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 Æ 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. Mutz

Department of Freshwater Conservation, Brandenburg

University of Technology, Cottbus, Seestrasse 45,

15526 Bad Saarow, Germany

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

123

Hydrobiologia (2009) 624:205–217

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 Muhlenfließ,

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) 624:205–217

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 positioned upstream of a weir to achieve

constant water excess in the container by means of

hydrostatic pressure. The openings of the mesocosm

were covered with a net (0.25 mm mesh size) to

prevent migration or drift of macroinvertebrates and

transport of particles with in- or outflow to simulate

the conditions within isolated pools after sudden

stream fragmentation, where macroinvertebrates are

not able to enter or escape. To assess the reaction of the

present aquatic community involved in the decompo-

sition process under sudden flow reduction the bottom

of the mesocosm was filled with a representative

density of macroinvertebrates and detritus (e.g., twigs

and some visually degraded leaf litter partly covered

and mixed with fine detritus and sand), which were

collected at the reference site with a Surber sampler

(25 9 25 cm, mesh size 0.2 mm).

Alder leaves collected from trees at the stream

margin during senescence (November 2005) were air-

dried at room temperature (21�C) and stored dry until

needed. Since this initial drying of the litter is

considered to affect leaching (Gessner, 1991; Gessner

et al., 1999), the leaves were preleached in distilled

water (24 h, 21�C) before the start of the experiment

in summer 2006. The moistened leaves were then cut

into disks (12 mm diameter) using a cork borer. 25

leaf disks were pinned to a stainless steel wire to

prevent handling loss at retrieval and to guarantee that

disks did not cover each other during exposure or

during respiration measurement. The disks were

placed in nylon mesh bags with either a coarse

(5 mm) or a fine (0.3 mm) mesh size.

In August 2006, 15 coarse and 15 fine-mesh bags

and an additional amount of preleached leaf litter were

placed in the reference and flow reduction system,

respectively. The additional litter (approximately

75 g/m2 stream bottom) was used to simulate a high

food supply. In the reference, it was spread over an

area of 3 m2 and fixed to the stream bottom with the

aid of a net (3 cm mesh size) and tent pegs. The litter

bags were evenly distributed on this litter layer and

fixed by tent pegs. In the FR system, the additional

litter was spread on the bottom and the bags were

evenly distributed on this layer. At week 1, 2, 3, 4, and

6 after exposure, sets of three coarse and three fine-

mesh bags were carefully retrieved with a hand net

(250 lm). An additional benthic organic matter

sample at the reference site was retrieved with the

Surber sampler to monitor macroinvertebrate coloni-

zation outside the exposed leaf litter. All samples

were placed in individual plastic boxes filled with

stream water, and transported to the laboratory.

Physical and chemical stream characteristics

Conductivity, temperature, dissolved oxygen (DO),

and pH were measured using portable meters

(WTWLF323, WTW Oxi 340-A, WTWpH325) four

to five times per week. In addition, the DO in the litter

bags and the surrounding water was measured on five

occasions by drawing water with a needle-type syringe

connected via rubber tubes with an oxygen microsen-

sor and an oxygen optode (Fibox 3, Precision Sensing

GmbH). Water samples (minimum 1 l) for nitrate,

ammonia, soluble reactive phosphorus (SRP), and

dissolved organic carbon (DOC) testing were also

taken on each sampling date. Samples for nitrate,

ammonium, and SRP analyses were filtered through

cellulose-acetate membrane filters (SARTORIUS,

0.45 lm pore size) and determined by segmented

flow analysis (PERSTORP-Analytical). The total

phosphorus (TP) concentration in unfiltered samples

was determined by flow analysis (FIA). For DOC,

samples were filtered through glass fiber filters

(WHATMAN GF/F, 0.7 lm pore size) and deter-

mined by elemental analysis (DIMATEC, DIMA-

TOC 100). All samples were stored in sealed plastic

jars at -20�C until analysis. Current velocity was

Hydrobiologia (2009) 624:205–217 207

123

measured three times per week. Current velocity in the

reference system was measured using a small hydro-

metric propeller (1.5 cm diameter, Mini-Air-2,

Schiltknecht), which was positioned 2.5 cm above

the stream bed and 5 cm in front of the litter bags.

Since current velocity inside the mesocosm was too

low to measure with a hydrometric propeller, it was

determined by assessing the discharge through the

mesocosm based on three consecutive measurements

of water volume per unit time at the mesocosm outlet.

Mean current velocity was calculated based on

discharge through the mesocosm and mesocosm cross

section.

Sample processing and macroinvertebrates

At each sampling date, leaf disks were carefully

removed from the bags and gently rinsed with filtered

(10 lm) stream water to remove sand, fine detritus,

and invertebrates. The additional benthic organic

matter samples were processed in the same manner.

The remaining slurry of each sample was sieved over

a 300 lm mesh, and the retained invertebrates were

preserved in 70% ethanol. Macroinvertebrates were

identified to the lowest practical taxonomic level,

counted, and classified as shredders or non-shredders

according to Merritt & Cummins (1996). The biomass

of each group, which was determined by weighing

dried animals (48 h, 60�C), was expressed based on

ash-free dry mass of leaf disks. The number of

invertebrates from the benthic organic matter samples

was expressed per m2 stream bottom.

Microbial parameters

Leaf-associated respiration and fungal sporulation

rates served as measures of microbial activity.

Respiration rates of all samples were measured within

24 h after sampling based on oxygen consumption in

a closed-chamber system with internal water circula-

tion adapted from Pusch & Schwoerbel (1994) and

Schlief (2004). For this purpose, the steel wire with 10

leaf disks was carefully placed in a cylindrical

respiration glass chamber (length 9 cm, diameter

3.2 cm), which was connected via plastic tubes to a

flow-through cell housing oxygen microsensor with

an oxygen optode (Fibox 3, Precision Sensing

GmbH). The system was filled with autoclaved water

from the sampling site and placed in a water bath. The

water bath had a constant temperature (21�C) and was

darkened to prevent photosynthesis. The circulation

inside the system was adjusted to 0.2 cm s-1 with an

electromagnetic pump. Blanks with autoclaved stream

water were measured concurrently for determination

of oxygen diffusion into the system and for correction

of leaf-associated respiration rates. Depending on the

level of activity, it took between 30 and 60 min to

complete the measurements. Respiration rates were

expressed as lg oxygen per unit weight of leaf disks.

To determine leaf-associated sporulation rates, five

leaf disks from each fine-mesh bag were aerated in

150 ml distilled water for 2 days at 15�C, and the

suspension was filtered through 8 lm pore membrane

filters (Millipore). Spores trapped on the filters were

stained with lactophenol cotton blue, counted and

identified under a microscope (200-4009 magnifica-

tions; minimum of 25 fields) and extrapolated to

numbers of spores per unit weight of leaf disks.

Leaf-associated ergosterol content, an indicator of

fungal biomass, was measured on 10 leaf disks from

each fine-mesh bag (Newell, 1993). Ergosterol was

extracted by 30-min reflux in an alcoholic base and

then purified by solid-phase extraction and quantified

by HPLC (Gessner & Schmitt, 1996). Ergosterol

content was expressed per unit weight of leaf disks.

Leaf mass loss

After respiration measurement, the leaf disks were

dried (48 h, 60�C) and ashed (550�C, 4 h) to determine

leaf ash-free dry mass (AFDM) according to standard

protocols (Benfield, 1996). Subsamples of initial leaf

disks were processed in the same way to obtain initial

AFDM. Data were expressed as %AFDM remaining.

Data analyses

An exponential decay model was fitted to the data on

mass loss through time using the SPSS curve estima-

tion method. The equation used for the model is

commonly applied to litter breakdown studies (e.g.,

Petersen & Cummins, 1974; Webster & Benfield,

1986):

Mt ¼ M0 e�kt

where Mt = AFDM remaining at time t, M0 = initial

AFDM, and k is the breakdown rate coefficient.

208 Hydrobiologia (2009) 624:205–217

123

Breakdown rates were calculated for coarse and fine-

mesh bags. We did not force the model through 100%

at time zero as an intercept. Breakdown rates were also

calculated on a degree-day basis (k0) by replacing time

(t) with cumulative degree days (�d), which were

computed from mean water temperature (Minshall

et al., 1983; Short et al., 1984). ANCOVA was used to

determine differences between breakdown coeffi-

cients on a degree-day basis to account for possible

temperature differences between the treatments. The

data were ln-transformed before analysis to satisfy

the assumptions of normality and homogeneity of

variance. Physicochemical water parameters, macro-

invertebrate abundance, respiration rates, fungal

sporulation, and fungal biomass were analyzed by

Kruskal-Wallis ANOVA and compared using a

Wilcoxon test to detect treatment effects. These non-

parametric tests were used because the data did not

have a normal distribution, even after transformation.

Non-transformed data were used to represent the mean

and standard error (SE) of the mean. All statistical

analyses were performed using SPSS (Vol. 14.0/SPSS

Inc., IL, USA) with significance levels set at P \ 0.05.

Results

Physical and chemical stream characteristics

The current velocity was much lower in the flow

reduction system (0.4 cm s-1) than in the reference

(33 cm s-1). Temperature was slightly but not signifi-

cantly higher (P = 0.2, Wilcoxon test) in the mesocosm

than in the reference (Table 1). Conductivity, pH, SRP,

NOx--N, NH4

?-N, and DOC were similar in the two

systems (Table 1). The TP concentrations indicated a

high trophic level of the stream (Dodds et al., 1998) and

were similar in both systems. The DO concentrations in

the two systems differed (P \ 0.05; Wilcoxon test).

Compared to 85% saturation in the reference, the DO

concentration in the low-flow environment was only

65% (Table 1). Both systems exhibited similar DO

concentrations within litter bags of both mesh sizes and

in the surrounding water.

Macroinvertebrates

Densities of shredders (147 ± 33 indiv. m-2) and

non-shredders (122 ± 27 indiv. m-2) associated to

benthic organic matter outside the exposed litter

remained constant throughout the experiment. These

densities had been taken to adjust macroinvertebrate

density at the beginning of litter bag exposure in the

mesocosm. Essentially the same taxa of macroinver-

tebrates were found in litter bags of the reference and

FR systems and associated to benthic organic matter

outside the exposed litter throughout the experiment

(Table 2). Two macroinvertebrate species, Gammarus

pulex L. and Asellus aquaticus L., were classified as

shredders. G. pulex was the dominant shredder

throughout the study period (Table 2). At the refer-

ence site, shredder abundance and biomass in litter

bags peaked at week 3 (Fig. 1) and remained higher

than non-shredder abundance and biomass throughout

the study (P \ 0.05; Wilcoxon-test). Shredder abun-

dance and biomass in litter bags in the FR system had

no distinct peaks throughout the study and decreased

to very low levels at the last two sampling dates

(Fig. 1). Shredder abundance and biomass values in

litter bags in the reference were higher than those in

the FR system (P \ 0.05; Wilcoxon-test), whereas

non-shredder abundance and biomass values did not

differ between the groups’ treatments. At week 3 of

exposure, we detected a high abundance of chirono-

mid larvae (Micropsectra spp.) in the fine-mesh bags

(up to 35 individuals per bag) with no difference

between the groups’ treatments. The Micropsectra

larvae were detected on the leaf surface and inside the

leaf matrix. At the last sampling date, most of the

Micropsectra larvae inside the bags had pupated.

Table 1 Physicochemical characteristics in the reference

(Ref) and under conditions of flow reduction (FR)

Ref FR

Temperature (�C) 13.5 ± 0.2 14.4 ± 0.4

Conductivity (lS cm-1) 969 ± 7.3 970 ± 12.2

pH 7.9 ± 0.1 7.8 ± 0.1

Velocity (cm s-1) 33.0 ± 4.6 0.4 ± 0.1

DO (mg l-1) 7.1 ± 0.5 5.6 ± 0.9

DO (% saturation) 84.9 ± 3.5 65.4 ± 9.2

SRP (lg l-1) 96.4 ± 21.1 93.5 ± 13.5

NOx--N (lg l-1) 40.4 ± 7.1 42.3 ± 6.8

NH4?-N (lg l-1) 107.4 ± 31.4 129.6 ± 78.1

TP (lg l-1) 137.5 ± 26.0 129.8 ± 18.8

DOC (mg l-1) 15.8 ± 2.4 18.2 ± 4.3

Values represent means (n = 15) ± standard error

Hydrobiologia (2009) 624:205–217 209

123

Microbial parameters

In fine-mesh bags, mean leaf-associated respiration

was higher (P \ 0.05, Wilcoxon test) in the reference

(1.43 ± 0.22 mg O2 g AFDM-1 h-1) than in the FR

system (1.26 ± 0.15 mg O2 g AFDM-1 h-1) (Fig. 2)

throughout the study. In coarse-mesh bags, mean

respiration did not differ between the reference

(1.53 ± 0.27 mg O2 g AFDM-1 h-1) and FR sys-

tems (1.63 ± 0.30 mg O2 g AFDM-1 h-1). Pair-

wise comparisons of the mesh size variants revealed

that, in the FR system, respiration was higher in

coarse-mesh bags (P \ 0.05, Wilcoxon test) than in

fine-mesh bags, whereas, in the reference system,

respiration did not differ between mesh sizes (Fig. 2).

Leaf-associated sporulation rates, number of spor-

ulating taxa and ergosterol content were higher in the

reference than in the FR system (P \ 0.05, Wilcoxon

test). The temporal pattern of sporulation, the number

of taxa, and the ergosterol content also differed

between the groups’ treatments (Figs. 3, 4). In the

reference, sporulation rates and the number of taxa

initially increased, peaked after week 2, and subse-

quently decreased. A total of 10 different taxa could

be identified from spore counts at this site (Table 3).

In the FR system, only five different taxa of aquatic

hyphomycete released a lower number of spores, with

the highest values occurring towards the end of the

study (Fig. 3). The leaf-associated ergosterol content

in the reference peaked after 3 weeks, whereas in the

FR system, ergosterol increased throughout the whole

experimental period (Fig. 4).

Leaf mass loss

During the first 2 weeks of exposure, almost no mass

was lost in bags of both systems (Fig. 5) presumably

Table 2 Mean relative abundance (%) of macroinvertebrates

in coarse-mesh litter bags during 6 weeks of exposure in the

reference (Ref) and flow reduction system (FR)

Taxon Ref FR

Gammarus pulex L. 51.1 ± 22.5 46.3 ± 25.4

Asellus aquaticus L. 21.1 ± 7.4 32.1 ± 19.2

Erpobdella octoculata L. 1.6 ± 0.7 2.6 ± 0.2

Erpobdella testacea Sav. 0.1 ± 0.1 1.7 ± 0.3

Helobdella stagnalis L. 0.8 ± 0.4 1.3 ± 0.1

Glossiphonia complanata L. 0.5 ± 0.2 1.5 ± 0.2

Chaetopterix villosa Fabr. 3.4 ± 1.3 1.3 ± 0.5

Platambus maculatus L. 2.5 ± 0.4 0.8 ± 0.3

Hydropsyche spp. 4.2 ± 2.3 0.4 ± 0.3

Psidium spp. 2.1 ± 0.3 0.2 ± 0.1

Ptychoptera spp. 1.2 ± 0.2 0

Micropsectra spp. 8.1 ± 6.2 14.1 ± 8.9

Baetis spp. 3.3 ± 0.3 1.3 ± 0.7

Values presented are means (n = 15) ± standard error

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6Exposure time (weeks)

Ab

un

dan

ce(I

nd

ivid

ual

s g-1

AF

DM

)

Shredder Ref Non-shredder RefShredder FR Non-shredder FR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 1 2 3 4 5 6

Exposure time (weeks)

Bio

mas

s(g

g-1

AF

DM

)

Shredder Ref Non-shredder RefShredder FR Non-shredder FR

A

B

Fig. 1 (A) Abundance and (B) ash-free dry mass (AFDM) of

shredder and non-shredder taxa on leaf disks in litter bags

exposed in coarse-mesh bags in the reference (Ref) and flow

reduction system (FR). Data presented are means (n = 3) ±

standard error

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 1 2 3 4 5 6Exposure time (weeks)

Res

pir

atio

n r

ate

(mg

O2

h-1

g-1

AF

DM

)

Ref coarse FR coarseRef fine FR fine

Fig. 2 Leaf-associated respiration rates based on ash-free dry

mass (AFDM) of leaf disks in fine and coarse-mesh bags

during 6 weeks of exposure in the reference (Ref) and flow

reduction system (FR). Data presented are means (n = 3) ±

standard error

210 Hydrobiologia (2009) 624:205–217

123

due to the use of preleached leaf material. After the

second week, substantial mass loss was measured in

all bags. After 4 weeks of exposure, most leaf disks

in the coarse-mesh bags exhibited extensive skelet-

onization, with only major leaf veins remaining,

whereas those in the fine-mesh bags exhibited partial

skeletonization, with major and small leaf veins

remaining. After 6 weeks, the leaf mass remaining

ranged from 18% of initial mass in coarse-mesh

bags in the reference to 57% of initial mass in the

fine-mesh bags in the FR system (Fig. 5).

The estimated breakdown rates based on time (k)

and degree days (k0) for each treatment group and the

determination coefficient of each regression are

reported in Table 4. Even though some determination

coefficient values (R2) seemed rather low, the regres-

sions were always highly significant (ANOVA

P \ 0.001). Time-based breakdown rates of leaf disks

ranged from k = 0.017 ± 0.004 in fine-mesh bags in

the FR system to k = 0.057 ± 0.007 for coarse-mesh

bags in the reference (Table 4). ANCOVA revealed

significant differences in breakdown rates between the

groups’ treatments. Breakdown rates of leaf disks

enclosed in coarse-mesh bags were higher in the

reference than in the FR system (P \ 0.05,

ANCOVA), whereas those of leaf disks in fine-mesh

bags rates did not differ between systems (Fig. 5). In

the reference system, leaf breakdown rates were

faster in coarse-mesh bags than in fine-mesh bags

0

2

4

6

8

10

0 1 2 3 4 5 6Exposure time (weeks)

Sp

oru

lati

on

rat

e(C

on

idia

µg

-1 A

FD

M d

-1) Ref FR

Ref FR

0

2

4

6

0 1 2 3 4 5 6

Exposure time (weeks)

Tax

a(N

o. b

ag-1

)

A

B

Fig. 3 (A) Fungal sporulation rates based on ash-free dry mass

(AFDM) and (B) number of sporulating aquatic hyphomycete

taxa on leaf disks in fine-mesh bags during 6 weeks of

exposure in the reference (Ref) and flow reduction system

(FR). Data presented are means (n = 3) ± standard error

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5 6Exposure time (weeks)

Erg

ost

ero

l (µg

g-1

AF

DM

) Ref FR

Fig. 4 Ergosterol content of leaf disks in fine-mesh bags

during 6 weeks of exposure in the reference (Ref) and flow

reduction system (FR). Data presented are means (n = 3) ±

standard error

Table 3 Aquatic hyphomycetes found (•) on alder leaf disks

decomposing in fine-mesh bags during 6 weeks of exposure in

the reference (Ref) and sudden flow reduction system (FR)

Taxon Ref FR

Anguillospora longissima •Articulospora tetracladia •Clavariopsis aquatica • •Flagellospora curvula •Flagellospora fusarioides • •Lemmoniera aquatica •Mycocentrospora acerina • •Tetrachaetum elegans • •ND1 •ND2 •ND3 •

0

20

40

60

80

100

120

140

0 1 2 3 4 5 6

Exposure time (weeks)

Lea

f m

ass

rem

ain

ing

(%

)

Ref fine FR fine

Ref coarse FR coarse

Fig. 5 Leaf mass remaining in leaf disks in coarse- and fine-

mesh bags during 6 weeks of exposure in the reference (Ref)

and flow reduction system (FR). Data presented are means

(n = 3) ± standard error

Hydrobiologia (2009) 624:205–217 211

123

(P \ 0.05, ANCOVA), whereas in the FR system, the

rates did not differ between mesh sizes.

Discussion

Our results revealed that sudden fragmentation into

isolated pools associated with severe flow reduction

reduces the importance of shredder-mediated litter

decomposition. Although invertebrate community

composition and densities at the start of the experi-

ment were identical, leaf breakdown rates in coarse

litter bags were lower in the low-flow environment

than in the reference. One possible explanation for this

difference could be that, during a season characterized

by limited litter supply, exposed leaf litter may have

attracted shredders from a wider section of the

reference reach, which in turn accelerated litter

breakdown rates. We observed increasing shredder

abundance and biomass in the reference during the

first 3 weeks of litter exposure, which could reflect an

attraction of shredders to this site. However, rapid

increases in shredder abundances during alder leaf

colonization were measured in several studies, either

during summer or autumn (Maloney & Lamberti,

1995; Haapala et al., 2001; Richardson et al., 2004).

Furthermore, the breakdown rates in our reference

system fall within the range reported for alder in

streams with similar trophic levels (Vought et al.,

1998; Baldy et al., 2007; Bergfur et al., 2007). These

rates were measured during or shortly after peak litter

input when food is not limited and, consequently, the

probability of shredders being attracted by exposed

litter was low. In addition, we measured similar

macroinvertebrate abundances associated to benthic

organic matter outside the exposed litter at the

reference site throughout the whole study period.

Accordingly, we suggest that the increase in shredder

colonization measured in the reference site was not

caused by an unusually high attraction. We conclude

that a reduction in leaf shredding in the mesocosm

accounted for the observed differences in breakdown

rates. It is possible that mesocosm effects, such as a

reduced diversity of habitats, predation refugia and

hyporheic interstitial, lack of groundwater inflow, or

the black color of the container, affected leaf-associ-

ated organisms and in turn breakdown rates. We tried

to partly prevent such effects and to mimic natural

conditions by providing coarse and fine detritus mixed

with sand in similar densities as found at the stream

reference to increase the complexity of the bottom

substrate in the mesocosm. However, the mesocosm

experiment had also several advantages as it allowed a

controlled low water in- and outflow and the simul-

taneous comparison of environmental conditions that

usually occurs subsequently, such as a stream reach

before and after its fragmentation into isolated pools.

Moreover, the mesocosm and the stream reference

received identical environmental influences such as

temperature, dark–light interval, precipitation and

quality of inflowing water.

We assume that shredders in the mesocosm were

negatively affected by unfavorable environmental

conditions that would also occur after a sudden

fragmentation into isolated pools. Severe flow reduc-

tion is known to cause changes in various abiotic and

biotic factors (Benbow et al., 2005; Acuna et al.,

2005). In our experiment, besides current velocity,

DO was the only abiotic factor that decreased in the

mesocosm. DO depletion is deleterious to macroin-

vertebrate species like G. pulex, which have a low

capacity to regulate oxygen consumption at low DO

levels (Meijering, 1991; Toman & Dall, 1998).

Nevertheless, we did not observe severe oxygen

depletion within the metabolically critical range

(O2 \ 2.7 mg l-1) for gammarid shredders (Vobis,

1973; Maltby, 1995). Moreover, A. aquaticus, which

coexisted with G. pulex in our experiment, is able to

shred at low DO levels. Bjelke (2005), for example,

found that even at hypoxic levels of 2 mg l-1, which

is far lower than the DO in the mesocosm, A.

aquaticus has the same leaf-shredding capacity as at

normoxia. We therefore conclude that besides abiotic

factors, biotic factors such as predation pressure also

influenced litter decomposition in the mesocosm.

Table 4 Summary of non-linear regression analyses of leaf

mass loss data for leaf disks in fine- and coarse-mesh bags

during 6 weeks of exposure in the reference (Ref) and sudden

flow reduction system (FR)

Site Mesh

bag

k days-1 ± SE R2 k0 �days-1 ± SE R2

Ref Fine 0.0322 ± 0.0049 0.77 0.0024 ± 0.0003 0.77

Ref Coarse 0.0571 ± 0.0066 0.85 0.0042 ± 0.0005 0.85

FR Fine 0.0168 ± 0.0036 0.63 0.0012 ± 0.0003 0.63

FR Coarse 0.0230 ± 0.0040 0.72 0.0017 ± 0.0003 0.72

Values presented are means (n = 15) ± standard error (SE)

212 Hydrobiologia (2009) 624:205–217

123

Accordingly, predation pressure in the mesocosm

could have decimated the shredder population, which

in turn resulted in reduced litter decomposition.

Similar observations were made by McIntosh et al.

(2002), who assume that invertebrate density

decreased in response to changes in competition and

predation in a Hawaiian stream after flow reduction

through water diversion. Indeed, we detected preda-

tory taxa among the non-shredders throughout the

whole experiment. For example Hirudinea such as

Erpobdella octoculata, which can significantly reduce

densities of prey organisms like G. pulex (Dahl &

Greenberg, 1997; Kutschera, 2003), were present in

both the reference and flow reduction systems. In the

FR system, however, extremely low shredder densi-

ties were observed in litter bags in the mesocosm

toward the end of the experiment. The macroinverte-

brates were apparently not able to escape predation

up- or downstream. In addition, reduced flow could

have removed their velocity-mediated predation refu-

gia in case predators and prey normally have different

velocity preferences (Dewson et al., 2007). As climate

change models predict a more frequent occurrence of

flow reduction and fragmentation for Central Euro-

pean lowland streams (Andersen et al., 2006), similar

effects may occur in those streams. In particular, at the

beginning of stream fragmentation, which often leads

to an abrupt cessation of surface flow and a discon-

nection of the formerly connected stream (Acuna

et al., 2005), macroinvertebrates would then be

restricted to isolated pools and, as in our mesocosm,

have low or no possibility to escape predation.

In addition to shredder-mediated decomposition,

sudden flow reduction also affected leaf-associated

microbial activity, which manifested as lower leaf

respiration in the fine litter bags in the mesocosm than

in the reference. This is in agreement with Acuna et al.

(2004), who found that the reduction of flow during

stream fragmentation caused a sharp decrease in

ecosystem respiration. In our experiments, the low-

ered respiration and the delayed sporulation and

number of sporulating taxa in the mesocosm could

indicate that aquatic hyphomycetes have played a

minor role in leaf decomposition. The reduction of

flow might have been too strong to support successful

dispersal and subsequent attachment of hyphomycete

spores or to induce spore production, which is

generally known to be stimulated by water current

(Ibqal & Webster, 1977; Smither-Kopperl et al., 1998;

Maamri et al., 2001). Observations made by Maamri

et al. (2003) support this assumption. The authors

reported that cessation of flow in an arid zone stream

decreased the likelihood of successful colonization of

aquatic hyphomycetes. Furthermore, Dang et al.

(2007) suggest that differences in successful attach-

ment of hyphomycete spores to leaf surfaces might

have implications for structuring aquatic hyphomyc-

ete communities on decomposing litter. Nevertheless,

our leaf respiration and ergosterol content data

indicate the activity and presence of microorganisms

and, in particular, fungi throughout the initial period

of the experiment as well. It is possible that in the

mesocosm reproduction of aquatic hyphomycetes did

not occur simultaneously to periods of biomass

growth or leaf consumption. Another possibility for

the non-correspondence between sporulation of aqua-

tic hyphomycetes and respiration could be that, at

least initially, bacteria or epiphytic terrestrial fungi

account for the measured activity. Thus, aquatic

hyphomycetes might have been initially outcompeted

by other microorganisms, e.g., through antagonistic

effects. Such antagonism has been reported by Mille-

Lindblom et al. (2006), who found depressed growth

of several fungal species in the presence of bacteria.

However, comparing to other studies, even in the

reference the number of leaf-associated aquatic

hyphomycete taxa and their sporulation was low or

intermediate, whereas respiration and ergosterol con-

tents were among the higher end of ranges reported for

alder (Haapala et al., 2001; Baldy et al., 2007;

Yoshimura et al., 2008). We assume this might have

been an effect of the higher trophic status of the stream

or an effect of the season, as our study was performed

during summer with higher water temperatures. Both,

nutrients and temperature, are generally considered to

stimulate leaf breakdown rates, microbial activity, and

biomass (Rowe et al., 1996; Barlocher & Corkum

2003; Gulis & Suberkropp, 2003; Baldy et al., 2007;

Bergfur et al., 2007), but may have variable effects on

aquatic hyphomycete sporulation (Sridhar & Barlocher,

1997; Chauvet & Suberkropp 2001).

Although we measured lowered respiration and

sporulation in the flow reduction system, the break-

down rates in fine-mesh bags were not lower than those

in the reference. This discrepancy can be attributed to

the occurrence of chironomid larvae (Micropsectra

spp.) on leaf skeletons in fine-mesh bags in the second

half of the experiment. Thus, based on the breakdown

Hydrobiologia (2009) 624:205–217 213

123

rates in our experiments, fine-mesh bags do not prevent

macroinvertebrate effect on leaf decomposition.

Although the fine mesh excluded most macroinverte-

brate adults, it did not exclude macroinvertebrate eggs.

Hence, we assume that the Micropsectra larvae

originated from eggs hatching inside the fine-mesh

bags. We also observed Micropsectra larvae inside the

matrix of alder leaf disks. Our findings indicate that the

leaf cuticle and tissue were sufficiently microbially

degraded in the second half of the experiment and that

Micropsectra larvae were able to enhance further

maceration of the litter. We suggest that, under

environmental conditions unfavorable for common

shredders in previously permanently flowing streams,

macroinvertebrates such as chironomid larvae, which

are not typically classified as shredders, may cause

substantial leaf fragmentation. Similar observations

had been made by other investigators, who reported an

exploitation of leaf litter by chironomid larva (Math-

uriau & Chauvet, 2002) or by tubificid Oligochaeta

(Chauvet et al., 1993) after the plant tissue has been

partly degraded. Stout & Taft (1985) found a chiron-

omid larvae (Brillia flavifrons), previously thought to

specialize on decaying wood, which consumed Alnus

rugosa leaves in two Michigan streams. Callisto et al.

(2007) evaluated the potential use of leaf litter by

chironomid larvae and observed a Micropsectra spe-

cies (M. apposita) feeding on A. glutinosa leaves. To

what extent chironomid larvae participate in leaf

decomposition certainly depends not only on the

synchronization of their life cycles with litter input,

but also on environmental conditions. For example,

high water temperatures during summer months favor

the fast life cycles of chironomids (Nolte & Hoffmann,

1992). Accordingly, we observed that Micropsectra

larvae frequently hatched and pupated within fine-

mesh bags. In coarse-mesh bags, they were found only

occasionally throughout the whole experiment. Inter-

specific effects may have influenced the colonization

of Micropsectra larvae in these bags. It is known that

macroinvertebrates, such as gammarids, additionally

act as efficient predators (Dick, 1992; Krisp & Maier,

2005) and, hence, indirectly control leaf decomposition

through predation and competition.

Another role of larger macroinvertebrates may be to

influence leaf-associated microbial activity. In the

mesocosm with experimental reduction of flow, leaf

respiration in coarse- was higher than in fine-mesh

bags, which suggests that the presence of large

macroinvertebrates enhanced leaf-associated micro-

bial respiration. This stimulation may have resulted

from the bioturbation effect of the locomotion activity

of macroinvertebrates. Previous studies in hyporheic

sediments suggest that bioturbating macroinverte-

brates, such as G. pulex and A. aquaticus, increase

oxygenation and stimulate aerobic microbial processes

(Mermillod-Blondin et al., 2000, 2003). However, we

found no differences in DO in litter bags including or

excluding these macroinvertebrates. The higher micro-

bial activity may also have resulted from an increase in

leaf surface to volume ratio caused by shredding

macroinvertebrates. Furthermore, it is possible that the

leaf feeding behavior of macroinvertebrates, in partic-

ular of Asellus aquaticus, which are known to feed by

scraping the leaf surface (Graca et al., 1993), stimu-

lated microbial activity. Thus, the macroinvertebrates

caused biomass losses to leaf-associated microorgan-

isms and simultaneously stimulated their growth and in

turn their activity (Suberkropp, 1992). However, as we

found differences in microbial activity between mesh

sizes only in the mesocosm, we conclude that in

particular under flow reduction shredders may have the

potential to stimulate microbially mediated leaf

decomposition through indirect effects.

In our experiment, large amounts of leaf litter were

used to guarantee a high food supply for shredders.

However, since preleached leaf material was used for

this purpose, a large amount of readily dissolvable

organic compounds in the leaf material was lost prior

to the experiment. Consequently, we could not study

the effects of high leachate input. Previous studies

have shown that high leachate concentrations may

strongly reduce microbially mediated litter decom-

position (Schlief & Mutz, 2007) and shredding

(Canhoto & Laranjeira, 2007). In addition, a high

leachate input is thought to have the potential to

lower DO through increased oxygen consumption in

the water column (Canhoto & Laranjeira, 2007).

Further studies are needed to determine whether such

leachate effects can interfere with, or accelerate

the effects of sudden flow reduction after stream

fragmentation into isolated pools.

Conclusion

A sudden reduction of flow and connectivity in a

previously permanently flowing stream can reduce

214 Hydrobiologia (2009) 624:205–217

123

direct shredder-mediated litter decomposition.

Nevertheless, macroinvertebrates not typically classi-

fied as shredders can participate in litter decomposition

even after a sudden flow reduction. Typical shredders,

in turn, may indirectly influence the decomposition

process by exerting interspecific effects or by stimu-

lating microbial activity. Although we found no

evidence of reduced microbially mediated decompo-

sition in our experimental model, we found that a

sudden reduction of flow lowered leaf respiration and

delayed leaf colonization by aquatic hyphomycetes.

We conclude that premature litter input after or during

a sudden reduction of flow in a normally permanently

flowing lowland stream can affect litter decomposition

and thus the supply of decomposition products for the

stream food web. Our results are essential in small

lowland stream scenarios that evaluate the effects of

drought events or water deficits in temperate climate

regions.

Acknowledgments This study was supported by the DFG

Priority Program 1162 AQUASHIFT. We are grateful to

Thomas Wolburg, Michael Seidel, and Susann Parsche for their

assistance that contributed to the success of this study.

References

Acuna, V., A. Giorgi, I. Munoz, U. Uehlinger & S. Sabater,

2004. Flow extremes and benthic organic matter shape the

metabolism of a headwater Mediterranean stream.

Freshwater Biology 49: 960–971.

Acuna, V., I. Munoz, A. Giorgi, M. Omella, F. Sabater & S.

Sabater, 2005. Drought and postdrought recovery cycles

in an intermittent Mediterranean stream: structural and

functional aspects. Journal of the North American

Benthological Society 24: 919–993.

Andersen, H. E., B. Kronvang, S. E. Larsen, C. C. Hoffmann, T. S.

Jensen & E. K. Rasmussen, 2006. Climate-change impacts

on hydrology and nutrients in a Danish lowland river basin.

Science of the Total Environment 365: 223–237.

Baldy, V., V. Gobert, F. Guerold, E. Chauvet, D. Lambrigot &

J.-Y. Charcosset, 2007. Leaf litter breakdown budgets in

streams of various trophic status: effects of dissolved

inorganic nutrients on microorganisms and invertebrates.

Freshwater Biology 52: 1322–1335.

Barlocher, F., 1983. Seasonal variation on the standing crop

and digestibility of CPOM in a Swiss Jura stream. Ecol-

ogy 64: 1266–1272.

Barlocher, F., 1985. The role of fungi in the nutrition of stream

invertebrates. Botanical Journal of the Linnean Society

91: 83–84.

Barlocher, F. & M. Corkum, 2003. Nutrient enrichment over-

whelms diversity effects in leaf decomposition by stream

fungi. Oikos 101: 247–252.

Benbow, M. E., M. D. McIntosh, A. J. Burky & C. M. Way,

2005. The influence of stream flow reduction on the

energetics of endemic Hawaiian torrenticolous aquatic

insects, Telmatogeton Schiner and Procanace Hendel.

Journal of Insect Conservation 9: 175–185.

Benfield, E. F., 1996. Leaf breakdown in stream ecosystems. In

Hauer, F. R. & G. A. Lamberti (eds), Methods in Stream

Ecology. Academic Press, San Diego: 579–589.

Benfield, E. F., J. R. Webster, J. J. Hutchens, J. L. Tank & P. A.

Turner, 2000. Organic matter dynamics along a stream-

order and elevational gradient in a southern Appalachian

stream. Verhandlungen der Internationalen Vereinigung fur

Theoretische und Angewandte Limnologie 27: 1341–1345.

Bergfur, J., K. R. Johnson, L. Sandin, W. Goedkoop &

K. Nygren, 2007. Effects of nutrient enrichment on boreal

streams: invertebrates, fungi and leaf-litter breakdown.

Freshwater Biology 52: 1618–1633.

Bjelke, U., 2005. Processing of leaf matter by lake-dwelling

shredders at low oxygen concentrations. Hydrobiologia

539: 93–98.

Bond, N. R., P. S. Lake & A. H. Arthington, 2008. The impacts

of drought on freshwater ecosystems: an Australian per-

spective. Hydrobiologia 600: 3–16.

Boulton, A. J., 2003. Parallels and contrasts in the effects of

drought on stream macroinvertebrate assemblages.

Freshwater Biology 48: 1173–1185.

Bunn, S. E. & A. H. Arthington, 2002. Basic principles and

ecological consequences of altered flow regimes for

aquatic biodiversity. Environmental Management 30:

492–507. doi:10.1007/s00267-002-2737-0.

Callisto, M., J. F. Goncalves Jr. & M. A. S. Graca, 2007. Leaf

litter as a possible food source for chironomids (Diptera)

in Brazilian and Portuguese headwater streams. Revista

Brasileira de Zoologia 24: 442–448.

Canhoto, C. & C. Laranjeira, 2007. Leachates of Eucalyptusglobulus in intermittent streams affect water parameters

and invertebrates. International Review of Hydrobiology

92: 173–182.

Chamier, A. C., 1985. Cell-wall-degrading enzymes of aquatic

hyphomycetes: a review. Botanical Journal of the Linnean

Society 91: 67–81.

Chauvet, E., N. Giani & M. O. Gessner, 1993. Breakdown and

invertebrate colonization of leaf litter in two contrasting

streams: significance of Oligochaeta in a large river. Cana-

dian Journal of Fisheries and Aquatic Sciences 50: 488–495.

Dahl, J. & L. Greenberg, 1997. Foraging rates of a vertebrate

and an invertebrate predator in stream enclosures. Oikos

78: 459–466.

Dang, C. K., M. O. Gessner & E. Chauvet, 2007. Influence of

conidial traits and leaf structure on attachment success of

aquatic hyphomycetes on leaf litter. Mycologia 99: 24–32.

Dewson, Z., A. B. James & R. G. Death, 2007. A review of the

consequences of decreased flow for instream habitat and

macroinvertebrates. Journal of the North American

Benthological Society 26: 401–415.

Dick, J. T. A., 1992. The nature and implications of differential

predation between Gammarus pulex and G. duebeni celti-cus larvae (Crustacea: Amphipoda). Journal of Zoology

227: 171–183.

Dodds, W. K., J. R. Jones & E. B. Welch, 1998. Suggested

classification of stream trophic state: distributions of

Hydrobiologia (2009) 624:205–217 215

123

temperate stream types by chlorophyll, total nitrogen, and

phosphorus. Water Research 32: 1455–1462.

Gelbrecht, J., E. Driescher, H. Lademann, J. Schonfelder &

H. J. Exner, 1996. Diffuse nutrient impact on surface

water bodies and its abatement by restoration measures in

a small catchment area in North-East Germany. Water

Science and Technology 33: 167–174.

Gelbrecht, J., E. Driescher & H. J. Exner, 2000. Long-term

investigations on nutrient input from catchment of the

brook Demnitzer Muhlenfließ and restoration measures to

reduce nonpoint pollution. Berichte des IGB 10: 151–160.

Gerstengarbe, F. W., F. Badeck, F. Hattermann, V. Krysanova,

W. Lahmer, P. Lasch, M. Stock, F. Suckow, F. Wechsung &

P. C. Werner, 2003. Studie zur klimatischen Entwicklung

im Land Brandenburg bis 2055 und deren Auswirkungen

auf den Wasserhaushalt, die Forst- und Landwirtschaft

sowie die Ableitung erster Perspektiven. Potsdam Institute

for Climate Impact Research (PIK). PIK Report 83: 1–96.

Gessner, M. O., 1991. Differences in processing dynamics of

fresh and air-dried leaf litter in a stream ecosystem.

Freshwater Biology 26: 387–398.

Gessner, M. O. & A. L. Schmitt, 1996. Use of solid-phase

extraction to determine ergosterol concentrations in plant

tissues colonized by fungi. Applied and Environmental

Microbiology 62: 415–419.

Gessner, M. O., E. Chauvet & M. Dobson, 1999. A perspective

on leaf litter breakdown in streams. Oikos 85: 377–384.

Graca, M. A. S., L. Maltby & P. Calow, 1993. Importance of

fungi in the diet of Gammarus pulex and Asellus aquati-cus. Oecologia 96: 304–309.

Gulis, V. & K. Suberkropp, 2003. Leaf litter decomposition

and microbial activity in nutrient-enriched and unaltered

reaches of a headwater stream. Freshwater Biology 43:

123–134.

Haapala, A., T. Muotka & A. Markkola, 2001. Breakdown and

macroinvertebrate and fungal colonization of alder, birch,

and willow leaves in a boreal forest stream. Journal of the

North American Benthological Society 20: 395–407.

Habdija, I., I. Radanovic, B. Primic-Habdija, R. Matonickin &

M. Kucinic, 2003. River discharge regime as a factor

affecting the changes in community and functional feed-

ing group composition of macroinvertebrates on a cobble

substrate in the Sava river. Biologia 58: 217–229.

Hieber, M. & M. O. Gessner, 2002. Contribution of stream

detrivores, fungi, and bacteria to leaf breakdown based on

biomass estimates. Ecology 83: 1026–1038.

Ibqal, S. H. & J. R. Webster, 1977. Aquatic hyphomycete spora

of some Dartmoor streams. Transactions of the British

Mycological Society 69: 233–241.

Kozlowski, T. T. & S. G. Pallardy, 2002. Acclimation and

adaptive responses of woody plants to environmental

stresses. The Botanical Review 68: 270–334.

Krisp, H. & G. Maier, 2005. Consumption of macroinverte-

brates by invasive and native gammarids: a comparison.

Journal of Limnology 64: 55–59.

Kutschera, U., 2003. The feeding strategies of the leech

Erpobdella octoculata (L.): a laboratory study. Interna-

tional Review of Hydrobiology 88: 94–101.

Lahmer, W. & A. Becker, 2000. Mogliche Auswirkungen von

Klimaveranderungen am Beispiel einer Untersuchung fur

das Land Brandenburg. In Auswirkungen von Klimaan-

derungen auf die Wasserwirtschaft. KA - Wasserwirtschaft,

Abwasser, Abfall 47: 170–175.

Lake, P. S., 2003. Ecological effects of perturbation by drought

in flowing waters. Freshwater Biology 48: 1365–2427.

Lengsfeld, H. & J. Gelbrecht, 2003. A first evaluation of res-

toration measures to reduce phosphorus losses from the

catchment of the brook Demnitzer Muhlenfließ. Berichte

des IGB 17: 105–114.

Maamri, A., F. Barlocher, E. Pattee & H. Chergui, 2001. Fungal

and bacterial colonisation of Salix pedicellata leaves decay-

ing in permanent and intermittent streams in eastern Morocco.

International Review of Hydrobiology 86: 337–348.

Maamri, A., E. Chauvet, H. Chergui, F. Gourbiere & E. Pattee,

2003. Microbial dynamics on dead leaves in a temporary

Moroccan river. I-Fungi. Archiv fur Hydrobiology 144:

41–59.

Maloney, D. C. & G. A. Lamberti, 1995. Rapid decomposition

of summer-input leaves in a northern Michigan stream.American Midland Naturalist 133: 184–195.

Maltby, L., 1995. Sensitivity of the crustaceans Gammaruspulex (L.) and Asellus aquaticus (L.) to short-term expo-

sure to hypoxia and unionized ammonia: observations and

possible machanisms. Water Research 29: 781–787.

Mathuriau, C. & E. Chauvet, 2002. Breakdown of leaf litter in

a neotropical stream. Journal of the North American

Benthological Society 21: 384–396.

McIntosh, M. D., M. E. Benbow & A. J. Burky, 2002. Effects

of stream diversion on riffle macroinvertebrate commu-

nities in a Maui, Hawaii, stream. River Research and

Applications 18: 569–581.

Meijering, M. P. D., 1991. Lack of oxygen and low pH as

limiting factors for Gammarus in Hessian brooks and

rivers. Hydrobiologia 223: 159–161.

Mermillod-Blondin, F., M. Creuze des Chatelliers, M. Gerino &

J. P. Gaudet, 2000. Testing the effect of Limnodrilus sp.

(Oligochaeta, Tubificidae) on organic matter and nutrient

processing in the hyporheic zone: a microcosm method.

Archiv fur Hydrobiology 149: 467–487.

Mermillod-Blondin, F., J. P. Gaudet, M. Gerino, G. Desrosiers

& M. Creuze des Chatelliers, 2003. Influence of macro-

invertebrates on physico-chemical and microbial

processes in hyporheic sediments. Hydrological Processes

17: 779–794.

Merritt, R. W. & K. W. Cummins, 1996. Trophic relations of

macroinvertebrates. In Hauer, F. R. & G. A. Lamberti

(eds), Methods in stream ecology. Academic Press, Lon-

don: 453–474.

Mille-Lindblom, C., H. Fischer & L. J. Tranvik, 2006.

Antagonism between bacteria and fungi: substrate com-

petition and a possible tradeoff between fungal growth

and tolerance towards bacteria. Oikos 113: 233–243.

Minshall, G. W., R. C. Petersen, K. W. Cummins & T. L. Bon,

1983. Interbiome comparison of stream ecosystem

dynamics. Ecological Monographs 53: 1–25.

Newell, S. Y., 1993. Membrane-containing fungal mass and

fungal specific growth rate in natural samples. In Kemp,

P. F., B. F. Sherr & J. C. Jonathan (eds), Handbook of

Methods in Aquatic Microbial Ecology. Lewis Publishers,

Boca Raton: 579–586.

216 Hydrobiologia (2009) 624:205–217

123

Nolte, U. & T. Hoffmann, 1992. Fast life in cold water: Diamesa

incalida (Chironomidae). Ecography 15: 25–30.

Pascoal, C. & F. Cassio, 2004. Contribution of fungi and

bacteria to leaf litter decomposition in a polluted river.

Applied and Environmental Microbiology 70: 5266–5273.

Petersen, R. C. Jr. & K. W. Cummins, 1974. Leaf processing in

a woodland stream. Freshwater Biology 4: 343–368.

Petersen, R. C., K. W. Cummins Jr. & G. M. Ward, 1989.

Microbial and animal processing of detritus in a woodland

stream. Ecological Monographs 59: 21–39.

Pusch, M. & J. Schwoerbel, 1994. Community respiration in

hyporheic sediments of a mountain stream (Steina, Black

Forest). Archiv fur Hydrobiology 130: 35–52.

Richardson, J. S., 1991. Seasonal food limitation of detritivores

in a montane stream: an experimental test. Ecology 72:

873–887.

Richardson, J. S., C. R. Shaughnessy & P. G. Harrison, 2004.

Litter breakdown and invertebrate association with three

types of leaves in a temperate rainforest stream. Archiv

fur Hydrobiology 159: 309–325.

Rowe, J. M., W. B. Perry & S. A. Perry, 1996. Leaf litter pro-

cessing in West Virginia mountain streams: effects of

temperature and stream chemistry. In Hom, J., B. Richard &

K. O’Brian (eds), Proceedings, 1995 meeting of the

Northern Global Change Program: Introduction/Environ-

mental Change. Department of Agriculture, Forest Service,

Northeastern Forest Experiment Station. Pittsburgh, PA.:

157–160.

Schlief, J., 2004. Leaf associated microbial activities in a

stream affected by acid mine drainage. International

Review of Hydrobiology 89: 467–475. doi:10.1002/iroh.

200410776.

Schlief, J. & M. Mutz, 2007. Response of aquatic leaf associ-

ated communities to elevated leachate DOC: a microcosm

study. International Review of Hydrobiology 92: 146–155.

Short, R. A., S. L. Smith, D. W. Guthrie & J. A. Stanford,

1984. Leaf litter processing rates in four Texas streams.

Journal of Freshwater Ecology 2: 469–473.

Smither-Kopperl, M. L., R. Charudattan & R. D. Berger, 1998.

Dispersal of spores of Fusarium culmorum in aquatic

systems. Phytopathology 88: 382–388.

Sridhar, K. R. & F. Barlocher, 1997. Water chemistry and

sporulation by aquatic hyphomycetes. Mycological

Research 101: 591–596.

Stout, R. J. & W. H. Taft, 1985. Growth patterns of a chironomid

shredder on fresh and senescent tag alder leaves in two

Michigan USA streams. Journal of Freshwater Ecology 3:

147–154.

Suberkropp, K., 1992. Interactions with invertebrates. In

Barlocher, F. (ed.), The Ecology of Aquatic Hyphomy-

cetes. Springer, Berlin: 118–134.

Toman, M. J. & P. C. Dall, 1998. Respiratory levels and

adaptations in four freshwater species of Gammarus(Crustacea: Amphipoda). International Review of Hydro-

biology 83: 251–263.

Vobis, H., 1973. Rheotaktisches Verhalten einiger Gammarus-

Arten bei verschiedenem Sauerstoffgehalt des Wassers.

Helgoland Marine Research 25: 495–508.

Vought, L. B.-M., A. Kullberg & R. C. Petersen, 1998. Effect

of riparian structure, temperature and channel morphom-

etry on detritus processing in channelized and natural

woodland streams in southern Sweden. Aquatic Conser-

vation Marine and Freshwater Ecosystems 8: 273–285.

Wallace, J. B., S. L. Eggert, J. L. Meyer & J. R. Webster, 1999.

Effects of resource limitation on a detrital-based ecosys-

tem. Ecological Monographs 69: 409–442.

Webster, J. R. & E. F. Benfield, 1986. Vascular plant break-

down in freshwater ecosystems. Annual Review of

Ecology and Systematics 17: 567–594.

Webster, J. R., E. F. Benfield, T. P. Ehrman, M. A. Schaeffer, J.

L. Tank, J. J. Hutchens & D. J. D’Angelo, 1999. What

happens to allochthonous material that falls into streams?

A synthesis of new and published information from

Coweeta. Freshwater Biology 41: 687–705.

Wendler, R. & P. Millard, 1996. Impacts of water and nitrogen

supplies on the physiology, leaf demography and nitrogen

dynamics of Betula pendula. Tree Physiology 16: 153–159.

Yoshimura, C., M. O. Gessner & K. Tockner, 2008. Chemical

properties, microbial respiration, and decomposition of

coarse and fine particulate organic matter. Journal of the

North American Benthological Society 27: 664–673.

Hydrobiologia (2009) 624:205–217 217

123