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PRIMARY RESEARCH PAPER
Effect of small reservoirs on leaf litter decompositionin Mediterranean headwater streams
Margarita Menendez • Enrique Descals •
Tecla Riera • Oscar Moya
Received: 14 April 2011 / Revised: 14 February 2012 / Accepted: 6 March 2012 / Published online: 23 March 2012
� Springer Science+Business Media B.V. 2012
Abstract Leaf litter decomposition is a crucial
process providing matter and energy to communities
inhabiting headwater streams. This process could be
affected by many man-made landscape transforma-
tions and its response can vary depending on the
climate setting. In this study, we test the hypothesis
that the presence of small headwater reservoirs
decreases litter decomposition downstream, as
reported for temperate Oceanic climatic regions, and
that this effect is more accentuated in the Mediterra-
nean. The effect of small dams on the decomposition
of alder (Alnus glutinosa) leaves was studied in four
headwater streams in Catalonia (NE Spain). The
presence of a dam affected litter decomposition rates
in three of the four streams studied, and this depended
on reservoir typology. In those with seasonal surface
release, decomposition rates were slower downstream
from the dams, but in the case of a continuous
hypolimnetic release, it was faster, with higher DIN
and temperature and abundance of shredders. Alder
litter decomposition rates were twice those reported
for Oceanic climatic conditions. In Mediterranean
headwaters, the effect of small dams will even be more
evident at an annual scale due to the diminished flow
rates in summer and this effect will be more
pronounced than in the more Oceanic.
Keywords Litter breakdown � Small reservoirs �Aquatic hyphomycetes � Invertebrates
Introduction
Headwater stream ecosystems are among the most
vulnerable to anthropogenic impacts, including dam
construction on previously free-flowing rivers. Dams
interfere with stream continuity and affect ecosystem
structure and processes (Short & Ward, 1980; Ward &
Stanford, 1983) such as sedimentation, nutrient levels,
temperature, and invertebrate density and diversity
(Cortes et al., 1998; Casas et al., 2000). Throughout
the world and especially in the Mediterranean Basin,
where river flow is seasonally variable, water volume
has been regulated for multiple purposes such as the
production of hydroelectric energy, domestic con-
sumption, or farmland irrigation. Spain has the world’s
highest number of dams per inhabitant and km2
(Garcıa de Jalon, 2003) and is the world’s fifth in
absolute numbers, after continental China, USA,
Japan, and India. It has more than 8,500 obstacles in
rivers, between dams and other small works such as
Handling editor: Nuria Bonada
M. Menendez (&) � T. Riera
Departmento de Ecologıa, Universidad de Barcelona,
Barcelona, Spain
e-mail: [email protected]
E. Descals � O. Moya
Instituto Mediterraneo de Estudios Avanzados
de las Baleares, IMEDEA (CSIC-UIB), Esporles,
Mallorca, Spain
123
Hydrobiologia (2012) 691:135–146
DOI 10.1007/s10750-012-1064-6
docks and waterwheels (WWF Spain, 2009). Of this
total, 14.5 % enter the category of the great dams
([15 m in height or[3 Hm3 in storage capacity if 5 to
15 m in height; World Commission on Dams, 2000).
The rest are small impoundments, some of them
constructed in the early twentieth century. These
regulate flow in headwaters, preventing a natural flow
regime and often turning them into temporary streams.
This problem could be aggravated under the predicted
global warming scenario. Climate change is expected
to have strong effects in the Mediterranean-climate
regions and, in some areas, this will include increases
in temperature and decreases in rainfall, especially in
the warm season (Giorgi & Lionello, 2008), hence
leading to prolonged periods of drought mainly in
summer and fall. Both severity and duration are
important in determining the outcome of drought on
aquatic communities (Beche et al., 2009). Therefore,
the effect of water control and regulation can render
the communities and processes in Mediterranean
streams especially vulnerable to global change.
The impact of such small dams has been studied
with regard to their effects on structure downstream, in
its effect on erosive and sedimentation processes
(Petts, 1979; Andrews, 1986; Lyons et al., 1992;
Magilligan et al., 2008), in the characteristics of
riparian vegetation (Hupp, 1992; Johnson, 1998;
Merritt & Cooper, 2000) and in the macroinvertebrate
community composition (Malmqvist & Englund,
1996; Pozo et al., 1997; Cortes et al., 1998; Breden-
hand & Samways, 2009). In spite of this, few studies
have evaluated the effect of fluvial regulation on
processes taking place in stream ecosystems, such as
leaf litter decomposition (Short & Ward, 1980; Casas
et al., 2000; Pomeroy et al., 2000; Muehlbauer et al.,
2009; Mendoza-Lera et al., 2010). Dams can modify
water temperature and chemical conditions such as
nutrient concentrations, both of which may affect
aquatic hyphomycete and macroinvertebrate commu-
nities, as well as the processing of organic matter. Leaf
litter decomposition is a crucial process in headwater
streams lined by riparian vegetation, since the greater
fraction of the energy sustaining the trophic web is
derived from allochthonous leaf litter inputs (Fisher &
Likens, 1973; Wallace et al., 1997; Webster et al.,
1999; Graca, 2001). Therefore, any disruption to this
process can negatively affect the entire food web.
In a study conducted in Portugal, Goncalves et al.
(2006) have showed higher decomposition rates, faster
hyphomycete colonization, and greater abundance of
shredders in a headwater stream in temperate Oceanic
than in Mediterranean climatic conditions. A recent
study in the Basque Country of five streams regulated
by small dams (Mendoza-Lera et al., 2010) established
that shredder macroinvertebrate biomass and density
as well as leaf litter decomposition diminished down-
stream from the dams. However, no significant
alterations in the physicochemical characteristics of
the water were observed. In this study, we explore the
effect of the presence of small dams along Mediter-
ranean headwater streams on leaf litter decomposition.
The Oceanic climate in the Basque Country is
characterized by its high rainfall throughout the year,
cool summers and temperate winters. However, the
Mediterranean is characterized by scarce precipita-
tion, taking place mainly in spring and fall, and drier
and warmer summers. Mediterranean streams are
characterized by severe changes in water flow, with
frequent floods and droughts (Gasith & Resh, 1999;
Sabater et al., 2008). Considering these results, our
expectation is that the effect of dams on leaf litter
decomposition would be more pronounced in our
Mediterranean streams than for those of the above
mentioned study. More specifically, our hypotheses
are that (1) the presence of a dam will decrease the
processing rate of organic matter downstream by
decreasing water flow variability, increasing temper-
ature and decreasing oxygen concentration in water,
which itself will decrease aquatic hyphomycete and
macroinvertebrate abundance and diversity and
(2) that this effect will be more accentuated in a
Mediterranean climate than in a more temperate
Oceanic conditions. For this purpose, we studied the
decomposition of alder (A. glutinosa (L.) Gaertner)
leaves up and downstream from small dams regulating
four Mediterranean headwater streams. Our results
will be compared with those reported for oceanic
climate streams.
Materials and methods
Study sites
This study was conducted in four low-order streams
(orders 1–3) where water flow is regulated by small
dams (as defined by the World Commission on Dams,
2000, i.e.,\15 m height or \ 3 Hm3 reservoir storage
136 Hydrobiologia (2012) 691:135–146
123
capacity for dams 5 to 15 m in height). The sites are
referred to as Santa Fe (SF), Fuirosos (FU) in the
Tordera, Avenco (AV), and Vallfornes (VF) in the
Besos, river basins of Catalonia (NE Spain; Table 1).
During the study period (November 2008 to February
2009) the mean air temperatures ranged from 6.5 to
17.5�C and the total rainfall was 613 mm. The streams
were similar in size (6.1 m ± 1.0; mean channel
width ± SE, the catchment area between 310 and
1,260 ha) and geology (siliceous bedrock). The ripar-
ian vegetation was dominated by alder, ash (Fraxinus
excelsior L.) and beech (Fagus sylvatica L.) in all
streams. The anthropogenic land use of the watersheds
was low (1–14%; Table 1). In each stream, two 50 m
reaches were selected, one 160–480 m downstream
from the dam and the other 210–360 m above the
headwater reservoir (Table 1).
Environmental variables
During the study period and in all four streams, water
temperature was continuously monitored with ACR
Smart-Button sensors (ACR Systems Inc., Surrey, BC,
Canada). Conductivity, pH, and dissolved oxygen were
measured with a WTW multiparametric sensor (Weil-
heim, Germany) on each sampling date (n = 5).
Instantaneous river flow was calculated from the
instantaneous water velocity measured with a digital
water velocity meter (FP311 flow probe, Global Water
Instrumentation Inc.) and from the average stream
section area on each sampling date and experimental
reach. Water was sampled and filtered through pre-
ashed glass fiber filters (Whatman GF/F) for nutrient
and alkalinity analyses. The latter was determined by
titration to a pH endpoint of 4.5 (APHA, 2005). Nitrate
concentration was determined by ion chromatography
(COMPACT IC1.1 Metrohm). Ammonium was mea-
sured by the manual salicylate method, nitrite by the
sulphanylamide method, and SRP by the molybdate
method (APHA, 2005).
The quality of the riparian vegetation was determined
by the Riparian Quality Index (QBR, Munne et al.,
2003), which considers qualitative aspects such as
presence of allocthonous species or the distribution and
species richness of trees, and tree canopy cover using a
spherical densiometer (Model-A, Forest Densiometers,
Bartlesville, OK, USA) in winter and summer. We
characterised the benthic habitat with the Fluvial Habitat
Index (IHF, Pardo et al., 2002), which measures the
habitat heterogeneity of the substrate. The granulomet-
ric composition of the stream bed was estimated visually
following the classification system by Allan & Castillo
(2009; ‘‘boulders’’: [25 cm, ‘‘cobbles’’: 6–25 cm,
‘‘sand’’:\6 cm).
Litter bags and decomposition
Alder leaves were collected just after abscission in the
Aguera River (Biscay-Cantabria) in autumn 2008.
Leaves were air-dried to constant weight and stored
until needed. Portions weighing 5.0 ± 0.25 g
(mean ± SE) were moistened with a garden atomizer
and inserted into mesh bags (15 9 20 cm, 5 mm
mesh). Leaf bags (16 in each stream site) were tied
with nylon lines to four iron bars driven into the
streambed along a 50 m reach. An extra set of four
bags was immersed for 24 h to correct the initial mass
values for leaching. Leaf submersion in the streams
was initiated on November 24, 2008.
Four bags were retrieved (one bag per bar) after
7 days and, thereafter, on dates that roughly corre-
sponded to 20 (t20), 50 (t50), and 70% (t70) loss of the
initial mass. These sampling dates were estimated from
exponential decomposition rates (k) recalculated from
previous sampling data for each experimental site. The
initial mass was considered to be the initial ash-free dry
mass (AFDM) corrected for leaching. After retrieval,
litter bags were placed in individual zip-lock bags and
transported in refrigerated containers to the laboratory,
where they were immediately processed. The leaf
material from each bag was rinsed with stream-filtered
water, and the fauna was separated on a 0.5 mm sieve
and preserved in 70% ethanol for later analysis.
Individuals were identified to family level with a
dissecting microscope, counted, and sorted into func-
tional feeding groups according to Merritt & Cummins,
(1996) and Tachet et al. (2002). At t50, individuals
within each feeding group were dried at 70�C to
constant weight (72 h) for biomass determination.
For aquatic hyphomycete sporulation rate determi-
nations, a set of five leaf disks (12 mm diam.) was cut
with a flamed cork borer from each bag at t20 (see
below). The remaining material was oven-dried
(70�C, 72 h), weighed, and the rest ashed (550�C,
4 h) to determine the ash content. This was done to
calculate the AFDM remaining in the bags by
subtracting the ash content of the dry mass (Menendez
et al., 2003).
Hydrobiologia (2012) 691:135–146 137
123
Ta
ble
1L
oca
tio
nan
dch
arac
teri
stic
so
fre
serv
oir
san
dca
tch
men
tar
eaaf
fect
ing
the
stu
die
dst
ream
s
Lat
itu
de,
N
Lo
ng
itu
de,
E
San
taF
e
4184
70 1
100
282
80 1
100
Fu
iro
sos
4184
00 3
200
283
50 1
100
Val
lfo
rnes
4184
30 1
400
282
00 3
000
Av
enco
4184
70 2
500
281
70 2
100
Sit
eU
pD
ow
nU
pD
ow
nU
pD
ow
nU
pD
ow
n
Alt
itu
de
(m)
11
48
10
87
22
82
14
56
24
60
53
05
20
Rea
chsl
op
e(%
)1
6.1
13
.81
5.8
13
.11
3.3
11
.23
3.9
30
.3
Wid
th(m
)1
2.5
7.8
3.3
3.6
4.9
5.9
7.1
4.1
Flo
w(L
s-1)
15
4.6
±1
23
.65
7.7
±2
9.2
54
.8±
11
.83
6.6
±6
4.4
31
1.3
±4
45
.32
8.6
±1
8.6
46
4.8
±4
16
.44
47
.7±
51
9.6
Dis
tan
ce(m
)2
60
29
01
60
21
04
80
37
12
32
36
0
Bas
in(H
a)3
10
44
04
30
48
07
25
11
60
12
60
15
20
Lan
du
se%
Nat
ive
veg
etat
ion
81
.98
4.3
98
.99
8.8
88
.38
2.7
92
.99
2.6
Aff
ore
stat
ion
4.1
3.8
1.1
1.2
1.0
9.4
00
Far
min
g1
41
0.9
00
10
.78
.67
.17
.4
Wat
erte
mp
erat
ure
(�C
)3
.7–
4.8
a4
.8–
8.5
b6
.4–
9.8
b6
.4–
8.9
b5
.4–
5.9
b5
.6–
10
.6c
4.2
–4
.5a
4.2
–4
.5a
SR
P(l
gP
L-
1)
3.7
±0
.8a
2.8
±0
.7a
2.3
±0
.6a
1.6
±0
.2a
11
.3±
7.5
b1
5.8
±7
.8b
6.0
±0
.3b
6.2
±0
.5b
DIN
(lg
NL
-1)
28
3.6
±1
2.2
a3
19
.4±
36
.4a
10
23
.4±
14
8b
72
8.6
±1
05
.2c
15
8.3
±7
5.4
d6
92
.9±
12
8.5
c8
5.3
±3
4.7
d7
8.6
±9
.9d
Alk
alin
ity
(meq
L-
1)
0.3
3±
0.0
3a
0.3
6±
0.0
3a
0.9
0±
0.0
2b
0.6
3±
0.1
2b
0.6
3±
0.1
4b
0.8
3±
0.0
5b
0.9
7±
0.2
1b
1.0
8±
0.2
4b
Co
nd
uct
ivit
y(l
Scm
-1)
63
±2
a6
6±
4a
19
5±
2b
17
1±
11
b1
14
±1
0b
14
2±
2b
16
8±
18
b1
77
±2
0b
%O
2S
atu
rati
on
10
6±
4a
10
4±
4a
10
0±
3a
99
±1
a1
03
±1
a1
00
±1
a1
02
±1
a9
9±
2a
Gra
nu
lom
etri
cco
mp
osi
tio
n(%
)
[2
5cm
61
.35
0.2
59
.82
1.2
49
.82
1.3
30
.61
5.3
6–
5cm
28
.52
2.4
25
47
.32
5.2
30
.11
9.9
50
.5
\6
cm1
0.2
27
.41
5.2
31
.52
54
8.6
49
.53
4.2
Tre
eca
no
py
cov
er(%
)
Win
ter
38
52
52
65
60
63
39
51
Su
mm
er7
8.7
94
.28
2.1
90
.68
2.5
82
.38
8.7
78
.7
IHF
ind
ex9
29
57
86
17
37
38
67
1
QB
Rin
dex
10
09
01
00
75
10
09
01
00
80
Th
ete
rmD
ista
nce
refe
rsto
lin
ear
dis
tan
ceb
etw
een
the
exp
erim
enta
lre
ach
fro
mth
ed
am(d
ow
nst
ream
)an
dto
the
rese
rvo
irh
ead
wat
er(s
trea
men
d).
Wat
erte
mp
erat
ure
(ran
ge)
,
inst
anta
neo
us
wat
erfl
ow
(mea
n±
SD
),an
do
ther
ph
ysi
coch
emic
ald
escr
ipto
rs(m
ean
±S
E)
of
exp
erim
enta
lsi
tes
are
sho
wn
(n=
5).
Dif
fere
nt
lett
ers
ind
icat
essi
gn
ifica
nt
dif
fere
nce
s(A
NO
VA
,T
uk
eyH
SD
test
,P
\0
.05
)
138 Hydrobiologia (2012) 691:135–146
123
Aquatic hyphomycete sporulation
Leaf disks were incubated in 100 ml Erlenmeyer
flasks with 25 ml filtered stream water (glass fiber
Whatman GF/F filters) on a shaker (60 rpm) for 48 h
at 10�C. The conidial suspensions were decanted into
50 ml centrifuge tubes, flasks rinsed twice with
distilled water, and conidia fixed with 2 ml 37%
formalin and stained with a few drops of Trypan Blue
in lactic acid (ca. 0.05%), to be later counted and
identified. For conidial identification, an aliquot of the
suspension (calibrated depending on the conidial
concentration) was filtered (with opaque Millipore
SMWP nitrocellulose filters 5 lm in pore size). Filters
were mounted on a drop of Trypan Blue in lactic acid
placed on a microscope slide and covered with 25 mm
diameter round cover slips. A quarter of the filter
surface was scanned for conidia, which were identified
and counted with bright field microscopy at least
2509. The counting effort was significantly reduced
with the assistance of voice recognition and Excel data
entry generator software. The leaf disk dry mass was
determined as described above for bulk leaf material.
Sporulation rates were expressed as numbers of
conidia released lg-1 AFDM day-1.
Statistical analyses
Differences in physicochemical variables, between up
and downstream reaches in each stream and between
streams, were tested by a General Linear Model
(GLM, mixed Type III) Nested analysis of variance
(ANOVA) for each variable, with stream location (up/
down) nested within streams. Paired Student’s t tests
were performed for examining differences in tree
canopy cover, QBR, and IHF indices, as well as for
those of flow between up and downstream reaches.
Water temperatures were compared separately with a
Nested ANOVA, considering the daily mean temper-
ature as a replica.
After correcting the leaf litter initial mass for
leaching, decomposition rates were estimated for
comparative purposes by linear regressions of both
ln-transformed (negative exponential model Mt =
M0 * e-kt, where M0 is the initial AFDM, Mt is the
remaining AFDM at time t, and k is the decomposition
rate) and non-transformed data (negative linear model
Mt = M0 – bt, where b is the decomposition rate).
As streams differed in their water temperatures,
decomposition rates were expressed in terms of degree
days (dd) by replacing time (t) by the sum of the mean
daily temperatures accumulated on the sampling
day. Differences in leaf decomposition rates among
streams and between up and downstream (dam)
experimental reaches were assessed by an analysis of
covariance (two-way ANCOVA, test for homogeneity
of slopes) on % AFDM remaining data and using dd as
a covariate.
Comparisons of macroinvertebrate numbers and
biomass as well as of sporulation rates of aquatic
hyphomycetes (at t20) were performed with a Nested
ANOVA with stream location (up/down) nested
within streams.
Relationships between density of macroinverte-
brates, aquatic hyphomycetes, or decomposition rates
and environmental variables were tested by ordinary
least square linear regressions.
Data were arcsine or log-transformed when needed
to ensure normality, and tested for homogeneity of
variance with the Levene test (P [ 0.05) (Legendre &
Legendre, 1998). Subsequent pair-wise comparisons
were performed using Tukey’s Honest Significant
Difference (HSD) comparisons (Zar, 1999). Statistical
calculations were made with the CSS Statistica
package, using the subprogram ANOVA/MANOVA.
Results
Physicochemistry of the stream water
The presence of a dam did not significantly affect
alkalinity, pH, conductivity, oxygen saturation, or
soluble reactive phosphorus (SRP) (Table 1). Water
temperature was not significantly different between up
and downstream reaches except in VF and SF, where it
was higher downstream (F4,152 = 50.4, Tukey HSD,
P \ 0.0001). The existence of the dam did not affect
dissolved inorganic nitrogen concentrations (DIN),
except in FU and VF, where they were respectively
27% lower and 77% higher downstream than upstream.
(F4,24 = 24.7, Tukey HSD P \0.01).
The IHF index was not significantly affected by the
presence of the dam (t3 = 1.42, P [ 0.05), indicating
a similar benthic heterogeneity up and downstream.
No significant differences were observed in stream
flow between the up and downstream reaches
(t3 = 2.3, P [ 0.05), although it was highly variable
Hydrobiologia (2012) 691:135–146 139
123
in the up reaches of the SF and VF streams (Table 1).
The QBR index showed a lower quality of the riparian
vegetation downstream (t3 = 4.18, P \ 0.05) and the
canopy cover also showed significant differences
between up and downstream reaches in winter
(t3 = 4.14, P \ 0.05) but not in summer (t3 = 0.62,
P [ 0.05), in relation to the existence of taller trees
downstream, mainly in SF and AV (Table 1).
Litter decomposition
After 24 h leaching, mass loss was 14.6 ± 1%
(mean ± SE) and in general the decomposition
dynamics were better adjusted to a linear (R2 =
0.86–0.99) than to an exponential model (R2 =
0.85–0.96). Litter decomposition differed signifi-
cantly between the up and downstream reaches
(F1,148 = 4.08, P \ 0.05) except at AV (Fig. 1).
However, the effect of the dam differed between
streams. Decomposition rates were significantly
slower downstream in FU and SF (ca. 34% reduction
in the linear rates; Tukey HSD, P \ 0.005); but in VF
a significantly faster rate was observed (with a 49.2%
increase in linear rates downstream; Tukey HSD, P \0.05). A significant relationship was found between
the linear decomposition rate in % AFDM day-1 and
the average water temperature during the study period
(R2 = 0.62, P \ 0.05).
Macroinvertebrates
Nested ANOVA revealed no significant differences in
macroinvertebrate abundance between up and down-
stream reaches. The percentage of shredders in
relation to total macroinvertebrates found inside litter
bags was inversely correlated with the average
(n = 5) water instantaneous flow at all sampling sites
along the experiment (R2 = 0.84, P \ 0.001, n = 8;
Fig. 2).
Shredder abundance was not significantly different
between up and downstream, except in VF (Table 2;
Fig. 3a), where it was higher downstream, where the
most abundant taxon Echinogammarus, which com-
prised 96% of shredders. The presence of a dam did
not affect the shredder biomass except in FU (Table 2;
Tukey HSD, P \ 0.05; Fig. 3b). No significant rela-
tionship was found between the lineal decomposition
rate and either the biomass or the abundance of
macroinvertebrates or shredders. However, the pro-
portion of shredders relative to total macroinverte-
brates was significantly correlated (R2 = 0.72,
P \ 0.001, n = 8) with the lineal decomposition rate
(Fig. 4a) and with the average DIN concentration in
water (R2 = 0.92, P \ 0.001, n = 8) (Fig. 4b). The
taxonomic richness (at family level) per bag ranged
from 2 to 14 taxa and no significant effect of the
presence of the dam was observed.
0
0.1
0.2
0.3
0.4
0.5
0.6
Santa Fe Avencó Fuirosos Vallfornés
Leaf
mas
s lo
ss (
% A
FD
M d
d-1
) UP
DOWN
**
*
Fig. 1 Alder litter linear decomposition rates (% AFDM dd-1)
up and downstream reaches of dams on the streams studied
(mean of four replicates ± SE). Significant Post hoc differences
are denoted by an asterisk (*) (two-way ANCOVA, P \ 0.05)
y = -0.073x + 41.6
R 2 = 0.84, P <0.00150
60
10
20
30
40
% S
hred
ders
00 200 400 600
Average flow L s-1
Fig. 2 Relationship between total shredders relative to total
macroinvertebrate abundance and average instantaneous flow
on the sampling dates
140 Hydrobiologia (2012) 691:135–146
123
Aquatic hyphomycete sporulation
The sporulation rate was not significantly affected by
the presence of the dam except in FU (Table 2; Tukey
HSD, P \ 0.005), where it was higher downstream
(average 3.2 conidia lg-1 AFDM d-1; Fig. 5a). No
significant effect of the dam on aquatic hyphomycete
richness was observed except in VF (Table 2; Tukey
HSD, P \ 0.0005), showing the highest value down-
stream (Fig. 5b). A total of 23 species were identified
(Table 3), and the dominant taxon was Flagellospora
curvula except at the downstream SF site, where the
most abundant one was Tetrachaetum elegans.
Discussion
The presence of a dam along the headwater streams
studied here influenced some physical and chemical
characteristics of water, thereby altering fluvial con-
tinuity, supporting in part our hypotheses. This
discontinuity was most obvious in VF and was also
noticed in SF and FU, although to a lesser extent. Our
results corroborate that the presence of small reser-
voirs in headwater streams alters the processing rate of
organic matter downstream, although no effect was
evident on the abundance and diversity of macroin-
vertebrates associated with leaf litter. Numerous
authors have shown that leaf litter decomposition
rates are higher where macroinvertebrates, and espe-
cially shredders, are abundant (e.g., Short & Ward,
1980). An interesting fining of the present study is that
a high proportion of shredders to total macroinverte-
brates instead of high shredder abundance was asso-
ciated with high decomposition rates. The proportion
of shredders is itself related to the DIN concentration.
Much research has suggested that the activity and
biomass of heterotrophic microorganisms may increase
when nutrient availability is high, which itself can
accelerate litter decomposition (e.g., Kaushik & Hynes,
1971; Suberkropp et al., 2010). Although we have not
quantified microbial biomass, our results suggest that N
availability enhanced detrital colonization by fungi and
bacteria, with a concomitant rise in detritus palatability
to shredders (as reported by Gessner & Chauvet,
1994), leading to accelerated decomposition rates.
Table 2 Summary of statics for GLMs comparing shredder
abundance, richness and biomass and aquatic hyphomycetes
sporulation rate and richness
Source d.f. F-value P-value
Shredder abundance (bag-1)
Stream 3 1.3 0.26
Up-down (stream) 4 3.2 0.02
Time (stream * up-down) 23 1.3 0.15
Shredder biomass (mg bag-1)
Stream 3 0.9 0.41
Up-down (stream) 4 3.4 0.02
Sporulation rate (spores lg-1 d-1)
Stream 3 12.2 0.03
Up-down (stream) 4 21.1 0.001
Hyphomycete richness
Stream 3 29.8 0.000001
Up-down (stream) 4 13.5 0.000007
0
10
20
30
40
50
60
70
Santa Fe Avencó Fuirosos Vallfornés
Shr
edde
rs b
iom
ass
mgD
M/b
ag UP
DOWN
0
10
20
30
40
50
60
70
80
Santa Fe Avencó Fuirosos Vallfornés
Shr
edde
r ab
unda
nce,
ind/
bag
UP
DOWN
b
*
*
a
Fig. 3 Shredder, abundance (a) and biomass (b), inside litter
bags at t50 (mean of four replicates ± SE). Significant Post hoc
differences are denoted by an asterisk (*) (two-way ANCOVA,
P \ 0.05)
Hydrobiologia (2012) 691:135–146 141
123
The negative relationship observed between water flow
and the proportions of shredders in leaf bags throughout
the study suggests also that the regulation of flow
intensity and its fluctuations caused by the presence of
the dam could affect the accumulation of litter in the
channel, which would itself affect shredder communi-
ties (Wallace et al., 1997; Dewson et al., 2007).
Armitage (1976) argued that community changes
downstream from the dams are not only affected by
flow regulation but also by the effect of the hydrological
variability in the transport of particulate material and
substrate stability. With regard to this, some studies
have observed a positive relationship between fluvial
discharge and the travel distance of CPOM (e.g., Jones
& Smock, 1991; Webster et al., 1999; Brookshire &
Drive, 2003).
In our Mediterranean streams, as opposed to those
in the Oceanic climatic region (Mendoza-Lera et al.,
2010), the presence of a dam did not affect the quality
of the benthic habitat. However, it did affect the
composition of the riparian vegetation, as shown by
the lower values in the QBR index downstream from
the dam, mainly characterized by the dominance of
one tree species and the presence of taller trees.
Gonzalez del Tanago & Garcıa de Jalon, (2007) have
also shown an increase in high-density alder and
poplar groves below dams, as a result of a reduction in
storm waters resulting from fluvial regulation. These
discrepancies between regulated Oceanic versus Med-
iterranean headwater streams could be related with the
different climatic conditions, characterized by a more
continuous yearly rainfall in the Basque Country.
y = 0.058x + 6.1775
R 2 = 0.92, P< 0.0005
0
10
20
30
40
50
60
70
0 500 1000 1500
DIN, µgN L-1
% S
hred
ders
y = 0.0036x + 0.1898
R 2 = 0.72, P< 0.05
0
0.1
0.2
0.3
0.4
0.5
0 20 40 60 80
% Shredders
b, %
AF
DM
dd
-1
a
b
Fig. 4 Relationship between percentage of shredders at t50 and
linear decomposition rate (% AFDM dd-1) (a) and average
dissolved inorganic nitrogen (DIN) concentration found along
the experiment in the studied reaches (b)
8
9
1 d-1
UP
a
2
3
4
5
6
7
ulat
ion
rate
spo
res
g-
DOWN
*
18
0
1
Spo
ru
b
8
10
12
14
16
omyc
etes
ric
hnes
s UP
DOWN
*
0
2
4
6
Santa Fe Avencó Fuirosos Vallfornés
Santa Fe Avencó Fuirosos Vallfornés
Aqu
atic
hyp
ho
µ
Fig. 5 Aquatic hyphomycete sporulation rates (a) and average
species richness (b) in alder leaf litter at t20 (mean of four
replicates ± SE). Significant Post hoc differences are denoted
by an asterisk (*) (two-way ANCOVA, P \ 0.05)
142 Hydrobiologia (2012) 691:135–146
123
Comparing the shredder community found in our
study with that reported by Mendoza-Lera et al. (2010)
in the Basque Country, we found almost a total
absence (except in VF) of crustaceans of the family
Gammaridae in our streams, probably due to the
drying up of the channel in summer, as commonly
observed in Mediterranean streams, which may be
enhanced by the presence of dams. The latter authors
and Bredenhand & Samways, (2009) pointed out that
shredders of the family Nemouridae are especially
sensitive to the presence of dams, as seen by their
diminishing numbers or even by their total absence
downstream. Our study partly corroborates this,
showing a density reduction of 12–90% of nemourid
stoneflies downstream, except in FU, where the
abundance increased by up to 80% downstream. In
contrast, the largest shredders (mainly trichopterans of
the family Limnephilidae, data not displayed) in this
stream were observed upstream. These results suggest
that, besides shredder abundance, low processing rates
can be also caused by the absence of key shredder
species or taxa (Pomeroy et al., 2000).
The effect of the presence of small reservoirs on
litter decomposition in our study varied depending on
reservoir typology. In the case of SF, AV, and FU, the
dams are built of stone and concrete, and the stored
water runs along an upper spillway. In this case, the
interruption of fluvial continuity reduces leaf litter
decomposition rates downstream by 20–37%. Men-
doza-Lera et al. (2010) reported similar results in the
Basque Country for headwater streams regulated by
small dams, although alterations in the physicochemical
Table 3 Percentage contribution of aquatic hyphomycete species in each experimental stream (based on the total number of conidia,
after a 20% mass loss in the litter bags (t20)
Species VF U VF D SF U SF D AV U AV D FU U FU D
Filosporella cf. annelidica Crane and Shearer 0.05 0.05 2.02 1.92 0.08
Alatospora acuminata Ingold: subulate morphotypea 1.96 2.66 4.90 0.20 0.04 0.99 0.15
Alatospora acuminata Ingold: pulchelloid morphotypea 1.39 0.13 1.04 0.12 0.10
Alatospora acuminata Ingold (sensu neotype) 0.34 0.13 0.88
Alatospora pulchella Marvanova 0.41 0.09
Articulospora tetracladia Ingold 0.34 0.22 2.00 2.90 21.06 0.16
Clavariopsis aquatica de Wild. 0.58 0.60 2.11 0.09
Clavatospora longibrachiata Ingold 1.35 1.94 0.75 0.13 0.08
Flagellospora curvula Ingold 98.07 61.10 80.48 32.12 96.04 99.5 57.63 93.06
Geniculospora inflata (Ingold) Marvanova and S.V. Nilsson 0.32 1.92
Goniopila/Margaritisporaa 0.04 0.32 0.25 1.59
Heliscella stellata (Ingold and Cox) Marvanova 2.66 1.02 3.91 0.09 0.14
Lemonniera alabamensis Sinclair and Morgan-Jones 0.10 0.12 0.23 0.56 0.37
Lemonniera aquatica De Wild. 0.2 0.03 1.32
Lemonniera cornuta Ranzoni 0.75 0.62 0.2 0.04 0.17
Lemonniera terrestris Tubaki 0.88 0.91 6.35 2.07 0.22 0.04 0.17 0.10
Lunulospora curvula Ingold 1.09 0.09 3.65 0.04
Mycocentrospora acerina (Hartig) Deighton 0.25 0.11
Stenocladiella neglecta (Marvanova and Descals) 1.61 1.12 3.39 0.10
Tetrachaetum elegans Ingold 25.03 1.66 42.5 0.3 0.1 16.3 4.7
Tetracladium marchalianum De Wild. 0.53 0.07
Tricladium chaetocladium Ingold 0.38 0.59 0.26 0.65 0.13
Tumularia tuberculata (Gonczol) Descals and Marvanova 0.26
Total number of species 8 18 16 17 13 7 10 13
U upstream; D downstream from the dama cf. footnote in table 6 of Pozo et al. (2011)
Hydrobiologia (2012) 691:135–146 143
123
characteristics of the water due to the reservoirs were
negligible. However, the decomposition rates of alder
leaves observed in the Basque Country were half
(0.05–0.17% AFDM dd-1; Mendoza-Lera et al., 2010)
than those in our study (0.17–0.45% AFDM dd-1).
These differences cannot be attributed to shredder
abundance, which varied between 5 and 40 ind bag-1
for both regions. However, the five times higher aquatic
hyphomycete sporulation rates in the Mediterranean
(maxima of 5.71 and 1.34 conidia lg-1 leaf d-1
respectively) could explain this increase in leaf litter
decomposition rates. The composition of the hyphomyc-
ete community was similar in both locations, as F.
curvula and T. elegans were also shown to be dominant
in Mendoza-Lera et al. (2010). These have been
considered pioneer species or fast colonizers of decom-
posing litter (Treton et al., 2004).
The observed effect of the reservoir on alder leaf
litter decomposition in VF was different. In this case,
the dam was constructed with compacted earth and,
besides having an upper spillway, its release was
hypolimnetic, which would explain the higher nutrient
concentrations, mainly of N, downstream. Nutrient
availability may affect organic matter processing rates
(Pozo, 1993), and numerous studies have shown that
nutrient enrichment increases decomposition rates
(e.g., Ferreira et al., 2006; Greenwood et al., 2007).
The higher decomposition rate downstream at VF was
probably due to the positive effect of the dam on DIN
concentration and water temperature. Casas et al.
(2000) found that nutrient loading and temperature
increased downstream from deep-release reservoirs,
which were related to the usually high nutrient
concentrations in the hypolimnion and to the thermal
regulation by the reservoir. Moreover, in a previous
study on a headwater stream in South Africa (Breden-
hand & Samways, 2009), a 2�C rise was also detected
downstream, and this was mainly attributed to a drop
in water flow. In our case, the water temperature at VF
was 2.3�C higher downstream, and the reduction in
water flow was of 81% due to fluvial regulation. All of
these factors may increase both the activity and the
richness of aquatic hyphomycetes as well as of
detritivores, as we observed in VF downstream, and
thus accelerate the decomposition rates downstream
(Short & Ward, 1980; Ferreira et al., 2006; Dewson
et al., 2007). The effect of aquatic hyphomycete
richness on litter breakdown by shredders has already
been considered by Lecerf et al. (2005), who suggest
that a species-rich assemblage of aquatic hyphomy-
cetes may enhance resource quality for shredders such
as amphipods, as these tend to eat hydrolyzed plant
tissue, rather than mycelia (Barlocher & Kendrick,
1975; Graca et al., 1993).
To summarize, we herein show that the presence of
small dams in Mediterranean headwater streams
affects the rates of leaf litter decomposition in three
of the four streams studied. This depends on the type
of dam, decreasing downstream for reservoirs that
empty superficially, as reported by Mendoza-Lera
et al. (2010), and increasing when release is hypolim-
netic. The interruption of fluvial continuity and the
modification of flow may influence riparian vegeta-
tion, N availability, and water temperature, which
have the potential of influencing macroinvertebrate
shredders and aquatic hyphomycete community struc-
ture. Flow reductions may reduce ecological resilience
in streams (Davey & Kelly, 2007) to hydroclimatic
extremes or other anthropogenic stressors (Daufresne
et al., 2007). The effect of dams through reducing
decomposition rates downstream under Oceanic cli-
mate conditions in the Basque Country (20% in
Mendoza-Lera et al., 2010) was slightly lower than
that observed in our study (20–37%). In Mediterra-
nean headwaters such as those studied here, the effect
of small dams on the structure and functioning of
streams will even be more pronounced at an annual
scale due to the diminished summer flow rates and the
deleterious effect of droughts on the benthic commu-
nities; the effect of water regulation would thus be
more evident than in the more temperate Oceanic
climates. A more detailed experiment conducted
during summer would confirm this hypothesis. The
effects of small dams on stream biota and processes in
headwater streams would be similar to those attributed
to the predicted climatic change and related stressors.
Acknowledgments This study was funded by the Spanish
Ministry of Education and Science (projects CGL2007-462
66664-C04 and CGL2011-30474-C02). We are grateful to
Milagros Barcelo, Maria Tauler, Cristina Moragues, Aina
Martınez, and Antoni Gili for help with field and laboratory
work. We thank the ‘‘Parc Natural del Montseny, Diputacio de
Barcelona’’ for sampling permits. We also thank Nuria Bonada
and two anonymous referees for helpful comments on the
article.
144 Hydrobiologia (2012) 691:135–146
123
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