Ecological Monographs, 53(1), 1983, pp. 1-25 1983 by the Ecological Society of America INTERBIOME COMPARISON OF STREAM ECOSYSTEM DYNAMICS G. WAYNE MINSHALL It Department of Biology, Idaho State University, Pocatello, Idaho 83209 USA ROBERT C. PETERSEN Department of Limnology, University of Lund, S-220 03 Lund, Sweden KENNETH W. CUMMINS Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon 97331 USA THOMAS L. BOTT Stroud Water Research Center, Academy of Natural Sciences of Philadelphia, Avondale, Pennsylvania 19811 USA JAMES R. SEDELL Forestry Sciences Laboratory, United States Forest Service, 3200 Jefferson Way, Corvallis, Oregon 97331 USA COLBERT E. CUSHING Ecological Sciences Department, Battelle-Northwest, Richland, Washington 99352 USA ROBIN L. VANNOTE Stroud Water Research Center, Academy of Natural Sciences of Philadelphia, Avondale, Pennsylvania 19811 USA Abstract. Studies were conducted in four distinct geographic areas (biomes/sites) in northern United States to examine changes in key ecosystem parameters: benthic organic matter (BOM), transported organic matter (TOM), community production and respiration, leaf pack decomposition, and functional feeding-group composition along gradients of increasing stream size. Four stations ranging from headwaters (1st or 2nd order) to midsized rivers (5th to 7th order) were examined at each site using comparable methods. The results for each parameter are presented and discussed in light of the River Continuum Concept of Vannote et al. (1980). The postulated gradual change in a stream ecosystems structure and function is supported by this study. However, regional and local deviations occur as a result of variations in the influence of: (1) watershed climate and geology, (2) riparian conditions, (3) tributaries, and (4) location-specific lithology and geomorphology. In partic- ular, the continuum framework must be visualized as a sliding scale which is shifted upstream or downstream depending on macroenvironmental forces (I and 2) or reset following the application of more localized "micro"-environmental influences (3 and 4). Analysis of interactions between BOM and TOM permitted evaluation of stream retentiveness for organic matter. Headwaters generally were most retentive and downstream reaches the least. Estimates of organic matter turnover times ranged between 0.2 and 14 yr, and commonly were 1-4 yr. Both turnover times and distances were deter- mined primarily by the interaction between current velocity and stream retention. Biological processes played a secondary role. However, the streams varied considerably in their spiraling of organic matter due to differences in the interplay between retentiveness and biological activity. Differences in the relative importance of retention mechanisms along the continuum suggest that headwater stream ecosystems may be functionally more stable, at least to physical disturbances, than are their inter- mediate river counterparts. Key words: benthic organic matter; carbon cycling; community metabolism; Idaho; invertebrate functional groups; Michigan; Oregon; Pennsylvania; River Continuum Concept; seston; spiraling; stream ecosystems; transported organic matter. INTRODUCTION munities have received particular attention. However, From the time biologists first began to examine there have been few attempts to treat streams and rivers f streams there has been a tendency to look at com- holistically, and particularly, to view them as discrete munities or organisms and their associated abiotic fac- ecological systems. It has been only within the last 25 tors. The periphytic, macroinvertebrate, and fish corn- yr that knowledge of the organization and functioning of streams has reached a stage that allows general- izations to be made about these ecosystems. Such ad- Manuscript received 22 January 1981: revised and ac- vances have been built on the early ideas about lon- cepted 5 March 1982. gitudinal succession and community structure (Mar- Purchased by USDA Forest Service, for Official Use G. WAYNE MINSHALL ET AL. Ecological Monographs Vol. 53, No. HEADWATER STREAM DOWNSTREAM REACH level processes (cycling of organic matter and nu- COMMUNITY COMMUNITY trients, ecosystem metabolism, net metabolism) in downstream areas are linked to in-stream processes in upstream areas; the concept provides a general frame- AI lochthonous work for dealing with streams as spatially heteroge- CPOM Alloch. neous systems (ONeill et al. 1979). CPOM The utility of visualizing an entire river system as a continuum of communities and material processes with I° Pro° their associated abiotic factors may be illustrated as FPOM Shredders — FPOM I° Prod. follows. Consider a simplified six-component model of Grazers a small woodland stream and larger sized reach farther downstream, as shown in Fig. 1. The relative size (e.g., dry mass in grams per square metre) of each compo- Col lectors Col lect - ors Grazers nent is illustrated by the size of the boxes. Both sys- tems have the same functional components, even FIG. 1. Simplified models for two hypothetical stream though the species present may be different; that is, ecosystems. Stream A is characteristic of most forest streams, general structure and function are the same. The dif- while Stream B is characteristic of large rivers and uncano- pied small streams. The components are the same in both ference between the two systems is in the relative cases but their relative importance (as reflected by the size magnitude of the components, the rates and amounts of the boxes) differs in the two systems. of transfer between components, and the actual species engaged in the transfers. But the two systems are es- sentially modifications of the same basic plan, and the galef 1960); the role of riparian vegetation in structuring entire stream-to-river complex can be viewed as one stream communities (Ross 1963); generalizations about ecosystem composed of a series of communities along ecosystem structure and function (Cummins 1974, a continuum. McIntire and Colby 1978); autotrophic production Four sets of hypotheses can be generated from the (Minshall 1978); and nutrient cycles in streams (New- conceptualization of streams as a continuum of com- bold et al. 1981). However, few unifying concepts have munities. The hypotheses may be grouped under the been proposed that can serve as a foundation for stream following sets of assumptions: ecosystem research. One reason for this missing foundation is the com- 1) If the particulate organic matter at one location in plex and diverse nature of flowing-water ecosystems. a stream is determined by what occurs upstream, It is more difficult to visualize streams of different size and if the biological community exploits this re- in the same drainage as belonging to the same ecosys- source, then: tem than to visualize a pond or a forest, no matter there will be a gradual reduction in particle size how large, as being a discrete ecosystem. For exam- as the material is metabolized and fragmented; ple, a reach of a large river such as the Mississippi there will be a decrease in the organic content typically has not been included in the same ecosystem of those particles in transport (seston); and as its headwater streams. The reluctance to view flow- c. there will be a downstream reduction in the frac- ing waters as ecosystems (Raiska 1978) may be due tion of a particle that is readily metabolized. partly to the difficulty in defining the term ecosystem 2) If the relative contribution of organic matter from (Mann 1979), but is more likely attributable to the way the riparian zone decreases downstream as com- streams or their entire drainage system (all streams pared to that present in a stream, and given the and rivers that are linked together) are visualized. This assumptions in (1), then: conceptualization is even more difficult without some there will be a reduction in coarse-to-fine parti- general unifying principles that apply to all flowing- cle size ratios; and water ecosystems. there will be a relative increase in particulate Recently, Vannote et al. (1980) proposed that the organics generated by in-stream processes. coalescing network of streams in a river drainage sys- 3) If stream channel morphology changes from nar- tem is a continuum of physical gradients and associ- row, shallow, and heavily shaded to wide, deep, ated biotic adjustments. This approach leads to useful and open, then: generalizations concerning the magnitude and varia- a. there will be a shift from benthic metabolism tion through time and space of the organic matter sup- dominated by heterotrophic processes to a dom- ply, the structure of the invertebrate community, and inance of photosynthesis, but shifting again to resource partitioning along the length of the river. The heterotrophic processes with increasing river River Continuum Concept views streams as longitu- depth and sediment load which interferes with dinally linked systems, i.e., those in which system- benthic primary production; and 1- March 1983 INTERBIOME COMPARISON OF STREAM DYNAMICS 3 b. there will be a shift from community metabolism Upper Salmon River, Idaho; (3) a tributary system of dominated by benthic processes to metabolism the Kalamazoo River, Michigan; and (4) White Clay dominated by water column processes. Creek and Buck Run, Pennsylvania (Figs. 2, 3, 4). 4) If the assumptions concerning food resources of the More detailed results and site-specific analyses for each invertebrates in (1) and (2) above are valid and the