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.

The snail Physella virgata virgata, a widely distributed freshwater pulmonate, was observed to change its life-history characteristics in the presence of the crayfish Orconectes virilis in spring-fed Oklahoma streams. These changes were apparently initiated by a water-borne cue released when crayfish fed on conspecific snails. In the presence of the cue, snails exhibited rapid growth rates and little reproduction until they reached a size of about 10 mm after 8 months. In the absence of the cue, snails typically grew to about 4 mm (3.5 months) and then began reproduction. The chemically inducible shift indicates that the life histories of these snails are phenotypically plastic. By increasing the variance associated with size and age of maturity, prey may increase the likelihood of coexisting with seasonal predators.

Estuaries and coastal wetlands are critical transition zones (CTZs) that link land, freshwater habitats, and the sea. CTZs provide essential ecological functions, including decomposition, nutrient cycling, and nutrient production, as well as regulation of fluxes of nutrients, water, particles, and organisms to and from land, rivers, and the ocean. Sedimentassociated biota are integral to these functions. Functional groups considered essential to CTZ processes include heterotrophic bacteria and fungi, as well as many benthic invertebrates. Key invertebrate functions include shredding, which breaks down and recycles organic matter; suspension feeding, which collects and transports sediments across the sediment-water interface; and bioturbating, which moves sediment into or out of the seabed. In addition, macrophytes regulate many aspects of nutrient, particle, and organism dynamics above-and belowground. Animals moving within or through CTZs are vectors that transport nutrients and organic matter across terrestrial, freshwater, and marine interfaces. Significant threats to biodiversity within CTZs are posed by anthropogenic influences; eutrophication, nonnutrient pollutants, species invasions, overfishing, habitat alteration, and climate change affect species richness or composition in many coastal environments. Because biotic diversity in marine CTZ sediments is inherently low whereas their functional significance is great, shifts in diversity are likely to be particularly important. Species introductions (from invasion) or loss (from overfishing or habitat alteration) provide evidence that single-species changes can have overt, sweeping effects on CTZ structure and function. Certain species may be critically important to the maintenance of ecosystem functions in CTZs even though at present there is limited empirical evidence that the number of species in CTZ sediments is critical. We hypothesized that diversity is indeed important to ecosystem function in marine CTZs because high diversity maintains positive interactions among species (facilitation and mutualism), promoting stability and resistance to invasion or other forms of disturbance. The complexity of interactions among species and feedbacks with ecosystem functions suggests that comparative (mensurative) and ma- 430nipulative approaches will be required to elucidate the role of diversity in sustaining CTZ functions.

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Cascading trophic interactions are important in many freshwater pelagic food webs, but their importance in more complex, omnivore—rich littoral—zone food webs is less well known. We tested the existence of a trophic cascade involving omnivorous crayfish (Orconectes rusticus), macroinvertebrates, periphyton, and macrophytes using 9—m2 cages in the littoral zone of Plum Lake, Wisconsin, USA. Treatments in the replicated (N = 4) experiment were crayfish enclosures, crayfish exclosures, and cageless references. During June—September, we measured macrophyte shoot numbers, macroinvertebrate numbers, and periphyton (on plastic strips) chlorophyll a, and dry mass (DM). We expected that crayfish foraging would directly reduce abundance and change species composition of macrophytes and macroinvertebrates and would indirectly enhance periphyton abundance by reducing the abundance of grazing snails. In enclosures, macrophyte and snail (but not nonsnail macroinvertebrate) densities declined significantly throughout the experiment, whereas densities of macrophytes, snails, and nonsnail macroinvertebrates increased in exclosures and cageless references. Some of the reduction in macrophytes resulted from nonconsumptive fragmentation of macrophytes by crayfish. Consistent with the cascading trophic interactions model, periphyton chlorophyll a per unit surface area increased in enclosures, but declined in exclosures. Periphyton quality (as indexed by chlorophyll a/DM) also increased in enclosures relative to exclosures and cageless references. However, because of large reductions in macrophyte surface area (which periphyton colonizes) in enclosures, total amount of periphyton chlorophyll a in enclosures (relative to exclosures) probably declined while periphyton quality per unit surface area and periphyton quality increased. Thus, the impacts of crayfish omnivory on periphyton, expressed in two conflicting indirect effects, confirm the possibility that omnivory can complicate cascading trophic predictions. Overall, results support the existence of strong trophic interactions in the littoral zone, in which omnivorous crayfish control abundance of macrophytes, snails, and periphyton.