We define disturbance in stream ecosystems to be: any relatively discrete event in time that is characterized by a frequency, intensity, and severity outside a predictable range, and that disrupts ecosystem, community, or population structure and changes resources or the physical environment. Of the three major hypotheses relating disturbance to lotic community structure, the dynamic equilibrium hypothesis appears to be generally applicable, although specific studies support the intermediate disturbance hypothesis and the equilibrium model. Differences in disturbance frequency between lentic and lotic systems may explain why biotic interactions are more apparent in lakes than in streams. Responses to both natural and anthropogenic disturbances vary regionally, as illustrated by examples from the mid-continent, Pacific northwest, and southeastern United States. Based on a generalized framework of climatic-biogeochemical characteristics, two features are considered to be most significant in choosing streams for comparative studies of disturbance: hydrologic regimes and comparable geomorphology. A method is described for quantifying predictability of the hydrologic regime based on long-term records of monthly maximum and minimum stream flows. Different channel forms (boulder and cobble, alluvial gravelbed, alluvial sandbed) have different responses to hydrologic disturbance from spates. A number of structural and functional components for comparing disturbance effects within regions and across biomes are presented. Experimental approaches to studying disturbance involve spatial-scale considerations, logistic difficulties, and ethical questions. General questions related to disturbance that could be addressed by stream ecologists are proposed.

Summary 1. Riparian structure and function were considered from a longitudinal perspective in order to identify multiscale couplings with adjacent ecosystems and to identify research needs. 2. We characterized functional zones (with respect to vegetation development in association with various biogeochemical processes) within geomorphological settings using a delineation based upon erosional, transitional and depositional properties. 3. Vegetation dynamics within the riparian corridor are clearly influenced substantially by hydrological disturbance regimes. In turn, we suggest that vegetation productivity and diversity may widely influence riverine biogeochemical processes, especially as related to the consequences of changing redox conditions occurring from upstream to downstream. 4. However, surface and groundwater linkages are the predominant controls of landscape connectivity within riparian systems. 5. The importance of riparian zones as sources and sinks of matter and energy was examined in context of structural and functional attributes, such as sequestering or cycling of nutrients in sediments, retention of water in vegetation, and retention, diffusion or dispersal of biota. 6. The consequences of interactions between different communities (e.g. animals and plants, micro‐organisms and plants) on biogeochemical processes are notably in need of research, especially with respect to control of landscape features. Multiscale approaches, coupling regional and local factors in all three spatial dimensions, are needed in order to understand more synthetically and to model biogeochemical and community processes within the river‐riparian‐upland landscape of catchments.

Disturbance regime is a critical organizing feature of stream communities and ecosystems. The position of a given reach in the river basin and the sediment type within that reach are two key determinants of the frequency and intensity of flow-induced disturbances. We distinguish between predictable and unpredictable events and suggest that predictable discharge events are not disturbances.We relate the dynamics of recovery from disturbance (i.e., resilience) to disturbance regime (i.e., the disturbance history of the site). The most frequently and predictably disturbed sites can be expected to demonstrate the highest resilience.Spatial scale is an important dimension of community structure, dynamics, and recovery from disturbance. We compare the effects on small patches (~<1 m 2) to the effects of large reaches at the river basin level. At small scales, sediment movements and scour are major factors affecting the distribution of populations of aquatic insects or algae. At larger scales, we must deal with channel formation, bank erosion, and interactions with the riparian zone that will affect all taxa and processes.Our understanding of stream ecosystem recovery rests on our grasp of the historical, spatial, and temporal background of contemporary disturbance events.

A synoptic study of photosynthetic and respiratory activity of plankton communities in different Amazon surface waters indicates that large-scale events such as flooding can have a major impact upon the cycling of carbon and nutrients in these aquatic ecosystems. During high water, the major factors influencing primary production appeared to be nutrient concentrations in the mouthbays and varzea (floodplain) lakes and high levels of suspended matter in the Amazon mainstem. In riverine systems, plankton primary production (PPP) averaged 4.04 mg C · m-3 • h-1 , and measures of respiration (R.) averaged 0.67 mg C · m-3 · h-1 • In the more productive varzea lakes and mouthbays, PPP averaged 26.37 mg C · m-3 · h-1 and R. averaged 2.30 mg C · m-a. h-1 • Bacterial densities, 14 C-acetate rate constants for uptake, and plankton carbon : ATP ratios implied that heterotrophic microbiota were important components of the plankton communities in riverine waters. The importance of terrestrial organics to metabolic activity in all waters was implied by high particulate carbon : nitrogen ratios (20: 1). These features were especially evident in riverine surface waters where integrated respiration rates exceeded those of plankton primary production. Riverine respiratory levels may be attributed to several factors: adequate supplies of terrestrial organic carbon, sufficient dissolved nutrient concentrations, increased surface area of suspended matter for microbial attachment and growth, and shading of phytoplankton by suspended matter which reduces photosynthetic activity.Observed supersaturation of Amazon waters by carbon dioxide was similar to observations for other rivers of the world. Shifts of C02 solute components to C02 in surface waters of varzea lakes and mouthbays and of some tributaries implied high partial pressures of carbon dioxide (=500-1500 Pa). The primary source of C02 is most likely decomposing organic matter in planktonic and benthic environments of the rivers, lakes, and flooded terrestrial lowlands.The hypothesis that respiratory input of C02 balanced by evasion (gas lost to atmosphere) is sufficient to explain high C02 vapor pressures in the Amazon River appeared true from our calculations but needs further examination. Particular attention should be given to periods of rising water, when planktonic respiration appears to be two orders of magnitude greater than at periods of high water. Subsequent seasonal studies of the Amazon and other large rivers are needed to determine how the plankton community, the chemistry of terrestrially derived organics and their biological oxidation in water, and inorganic factors control C02 supersaturation and exchange with the atmosphere.