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FILE COPYStream ecosystem theory

K. W. CUNIUNS, G. W. Mussa&u, J. R. SEDE11, C. E. CUSHING and R. C. Prrum

With 1 figure and 6 tables in the text

Introduction

At one level of resolution all streams and rivers are the same; at another they all differ. The for-mer extreme is tether uninteresting. All contain water and eventually find their way downslope tothe next sized tributary, a lake, or the ocean. At the other =nose, recognition that each is different(e. g. no two have exactly the same species composition and abundances), ignores many patternsthat transcend streams, watersheds, whole basins, biomes (i. e. with latitude) and even continents

e. with longitude). The preoccupation with classification and the economic incentive for manage-ment of ecosystems has provided impetus for recognition of patterns and understanding theirunderlying bases. -

Similarities among running waters ecosystems have been recognized for some time (e.g.. Timm-MANN 1925; Lamm at al. 1964; Hum 1970). Biologically. relevant geomorphic similarities resultfrom a limited number of possibilities: channel downcutung and lateral excursion, and sedimenttransport and dePasitian. I.otic organisms have adapted, through millions of years of evolution and10,000 years or more of population acclimatization, to occupy one of several habitat types, broadlyclassed as either erosional or depositional, and feed on one of the food resource categories available(Table 1). These limited combinations of general physical channel conditions and biotic adaptationsare the basis of pattern similarity. The major tenet of this paper is that stream and river-side veleta-

Table 1. Invertebrate examples of basic adaptations in prototype lode systems (modified from CUM-MINS 1973,1974; Gummy* & Kum 1979; Mmarrr & Cuomo 1984).Lode habitat Dominant habitat Dominant functional Dominant organic food

adaptations feeding groups resources

Erosional Clingers(riffles, rapids,cascades nun, glides)

Scraper Periphyton

Filtering collecton FPOM (sloughedperiphyton, transport-suspended and bed-load)

SwimmersBurrowers (crevicedwellers)

Gathering collectors Depositional FPOM Gathering collectors Depositional FPOM Shredders Depositional CPOM

macrophytes

•

Sprawlers Gathering collectors Depositional FPOM Shredders Leaf packs

Burrowers

Climbers

Depositional(pools, margins,off channel, sidechannel)

Gathering collectorsShreddersShreddersPiercers

Depositional FPOM Leaf packs, woodVascular hydrophytesMacroalgae

•

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tion, the riparian zone, exerts the primary control over biotic associations. This control is mediatedboth through physical channel influences (e. g. large woody debris, root bank stabilization, shading,etc.) and the nature of the organic inputs (solutions or particles) from outside the wetted channel(allochthonous) or in-stream plant growth (autochtho yous). Such corridors of stream-side vegeta-tion are inseparable from the biology in the channel and constitute a ribbon of continuity ripsponsible for many universally discernable patterns.

Lout paradigms

Some selected examples of general attempts at classifying patterns in running waters and under.standing their underlying controls are summarized in Tables 1 and 3. The riparian zone has notbeen specifically included as a critical associated component in most manning water ecosystemparadigms. Geomorphic (especially altitude and gradient) and hydrologic (average and peak flows)associated thermal regimes (with distance along the drainage profile) have been used most Ereesquently in classification schemes. For example, these have been viewed as the major components

Table 2. Examples of control parameters in some exemplary lotic ecosystem paradigms.Paradigm Geomorphic Hydrologic Thermal

Riparian Temporal References

(pediem, influence perspectivestream order)

Longitudinal Hurt 1954;zonation X X X Sonars 1955:

BODWAIGANU 1979Succession Maaaase 1960RiverContinuum X

X

Munnau. at aL 1983&vow a aL 1983

Serial WARD et Sumo=discontinuity X 1913Ripariancontrol X Cuomo et al. 1983

Table 3. Examples of biotic components in some exemplary lotic ecosystem paradigms.Primary Species Functional Referenceproduction, diversity, organizationcommunity community (groups)respiration structure(P/R) ,tributes

Paradigm Organicmatterprocessing(budgets)

a.* • •,1.1111,.'..

Longitudinalzonation

SuccessionRivercontinuum +SpirallingSerialdiscontinuity +

Ripariancontrol +

+ (fish;invertebrates)

+ (primarilyalgae)

(+)

Hurt 1954;Sonars 1955;1r^s tit Bozosemenu1963ivIencaz.ze 1960

Monous. a al. 1983ELWOOD et al. 1983

WARD lit Szniaoim1983

Cusaarn et al. 1983

1'

1820 IX. Running Waters

controlling patterns in fish and invertebrate zonation (Ina 1935; Burma 1952; Huai 1954;SOO1172 1955; Tins & Borowiweu 1963; BOTOSANEANU 1979). ?AARGAU! , (1960), relying almostexclusively on algal associations, viewed the succession of pioneer to climax communities as a basicpattern transcending stream size and recognizable on an extra-regional level.

In the last two daada there has been the simple, but important, recognition that running wa-ters differing in expression of the basic geomorphic (e. g. size, gradient), hydrologic (e. g. discharge),and biotic (e. g. community organization) characteristics are linked together in drainage networks(aquatic-aquatic) and functionally are inseparably linked to the stream-side vegetated zone (riparianzone, upper bank, floodplain, etc.) (e. g. CumuNs 1974; HYNES 1975; Vamorz et al. 1980). Thegradual changes along the drainage net have been viewed as a continuum of physical gradients andassociated biotic adjustments; organisms become adapted and remain acclimatized to the mostprobable physical state (quasi-equilibrium; Sieuwe & Lzarrr 1956) along such continua (Varniceraet al. 1980; MINSHAu et al. 1983; CUSHING et al. 1983).

Paradigms related to the river continuum concept specifically address the transport, processingand storage, and further transport of particulate and dissolved organic matter (POM, DOM) and in-organic nutrients along a sequence of drainage nets (Es.woon et al. 1983), and the displacement offunctional relationships rating from impoundment (WALD & STANFORD 1983). Spiralling lengthhas been proposed as the length of lotic reach required for one turnover cycle of a given nutrientatom. Thus, higher production rates, as well as slower exchange (greater retention) between waterand bottom, decreases spiralling length. As WI= & STANK= (1983) have discussed, impoundmentresults in a "serial discontinuity" in the continuum as defined by a predictable change in biotic as-sociations along the physical gradient from small streams to large rivers in a drainage basin. Discon-tinuity distance is the channel length of displacement (towards the head or mouth) of a given bioticfunctional association.

As proposed above, the characteristics of the atreamside vegetation system, the riparian zone,constitute the major organizing function for biotic associations in running waters. This followslogically from the paradigms summarized in Tables 2 and 3. Succession of riparian plant communi-ties following natural disturbances (fire, flood) insured that forested headwater streams returned topredisturbance status of terrestrial-aquatic linkage in aboriginal (prototype) streams and rivers.Both physical and biological conditions described as gradually changing along the continuum ofdrainage nets are shaped to a major extent by conditions in, and products from, the riparian zone.Spiralling length, i.e. the pattern of storage and processing (turn-over) within, and transport intoand out of, a reach is under riparian influence, particularly through the effect of retention. A case inpoint is the highly retentive nature of very steep gradient streams in the Pacific Northwest(U.S.A.) due largely to breaks in slope attributable to large logs in the channel (Gazzoa y et al.1984), many of which have been in place for over a century (SwaisoN & LIENIAILIOU 1982). Thisconstitutes an example, undoubtedlytypicalof forested aboriginal running waters, in which ex-tremes in channel roughness are directly related to wood debris, particularly in high gradientMUMS.

Prototype streams

Presumably most attempts at lotic paradigm building have been intended as models ofat least classes of prototype streams. The most probable natural physical state, as en-joined in the river continuum concept, is no longer expressed in most river basins as itwas in pre-aboriginal or aboriginal times (Table 4). Pristine conditions are taken as min-imally influenced by mechanized man and closely matching the semi-stable (quasi-equi-librium) conditions extant 10,000 years or more ago following the last glacial episodesand general broad scale climatic-terrestrial vegetative biome changes. The extent ofhuman impact on lotic systems can be viewed in three general phases: pre-aboriginal,aboriginal and post-aboriginal (Table 4). The primary impact of aboriginal, and post-aboriginal man has been to increase or decrease the impacts of natural events that changestream ecosystems. Major examples of natural events include flood, drought, fire, effectson banks and sediments by large grazing mammals, alterations in channel structure,

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Table 4. Natural disturbance events altering channel form and community functional organiza-tion and spatiakemporal alterations attributable to aboriginal and post-aboriginal man.

Alterations by post-aboriginal manRegulation — supression and enhance-mentRegulation — impoundment, dews-tering, reduction of peak flows and in-crease in low flowsAcceleration — greatly increasedsediment loading (agricuhure, logging-road building, draining)Large scale depletion and extinction(beaver► large grazing manunals, e.g.bison, elephants, hippopotomi)Wide scale reductions, extinctions andintroduction of exotics

Alteration of major plant bio-mes and associated alteration of ri-parian zones

Over-exploitation of nativespecies3. Waxer quality, thermal effects

produced for example by hippopotomi and beaver, and defoliating invertebrate out-breaks (Sznzu. 1984).

Man often has increased the spatial and/or decreased the temporal scale of these es-sentially natural events. Human influence on stream blockage is a good example. Inaboriginal times, natural blockage, and resulting impoundment, occurred on all sizedstreams and rivers, produced by mass earth movement and derangement of drainage netsby lakes, such as those formed in glacial depressions. Blockage of smaller channels bytrees toppled by wind throw or by animals such as the North American beaver also wereundoubtedly common (Sznzu. 1984). Aboriginal man probably had little effect on thesephenomena, although their use of fire, animal drives, or removal of trees from navigablestreams may have had local influences. With the advent of mechanized, and more drama-tically, industrialized man, the scope of dam construction has increased exponentially.Essentially every major world river system has been impounded to some extent (Wan 8c&m eon 1979). In some areas, the number of small dams is nearly equal to the numberof second to fourth order streams. The wide-scale building of splash dams in streams ofthe Pacific Northwest (U. S. A.) in the late 1800's to early 1900's is a good case in point(Sluxu. 1984). In North America the virtual elimination of the beaver from large areasand consequent loss of their dams certainly influenced the existing morphology of small,lower gradient channels. Such headwaters would have been more open, allowing greaterprimary production (MnanAu. 1978), and the retention of fine organics in ponds was un-doubtedly greater (SEDEu. 1984).

Within any of these periods of historical importance to stream ecosystems, from pre-to post-aboriginal, a temporal perspective is essential (Custums et al. 1983). Given the ap-propriate time and spatial scales, essentially all stream drainage basins with vegetativecover have experienced fire and, with absolute certainty, floods. Periods of annual peak

Pre-aboriginaldisturbanceFire

Flood (drought)

Mass movement (masswasting debris tor-rents, etc.)Mammal effects

Organism extinctionsand introductions

Alterations byaboriginal man Local increased burnbig

Small scale effects — treeremoval and damming

Minor — increased sedi-ment loading atencampmentsLocally important — ani-mal drives and harvest

Minor — increased dispersalby nomadic tribes

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MODERATE TO LOW HIGHER GRADIENTGRADIENT

CANYON CONTROL

(BROAD FLOOD PLAIN)

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1822 IX. Running Waters

FOREST FOREST HIGH or LOW LATITUDESTEEPER GRADIENT LOWER GRADIENT HIGH ALTITUDE

X ER IC

rnr

ORDERS1-4

-

Fig.1. Comparison of riparian relationships for streams (orders 1-4) and riven (orders > 4) inaboriginal times. Headwater streams in wooded areas shown dominated by large wood debris andstream side vegetation with ponding and springs at lower gradients. At 'high and low latitudes and athigh altitudes and in uric areas or seasons, streams less controlled by large vegetation. Many chan-nels may be intermivant. Large rivers also influenced by large wood or canyon constricted. Allstreams and riven generally more braided and complex than at present, with greater riparian in-fluence.

ATITUDEITUDE

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H. W. Cummins et al., Stream ecosystem theory 1823

discharge are a basic characteristic of running waters. The pattern of the runoff cycle(hydrograph) and its relation to the "growing" season, with warm periods at high andlow latitudes and high altitudes, wet periods at mid-latitudes, are important to the timingof organic matter inputs, primary production and the life cycles of stream-riverorganisms (Fig. 1).

Annual peak discharge or drought (drying up of intermittent channels) can be viewedas reset mechanisms; events that confer certain pioneer community characteristics onstreams (Commis et a1. 1983). For example, algal communities are scoured or dried in in-termittent channels to very low population levels and resting stages, initiating an annualrecolonization and sequential community development. Greater than annual or bankfulflows have a proportionately larger effect on the physical habitat (depth of sedimentscour and fill, size of channel wood debris moved, extent of upperbank, floodplain, cap-tured) and on the lotic biota. Description of each flow year as a recurrance interval(MonsAvA 1968) places it in perspective with regard to long-term changes (Common etal. 1983).

Functional relationships in running water systemsThe extensive use of invertebrates in characterizing various functional relationships in

running waters reflects their favorable relative size and abundance, conferring greaterease of sampling and observation. Clearly, most energy and material fluxes are accountedfor by microorganisms: algal primary producers and bacterial and fungal consumers. Thesmall size and rapid turnover of the microbes is often restrictive in investigations of com-munity organization (although they lend themselves admirably to measures of commu-nity metabolism) while fish, although relatively large, are long-lived, proportionatelylow in density, and are of negligible importance in community metabolism. The inver-tebrates constitute a useful link between the quantitatively significant microorganismsand the economically important fish. Individuals are relatively large and numerous,usually have annual or seasonal life cycles, and associations are functionally organized inrelation to habitat and nutritional resources (Tables 2 and 3).

Indices based on invertebrate species composition (e. g. diversity, richness, evenness,etc.) are compromised by post-aboriginal disturbance of lotic ecosystems. Invertebrates,particularly in streams and rivers below orders five or six (Smut= 1957) in a givendrainage network where insects predominate, have always posed significant taxonomicproblems. Separations of immature forms, often the only life stages collected, are diffi-cult. Capability for reasonable taxonomic completeness has come only after extensivecultural expansion and establishment of scientific institutions. In short, by the time spe-cies can be separated with certainty, essentially all the stream-river systems have been sig-nificantly altered from the aboriginal condition. Thus, only in a few special cases cansuch indices be taken as reflective of prototype (aboriginal) systems.

Given state-of-the-art limitations in taxonomy together with undetermined loss oftaxa, functional categorizations of invertebrate associations have been proposed. For ex-ample, functional feeding groups based on morpho-behavioral adaptation of food aquisi-tion (Table 1) emphasize the match between food resource categories (e.g. coarse and fineparticulate detritus and attached algae) and feeding methods (Cuustms 1973, 1974, 1978;Culams dt KI.uc 1979; Mmuirrr 8c CUMMINS 1984). This functional organization ofstream communities also has been incorporated into a process oriented simulation model(Manua 1983).

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IX. Running Waters

As the resource bases have become more fluctuating or permanently changed fromthe aboriginal state — a condition strongly influenced by the riparian system — theproportion of invertebrate taxa having general feeding strategies probably has increased.The obligate (specialist) or facultative (generalist) adherence of a given invertebrate taxonto a functional category is related both to flexibility of feeding mode and the accom-panying assimilation system. The importance of food assimilation systems, especially therole played by microorganisms, both in the food (food biomass, alteration of substrate,acquired enzymes) and as a resident gut flora, is presently one of the most active areas ofresearch in stream ecology (e. g. Mzurrr & CUMMINS 1984).

New perspectives

The general concept of running water ecosystems (drainage nets or watersheds inbasins) as continua of adjustments of lotic biota to quasi-equilibrium (high probability)states of the physical systems remains intuitively satisfying. It is well grounded in theearlier literature (e. g. Timuswass 1925; Harr 1954; MARGALEF 1960) and remains a use-ful conceptual model (CubuuNs 1974; MINSHALL 1978; Vongrre et al. 1980; Mnssitsu. etal. 1983), with modifications and elaborations (e. g. ELWOOD et al. 1983; WARD & STAN-

FORD 1983; CUSHING et a1.1983) dealing with special aspects, particularly as applicable topost-aboriginal stream-rivers.

The role of the riparian zone has received increasing attention in emerging loticecosystem paradigms. The plant species constituting the streamside vegetation — trees,shrubs, herbaceous plants — differ between watersheds but they represent physicalanalogues, that have similar impacts on channel structure (shading, retention of al-lochthonous organic matter, sediment routing, etc.), and biological analogues with regardto the fate of their organic substance once it is entrained in the stream-river network.Critical aspects conferring analogue status to riparian vegetative components are size oftrees relative to size of channel (e. g. large or small wood) and rate of biological process-ing (that,is breakdown or turnover, e.g. conversion of leaf litter to DOM, CO2, bioticbiomass, and FPOM, CUMMINS et a1.1980). Thus riparian vegetation species can be clas-sified as fast, medium, or slow with regard to processing rate. The rates are a function ofphysical nature of the leaf (e. g. cuticle thickness) and biochemical constituents. Thus,Oven the temperature regime, the rate of processing for a given leaf type is predictablewhen introduced into a stream along which the species does not occur (e. g. MINSHALL etal. 1983). The basic physical-biochemical differences control the rates of microbial colo-nization and metabolism of the organic substrates which in turn controls the rate of pro-cessing both directly by the microbes (especially aquatic hyphomycete fungi) and indi-rectly through the action of shredders (a functional group of invertebrates specialized forutilizing coarse particulate organic matter or CPOM).

Examples of the contrasting rates of processing of wood and leaf litter in the condi-tions encountered from headwaters to mouth (terrestrial to marine) of lotic systems aresummarized in Tables 5 and 6. Extensive adaptive radiation of wood-boring, gallery-forming insects and large rot fungi in the terrestrial system and many wood boring ma-rine invertebrates (some with cellulase enzymes) account for the faster processing rates onland and in the estuaries. The slower processing rate of wood in freshwater streams max-imizes their role as features shaping channel structure. In contrast the more rapid turn-over of leaf litter in streams reflects the preponderance of leaf (CPOM) shredder inver-

IL W. Cummins et al., Stream ecosystem theory 1825

Table 5. Comparison of processing of large wood debris in forested, stream and estuarine habitats.

Forest floorFast'Rot fungi diverse andabundant

StreamSlow'Aquatic hyphomycetes(rot fungi, virtuallyabsent)

EstuaryFast'Fungi minor or absent

Processing rateMicrobes(fungi)

Invertebrates

Insects Diverse and abundant Few species (e.g. Absentincluding gallery- Lora, Brills, Tipulidae),forming social insects no social insects

Others Oligochaeta important Absent Annelids, Mollusc* andin later uages of pro- Crustacea diverse andceasing abundant (some boring

forms with cellulase)Vertebrates Gallery or destroy Absent Absent

wood in search ofinvertebrate food

Environmental Temperature, mob-

controls tine and oxygen2I Hardwoods faster than conifers.

Table 6. Comparison of processing of leaf litter in terrestrial, stream and estuarine habitats.Terrestrial litter SUOMI Esniary

Temperature and Temperature andoxygen2 OXypII2

2 Lignin decomposed little anaerobically.

Sources oflitterProcessing rateAnnual timingof processingFungi

InvertebratesInsects

General forest or grass-landMediums

Spring—summer (warmand/or wet)Rot fungi diverse andabundant

Some species (Tipulidae,Collembola)

Riparian zone and ma-crophyu mortalityFast'Pall—winter andspring—summer pulsesAquatic hyphomycetes

Diverse and abundant(especially Plecoptera,Tricboptera, Tipulidae)Few species (Amplii-pods, bopoda, Gasun-Poda)Temperature andoxyrn2

Streams, rivers and estuari-ne manhesMedium'Year around

Minor role

Absent

Common (Annelid*, Gas-tropoda, Crustacea)

Temperature and oxygen2

Others Abundant (Oligochans,Diplopoda, bopoda)

Environmental Temperature and snois-controls sure'Species differences, e.g. alder fast, oaks and2 Little anaerobic lignin decomposition.

conifers slow.

tebrates and the presence of aquatic hyphomycete fungi and other microorganisms thatcan grow at low temperatures.

Even in more xeric regions (e.g. grasslands and savannahs) the maximum develop-ment of trees and/or shrubs and large herbaceous species has been along aboriginalstream-river courses. Thus, tree and/or shrub and/or at least herbaceous vegetation (evenat high altitudes and high or low latitudes) are essentially universal components of loticecosystems. The influences are readily categorized by plant size (physical) and physical-

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1826 DC. Running Waters

biochemically controlled biotic processing rates. Thus, inasmuch ..as these fundamentalcharacteristics of the riparian zone can be recognized or predicted, the associated bioticadjustments should follow.

The final point is that the major shifts from pre-aboriginal and aboriginal conditionshave been highly effective in altering the physical characteristics of channels and the asso-ciated riparian system. If running water ecosystems are viewed as terrestrial-aquatic in-separably linked components of a single system to which the lotic biota was adapted inaboriginal (prototype) streams, then reconstruction of their most probable condition isessential if the goal is description of prototype stream-riven.

As shown in Fig. 1, the aboriginal running waters were probably generally character-ized by a much greater influence of riparian- vegetation further along the drainage net-work. Records within post-aboriginal historical time indicate that rivers of order sixthrough nine were highly braided, complex rhennels with greatly expanded opportunityfor direct riparian influence. This complexity was maintained, as in the headwaters, bythe presence of large amounts of woody debris (Swim. 1984). Further, North Americanheadwater streams of order less than three or four were probably more open as a featureof extensive beaver damming in lower gradient systems.

Thus, it appears that given the river continuum concept, and its more recent corol-laries, when adjusted to reflect more acurately aboriginal condition of the lotic-riparianlinkages, will go a long way toward improving our conceptual models of prototyperunning water ecosystems.

AcknowledgementsRiparian Contribution no. 3. Research supported by grant from the ecosytems Study Program,

National Science Foundation. This paper benefitted greatly from discussions with M. A. Witzluatand S. V. GRIMM.

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•‘04A44. •

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263: 110.-119.Slum, J. R. & Fawn?, J. L., 1984: The importance of streamside forests to large rivers: the isola-

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Smarm, R. M., Moons, K. & Guam, S. V., 1984: Analysis of the process of retention of organicmatter in stream ecosystems. — Veit. Internat. Vasil& LimnoiL 22: 1835-1841.

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SWANSON, F. J. & LISNIADOWS, G. W., 1978: Physical consequences of large organic debris in PacificNorthwest streams. — USDA Gen. Tech. Rep. Forest Serv. PNW-69: 1-12.

Tnuonowel, A., 1925: Die BinnengewIsscr Mitteleuropas. — Die Binnengenister 1: 1-390. Stutt-Pvt.

VArmarz, R. L, himstuz., G. W., Gnome, K. W., UDELL, J. R. & CUSHING, C. E., 1980: Theriver continuum concept. — Gin. I. Fish. Aqua. Sri 37:370-377.

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BARTELS., S. M. (ads.), Dymnia ciflotic moue= 29-42. Ann Arbor Sci., 494 pp.

Authors' addresses:W. Cum:Ns, Dept. Fisheries and Wildlife, Oregon State Univ., Corvallis, Oregon 97331,

G. W. MINSHALL, Dept. Biology, Idaho State Univ., Pocatello, Idaho 83201, U.S.A.R. Siamu, Forest Sciences Lob., U.S. Forest Service, Corvallis, Oregon 97331, U.S.A.

C. E. Cuomo, Ecosystem Dept., BatellePacific N.V. Lab., Richland, WA 99352, U.S.A.R. C. Fertaran, Dept. Limnolou, Univ. Lund, S-22003 Lund, Sweden

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