SUMMARY Introduction
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FreshwaterBiology (2001)46, 166~1672 C. T. ROBINSON, U. UEHLINGER and M. HIEBER Departmentof Limnology, EA WAG/ETH, UeberlandstrasseDuebendorf, Switzerland SUMMARY 1. Changes in water chemistry, benthic organic matter (BOM), and macroinvertebrates were examined in four different glacial streams over an annual cycle. The streams experienced strong seasonalchanges in water chemistry that reflected temporal changes in the influence from the source glacier, especially in water turbidity, particulate phosphorus and conductivity. 2. Nitrogen concentrations were high (nitrate-N values were 130-274 ~g L -1), especially during spring snowmelt runoff. Benthic organic matter attained >600 g m-2 dry mass at certain times, peaks being associated with seasonalblooms of the alga Hydrurus foetidus. 3. Macroinvertebrate taxon richness was two to three times higher (also numbers and biomass) in winter than summer suggesting winter may be a more favourable period for these animals. Benthic densities averaged 1140-3820indo m-2, although peaking as high as 9000 indo m-2. Average annual biomass ranged from 102 to 721 mg m-2, and reached >2000 mg m-2 at one site in autumn. 4. Taxa common to all sites included the dipterans Diamesaspp. and Rhypholophussp., the plecopterans Leuctra spp. and Rhabdiopteryxalpina, and the ephemeropterans Baetisalpinus and Rhithrogenaspp. Principal components analysis clearly separated winter assemblages from those found in summer. Keywords:alpine, Chironomidae,Ephemeroptera, insect, kryal, Plecoptera,seasonality, stream instability of kryal streams limit most assemblagesto Introduction species of Diamesa (Chironomidae) (Steffan, 1971; Glacier-fed streamsand rivers are ubiquitous features Milner & Petts, 1994). of arctic and alpine environments at high altitudes The biota of kryal streams has been little studied; and latitudes, and are coined kryal systems (Steffan, the first report of the macroinvertebrate fauna being 1971).Kryal streams are distinctive, being character- that of Steinbock (1934). Other early authors, inclu- ized by low temperatures, high seasonaland diel ding Thienemann (1941),lIlies (1961),Saether (1968), dischargefluctuations, high loads of suspendedsolids Bretschko (1969), and Steffan (1971), found that from glacial flour, and low channel stability (Milner & glacier-fed streams were dominated by chironomids Petts, 1994;Ward, 1994;Fiireder, 1999).These habitat of the genus Diamesa.Later researchersalso docu- features of glacial streamsare important 'filters' (sensu mented the predominanceof Diamesain kryal systems Tonn, 1990)that. constrain the kinds and abundances (e.g.Kawecka, Kownacka & Kownacki, 1971;Kownacka of aquatic insects able to persist in them (Milner & & Kownacki, 1972)and provided the rationale for the Petts, 1994; Ward, 1994). Indeed, it is generally conceptual model proposed by Milner & Petts (1994) believed that the low temperature «2 DC)and channel for the longitudinal zonation of glacial stream benthos. The latter proposed that changes in the Correspondence:C. T. Robinson,Department of Limnology, longitudinal distribution of benthic invertebrate com- EAWAG/ETH,Ueberlandstrasse 133, 8600 Duebendorf, munities were a function of increasing temperature Switzerland.E-mail: [email protected] and channel stability, with Orthocladiinae, Baetidae, @ 2001 Blackwell Science Ltd 1663 1664 C.T. Robinson et al. Simuliidae, Nemouridae and Chloroperlidae succes- Morteratsch. All sites had sparse riparian vegetation sively becoming more common further downstream. exceptfor Grindelwald where the glacier-snout termi- Milner & Petts (1994)suggested that deviations from nus was below the treeline. The percentage of the this qualitative model may be caused by time since catchmentglaciated varied from 41 % at Steinlimi to glaciation (e.g. Milner, 1987,1994) or by resetscaused 62% at Grindelwald. Annual discharge exhibited by the presence of tributaries and lakes, or changes typical maxima in summer during glacial ablation in catchment morphology. In line with this idea, and minima in winter (Fig. 2). Water temperature Burgherr & Ward (2000)found distinct differences in rarely exceeded2 °C in summer becauseof glacial ice zoobenthic assemblagestructure in a kryal and an melt; temperatures are typically near 0 °C in winter. adjacent proglacial lake (i.e. a lake at the glacier Streamwidths during summer ranged from about 5 m terminus) outlet. at Steinlimi to over 15 m at Morteratsch and Lang. Other than the studies cited above,there is a general Threeof the glacierslie on crystalline granite, while the paucity of information on the zoobenthos of glacier- Grindelwald glacier lies over dolomite bedrock, thus fed streams (but see Flory & Milner, 2000).Most of the increasingthe specific conductanceof its stream. earlier studies also were limited in geographic breadth, many being completed, for example, in the Tatra mountains of Poland (Kaweckaet al., 1971).We Field and laboratory techniques undertook the present study to examine spatial and Study sites were visited monthly (when accessible) temporal dynamics of macroinvertebrate communi- beginning in August or September1998 and ending in ties in a number of different kryal streamsin the Swiss September1999. Lang and Steinlimi were inaccessible Alps over an annual cycle. The results provide in late winter. On the initial visit to each stream, two seasonal information on the biotic variation found sites were chosen for study: an upper site about SO- among glacial streams in the Swiss Alps. 100 m from the glacial snout and a lower site 300- 500 m downstream of the upper site. Only an upper site was establishedat Steinlimi. A water temperature Methods logger (StowAway XTI, Onset Corp., Pocasset,MA, U.S.A., or Minilog, Vemco Ltd, Shad Boy, Nova Scotia, Study sites Canada) recording hourly was installed on the first The four glacial streams examined were in the Swiss sampling date at eachsite. Conductivity (WTW LF323 Alps (Fig. 1). Site names refer to the glacier that at 20 °C), turbidity (Cosmos, Fa. Zullig, Switzerland) provided the primary source of water to eachstream. as nephelometric turbidity units (NTU), and water Sites ranged in altitude from 1320 (Oberer Grindel- temperature were determined in the field on each wald) to 2070 m a.s.l. (Lang) (Table 1). Catchment visit. In addition, alL water sample was collected, areasranged from 6.2 km2 at Steinlimi to 35.3 km2 at placed on ice, returned to the laboratory, and illtered for analysis of nitrogen species (~, NOZI NO2 + NO3, dissolved~N, and particulate-N), phos- phorus species (soluble reactive phosphorus, dis- solved-P and particulate-P), dissolved organic carbon (DOC),particulate organic carbon (POC),total solids {TS), and ash-free dry mass (AFDM) following methods detailed in Tockner et al. (1997). On each sampling date, five benthic samples were collected using a modified Hess sampler (0.042m2, 100 IlIn mesh). Sample locations at each site were selected haphazardly in riffle/run habitats. Samples were stored individually in labelled polyethylene Fig. 1 Map of Switzerland with locations of study streams. Dark bottles, and preserved with 4% formalin in the field. lines delineate the major drainage basins within which each In the laboratory, macroinvertebrates were hand- study glacier is situated. picked from eachsample using a dissectingmicroscope @2001 Blackwell Science Ltd, FreshwaterBiology, 46, 1663-1672 Variation in glacial stream macroinvertebrates 1665 Table 1 Location and general characteristics of the study systems. Coordinates Elevation is at location of the glacial snout Altitude Catchment % Area Glacier Drainage Latitude Longitude (m a.s.l.) area(km1 glaciated Morteratsch Danube 46°26' 9°56' 2050 35.3 52 Steinlimi Rhine 46°42' g024' 1920 6.2 41 Grindelwald Rhine 46°39' go05' 1320 17.3 62 Lang Rhone 46°26' 7°54' 2070 29.3 50 at lOx magnification. All invertebrates were identified summarized as mean values, standard deviations (SD) to the lowest possible taxonomic unit (usually genus), and coefficients of variation (CV). Turbidity, specific counted, and dried at 60 °C for biomass determina- conductanceand particulate phosphorus were graph- tions. The remaining benthic organic matter (BOM) ically presented to illustrate the seasonalchanges in was separated from the inorganic fraction of each water quality. The most common groups of macroin- sample by elutriation, dried at 60 °C until constant vertebrateswere described by their average densities weight (c. 3-7 days), and weighed as dry mass. and relative abundances(%). Benthic organic matter, and macroinvertebratedensity and biomass also were plotted as monthly averages over the study period. Data analysis Taxon richnesswas summarized for upper and lower The physical and chemical measures recorded for sites for samples collected in summer and winter. each site on different sampling dates, and macroin- Finally, principal components analysis was under- vertebrate taxon richness, density and biomass were taken on log(x + 1) transformed density data (varimax rotated) of the most common macroinvertebrate taxa to further illustrate seasonalchanges in assemblage 30 structure among sites. 25 20 Results 15 Physicaland chemicalcharacteristics of the study streams 10 Average streamtemperatures were <2.1 °C at all sites, 0., 5 being lowest at Morteratsch and highest at Steinlimi 1 Q) and lower Lang (Table 2). Conductivity was typically C) ffi <50 ~ cm-1 except at Grindelwald where mean val- -5., [5 ues reached >80 ~ cm-1 because of its dolomite dominated