As Brundin's (1958) statements imply, lake productivity is partly related to climate. Higher temperature and longer growing seasons facilitate greater biological activity and pro- ductivity. Increased chemical weathering rates at high

Chironomidae (Diptera) in Paleoecology 37

temperatures yield higher nutrient levels. The catch- ment vegetation, also climatically dependent, may significantly influence nutrient flux in a watershed.

Climate may have direct effects upon midges. Many taxa characteristic of ultra-oligotrophic and oligo- trophic lakes are cold stenothermous. Williams et al. (1981) suggest 'that both aquatic and terrestrial insects are good indicators of macroclimate'. Most Chiron- ominae and Tanypodinae are warm-adapted. The Orthocladiinae and Diamesinae are predominantly cold-adapted. Cold-adapted taxa appear to grow prin- cipally in winter; warm-adapted taxa grow rapidly through summer (Oliver, 1971). Emergence in cold- stenothermous taxa usually occurs immediately follow- ing ice-melt (Oliver, 1968).

The duration of ice-cover and the interval between mixing episodes will influence profundal oxygen deficits. Shallow lakes in cold climates may experience pronounced oxygen depletion beneath winter ice (Nagell and Brittain, 1977). Similar lakes without ice cover would be continuously well-oxygenated. Warm monomictic lakes will experience a long spring to autumn interval without renewal of oxygen-depleted hypolimnetic water, as compared to dimictic lakes, where both spring and fall overturns occur.

Andersen's (1938) early investigation of the Danish late-Pleistocene midge fauna reveals that midges responded rapidly to the known late-glacial climatic oscillations of Europe. Corynocera (as Dryadotany- tarsus), Chironomus, and a group of undetermined Orthocladiinae (including Heterotrissocladius?) were prominent during the Older and Younger Dryas, but disappeared during the intervening Aller0d phase.

It is unfortunate that Megard's (1964) analyses of Pleistocene sediments in Dead Man Lake, New Mexico, did not include a more thorough systematic analysis. Palynological results are interpreted as in- dicating alpine conditions. Thus, the Pleistocene midge fauna may have included cold-stenothermous taxa. Where recent studies of post glacial succession have included thorough systematic analysis, Heterotrisso- cladius, Corynocera, and members of the Tanytarsus lugens community are prominent in the late-glacial sediments (Giinther, 1983; Hofmann, 1971b, 1978, 1979b, 1983a, b, 1984, 1985; Walker and Paterson, 1983; Walker and Mathewes, in press).

Late-glacial sediments of two small lakes in Atlantic Canada include large numbers of Heterotrissocladius remains with Paracladopelma (Walker and Paterson, 1983). This assemblage is strikingly similar to that of Brundin's (1958) arctic Heterotrissocladius subpilosus lakes. The peak abundance of this fauna may relate to a particularly cold phase, which Mott (1985) has been tempted to correlate with the European Younger Dryas. The more organic basal sediments, in which Heterotrissocladius and Paracladopelma are less com- mon, contain an anomolously high representation of thermophilous pollen. These sediments could relate to an earlier warm phase in which pollen from the sparsely-vegetated, recently-deglaciated terrain was

mixed with pollen transported long distances by warm south-westerly winds.

Similarly, Hofmann (1983a) noted that the faunal changes observed in a shallow North German lake do not reflect trophic conditions, but were related to climate and siltation. Walker and Mathewes (in press, unpubl, data) derive similar conclusions for lakes of coastal British Columbia, Canada. They demonstrate (Walker and Mathewes, in press) that climatic infer- ences suggested by the chironomid data correspond well to trends indicated by pollen-climate transfer functions. They also note that a fauna in which Heterotrissocladius was prominent seems to have been widespread near the glacial margins.

These studies illustrate that chironomids may yield valuable information for paleoclimatologists. The re- lationships between midge taxa and climate are not well understood, and it is unlikely that such environmental requirements will be as closely defined as those relating climate and vegetation. However, where equivocal interpretations of pollen data exist, Chironomidae may help to clarify past conditions. An arctic lake possessing a normal eutrophic fauna, or a small warm-temperate lake dominated by Heterotrissocladius would be highly unusual.

Chironomidae, as adults, are capable of rapid dispersal. As with Coleoptera (Birks and Birks, 1980; Morgan and Morgan, 1980), Chironomidae may respond more rapidly to climate than is possible for vegetation. Significantly, Paterson and Fernando (1970) have noted that midge colonization of a reser- voir was essentially complete within a single season!

SURFACE SPECTRA

Perhaps the greatest difficulty inherent to using midges as paleoecological indicators is the limited ecological information available for many taxa. This is perhaps less of a problem in Europe than North America. Studies defining the ecological range of chironomid taxa have proven very useful (S~ether, 1975, 1979; Beck, 1977; Timms, 1983; Mossberg and Nyberg, 1979; Walker et al., 1985). However, because many of these studies are based upon few collections per season, and since taphonomic processes may influence the composition of subfossil associations, such studies yield information not easily comparable to fossil data.

To overcome similar difficulties with fossil analyses, palynologists and diatomists have examined large numbers of subfossil remains from surficial sediments of extant lakes. Comparison of fossil data with these modern samples from known environments has greatly assisted interpretation of paleoecological data. Similar work with chironomid surface spectra would establish a more quantitative basis for chironomid investigations and greatly enhance their paleoecological value. Cris- man (pers. comm. 1984) has collected a large set of chironomid surface spectra from Florida lakes. I have limited data regarding occurrence of chironomid re-

38 I.R. Walker

mains in New Brunswick and British Columbia lakes in Canada. The growing bank of such data should greatly assist future chironomid paleoecological studies.

CONCLUSION

Subfossil Chironomidae have provided, and will continue to provide significant information regarding past environments. To date, such studies have per- mitted reconstruction of past trophic variations, acidi- fication trends and salinity fluctuations. Changes in chironomid communities appear also to reflect past climatic change. The potential of Chironomidae in each of these areas will, in future, be greatly enhanced by improved ecological information, systematic knowl- edge, and extensive analyses of surface spectra.

Acknowledgements - - I thank P.S. Cranston, Rolf Mathewes, Colin Paterson, Geoff Quickfall, and Bob Vance for critically reviewing this manuscript. This effort has been supported by the Natural Sciences and Engineering Research Council of Canada through grant A3835 to R. Mathewes, and a postgraduate scholarship held by the author.

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