Effect of sudden flow reduction on the decomposition of alder leaves (Alnus glutinosa [L.] Gaertn.) in a temperate lowland stream: a mesocosm study

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<ul><li><p>PRIMARY RESEARCH PAPER</p><p>Effect of sudden flow reduction on the decompositionof alder leaves (Alnus glutinosa [L.] Gaertn.) in a temperatelowland stream: a mesocosm study</p><p>Jeanette Schlief Michael Mutz</p><p>Received: 29 April 2008 / Revised: 7 December 2008 / Accepted: 15 December 2008 / Published online: 2 February 2009</p><p> Springer Science+Business Media B.V. 2009</p><p>Abstract Climate change leads to summer low flow</p><p>conditions and premature litter input in lowland</p><p>streams in Central Europe. This may cause a sudden</p><p>reduction of flow and fragmentation into isolated pools</p><p>of permanently flowing streams, with a simultaneous</p><p>increase in the food supply for detrivores during</p><p>summer months. We performed a mesocosm study to</p><p>investigate shredder and microbial mediated litter</p><p>decomposition under these conditions. Leaf litter was</p><p>placed in a lowland stream with a natural flow regime</p><p>(reference) and in a stream mesocosm with significant</p><p>flow reduction (FR) and a representative density of</p><p>macroinvertebrates and detritus. Physicochemical</p><p>parameters, leaf mass loss, macroinvertebrate abun-</p><p>dance and biomass, leaf-associated respiration, fungal</p><p>sporulation, and biomass were measured at regular</p><p>intervals for 6 weeks. Coarse and fine-mesh bags were</p><p>used to include or exclude macroinvertebrate shred-</p><p>ders. In the coarse-mesh bags, leaf mass loss was</p><p>significantly lower in the FR system than in the</p><p>reference regime. In the fine-mesh bags, leaf respira-</p><p>tion, fungal sporulation, and biomass but not leaf mass</p><p>losses were substantially lower with flow reduction.</p><p>Chironomid larvae (Micropsectra spp.) appeared to</p><p>effectively fragment leaf litter in fine-mesh bags. In the</p><p>FR system, leaf respiration was higher in the coarse-</p><p>than in the fine-mesh bags. Our results suggest that, in</p><p>temperate lowland streams, premature litter input</p><p>during or after a sudden fragmentation into isolated</p><p>pools and a reduction of stream flow reduces direct</p><p>shredder-mediated litter decomposition, but shredders</p><p>may indirectly influence the decomposition process.</p><p>Keywords Litter decomposition Stream flow Litter input Shredder Microbial activity</p><p>Introduction</p><p>Leaf litter is a major source of energy for food webs in</p><p>small forested streams (Wallace et al., 1999; Webster</p><p>et al., 1999). Leaf litter decomposition in streams</p><p>consists of abiotic processes, such as leaching and</p><p>mechanical leaf fragmentation by the physical envi-</p><p>ronment, decomposition by bacteria and fungi, and</p><p>consumption by invertebrate shredders. Shredders</p><p>often stimulate breakdown rates in temperate streams</p><p>(Barlocher, 1985; Hieber &amp; Gessner, 2002). Among</p><p>the fungi, the aquatic hyphomycetes, which are able to</p><p>degrade plant cell polymers (Chamier, 1985), play a</p><p>predominant role in litter decomposition (Hieber &amp;</p><p>Gessner, 2002; Pascoal &amp; Cassio, 2004). In temperate</p><p>climate regions leaf litter decomposition has been</p><p>most intensively studied in autumn and winter periods</p><p>Handling editor: B. Oertli</p><p>J. Schlief (&amp;) M. MutzDepartment of Freshwater Conservation, Brandenburg</p><p>University of Technology, Cottbus, Seestrasse 45,</p><p>15526 Bad Saarow, Germany</p><p>e-mail: jeanette.schlief@tu-cottbus.de</p><p>123</p><p>Hydrobiologia (2009) 624:205217</p><p>DOI 10.1007/s10750-008-9694-4</p></li><li><p>when the leaves of deciduous trees senesce and fall</p><p>(Petersen et al., 1989). In temperate lowland streams</p><p>with pluvial hydrological regimes, high autumn to</p><p>winter discharge coincides with peak litter input, thus</p><p>creating an environment favorable for litter decom-</p><p>position (Benfield et al., 2000; Habdija et al., 2003).</p><p>Large water volumes and fast turbulent flows dilute</p><p>leaf leachates, transport leaves, distribute stream</p><p>organisms, and guarantee a high oxygen supply from</p><p>the atmosphere. Most organisms involved in litter</p><p>decomposition are well adapted to these environmental</p><p>conditions (Bunn &amp; Arthington, 2002).</p><p>Climate change may affect the stream hydrology as</p><p>well as the timing of leaf abscission, thereby influ-</p><p>encing litter input into streams. In Central Europe,</p><p>scientists predict a regional decrease in precipitation</p><p>during summer and early autumn, which could lead to</p><p>more frequent dry periods and droughts during this</p><p>time (Gerstengarbe et al., 2003). Particularly in first to</p><p>third-order streams, flow will presumably diminish</p><p>during summer months (Lahmer &amp; Becker, 2000).</p><p>These streams may lose their flow continuum and</p><p>fragment into a series of isolated pools without</p><p>connection via surface flow (Boulton, 2003; Acuna</p><p>et al., 2004; Andersen et al., 2006). Since Central</p><p>European lowland streams often have a strong link to</p><p>the groundwater, and isolated pools may be connected</p><p>by hyporheic water exchange, the water within such</p><p>pools is not stagnant but has extremely low current</p><p>velocities. A reduction of current may be associated</p><p>with changes in environmental conditions such as</p><p>water temperature, habitat for aquatic fauna and</p><p>dissolved oxygen (DO) concentrations (Lake, 2003;</p><p>Bond et al., 2008). The fragmentation into isolated</p><p>pools with changes in the aquatic environment occur</p><p>abruptly at the beginning of a fragmentation phase</p><p>(Boulton, 2003; Acuna et al., 2005) and may have a</p><p>negative impact on stream organisms involved in litter</p><p>decomposition.</p><p>Furthermore, drought-related stress may result in</p><p>premature leaf abscission (Wendler &amp; Millard, 1996;</p><p>Kozlowski &amp; Pallardy, 2002). Particularly in small</p><p>streams, premature litter input during summer months</p><p>would provide an unnaturally high food supply to</p><p>aquatic detrivores during a season usually character-</p><p>ized by limited supply (Barlocher, 1983; Richardson,</p><p>1991). Thus, under the predicted climate change</p><p>scenario, premature litter input would coincide with</p><p>low to zero flow and stream fragmentation into</p><p>isolated pools. The effects of such a sudden flow</p><p>reduction within an isolated pool in association with a</p><p>high food supply on leaf-associated organisms and on</p><p>the dynamics of litter decomposition are unknown.</p><p>In the present study, we simulated conditions of</p><p>sudden fragmentation into an isolated pool with strong</p><p>flow reduction in association with high litter input</p><p>during summer months to test their effect on litter</p><p>breakdown rates, leaf-associated microbial activity,</p><p>and biomass and to elucidate the role of invertebrate</p><p>shredders in litter decomposition. We hypothesized</p><p>that breakdown rates would be lower in the system</p><p>with reduced flow, and that the significance of</p><p>shredders in controlling litter decomposition would</p><p>decrease in the low-flow environment. To distinguish</p><p>the effects of shredders, we compared leaf breakdown</p><p>rates and leaf-associated microbial activity of exposed</p><p>litter bags with versus without shredder access</p><p>(Webster &amp; Benfield, 1986; Benfield, 1996). We also</p><p>hypothesized that leaf-associated microbial activity</p><p>would be reduced. In particular, we expect lower</p><p>colonization by aquatic hyphomycetes under flow</p><p>reduction, since their sporulation and dispersal are</p><p>known to be stimulated by the current (Smither-</p><p>Kopperl et al., 1998; Maamri et al., 2001).</p><p>Materials and methods</p><p>Study site</p><p>The study was performed in Demnitzer Muhlenflie,</p><p>a third-order stream located 60 km southeast of</p><p>Berlin in the State of Brandenburg, Germany. It is a</p><p>tributary of the Spree River. The stream water is</p><p>influenced by nutrients (mainly phosphorus and</p><p>nitrogen) originating from agricultural land use of</p><p>the headwater catchment (Gelbrecht et al., 1996,</p><p>2000; Lengsfeld &amp; Gelbrecht, 2003). Our study site</p><p>was located 50 m downstream of a weir. The</p><p>streambed was dominated by sandy substrates partly</p><p>covered with fine or coarse detritus. In the summer,</p><p>coarse detritus mainly consisted of small twigs and a</p><p>few green leaves that had been shed during storm</p><p>events, etc. Visually degraded leaf litter was scarce</p><p>during this season. The riparian zone was dominated</p><p>by alder (Alnus glutinosa [L.] Gaertn.) and to a lesser</p><p>extent by ash (Fraxinus excelsior L.) and hornbeam</p><p>(Carpinus betulus L.).</p><p>206 Hydrobiologia (2009) 624:205217</p><p>123</p></li><li><p>Experimental set-up</p><p>We created an experimental design that should</p><p>simulate the conditions of drastic flow reduction</p><p>accompanied by sudden fragmentation and isolation</p><p>of invertebrates in an isolated pool. The experiment</p><p>compared leaf decomposition under these conditions</p><p>of sudden flow reduction (FR) with reference condi-</p><p>tions (Ref). At the reference site the stream was 3 m</p><p>wide, 0.35 m deep, and characterized by a natural flow</p><p>regime. Directly adjacent to this site, we placed a</p><p>mesocosm in the bed close to the stream margin to</p><p>induce considerable flow reduction. The mesocosm</p><p>consisted of a box (67 9 36 9 30 cm) that was</p><p>immersed about 20 cm into the water column to</p><p>ensure that the water inside was at stream water</p><p>temperature. The mesocosm contained a total water</p><p>volume of 50 l and had two opposite openings for</p><p>inflow and outflow near the upper margin. The inflow</p><p>was connected with a plastic pipe, the upper opening</p><p>of which was positioned upstream of a weir to achieve</p><p>constant water excess in the container by means of</p><p>hydrostatic pressure. The openings of the mesocosm</p><p>were covered with a net (0.25 mm mesh size) to</p><p>prevent migration or drift of macroinvertebrates and</p><p>transport of particles with in- or outflow to simulate</p><p>the conditions within isolated pools after sudden</p><p>stream fragmentation, where macroinvertebrates are</p><p>not able to enter or escape. To assess the reaction of the</p><p>present aquatic community involved in the decompo-</p><p>sition process under sudden flow reduction the bottom</p><p>of the mesocosm was filled with a representative</p><p>density of macroinvertebrates and detritus (e.g., twigs</p><p>and some visually degraded leaf litter partly covered</p><p>and mixed with fine detritus and sand), which were</p><p>collected at the reference site with a Surber sampler</p><p>(25 9 25 cm, mesh size 0.2 mm).</p><p>Alder leaves collected from trees at the stream</p><p>margin during senescence (November 2005) were air-</p><p>dried at room temperature (21C) and stored dry untilneeded. Since this initial drying of the litter is</p><p>considered to affect leaching (Gessner, 1991; Gessner</p><p>et al., 1999), the leaves were preleached in distilled</p><p>water (24 h, 21C) before the start of the experimentin summer 2006. The moistened leaves were then cut</p><p>into disks (12 mm diameter) using a cork borer. 25</p><p>leaf disks were pinned to a stainless steel wire to</p><p>prevent handling loss at retrieval and to guarantee that</p><p>disks did not cover each other during exposure or</p><p>during respiration measurement. The disks were</p><p>placed in nylon mesh bags with either a coarse</p><p>(5 mm) or a fine (0.3 mm) mesh size.</p><p>In August 2006, 15 coarse and 15 fine-mesh bags</p><p>and an additional amount of preleached leaf litter were</p><p>placed in the reference and flow reduction system,</p><p>respectively. The additional litter (approximately</p><p>75 g/m2 stream bottom) was used to simulate a high</p><p>food supply. In the reference, it was spread over an</p><p>area of 3 m2 and fixed to the stream bottom with the</p><p>aid of a net (3 cm mesh size) and tent pegs. The litter</p><p>bags were evenly distributed on this litter layer and</p><p>fixed by tent pegs. In the FR system, the additional</p><p>litter was spread on the bottom and the bags were</p><p>evenly distributed on this layer. At week 1, 2, 3, 4, and</p><p>6 after exposure, sets of three coarse and three fine-</p><p>mesh bags were carefully retrieved with a hand net</p><p>(250 lm). An additional benthic organic mattersample at the reference site was retrieved with the</p><p>Surber sampler to monitor macroinvertebrate coloni-</p><p>zation outside the exposed leaf litter. All samples</p><p>were placed in individual plastic boxes filled with</p><p>stream water, and transported to the laboratory.</p><p>Physical and chemical stream characteristics</p><p>Conductivity, temperature, dissolved oxygen (DO),</p><p>and pH were measured using portable meters</p><p>(WTWLF323, WTW Oxi 340-A, WTWpH325) four</p><p>to five times per week. In addition, the DO in the litter</p><p>bags and the surrounding water was measured on five</p><p>occasions by drawing water with a needle-type syringe</p><p>connected via rubber tubes with an oxygen microsen-</p><p>sor and an oxygen optode (Fibox 3, Precision Sensing</p><p>GmbH). Water samples (minimum 1 l) for nitrate,</p><p>ammonia, soluble reactive phosphorus (SRP), and</p><p>dissolved organic carbon (DOC) testing were also</p><p>taken on each sampling date. Samples for nitrate,</p><p>ammonium, and SRP analyses were filtered through</p><p>cellulose-acetate membrane filters (SARTORIUS,</p><p>0.45 lm pore size) and determined by segmentedflow analysis (PERSTORP-Analytical). The total</p><p>phosphorus (TP) concentration in unfiltered samples</p><p>was determined by flow analysis (FIA). For DOC,</p><p>samples were filtered through glass fiber filters</p><p>(WHATMAN GF/F, 0.7 lm pore size) and deter-mined by elemental analysis (DIMATEC, DIMA-</p><p>TOC 100). All samples were stored in sealed plastic</p><p>jars at -20C until analysis. Current velocity was</p><p>Hydrobiologia (2009) 624:205217 207</p><p>123</p></li><li><p>measured three times per week. Current velocity in the</p><p>reference system was measured using a small hydro-</p><p>metric propeller (1.5 cm diameter, Mini-Air-2,</p><p>Schiltknecht), which was positioned 2.5 cm above</p><p>the stream bed and 5 cm in front of the litter bags.</p><p>Since current velocity inside the mesocosm was too</p><p>low to measure with a hydrometric propeller, it was</p><p>determined by assessing the discharge through the</p><p>mesocosm based on three consecutive measurements</p><p>of water volume per unit time at the mesocosm outlet.</p><p>Mean current velocity was calculated based on</p><p>discharge through the mesocosm and mesocosm cross</p><p>section.</p><p>Sample processing and macroinvertebrates</p><p>At each sampling date, leaf disks were carefully</p><p>removed from the bags and gently rinsed with filtered</p><p>(10 lm) stream water to remove sand, fine detritus,and invertebrates. The additional benthic organic</p><p>matter samples were processed in the same manner.</p><p>The remaining slurry of each sample was sieved over</p><p>a 300 lm mesh, and the retained invertebrates werepreserved in 70% ethanol. Macroinvertebrates were</p><p>identified to the lowest practical taxonomic level,</p><p>counted, and classified as shredders or non-shredders</p><p>according to Merritt &amp; Cummins (1996). The biomass</p><p>of each group, which was determined by weighing</p><p>dried animals (48 h, 60C), was expressed based onash-free dry mass of leaf disks. The number of</p><p>invertebrates from the benthic organic matter samples</p><p>was expressed per m2 stream bottom.</p><p>Microbial parameters</p><p>Leaf-associated respiration and fungal sporulation</p><p>rates served as measures of microbial activity.</p><p>Respiration rates of all samples were measured within</p><p>24 h after sampling based on oxygen consumption in</p><p>a closed-chamber system with internal water circula-</p><p>tion adapted from Pusch &amp; Schwoerbel (1994) and</p><p>Schlief (2004). For this purpose, the steel wire with 10</p><p>leaf disks was carefully placed in a cylindrical</p><p>respiration glass chamber (length 9 cm, diameter</p><p>3.2 cm), which was connected via plastic tubes to a</p><p>flow-through cell housing oxygen microsensor with</p><p>an oxygen optode (Fibox 3, Precision Sensing</p><p>GmbH). The system was filled with autoclaved water</p><p>from the sampling site and placed in a water bath. The</p><p>water bath had a constant temperature (21C) and wasdarkened to prevent photosy...</p></li></ul>

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