ALBERT

Notes: SUMMARY 1. We examined the thermal patterns of the surface waters in the catchment of the Roseg River, which is fed by the meltwaters of two valley glaciers. One of the glaciers has a lake at its terminus. The river corridor comprised a proglacial stream reach below one glacier, the glacier lake outlet stream, a 2.5-km long complex floodplain and a constrained reach extending to the end of the catchment.2. Temperatures were continuously measured with temperature loggers at 27 sites between 1997 and 1998. Moreover, from 1997 to 1999, spot measurements were taken at 33–165 floodplain sites (depending on water level) at monthly intervals.3. The temperature regime of glacial streams, including the glacier lake outlet, was characterised by rapidly increasing temperatures in April and May, a moderate decline from June to September (period of glacial melt) and a subsequent fast decline in autumn. During summer, the lake increased temperatures in the outlet stream by 2–4 °C, compared with the adjacent proglacial stream reach.4. In the main channel (thalweg) of the Roseg River, annual degree-days (DD) ranged from 176 DD in the upper proglacial reach to 1227 DD at the end of the catchment.5. Thermal variation among different channels within the floodplain was higher than the variation along the entire main channel. Floodplain channels lacking surface connection to the main channel accumulated up to 1661 annual DDs.6. Thermal heterogeneity within the floodplain was linked to the glacial flow pulse. With the onset of ice melt, temperatures in the main channel and in channels surface-connected to the main channel began to decline, whereas in surface-disconnected channels temperatures continued to increase; as a consequence, thermal heterogeneity at the floodplain scale rose slightly until September.7. High thermal heterogeneity was not anticipated in the harsh environment of a largely glacierised alpine catchment. The relatively wide range of thermal environments may contribute to the highly diverse zoobenthic community.

Notes: 1. This paper is an introduction to a special issue of Freshwater Biology containing selected papers from the First International Symposium on Riverine Landscapes held in March 2001 in Switzerland.2. The primary goal of the symposium was to synthesise present understanding of riverine landscapes from the perspectives of different disciplines. A landscape approach was used to address interactions between patterns and processes, in the context of spatial heterogeneity, across scales in physical and biological systems.3. The three main themes were: (i) hydrogeomorphic processes, (ii) biological dynamics and (iii) human influences in riverine landscapes.

Notes: 1.  Riverine landscapes are heterogeneous in space (complex mosaic of habitat types) and time (expansion and contraction cycles, landscape legacies). They are inhabited by a diverse and abundant fauna of aquatic, terrestrial and amphibious species.2.  Faunal distribution patterns are determined by interactive processes that reflect the landscape mosaic and complex environmental gradients. The life cycles of many riverine species rely upon a shifting landscape mosaic and other species have become adapted to exploit the characteristically high turn-over of habitats.3.  The complex landscape structure provides a diversity of habitats that sustains various successional stages of faunal assemblages. A dynamic riverine landscape sustains biodiversity by providing a variety of refugia and through ecological feedbacks from the organisms themselves (ecosystem engineering).4.  The migration of many species, aquatic and terrestrial, is tightly coupled with the temporal and spatial dynamics of the shifting landscape mosaic. Alternation of landscape use by terrestrial and aquatic fauna corresponds to the rise and fall of the flood. Complex ecological processes inherent to intact riverine landscapes are reflected in their biodiversity, with important implications for the restoration and management of river corridors.

Notes: 1. This review is presented as a broad synthesis of riverine landscape diversity, beginning with an account of the variety of landscape elements contained within river corridors. Landscape dynamics within river corridors are then examined in the context of landscape evolution, ecological succession and turnover rates of landscape elements. This is followed by an overview of the role of connectivity and ends with a riverine landscape perspective of biodiversity.2. River corridors in the natural state are characterised by a diverse array of landscape elements, including surface waters (a gradient of lotic and lentic waterbodies), the fluvial stygoscape (alluvial aquifers), riparian systems (alluvial forests, marshes, meadows) and geomorphic features (bars and islands, ridges and swales, levees and terraces, fans and deltas, fringing floodplains, wood debris deposits and channel networks).3. Fluvial action (erosion, transport, deposition) is the predominant agent of landscape evolution and also constitutes the natural disturbance regime primarily responsible for sustaining a high level of landscape diversity in river corridors. Although individual landscape features may exhibit high turnover, largely as a function of the interactions between fluvial dynamics and successional phenomena, their relative abundance in the river corridor tends to remain constant over ecological time.4. Hydrological connectivity, the exchange of matter, energy and biota via the aqueous medium, plays a major though poorly understood role in sustaining riverine landscape diversity. Rigorous investigations of connectivity in diverse river systems should provide considerable insight into landscape-level functional processes.5. The species pool in riverine landscapes is derived from terrestrial and aquatic communities inhabiting diverse lotic, lentic, riparian and groundwater habitats arrayed across spatio-temporal gradients. Natural disturbance regimes are responsible for both expanding the resource gradient in riverine landscapes as well as for constraining competitive exclusion.6. Riverine landscapes provide an ideal setting for investigating how complex interactions between disturbance and productivity structure species diversity patterns.

Notes: 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.

Notes: 1.  River corridors can be visualised as a three-dimensional mosaic of surface–subsurface exchange patches over multiple spatial scales. Along major flow paths, surface water downwells into the sediment, travels for some distance beneath or along the stream, eventually mixes with ground water, and then returns to the stream.2.  Spatial variations in bed topography and sediment permeability result in a mosaic of patch types (e.g. gravel versus sandy patches) that differ in their hydrological exchange rate with the surface stream. Biogeochemical processes and invertebrate assemblages vary among patch types as a function of the flux of advected channel water that determines the supply of organic matter and terminal electron acceptors.3.  The overall effect of surface–subsurface hydrological exchanges on nutrient cycling and biodiversity in streams not only depends on the proportion of the different patch types, but also on the frequency distribution of patch size and shape.4.  Because nutrients are essentially produced or depleted at the downwelling end of hyporheic flow paths, reach-scale processing rates of nutrients should be greater in stretches with many small patches (e.g. short compact gravel bars) than in stretches with only a few large patches (e.g. large gravel bars).5.  Based on data from the Rhône River, we predict that a reach with many small bars should offer more hyporheic refugia for epigean fauna than a reach containing only a few large gravel bars because benthic organisms accumulate preferentially in sediments located at the upstream and downwelling edge of bars during floods. However, large bars are more stable and may provide the only refugia during severe flood events.6.  In river floodplain systems exhibiting pronounced expansion/contraction cycles, hyporheic assemblages within newly created patches not only depend on the intrinsic characteristics of these patches but also on their life span, hydrological connection with neighbouring patches, and movement patterns of organisms.7.  Empirical and theoretical evidence illustrate how the spatial arrangement of surface–subsurface exchange patches affects heterogeneity in stream nutrient concentration, surface water temperature, and colonisation of dry reaches by invertebrates.8.  Interactions between fluvial action and geomorphic features, resulting from seasonal and episodic flow pulses, alter surface–subsurface exchange pathways and repeatedly modify the configuration of the mosaic, thereby altering the contribution of the hyporheic zone to nutrient transformation and biodiversity in river corridors.