G. Wayne Minshall

The Role of Disturbance in Stream Ecology

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.

Autotrophy in Stream Ecosystems

where the import of organic matter from outside the system is the predominant feature. Little has been done to test the hypotheses derived from these systems against information obtained from other lotic systems in different vegetational regimes (biomes) or arising along a single river system from its source to the sea. The purpose of this paper is to examine the relative contribution from allochthonous and autochthonous sources of

Microdistribution of benthic invertebrates in a rocky mountain (U.S.A.) stream

A study of the benthic invertebrate community inhabiting a small, foothill trout stream in the Rocky Mountains of Idaho was conducted over a two-year period. Monthly Hess samples and short-term experiments using substratum-filled trays were used to describe the spatial dispersion of the benthos and to examine the response of invertebrate populations to substratum and current. A method was devised for measuring available surface area which involved coating individual stones with latex and measuring the area of the ‘print’ resulting from inking the impression left on the latex mold.The dispersion of all populations was clumped throughout the year. Alteration of the cross-sectional pattern of current velocity and stream bed composition changed the pattern of distribution but not the extent of clumping. Collections made in areas of depositing and eroding substrata revealed a more diverse fauna in the latter. Most groups of organisms found in the riffle were scarcer in the pools or absent from them. The pool fauna contained no important additions over those found in the riffles.After a years study of invertebrate populations in an otherwise undisturbed riffle, the substratum was altered and the flow made more uniform; an increase in the abundance of most of the benthic invertebrates followed. No single factor was responsible for the increase, but the change in substratum size and degree of compaction accounted for most of the change. Interpretation of the results was aided by findings from experiments using substratum-filled trays.Two series of stream experiments using the trays were conducted: one to test the relative importance of current and substratum and the other to test the effect of particle size on the distribution of the benthic fauna. In the first series, placement of trays of stones in a pool resulted in an increase in numbers of some but not all of the invertebrates over numbers usually occurring in the pool. Trays filled with stones and placed in a riffle supported fewer animals than found on the adjacent stream bed but more than in the pool. Variations are attributed to differences in current velocity and amounts of imported organic and inorganic debris. Three different relationships of population numbers to current velocity were found for different members of the community (direct, indirect, and parabolic) over the range of 10 to 60 cm/sec. The second series of experiments consisted of two sets of trays filled with stones of medium or large pebbles, respectively. Nine taxa, as well as all of the combined taxa, showed a preference for trays of small stones over the natural stream bed. A few taxa were noticeably more abundant on the small substratum than on the large but most of the fauna showed only slight increases in numbers or remained constant on the two substrata. Only three taxa showed a direct relation of numbers to total surface area presented by the stones.Number and kinds of organisms found in trays filled with a uniform size of substratum did not correspond to those taken in Hess samples from the natural stream bed. This has important implications in terms of currently recommended pollution monitoring techniques. However, it is suggested that if the substratum composition of the trays more nearly matched that of the stream, the correspondence would be much better. The results of the present study also throw considerable doubt on the adequacy of generalizations derived from earlier studies of responses to substratum size and suggest several reasons for reevaluating current ideas regarding the influence of substratum on invertebrate distribution.

Species richness in streams of different size from the same drainage basin

The species richness of stream benthic invertebrates was studied along a longitudinal profile of the Salmon River, Idaho, during spring, summer, and autumn. Sampling was done using replicate rocks and the analytical approach of Stout and Vandermeer (1975) was used to calculate theoretical number of species present, relative immigration rate, and relative spatial heterogeneity. Species richness varied with stream size, being highest in midorder streams and lower in headwater and high order streams. This downstream shift in species richness conforms to the river continuum concept (Vannote et al. 1980). Possible cause is associated with varying temperature regimes in the different-sized streams as well as other factors. Species richness also varied with season depending on stream size. Low order streams were more individualistic, probably because of a greater influence of local (terrestrial) environmental conditions, but showed a higher richness in summer than in autumn. Larger streams were more similar and showed a higher richness in autumn than in summer. Other community-level indicators, such as relative immigration rate and extinction rate, also showed seasonal differences. We conclude that different macroinvertebrate community types having characteristics of either nonequilibrium (density-independent, opportunistic) or equilibrium conditions can be found in streams from the same drainage basin depending on location along the river continuum and time of the year. These findings have important implications for future studies of stream communities as well as for those of predation and competition in streams. Among other things, we propose that the interactive/noninteractive model of Wilson (1969) may be more appropriate for stream invertebrate communities than those assuming strictly nonequilibrium or equilibrium conditions. We further suggest that any attempt to classify or compare stream communities, such as was done for tropical versus midlatitude streams by Stout and Vandermeer (1975), must take seasonality and position along the downstream gradient into account.

Interpreting the Yellowstone Fires of 1988Ecosystem responses and management implications

Norman L. Christensen is a professor in the Department of Botany, Duke University, Durham, NC 27706. James K. Agee is a professor in the College of Forest Resources, University of Washington, Seattle, WA 98195. Peter F. Brussard is a professor in and the chairman of the Biology Department, University of Nevada, Reno, NV 89557. Jay Hughes is a professor in and dean of the College of Forestry and National Resources, Colorado State University, Fort Collins, CO 80523. Dennis H. Knight is a professor in the Department of Botany, University of Wyoming, Laramie, WY 82071. G. Wayne Minshall is a professor in the Department of Biology, Idaho State University, Pocatello, ID 83209. James M. Peek is a professor in the College of Forest Resources, Wildlife, and Range Science, University of Idaho, Moscow, ID 83843. Stephen J. Pyne is a professor in the Department of History, Arizona State University, West Campus, Phoenix, AZ 85017. Frederick J. Swanson is a senior research scientist in the USDA Forest Ser-

The Effect of Reduction in Stream Flow on Invertebrate Drift

Artificial reduction of stream discharge resulted in an increase in benthic invertebrates in the drift. Virtually all bottom—dwelling forms were affected. Entry into the drift seemed an active process initiated by changes in current velocity and depth, and resulting in reversal of the normal avoidance response to light. See full-text article at JSTOR

Wildfires and Yellowstone's Stream EcosystemsA temporal perspective shows that aquatic recovery parallels forest succession

ew studies have examined the effect of fire on the aquatic biota, and none has adequately addressed major aspects of aquatic ecosystem function. Most of the research has examined the effects of fire on water chemistry (Schindler et al. 1980, Tiedemann et al. 1979). Nevertheless, it is possible to develop a set of predictions regarding the immediate, nearterm, and long-term consequences of the 1988 fires in the Greater Yellowstone Area (GYA) by supplementing the existing information base on fire response and general ecological behavior of aquatic ecosystems with knowledge of the response of aquatic systems to logging and to physical disturbances within the channel. Of the 0.57 million ha of the GYA that burned (Burned Area Survey Team 1988), most (95%) of the area was forest, and the remainder was meadow, grassland, and sagebrush scrubland. Twenty separate river basins or major subbasins were affected by the fires to various degrees (Figure 1). Within Yellowstone National Park (YNP), approximately 32% (1380 km2) of the stream system was influenced by the fires. In addition, the four large oligotrophic lakes (Yellowstone, Shoshone, Lewis, and Heart lakes), which together make up Effects of the fires are

Responses of stream benthic macroinvertebrates to fire

Synthesis of published research on the responses of stream benthic macroinvertebrates to fire in western United States indicates a consistent pattern of response that can guide resource management and future research. Direct effects of fire generally are minor or indiscernible. Indirect effects, resulting primarily from increased rates of runoff and channel alteration, have the greatest impacts on macroinvertebrate community metrics and foodweb responses. Postfire effects are variable in time and space, but in smaller size streams (first to fourth order) that are otherwise undisturbed, changes generally are restricted to the first 5‐10 years following fire and are associated with the more intense burns (crown fires with � 50% of the catchment involved). In unfragmented habitats, initially supporting intact, functioning stream ecosystems, recovery from fire appears to be relatively rapid and to contribute to enhanced aquatic productivity and biodiversity. However, in poorly managed watersheds and those subjected to indiscriminate salvage logging, impacts from fire are expected to be greater and recovery of the macroinvertebrate communities and stream ecosystems more protracted. # 2003 Elsevier Science B.V. All rights reserved.

Nitrogen and phosphorus uptake in two Idaho (USA) headwater wilderness streams

Abstract Nitrate and phosphate solutions were released into two reaches of two central Idaho streams to determine within- and between-stream variability in uptake lengths, uptake rates, and mass transfer coefficients. Physical and biotic stream characteristics and periphyton nitrate-uptake rates in recirculating chambers were measured to determine their influence on nutrient dynamics. Phosphate uptake length did not differ among the four reaches. There were no within-stream differences in nitrate uptake lengths but they did differ between the two streams. Long nitrate uptake lengths likely were due to instream concentrations above saturation but also may have been influenced by differences in active surface area and algal abundance. Nitrate and phosphate uptake lengths were longer, and uptake rates higher, than most other published values. However, mass transfer coefficients were comparable to measurements in other streams. Mass transfer coefficients may be a better parameter for temporal and spatial comparisons of instream nutrient dynamics, and for determining the underlying causes of variability in uptake length.

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.