J. V. Ward

Biodiversity of floodplain river ecosystems: ecotones and connectivity1

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

The Four-Dimensional Nature of Lotic Ecosystems

The dynamic and hierarchical nature of lotic ecosystems may be conceptualized in a four-dimensional framework. Upstream-downstream interactions constitute the longitudinal dimension. The lateral dimension includes interactions between the channel and riparian/floodplain systems. Significant interactions also occur between the channel and contiguous groundwater, the vertical dimension. The fourth dimension, time, provides the temporal scale. Lotic ecosystems have developed in response to dynamic patterns and processes occurring along these four dimensions. An holistic approach that employs a spatio-temporal framework, and that perceives disturbances as forces disrupting major interactive pathways, should lead to a more complete understanding of the dynamic and hierarchical structure of natural and altered lotic ecosystems.

An extension of the flood pulse concept.

The flood pulse concept of Junk, Bayley and Sparks is a major contribution to our understanding of river–floodplain interactions and has become an important paradigm in lotic ecology. The concept is based mainly on large tropical lowland rivers. Floodplains may, however, develop in all geographical areas and at different locations along a river corridor. We extend this concept to temperate areas by including information derived from near-natural proglacial, headwater and lowland floodplains. Specific attention is directed to the role of temperature as a major determinant of floodplain ecology. Further attention is directed to the importance of expansion–contraction cycles occurring well below bankfull (‘flow pulse’ versus ‘flood pulse’). Selected examples are presented that highlight the complexity of expansion–contraction events and their consequences on habitat heterogeneity and functional processes. Habitat heterogeneity is mainly a product of shifting water sources, different flow paths and the relative importance of autogenic processes. In different floodplain systems, expansion may enhance habitat heterogeneity (e.g. glacial floodplain) or create homogeneity (e.g. Danubian floodplain). Further, the ecological consequences of episodic flow and flood pulses are discussed. Finally, a landscape approach is suggested in order to document expansion and contraction processes and to elucidate how these processes influence landscape heterogeneity and biodiversity patterns. Such a landscape-based ecosystem model can be applied to rigorously assess the ecological integrity of river–floodplain systems. Copyright

An ecosystem perspective of alluvial rivers: connectivity and the hyporheic corridor

Floodplains of large alluvial rivers are often expansive and characterized by high volume hyporheic flow through lattice-like substrata, probably formed by glacial outwash or lateral migration of the river channel over long time periods. River water downwells into the floodplain at the upstream end; and, depending on bedrock geomorphology and other factors, groundwater from the unconfined aquifer upwells directly into the channel or into floodplain springbrooks at rates determined by head pressure of the water mass moving through the floodplain hydrologic system. These large scale (km3) hyporheic zones contain speciose food webs, including specialized insects with hypogean and epigean life history stages (amphibionts) and obligate groundwater species (stygobionts). Biogeochemical processes in the hyporheic zone may naturally load groundwaters with bioavailable solutes that appear to exert proximal controls on production and biodiversity of surface benthos and riparian vegetation. The effect is especially evident in floodplain springbrooks. Dynamic convergence of aquifer-riverine components adds physical heterogeneity and functional complexity to floodplain landscapes. Because reaches of aggraded alluvium and attendant ecotonal processes occur serially, like beads on a string, along the river continuum, we propose the concept of a hyporheic corridor in alluvial rivers. We expect predictable zonation of groundwater communities and other aquifer-riverine convergence properties within the corridor from headwaters to river mouth. The landscape-level significance and connectivity of processes along the hyporheic corridor must be better understood if river ecosystems, especially those involving large floodplain components, are to be protected and/or rehabilitated.

A General Protocol for Restoration of Regulated Rivers

Large catchment basins may be viewed as ecosystems in which natural and cultural attributes interact. Contemporary river ecology emphasizes the four-dimensional nature of the river continuum and the propensity for riverine biodiversity and bioproduction to be largely controlled by habitat maintenance processes, such as cut and fill alluviation mediated by catchment water yield. Stream regulation reduces annual flow amplitude, increases baseflow variation and changes temperature, mass transport and other important biophysical patterns and attributes. As a result, ecological connectivity between upstream and downstream reaches and between channels, ground waters and floodplains may be severed. Native biodiversity and bioproduction usually are reduced or changed and non-native biota proliferate. Regulated rivers regain normative attributes as distance from the dam increases and in relation to the mode of dam operation. Therefore, dam operations can be used to restructure altered temperature and flow regimes which, coupled with pollution abatement and management of non-native biota, enables natural processes to restore damaged habitats along the river’s course. The expectation is recovery of depressed populations of native species. The protocol requires: restoring peak flows needed to reconnect and periodically reconfigure channel and floodplain habitats; stabilizing baseflows to revitalize food-webs in shallow water habitats; reconstituting seasonal temperature patterns (e.g. by construction of depth selective withdrawal systems on storage dams); maximizing dam passage to allow recovery of fish metapopulation structure; instituting a management belief system that relies upon natural habitat restoration and maintenance, as opposed to artificial propagation, installation of artificial instream structures (river engineering) and predator control; and, practising adaptive ecosystem management. Our restoration protocol should be viewed as an hypothesis derived from the principles of river ecology. Although restoration to aboriginal state is not expected, nor necessarily desired, recovering some large portion of the lost capacity to sustain native biodiversity and bioproduction is possible by management for processes that maintain normative habitat conditions. The cost may be less than expected because the river can do most of the work.

The hyporheic habitat of river ecosystems

Contemporary river ecology is based primarily on biogeochemical studies of the river channel and interactions with shoreline vegetation, even though most rivers have extensive floodplain aquifers that are hydraulically connected to the channel. The hyporheic zone, the interstitial habitat penetrated by riverine animals, is characterized as being spatially limited to no more than a few metres, in most cases centimetres, away from the river channel1–9. However, riverine invertebrates were collected in hundreds per sample within a grid of shallow (10 m) wells located on the flood-plain up to 2 km from the channel of the Flathead River, Montana, USA. Preliminary mass transport calculations indicate that nutrients discharged from the hyporheic zone may be crucial to biotic productivity in the river channel. The strength and spatial magnitude of these interactions demonstrate an unexplored dimension in the ecology of gravel-bed rivers.

Inundation Dynamics in Braided Floodplains: Tagliamento River, Northeast Italy

AbstractThe relationships among water level, inundated area, and shoreline dynamics were investigated in a bar-braided and an island-braided floodplain of the Tagliamento River in northeast Italy. Ground-based surveys with a differential global positioning system (aGPS) unit were used to delineate all aquatic–terrestrial interfaces (shorelines) in the active floodplain at different water levels. Despite complex inundation patterns, a highly significant (P < 0.00001) linear relationship between water level and arcsine square root of inundated area was found in both reaches (y = 0.49x + 0.07). A highly significant (P < 0.00009) second-order polynomial relationship occurred between water level and shoreline length (y = 87.83 − 65.85x2 + 169.83x). Using these relationships as simple predictive models, we converted several years of water-level data into predictions for degree of inundation and shoreline length. The plot of the simulated degree of inundation strongly resembled the actual hydrograph. Complete inundation of the active floodplains occurred one or two times per year; however, the degree of inundation at lower water levels was highly dynamic during most of the year. Simulated shoreline length averaged 171 m ha−1 (13.6 km km−1), with a maximum of 197 m ha−1 (15.6 km km−1) occurring during periods with intermediate water levels. The corresponding values determined with GPS were somewhat higher, with an average value of 181 m ha−1 (14.4 km km−1) and a maximum of 214 m ha−1 (16.3 km km−1). During major flood events, actual shoreline length decreased to 28 m ha−1 (2.1 km km−1). Braiding index and upstream surface hydrologic connectivity were positively related to water level, whereas total area of isolated water bodies was negatively related to water level. The number of nodes remained high most of the time during the 2-year study period.

Ecological Factors Controlling Stream Zoobenthos with Emphasis on Thermal Modification of Regulated Streams

A myriad of factors, including temperature, flow, substrate, aquatic and riparian vegetation, dissolved substances, food and bio-tic interactions, determine the composition and abundance of stream zoobenthos (Macan, 1961, 1974; Hynes, 1970a, b). The influence of the watershed on many of these factors has only recently been fully appreciated (e.g., Hynes,, 1975; Cummins, this volume).

Conservation by restoration: the management concept for a river‐floodplain system on the Danube River in Austria

1. One of the last remnants of a functional alluvial landscape on the Danube extends from Vienna to the Slovakian frontier. It is recognized as an ecosystem extremely worthy of protection and therefore has been designated as a National Park (‘Alluvial Zone National Park’). 2. However, surface connectivity has been reduced and floodplain habits have been fragmented. At present, lateral exchange processes of matter are restricted to short-term flood pulses, while most of the year backwater processes are de-coupled from the river system. 3. A very high species diversity is recorded for this section, with a high proportion of endangered species in all groups, ranging from 16% for riparian vascular plants to 100% for amphibians and reptiles. High diversity is mainly a result of the remaining spatial array of water bodies of different age across the river-floodplain complex (between-channel diversity). 4. A successful conservation strategy for this floodplain area requires a management scheme based on a solid conceptual foundation of the key processes in river-floodplain systems. Re-establishing hydrological dynamics is recognized as the most vital step, because other processes are influenced by the flow regime and resulting connectivity. Therefore, a large-scale pilot project has been developed for a segment of the free-flowing section to restore gradually the hydrological connectivity between the river and its floodplain. 5. The side-arm system will be reconnected to the main channel by lowering parts of the riverside embankments. After implementation, the side-arm system will be integrated with the flow regime of the river for more than half of an average year (at present: <8 days per year). 6. A key challenge in the evaluation of the effects of restoration is the development and testing of an appropriate monitoring scheme, which has to include a wide range of physical, chemical, geomorphic, and ecological parameters.

The Danube restoration project: species diversity patterns across connectivity gradients in the floodplain system

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