The Timing of Winter-growing Shredder Species and Leaf Litter Turnover Rate in an Oligotrophic Lake, SE Sweden

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<ul><li><p>Primary Research Paper</p><p>The timing of winter-growing shredder species and leaf litter turnover ratein an oligotrophic lake, SE Sweden</p><p>Irene Marita Bohman* &amp; 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)</p><p>Received 3 November 2005; in revised form 22 June 2005; accepted 24 June 2005</p><p>Key words: shredders, leaf litter processing, allochthonous subsidies, winter, Limnephilidae, Asellus, Leptophlebia</p><p>Abstract</p><p>Small freshwater systems often depend on allochthonous organic subsidies to sustain productivity. Benthicinvertebrates consuming coarse detritus maintain the energy ow 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 inuenced byfreezing in late winter the second year. Further, the seasonal variation patterns in abundance in litterbagswere quite dierent 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.</p><p>Introduction</p><p>Productivity in small freshwater systems oftendepends upon ecotonal transfer of allochthonousorganic subsidies (Gasith &amp; Hasler, 1976; Oertli,1993; France &amp; Peters, 1995; Caraco &amp; Cole,2002). Hence, disturbances aecting detritus inputand processing could potentially inuence the lakeecosystem. Regarding coarse detritus the eects 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</p><p>ecosystem (Palmer et al., 2000). Shredder pro-cessing of coarse detritus can control the furtherconsumption of leaf-derived detritus through themobilization of ne particulate matter (Cumminset al., 1989; Palmer &amp; OKeee, 1992; Wallace &amp;Webster, 1996). In addition, all detritivorousbenthic invertebrates maintain the energy ow inthe food web by converting dead organic matterinto prey items (Vanni, 1997; Covich et al., 1999).</p><p>The riparian zone plays an essential role foraquatic ecosystem functions and thereby also forrestoration and management of both small andlarger freshwater habitats (Naiman &amp; Decamps,1997). The leaf breakdown rate and shredder</p><p>Hydrobiologia (2006) 556:99108 Springer 2006DOI 10.1007/s10750-005-1052-1</p></li><li><p>distribution have been suggested to be helpfulinstruments for evaluating the impact of land useand climate change on the ecosystem function instreams (Wallace &amp; Webster, 1996; Buzby &amp; Perry,2000; Sponseller &amp; Beneld, 2001; Gage et al.,2004). Additionally, the interest focused on bio-diversity and potential eects 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 identied as ecologicalkey species, e.g. the functional group of shredders(Palmer et al., 1997; Covich et al., 1999).</p><p>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 eects of acidication in lakes(Fjellheim &amp; Raddum, 1988; Tuchman, 1993; Kok&amp; van der Velde, 1994; Henrikson, 1996).</p><p>In running waters a wide range of invertebratetaxa are classied as shredders, (see e.g. Meritt &amp;Cummins, 1984; Cummins et al., 1989; Gessner &amp;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, dierent 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 &amp;Cummins, 1974; Grubbs &amp; 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).</p><p>The specic 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</p><p>abundance in litterbags is positively correlated tothe food resource available, i.e. remaining weightof the exposed leaf litter during winter and springseason.</p><p>Methods</p><p>Site description</p><p>The experimental eldwork was performed in thenon-acidied (pH 7.0, alkalinity 0.20 meq/l) oligo/mesotrophic (tot-N 0.48 mg/l, tot-P 5.0 lg/l) lakeValen (5794 N, 153920 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.</p><p>Experimental design</p><p>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.</p><p>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 identication to species orgenus level (chironomids and microoligochaetesexcepted). The leaf litter was gently rinsed fromsediment and biolm, and then dried in 50 C for48 h.</p><p>100</p></li><li><p>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 classication(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 &amp;Giller, 2000; Dangles &amp; Guerold, 2001; Basagurenet al., 2002) and could not be quantitatively sam-pled with this method.</p><p>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.</p><p>Statistical analysis</p><p>We used Spearmans correlation coecient (Sokal&amp; Rohlf, 1995), Statistica 6.1 software for PC andsignicance level 5% to evaluate the predictedassociation between remaining leaf litter quantityand shredder abundance in the litterbags.</p><p>Results</p><p>Ice conditions and litter turnover</p><p>Water temperature and ice cover duration dieredconsiderably between the two experimental peri-ods (Fig. 1). Maximum ice layer in winter the rstyear was 200 mm and the second, &gt;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</p><p>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 eect 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.</p><p>Macroinvertebrate colonization</p><p>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 ve trichopteran species belonging tothe family Limnephilidae (G. pellucidus, Halesusradiatus, Limnephilus avicornis, 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).</p><p>The seasonal colonization patterns were quitedierent 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</p><p>02468</p><p>101214161820</p><p>Nov Dec Jan Feb Mar Apr May Jun Jul</p><p>oC</p><p>Year 1Year 2</p><p>Year 1</p><p>Year 2</p><p>Figure 1. Water temperature and ice cover (dotted rectangles</p><p>indicate both duration and thickness) throughout litterbag</p><p>experiments year 1 and 2.</p><p>101</p></li><li><p>abundance vary both among the shredder groupsand between the 2 years (Table 2). The abundanceof G. pellucidus was signicantly positively corre-lated with both birch and oak litter quantity bothyears, while the abundance of A. aquaticus showedno signicant correlation at all. The abundance ofLeptophlebia spp. was signicantly correlated withboth birch and oak litter quantity both years, but</p><p>the association was negative year 1 and positiveyear 2.</p><p>Temporal coupling of loss rate and shredderabundance</p><p>As a measure of decomposition rate, the averageloss of dry weight per day since previous sampling</p><p>0.00.20.40.60.81.01.21.41.61.82.0</p><p>Dec</p><p>Jan</p><p>Feb</p><p>Mar</p><p>Apr</p><p>May</p><p>Rem</p><p>ain</p><p>ing </p><p>dry </p><p>weig</p><p>ht (g</p><p>) BirchOak</p><p>0.00.20.40.60.81.01.2</p><p>1.41.61.82.0</p><p>Oct</p><p>Nov</p><p>Nov</p><p>Dec</p><p>Jan</p><p>Feb</p><p>Mar</p><p>Mar</p><p>Apr</p><p>May</p><p>Jun Ju</p><p>lJu</p><p>l</p><p>Rem</p><p>ain</p><p>ing </p><p>dry </p><p>we</p><p>ight</p><p> (g)</p><p>BirchOak</p><p>(b)</p><p>(a)</p><p>Figure 2. (a) Remaining dry weight of 2.0 g of each birch and oak leaves in litterbags year 1 (n = 10). (b) Remaining dry weight of</p><p>1.5 g of each birch and oak leaves in litterbags year 2 (n = 6). Error bars show 95% condence interval.</p><p>102</p></li><li><p>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</p><p>decrease during mid-winter, when weight loss rateof birch litter peaks. In the rst 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</p><p>Table 1. List of invertebrates found in litterbags. The taxa considered as potential shredders are denoted with S and the total number</p><p>of individuals &gt; 6 mm are given for each sampling period</p><p>TAXON Shredder Total number of observations</p><p>Leaves Year 1 Leaves Year 2 Rush Year 2</p><p>TURBELLARIA Dendrocoelum lacteum (Mueller) 11 33</p><p>OLIGOCHAETA Stylaria lacustris (L.) 8 2</p><p>Indet 2 6</p><p>GASTROPODA Radix ovata/peregra 5 7</p><p>Indet 16</p><p>ISOPODA Asellus aquaticus L. &gt;6 mm S 91 52 8</p><p>ODONATA</p><p>Zygoptera Coenagrion sp. 6 1 1</p><p>Erythromma najas (Hansemann) 8 6 22</p><p>Ischnura elegans (van der Linden) 1 1</p><p>Platycnemis pennipes (Pallas) 27 12 60</p><p>Indet 2 1 7</p><p>Anisoptera Aeshna spp. 7 14 28</p><p>Somatochlora avomaculata (van der Linden) 1 1</p><p>Somatochlora metallica (van der Linden) 2 1</p><p>EPHEMEROPTERA Caenis sp. 14 7</p><p>Cloeon dipterum Eaton 17 1 1</p><p>Ephemera vulgata (L.) 2</p><p>Heptagenia fuscogrisea (Retzius) 29 12 15</p><p>Heptagenia sp. 1</p><p>Leptophlebia marginata (L.) &gt;6 mm S 171 65</p><p>Leptophlebia vespertina (L.) &gt;6mm S 2</p><p>Leptophlebia spp. &gt;6 mm S 5 12</p><p>PLECOPTERA Nemoura cinerea (Retzius) &gt;6 mm S 29 3</p><p>MEGALOPTERA Sialis lutaria (L.) 5 2</p><p>TRICHOPTERA</p><p>Caseless Cyrnus avidus MacLachlan 17 14 11</p><p>Holocentropus dubius (Rambur) 67 43 18</p><p>Polycentropodidae indet 1 1</p><p>Case-bearing Glyphotaelius pellucidus (Retzius) S 37 10</p><p>Halesus radiatus (Curtis) S 6 2</p><p>Limnephilus avicornis (Fabricius) S 3 4</p><p>Limnephilus marmoratus Curtis S 105 20 4</p><p>Limnephilus rhombicus (L.) S 1 1</p><p>Limnephilus spp. S 9 14 5</p><p>Mystacides spp. 4 1</p><p>DIPTERA Chironomidae &gt;6 mm 24 47 28</p><p>103</p></li><li><p>Asellus aquaticus</p><p>012345678</p><p>Dec</p><p>Jan</p><p>Feb</p><p>Mar</p><p>Apr</p><p>May</p><p>Num</p><p>bero</p><p>f ind</p><p>ividu</p><p>als</p><p>/ litt</p><p>er b</p><p>agN</p><p>umbe</p><p>rof i</p><p>ndivi</p><p>dual</p><p>s/ l</p><p>itter</p><p> bag</p><p>Num</p><p>bero</p><p>f ind</p><p>ividu</p><p>als</p><p>/ litt</p><p>er b</p><p>agN</p><p>umbe</p><p>rof i</p><p>ndivi</p><p>dual</p><p>s/ l</p><p>itter</p><p> bag</p><p>Num</p><p>bero</p><p>f ind</p><p>ividu</p><p>als</p><p>/ litt</p><p>er b</p><p>agN</p><p>umbe</p><p>rof i</p><p>ndivi</p><p>dual</p><p>s/ l</p><p>itter</p><p> bag</p><p>N...</p></li></ul>

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