A high level of spatio‐temporal heterogeneity makes riverine floodplains among the most species‐rich environments known. Fluvial dynamics from flooding play a major role in maintaining a diversity of lentic, lotic and semi‐aquatic habitat types, each represented by a diversity of successional stages. Ecotones (transition zones between adjacent patches) and connectivity (the strength of interactions across ecotones) are structural and functional elements that result from and contribute to the spatio‐temporal dynamics of riverine ecosystems. In floodplain rivers, ecotones and their adjoining patches are arrayed in hierarchical series across a range of scales. At a coarse scale of resolution, fringing floodplains are themselves complex ecotones between river channels and uplands. At finer scales, patches of various types and sizes form habitat and microhabitat diversity patterns. A broad spatio‐temporal perspective, including patterns and processes across scales, is needed in order to gain insight into riverine biodiversity. We propose a hierarchical framework for examining diversity patterns in floodplain rivers. Various river management schemes disrupt the interactions that structure ecotones and alter the connectivity across transition zones. Such disruptions occur both within and between hierarchical levels, invariably leading to reductions in biodiversity. Species richness data from the connected and disconnected floodplains of the Austrian Danube illustrate this clearly. In much of the world, species‐rich riverine/floodplain environments exist only as isolated fragments across the landscape. In many large rivers, these islands of biodiversity are endangered ecosystems. The fluvial dynamics that formed them have been severely altered. Without ecologically sound restoration of disturbance regimes and connectivity, these remnants of biodiversity will proceed on unidirectional trajectories toward senescence, without rejuvenation. Principles of ecosystem management are necessary to sustain biodiversity in fragmented riverine floodplains. Copyright © 1999 John Wiley & Sons, Ltd.

Summary 1. The relationship between hydrological connectivity, and the exchange processes of suspended sediments, organic matter and nutrients (NO3‐N) was investigated in a dynamically connected river–floodplain segment of the Danube over a 15‐month period in 1995 and 1996 in the Alluvial Zone National Park, Austria. 2. Based on water level dynamics and water retention times, three phases of river–floodplain connectivity were identified: disconnection (phase I), seepage inflow (phase II) and upstream surface connection (phase III). The frequency of occurrence of these phases was 67.5%, 29.3% and 3.2%, respectively, during the study period. 3. A conceptual model is presented linking hydrological connectivity with ecological processes. Generally, the floodplain shifts from a closed and mainly biologically controlled ecosystem during phase I to an increasingly open and more hydrologically controlled system during phases II and III. Phase I, with internal processes dominating, is designated the ‘biotic interaction phase’. 4. Phase II, with massive nutrient inputs to the floodplain yet relatively high residence times, and therefore, high algal biomass, is classified as the ‘primary production phase’. This demonstrates that water level fluctuations well below bankfull may considerably enhance floodplain productivity. 5. Finally, since transport of particulate matter is mainly restricted to short flood pulses above bankfull level, phase III has been defined as the ‘transport phase’. 6. The floodplain served as a major sink for suspended sediments (250 mt ha−−1 year−−1), FPOM (96 mt ha−−1 year−−1), particulate organic carbon (POC; 2.9 mt ha−−1 year−−1) and nitrate‐nitrogen (0.96 mt ha−−1 year−−1), but was a source for dissolved organic carbon (DOC; 240 kg ha−−1 year−−1), algal biomass (chlorophyll‐a; 0.5 kg ha−−1 year−−1) and CPOM (21 kg ha−−1 year−−1). Considerable quantities of DOC and algal biomass were exported to the river channel during phase II, whereas particulate matter transport was largely restricted to the short floods of phase III. 7. The Danube Restoration Project will create a more gradual change between the individual phases by increasing hydrological connectivity between the river channel and the floodplain, and is predicted to enhance productivity by maintaining a balance between retention and export of nutrients and organic matter.

Through functional analyses, integrative physiology is able to link molecular biology with ecology as well as evolutionary biology and is thereby expected to provide access to the evolution of molecular, cellular, and organismic functions; the genetic basis of adaptability; and the shaping of ecological patterns. This paper compiles several exemplary studies of thermal physiology and ecology, carried out at various levels of biological organization from single genes (proteins) to ecosystems. In each of those examples, trade-offs and constraints in thermal adaptation are addressed; these trade-offs and constraints may limit species' distribution and define their level of fitness. For a more comprehensive understanding, the paper sets out to elaborate the functional and conceptual connections among these independent studies and the various organizational levels addressed. This effort illustrates the need for an overarching concept of thermal adaptation that encompasses molecular, organellar, cellular, and whole-organism information as well as the mechanistic links between fitness, ecological success, and organismal physiology. For this data, the hypothesis of oxygen- and capacity-limited thermal tolerance in animals provides such a conceptual framework and allows interpreting the mechanisms of thermal limitation of animals as relevant at the ecological level. While, ideally, evolutionary studies over multiple generations, illustrated by an example study in bacteria, are necessary to test the validity of such complex concepts and underlying hypotheses, animal physiology frequently is constrained to functional studies within one generation. Comparisons of populations in a latitudinal cline, closely related species from different climates, and ontogenetic stages from riverine clines illustrate how evolutionary information can still be gained. An understanding of temperature-dependent shifts in energy turnover, associated with adjustments in aerobic scope and performance, will result. This understanding builds on a mechanistic analysis of the width and location of thermal windows on the temperature scale and also on study of the functional properties of relevant proteins and associated gene expression mechanisms.

1. Zooplankton density and biomass was examined in a Danube River floodplain section with highly variable hydrological dynamics. Temporal patterns were analysed to assess the effects of hydrological conditions on zooplankton community structure and the differential response of the two major zooplankton taxa, rotifers and crustaceans. 2. Calculated floodplain water age was used as an integrated parameter describing hydrological conditions and connectivity. 3. Total zooplankton biomass, crustacean biomass and crustacean species number were significantly positively related to water age. Rotifer biomass followed a hump‐shaped relationship with water age, and rotifer species number decreased with increasing water age. 4. Rotifers dominated the community in periods of low to medium water ages. In periods of higher water ages the community was dominated by crustaceans. 5. We propose that the hydrological regime of floodplains is crucial for zooplankton biomass patterns and succession, through the alternation of washing‐out effects, taxon‐specific potential of reproduction and biological interactions. Flood events and high water levels reset the community to an early successional phase.

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Preparations are currently under way to establish a river restoration concept for the free-flowing section of the Austrian Danube downstream of Vienna. This should serve as a basis for a National Park management. Studies have been initiated to assess the effects of hydrological connectivity, flood pulses and successional processes on backwaters with regard to their hydrochemistry and suspended solid load. Water level fluctuations in the ground-and surface waters of the floodplain are dynamic because of the porous aquifer structure and several inflow areas. In a lateral transect, increasing distance from the Danube correlates with decreasing connectivity. Different influences in a longitudinal transect are due to the positions of the inflow areas. At low water level, the Danube and its backwaters have no surface connection. Internal processes determine the conditions within the backwaters, which are typically characterized by low concentrations of inorganic nutrients and particles versus the Danube. At higher water levels, local surface inflow and seepage water lead to periodic nutrient pulses and eutrophication. The input of inorganic nutrients and particles, as well as the flushing of phytoplankton during floods, establish an initial setpoint in the backwaters. The significance of flood pulses for the dynamics of biological processes is emphasized by short-term changes, e.g. in the relationship between nutrients, inorganic particles and chlorophyll. The response in different water bodies depends on the grade of connectivity to the Danube.

SUMMARY 1. The elemental composition, the proportion of living organic carbon and the carbon stable isotope signatures of particulate organic matter (POM) were determined in a large river floodplain system in order to elucidate the major carbon sources in relation to the hydrological conditions over a 13‐month period. 2. Two floodplain segments and the main channel of the River Danube downstream of Vienna (Austria), were compared on the basis of discharge and water age estimations. The more dynamic floodplain was connected to the main channel for 46% of the study period and drained up to 12% of total discharge at high water. 3. The mean C : N ratio and δ13C signature of the POM increased from the floodplain site that was more isolated from the river (6.6; −33‰) to the main channel (8.4; −25‰). At the dynamic floodplain site, the C : N ratio and the δ13C signature of the POM increased with hydrological connectivity (expressed as water age). 4. Only during flood events (4% frequency of occurrence), a considerable input of riverine POM was observed. This input was indicated by a C : N ratio of the POM pool of more than 10, the amount of detrital carbon (>80% of the total POM pool) and a δ13C signature of POM of more than −25‰ in the dynamic floodplain. 5. Plankton derived carbon, indicated by C : N ratios less than eight and δ13C values lower than −25‰, dominated the particulate organic carbon (POC) pool at both floodplain sites, emphasising the importance of local (autochthonous) production. Phytoplankton was the major plankton compartment at the dynamic site, with highest biomasses at medium water ages. 6. At the dynamic floodplain site, the Danube Restoration Project has enhanced the duration of upstream surface connection with the main channel from 4 to 46% frequency of occurrence. Therefore, the export of living POC to the main channel is now established during phases of maximum phytoplankton production and doubled the estimated total export of non‐refractory POM compared with prerestoration conditions.

The relationship between hydrological connectivity and species diversity patterns (alpha and beta diversity) of macrophytes, molluscs, odonates and amphibians was investigated in a semi‐natural floodplain segment in the ‘Alluvial Zone National Park’ of the Danube River in Austria. Based on environmental variables, we distinguished four major channel types (inflow channel, parapotamal, plesiopotamal and palaeopotamal) that reflected a lateral connectivity gradient. In addition, a longitudinal environmental gradient along the parapotamal channel was found. Connectivity, rather than the surface area of individual floodplain water bodies, explained local species richness. Species diversity patterns varied among taxa: the highest species richness values for molluscs occurred in the parapotamal channels, for odonates in the para‐ and plesiopotamal channels, for macrophytes in the plesiopotamal channels and for amphibians in the palaeopotamal channels. Within the parapotamal channels, the species richness of odonates and amphibians increased moving upstream. Beta diversity displayed an almost inverse relationship with alpha diversity, with highest average values in isolated and fragmented floodplain channels. Habitat fragmentation favoured the beta diversity of most groups, although connectivity favoured the beta diversity of amphibians. The highest proportion of endangered species (mainly rheophilic forms) was found in the parapotamal channels. It is concluded that preservation of the high diversity of this alluvial flood plain would be more fully realised by reconstitution of fluvial dynamics and the associated connectivity gradients, rather than by restoration strategies for individual groups or endangered species. Copyright © 1999 John Wiley & Sons, Ltd.