Norwegian Journal of Entomology Volume 47 No. 2 • 2000 Published by the Norwegian Entomological Society Oslo and Stavanger NORWEGIAN JOURNAL OF ENTOMOLOGY A continuation of Fauna Norvegica Serie B (1979-1998), Norwegian Journal ofEntomology (1975­ 1978) and Norsk Entomologisk Tidsskrift (1921-1974). Published by The Norwegian Entomological Society (Norsk entomologisk forening). Norwegian Journal of Entomology publishes original papers and reviews on taxonomy, faunistics, zoogeography, general and applied ecology of insects and related terrestrial arthropods. Short com­ munications, e.g. less than two printed pages, are also considered. Manuscripts should be sent to the editor. Editor Lauritz Sl/lmme, Department of Biology, University of Oslo, PO.Box 1050 Blindem, N-0316 Oslo, Norway. E-mail: [email protected]. Editorial secretary Lars Ove Hansen, Zoological Museum, University of Oslo, Sarsgate I, N-0562 Oslo. E-mail: [email protected]. Editorial board Ame C. Nilssen, Tromsl/l John O. Solem, Trondheim Lita Greve Jensen, Bergen Knut Rognes, Stavanger Ame Fjellberg, Tjl/lme The goal of The Norwegian Entomological Society is to encourage the study of entomology in Norway and to provide a meeting place for those who are interested in the field. Annual membership fees are NOK 200 (juniors NOK 100) for members with addresses in Norway, and NOK 220 (Juniors NOK 110) for members abroad. Inquiries about membership should be sent to the secretary: Jan A. Stenll/lkk, P.O.Box 386, N-4002 Stavanger. Norway. E-mail: [email protected]. Norsk entomologisk forening (NEF) ser som sin oppgave a fremme det entomologiske studium i Norge, og danne et bindeledd mellom de interesserte. Medlemskontingenten er kr. 200 (junior kr. 100) pr. ar for innenlandsmedlemmer og kr 220 (junior kr. 110) for medlemmer bosalt utenfor Norge. Forespl/lrsel om medlemsskap i NEF sendes sekret<eren: Jan A. Stenll/lkk, Postboks 386, N-4002 Stavanger. E-mail: [email protected]. NEF web site: http://www.entomologi.no/ Subscription. Members of The Norwegian Entomological Society receive Norwegian Journal of Entomology and Insekt-Nytt free. Institutional and non-member subscription NOK 250. Subscriptipn and membership fees should be sent to the treasurer: Bjl/lm 0kland, Brekkeskog 31, N-1430 As, Norway; account no.: 08065440920. Front cover: Boreophilus henningianus Sahlberg, 1832 (Coleoptera, Staphylinidae). Artist: Frode 0degaard. Printed by: Reprografisk Industri AS, Oslo. ISSN 1501-8415. Norw. J. EntomoJ. 47, 101-112.2000 Diel fluctuations of invertebrate drift in a Norwegian stream north of the Arctic Circle Morten Johansen, J. Malcolm Elliott & Anders Klemetsen Johansen, M., Elliott, J.M. & Klemetsen, A. 2000. Die1 fluctuations of invertebrate drift in a Norwe­ gian stream north of the Arctic Circle. Norw. J. Entomol. 47, 101-112. Sreterelva is a small, third order, stream situated north of the Arctic Circle (latitude 68° N). Drift samples were collected in 1996 during four periods (22-24 May, 23-25 June, 18-20 August and 6-7 October) to determine whether diel fluctuations in drift changed with the changing light regimes of the Arctic summer. Two of the sampling periods (May and June) had continuous daylight, while August and October had dark nights. The taxonomic composition ofthe drift in Sreterelva was similar to that ofother northern temperate streams, being dominated by larvae of Ephemeroptera, Plecoptera and Chironomidae. Total 24 h drift density (± SE) was high, ranging from 242 ± 45 individuals per 100 m) in May to 772 ± 66 per 100 rn' in October. Terrestrial invertebrates contributed a high proportion to the total drift (7-39 %), probably as a result ofthe dense riparian vegetation along the stream. Total drift density of aquatic invertebrates was greater at night than during the day in August and October, while there were no differences between day and night in May and June. Most individual taxa of aquatic invertebrates followed this pattern. The major exception was Hydracarina, which consistently drifted with highest density during the day. It is concluded that simple classifications of drift are difficult to apply to Arctic streams with periods of continuous daylight. Key words: invertebrate drift, aquatic invertebrates, die1 fluctuations, streams. Morten Johansen & Anders Klemetsen, Department ofMarine and Freshwater Biology, University ofTromso, N-9037 Tromso, Non1/ay. J. Malcolm Elliott, Freshwater Biological Association, The Ferry House, Far Sawrey, Ambleside, Cumbria LA22 aLp, United Kingdom. INTRODUCTION causes have been studied extensively (reviews by Waters 1969, 1972, Muller 1974, Statzner et al. Rivers and streams differ from other aquatic en­ 1984, Brittain & Eikeland 1988, Allan 1995). vironments in having a strong unidirectional flow of water that transports materials from upstream Elliott (l965a) published the first account of in­ to downstream areas. This transport forms the ba­ vertebrate drift in Norway. Since then, there have sis of the River Continuum Concept (Vannote et been relatively few publications on this phenome­ al. 1980, Minshall et al. 1985). Physical forces non in Norwegian streams (Steine 1972, Fjellheim associated with the current are perhaps the most 1980, Brekken et al. 1981, Sandlund 1982, Lil­ important factor affecting organisms in running lehammer et al. 1995). Invertebrate drift was in­ water and many invertebrates possess morphologi­ vestigated over several years in Kaltisjokk, a cal adaptations which help them avoid being swept Swedish stream just north of the Arctic Circle (66 away. Nevertheless, many of these organisms en­ ON) (Muller 1966, 1970). The present account is ter the water column (either actively or passively) the first on invertebrate drift in a Norwegian stream and are displaced downstream by the current. This north of the Arctic Circle. phenomenon (termed invertebrate drift) and its 101 Johansen et al.: Diel fluctuations of invertebrate drift in a Norwegian stream Since Tanaka (1960) described die1variations in S~terelva is an important spawning site for brown the composition and the intensity ofthe drift, simi­ trout (Salmo trutta L.), and the fish community lar patterns have been observed frequently all over consists mainly ofjuvenile trout with a few three­ the world (e.g. Waters 1962, Elliott 1965a, b, 1967, spined sticklebacks (Gasterosteus aculeatus L.). McLay 1968, Bishop & Hynes 1969, Clifford 1972, Hynes 1975, Cowell & Carew 1976, Bailey METHODS 1981, Allan et al. 1988, Brewin & Ormerod 1994). Several studies have shown that light is the major The drift of invertebrates was sampled over two cue for the diel periodicity ofthe behavioural drift nights and one day during each of three periods: (e.g. Muller 1965, Elliott 1967, Holt & Waters 22-24 May, 23-25 June, 18-20 August 1996. One 1967, Pearson & Franklin 1968, Bishop 1969, day and one night was also sampled on 6-7 Octo­ CoweIl & Carew 1976). North of the Arctic Cir­ ber 1996. Drift was sampled using four floating cle, daylight is continuous in summer, but progres­ drift nets with a mesh size of 430 Ilm (a detailed sively shortens in autumn to a winter minimum. description is given in Elliott 1967). The nets were emptied every 3 h. Each net sampled to a depth of The main objectives of this study were to exam­ 6 cm and the total submerged area was 288 cm'. ine how the composition and the dieI fluctuations An Ott miniature current meter was used to meas­ of the invertebrate drift changed with season. ure water velocity at the middle ofthe net mouth, at the beginning and end of each 3 h-sample. The STUDY SITE volume of water filtered was estimated from the product of the area of the submerged net mouth, S~terelva is a small third order stream which is current velocity and duration ofsampling. All drift the main tributary of the lake Mokkelandsvatn in samples were preserved in 70 % ethanol, and later southern Troms (northern Norway) north of the sorted in the laboratory using a dissecting micro­ Arctic Circle (68±49DN, l6±27DE) (Figure 1). scope at 6x magnification. The invertebrates were The stream flows through mixed deciduous for­ handpicked, identified and counted. ests, with riparian vegetation dominated by grey alder (Alnus incana (L.) Moench), birch (Betula Flow through the nets was used to calculate drift pubescens Ehrhart) and willow (Salix spp.). The density as the number ofdrifting invertebrates per catchment geology is dominated by micaceous 100 m3 sampled (Waters 1969, Elliott 1970, Al1an gneiss and slate. The flow regime is characterised & Russek 1985). Mean drift densities per samp­ by wide seasonal fluctuations, with peak flows ling date (referred to as 24 h drift density) were during snowmelt in May-June and the lowest flows calculated by pooling all samples from each net 3 during winter. The mean annual discharge is 0.5 m / and dividing by the total number of hours sam­ s. Water temperature falls below 1 DC in early No­ pled at that time. Before statistical analyses, all vember, and the stream is ice covered from No­ drift densities were log10 transformed to ensure vember to May. The area has a coastal climate, that the components of variance were additive with relatively mild winters and cool summers. before using analysis ofvariance (ANOVA) to test The stream is oligotrophic (total nitrogen 85-130 for temporal differences in drift (Elliott 1971). A flg/l, total phosphorus 2-6 flg/l) and circumneutral two-way ANOVA using the General Linear Model (mean pH 7). The stream is mildly polluted, mainly (GLM) was performed with day versus night as from sewage and agricultural runoff (Muladal & one main effect and sampling date as the other Skotvold 1993). main effect. Night in August and October was de­ fined as the period ofdarkness between sunset and A 200 m-long stretch of the stream just upstream sumise. Even though there was no darkness on from Mokkelandsvatn was chosen as the study the sampling occasions in May and June, there was reach.