Effects of snow cover on the benthic fauna in a glacier-fed stream

1. Alpine streams above the tree line are covered by snow for 6–9 months a year. However, winter dynamics in these streams are poorly known. The annual patterns of macroinvertebrate assemblages were studied in a glacial stream in the Austrian Alps, providing information on conditions under the snow.
2. Snow cover influenced water temperature, the content of benthic organic matter and insect development. Taxa richness and abundance of macroinvertebrates did not show a pronounced seasonal pattern. The duration of the autumn period with stable stream beds was important in determining the abundance and composition of the winter fauna.
3. There were significant differences in species composition between summer and winter. Two potential strategies in larval survival were evident: adaptation to the extreme abiotic conditions in summer (e.g. Diamesa spp.) or avoidance of these conditions and development during winter (e.g. Ephemeroptera and Plecoptera).
4. A comparison of a stream reach with continuous snow cover and a stream reach that remained open throughout winter showed that conditions under snow are suboptimal. At the open stream site, with higher water temperatures and greater food supply (benthic organic matter content), abundance and taxa richness was higher and larval growth was faster. Several taxa were found exclusively at this site.
5. Winter conditions did not provide an entirely homogeneous environment, abiotic conditions changed rapidly, especially at the onset of snowfall and at snowmelt. Continuous monitoring is necessary to recognize spatial and temporal heterogeneity in winter environments and the fauna of alpine streams.     

Multi-scale effects of resource patchiness on foraging behaviour and habitat use by longnose dace, Rhinichthys cataractae

1. We examined the response of a predatory benthic fish, the longnose dace ( Rhinichthys cataractae ), to patchiness in the distribution of benthic macroinvertebrates on cobbles at three hierarchical spatial scales during summer and autumn 1996, and spring 1997 in a southern Appalachian stream. 2. At the primary scale (four to five individual cobbles separated by <1 m), the intensity of foraging was not correlated with the biomass of benthic macroinvertebrates/cobble, regardless of season. 3. At the secondary scale (i.e. foraging patches <5 m in diameter) we found that benthic macroinvertebrates were patchily distributed in summer, but not in autumn or spring. Concomitantly, in summer, longnose dace foraged on cobbles with a significantly higher biomass of benthic macronvertebrates than nearby, randomly selected cobbles with similar physical conditions (i.e. longnose dace tended to avoid low-prey foraging patches). In contrast, when benthic macroinvertebrates were distributed homogeneously (spring and autumn), dace did not select patches with a significantly higher biomass of benthic macroinvertebrates than that available on randomly selected cobbles. 4. At the tertiary scale (i.e. stream reaches 11–19 m long), the biomass of benthic macroinvertebrates (per cobble per reach) was patchily distributed (i.e. differed significantly among reaches) in all seasons. Among reaches with physical characteristics preferred by longnose dace, (i.e. erosional reaches dominated by cobble/boulder substratum and high current velocity), we detected a significant, positive correlation between the biomass of benthic macroinvertebrates/cobble and longnose dace density in all seasons. 5. Our results demonstrated that both spatial and temporal patchiness in resource availability influenced significantly the use of both foraging patches and stream reaches by longnose dace.     

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.     

The seasonal dynamics and distribution of Chaoborus flavicans larvae in adjacent lake basins of different morphometry and degree of eutrophication

1.  Seasonal dynamics, spatial distribution and population size of the phantom midge Chaoborus flavicans in different parts of the eutrophic Lake Hiidenvesi (30.3 km²) were studied.
2.  Density of larvae was low in the shallow, most eutrophic parts of the lake, while the deep Kiihkelyksenselkä basin was inhabited by a dense population. In the deepest part of Kiihkelyksenselkä (33 m) density was 13 989 ± 3542 m^–2 in May, declined to 1102 ± 274 m^–2 in July and recovered to 7225 ± 1314 m^–2 by October. In spring and autumn the majority of larvae were benthic while, during high summer, few larvae were found in the sediment.
3.  Horizontal distribution fluctuated seasonally. On 3 June < 5% of the population inhabited areas shallower than 10 m. On 6 July the limnetic fraction was still restricted to regions deeper than 10 m, but 43% of benthic larvae were found between 6 and 10 m depths. In October both limnetic and benthic larvae were concentrated in areas deeper than 20 m.
4.  Within the lake, distribution was mainly regulated by stratification characteristics, degree of eutrophy being less important. The seasonal horizontal movements were probably induced by food shortage. Larvae could not meet their energetic demands in stratified areas and dispersed to shallower water, reducing predation risk by use of the benthic habitat.     

Recovery of benthic macroinvertebrate and adult dragonfly assemblages in response to large scale removal of riparian invasive alien trees

Invasive alien organisms can impact adversely on indigenous biodiversity, while riparian invasive alien trees (IATs), through shading of the habitat, can be a key threat to stream invertebrates. We ask here whether stream fauna can recover when the key threat of riparian IATs is removed. Specifically, we address whether IAT invasion, and subsequent IAT removal, changes benthic macroinvertebrate and adult dragonfly assemblages, for the worse or for the better respectively. Natural riparian zones were controls. There were statistically significant differences between stream reaches with natural, IAT-infested and IAT-cleared riparian vegetation types, based on several metrics: immature macroinvertebrate taxon richness, average score per macroinvertebrate taxon (ASPT), a macroinvertebrate subset (Ephemeroptera, Plecoptera, Trichoptera and Odonata larvae; EPTO), and adult dragonfly species richness. Reaches with natural vegetation, or cleared of IATs, supported greater relative diversity of macroinvertebrates than reaches shaded by dense IATs. Greatest macroinvertebrate ASPT and EPTO were in reaches bordered by natural vegetation and those bordered by vegetation cleared of IATs, and the lowest where the riparian corridor was IATs. Highest number of adult dragonflies species was along streams cleared of dense IATs. Overall, results showed that removal of a highly invasive, dense canopy of alien trees enables recovery of aquatic biodiversity. As benthic macroinvertebrate scores and adult dragonfly species richness are correlated and additive, their combined use is recommended for river condition assessments.     

Bottom fauna and littoral vegetation fauna in Lake Maarsseveen

Summary The littoral vegetation starts its development only in June and reaches its highest expansion in autumn. By that time the organic debris from these plants starts to play an important role for detritivorous organisms. The quantitative development of the fauna in the littoral is highest in autumn and early winter. The bottom fauna in the hypolimnetic parts of the lake is less dependent of the season. The experiments with artificial plants have shown that most species are present when the littoral vegetation is absent. Owing to the absence of substrate in the form of vegetation with epiphyton as a major food resource the numbers in spring and early summer are low. The most productive invertebrate isDreissena polymorpha.     

Taxonomic composition and biomass of heterotrophic flagellates in relation to lake trophy and season

1. Taxonomic composition and abundance of heterotrophic flagellates (HF) were studied in 55 lakes with different trophy in northern Germany using a live-counting technique.
2. Mean abundances and biomasses of HF ranged from 169 cells L⁻¹ and 22 μg L⁻¹ in mesotrophic lakes to 2439 cells mL^–1 and 475 μg L⁻¹ in hypertrophic lakes, respectively. Highest values were generally observed in spring, but mesotrophic lakes showed maximum values in early summer.
3. The taxonomic composition of HF was not significantly influenced by lake trophy and season. The major fraction of HF consisted of chrysomonads and Protista incertae sedis; other important groups were choanoflagellates and bicosoecids. The size distribution of HF changed with lake trophy and season, with a higher proportion of large HF (> 10 μm) in hypertrophic lakes and in spring, respectively.
4. Correlation analyses revealed a strong negative impact of cladocerans on total HF biomass and especially on large HF. Ciliates and large bacteria (> 10 μm) were strongly positively correlated with HF biomass; small bacteria (< 2 μm) showed a weak positive correlation.
5. Analyses at the level of species and genera revealed distinct distribution patterns of some taxa. Paraphysomonas , Aulacomonas and Quadricilia as large-bodied HF showed highest abundance in hypertrophic lakes and in spring. Attached taxa (e.g. Monosiga , Salpingoeca amphoridium ) were highly abundant in late summer and autumn, whereas Spumella and Kathablepharis occurred frequently in most samples.     

Physico-chemistry and aquatic insects of a glacier-fed and a spring-fed alpine stream

1. Physico-chemical conditions and benthic macroinvertebrates were studied in two adjacent alpine streams in the Tyrolean Alps, Austria, for 2 years, and aquatic insect emergence was recorded for 1 year.
2. In the spring-fed system, maximum discharge and increased concentrations of suspended solids, nitrate and particulate phosphorus occurred during snowmelt in June. In the glacier-fed stream, high discharge and strong diel fluctuations in flow and concentrations of suspended solids created a harsh and unstable environment during summer. Glacial ablation, variation in groundwater inflow, and water inputs from tributaries draining calcareous rocks caused water chemistry to vary both seasonally and longitudinally in glacier-fed Rotmoosache.
3. A total of 126 aquatic or semi-aquatic invertebrate taxa were collected, 94 of which were found in the glacier-fed stream and 120 in the spring-fed stream. Chironomid abundance was 2–8 times and taxa richness 2–3 times lower in the glacier-fed stream than in the spring-fed stream, as was the number of chironomid taxa (72 versus 93 total).
4. These results broadly support the conceptual model by Milner & Petts (1994) concerning glacier-fed stream systems. However, single samples and seasonal means showed relatively high invertebrate abundance and richness, especially during winter, indicating a considerable degree of spatial and temporal variability.
5. We suggest that the seasonal shifts from harsh environmental conditions in summer to less severe conditions in autumn and a rather constant environment in winter are an important factor affecting larval development, life-history patterns and the maintenance of relatively high levels of diversity and productivity in glacier-fed streams.     

Colonisation of pitcher plant leaves at several spatial scales

Abstract.  1. The effect of meso-scale (zone within bog and local plant density) and fine-scale (leaf length and resource availability) factors on the colonisation of pitcher plant leaves by arthropods was examined in an eastern Canadian bog.
2. In spring, the abundances of three arthropods, the mosquito Wyeomyia smithii , the midge Metriocnemus knabi , and the mite Sarraceniopus gibsoni , were determined for plots with low, moderate, and high densities of pitcher plants. All overwintering inhabitants were then removed from the plots. Newly opening leaves were colonised from outside the plots, and arthropod abundances were assessed again in autumn.
3. Pitcher plant fauna varied in their response to the meso-scale factors. In autumn (soon after colonisation), midges were more abundant in areas with high densities of pitcher plants. The relationship between mosquito abundance and plant density, and the variation in abundance among zones within the bog in the spring, were probably due to overwintering mortality.
4. All taxa responded to the fine-scale factors, leaf length, and capture rate, in the autumn, but the strength of the responses frequently depended on a meso-scale factor (plant density), in which responses were usually strongest where plants were sparse. Thus, the interaction between meso- and fine-scale processes needs to be considered when interpreting patterns of species abundance within arthropod assemblages in pitcher plant leaves.     

Longitudinal variation in recolonization rates of macroinvertebrates along an upland river in south-eastern Australia

SUMMARY. 1. Recolonization rates of benthic invertebrates were estimated at five sites in the catchment of the Acheron River, in spring (October) and in summer (January), The sites ranged from those that experience short floods and high shear stress at the streambed (upstream sites) to those that experience prolonged floods and low shear stress (downstream sites). We hypothesized that these differences should affect recolonization rate.
2. In October, absolute rates of recolonization of taxa (number of taxa 0.05 m⁻² d⁻¹) onto 1-m² patches of substratum, which had been raked to remove fauna, did not vary between the three sites studied, nor did the relative rates of recolonization of taxa (absolute rate/mean number of taxa in control samples, which were taken from adjacent undisturbed patches of substratum). Absolute rates of recolonization of individuals (number of individuals 0.05 m⁻² d⁻¹) were proportional to the mean number of individuals in control samples; relative rates of recolonization of individuals (absolute rate/mean number of individuals in control samples) did not vary between sites.
3. In January, absolute rates of recolonization either of taxa or individuals were positively correlated with the mean densities of taxa or individuals in control samples; relative rates did not vary between the four sites studied.
4. We conclude that the benthic invertebrate communities at the various sites do not adapt to variations in flooding regime by altering relative recolonization rates. Absolute recolonization rates are directly proportional to the prevailing number of taxa or individuals at a site.     

Spatio-temporal variation in Lepidopteran larval assemblages associated with oak, Quercus crispula: the importance of leaf quality

Abstract.  1. Lepidoptera larval abundance and diversity in the canopies of oak ( Quercus crispula ) trees and saplings were surveyed in a cool-temperate, deciduous broadleaf forest in northern Japan.
2. In general, newly developed leaves were soft, rich in water and nitrogen, and low in tannin, whereas they became tough, poor in water and nitrogen, and high in tannin as the season proceeded. Leaf quality also varied among forest strata, such variations resulting in seasonal and among-strata differences in the structure of the Lepidoptera larval assemblage.
3. The greater Lepidoptera larval abundance and species richness may related to the higher leaf quality on spring foliage compared with summer foliage. On the other hand, diversity (Shannon's H' ) and evenness (Pielou's J' ) were greater on summer foliage than on spring foliage. Strengthened defences of the host plants against herbivory may cause these differences by filtering the larvae of Lepidoptera species and by constraining the super-dominance of a few species on summer foliage.
4. Canonical Correspondence Analysis (CCA) ordination also revealed a stratified structure of the Lepidoptera larval assemblage in the forest. In both spring and summer, the assemblage composition was more similar between sunlit and shaded canopies than between canopies and saplings. Such assemblage stratification was highly correlated with toughness and tannin content (in spring and summer) or water content (in summer).
5. This study emphasised the importance of spatio-temporal variations in leaf quality, even within the same host plant species, for promoting herbivore diversity in forests.     

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.     

唐家河国家级自然保护区川金丝猴生境适宜性评价

生境适宜性评价是濒危物种保护的重要基础。川金丝猴（Rhinopithecus roxellana）是栖息于温带森林的、中国特有的珍稀灵长类动物。位于岷山山系的四川唐家河国家级自然保护区是川金丝猴的重要分布地之一,但涉及该地区川金丝猴的生境信息却较缺乏。运用最大熵（Maximum entropy,MaxEnt）模型对四川唐家河国家级自然保护区川金丝猴不同季节的生境适宜性进行了研究,发现四个季节的训练集和验证集的受试者工作特征曲线下面积（Area under the receiver operator characteristic curve,AUC）值均超过0.8,说明模型预测结果较好。结果显示：（1）影响不同季节川金丝猴分布的主要因子是海拔、河流和道路。（2）川金丝猴的适宜生境面积存在季节性变化。其中,春季的适宜生境面积最大,为233.94 km²,占全区面积的58.48%;夏季的次之,为192.75 km²,占48.19%;秋冬季的适宜生境面积相对较低,分别为145.54 km²（占36.39%）和142.63 km²（占35.66%）。（3）川金丝猴的适宜生境分布具有明显的季节性垂直变化。研究揭示保护好完整的森林植被带对川金丝猴的生存具有重要意义,尤其要重视对人为干扰较强的低海拔生境的保护。     

The independent and interactive effects of fine sediment and flow on benthic invertebrate communities characteristic of small upland streams

SUMMARY 1. Floods are an important mechanism of disturbance operating in streams that can markedly influence the abundance and diversity of benthic fauna. In upland streams many studies cite the scouring effects of fine transported sediments as a potential source of disturbance to the biota during spates, but few studies have sought to test this hypothesis critically.
2. Here we used a series of eight artificial streams to test whether high suspended-sediment concentrations influenced the short-term response of benthic invertebrate fauna to increases in flow. In an experiment designed to simulate a small spate, flow and sediment loads were each manipulated to examine their independent and interactive effects. Benthic invertebrates were sampled before and after the manipulation, and drift samples were taken at regular intervals during the experiment. The experiment was repeated twice, once at the end of winter (June, 1998), and once in summer (February, 1999).
3. Flow increases caused large increases in the number and diversity of drifting animals, and significant declines in the numbers and diversity of organisms found in benthic samples, but these declines were apparently not affected by the addition of fine sediment. The addition of sediment alone had little effect on the fauna. These results were consistent across both experiments.
4. The results suggest that flow increases alone can disturb benthic fauna, and that neither substrate movement nor suspended sediment increases are necessary for floods and spates to disturb the benthic assemblage. However, as argued elsewhere, the effects of flow increases are likely to be contingent upon the presence or absence of local flow refugia, which can allow animals to escape the shear forces that would otherwise remove them from the surface or interstitial areas of the streambed.     

Influences of seasonal flooding on macroinvertebrate abundance in wetland habitats

SUMMARY. 1. We studied the effects of seasonal flooding on macro-invertebrate abundance by manipulating water regime and detrital level within three contiguous experimental marshes in Manitoba, Canada, over 2 years. One area was seasonally flooded (standing water present through midsummer) with emergent vegetation left undisturbed throughout the study, one was semipermanently flooded (standing water present through the ice-free season) with the vegetation left undisturbed, and one was seasonally flooded with the vegetation harvested at the end of the first summer.
2. Abundances of frequent macroinvertebrate taxa were compared between the seasonally flooded-undisturbed treatment area and each of the other areas.
3. Densities of total invertebrates and of the dominant taxa (Cladocera, Ostracoda and Culicidae) were reduced dramatically by a year of semipermanent flooding, despite high levels of paniculate organic food resources and low populations of predators. Densities were not reduced by lowering the availability of detritus under seasonally flooded conditions.
4. Taxa unaffected by water regime included Dytiscidae, Corixidae, Chironomidae, Ceratopogonidae and Ephydridae.
5. Semi-permanent flooding may have eliminated environmental cues necessary for oviposition, embryonic development and hatch among dominant taxa. High invertebrate densities in temporary waters may be more dependent upon life history traits of resident fauna than upon habitat features such as food availability or predation pressure.     

Changing temperature regimes have advanced the phenology of Odonata in the Netherlands

Abstract.  1. Responses of biota to climate change have been well documented for a restricted number of taxa. This study examined shifts in phenology of 37 species of the aquatic insect order Odonata in the Netherlands over the last decade.
2. The present study shows that adults of the Dutch dragonflies and damselflies have advanced their flight dates over recent years due to complex effects of changing temperature regimes on the timing of adult flight dates.
3. Flight dates did not respond to changes in autumn/winter temperatures, advanced with increases in spring temperatures of the focal and previous year, and delayed with increases in summer temperatures of the previous year. Climate change consequently advanced the flight dates of the Odonata because only spring temperatures have increased during the study period.
4. The findings imply that climate change can evoke strong phenological responses in aquatic insects. Moreover, shifts in phenology due to climate change are likely to vary both spatially or temporally, depending on the exact nature of climate change.     

广东罗坑自然保护区鳄蜥生境选择的季节性差异

2006年3-10月,在广东省罗坑鳄蜥自然保护区利用直接观察法对鳄蜥春、夏、秋3季的生境选择进行研究。春、夏、秋3季各测定了31、71、31个有鳄蜥样方以及50、90、51个对照样方的14种生态因子。利用生态因子对比分析和逐步判别分析确定各季节生境选择的主要影响因素。结果表明,春季影响鳄蜥生境选择的主要影响因素是溪沟底质中沙的含量和植被盖度,正确判别率为97.5%;夏季影响鳄蜥生境选择的主要影响因素是溪沟底质中沙的含量、植被盖度、溪流宽度和枯枝百分比,正确判别率为94.4%;秋季影响鳄蜥生境选择的主要影响因素是溪沟底质中沙的含量、植被盖度和可利用食物量,正确判别率为97.6%。在区分春、夏、秋3季鳄蜥生境选择方面有一系列生态因子发挥作用,依照贡献值的大小依次为可利用食物量、干扰距离、枯枝百分比、溪水流速、溪沟坡度和溪宽等6个因子,由这6个变量构成的方程对春、夏、秋3季鳄蜥生境的正确区分率达到76.7%。判别函数的分析表明,春、夏、秋3季鳄蜥的生境选择具有较高的差异性,以判别函数建立的判别分类图表明,春、夏季以及夏、秋季鳄蜥的生境选择重叠较多,而秋季与春季的重叠较少。     

Seasonal variation in the abundance, biomass and biodiversity of earthworms in soils contaminated with metal emissions from a primary smelting works

1. Earthworms (Annelida: Oligochaeta) were sampled on four occasions (spring, summer, autumn and winter) at 14 sites along two transects from a primary lead/ zinc/cadmium smelting works at Avonmouth, UK.
2. Total abundance and biomass of earthworms decreased with proximity to the smelter. No worms were collected from the two sites closest to the factory (<0·6 km) and catches were significantly lower than controls at a further five sites (<3 km).
3. Seasonal composition of sampled communities differed only for summer with lower numbers of individuals and species collected at all sites. Reduced catches in the summer sample is a response to drought.
4. Species richness was lowest at sites close to the factory. For example, worms such as Aporrectodea caliginosa (Savigny) and Allolobophora chlorotica (Savigny) that were dominant at relatively clean sites further from the smelter are absent from the most contaminated soils.
5. Reduced species richness resulted in lower Shannon–Weiner diversity and higher Berger–Parker dominance. Multivariate cluster analysis for spring, summer and winter indicated that sites could be split into three groups based upon relative species composition. In autumn, two clusters were identified.
6. The absence of sensitive species from sites close to the smelting works supports the inclusion of earthworms as a key group in a terrestrial prediction and classification scheme for quantifying the effects of pollutants on soil biodiversity. However, sampling should be carried out in spring or autumn to obtain an accurate picture of community structure.     

云南大中山黑颈长尾雉栖息地选择周年变化

采用比较利用和可利用栖息地的方法,分别对云南大中山黑颈长尾雉秋冬季、春季和夏季栖息地展开调查。检验和Bonferroni置信区间分析结果表明,黑颈长尾雉不同季节均偏向选择常绿阔叶林,偏向选择或回避选择的乔木、灌木和草本的种类不尽相同。利用和可利用样方差异性检验显示,不同季节对地形因素的选择性不明显,仅夏季距水源距离一个因子差异显著;植被因子中,春季差异显著的仅有乔木层盖度因子,夏季有乔木密度、乔木层盖度、落叶层盖度和草本层盖度等4个因子,秋冬季有乔木层盖度、草本层盖度和藤本密度等3个因子。主成分分析表明,不同季节利用样方负荷绝对值较大的因子在各主成分中的序位不尽相同,即栖息地选择的主要生态因子随季节发生变化。单因素方差分析和逐步判别分析表明,秋冬季与春季栖息地特征较接近,而与夏季差别较大。χ2     

Seasonal and diel patterns of invertebrate drift in different alpine stream types

1. We examined the seasonal and diel patterns of invertebrate drift in relation to seston and various habitat characteristics in two each of four different kinds of alpine streams [rhithral (snow‐fed) lake outlets, rhithral streams, kryal (glacial‐fed) lake outlets and kryal streams]. Samples were collected at four times of the day (dawn, midday, dusk and midnight) during three seasons (spring, summer and autumn). 2. Habitat characteristics differed mainly between rhithral and kryal sites, with the latter having higher discharge and turbidity, lower water temperature, and higher concentrations of ammonium, and particulate and soluble reactive phosphorus. Seasonality in habitat characteristics was most pronounced for kryal streams with autumn samples being more similar to rhithral sites. 3. The concentration of seston was lowest in the glacial‐influenced lake outlets and slightly higher in the stream sites; no seasonal or diel patterns were evident. 4. The density of drifting invertebrates averaged less than 100 m^?3 and was lowest (<10 m^?3) at three of the four kryal sites. Taxon richness and diversity were lowest at rhithral lake outlets. Chironomidae dominated the drift as well as benthic communities and <30% of benthic taxa identified were found in the drift. 5. Drifting invertebrates showed no consistent seasonal pattern. However, density tended to be highest in spring at rhithral sites and in autumn at kryal sites. No diel periodicity in drift density was found at any site and the lack of diel pattern may be a general feature of high altitude streams. 6. Glacially influenced habitat parameters were a major factor affecting drift in these alpine streams, whereas no clear differences were observed between streams and lake outlets. Our findings indicate that invertebrate drift in alpine streams is primarily influenced by abiotic factors, and therefore, substantially differs from patterns observed at lower altitude.