+$ BOREAS DISCUSSION b h 'Q" 5oreu.s. Vol. 16. pp. 3lfi-321 Oslo. 1987 09 01

Chironomid larvae trails in proglacial lake sediments ALAN MORRISON

Duck& McManuc( 19x4) rcportcdohscr\;rtionsoftraccson~edi- merits rccob ered in traps from Briksdal Lake, Norway. Reference ivas made t o the traces tound in Pleistocene proglacial lake sedi- ments (Gibhard Xi Stuart 1974: G h h a r d 1977) which were ident- ical in form hut were not ;ittrihutcd 10 chirnnornid larvae. Duck X i McManu\ ascrihcd the t i ace\ to chitonomid larvae captured within the traps and they aincluded. 'in ab\encc nf i n ~ i r u obser- bation\ in ;ictive proelasi;il lakes i t I\ \ugpc\ted that they m a y be of use in the interpretation of tracc fowls t i urn ;rncicnt proglacial lake dcpo\it\' In thi\ paper itt wtl trace\ Irorn wdirncnts ot a n active ice-content lakc are devxihed and discusvxi: ohscr\ations made i n the field xiid lahorstor! \hob$ that the oripinators of the trace\ dre i.hironnrnid kirbde.

Field site Austerdahvatnet lies in ;I hedroch basin aligned east-west ;it

latitude flh'11' and 7 0 X m ~iltituclc IFIS I ) . Thc de\elopnient of

thc lake has been well documented (Liestol 1956; Thcakstone 1964: Knudscn & Theakstonc 19x1): at the time of the obser- \ations (1080) the lake was 1 . I X 2.2 km with a maximum depth of 70 in and an average dcpth o f 40 m. Subsequently the glacier has retreated and the lake has lengthened (Theakstone 19x5) hut this has not affected the site iit which the traces were observed.

The geology of the catchment is predominantly garnet-mica \chi51 of the Nonvegian Calcdonides suite and this provides the \cdiment whichisdelivercdtuthe lake. In Austerdalsvatnetsedi- nient size andsedimentation ratcsdecrcscaway from the ice front (,Morrison 1981 ) towards the western end of the lake. Here, as a result ot a swi tch in local-river drainage, fornicr lake bed sedi- ment, (exposed when the lake lcvel was lowered by 70 m as part of i i flood control tchcnie) wcrc exploited and formed a rapidly pr-t)gr;iding delta into the lake (Theakstone 1976). Consequently

wpply of c o a r w (sand size) sediment at the distal end of the lakc mostly by slumping at the delta front. Evidence tor this I\ provided hy increased sedimentation up to 200 m east

Fig. I . Mapot thc glaci;il litkc Au,terdals\ atnct. north is tu the top of the map. The delta at the ca\tcrn end of the lake. where the trails w r r &scr\cd. I \ acti\cl! prograding N o trails were ohscr\ed o n other areas of the lake margin when these were exposed by falling uatci I C V C I .

BOREAS 16 (1987) Chironomid larvae trails 319

from the delta front and the presence of slump structures along the delta foresets. Loading by ice in winter when the lake level naturally falls by 2 m provides a mechanism for slump production in addition to rapid sedimentation rates during the melt season.

The traces described in this paper were observed in the topset beds of this delta where the water temperatures achieved 17°C in contrast to the adjacent body of the lake which reached a maxi- mum of 3.9"C. Austerdalsvatnet, like Briksdal Lake, is cold monomictic (Hikanson & Jansson 1984): traces were observed only on the deltaic sediments and not in other shoreline sediments.

Trace description Two distinct forms of trace were observed but all were semi- circular in cross section. Without the constraint of trap walls, as in the case of Duck & McManus, the trails were extensive, usually circular in plan and at places crossed frequently (Fig. 2) . Previous tracks are also discernible in Fig. 3 as fainter impressions. With change of grain size (Fig. 3) the depth of the trails varied, being deeper and wider in the slightly coarser (fine-sand) fraction and more restricted in the fine-silt/clay sediments. Also apparent in Fig. 2 are the holes where the chironomids emerged from their tubes. The appearance of the trace in the fine sediment is very similar to that described by Duck & McManus (1984:92): 'How- ever at numerous discrete parts l to 10 mm apart, along their lengths they broadened to double their normal width. Thus they had abeaded appearance'. This can he seen in Fig. 3 and isthought to be a function of the activity of the chironomid rather than of the nature of the sediment. The beaded nature was not observed in the coarser sediment and coupled with the holes described above, the pattern of behaviour ofthe chironomid is different in

the finer sediment: it appears to burrow in addition to moving on the surface.

More regular, parallel sided tracks were also noted (Fig. 4) where the nature of the sediment is coarser with ripples. Therc- fore it is not clear if this reflected another form of movement or merely a sediment control on the form of the track. Fig. 3 shows the nature of the traces and change of form related to sediment size with an apparent preference shown by chironomids for finer grains. However a medium sand grain-sized sample taken back for laboratory analysis was found to contain a chironomid larva which lived for several weeks in the sediment. Identification was made by the University of Manchester Museum where it was identified simply as a chironomid larva.

Discussion Theauthor had previously observed chironomids in thesediment ofaglacierfedhighalpinelakein Switzerland and their ubiquitous nature was indicated by Duck & McManus (1984). The traces observed by Duck & McManus were observed after 27 days of storage in the laboratory and some continued to be produced for over 37 days of further storage. The water temperatures were the same as those of the lake (3°C 2 1°C) but the turbulent nature of the water was not reproduced. In Austerdalsvatnet the traces were observed only where the water temperature was above 4°C and the sedimentation rates very low producing almost clear water conditions as seen in the above figures. The activity of the chironomid larvae appears to depend on temperature and alsoon sedimentgrain size. Thelarvae required warmer water conditions in order to travel on the surface and the nature of the resultant traces depends on the burrowing and rotation of the insect in its

Fig. 2. Two distinctive trails are visible. One follows a straight path and loops back upon itself, whilst the second has a headed appearance and follows a much more sinuous course. The 'beading' is due to burrowing in the sediment.

320 AIun Morrison BOREAS 16 (1987)

Fig. 3. Detail of the trail\ showing hurrows on the track which runs left to right across the picture Some burrows (immediately above thepenci1point)lieoff thc track O n the track which runsdiagonally (parallel to the pencil) several hurrowsoccurmore widelyspaced, and the appearance irf the trail changes when the animal moles intn COarser grained sediment. The darker material was coarse silt whilst the lightcr ripplcd sediment wa\ fine sand.

Fig. 4. Parallel sided tracks formed o n rippled sediment - lack of clarity is due to depth of water (8 cm)

BOREAS 16 (1987) Chironomid larvae trails 321

tube (Macan & Worthington 1972). This last observation must References necessarily be speculative because the creatures were not observed producing the traces. Fig. 3 shows subaerial exposure of sediments by a drop in lake level although their formation was subaqueous. Inconclusion,chironomidlarvae traceswerefoundat thedistal

end of an ice-contact lake. The activity of the larvae appears (on the distribution of the traces) to be controlled by water tem- perature and sediment size. The larvae showed a preference for tine-grained, homogeneous sediment with a smooth surface and tended to move out of areas of slightly coarser (medium-sand sized) sediment with surface structures. Absence of evidence of tubc entrancesat thesurfaceof acoarser sediment may be related to their preservation potential in such material: it was in this material that a larva was found in its burrow. Gibbard & Stuart (1974) stated ‘Any meaningful interpretation of the fossil data must await studies on the ecology of modern arctic lowland and temperatc mountain proglacial lakes. However, these may not be strictly comparable because of differences in latitude and atti- tude.’ It is hoped that these obscrvations of traces in situ will go some way in helping elucidate the tracesin ancient proglaciallake sedimcnts.

Duck, R. W. & McManus, J. 1984: Traces produced by chi- ronomid larvae in sediments of an ice-contact proglacial lake. Boreus 13,89-93.

Gibbard, P. L. 1977: Fossil tracks from varvcd scdimcnts near Lammi, south Finland. Bulletin of the Geological Society of Finland 49,53-57.

Gibbard, P. L. &Stuart, A. J. 1974: Tracefossilsfromproglacial lake sediments. Boreas 3,69-74.

HBkanson, L. & Jansson, C. 1984: Principles of Lake Sedi- mentology. Springer.

Hutchinson, G. E. 1957: A Treatise on Limnology, uol 1: Geo- graphy, Physics and Chemistry. Wiley and Sons, New York.

Knudsen, N. T. & Theakstone, W. H. 1981: Recent changcs of the glacier Osterdalsisen, Svartisen, Norway. Geografiska Annaler 63A, 23-30.

Liestol, 0 . 1956: Glacier dammed lakes in Norway. Norsk Geo- grafrsktidsskrift 15,122-149.

Macan, T. T. & Worthington, E . B . 1972: Life in Lakes and Riuers, 320 pp. Collins/Fontana, London.

Morrison, A. A. 1981: Sedimentation patternsin a proglacial lake with particular reference to the influence of windstress and

Acknowledgements. -The work was carried out while the author was in receipt of an NERC grant. Thanks to Eric Milton for his work on some difficult photographic material.

mineralogy. 220 pp. Unpublished Ph.D. thesis, University of Manchester, U.K.

Theakstone, W. H. 1964: Recent studies in the Svartisen area. Norsk Geograjisk tidsskrift 19,31&334.

Theakstone, W . H. 1976: Glacial lake sedimentation, Auster- dalsvatnet Norway. Sedimentology 23,671-688

Alan Morrison, Luton CollegeofHigher Education, Park Square. Luton LU13JU, U. K.; 20th October 1986.