water depths do not exceed 2.5 m (Alm, 1986). Vorren et al. (1988) have a detailed description of the Late Weichselian sediments in L. Nedre. A'Erdsvatn.
Hydrobiologia 264: 21-32, 1993. © 1993 Kluwer Academic Publishers. Printed in Belgium.
21
Late Weichselian Chironomidae (Diptera) stratigraphy of Lake Nedre iErasvatn, And0ya, Northern Norway Torbjorn Aim & Endre Willassen' Institute of biology and geology, University of Tromso, N-9037 Tromso, Norway; Museum of Zoology, University of Bergen, Museplass 3, N-5007 Bergen, Norway Received 30 October 1990; in revised form 12 November 1992; accepted 14 January 1993
Key words: Chironomidae, fossils, lake ontogeny, palaeoenvironment, palaeoclimate
Abstract A 5.6 m long sediment column from Lake Nedre AErsvatn, Andoya, Northern Norway, spanning ca 20000 to 11000 14C yr B.P. (Before Present) has been analysed with respect to chironomid remains. Head capsules are absent or rare in the lower, marine part of the sequence (ca 20000 to 15 500 14 C yr B.P.). From ca 15 500 14C yr B.P. onwards, a continuous representation of hygropetric or rheophilous taxa indicate the existence of stream habitats. The lake itself seemingly had no significant chironomid fauna until ca 12800 14 C yr B.P. The main taxa recorded in pre-12000 14C yr B.P. samples are Micropsectra and Metriocnemus. Increases of Micropsectra are positively correlated with climatic improvements, especially the Blling amelioration (ca 12800 to 12000 14C yr B.P.). This may be due to a denser vegetation cover, and an increased influx of organic detritus. Chironomid concentration and influx values increased markedly from ca 12 500 '4 C yr B.P. onwards, attaining a maximum of 450 head capsules cm-3 in late Bolling. At ca 12000 14C yr B.P., dominance shifted to Tanytarsus lugens Kieffer. Increases of T. lugens, Psectrocladius,Procladius sp. and Heterotris-
socladius subpilosus Kieffer at ca 11400 14C yr B.P. may point to slightly more mature lacustrine conditions.
Introduction Lake Nedre Arsvatn is a small lake near the northern tip of Andoya in Northern Norway (Fig. 1), at 69 ° 15' N, 16 ° 5' E. Corings in 1982 yielded a 5.60 m long Late Weichselian sediment sequence, spanning ca 20000 to 11000 14 C yr B.P. (Vorren et al., 1988). 16 radiocarbon datings were carried out at the Radiological Dating Laboratory in Trondheim,
Norway (Vorren etal., 1988, Table 1). Supplemented by numerous datings from neighboring lakes, L. Endletvatn (Vorren, 1978; Alm, 1986) and L. vre Ersvatn (Alm, 1990), they provide a detailed chronology. Previous biostratigraphical studies in L. Nedre iErsvatn include analyses of pollen, spores and other acid-resistant microfossils (Vorren et al., 1988; Alm, 1990); diatoms and marine macroalgae (Vorren etal., 1988). Detailed macrofossil
22
Fig. 1. Key map, showing the position of Andoya in Northern Norway (A), and map of northern Andoya (B), with location of coring sites. I: Lake Endletvatn SW (Vorren, 1978), II: L. Endletvatn NE (Alm, 1986), III: L. Nedre AErisvatn (Vorren etal., 1988, this paper), IV: L. Ovre AErhsvatn (Alm, 1990).
studies were started in 1987, including seeds and fruits (Alm & Birks, 1991), marine algae and various animal remains. The upper, lacustrine part of the sequence (ca 15 500 to 11000 14 C yr B.P.) contains numerous chironomid remains, mainly head capsules. Such remains have proved useful in reconstructing environments of the past (Brodin, 1986; Hofmann, 1979, 1988; Walker etal., 1984; Walker, 1987; Walker & Mathewes, 1987, 1988; Warwick, 1980). This paper is a study of the chironomid record in L. Nedre AErsvatn, and aims at eluci-
dating the basin's palaeoenvironmental development. Physical and geological setting L. Nedre irsvatn (35 m above sea level) occupies a depression in the gently undulating lowland north of the Endleten mountain (Fig. 1). Except for a rocky western hill (Store Arhsen, 111 m), the lake surroundings are extensively covered by tills and Holocene mire deposits.
23 The lake is rather small, measuring ca 600 x 450 m. A brook from L. Ovre (Upper) iErfsvatn is the only appreciable inlet. The basin has largely been filled in with sediments during the Late Weichselian and Holocene. Present water depths do not exceed 2.5 m (Alm, 1986). Vorren et al. (1988) have a detailed description of the Late Weichselian sediments in L. Nedre A'Erdsvatn. Six lithological units (I-VI) were discerned. Unit I is of uncertain, possibly glaciolacustrine origin. Units II and III (10.49-6.60 m, ca 19500 to 15 500 14C yr B.P.) comprise marine sediments, mainly silt and clay. They are superposed by lacustrine sediments (units IV, V and VI, 6.60-4.97 m), deposited from ca 15 500 14 C yr B.P. onwards. The combined records of L. Endletvatn (Vorren, 1978; Alm, 1986), L. Nedre rsvatn (Vorren et al., 1988; Aim, 1990; Alm & Birks, 1991) and L. Ovre AErdsvatn (Alm, 1990) allow a detailed reconstruction of the Late Weichselian palaeoenvironment of Andoya. The L. 0vre Arsvatn area has been unglaciated since at least 22000 14C yr B.P. (Alm, 1990), whereas L. Endletvatn and L. Nedre Erisvatn were overridden by a glacial advance at ca 18 500 14C yr B.P. (Vorren et al., 1988; Alm, 1990). A dry climate prevailed at ca 22000 to 12000 14C yr B.P. Temperature conditions were much more variable (Fig. 3). Cold (High Arctic) intervals are indicated at ca 22000 to 21000 14C yr B.P., 19000 to 18500 14C yr B.P., 16800 to 16000 14C yr B.P. and 13700 to 12800 14C yr B.P. Warmer (Low Arctic) intervals occurred at ca 19500 to 19000 14C yr B.P., 18300 to 17900 14C yr B.P., 16000 to 15000 14 C yr B.P. and 12800 to 12000 14C yr B.P. (Alm, 1990). At ca 12000 14C yr B.P., a shift to more humid, Low Arctic conditions is recorded (Vorren et al., 1988).
iments treated with 10% KOH for 24-48 hours. They were subsequently sieved through 2.0 and 0.125 mm sieves and the fraction retained on the sieve was examined. Macrofossil remains were picked out under a Wild M3 stereo binocular (40 x magnification), using a pair of fine forceps. A total of 45 macrofossil samples have been analysed (Fig. 2). Chironomids were restricted to the upper part of the sequence. Concentration and influx values have been calculated (Figs. 2 & 3); influx according to the sedimentation rates from the "4C years indicated by Vorren et al. (1988, Fig. 17). Head capsules have been mounted in Euparal following standard procedure (Hofmann, 1986). Species identifications are restricted to a selection of 11 samples (Fig. 3). The fossils are deposited in the Museum of Zoology, University of Bergen.
Results and discussion Taxonomic notes
The only representative of the subfamily Tanypodinae in the sediments is Procladius(Holotanypus)
sp. The taxon is frequently found in Quaternary sediments, although there is reason to believe that Tanypodinae remains are less well preserved and poorly represented in fossil assemblages (Walker etal., 1984; Walker, 1987). The taxonomy and ecology of the genus is presently not well understood. The larval head capsules found in the cores (Fig. 4a-d) are very similar to those of a species occurring in Norwegian high mountain reservoirs. Two species of Diamesa Meigen have been found. Diamesa sp. I (Fig. 4e-g) probably belongs to the Diamesa zernyi or Diamesa tonsa
Material and methods
groups (see figures in Ferrarese & Rossaro, 1981). It cannot be identified with certainty, but it may belong to Diamesahyperborea Holmgren ( = D. ur-
Macrofossil studies were carried out on 1-5 cm thick half-core slices taken from the cores studied by Vorren et al. (1988). Sample volumes were measured by displacement in water, and the sed-
sus (Kieffer)). Diamesa sp. 2 (Fig. 4h) most probably belongs to D. aberrataLundbeck (Rossaro, 1980, Fig. 3a). The species is well known from Arctic areas
24
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r"/iLL Fig. 4. Drawings of some chironomid fossils encountered in Lake Nedre iErdsvatn. a-d, Procladius(Holotanypus)sp.; e-g, Diamesa sp. 1; h, Diamesa sp. 2; i, Corynoneura sp.; j-m, Metriocnemus obscuripes (Holmgren); n, Orthocladius consobrinus Holmgren; o, Psectrocladius/Zalutschiasp.; p, Psectrocladius sp.; q-t, Micropsectra sp.; u-v, Tanytarsus lugens Kieffer.
28 although the mental teeth in the latter specimen are worn to some extent. The specimen in Fig. 4o is also similar to Orthocladiinae genus? of Styczynski & Rakusa-
Suszczewski (1963), and to the undescribed larvae of Zalutschia tornetraeskensis (Edwards). Larvae of Z. tornetraeskensis have recently been
collected from a river and a shallow pond in the vicinity of Lake Gjende of S. Norway (Schnell & Willassen, pers. obs.). However, the fossils are inadequately preserved for a resolute species identification. A dominant proportion of the fossil assemblage belongs to the tribe Tanytarsini. Taxonomic uncertainty makes the indicator value of Tanytarsini fossils somewhat limited. However, our identifications are based on a few, almost complete larval exuviae, including premandibles. They belong
gens. The species is known as more or less coldstenothermic. In oligotrophic to weakly eutrophic middle European lakes, it occurs as a profundal species. However, in northern European lakes situated in the birch zone, the larvae may ascend to the littoral (Brundin, 1949; Reiss & Fittkau, 1971).
Palaeoenvironment
The occurrence of chironomid remains in the sediments of L. Nedre iErisvatn is strongly correlated with the depositional environment (Fig. 2). Head capsules are rare in the lower, marine part of the sequence (10.49-6.60 m), before 15 500
14
C
cera oliveri Lindeberg (see Hofmann, 1984b, Fig. 3). However, a different mentum and one less dorsal tooth on the mandible is displayed in
yr B.P. Five specimens of M. obscuripes were found in the sample at 6.75-6.70 m (ca 16000 to 15 800 14C yr B.P.). Our data support the palaeoenvironmental reconstruction of Vorren etal. (1988). L. Nedre AErdsvatn was a marine bay at ca 19 500 to 15 500 4C yr B.P. There is no indication of an autochtonous lentic chironomid fauna before 15 500 14 C yr B.P. Since M. obscuripes is a rheobiont freshwater species, the specimens at 6.75-6.70 m have clearly been flushed in from surrounding hygropetric habitats. This first, low-frequent occurrence of chironomids in L. Nedre AErisvatn is correlated with a pronounced climatic amelioration at ca 16000 14 C yr B.P. (Vorren etal., 1988; Alm, 1990), but it may not reflect more than the basin's development into an increasingly effective fossil 'trap' shortly before the isolation from the sea at
T. lugens.
ca 15 500 14C yr B.P. This is also indicated for
to Micropsectra and Tanytarsus.
The larvae of most Micropsectra are coldstenothermic and associated with sediments (Sawedal, 1982). The genus is particularly characteristic of muddy deposits in slack regions of streams and rivers (Pinder & Reiss, 1983). However, some species are dominant in Arctic and high altitude lakes. It is likely that the fossils (Fig. 4q-t) may belong to either Micropsectraborealis (Kieffer) (Sawedal & Willassen, 1980) or Micropsectra coracina (Kieffer), although a few
other species of Micropsectra cannot be ruled out. To some extent, the species identified as Tanytarsuslugens Kieffer (Fig. 4u-v) resembles Coryno-
Reiss & Fittkau (1971) include two species in the Tanytarsus lugens group: T. lugens and T. ba-
thophilus (Kieffer). Hofmann (1971) refers to two additional, similar larval types: Tanytarsus sp. C and Tanytarsus radens Krtiger. The latter species was described from a brackish pond near Waternevers torfer Binnensee in Germany. The premandible of T. radens apparently has a larger number of teeth (KrUger, 1944, Fig. 6) than in the species found by us. We assume that the species in L. Nedre Arsvatn belongs to Tanytarsus lu-
seeds and fruits (Alm & Birks, 1991). Chironomids are present in all post-15 500 14 C yr B.P. lacustrine samples, at first with low concentration and influx values (Fig. 2). Based on marked shifts in dominant taxa (Fig. 3), we have erected three Chironomidae assemblage zones (CAZ 1-3): Ca 15500 to 12600 '4 C yr B.P. - CAZ 1 Metriocnemus - Micropsectra assemblage zone
comprises five samples at 6.50-6.45 to 5.91-
29 5.90 m. All are dominated by Metriocnemus (1981%) and Micropsectra(0-69%), but the absence
of Micropsectra at 6.30-6.25 m makes the zone heterogeneous. Concentration (0.4-12 head capsules cm-3) and influx (0.01-0.60 head capsules
ditions (Alm, 1990). The climate was still arid, but a low-frequency representation of Oxyria pollen may suggest a slightly increased humidity (Vorren et al., 1988).
The chironomid record supports this picture.
cm- 2 yr - 1) values are low (Fig. 3). CAZ 1 spans
Presence of Diamesa and Orthocladiusfrigidus
the first 2 900 years of L. Nedre A)Ersvatn's lacustrine history. According to the L. Ovre A-rsvatn record, this was a period of arid climate and limited surface runoff (Alm, 1990). The chironomids record the existence of at least some brooks. Both
during the last phase of CAZ 1 indicates a water regime characterized by increased flow intensity. The re-increase of Micropsectra at 6.00-5.95 m (ca 12800 to 12700 14C yr B.P.) and 5.91-5.90 m (ca 12650 14 C yr B.P.) suggests an increased influx of organic detritus. Concentration (5-12 head capsules cm- 3) and influx (0.24-0.60 head capsules cm- 2 yr- ) values are higher than before, indicating an increased overall productivity. Several new taxa appear at this level, contributing to an increased diversity. The dominant taxon at ca 12800 14C yr B.P., Metriocnemus, represents a brook or river element. At the same level, an algal (Pediastrum)peak suggests increased lacustrine productivity (Fig. 2). Subsequent samples are characterized by declin-
Metriocnemus and Diamesa (1.9%
at 6.30-
6.25 m) are mainly rheobiontic taxa, preferring running waters. Micropsectra are collectors, feeding on fine organic detritus particles. The high frequency (69 % ) of Micropsectra at 6.50-6.45 m (ca 14 800
14
C yr
B.P.) points to an increased influx of detritus. The pollen record indicates Low to Middle Arctic temperature conditions at this level (Alm, 1990), cf. Fig. 3. Fairly well preserved remains of M. obscuripes pupae indicate that the larval habitat of this species was close to L. Nedre AErfsvatn. The absence of Micropsectra at 6.30-6.25 m (ca
13 600 14 C yr B.P.), and the rather low frequency at 6.10-6.05 m (13000 14 C yr B.P.) fall within an interval of pronouncedly cold, High Arctic climate (Vorren et al., 1988). A strong decline of the pollen influx may indicate a reduced vegetation cover (Alm 1990). A reduced influx of organic detritus may explain the decrease of Micropsectra. Metriocnemus, feeding on epiphytic algae, seemingly fared better. Nonetheless, the total influx of chironomid head capsules is at a minimum, suggesting reduced populations and low overall productivity. The number of taxa show an upwards increasing trend throughout CAZ 1, independent of the climatic depression at 13 700 to 12 800 14 C yr B.P. This may reflect a relatively slow immigration of new species. According to the pollen record, a pronounced climatic amelioration occurred at ca 12 800 14C yr B.P. (Bolling). A surging pollen influx reflects a denser and more productive vegetation cover, and an abrupt change to Low Arctic temperature con-
ing Metriocnemus. The appearance of Procladius,
and increasing Micropsectra frequencies may indicate the first stage towards development of a true lacustrine benthic fauna. Ca 12500 to 12300 (12 000)
14
C yr B.P. - CAZ 2
Micropsectra assemblage zone comprises two samples at 5.80-5.75 and 5.71-5.70 m. Both are entirely dominated by Micropsectra (94-96%).
Concentration (34-540 head capsules cm- 3) and influx (1.7-24.8 head capsules cm - 2 yr - 1) values
increase strongly within the zone, attaining a Late Weichselian maximum at ca 12300 14C yr B.P.
Both samples belong to the upper part of the Bolling amelioration. Abundant chironomids point to an increased productivity. The dominance of Micropsectra suggests ample supply of organic detritus. Unless the Micropsectrarecorded in L. Nedre JArhsvatn was a specialized, running water species, the contribution of lotic elements to the fossil record is reduced. This may explain why the number of taxa is lower than in the uppermost part of CAZ 1.
30 According to Vorren et al. (1988), the first ca 300 years of the Bolling amelioration (ca 12 800 to 12500 14 C yr B.P.) were the climatically most favourable, with a declining trend towards 12 000 14C yr B.P. Pollen influx data, on the other hand, do not indicate more than a modest retardation (Alm, 1990). This may also be indicated by the chironomid record (Fig. 3). Ca 12000 to 11400
14 C yr
B.P. - CAZ 3
Tanytarsus assemblage zone comprises three samples at 5.51-5.50 to 5.26-5.25 m. The lower zone boundary is characterized by a steep rise of Tanytarsus, attaining high frequencies (33-69%) in all three samples. Some new taxa, including the littoral species Psectrocladius and Corynoneura
appear within the zone, contributing to a diversity maximum at ca 11400 1 4C yr B.P. In the lake typology of Brundin (1956), T. lugens is a characteristic species of moderately oligotrophic lakes. The predatory Procladius shows a pronounced increase (7 %) at 5.26-5.25 m. The
characteristic species of ultra-oligotrophic profundal faunas, H. subpilosus (Brundin, 1949), appears for the first time at this level, but with low frequency. CAZ 3 points to development of a more 'mature' lacustrine environment. However, temperate, moderately oligotrophic lakes are often inhabited by at least 100 species of chironomids (Brundin, 1949; Reiss, 1968). It is striking that the fossil record from L. Nedre AErdsvatn shows no remains of other characteristic members of the lake-typological
Tanytarsus lugens community
(Brundin, 1956; Saether, 1979), e.g. Protanypus morio (Zetterstedt), Monodiamesa bathophila Kieffer, Paracladopelma nigritula Goetghebuer, Sergentia coracina (Zetterstedt), and Stictochironomus rosenschoeldi (Zetterstedt). Even at ca
11400 14C yr B.P., the chironomid species composition points to a species-poor, oligotrophic lake. Contrary to this, palynological (algal) data indicate increasingly productive lacustrine conditions during late Allerod (Vorren et al., 1988; Alm, 1990). Apparently, the environment was more favourable for algal growth than for the chirono-
mids. At ca 11 300 14 C yr B.P., eutrophic conditions are witnessed by large numbers of green algae (Pediastrumboryanum, Botryococcus braunii, Scenedesmus, Tetraedron minimum, cf. Fig. 2).
The algal maximum recorded during late Allerod - early Younger Dryas (ca 11300 to 10 500 14 C yr B.P.) may be due to a lowering of the water level, possibly coupled with an increased leaking of nutrients from surrounding mineral soils (Vorren et al., 1988; Alm, 1990). The sediments contain abundant pyrite (Fig. 2), indicating at least seasonal oxygen depletion and stagnating bottom conditions. This would not have any negative effect on planktonic algae (e.g. Pediastrum),but may have restricted the areas suitable for chironomid larvae. There is no indication of a genuine profundal chironomid fauna in the L. Nedre AErisvatn sequence. Comparison with large Arctic lakes (e.g. Oliver, 1964; Welch, 1976) thus seems inappropriate. The chironomid record gives the impression of a relatively shallow lake, sharing characteristics with those described by Andersen (1946) and Styczynski & Rakusa-Suszczewski (1963) from Greenland and Svalbard. There is a general trend in the lake succession from a 'lotic' to a 'lentic' chironomid fauna, which is clearly coupled with deposition of organic sediments in the substrate. The number of chironomids is strongly reduced in the uppermost macrofossil sample, at 5.205.15 m (Fig. 2). This could be an artefact of the sample studied. Consisting mainly of moss (Drepanocladus)remains, it may represent material flushed into the basin in a single episode, thus embedding only a few head capsules. An alternative interpretation is that the lake entered a phase with low water level and in situ moss growth, a limnological situation often characteristic of the terminal successional stage in the ontogeny of shallow lakes. Summary and conclusions (1) A Late Weichselian core sequence from Lake Nedre iErdsvatn, Andoya, Northern Nor-
31 way has been studied with respect to chironomid remains. (2) Chironomid head capsules are rare in the lower, marine part of the sequence, deposited at ca 19500 to 15500 14 C yr B.P. (3) Head capsules occur continuously in the post-15500 14C yr B.P. lacustrine record. Low concentrations and a relative abundance of Metriocnemus at ca 15500 to 12800 14 C yr B.P. indicate a fossil assemblage dominated by input from stream habitats. The lake itself seemingly lacked a significant chironomid fauna. (4) At ca 12800 14C yr B.P., increasing Micropsectrafrequencies may point to establishment of a lentic chironomid fauna. High head capsule concentration and influx values are recorded at ca 12500 to 12000 14 C yr B.P., indicating an increased lacustrine productivity. (5) At ca 12 000 14 C yr B.P., dominance shifted to Tanytarsuslugens. A slightly increased diversity with Corynoneura, Psectrocladius and rising Pro-
cladius at ca 11400 14C yr B.P. may point to somewhat more 'mature' lacustrine conditions. However, several species which are characteristic elements in present-day Arctic lakes are missing, and the fossil record of L. Nedre Airtsvatn has no indication of genuine profundal communities.
Acknowledgements Thanks are due to 0. A. Schnell and I. R. Walker for valuable discussions. The suggestions of two anonymous reviewers are acknowledged. T. Hemma made the illustrations of the fossils.
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