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Leaf decomposition and invertebrate colonization responses to manipulated litterquantity in streamsAuthor(s): S. D. Tiegs, F. D. Peter, C. T. Robinson, U. Uehlinger, and M. O. GessnerSource: Journal of the North American Benthological Society, 27(2):321-331. 2008.Published By: The Society for Freshwater ScienceDOI: http://dx.doi.org/10.1899/07-054.1URL: http://www.bioone.org/doi/full/10.1899/07-054.1

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J. N. Am. Benthol. Soc., 2008, 27(2):321–331� 2008 by The North American Benthological SocietyDOI: 10.1899/07-054.1Published online: 25 March 2008

Leaf decomposition and invertebrate colonization responses tomanipulated litter quantity in streams

S. D. Tiegs1AND F. D. Peter2

Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, andInstitute of Integrative Biology (IBZ), ETH Zurich, 6047 Kastanienbaum, Switzerland

C. T. Robinson3AND U. Uehlinger4

Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, andInstitute of Integrative Biology (IBZ), ETH Zurich, 8600 Dubendorf, Switzerland

M. O. Gessner5

Department of Aquatic Ecology, Eawag: Swiss Federal Institute of Aquatic Science and Technology, andInstitute of Integrative Biology (IBZ), ETH Zurich, 6047 Kastanienbaum, Switzerland

Abstract. Resource availability is an important ecosystem attribute that can influence species distributionsand ecosystem processes. We manipulated the quantity of leaf litter, a critical resource in streams, in areplicated field experiment to test whether: 1) greater litter quantity promotes microbial leaf decomposition(through greater microbial inoculum potential), and 2) reduced litter quantity enhances decomposition byleaf-shredding invertebrates (because shredders aggregate on rare resource patches). In each of 3 streams,we identified reaches in which litter quantity was either: 1) augmented, 2) depleted, or 3) left unchanged. Wedetermined decomposition rates and macroinvertebrate colonization of alder leaves placed in coarse- andfine-mesh litter bags, an approach intended to allow or prevent access to leaves by leaf-shreddingmacroinvertebrates. Responses to litter manipulations were complex. In 2 streams, litter quantities differedamong treatments, but high quantities of litter in the control reach of the 3rd stream produced an overallvariable pattern. Microbial decomposition was similar across litter treatments. In contrast, in the 2 streamswhere litter manipulation was successful, decomposition in coarse-mesh bags tended to be faster wherelitter was scarce than where it was abundant. Abundances of total and leaf-shredding macroinvertebrates inlitter bags did not differ among litter manipulations in these 2 streams. However, a litter-consumingamphipod (Gammarus fossarum) tended to be most abundant in bags placed in litter-depleted reaches in the 2streams, indicating that this large and highly mobile shredder might have been instrumental in causingdifferences in decomposition in response to litter manipulations. Overall, the effects caused by alteration oflitter quantities on leaf decomposition and macroinvertebrate colonization were relatively weak.Nevertheless, results from 2 of the 3 streams where litter manipulation was successful were consistentwith the hypothesis that short-term changes in resource availability might influence ecosystem processes bydetermining the spatial distribution of key consumers.

Key words: leaf retention, invertebrate aggregation, benthic organic matter, leaf breakdown, shred-ders, Gammarus, ecosystem process.

The quantity of resources available in ecosystems is

a key factor that determines the spatial distribution of

organisms, which, in turn, might govern ecosystem

processes. A fundamental resource in many streams is

leaf litter derived from terrestrial vegetation (Wallace

et al. 1999). Leaves in the canopies of streamside trees

cast shade that strongly limits instream primary

1 Current address: Department of Biological Sciences,University of Notre Dame, Notre Dame, Indiana 46556USA. E-mail: stiegs@nd.edu

2 E-mail addresses: fabianpeter@gmx.net3 christopher.robinson@eawag.ch4 urs.uehlinger@eawag.ch5 mark.gessner@eawag.ch

321

production (Sabater et al. 2000). When shed andretained in stream channels, these leaves providehabitat and food or substrate to benthic invertebratesand microorganisms (Baldy et al. 2007, Greenwood etal. 2007). As a result, terrestrially derived leavesconstitute the major basal resource of food webs insmall forest streams (Wallace et al. 1999), and thequantity of leaf litter present is likely to be a criticalfactor governing stream ecosystem structure andfunction.

Quantities of litter present at a given time andlocation hinge on 3 processes: input, retention, anddecomposition. Litter input depends on the density,composition, and productivity of riparian vegetation(Benfield 1997) and on the hillslope transport (i.e.,lateral input) that delivers material to the stream fromthe forest floor (Webster et al. 1995). Retention of litterin streams is determined by interactions betweenhydrologic and geomorphic features, such as channeldepth, grain size of the substratum, and abundance oflarge wood (Jones 1997, Hoover et al. 2006). Becausethese factors vary markedly across time and space,litter retention is also highly variable (Webster et al.1999, Larranaga et al. 2003). Litter retention might beas important as input in determining quantities ofbenthic litter. For example, in a multiple regressionanalysis with data from 19 streams located throughoutthe USA, variables related to channel retentivenessexplained more variability in benthic litter quantitiesacross streams than did variables related to input(Jones 1997). When retained in stream channels, litter iscolonized and used by detritivorous macroinverte-brates (shredders) and by microbial decomposers,particularly fungi (Anderson and Sedell 1979, Hieberand Gessner 2002). The interplay of these organismsdetermines the biological decomposition of leaf litter instreams (Gessner et al. 1999).

Experimental manipulation of litter quantity instreamside channels has shown that shredders cantrack litter resource patches. Such resource trackingcan lead to aggregation of shredders on leaf packs andconsequent acceleration of decomposition (Rowe andRichardson 2001). Results from this small-scale andshort-term experiment have been corroborated by datafrom resource-depleted streams, such as those thatdrain clear-cut catchments (Benfield et al. 2001) orthose located above tree line (Robinson et al. 1998). Inthis and other situations where litter resources instreams are rare, aggregation of shredders on exper-imental leaf packs can be massive and can dramati-cally accelerate decomposition beyond the ratescaused by microbial activity alone (e.g., Baldy andGessner 1997, Robinson et al. 1998).

Effects of shredder aggregation on decomposition

have been difficult to demonstrate by manipulatinglitter quantity in whole-stream experiments. Reice(1991) altered litter quantities in a series of 30-mstream reaches and found no evidence for changes indecomposition rates in either litter-augmented orlitter-depleted reaches. Leaf decomposition rate didnot differ between a headwater stream from whichlitter was experimentally excluded and a referencestream during the 1st year after exclusion (Eggert andWallace 2003). The absence of immediate responses tolitter manipulations in these 2 studies is surprising inview of numerous field observations of shredderaggregation, corresponding rapid decomposition, andclear effects on decomposition in experimental streamchannels. Thus, the extent to which shredder aggrega-tion responses to altered quantities of benthic litterdetermine leaf litter colonization and decomposition isunclear.

Microbial decomposers also might respond tochanges in litter availability, which would haveconsequences for decomposition. Experimentally in-troduced litter might immobilize nutrients and slowmicrobial decomposition when dissolved nutrientconcentrations in stream water are low (Eggert andWallace 2003). In contrast, abundant benthic littermight promote, rather than curb, microbial decompo-sition when nutrient supply is less critical. Such aneffect could arise as a consequence of the life-cyclecharacteristics of aquatic hyphomycete fungi, the keymicrobial decomposers of leaf litter in streams (Gess-ner et al. 2007). Aquatic hyphomycetes are character-ized by rapid and dense sporulation following initialestablishment and growth in decomposing litter(Gessner and Chauvet 1994). As a result, sporedensities in stream water should be greater whenbenthic litter is abundant than when it is sparse, andthe difference is likely to be large, as indicated byoften-dramatic increases in spore concentrations fol-lowing autumn leaffall (Barlocher 2000) and greaterspore concentrations following experimental enhance-ment of litter retention (Laitung et al. 2002). Given thatleaf decomposition in microcosms is considerablyfaster when spore densities are high than when theyare low (Treton et al. 2004), greater fungal colonizationand decomposition of leaves would be expected whenquantities of benthic litter are high.

We present results from an experimental manipula-tion of benthic litter quantity at the scale of the streamreach (sensu Bisson and Montogomery 2006) in 3streams. We designed this replicated experiment to test2 hypotheses related to the effect of benthic litterquantity on macroinvertebrates colonization anddecomposition. First, we hypothesized that, in theabsence of pronounced nutrient limitation, augmenta-

322 [Volume 27S. D. TIEGS ET AL.

tion of benthic litter would increase microbial decom-position relative to controls. We addressed thishypothesis by testing whether rates of leaf decompo-sition in fine-mesh litter bags, which restrict shredderaccess, varied across stream reaches in response toexperimental augmentation or depletion of the quan-tity of benthic litter. Second, we hypothesized thatshredding invertebrates would promote decomposi-tion, particularly in stream reaches where benthic litterwas scarce. We tested this hypothesis by comparingshredder colonization and decomposition of leaf packsin coarse-mesh litter bags placed in reaches withexperimentally augmented, depleted, or unmanipulat-ed litter.

Methods

Study sites

Three streams (Andelsbach, Fohrenbach, Muhle-bach) were selected in the southern Black Forest,Germany. Streams had similar watershed geology(granitic), land cover (primarily managed forests),and size (3rd-order). Channels were characterized bypool and riffle sequences, substrata were dominatedby coarse gravel and small cobble (sensu Wentworth1922), and similar volumes of large wood werepresent, as judged by visual inspection along thestudy sections. Riparian vegetation consisted of aclosed-canopy mixed community, including alder,maple, ash, and beech. Table 1 summarizes the majorchemical and physical attributes of the streams.

Experimental design

Three reaches were delineated in each stream.Reaches were 40 m long and homogeneous in termsof morphological characteristics and riparian vegeta-tion. One of 3 treatments was assigned to each reach ineach stream: 1) litter augmentation above backgroundlevels, 2) litter depletion, and 3) unmanipulatedcontrol. Potentially confounding upstream–down-stream effects were mitigated by constraining randomassignment of treatments to reaches using the criterionthat each treatment was replicated in an upstream,

middle, and downstream reach, and each treatmentwas replicated only once within a stream. Reacheswere separated by .100 m to minimize influences ofupstream treatments on treatments in middle ordownstream reaches. Thus, the result was a con-strained complete block design using the stream as theblocking factor (Fig. 1).

Litter augmentation was achieved by means of littertraps (Dobson and Hildrew 1992, Dobson 2005). Trapsconsisted of plastic mesh (20 3 20 cm, 1-cm mesh size)held vertically in the stream by 2 rebars hammeredinto the streambed and oriented perpendicular tostream flow. In each augmented reach, 105 to 140 trapswere added (average trap density ¼ 1/m2) just beforeleaffall (Dobson 2005).

Litter depletion was accomplished by removing allvisible leaf material from the reach by hand at weeklyintervals. Each handful of leaf material was rinsedgently in the stream to minimize removal of residentinvertebrates. The first depletion campaign occurredthe same day that litter traps were installed. Littervolume removed during each depletion campaign wasquantified by placing the collected leaf material into arigid plastic bin (volume¼ 0.20 m3) and measuring theheight of leaf material in the bin. Numerous holes (2-cm diameter) drilled into the walls of the bin allowedwater to drain. The content of the bin was mixedthoroughly by hand, and three 1.6-L subsamples weretaken to the laboratory where they were oven-dried(1058C, 5 d) and weighed. Dry mass of subsampleswas extrapolated to the volume of leaf material in thebin to estimate the total mass of litter collected inreaches during each removal campaign.

Effectiveness of litter augmentation and depletion inmodifying the quantity of benthic litter on thestreambed was determined with a cylindrical sampler(area¼0.071 m2) 58 d after initiation of the experiment.The sampler was placed at 20 locations that wererandomly selected within a grid along each streamreach. The enclosed litter on the streambed wasgathered, placed in plastic bags, transported to thelaboratory, oven-dried (1058C, 5 d), and weighed.

TABLE 1. Physical and chemical characteristics of the 3 streams at low-flow conditions. Temperature data refer to daily meansduring the study period. Other measurements were taken on a single sampling date during the study. Values in parentheses arestandard deviations; n/a indicates data were not available.

StreamElevation

(m asl)Width

(m)Discharge

(L/s)Temperature

(8C)Turbidity

(NTU)Alkalinity(mmol/L)

Conductivity(lS/cm)

PO4-P(lg/L)

NH4-N(lg/L)

NO3-N(lg/L)

Andelsbach 530 2.3 (0.53) 30 5.5 (1.5) 2.3 0.67 108 3.5 5.0 961Fohrenbach 740 3.8 (0.71) 65 5.3 (1.6) n/a 0.47 115 81.1 4.3 649Muhlebach 420 2.9 (0.65) 33 6.9 (1.2) 1.3 1.08 151 5.3 3.0 2500

2008] 323LEAF DECOMPOSITION AND LITTER QUANTITY

Litter-bag preparation, installation, and sample processing

A litter-bag approach was used to determinedecomposition rates. Bags were constructed of eithercoarse-mesh (10-mm mesh size) or fine-mesh (0.5-mmmesh size) plastic netting to allow or prevent,respectively, macroinvertebrate access to enclosedleaves. Recently senesced leaves of alder (Alnusglutinosa [L.] Gaertn.), a common riparian speciesthroughout most of Europe, were collected, air-dried,and weighed into batches of 5.00 6 0.25 g. Afterweighing, each batch was remoistened to render theleaves pliant, and the leaves were placed into the meshbags. Five randomly selected litter bags were used toestimate initial leaf moisture content by drying (1058C,24 h) and reweighing the material.

Litter bags were taken to the field the next day, and 6coarse-mesh and 6 fine-mesh bags were placed in themiddle of each of the 9 study reaches. Steel rods werehammered into the streambed, and a coarse- and fine-mesh bag was attached to each rod with nylon cord.Flat cobbles were placed on the cord immediately

upstream of each bag to prevent bags from moving inthe current and to ensure contact with the sediment.After 41 d of exposure in the streams, all leaf bags wereretrieved and placed in plastic bags. Bags werereturned to the laboratory in a cooler and frozen forlater processing.

In the laboratory, the contents of each plastic bagwere emptied into a shallow tray with a small amountof water and allowed to thaw. Each leaf was cleanedindividually with a soft-bristled paint brush to removeadhering debris and macroinvertebrates, placed in analuminum tray, oven-dried (1058C, 48 h), and weighedto the nearest 0.01 g. Invertebrates were collected on a500-lm mesh screen, preserved in 70% ethanol,identified to the lowest practicable taxon, counted,and assigned to functional feeding groups (Gessnerand Dobson 1993, Merritt and Cummins 1996).

Statistical analysis

Differences in log10(xþ 1)-transformed quantities ofbenthic litter on the streambed were tested with

FIG. 1. Schematic diagram of the randomized block design used to test the effect of benthic litter quantity on leaf decompositionand macroinvertebrate abundance. Litter quantity was manipulated in 3 reaches (litter depletion, litter augmentation, andunmanipulated control) in each of 3 study streams. Random assignment of litter manipulations to reaches was constrained toensure that each treatment was replicated in an upstream, middle, and downstream reach, and each treatment was replicated onlyonce within a stream. Stream-flow direction is from the top of the figure downward. Minimum distance between reaches was 100m.

324 [Volume 27S. D. TIEGS ET AL.

analysis of variance (ANOVA) using the stream as ablocking factor. When the effect of litter manipulationwas not consistent across streams, the litter manipu-lation 3 stream interaction term was included in theANOVA model, following the rationale of Newman etal. (1997) and Quinn and Keough (2002). Differences inthe percentage of leaf dry mass remaining in litter bagsalso were tested with ANOVA (using litter manipula-tion and mesh size as the main factors of interest, withstream as a blocking factor), and when streamsdiffered in their responses, the interaction terms withstream were included in the model as well (Newmanet al. 1997, Quinn and Keough 2002). Differences inlog10(x þ 1)-transformed invertebrate abundance incoarse-mesh litter bags were tested with ANOVA(using litter manipulation as the main factor of interestand stream as a blocking factor). In this case, the littermanipulation 3 stream interaction was never signifi-cant, so all p-values reported for invertebrates refer toANOVA models without this interaction term. Whensignificant (i.e., p , 0.05) differences were observedamong litter manipulations, Tukey’s post-hoc testswere used to identify the means that differed.

Results

Quantity of benthic litter on the streambed

The quantity of benthic litter on the streambeddiffered significantly among streams (F2,175 ¼ 8.6, p ,

0.001) and among litter manipulations (F2,175¼ 8.0, p ,

0.001). Depleted reaches had consistently less benthiclitter than control or augmented reaches (Fig. 2).However, the litter manipulation 3 stream interactionterm was significant and was included in the ANOVAmodel (model 1 of Newman et al. 1997; Table 2),

indicating an inconsistent effect of litter manipulationamong streams. This effect was caused by largequantities of naturally accumulated benthic litter inthe control reach of Muhlebach (Fig. 2). WhenMuhlebach was excluded from analysis, the littermanipulation 3 stream interaction term was notsignificant (F2,114 ¼ 1.08, p ¼ 0.34), litter manipulationhad a marginally significant effect on the quantity ofbenthic litter on the streambed (F2,2 ¼ 16.9, p ¼ 0.056),and the quantity of benthic litter was greater inaugmented than in control and depleted reaches(Tukey’s post-hoc comparisons, p , 0.001) but didnot differ significantly between control and depletedreaches (p ¼ 0.08).

Benthic litter removed from depletion reaches

The quantity of benthic litter removed from depletedreaches varied among streams. A total of 7.2 kg drymass was collected in Andelsbach, 7.7 kg in Fohren-bach, and 21.8 kg in Muhlebach, corresponding to adepletion of 72, 51, and 189 g/m2 in each streamchannel. The mass of benthic litter removed fromdepleted reaches declined over the first month afterlitter traps were installed and subsequently remainedlow (Fig. 3), except in Muhlebach in late November,when the quantity of benthic litter removed increasedat a time that coincided with a minor rainfall andstream-flow event that occurred in this watershed butnot the 2 others.

Leaf decomposition

Leaf decomposition was significantly faster incoarse-mesh than in fine-mesh litter bags (F1,100 ¼31.4, p , 0.001) and differed significantly amongstreams (F2,100¼ 8.1, p , 0.001; Fig. 4A, B). In contrast,litter manipulation had no significant effect on leafdecomposition (F2,100¼ 1.9, p¼ 0.15), nor was the littermanipulation 3 mesh size interaction term significant(F2,100 ¼ 0.74, p ¼ 0.48). Leaf decomposition was more

FIG. 2. Mean (61 SE) quantity of benthic litter on thestreambed in litter-depleted, unmanipulated control, andlitter-augmented reaches in 3 streams; n¼ 20 samples in eachreach. Note that the y-axis begins at 0.1 g.

TABLE 2. Results of analysis of variance for the effect oflitter manipulation and stream on benthic litter quantity.Stream is treated as a blocking factor according to Newmanet al. (1997). – indicates F was not calculated because thestream 3 litter manipulation term was significant.

Source of variation

Sum ofsquares

(%) df

Meansquare

F p

Stream 8.6 2 6.4 – –Litter manipulation 15.1 2 11.2 5.44 0.07Litter manipulation

3 stream 5.5 4 2.1 3.35 0.011Error 70.8 171 0.6

2008] 325LEAF DECOMPOSITION AND LITTER QUANTITY

variable across streams and litter manipulations incoarse-mesh (Fig. 4A) than in fine-mesh litter bags(Fig. 4B). Patterns of leaf decomposition were similaramong litter manipulations in 2 streams (Andelsbachand Fohrenbach), whereas the pattern in Muhlebachdiffered (Fig. 4A). When Muhlebach was excludedfrom the analysis, the litter manipulation did notsignificantly affect leaf decomposition (F2,65 ¼ 2.6, p ¼0.08), but the litter manipulation 3 mesh size interac-tion term was significant (F2,65 ¼ 3.2, p ¼ 0.045). InAndelsbach and Fohrenbach, leaf decomposition ratein coarse-mesh bags decreased with the quantity ofbenthic litter in the reach (augmented , control ,

depleted; Fig. 4A). Leaf decomposition in fine-meshbags varied little overall (Fig. 4B); the differencebetween mean leaf decomposition across all streamsand litter manipulations was ,4% (Fig. 4B).

Macroinvertebrate colonization

Thirty-two taxa of macroinvertebrates were identi-fied from coarse-mesh bags. Shredders accounted for46% of all macroinvertebrates and consisted of stone-flies (Amphinemura, Leuctra, Nemoura, Protonemura,Taeniopteryx), limnephilid caddisflies, and the amphi-pod Gammarus fossarum. Stonefly shredders accountedfor 77%, and the genus Nemoura accounted for 60% ofall shredders.

Large macroinvertebrates were almost never en-countered in fine-mesh bags, indicating that fine-meshbags effectively excluded the most important shred-ders. Very small nemourids were found in fine-meshbags (449 individuals in fine-mesh bags; 1732 individ-uals in coarse- and fine-mesh bags combined). Thissuggests that an appreciable fraction of nemourids in

fine-mesh bags and, by inference, also in coarse-meshbags, were very early instars, small enough to passthrough a 0.5-mm mesh. In contrast, only 2 individualsof Gammarus (,0.5% of the total number) wereobserved in fine-mesh bags, indicating that Gammaruswere typically larger than other abundant shredders.Most of the remaining individuals in fine-mesh bags(55% of all macroinvertebrates) were chironomids.Numbers of other invertebrates (collector–gatherers,collector–filterers, scrapers, and nonchironomid pred-ators) were consistently low across streams and littermanipulations.

The total number of invertebrates per coarse-meshlitter bag and number of shredders per coarse-meshlitter bag did not differ significantly among streams(total invertebrates: F2,49 ¼ 0.73, p ¼ 0.49; shredders:F2,49 ¼ 0.25, p ¼ 0.78) or litter manipulations (totalinvertebrates: F2,49 ¼ 0.24, p ¼ 0.79; shredders: F2,49 ¼0.20, p ¼ 0.81) (Fig. 5A, B). Numbers of individuals incoarse-mesh litter bags from other functional feedinggroups did not differ significantly among litter

FIG. 3. Dry mass of benthic litter removed from eachlitter-depleted reach through time. The abrupt increaseobserved in Muhlebach on 25 November coincided with arainfall event in this watershed and not the others.

FIG. 4. Mean (61 SE) leaf dry mass remaining in coarse-mesh (A) and fine-mesh (B) litter bags after 41 d ofdecomposition in litter-depleted, litter-augmented, andunmanipulated control reaches of three streams; n ¼ 6 litterbags for each mesh size in each reach. Note that the y-axesbegin at 40%.

326 [Volume 27S. D. TIEGS ET AL.

manipulations (F2,49 , 1.29, p . 0.28). The number ofnemourid stoneflies per coarse-mesh litter bag did notdiffer significantly among streams (F2,49 ¼ 2.38, p ¼0.10) or litter manipulations (F2,49¼1.78, p¼0.18), eventhough nemourids were rare in Fohrenbach (Fig. 5C).In contrast, the number of Gammarus individuals percoarse-mesh litter bag differed strongly among streams(F2,49 ¼ 35.3, p , 0.001), although differences among

litter manipulations were not significant (F2,49¼ 2.86, p¼ 0.067). However, when data from Muhlebach (whereonly a few Gammarus colonized litter bags) wereexcluded from the analysis, the number of Gammarusper coarse-mesh litter bag also differed significantlyamong litter manipulations (F2,32¼ 3.47, p¼ 0.043; Fig.5D). In Andelsbach and Fohrenbach, the number ofGammarus per coarse-mesh litter bag was significantlylower in augmented than in depleted litter manipula-tions (Tukey’s post-hoc comparison, p ¼ 0.033).

Discussion

Test of the shredder aggregation hypothesis

Previous studies on ecosystem effects of litterdepletion on decomposition have yielded equivocalresponses. Results from observational field studies andsmall-scale experiments in streamside channels haveindicated an acceleration of decomposition when litteris scarce (e.g., Benfield et al. 1991, 2001, Robinson et al.1998, Rowe and Richardson 2001), but this responsehas not been evident in field experiments (e.g., Reice1991). The overall effects of benthic litter manipula-tions in our experiment were subtle; however, resultsfrom coarse-mesh bags in 2 of the 3 study streamswere consistent with the predicted pattern: the leafdecomposition rate tended to be fastest in depletedreaches and slowest in augmented reaches. Differencesin shredder colonization of experimental litter bagshave been proposed as the mechanism underlyingvarying rates of leaf decomposition in response to litterquantity. This hypothesis proposes that shredderswould aggregate most in litter bags exposed inresource-depleted environments (e.g., Benfield et al.2001, Rowe and Richardson 2001), whereas shredderswould be distributed across a larger number ofresource islands in control reaches and, especially, inresource-augmented reaches.

Data consistent with these hypotheses must showthat: 1) shredders cause significant litter mass loss, and2) shredders aggregate in experimental leaf bags inreaches where benthic litter is scarce, and they do notaggregate in experimental litter bags in reaches wherebenthic litter is abundant. The first requirement wasmet in our study streams. Leaf decomposition wassignificantly faster in coarse-mesh than in fine-meshbags. More rapid decomposition in coarse-mesh thanin fine-mesh litter bags can be caused by factors otherthan shedders (Boulton and Boon 1991), but sucheffects were unlikely to be important in our studybecause no indications of mechanical fragmentation incoarse-mesh bags or O2 depletion in fine-mesh bagswere observed. Moreover, controlled experimentsunder various hydraulic conditions in experimental

FIG. 5. Mean (61 SE) number of total macroinvertebrates(A), total shredders (B), nemourid stoneflies (C), andGammarus (D) in coarse-mesh litter bags after 41 d of leafdecomposition in litter-depleted, litter-augmented, andunmanipulated control reaches of 3 streams; n ¼ 6 coarse-mesh litter bags in each reach.

2008] 327LEAF DECOMPOSITION AND LITTER QUANTITY

stream channels have shown that litter mass loss doesnot differ between coarse-mesh and fine-mesh litterbags when shredders are absent, but it does differsignificantly between the 2 types of litter bags whenshredders are present (Ferreira et al. 2006).

The 2nd requirement, aggregation of shredders incoarse-mesh bags placed in litter-depleted reaches,was less well met in our study because total numbersof macroinvertebrates, shredders, and nemourid stone-flies did not follow the predicted pattern. However, thedistribution of Gammarus across reaches supports theidea—this shredder did converge on resource islandsprovided by our experimental litter bags. Specifically,Gammarus was more abundant in litter bags indepleted reaches and rarer in litter bags in augmentedreaches. This pattern matched the patterns in benthiclitter quantities and leaf decomposition rate in the 2study streams where litter manipulations were suc-cessful (i.e., in Andelsbach and Fohrenbach).

Gammarus are very effective leaf shredders (e.g.,Groom and Hildrew 1989, Baldy and Gessner 1997,Dangles et al. 2004) that feed very selectively(Barlocher and Kendrick 1973, Arsuffi and Suberkropp1989, Graca et al. 2001). Moreover, they are extremelymobile relative to other invertebrates in our study, and,thus, they are capable of seeking out and making useof resource islands. Gammarus were almost neverencountered in fine-mesh litter bags (0.5-mm meshsize), suggesting that specimens were larger and moreeffective at consuming leaves than other shredderspecies in our study streams. For example, thenumerically abundant nemourid stoneflies often werefound in fine-mesh litter bags, indicating that a largeproportion of them were early instars with lowbiomass (2-mm length, ,0.05 mg). These early instarswould have had a limited shredding capacity, even ifleaves were their main diet.

Mass loss also was significantly faster in coarse-mesh litter bags than in fine-mesh bags in the streamwhere Gammarus was rare. This result suggests thatother shredders contributed to litter mass loss as well.However, the lower mobility of those taxa mightrequire greater differences in benthic litter quantitiesthan those achieved in our study to demonstrateaggregation effects on leaf decomposition in fieldsituations. Collectively, this evidence indicates thataggregation in experimental litter bags of highlymobile Gammarus, but not other shredders, could havebeen instrumental in causing the decompositionpatterns observed across litter-manipulated streamreaches. The general implication is that resourceavailability might influence ecosystem functioning bymodulating aggregation of key consumer species.

In the long term, shredder-mediated changes of

decomposition rates also might occur as a result ofchanges in shredder production, an idea that issupported by data from a whole-stream litter-exclu-sion experiment (Eggert and Wallace 2003). Noimmediate response to litter exclusion was observedin that study, but decomposition of red maple leaves 1and 2 y later was much slower in the litter-exclusionstream than in the reference stream, which receivednormal litter inputs (k ’ �0.010/d compared to�0.017/d). The suggested mechanism was severe foodlimitation, which restrained recruitment of shreddersin the years following litter depletion. This mechanismis in accordance with data from another study, whichshowed that production of some large shredder taxawas markedly lower in the litter-exclusion stream thanin the reference stream 1 y after litter inputs wereprevented (Wallace et al. 1999, Eggert and Wallace2003). Such an effect, while possible, would not havebeen captured by our single-season experiment.

In contrast to litter-depletion experiments done at alarge scale, litter-augmentation experiments typicallyhave been conducted at smaller spatial scales. Fur-thermore, most litter-augmentation experiments haveconsidered macroinvertebrate responses, rather thanresponses of litter decomposition or other processes, toaltered litter availability. Shredders have shownpositive responses when benthic litter quantities havebeen experimentally elevated (Dobson and Hildrew1992). For example, in streamside experimental chan-nels with varied quantities of benthic leaf litter, densityand biomass of shredders increased in response togreater litter availability (Richardson 1991). Further-more, abundances of invertebrates increased relative tocontrols when boulders or litter traps installed instream channels increased litter quantity relative tocontrols (Dobson and Hildrew 1992, Negishi andRichardson 2003). However, shredder abundanceremained unchanged in a similar boulder-introductionexperiment that enhanced litter retention (Lepori et al.2005, see also Wallace et al. 1995), and leaf decompo-sition also remained unchanged (Lepori et al. 2005).The results of these augmentation studies lend indirectsupport to the hypothesis that shredders accelerate leafdecomposition in resource-limited environments.

Test of the microbial decomposition hypothesis

Our hypothesis that larger quantities of benthic litterwould lead to faster microbial decomposition inaugmented than in control or depleted reaches wasnot supported by our results. The rationale behind thishypothesis was that larger quantities of decomposingbenthic litter should lead to higher concentrations offungal spores in stream water. A greater fungal

328 [Volume 27S. D. TIEGS ET AL.

inoculum would accelerate fungal colonization of freshlitter and, therefore, microbial decomposition. Such anoutcome has been observed in a microcosm experi-ment (Treton et al. 2004). In one field study, fungalspore concentrations in stream water were greater instream reaches to which logs or litter traps (identical tothose used in our study) had been added as retentionstructures (Laitung et al. 2002). Some of the controland depleted reaches in our study could have beenexposed to elevated spore concentrations from aug-mented upstream reaches, but our experiment wasdesigned to prevent the same systematic upstream–downstream effect in all 3 streams (Fig. 1). Further-more, even if spore concentrations in our experimentalreaches were influenced by upstream litter manipula-tions, the effect on decomposition was negligiblebecause mass loss of leaves in fine-mesh litter bagswas highly consistent across litter manipulations in all3 streams.

Effect of differences among streams on results of littermanipulations

Macroinvertebrate responses to litter manipulationwere different in Muhlebach than in the other 2streams. The control reach of this stream flowed alongthe base of a steep hillslope that delivered large lateralinputs of litter to the channel, so quantities of benthiclitter were higher in the control than in the augmentedreach. As a consequence, relative quantities of benthiclitter in the 3 reaches of this stream differed from thoseintended by our manipulations. However, even if aless extreme control reach had been chosen, theresponse pattern to our litter manipulation probablywould have been different from the responses in theother 2 streams because Gammarus was rare in all 3reaches of Muhlebach. If our conclusion regarding thecritical role of Gammarus in leaf decomposition iscorrect, then the low abundance of this species in theMuhlebach probably explains why decompositionpatterns across reaches did not reflect shredderabundances.

The deviating pattern among streams in our studyillustrates the importance of replicating manipulativeecosystem experiments and of exercising great carewhen extrapolating results to other ecosystems, evenwhen they appear to be similar. Whole-ecosystemmanipulations are among the best means to assesseffects of environmental or biotic factors on ecosystemprocesses and properties (Carpenter et al. 1989, 1995).A drawback of the approach is that practical con-straints often preclude replication of treatments (sensuHurlbert 1984). A suite of methods has been proposedto alleviate this difficulty (e.g., Carpenter et al. 1989,

1995, Wallace et al. 1999), but none of the methodsfully resolves the problem (e.g., Murtaugh 2002).However, although not always practical, use ofreplicated designs often is possible, even in manipu-lative ecosystem experiments (e.g., Maron et al. 2006,Entrekin et al. 2008). Headwater streams are primecandidates for this approach because of their relativelysmall size.

In summary, we observed that the quantity of leaflitter in stream channels influenced the colonization ofexperimental litter bags by Gammarus, and theabundance of Gammarus in litter bags, in turn, mighthave influenced leaf decomposition rate. In litter-depleted reaches, Gammarus appeared to aggregate inlitter bags and to accelerate decomposition. However,these results were not consistent among all streamsexamined, which illustrates the importance of treat-ment replication when conducting manipulative eco-system experiments.

Acknowledgements

We thank Markus Schindler, Simone Graute, TorstenDiem, Catherine Hoyle, Lucia Klauser, Caroline Joris,Angelika Rohrbacher, Michael Siegrist, and MichaelVock for their help in the field. Andrew Boulton, AlanCovich, Pamela Silver, and anonymous referees pro-vided many useful comments on previous drafts of thepaper. This research was funded by the Swiss StateSecretariat of Education and Research (SBF No.01.0087) as part of the European Union projectRivFunction (contract no. EVK1-CT-2001–00088).

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Received: 11 June 2007Accepted: 5 February 2008

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