This paper applies concepts of landscape ecology and patch dynamics to lotic systems. We present a framework for the investigation of pattern and process in lotic ecosystems that considers how specific patch characteristics determine biotic and abiotic processes over various scales. Patch characteristics include: size, size distribution within the landscape, juxtaposition, diversity, duration, and mechanisms affecting patch formation. Several topics of current interest in lotic ecology are examined from a patch-dynamics perspective: (1) response of periphyton communities to nutrient patches; (2) effects of patch dynamics on nutrient spiralling; (3) riparian patch dynamics and effects of leaf litter characteristics on lotic food webs; (4) beaver-induced patch dynamics; and (5) patch dynamics of river floodplains. We conclude that a patch-dynamics perspective coupled with a strong experimental approach can enhance the utility and predictive power of unifying concepts in lotic ecology, such as the river continuum hypothesis and nutrient spiralling, through its focus on organismal and process-specific building blocks of lotic systems. The effectiveness of a patch-dynamics approach as a framework for the study of lotic systems lies in the strength of the linkage between reductionist and wholestream perspectives.

Summary 1. This overview of metazoans associated with the riparian/groundwater interface focuses on the fauna inhabiting substratum interstices within the stream bed and in alluvial aquifers beneath the floodplain. The objective is to integrate knowledge of habitat conditions and ecology of the interstitial fauna into a broad spatiotemporal perspective of lotic ecosystems. 2. Most aquatic metazoans of terrestrial ancestry, secondarily aquatic forms including insects and water mites (Hydracarina), are largely confined to surface waters (epigean), most of the time penetrating only the superficial interstices of the stream bed. 3. Primary aquatic metazoans include crustaceans and other groups whose entire evolutionary histories took place in water. Some species are epigean, whereas other members of the primary aquatic fauna are true subterranean forms (hypogean), residing deep within the stream bed and in alluvial aquifers some distance laterally from the channel. 4. The hypogean/epigean affinities of interstitial animals are reflected in repetitive gradients of species distribution patterns along vertical (depth within the stream bed), longitudinal (riffle/pool), and lateral (across the floodplain) spatial dimensions, as well as along recovery trajectories following floods (temporal dimension). 5. Fluvial dynamics and sediment characteristics interact to determine hydraulic conductivity, oxygen levels, pore space, particle size heterogeneity, organic content and other habitat conditions within the interstitial milieu. 6. Multidimensional environmental gradients occur at various scales across riparian/groundwater boundary zones. The spatiotemporal variability of hydrogeomorphological processes plays an important role in determining habitat heterogeneity, habitat stability, and connectivity between habitat patches, thereby structuring biodiversity patterns across the riverine landscape. 7. The erosive action of flooding maintains a diversity of hydrarch and riparian successional stages in alluvial floodplains. The patchy distribution patterns of interstitial communities at the floodplain scale reflect, in part, the spatial heterogeneity engendered by successional processes. 8. Interstitial metazoans engage in passive and active movements between surface waters and ground waters, between aquatic and riparian habitats, and between different habitat types within the lotic system. Some of these are extensive migrations that involve significant exchange of organic matter and energy between ecosystem compartments. 9. The generally high resilience of lotic ecosystems to disturbance is attributable, in part, to high spatiotemporal heterogeneity. Habitat patches less affected by a particular perturbation may serve as ’refugia ‘; from which survivors recolonize more severely affected areas. Mechanisms of refugium use may also occur within habitats, as, for example, through ontogenetic shifts in microhabitat use. Rigorous investigations of interstitial habitats as refugia should lead to a clearer understanding of the roles of d...

A major deterrent to a full understanding of the ecological ramifications of river regulation at the catchment scale is a lack of fundamental knowledge of structural and functional attributes of morphologically intact river systems. For example, both the River Continuum and the Serial Discontinuity Concepts, in their original formulations, had the implicit assumption of a stable, single‐thread channel from headwaters to the sea. The Fiume Tagliamento traverses a course of 172 km from its headwaters in the Italian Alps to the Adriatic Sea. No high dams impede the river's passage as it flows through the characteristic sequence of constrained, braided, and meandering reaches. The Tagliamento, the only large morphologically intact Alpine river remaining in Europe, provides insight into the natural dynamics and complexity that must have characterized Alpine rivers in the pristine state. The Tagliamento has a flashy pluvio‐nival regime (mean Q=109 m3 s−1, with flood flows up to 4000 m3 s−1). Thousands of newly‐uprooted trees were strewn across the active bed and floodplain along the river's course following a major flood in the autumn of 1996. The active floodplain is up to 2 km wide and contains a riparian vegetation mosaic encompassing a range of successional stages. Up to 11 individual channels per cross section occur in the braided middle reaches. Islands are a prominent feature of the riverine landscape and island dynamics are postulated to play a key role in determining pattern and process across scales. Future studies will examine the roles of island dynamics and large woody debris in structuring biodiversity patterns of aquatic biota and successional trajectories of riparian vegetation. The high levels of spatiotemporal heterogeneity exhibited by the Fiume Tagliamento provide a valuable perspective for regulated river ecologists and those engaged in conservation and restoration. Copyright © 1999 John Wiley & Sons, Ltd.

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