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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  • PRIMARY RESEARCH PAPER

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

    Jeanette Schlief Michael Mutz

    Received: 29 April 2008 / Revised: 7 December 2008 / Accepted: 15 December 2008 / Published online: 2 February 2009

    Springer Science+Business Media B.V. 2009

    Abstract Climate change leads to summer low flow

    conditions and premature litter input in lowland

    streams in Central Europe. This may cause a sudden

    reduction of flow and fragmentation into isolated pools

    of permanently flowing streams, with a simultaneous

    increase in the food supply for detrivores during

    summer months. We performed a mesocosm study to

    investigate shredder and microbial mediated litter

    decomposition under these conditions. Leaf litter was

    placed in a lowland stream with a natural flow regime

    (reference) and in a stream mesocosm with significant

    flow reduction (FR) and a representative density of

    macroinvertebrates and detritus. Physicochemical

    parameters, leaf mass loss, macroinvertebrate abun-

    dance and biomass, leaf-associated respiration, fungal

    sporulation, and biomass were measured at regular

    intervals for 6 weeks. Coarse and fine-mesh bags were

    used to include or exclude macroinvertebrate shred-

    ders. In the coarse-mesh bags, leaf mass loss was

    significantly lower in the FR system than in the

    reference regime. In the fine-mesh bags, leaf respira-

    tion, fungal sporulation, and biomass but not leaf mass

    losses were substantially lower with flow reduction.

    Chironomid larvae (Micropsectra spp.) appeared to

    effectively fragment leaf litter in fine-mesh bags. In the

    FR system, leaf respiration was higher in the coarse-

    than in the fine-mesh bags. Our results suggest that, in

    temperate lowland streams, premature litter input

    during or after a sudden fragmentation into isolated

    pools and a reduction of stream flow reduces direct

    shredder-mediated litter decomposition, but shredders

    may indirectly influence the decomposition process.

    Keywords Litter decomposition Stream flow Litter input Shredder Microbial activity

    Introduction

    Leaf litter is a major source of energy for food webs in

    small forested streams (Wallace et al., 1999; Webster

    et al., 1999). Leaf litter decomposition in streams

    consists of abiotic processes, such as leaching and

    mechanical leaf fragmentation by the physical envi-

    ronment, decomposition by bacteria and fungi, and

    consumption by invertebrate shredders. Shredders

    often stimulate breakdown rates in temperate streams

    (Barlocher, 1985; Hieber & Gessner, 2002). Among

    the fungi, the aquatic hyphomycetes, which are able to

    degrade plant cell polymers (Chamier, 1985), play a

    predominant role in litter decomposition (Hieber &

    Gessner, 2002; Pascoal & Cassio, 2004). In temperate

    climate regions leaf litter decomposition has been

    most intensively studied in autumn and winter periods

    Handling editor: B. Oertli

    J. Schlief (&) M. MutzDepartment of Freshwater Conservation, Brandenburg

    University of Technology, Cottbus, Seestrasse 45,

    15526 Bad Saarow, Germany

    e-mail: jeanette.schlief@tu-cottbus.de

    123

    Hydrobiologia (2009) 624:205217

    DOI 10.1007/s10750-008-9694-4

  • when the leaves of deciduous trees senesce and fall

    (Petersen et al., 1989). In temperate lowland streams

    with pluvial hydrological regimes, high autumn to

    winter discharge coincides with peak litter input, thus

    creating an environment favorable for litter decom-

    position (Benfield et al., 2000; Habdija et al., 2003).

    Large water volumes and fast turbulent flows dilute

    leaf leachates, transport leaves, distribute stream

    organisms, and guarantee a high oxygen supply from

    the atmosphere. Most organisms involved in litter

    decomposition are well adapted to these environmental

    conditions (Bunn & Arthington, 2002).

    Climate change may affect the stream hydrology as

    well as the timing of leaf abscission, thereby influ-

    encing litter input into streams. In Central Europe,

    scientists predict a regional decrease in precipitation

    during summer and early autumn, which could lead to

    more frequent dry periods and droughts during this

    time (Gerstengarbe et al., 2003). Particularly in first to

    third-order streams, flow will presumably diminish

    during summer months (Lahmer & Becker, 2000).

    These streams may lose their flow continuum and

    fragment into a series of isolated pools without

    connection via surface flow (Boulton, 2003; Acuna

    et al., 2004; Andersen et al., 2006). Since Central

    European lowland streams often have a strong link to

    the groundwater, and isolated pools may be connected

    by hyporheic water exchange, the water within such

    pools is not stagnant but has extremely low current

    velocities. A reduction of current may be associated

    with changes in environmental conditions such as

    water temperature, habitat for aquatic fauna and

    dissolved oxygen (DO) concentrations (Lake, 2003;

    Bond et al., 2008). The fragmentation into isolated

    pools with changes in the aquatic environment occur

    abruptly at the beginning of a fragmentation phase

    (Boulton, 2003; Acuna et al., 2005) and may have a

    negative impact on stream organisms involved in litter

    decomposition.

    Furthermore, drought-related stress may result in

    premature leaf abscission (Wendler & Millard, 1996;

    Kozlowski & Pallardy, 2002). Particularly in small

    streams, premature litter input during summer months

    would provide an unnaturally high food supply to

    aquatic detrivores during a season usually character-

    ized by limited supply (Barlocher, 1983; Richardson,

    1991). Thus, under the predicted climate change

    scenario, premature litter input would coincide with

    low to zero flow and stream fragmentation into

    isolated pools. The effects of such a sudden flow

    reduction within an isolated pool in association with a

    high food supply on leaf-associated organisms and on

    the dynamics of litter decomposition are unknown.

    In the present study, we simulated conditions of

    sudden fragmentation into an isolated pool with strong

    flow reduction in association with high litter input

    during summer months to test their effect on litter

    breakdown rates, leaf-associated microbial activity,

    and biomass and to elucidate the role of invertebrate

    shredders in litter decomposition. We hypothesized

    that breakdown rates would be lower in the system

    with reduced flow, and that the significance of

    shredders in controlling litter decomposition would

    decrease in the low-flow environment. To distinguish

    the effects of shredders, we compared leaf breakdown

    rates and leaf-associated microbial activity of exposed

    litter bags with versus without shredder access

    (Webster & Benfield, 1986; Benfield, 1996). We also

    hypothesized that leaf-associated microbial activity

    would be reduced. In particular, we expect lower

    colonization by aquatic hyphomycetes under flow

    reduction, since their sporulation and dispersal are

    known to be stimulated by the current (Smither-

    Kopperl et al., 1998; Maamri et al., 2001).

    Materials and methods

    Study site

    The study was performed in Demnitzer Muhlenflie,

    a third-order stream located 60 km southeast of

    Berlin in the State of Brandenburg, Germany. It is a

    tributary of the Spree River. The stream water is

    influenced by nutrients (mainly phosphorus and

    nitrogen) originating from agricultural land use of

    the headwater catchment (Gelbrecht et al., 1996,

    2000; Lengsfeld & Gelbrecht, 2003). Our study site

    was located 50 m downstream of a weir. The

    streambed was dominated by sandy substrates partly

    covered with fine or coarse detritus. In the summer,

    coarse detritus mainly consisted of small twigs and a

    few green leaves that had been shed during storm

    events, etc. Visually degraded leaf litter was scarce

    during this season. The riparian zone was dominated

    by alder (Alnus glutinosa [L.] Gaertn.) and to a lesser

    extent by ash (Fraxinus excelsior L.) and hornbeam

    (Carpinus betulus L.).

    206 Hydrobiologia (2009) 624:205217

    123

  • Experimental set-up

    We created an experimental design that should

    simulate the conditions of drastic flow reduction

    accompanied by sudden fragmentation and isolation

    of invertebrates in an isolated pool. The experiment

    compared leaf decomposition under these conditions

    of sudden flow reduction (FR) with reference condi-

    tions (Ref). At the reference site the stream was 3 m

    wide, 0.35 m deep, and characterized by a natural flow

    regime. Directly adjacent to this site, we placed a

    mesocosm in the bed close to the stream margin to

    induce considerable flow reduction. The mesocosm

    consisted of a box (67 9 36 9 30 cm) that was

    immersed about 20 cm into the water column to

    ensure that the water inside was at stream water

    temperature. The mesocosm contained a total water

    volume of 50 l and had two opposite openings for

    inflow and outflow near the upper margin. The inflow

    was connected with a plastic pipe, the upper opening

    of which was pos