Chironomid (Diptera: Chironomidae) communities in six European glacier-fed streams

1. A study on glacial stream ecosystems was carried out in six regions across Europe, from Svalbard to the French Pyrenees. The main aim was to test the validity of the conceptual model of 38 with regard to the zonation of chironomids of glacier‐fed rivers along altitudinal and latitudinal gradient. 2. Channel stability varied considerably, both on the latitudinal and altitudinal scale, being lowest in the northern regions (Svalbard, Iceland and Norway) and the Swiss Alps. Water temperature at the upstream sites was always <2 °C. 3. There was a prominent difference in taxonomic richness between the Alpine and the northern European regions, with a higher number of taxa in the south. In all regions, the chironomid community was characterized by the genus Diamesa and the subfamily Orthocladiinae. Of a total of 63 taxa recorded, two (Diamesa bertrami and Orthocladius frigidus) were common in all the regions except Svalbard. 4. On the basis of cluster analysis, seven distinct groups of sites were evident amongst glacial‐fed systems of the five regions (Pyrenees excluded). This classification separated the glacier‐fed streams on geographical, latitudinal and downstream gradients. 5. Canonical Correspondence Analysis (CCA) of environmental variables was carried out using 41 taxa at 105 sites. Slope, water depth, distance from source, water temperature and the Pfankuch channel stability index were found to be the major explanatory environmental variables. The analysis separated Diamesinae and typical upstream orthoclads from the other chironomids by low temperature and high channel instability. 6. In all six regions, Diamesa was present closest to the glacier. Within 200 m of the glacier snout, other genera of Diamesinae were found together with Orthocladiinae. Pioneer taxa like Diamesa species coexisted with later colonizers like Eukiefferiella minor/fittkaui in relatively unstable channels. 7. The longitudinal succession of chironomid assemblages across altitudinal and latitudinal gradients in glacial streams followed the same pattern, with similar genera and groups of species. The general aspects of the conceptual model of 38 were supported. However, Diamesa species have wider temperature limits than predicted and other Diamesinae as well as Orthocladiinae colonize metakryal habitats.     

Longitudinal zonation of macroinvertebrates in an Ecuadorian glacier‐fed stream: do tropical glacial systems fit the temperate model?

1. The ecology of glacier‐fed streams at temperate latitudes has been intensely studied in recent years, leading to the development of a well‐validated conceptual model on the longitudinal distribution of macroinvertebrate communities downstream of the glacier margin (Freshwater Biology, 2001a; 46 , 1833). However, to our knowledge, the ecology of tropical glacier‐fed streams has not yet been studied. 2. We sampled benthic macroinvertebrates and measured environmental variables at nine sites between 4730 and 4225 m altitude along a 4.3 km stretch of a glacier‐fed stream 40 km south of the equator in the Ecuadorian Andes. Our goal was to study the longitudinal distribution of the fauna in relation to environmental factors and to compare this with the conceptual model based on temperate–arctic glacier‐fed streams. 3. Total density of invertebrates differed considerably at the two highest altitude sites; 4600 m^?2 at a pro‐glacial lake outlet and only 4 m^?2 at a site originating directly from the glacier snout. Otherwise, there was a downstream decrease in density to about 825 m^?2 at the three lowest sites. Taxon richness increased with distance from the glacier, very similar to the pattern predicted. A total of 28 taxa were collected; two at the glacier snout, seven at the nearby pro‐glacial lake outlet, 13 at site 2 (<400 m from the glacier) and 20 at the lowest sites. 4. The numerical percentage of Chironomidae (Diptera) decreased downstream from 100 to 44%. The subfamily Podonominae was numerous at the highest sites but became much less important further downstream. The Orthocladiinae were important both in numbers and species at all sites, while Diamesinae were numerous only in the middle of the reach studied and were completely absent from the upper three sites. The limited importance of the Diamesinae, and its replacement by Podonominae, is different from the pattern typically observed in north‐temperate glacier‐fed streams. This could be because of the fact that the genus Diamesa is missing from the Neotropics. 5. Stream temperature and channel stability explained most of the variability in faunal composition and richness, supporting the model. Stability increased systematically downstream while temperature did not. Surprisingly, no classical kryal zone (T_max < 4 °C) was found, as even the site closest to the glacier snout (50 m) had a T_max of 15 °C and no site had T_max < 8 °C. We propose that this might be a general feature of equatorial glacial streams.     

Disruption of a longitudinal pattern in environmental factors and benthic fauna by a glacial tributary

SUMMARY 1. In the upper Rhône catchment (Swiss Alps), modifications in the longitudinal pattern of environmental conditions and the benthic macroinvertebrate fauna were investigated in a glacier-fed stream (Rhône) at its confluence with a smaller glacier-fed tributary (Mutt) in June, August and September 1998. The distance to the source glacier was greater for the Mutt than for the Rhône.
2. Environmental conditions were harsher for the biota in the main stream upstream of the confluence than in the tributary. The tributary upstream of the confluence was characterised by higher taxonomic richness and abundance of the zoobenthos than the Rhône upstream.
3. Although environmental conditions in the main stream were little modified by the tributary, the fauna was richer and more diverse below the confluence. During the period of ice melt, colonisation from the Mutt led to the occurrence of faunal elements atypical of glacial streams in the main glacial stream upstream of the confluence, where water temperature remains below 4 °C.
4. Although contributing an average of only 10% to the Rhône discharge, the Mutt tributary is suggested to be the faunal driver of the system.     

When the going gets tough,the tough get going: The enigma of survival strategies in harsh glacial stream environments

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Longitudinal distribution of macroinvertebrates in two glacier-fed New Zealand rivers

1. Environmental variables, benthic algal biomass and macroinvertebrate fauna were examined from September 1999 to January 2000 (austral summer) along two glacier-fed rivers in South Island, New Zealand.
2. The rivers were characterized by high flow variability, high turbidity and physically disturbed beds. Water temperature ranged from <1 °C near the glacier margin to 10 °C further downstream.
3. Epilithic algal biomass was very low (<0.1 mg m^–2) in months characterized by heavy rainfall, but ranged from 1.1 to 14.4 mg m^–2 following an extended period with negligible precipitation.
4. Abundance and diversity of invertebrates in both rivers was low. Dominant taxa were Chironomidae (Orthocladiinae, Podonominae, Diamesinae), although mayfly species ( Deleatidium : Leptophlebiidae) also occurred at most sites. A species of Eukiefferiella (Orthocladiinae) was collected at all sites and was the most abundant invertebrate close to the glacier margins. No meiofauna were found in either river.
5. Faunal diversity increased at the lowermost stations where species of Plecoptera, Trichoptera, Coleoptera and non-chironomid Diptera also occurred.
6. The faunas of the two New Zealand rivers conformed to the conceptual model of Milner & Petts (1994) in that taxon richness increased downstream with water temperature. However, invertebrate abundance increased downstream in only one of the two rivers. Also in contrast to the model predictions, Leptophlebiidae and Orthocladiinae, rather than Diamesinae, dominated the fauna at the coldest sites.     

The macroinvertebrate communities of two contrasting Norwegian glacial rivers in relation to environmental variables

1. Macroinvertebrate communities in two Norwegian glacial rivers, one in the western fjords (Dalelva) and one in the eastern mountains (Leirungsåi), were investigated during three time periods in 1996 and 1997.
2. Channel stability variables (substratum heterogeneity/Pfankuch index/hydraulic stress) and water temperature accounted for 54% of the total inertia in the principal components analysis (PCA) ordination of environmental variables. The importance of these variables was confirmed by cluster analysis.
3. The two rivers were well separated in the ordinations, with Leirungsåi showing much greater heterogeneity. This is explained by differences in altitudinal range, terrestrial vegetation and the importance and nature of tributary inputs.
4. Channel stability and temperature were also important in determining faunal communities in the two glacial rivers, supporting the main determining variables in the conceptual model of glacial streams ( Milner & Petts, 1994 ). However, clear temporal differences were apparent in the data, the two rivers being more similar during the summer period of high discharge dominated by glacial meltwater. During spring and especially during autumn environmental conditions and the macroinvertebrate fauna differed both within and between rivers.
5. Diamesinae dominated in the upper reaches of both rivers, with Orthocladiinae becoming more common downstream. The dominance of Diamesinae persisted further down Dalelva because of the continued influence of glacial tributaries, whereas in Leirungsåi the influence of non-glacial tributaries led to a change towards a greater proportion of Orthocladiinae. Lakes modified macroinvertebrate communities in both river systems.     

Macroinvertebrate community structure in relation to environmental variables in a Swiss glacial stream

1. Benthic macroinvertebrate distribution was examined in relation to channel characteristics (including stability), substratum, hydraulic variables, primary production (chlorophyll a ) and coarse particular organic matter (CPOM) in an alpine glacial stream, the Mutt (Upper Rhône valley, Switzerland). Co-inertia analysis and canonical correspondence analysis were used to identify the major environmental gradients influencing community variations.
2. The Mutt (length: 3.6 km, altitudinal range: 1800–3099 m a.s.l.) exhibited typical characteristics of a kryal stream. Average summer temperature remained below 2 °C immediately downstream from the snout but was on average 5 °C higher 1700 m downstream. Seasonal variations in water sources were evidenced by the high late-summer (September) contribution of groundwater with increased conductivity.
3. Sixty-six taxa were recorded from the five reaches sampled at three periods (snowmelt, ice melt and low water in late summer) in 1996 and 1997, of which 29 were Chironomidae. Three taxa of Diamesinae were the first colonizers of the stream below the glacier, but 16 taxa, including Ephemeroptera, Plecoptera and Trichoptera, were already recorded 200 m downstream. Water depth, channel slope and Pfankuch's Index of channel stability were strongly correlated with the longitudinal faunal gradient. Maximum temperature, current velocity and water conductivity were also correlated, but to a lesser extent.
4. The rapid incorporation of non-chironomid taxa into the stream community represented a departure from Milner & Petts's (1994) conceptual model of invertebrate succession downstream of glacial margins. The results confirmed that glacial stream communities are primarily driven by physical determinants.     

Macroinvertebrate stream communities along regional and physico-chemical gradients in Western Greenland

1. Macroinvertebrates were collected and physico‐chemical variables measured at 16 stream sites in Western Greenland during July 1999. Eight sites were located on Disko Island in an arctic oceanic climate and eight sites in the Kangerlussuaq area close to the icecap where the climate is arctic continental. The streams had different water sources (glacial, groundwater, snowmelt and lake water). 2. The streams showed pronounced differences in water temperature (2.2–17.3 °C), concentrations of suspended solids (0–2400 mg L^?1), and conductivity (10–109 μS cm^?1). Principal component analysis (PCA) analysis of the physico‐chemical variables separated the Disko Island sites into a distinct group, whereas the sites in the Kangerlussuaq area were more dispersed. 3. A total of 56 macroinvertebrate species were found, including 31 species of Chironomidae, the most abundant of which was Orthocladius thienemanni. Diamesa sp. was only the sixth most abundant chironomid taxon. Species composition varied between sites, and abundance varied from about 20 individuals m^?2 in a glacier fed stream to more than 16 000 m^?2 in a lake outlet. 4. The macroinvertebrate communities of the 16 streams were separated into five TWINSPAN groups reflecting water source, irrespective of region. Lake outlets and ground‐water‐fed streams had the highest species richness and abundance, temperature and bed stability, while glacier‐fed streams were characterized by low species richness, abundance, temperature, bed stability and high concentrations of suspended solids. Macroinvertebrate species richness was positively correlated with water temperature and negatively with bed stability. Conductivity was positively correlated with invertebrate abundance. 5. The results of this study suggest that the source of stream water can be used to predict invertebrate community composition in Greenlandic streams and thus the effects of changes in water balance and flow regime, and to identify sites of special conservation interest.     

Spatial variability in macroinvertebrate assemblages along and among neighbouring equatorial glacier‐fed streams

1. During the past two decades, understanding of the structure and function of glacier‐fed stream ecosystems at temperate latitudes has increased substantially. In contrast, information on their tropical counterparts is very limited. We studied three neighbouring glacier‐fed streams in the tropical Andes of Ecuador. Our main goals were (i) to determine overall longitudinal patterns in density, taxon richness and the composition of macroinvertebrate assemblages and driving factors in equatorial glacial streams and (ii) to examine variability among replicate streams in faunal metrics and assemblages, and stream‐specific effects of supposed environmental key factors. 2. We measured four geographical and 17 environmental factors and collected five Surber samples (500 cm²) of macroinvertebrates at each of nine sites, three sites along three streams. The streams were located 1–5 km apart. In each stream, the three sites were placed at comparable distances from the glacier and were grouped as ‘upper’ (50–200 m), ‘middle’ (1.5 km) and ‘lower’ sites (3.5–5.6 km). 3. In total, 2200 individuals (64% chironomids) were collected and 47 taxa (30 dipterans, 18 of these Chironomidae) identified. Density ranged from 176 to 372 ind. m^?2, and the number of taxa ranged from 2 to 6 at the upper sites and 868–3044 ind. m^?2 and 21–27 taxa at the lower sites. Density, number of taxa, rarefied richness and axis‐1 coordinates from a MDS ordination increased logarithmically with distance from the glacier. These faunal metrics were equally related to altitude and glacier per cent of catchment and correlated with maximum conductivity, mean temperature, mean daily maximum temperature and a channel stability index. As expected, the mean difference in distance decay in similarity was higher at the upper (47% km^?1) than at the lower reaches (20% km^?1) of the streams. 4. The number of taxa varied among sites within the upper and middle groups, but not among the lower sites. In contrast, but in accordance with our expectation, assemblage composition did not differ among upper sites but did so at middle and lower sites, following a supposed decrease in environmental harshness along the streams. Relationships between faunal metrics and the four environmental variables mean temperature, the stability index, chlorophyll a and coarse particulate organic matter also varied among the three streams. Generalised linear model analyses revealed that temperature interacted with stream on macroinvertebrate density, while chlorophyll a had a significant effect on the number of taxa in interaction with stream and stability. 5. The basic predictions of the Milner et al. (2001a) , model regarding longitudinal faunal patterns and temperature and stability as main driving factors were met by our three replicate equatorial glacial streams. Qualitative departures from the model were mainly because of zoogeographical differences. We demonstrated that variability in assemblages between comparable sites in closely situated streams was considerable, and the effect of key environmental factors varied among streams and interacted with other factors. Quantifying spatial variation in benthic assemblages may help us foresee possible consequences for biodiversity as a result of glacial retreat.     

Longitudinal and seasonal distribution patterns of the benthic fauna of an alpine glacial stream (Val Roseg, Swiss Alps)

1. Seasonal changes in longitudinal patterns of environmental conditions and macroinvertebrate community distributions were examined in an alpine glacial stream (Roseg River, Switzerland). 2. Physico‐chemical parameters reflected seasonal changes in glacial influence via shifts in water sources and flowpaths (glacial meltwater versus ground water), and were best described by turbidity, particulate phosphorus and specific conductance. High nitrogen concentrations indicated snowmelt was the main water source in June. 3. Macroinvertebrate densities and taxon richness were highest during spring (4526 m^–2 and 16 taxa, all sites combined) and late autumn/early winter (8676–13 398 m^–2 with 16–18 taxa), indicating these periods may be more favourable for these animals than summer when glacial melting is maximal. Diamesa spp. (Chironomidae) dominated the fauna at the upper three sites (>95% of zoobenthos) and were abundant at all locations. Other common taxa at lower sites (1.2–10.6 km downstream of the glacier terminus) included other chironomids (Orthocladiinae, Tanytarsini), the mayflies Baetis alpinus and Rhithrogena spp., the stoneflies Leuctra spp. and Protonemura spp., blackflies (Simulium spp., Prosimulium spp.), and Oligochaeta. 4. Co‐inertia analysis revealed a strong relationship between environmental conditions and benthic macroinvertebrate assemblages. Furthermore, it elucidated temporal variability in longitudinal response patterns, as well as a similarity in temporal patterns among individual sites. 5. Our results suggest that zoobenthic gradients are not solely related to temperature and channel stability. Seasonal shifts in sources and pathways of water (i.e. extent of glacial influence), and periods of favourable environmental conditions (in spring and late autumn/early winter) also strongly influenced zoobenthic distributions.     

Why on the snow? Winter emergence strategies of snow‐active Chironomidae (Diptera) in Poland

A long‐term study of adult non‐biting midges (Chironomidae) active in winter on the snow in mountain areas and lowlands in Poland yielded 35 species. The lowland and mountain communities differed significantly in their specific composition. The mountain assemblage was found to be more diverse and abundant, with a substantial contribution from the subfamily Diamesinae, whereas Orthocladiinae predominated in the lowlands. Orthocladius wetterensis Brundin was the most characteristic and superdominant species in the winter‐active chironomid communities in both areas. Only a few specimens and species of snow‐active chironomids were recorded in late autumn and early winter. The abundance of chironomids peaked in late February in the mountain and lowland areas with an additional peak in the mountain areas in early April. However, this second peak of activity consisted mainly of Orthocladiinae, as Diamesinae emerged earliest in the season. Most snow‐active species emerged in mid‐ and late winter, but their seasonal patterns differed between the 2 regions as a result of the different species composition and the duration of snow cover in these regions. Spearman's rank correlation coefficient tests yielded positive results between each season and the number of chironomid individuals recorded in the mountain area. A positive correlation between air temperature, rising to +3.5 °C, and the number of specimens recorded on the snow in the mountain community was statistically significant. The winter emergence and mate‐searching strategies of chironomids are discussed in the light of global warming, and a brief compilation of most important published data on the phenomena studied is provided.     

Phylogenetic and Functional Heterogeneity of Sediment Biofilms along Environmental Gradients in a Glacial Stream

We used in situ hybridization with fluorescently labeled rRNA-targeted oligonucleotide probes concurrently with measurements of bacterial carbon production, biomass, and extracellular polymeric substances (EPS) to describe the bacterial community in sediments along a glacial stream. The abundance of sediment-associated Archaea, as detected with the ARCH915 probe, decreased downstream of the glacier snout, and a major storm increased their relative abundance by a factor of 5.5 to 7.9. Bacteria of the Cytophaga-Flavobacterium group were also sixfold to eightfold more abundant in the storm aftermath. Furthermore, elevated numbers of Archaea and members of the Cytophaga-Flavobacterium group characterized the phylogenetic composition of the supraglacial ice community. We postulate that glacial meltwaters constitute a possible source of allochthonous bacteria to the stream biofilms. Although stream water temperature increased dramatically from the glacier snout along the stream (3.5 km), sediment chlorophyll a was the best predictor for bacterial carbon production and specific growth rates along the stream. Concomitant with an increase in sediment chlorophyll a, the EPS carbohydrate-to-bacterial-cell ratio declined 11- to 15-fold along the stream prior to the storm, which is indicative of a larger biofilm matrix in upstream reaches. We assume that a larger biofilm matrix is required to assure prolonged transient storage and enzymatic processing of allochthonous macromolecules, which are likely the major substrate for microbial heterotrophs. Bacteria of the Cytophaga-Flavobacterium cluster, which are well known to degrade complex macromolecules, were most abundant in these stream reaches. Downstream, higher algal biomass continuously supplies heterotrophs with easily available exudates, therefore making a larger matrix unnecessary. As a result, bacterial carbon production and specific growth rates were higher in downstream reaches.     

Runoff and the longitudinal distribution of macroinvertebrates in a glacier‐fed stream: implications for the effects of global warming

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Seasonal changes in stream salmonid population densities in two tributaries of a boreal river in northern Japan

We examined seasonal changes in population densities of stream salmonids (masu salmon Oncorhynchus masou, white-spotted charr Salvelinus leucomaenis, and rainbow trout O. mykiss) in two tributaries of the Shoro River, eastern Hokkaido, Japan. In one small tributary, water temperature was relatively high during the winter, and populations of salmon and trout increased through immigration at this time of the year, becoming dominant components of the salmonid assemblage; the density of charr in this stream decreased during the winter, but charr was dominant during the summer. In another medium-sized tributary, the water temperature fell to close to 0°C during the winter, and densities of salmon and charr decreased in this season, through emigration; trout were very rare in this stream. Seasonal patterns of stream salmonid densities vary among species and between localities, resulting in seasonal changes in species composition. For a comprehensive understanding of population processes, a whole-river survey across seasons will be necessary.     

Global warming effects on benthic macroinvertebrates: a model case study from a small geothermal stream

The aim of this study was to predict global warming effects on benthic macroinvertebrate community structure by using a small temperate geothermal stream as a model system. We collected benthic macroinvertebrates, measured physical and chemical water properties at eight localities up the Kudo?ki stream steep water temperature gradient, and used 11 metrics and indexes to characterize community structure. Species richness and evenness decreased, but total abundance increased with the increase of average annual water temperature (t _av), with species richness being most and total abundance least sensitive to this parameter. The increase of Gastropoda relative abundance and the decrease of Ephemeroptera, Plecoptera and Trichoptera richness, respectively, were the earliest responses of taxonomic groups to t _av increase. Relative abundance of Orthocladiinae decreased and that of Chironomini increased with the increase of t _av. This indicates that Chironomidae are not reliable predictors of global warming effects in running waters, and that lower taxonomic levels, subfamily or tribe, are more suitable for that purpose. Changes in community structure did not linearly follow t _av increase, since a great community shift was observed at t _av ≈ 20°C indicating that present trends of community responses to changes in climatic conditions should not be linearly extrapolated to future warming.     

Ecology of alpine streams

1. This review examines ecological conditions and zoobenthic communities of kryal, krenal and rhithral streams of the alpine zone. Altitudinal and biogeographical faunal patterns are also analysed. 2. Kryal segments, fed by glacial meltwater, are characterized by low temperatures (T_max4°C) and large diel flow fluctuations in summer. The water may be clear or turbid from suspended rock flour. Fishes and higher plants are absent. The macroalga Hydrurus foetidus may be abundant in kryal and other alpine stream types of the Holarctic. The highly restricted cosmopolitan fauna of glacial brooks consists of diamesine chironomids, sometimes accompanied by simuliids. Sparse food resources include algae and allochthonous (aeolian) organic matter. 3. Rhithral segments in alpine catchments are characterized by soft water, a hydrograph dominated by an extended period of snowmelt runoff, and a broader temperature range than kryal or krenal biotopes. Bryophytes, macroalgae (chrysophytes, chlorophytes, cyanophytes, rhodophytes) and epiphytic and epilithic diatoms constitute the flora. A relatively diverse zoobenthos includes four orders of insects (Flecoptera, Ephemeroptera, Trichoptera, Diptera), turbellarians, acarines, oligochaetes and nematodes. 4. Krenal segments, fed by groundwater, are typically calcareous with summer-cool and winter-warm thermal conditions, high water clarity, and constant flow regimes. Bryophytes and macroafgae are accompanied by a rich diatom flora. The zoobenthos consists of a composite of kryal and rhithral elements with few crenobionts. Zoobenthos species richness values are intermediate between those of kryal and rhithral segments, whereas densities in perennial, well-oxygenated springbrooks far exceed those in other alpine stream types. 5. Downstream faunal changes are most predictable in kryal segments where chironomids of the genus Diamesa are the predominant, if not sole, members of the zoobenthos in the upper zone of glacial brooks, the metakryal. Where T_max exceeds about 2°C the transition to the hypokryal occurs and Diamesa is co-dominant with simuliids. These largely stenozonal headwater forms decline downstream where T_max exceeds about 4°C, concomitant with a marked increase in the euryzonal mountain fauna. 6. Species occurring in alpine rhithral biotopes tend to be euryzonal forms at their upper alrirudinal limits, whereas the lower elevation mountain stream fauna consists of species with narrower distribution limits. There is, however, a precipitous drop in mean altitudinal range from the alpine rhithral to the kryat because of the stenozonal nature of the glacial brook fauna. 7. The view that effects of temperature on generation time and mutation rate determine the speed at which selection proceeds is consistent with altitudinal species richness patterns exhibited by zoobenthos along the alrirudinal gradient and may provide an evolutionary explanation for the low faunal diversity in alpine headwaters. 8. With increasing altitude, mountain ‘islands’ become progressively insular as area decreases and isolation increases. For a cold-adapted stream fauna the insular nature of mountain tops is greatest in the tropics. Nonetheless, alpine stream faunas generally     

Longitudinal changes in macroinvertebrate assemblages along a glacial river system in central Iceland

1. Macroinvertebrate assemblages were studied in the glacial river West-Jökulsá, originating from the Hofsjökull Ice Cap in central Iceland at an altitude of 860 m. Sampling sites were distributed from the source to 45 km downstream at 160 m a.s.l. Comparative studies were carried out on non-glacial rivers and tributaries in the area, at similar altitudes and distances from the glacial source.
2. Detrended correspondence analysis (DCA) demonstrated that species composition of benthic macroinvertebrates was related to the distance from the glacier. Assemblages at sampling sites furthest from the glacier were similar in species composition to sites in non-glacial rivers. Temporal variation was small compared with longitudinal zonation.
3. Based on canonical correspondence analysis (CCA) of data from the main glacial river, distance from the glacier, altitude, bryophyte biomass and the Pfankuch Index of channel stability were the measured explanatory variables having a significant effect on the structure of macroinvertebrate assemblages, accounting for 31% of the total variation in the data set. When data from all the rivers were analysed, altitude, bryophyte biomass, channel slope, suspended sediment concentration and maximum water temperature explained 21% of the variance.
4. Macroinvertebrate communities were in general agreement with the predictions of the conceptual model of Milner & Petts (1994) for the upstream reaches. The assemblages consisted mainly of Orthocladiinae and Diamesinae (Chironomidae), although other taxa such as Simuliidae, Plecoptera and Trichoptera were also found in low numbers. Shredders were lacking from the benthic communities, apparently because of continued glacial influence in the river even 45 km downstream from the glacier and lack of allochthononus inputs from riparian vegetation.     

The response of Chironomidae (Diptera) to a long-term exclusion of terrestrial organic matter

We examined the effects of a seven-year detrital exclusion on chironomid assemblages in an Appalachian headwater stream. We hypothesized that litter exclusion would lead to a reduction in all chironomids at both the subfamily and generic levels because organic matter serves as both food and habitat in these headwater streams. Tanytarsini total abundance and biomass significantly declined after litter exclusion. Before litter exclusion, Tanytarsini average abundance was 4271 ± 1135 S.E. m⁻² and 625 ± 98 after litter exclusion. Biomass was 3.57 ± 0.96 mg AFDM m⁻² before litter exclusion and 1.03 ± 0.9 after exclusion. In contrast, Orthocladiinae abundance and biomass did not change because a psammanophilic chironomid, Lopescladius sp., and other Orthocladiinae genera did not decline significantly. Overall chironomid taxa richness and diversity did not change as a result of litter exclusion. However, Canonical Correspondence Analysis (CCA) of genus-level biomass did show a clear separation between the litter exclusion stream and a reference stream. Separation of taxa between the two streams was due to differences in fine (r ² = 0.39) and coarse (r ² = 0.36) organic matter standing stocks and the proportion of small inorganic substrates (r ² = 0.39) present within a sample. As organic matter declined in the litter exclusion stream, overall chironomid biomass declined and the chironomid community assemblage changed. Tanytarsini were replaced by Orthocladiinae in the litter exclusion stream because they were better able to live and feed on biofilm associated with inorganic substrates. Handling editor: K. Martens     

Watershed Glacier Coverage Influences Dissolved Organic Matter Biogeochemistry in Coastal Watersheds of Southeast Alaska

The Coast Mountains of southeast Alaska are currently experiencing some of the highest rates of glacier volume loss on Earth, with unknown implications for proglacial stream biogeochemistry. We analyzed streamwater for δ¹⁸O and dissolved organic matter (DOM) biogeochemistry (concentration, δ¹³C-dissolved organic carbon (DOC), and fluorescence characterization) during the 2012 glacial runoff season from three coastal watersheds in southeast Alaska that ranged in glacier coverage from 0 to 49% and a glacier outflow stream. Our goal was to assess how DOM biogeochemistry may change as receding glaciers are replaced by forests and glaciers contribute less meltwater to streamflow. Discharge and streamwater δ¹⁸O varied seasonally reflecting varying contributions of rainfall and snow/icemelt to streamflow over the runoff season. Mean DOC concentrations were lowest in the glacial outflow and highest in the non-glacial stream reflecting an increasing contribution of vascular plant-derived carbon with decreasing watershed glaciation. Fluorescence and δ¹³C-DOC signatures indicated that DOM shifted from vascular plant-derived, humic-like material in the non-glacial stream toward more δ¹³C-DOC enriched, glacier-derived DOM in the glacial outflow. Streamwater δ¹⁸O was significantly correlated to DOC concentration, δ¹³C-DOC, and protein-like fluorescence of streamwater DOM (all P < 0.05), demonstrating that changes in the source of streamwater across the glacial watershed continuum have important implications for the amount and quality of stream DOM export. Overall, our findings show that continued glacial recession and subsequent changes in glacial runoff could substantially influence the biogeochemistry of coastal temperature watersheds by altering the timing, magnitude, and chemical signature of DOM delivered to streams.     

Environmental harshness and global richness patterns in glacier‐fed streams

Aim To test for a possible effect of environmental harshness on large‐scale latitudinal and elevational patterns in taxon richness of macrofauna in arctic and alpine glacier‐fed streams. Location Svalbard (79° N), Iceland (65° N), Norway (62° N), Switzerland and Italy (46° N), France (43° N), New Zealand (43° S) and Ecuador (0°), covering an elevational gradient from sea level to 4800 m a.s.l. Methods We gathered data from 63 sites along 13 streams and created an index of glacial influence (the glacial index, GI) as an integrative proxy for environmental harshness. The explicative power of the GI, environmental variables, latitude and elevation on taxon richness was tested in generalized linear models. Taxon richness along geographical gradients was analysed at standardized levels of GI in contour plots. Beta diversity and assemblage similarity was calculated at different GI intervals and compared with a null‐model. Results Overall, taxon richness decreased exponentially with increased GI (r²= 0.64), and of all included factors, GI had the highest explicative power. At low values of GI we found that local taxon richness varied along the coupled gradients of latitude and elevation in a hump‐shaped manner. However, this pattern disappeared at high values of GI, i.e. when environmental harshness increased. Beta diversity increased, while similarity among assemblages decreased towards high GI values. Main conclusions In our study system, the number of taxa able to cope with the harshest conditions was largely independent of the regional taxon pool, and environmental harshness constituted a ‘fixed’ constraint for local richness, irrespective of latitude and elevation. Contrary to expectations, we found that beta diversity was highest and similarity lowest among the harshest sites, suggesting that taxon richness was not solely driven by niche selection based on environmental tolerances, but also stochastic ecological drift, leading to dispersal‐limited communities.