Primary Research Paper

The timing of winter-growing shredder species and leaf litter turnover rate

in an oligotrophic lake, SE Sweden

Irene Marita Bohman* & Jan HerrmannFreshwater Ecology Group, Dept. of Biology and Environmental Science, University of Kalmar, S-391 82, Kalmar(*Author for correspondence: Tel: +46 480 44 73 23; Fax: +46 480 44 73 05; E-mail: Irene.Bohman@hik.se)

Received 3 November 2005; in revised form 22 June 2005; accepted 24 June 2005

Key words: shredders, leaf litter processing, allochthonous subsidies, winter, Limnephilidae, Asellus, Leptophlebia

Abstract

Small freshwater systems often depend on allochthonous organic subsidies to sustain productivity. Benthicinvertebrates consuming coarse detritus maintain the energy flow by conveying dead organic matter intoprey items and increase the food availability for other consumers. Compared to lotic systems, the dynamicsof coarse detritus decomposition has not received much attention in lakes. The objectives of this study wereto investigate the seasonality of leaf litter turnover and the timing of abundance of potential shredderspecies in a typical oligotrophic boreal lake. Leaf litter was experimentally exposed in litterbags in thelittoral zone in Lake Valen from autumn to late spring two consecutive years. The weight loss rate of leaflitter initially followed the same pattern during both winter periods, but was markedly influenced byfreezing in late winter the second year. Further, the seasonal variation patterns in abundance in litterbagswere quite different among the potential shredder species. Only the limnephilid caddis larvae showed adensity variation pattern possible to connect to the weight loss of leaf litter in litterbags. Otherwise frequentdetritivores such as Asellus aquaticus and Leptophlebia marginata displayed lowest density in litterbagsduring the main weight loss period. However, after the long ice period the second winter the remaining leaflitter seemed to be consumed by A. aquaticus. With increasing knowledge of the initial leaf breakdownprocess and the guild of shredders in lakes, the decomposition rate may also in this habitat become a usefulinstrument when evaluating the impact from perturbations on ecosystem function.

Introduction

Productivity in small freshwater systems oftendepends upon ecotonal transfer of allochthonousorganic subsidies (Gasith & Hasler, 1976; Oertli,1993; France & Peters, 1995; Caraco & Cole,2002). Hence, disturbances affecting detritus inputand processing could potentially influence the lakeecosystem. Regarding coarse detritus the effects ofchanges are expected to be most pronounced inlittoral zones (Pieczynska, 1990; Wetzel, 1990).Shredders, invertebrates feeding on coarse organicmatter, constitute a link between riparian vegeta-tion and many compartments of the lake

ecosystem (Palmer et al., 2000). Shredder pro-cessing of coarse detritus can control the furtherconsumption of leaf-derived detritus through themobilization of fine particulate matter (Cumminset al., 1989; Palmer & O’Keeffe, 1992; Wallace &Webster, 1996). In addition, all detritivorousbenthic invertebrates maintain the energy flow inthe food web by converting dead organic matterinto prey items (Vanni, 1997; Covich et al., 1999).

The riparian zone plays an essential role foraquatic ecosystem functions and thereby also forrestoration and management of both small andlarger freshwater habitats (Naiman & Decamps,1997). The leaf breakdown rate and shredder

Hydrobiologia (2006) 556:99–108 � Springer 2006DOI 10.1007/s10750-005-1052-1

distribution have been suggested to be helpfulinstruments for evaluating the impact of land useand climate change on the ecosystem function instreams (Wallace & Webster, 1996; Buzby & Perry,2000; Sponseller & Benfield, 2001; Gage et al.,2004). Additionally, the interest focused on bio-diversity and potential effects of species loss infreshwaters calls for better knowledge of thefunctional roles that species actually play in nature(Palmer et al., 2000; Sala, 2000). Research priorityshould be given to species identified as ecologicalkey species, e.g. the functional group of shredders(Palmer et al., 1997; Covich et al., 1999).

Most knowledge of the mechanisms behind thedecomposition of leaf litter is derived from streamhabitats. Despite the awareness of a tight linkbetween riparian vegetation and ecosystem func-tioning also in lakes, the mechanisms and temporaldynamics in this link have not been widely studied.Only a few authors have dealt with these or similarissues in boreal lakes (e.g. Casper, 1987; Oertli,1993; Kiss et al., 2003). However, in some studies,leaf litter processing is used as a tool for evaluatingfunctional effects of acidification in lakes(Fjellheim & Raddum, 1988; Tuchman, 1993; Kok& van der Velde, 1994; Henrikson, 1996).

In running waters a wide range of invertebratetaxa are classified as shredders, (see e.g. Meritt &Cummins, 1984; Cummins et al., 1989; Gessner &Dobson, 1993; Dobson, 1994). This contrasts withthe scarce information from lakes. It seems there-fore important to recognize the potential lakespecies performing coarse detritus processing. Weexpect that a certain set of shredder species arelikely to be present in boreal lakes where leaf litteris available, different from the set of shredders instreams in the same region. Further, the decom-position dynamics of leaf litter has been shown tovary among leaf species in streams (Petersen &Cummins, 1974; Grubbs & Cummins, 1996;Haapala et al., 2001). To be able to study thedecomposition process for an extended time peri-od, we chose leaf litter from one easy-degradable(birch) and one more resistant species (oak).

The specific objectives of this study were toexperimentally investigate the leaf litter decom-position rate for two common tree species andthe timing of the colonization of shredder specieson the leaf litter in a typical lake over two con-secutive years. We hypothesized that shredder

abundance in litterbags is positively correlated tothe food resource available, i.e. remaining weightof the exposed leaf litter during winter and springseason.

Methods

Site description

The experimental fieldwork was performed in thenon-acidified (pH 7.0, alkalinity 0.20 meq/l) oligo/mesotrophic (tot-N 0.48 mg/l, tot-P 5.0 lg/l) lakeValen (57�9¢4¢¢ N, 15�39¢20¢¢ E, surface area2.70 km2). The catchment area (22.9 km2) is domi-nated by coniferous forests (spruce Picea abies (L.)H. Karst and pine Pinus silvestris L.), while theriparian zone is a mixture of birch Betula pendula(Roth), oak Quercus robur L., aspen Populus tremulaL., alder Alnus glutinosa (L.) Gaertn., willow Salixand bog myrtle Myrica gale L.

Experimental design

Leaf litter was exposed to decomposition in thelittoral zone using litter bags, made of a doublelayer plastic net in a square (20� 20 cm) form,with bottom mesh size of 6 mm and top mesh sizeof 12 mm. This construction retained leaf partsover 6 mm, but still allowed the entrance of case-bearing trichopteran shredders. In November1994, 60 litterbags, each containing 2.0 g dw (dryweight) of birch and oak litter, were dispersed at50 cm depth every second meter along essentiallyalike littoral zones.

The leaves were collected in the riparian zone ofthe lake after senescence, but before abscission anddried at 50 �C until constant weight. Ten litterbagswere retrieved randomly once a month until May1995, resulting in a maximum exposure time of178 days. To minimize the loss of colonisinginvertebrates, a specially constructed sampler grabwas used. Both grab and lid were made of 0.5 mmmetal net. At the same time all invertebrates werecollected alive from the samples and preserved in80% ethanol for later identification to species orgenus level (chironomids and microoligochaetesexcepted). The leaf litter was gently rinsed fromsediment and biofilm, and then dried in 50 �C for48 h.

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Two criteria were applied to decide which taxaand size classes should be considered as potentialleaf shredders in this study; literature data onfeeding habits/functional group classification(Brinck, 1952; Cummins et al., 1989; Wallaceet al., 1990; Hargeby, 1993; Monakov, 2003), anda body length more than 6 mm. This arbitrary sizelimit excludes the earliest stages in the life cycle ofthe shredder species, which were not assumed to becapable of consuming or tearing apart leaves(supported by recent publications as Murphy &Giller, 2000; Dangles & Guerold, 2001; Basagurenet al., 2002) and could not be quantitatively sam-pled with this method.

The study was repeated the following year, thewinter period 1995/1996, using essentially the sametechnique, but the litterbags were prepared with1.5 g dw of birch and oak, respectively. Themaximum exposure time was extended to eightmonths, 248 days, while the number of replicateswas reduced to six. At the same time 24 litterbags(3 replicates at 8 sampling occasions) containing6.0 g dw of rush (Schoenoplectus lacustris L.) wereused to evaluate whether the bags themselves wereattractive refuges for the shredders to escape pre-dation. The rush stems were supposed not to bepreferred food for shredders.

Statistical analysis

We used Spearman’s correlation coefficient (Sokal& Rohlf, 1995), Statistica 6.1 software for PC andsignificance level 5% to evaluate the predictedassociation between remaining leaf litter quantityand shredder abundance in the litterbags.

Results

Ice conditions and litter turnover

Water temperature and ice cover duration differedconsiderably between the two experimental peri-ods (Fig. 1). Maximum ice layer in winter the firstyear was 200 mm and the second, >500 mm, thelatter a very unusual ice situation in this lake. InMarch the second year the ice cover extendeddown to the sediment at the sampling sites,resulting in frozen litter bags impossible to sample.The long episode with ice layer also caused a

period of locally low oxygen concentrations atsome sampling sites, observed as black colouredleaves in litterbags. Weight loss of leaf litter ini-tially followed the same temporal pattern bothwinter periods (Fig. 2), but the effect on the frozenlitterbags was seen as no weight loss between Apriland May year two and a greater variation betweenreplicate litterbags after the ice-break in latespring.

Macroinvertebrate colonization

Altogether 35 taxa of invertebrates colonized thelitterbags (Table 1), among which Asellus aquati-cus, family Chironomidae, Glyphotaelius pelluci-dus, Heptagenia fuscogrisea, Holocentropus dubius,Leptophlebia marginata, Limnephilus marmoratusand Platycnemis pennipes were most frequent. Thepotential shredder species found in litterbags wereA. aquaticus, L. marginata, L. vespertina, Nemouracinerea and five trichopteran species belonging tothe family Limnephilidae (G. pellucidus, Halesusradiatus, Limnephilus flavicornis, L. marmoratusand L. rhombicus). Only A. aquaticus, G. pelluci-dus, Limnephilus spp. and Leptophlebia spp. werefound numerous enough to further consider.Among the species of the genera Limnephilusand Leptophlebia, Limnephilus marmoratus andLeptophlebia marginata were strongly dominant(Table 1).

The seasonal colonization patterns were quitedifferent among the most frequent potentialshredder species, although high variation betweenreplicates and between the 2 years was observed(Fig. 3). The correlation between food resource,i.e. remaining dry weight of leaves and shredder

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Figure 1. Water temperature and ice cover (dotted rectangles

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abundance vary both among the shredder groupsand between the 2 years (Table 2). The abundanceof G. pellucidus was significantly positively corre-lated with both birch and oak litter quantity bothyears, while the abundance of A. aquaticus showedno significant correlation at all. The abundance ofLeptophlebia spp. was significantly correlated withboth birch and oak litter quantity both years, but

the association was negative year 1 and positiveyear 2.

Temporal coupling of loss rate and shredderabundance

As a measure of decomposition rate, the averageloss of dry weight per day since previous sampling

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1.5 g of each birch and oak leaves in litterbags year 2 (n = 6). Error bars show 95% confidence interval.

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was calculated for both leaf types each month.Decomposition rate and the average number ofindividuals of the four shredder groups were thencompared between years (Fig. 3). The data fromboth years demonstrate that the abundances of A.aquaticus and Leptophlebia spp. in litterbags

decrease during mid-winter, when weight loss rateof birch litter peaks. In the first year abundance ofG. pellucidus shows proper timing with weight lossrate of birch, while the abundance of Limnephliusspp. is relatively high during the whole period ofleaf decomposition. Immediately after the period

Table 1. List of invertebrates found in litterbags. The taxa considered as potential shredders are denoted with S and the total number

of individuals > 6 mm are given for each sampling period

TAXON Shredder Total number of observations

Leaves Year 1 Leaves Year 2 Rush Year 2

TURBELLARIA Dendrocoelum lacteum (Mueller) 11 33

OLIGOCHAETA Stylaria lacustris (L.) 8 2

Indet 2 6

GASTROPODA Radix ovata/peregra 5 7

Indet 16

ISOPODA Asellus aquaticus L. >6 mm S 91 52 8

ODONATA

Zygoptera Coenagrion sp. 6 1 1

Erythromma najas (Hansemann) 8 6 22

Ischnura elegans (van der Linden) 1 1

Platycnemis pennipes (Pallas) 27 12 60

Indet 2 1 7

Anisoptera Aeshna spp. 7 14 28

Somatochlora flavomaculata (van der Linden) 1 1

Somatochlora metallica (van der Linden) 2 1

EPHEMEROPTERA Caenis sp. 14 7

Cloeon dipterum Eaton 17 1 1

Ephemera vulgata (L.) 2

Heptagenia fuscogrisea (Retzius) 29 12 15

Heptagenia sp. 1

Leptophlebia marginata (L.) >6 mm S 171 65

Leptophlebia vespertina (L.) >6mm S 2

Leptophlebia spp. >6 mm S 5 12

PLECOPTERA Nemoura cinerea (Retzius) >6 mm S 29 3

MEGALOPTERA Sialis lutaria (L.) 5 2

TRICHOPTERA

Caseless Cyrnus flavidus MacLachlan 17 14 11

Holocentropus dubius (Rambur) 67 43 18

Polycentropodidae indet 1 1

Case-bearing Glyphotaelius pellucidus (Retzius) S 37 10

Halesus radiatus (Curtis) S 6 2

Limnephilus flavicornis (Fabricius) S 3 4

Limnephilus marmoratus Curtis S 105 20 4

Limnephilus rhombicus (L.) S 1 1

Limnephilus spp. S 9 14 5

Mystacides spp. 4 1

DIPTERA Chironomidae >6 mm 24 47 28

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Asellus aquaticus

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Figure 3. Temporal pattern of decomposition rate for birch (m–m) and oak (D–D) litter, represented as mean weight loss in g per day

since previous sampling occasion, combined with mean values of number of individuals per litterbag (d–d) of the four most frequent

shredder groups (year 1, n = 10; year 2, n = 6). Error bars show 95% confidence interval. Scales of number per litterbag are adjusted

to increase visibility.

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with frozen litterbags in late winter the secondyear, very low numbers of invertebrates werefound in litterbags. Later the same spring highnumbers of A. aquaticus and pronounced peaks ofleaf weight loss rate were recorded (Fig. 3).

In this study seasonal comparisons of thenumber of individuals of shredders were made,without biomass taken into account. The biomassper individual during the year differs considerablybetween the studied species. Maximum individualdry weight over the year in lake Valen was 2.9 mgfor A. aquaticus, 2.7 mg for Leptophlebia spp,28 mg for L. marmoratus, and 50 mg for G. pel-lucidus (own data from other studies). At equalabundances, this suggests a possibly much higherleaf litter processing capacity for trichopteranlarvae than for A. aquaticus and Leptophlebia spp.

During the 8-month exposure time the totalweight loss of Schoenoplectus lacustris was34.5±4.5% (95% CI, n = 3), while total weightloss for birch and oak litter were 96.6±0.07%(95% CI, n = 6) and 78.4±0.35% (95% CI,n = 6), respectively. There were very low numbersof shredders in the rush litterbags over the wholeperiod (Table 1).

Discussion

The most important observations in this study arethat leaf litter in lake Valen is a food resourcenormally processed during 6–8 months and thatthe temporal pattern for the weight loss of leaflitter coincided well with the maximum occurrence

of trichopteran winter-active shredders. Among allthe potential shredders in lake Valen, only thetrichopteran species showed a seasonal densityvariation pattern possible to connect to weight lossof leaf litter in litterbags. A. aquaticus and Lep-tophlebia spp. displayed the lowest density in lit-terbags during the main weight loss period and aretherefore suggested not to considerably contributeto transformation of leaf litter in this lake duringnormal winters. But severe winters with thick ice-cover seem to inhibit the consumption of leaf litterby trichopteran shredders. This kind of wintersituation can result in well-conditioned leaf litterremaining in late spring, providing an attractivefood resource for A. aquaticus. Some authorsconsider A. aquaticus to be one of the mostimportant shredder species in lake littorals (Prus,1981; Andersson, 1985; Henrikson, 1996), astatement not supported by this study. Yet, factorsother than food preference can possibly makeA. aquaticus to leave the uppermost parts of thebottom substrate during the mid winter period,e.g. intolerance to low temperatures. A. aquaticusdoes not grow at temperatures lower than 3 �Cand it has been suggested to migrate deeper waterwhen ice formation occurs in the littoral zone(Berglund, 1968; Andersson, 1969).

The set of shredder species in this study isdifferent from the shredder guilds found in streams,but resembles the guild Oertli (1995) found in asmall woodland pond in Switzerland. Of thetrichopterans only L. rhombicus and H. radiatushave been classified as shredders in streams(Malicky, 1990). The lifecycle of trichopteran

Table 2. Correlation between amount of leaf litter (g DW) per litterbag and number of individuals of the most abundant potential

shredders in litterbags from January to end of experiment (year 1, n = 10 and year 2, n = 6)

Shredder Leaf litter Spearmans rho p

Year 1 Year 2 Year 1 Year 2

Asellus aquaticus Birch )0.104 0.029 0.593 0.881

Asellus aquaticus Oak )0.006 )0.192 0.974 0.319

Glyphotaelius pellucidus Birch 0.728 0.472 <0.001* 0.010*

Glyphotaelius pellucidus Oak 0.662 0.546 <0.001* 0.002*

Leptophlebia spp. Birch )0.664 0.632 <0.001* <0.001*

Leptophlebia spp. Oak )0.554 0.551 0.002* 0.002*

Limnephilus spp. Birch )0.159 0.459 0.410 0.012*

Limnephilus spp. Oak )0.062 0.416 0.748 0.025*

Significant correlation (p<0.05) is indicated by *.

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winter shredder species is correlated to litter inputand larval growth occurs even at very low tem-peratures (Grubbs & Cummins, 1996; Bjelke et al.,2005). In the present study we suggest that freezingand oxygen deficit are probably insuperable envi-ronmental constraints for such species. Sensitivityto low oxygen concentrations has been shown forsome stream living caddis larvae in still waters(Philipson, 1954).

After ice break in spring the second year, thetrichopterans and mayflies were still scarce, whileA. aquaticus occurred in relatively high numbers inthe litterbags. The opposite pattern was recordedthe previous spring; the trichopteran wintershredders were still abundant in litterbags in Maywhereas A. aquaticus showed very low density.This may indicate a competitive relationship be-tween trichopteran winter shredders and A. aquat-icus for available leaf litter during spring.

In this study we made the seasonal couplingbetween litter turnover and the shredder occur-rence based on the number of individuals in lit-terbags. With respect to the difference in size of theshredder species in this study, the found differencesin temporal relation to litter processing appeareven more significant. But, neither biomass norabundance measurements of invertebrates fullyexplain differences in decomposition rates betweensites, because population dynamics, feeding man-ners and metabolic capacities of shredders are alsoof vital importance for the overall processing rate(Casas, 1996). Jonsson et al., (2001) showed thateven biodiversity could be more strongly corre-lated with breakdown rates than shredder bio-mass. Some of these factors are likely to explainthe inconsistent results of correlation betweendensity of macroinvertebrates and remainingamount of detritus in this study.

Conclusions

In this study, trichopteran shredder species,G. pellucidus and L. marmoratus appear responsi-ble for leaf litter turnover during winter, whileA. aquaticus and L. marginata do not contributesignificantly. Further, we show that in bothdecomposition rate of leaf litter and abundance ofshredder species are sensitive to disturbances, herefreezing. We suggest that experiences from leaf

litter dependent stream ecosystems can be trans-ferable to small oligotrophic lakes within the sameecoregion (Johnson et al., 2004), when trying topredict the effects of various disturbances on lakefunctions, e.g. changes in the drainage area vege-tation or climate change. However, the unique lakefactors must be considered; such as wind inducedleaf accumulation patterns, ice formation, risk ofoxygen deficit and the life cycle and feeding capacityof the specific lake shredder species.

Acknowledgements

Anders Bostrom, Stefan Hagberg, Leif Jeangson,Thomas Jonsson, and the late Peter Karlssonprovided technical assistance. We also thank UlfBjelke for fruitful discussions, John Airey for lin-guistic advice and the University of Kalmar forfinancial support.

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