ryn and Wallace 1987a), and geomorphic characteristics thereby influence ...... Stream ecosys- tem dynamics of the Salmon River, Idaho: an 8th-order system. J.
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Longitudinal changes of macroinvertebrate communities along an Appalachian stream continuum J.W. Grubaugh, J.B. Wallace, and E.S. Houston
Abstract: Richness and mean annual abundance and biomass of benthic taxa were measured in prevalent habitats along a first- through seventh-order stream continuum in the southern Appalachian Mountains (United States). Richness was greatest in midorder reaches, and benthic densities were highest in cobble habitats, followed by bedrock, pebble–gravel, and depositional areas. Abundance-based estimates of functional feeding-group composition were dominated by collector–gatherers and changed little with habitat. In contrast, biomass-based estimates differed greatly among habitats: collector–filterers generally dominated cobble and bedrock areas; collector–gatherers, in pebble–gravel; and shredders and collector–gatherers, in depositional habitats. When functional-group biomass estimates were weighted for relative habitat availability along the continuum, patterns of benthic community composition generally matched predictions of the river continuum concept (RCC), although localized changes in stream geomorphology also influenced community structure. Habitat-weighted abundance estimates did not produce similar trends. Because RCC predictions are based on benthic biomass, caution and qualification must be exercised when using abundance data to test RCC predictions. Résumé : On a déterminé la richesse ainsi que l’abondance et la biomasse annuelle moyenne des taxons benthiques dans des habitats prévalants le long d’un continuum fluviatile d’ordre une à sept dans le secteur sud des Appalaches (É.-U.). La richesse était maximale dans les passages d’ordre moyen, tandis que les habitats à cailloux, suivis des aires constituées du substratum rocheux, de galets–gravier, et les aires sédimentaires présentaient la densité benthique la plus élevée. Des valeurs estimatives, basées sur l’abondance, de la composition des groupes d’alimentation fonctionnels indiquaient qu’il y avait surtout des organismes collecteurs et que la composition ne changeait guère d’un habitat à l’autre. Par contraste, les valeurs estimatives basées sur la biomasse variaient considérablement d’un habitat à l’autre; les collecteurs–filtreurs dominaient généralement dans les aires recouvertes de cailloux et celles constituées du substratum rocheux, tandis que ce sont les collecteurs et les déchiqueteurs et collecteurs qui dominaient dans les aires de galets–gravier et les habitats sédimentaires, respectivement. Lorsque les valeurs estimatives de la biomasse des groupes fonctionnels étaient pondérées pour tenir compte de la disponibilité relative des habitats le long du continuum, la composition de la communauté benthique correspondait généralement aux prévisions du concept du continuum fluviatile (RCC), même si des changements localisés dans la géomorphologie du cours d’eau influaient également sur la structure de la communauté. Les valeurs estimatives pondérées pour tenir compte de l’habitat n’ont pas indiqué des tendances semblables. Comme les prévisions du concept du continuum fluviatile (RCC) sont fondées sur la biomasse benthique, il faut faire preuve de prudence et nuancer les conclusions lorsqu’on utilise des données d’abondance pour vérifier les prévisions RCC. [Traduit par la Rédaction]
Introduction Stream ecosystems provide an array of changing physical conditions that influence benthic fauna. Some characteristics such as elevation, thermal regime, and stream size change predictably along undisturbed stream gradients and sequentially alter Received June 24, 1994. Accepted October 27, 1995. J12450 J.W. Grubaugh,1 J.B. Wallace, and E.S. Houston.2 Department of Entomology and Institute of Ecology, The University of Georgia, Athens, GA 30602, U.S.A. 1
2
Author to whom all correspondence should be addressed: Department of Biology, Division of Ecology and Organismal Biology, The University of Memphis, Memphis, TN 38152, U.S.A. Present address: Alabama Department of Environmental Management, Ecological Studies Field Operations, 1890 Dickinson Drive, Montgomery, AL 36107, U.S.A.
Can. J. Fish. Aquat. Sci. 53: 896–909 (1996).
benthic community structure (Vannote et al. 1980; Minshall et al. 1985). Geomorphic characteristics such as streambed slope, substratum composition, and current velocity often change more locally and with less predictability (Statzner and Higler 1986). These characteristics determine the relative availability of stream habitats such as bedrock outcrops, cobble riffles, pebble–gravel runs, and depositional areas (Brussock et al. 1985). Benthic communities are adapted to utilize the structural and resource features of particular habitats (Huryn and Wallace 1987a), and geomorphic characteristics thereby influence benthic community structure on a local or stream-segment scale. The interplay of longitudinal and localized physical characteristics produce distinct patterns in the structure of benthic communities along stream continua. Changes in benthic community structure can be assessed by examining taxonomic composition along stream continua. Allan (1975a, 1975b) characterized species composition along an elevational stream gradient in Colorado, U.S.A., and used an index of community similarity to identify specific stream © 1996 NRC Canada
Grubaugh et al. Fig. 1. Longitudinal profile of the Ball Creek – Coweeta Creek – Little Tennessee River continuum. Locations of sampling stations in small streams (S-1 through S-3), midorder (M-1 through M-5), and river (R-1 through R-3) reaches are denoted along the gradient.
reaches exhibiting rapid faunal replacement. Statzner and Higler (1986) applied a similar approach to published lists of benthic taxa from a worldwide assortment of elevational gradient studies. They also identified abrupt changes in species composition and attributed resulting zonation patterns to localized changes in “stream hydraulics,” or the physical characteristics of streamflow (Statzner and Higler 1986). Longitudinal patterns of community structure are also indicated by changes in the functional feeding-group composition of the benthos. These functional feeding groups refer primarily to modes of feeding (Cummins 1973; Wallace et al. 1992) or the food acquisition system (sensu Cummins 1986). This approach was used in formulating the river continuum concept (RCC), a series of hypotheses that suggest a general, predictable pattern of changing benthic community structure in stream size continua (Vannote et al. 1980; Minshall et al. 1985). Accordingly, subsequent studies that support, dispute, or rely on RCC predictions as an appropriate description of lotic ecosystems also are based on functional feeding-group composition. Minshall et al. (1983) examined functional composition of benthic communities along four North American stream-size gradients and found that longitudinal patterns agreed with those predicted by the RCC. Community composition along an extensive continuum in the Salmon River system (572 km, second- to eighth-order reaches) also supported the basic premises of the RCC (Minshall et el. 1992). Conversely, Ward (1986) found little conformance of functional feeding-group composition in St. Vrain River to the RCC. Townsend and Hildrew (1984) found a predictable arrangement of functional composition in small-stream continua in England but hypothesized it was in response to food resource and predation factors rather than evolutionary processes suggested by the RCC. In the Rhine River, van der Velde and van den Brink (1994) compared longitudinal composition of benthic functional structure with RCC predictions to assess whether this anthropogenically impacted river retained the characteristics of a river ecosystem. There are limitations in some studies that use functional composition to assess benthic community structure. Benthic sampling has often been confined to a single habitat, such as
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cobble riffles, at all sampling stations along a stream continuum. This approach provides information on how taxonomic or functional structure within a single habitat changes along the gradient but does not account for benthic populations in unsampled habitats (Minshall et al. 1985). As habitat availability changes both locally and longitudinally, it is doubtful that samples from a single habitat can adequately reflect community composition throughout a stream continuum. An alternative approach is to assess relative habitat availability at each sampling station, focus sampling efforts in the dominant habitats at each station, and proportionally weight results by relative habitat availability to reflect the total benthic community (sensu Huryn and Wallace 1987a). Predictions of the RCC concerning functional composition of communities along stream-size continua were based on measures of macroinvertebrate biomass (see Vannote et al. 1980, p. 132). Despite this, many studies that test RCC predictions use measures of benthic abundances rather than biomass (see also Towers et al. 1994). Estimates of functional composition based on abundance and biomass data can be very different (Lugthart and Wallace 1992); therefore, using benthic abundance data to test predictions of the RCC is inappropriate and even possibly misleading. The objective of this study is to characterize the structure of the benthic community along a continuous elevational and stream-size gradient in the southern Appalachian Mountains. Our approach examines three aspects of the benthic community structure. First, we assess taxonomic composition and patterns of faunal replacement with respect to changes in stream elevation along the continuum. Second, using measures of macroinvertebrate biomass, we describe longitudinal trends in functional-group composition for specific habitats along the stream-size gradient. Finally, we characterize total (habitatweighted) functional composition of benthic communities along the continuum, using estimates of both macroinvertebate biomass and abundance. We will compare these results with predicted longitudinal patterns of benthic community structure suggested by the hypotheses of the RCC. Study area The study was conducted in Macon and Swain counties of western North Carolina in the Blue Ridge province of the southern Appalachian Mountains. Eleven sampling stations were established along a continuous, 65-km stream continuum that included Ball Creek from its headwaters, the entire length of Coweeta Creek, and a reach of the Little Tennessee River between the Coweeta Creek confluence and Fontana Reservoir (Fig. 1). The Ball Creek –Coweeta Creek – Little Tennessee River continuum flows northwest over granitic parent materials and ionic concentrations in streamwater are persistently low (see Swank and Waide 1988; Swank and Bolstad 1994). Sampling stations can be partitioned into three reaches: three stations (S-1 through S-3) in the small-stream reach, five stations (M-1 through M-5) in the midorder reach, and three stations (R-1 through R-3) in the river reach (Fig. 1). Stations in the small-stream reach include first- (headwater) and second-order sites along Ball Creek (Table 1). The stream is narrow (
