J. Bruce Wallace

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.

EFFECTS OF RESOURCE LIMITATION ON A DETRITAL-BASED ECOSYSTEM ' ~:'

We examined the importance of terrestrial detrital inputs to secondary productivity of a headwater stream. Following a year of pretreatment studies on two headwater streams, we excluded terrestrial litter inputs (=treatment) to one stream while using the other as a reference. We excluded litter for 3 yr followed by 1 yr of small woody debris (≤10 cm diameter) removal and litter exclusion. Monthly benthic samples were collected from dominant mixed substrate (cobble, pebble, and sand-silt) as well as from moss-covered bedrock outcrop substrates. We used randomized intervention analysis (RIA) to test the null hypotheses that no change in abundance or biomass of functional feeding groups or specific taxa occurred in the treatment stream relative to the reference stream. Benthic organic matter was significantly lower in mixed substrate habitats of the treatment stream; however, small woody debris did not show a significant reduction prior to manual removal during year 4. At the end of the treatment period, tot...

Stream detritus dynamics: Regulation by invertebrate consumers

SummaryInsecticide treatment of a small, Appalachian forest stream caused massive downstream insect drift and reduced aquatic insect densities to <10% of an adjacent untreated reference stream. Reduction in breakdown rates of leaf detritus was accompanied by differences in quantity and composition of benthic organic matter between the two streams. Following treatment, transport of particulate organic matter was significantly lower in the treated stream than in the reference stream whereas no significant differences existed prior to treatment. Our results indicate that macroinvertebrate consumers, primarily insects, are important in regulating rates of detritus processing and availability to downstream communities.

Biotic Indices and Stream Ecosystem Processes: Results from an Experimental Study

We investigated the ability of the North Carolina Biotic Index (NCBI) and the Ephemeroptera + Plecoptera + Trichoptera (EPT) index to track an experimental ma- nipulation of the invertebrate community and resultant alteration of several ecosystem-level processes in a headwater stream at the Coweeta Hydrologic Laboratory in western North Carolina. Indices were calculated from quantitative monthly or bimonthly benthic samples of moss-covered rockface and mixed substrate habitats, as well as habitat-weighted values based on the proportion of each habitat in the two streams. One stream (C 55) served as a reference stream over the 6-yr period of late 1984 through 1990, whereas the other (C 54) received seasonal treatments with an insecticide for 3 yr (1986-1988). Throughout pretreatment, treatment, and recovery, both the NCBI and EPT indices tracked the distur- bance regime of the treatment stream. Indices for the reference stream varied little during the 6-yr period. Both the NCBI and EPT suggested strong changes in the treatment stream during treatment relative to both pretreatment and the reference stream. Following cessation of insecticide treatments, both indices reflected improved biotic conditions during first and second years of recovery in C 54. Compared with fauna of mixed substrates, rockface fauna had lower (better) NCBI values during pretreatment, and exhibited a greater proportional increase in tolerant taxa during treatment than mixed substrates, emphasizing the importance of including rockface communities in environmental monitoring programs. Changes in both the EPT and NCBI indices closely corresponded to changes in ecosystem- level processes observed in C 54 from pretreatment to treatment, and recovery periods. These processes include: leaf litter processing rates, organic matter storage, fine particulate organic matter generation and export, and secondary production. With the exception of organic matter storage, all of these processes declined during treatment of C 54, and subsequently increased during recovery. Our results demonstrate the potential of such indices to detect and monitor stream ecosystem changes during and following disturbance. The EPT index was by far the easiest to use from both the standpoint of time required for sample processing and ease of application. Compared with the labor-intensive sample processing, specimen identification and measurement, and data entry required for secondary production calculations, the EPT index was relatively simple and displayed a remarkable ability to track secondary production of invertebrates in the treatment stream. Our data strongly support the inclusion of the EPT and NCBI indices in these southern Appalachian headwater streams as indicators of both degradation and recovery of stream ecosystem processes from chemical-induced disturbance.

Organic matter flow in stream food Webs with reduced detrital resource base

Food webs based on flows of organic matter were developed for two small streams to examine food web response to a large reduction in detrital inputs. At the study site, Coweeta Hydrologic Laboratory in the southern Appalachians, leaf litter inputs and associated microbial assemblages are the main energy source for food webs in headwater streams. We eliminated leaf litter inputs to one stream using a net placed over the first 180 m of stream from its origin. Food webs based on flow of organic matter were developed for a reference stream and the litter-excluded stream for two months, July and December of year 1 of the litter exclusion, to examine effects of leaf litter exclusion on the trophic base of the food web, size distribution of flows, predator-prey interactions, and trophic structure. Flows (mg AFDM·m 22 ·d 21 ; AFDM 5 ash-free dry mass) were estimated using gut content analyses for detritus and prey items, coupled with secondary production esti- mates. We used a whole-stream 13 C tracer method to estimate assimilation of bacteria by invertebrates. The food webs encompassed most (84-91%) of invertebrate secondary pro- duction, but ,30% of the estimated total links. The primary sources of organic matter for the food web in the reference stream were leaf tissue, bacterial carbon, and animal prey, with ;25-30% of total secondary production derived from each. In-stream primary pro- duction led to ,1% of invertebrate secondary production. A higher fraction of food web production in the litter-excluded stream was derived from wood. Magnitudes of detrital flows were lower in the litter-excluded stream, and some taxa were missing compared to the reference stream. The fraction of predator ingestion approached 100% of total secondary production for both streams, but this predation was distributed diffusely among several taxa. Flows to predators were fewer and smaller in the litter-excluded stream, yet these flows had higher per-biomass consumption coefficients, suggesting stronger interactions among the remaining common taxa. These food webs enabled us to examine interactions among taxa in the streams; hence, we found responses of the stream ecosystem to litter- exclusion that we would not have considered had we only measured changes in invertebrate population sizes or system-level changes in organic matter flow.

Local Geomorphology as a Determinant of Macrofaunal Production in a Mountain Stream

By comparing distributions of functional group production among different habitats in an Appalachian mountain stream, the influence of site-specific geomorphology upon the overall functional group composition of the animal community was demonstrated. By replicated monthly sampling, substrate particle size distributions, current velocity, standing crops of benthic organic matter, and production of macrofauna were measured in each of three principal habitats: bedrock-outcrop, riffle, and pool. Samples were taken at randomly assigned locations and the relative number of samples taken from each habitat was assumed to be proportional to the area of the habitat within the stream. These proportions were used to weight production measured in each habitat and the resulting values were summed to obtain production per unit area of average stream bed. The bedrock-outcrop habitat was characterized by high material entertainment and export as indicated by significantly higher current velocities and lower standing crops of detritus compared to the riffle and pool habitats. Pools were sites of low entertainment and high retention of organic matter as demonstrated by significantly lower current velocities and higher accumulations of detritus than other habitats. The riffle habitat was intermediate to the bedrock-outcrop and pool habitats in all parameters measured. Annual production of collector-filterers was highest in the bedrock-outcrop (ash-free dry mass 1920 mg/m2 ), followed by riffle (278 mg/m2 ) and pool (32 mg/m2 ). Although constituting only 19% of the stream area, the bedrock-outcrop habitat contributed 68% of the habitat-weighted collector-filterer production. Annual production of shredders was highest in pools (2616 mg/m2 ), followed by riffles (1657 mg/m2 ) and bedrock-outcrop (579 mg/m2 ). The pool habitat, constituting 23% of stream area, contributed 36% of shredder production. Annual production of scrapers was highest in the riffle habitat (905 mg/m2 ), followed by bedrock-outcrop (517-mg/m2 ) and pool (238 mg/m2 ). Riffles constituted 58% of total stream area and were the source of 77% of the habitat-weighted scraper production. Annual production of engulfing predators was greatest in the pool habitat (2313 mg/m2 ), followed by riffles (1765 mg/m2 ) and bedrock-outcrop (687 mg/m2 ). The relatively lower production of engulfing predators in the bedrock-outcrop habitat reflects a functional shift in mode of resource acquisition by predators, with predaceous collector-filterers (Arcto-psychinae: Trichoptera) predominating in the bedrock-outcrop. Collector-gatherer production was more evenly distributed, with the bedrock-outcrop, riffle, and pool habitats each contributing 14, 54, and 33% to the habitat-weighted production, respectively. Unlike all other functional groups, this distribution was not significantly different from the distribution of stream area among habitats and reflected lack of dependence on specific physical attributes of the local environment for access to food by members of this functional group. Local geomorphology determined the diversity and spatial distribution of bedrock-outcrops, riffles, and pools in the study stream. In turn, the functional structure of the macrofauna, when viewed holistically, was the result of the integration of the relative contributions of each habitat type of total stream area. Total habitat-weighted annual production in the study stream was estimated at 5093 and 1921 mg/m2 for primary and secondary consumers, respectively. The distribution of habitat-weighted production among functional groups was: collector-gatherers (39%), followed by shredders (225), engulfing predators (22%), scrapers (13%), and collector-filterers (8%). This functional structure agrees favorably with current conceptual models of head water streams draining forested catchments.

Leaf Litter as a Source of Dissolved Organic Carbon in Streams

ABSTRACT Dissolved organic carbon (DOC) is an abundant form of organic matter in stream ecosystems. Most research has focused on the watershed as the source of DOC in streams, but DOC also comes from leaching of organic matter stored in the stream channel. We used a whole-ecosystem experimental approach to assess the significance of leaching of organic matter in the channel as a source of DOC in a headwater stream. Inputs of leaf litter were excluded from a forested Appalachian headwater stream for 3 years. Stream-water concentration, export, and instream generation of DOC were reduced in the litter-excluded stream as compared with a nearby untreated reference stream. The proportion of high molecular weight (HMW) DOC (more than 10,000 daltons) in stream water was not altered by litter exclusion. Mean DOC concentration in stream water was directly related to benthic leaf-litter standing stock. Instream generation of DOC from leaf litter stored in the stream channel contributes approximately 30% of daily DOC exports in this forested headwater stream. This source of DOC is greatest during autumn and winter and least during spring and summer. It is higher during increasing discharge than during base flow. We conclude that elimination of litter inputs from a forested headwater stream has altered the biogeochemistry of DOC in this ecosystem.

SUBSTRATE-MEDIATED RESPONSE OF STREAM INVERTEBRATES TO DISTURBANCE'

The response of aquatic invertebrates to a major watershed (catchment) disturbance, clear—cutting, was examined in a second—order stream in the southern Appalachian Mountains. For 21 mo after the start of logging, invertebrates were sampled in four substrate types: rock face (moss—covered boulders and outcrops), cobble riffles, pebble riffles, and sand. The type of substrate was an important factor in determining the direction and magnitude of the response of many taxa. In the stream that drained the clear—cut watershed, more taxa increased in density (compared with a nearby reference stream) in moss—covered rock face than in any other substrate; cobble riffles were next, followed by pebble riffles and sand. Conversely, the number of taxa with significant reductions in density was highest for sand substrates, followed by pebble, cobble, and rock face. Among functional groups of insects, collector—gatherers and scrapers increased, while the dominant shredder, Peltoperla, declined. The differential response of invertebrates among substrates suggests that biological stability is closely coupled with physical stability; however, moss associated with larger particles may be a factor in enhancing the biological stability of these substrates. Clear—cutting of a watershed is a largescale, low—frequency, anthropogenic disturbance to associated stream ecosystems. Certain successional characteristics of these headwater streams following a major anthropogenic disturbance differ from those observed in streams in which succession may be truncated by frequent disturbances such as flash flooding.

TROPHIC BASIS OF PRODUCTION AMONG RIVERINE CADDISFLIES: IMPLICATIONS FOR FOOD WEB ANALYSIS

We determined the trophic basis of production and quantified the food web of caddisfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Production was dominated by three net-spinning taxa (Cheuma- topsyche spp., Hydropsyche rossi, and Chimarra moselyi), comprising 97-98% of the total among at least 14 trichopteran species. Annual production (as dry mass) was among the highest estimates reported for caddisflies, ranging from 43.5 to 63.9 g/m2 of snag surface (or from 14.2 to 25.5 g/m2 of channel bottom) in two consecutive years. Although all taxa were present throughout the year, C. moselyi had much higher production in summer, H. rossi was highest from late summer to winter, and Cheumatopsyche spp. showed no pattern. Ingestion was determined from production estimates, bioenergetic efficiencies, and quan- titative gut analyses. The two macrofiltering taxa (Cheumatopsyche spp. and H. rossi) were omnivorous, with 50.6 and 64.4% of their production, respectively, due to eating animals. Somewhat lower amounts of their production (40.7 and 23.5%) were due to ingestion of amorphous detritus. Production of the microfiltering species Chimarra moselyi was pri- marily due to eating amorphous detritus (91%). Removal of amorphous detritus from the system by the major taxa was highest in summer, and diatom removal was highest in fall, whereas animals were eaten consistently throughout the year. A quantitative food web showed that while the linkages among nine taxa were complex, the ingestion pathways were dominated by amorphous detritus (total consumption = 62.5 g.m-2.yr-1) and animal prey (22.3 g.m-2.yr-1), with >99% of food resources being consumed by the three dominant filter-feeding taxa. A connectivity food web was misleading because it implied equivalence of all food resources and consumers when great variation in strength of linkages actually existed. These comparisons provide a strong argument for considering energy flow as a measure of linkage strength when evaluating food webs, and they cast doubt on the use- fulness of oversimplified connectivity webs as a basis for food web theory.

Effects of Disturbance on Benthic Functional Structure and Production in Mountain Streams

To assess the role of macrofauna consumers in organic matter dynamics of headwater streams, we applied seasonal insecticide treatments to a southern Appalachian Mountain stream and compared benthic community structure between this and two other streams. Production was estimated in the two major habitats: mixed cobble-gravel-sand substrate and bedrock outcrop. Using the proportional availability of the habitats in each stream, production over the entire stream was measured. Annual habitat-weighted production in the untreated streams during the two study years ranged from 8.9 to 14.0 g AFDM m-2 yr-1, 81 to 86% of which was attributed to insects. Habitat-weighted production was distributed rather evenly among the collector-gatherer (32-41%), shredder (23-31%), and predator (28-33%) functional groups. Collector-filterer and scraper groups accounted for less than 9% of total habitat-weighted production in all three streams. Insecticide applications resulted in dramatic changes in the macrofaunal community. Annual habitat-weighted production (4.4 g m-2 yr-1) in the treated stream decreased by 62% from Yr 1 to Yr 2, with insects contributing only 45% to total production. Oligochaetes, several Diptera, dragonflies, and copepods composed most of the production during treatment. Although habitat-weighted production of all functional groups was lower in the treatment year than in the pre-treatment year, collector-gatherer production decreased the least (21% reduction) and dominated overall production during treatment. Production of the other functional groups decreased by 71-94% between the two years. The changes in macrofaunal community structure and production observed in the insecticide treated stream were much greater than between-year changes in the reference stream, despite the occurrence of a record drought in Yr 2. The effect of the two different disturbances on overall production differed not only in magnitude but also in direction, with production in the reference stream increasing (12% over Yr 1) during the drought year. Resource ingestion was estimated using values of functional group production. Estimates of ingestion suggested that the major functional groups were ingesting a fairly large portion of available resources in the untreated streams. Collector-gatherers consumed 34-64% of total annual habitat-weighted fine particulate organic matter. Shredders consumed an estimated 28-47% of average annual standing crop of leaves, and predators ingested 67-79% of total macrofaunal production. Insecticide treatments resulted in much lower levels of resource ingestion: 26%, 4%, and 50% for collector-gatherers, shredders, and predators, respectively. Our results show the importance of using estimates of production when assessing disturbance effects on macrofaunal communities, since abundances greatly underestimated the impact of insecticide treatments. Biomass estimates, though providing better measures of disturbance than abundances, tended to overestimate overall macrofaunal losses because of the disproportionate influence of large, slow growing taxa such as crayfish.