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The main purpose of our work was to elucidate factors responsible for the geographical differences in leaf-litter decomposition rates in Spanish oligotrophic headwater streams. Decomposition experiments with alder (Alnus glutinosa) leaf litter were carried out in 22 headwater streams in 4 different climatic regions across the Iberian Peninsula (Cornisa Cantábrica, Cordillera Litoral Catalana, Sierra de Guadarrama, and Sierra Nevada). Streams that were similar in size, flowed mainly over siliceous substrate in catchments with scarce human settlements and activities, and fell within a range of low nutrient concentrations were chosen in each region. Breakdown rates were regionally variable and were low (0.109–0.198% ash-free dry mass [AFDM]/degree day [dd]) in the Cornisa Canta´brica, the most mesic and Atlantic region, and high (0.302–0.639% AFDM/dd) in Sierra de Guadarrama, one of the coldest and most inland areas. Temperature was not the determining factor affecting differences in breakdown rates among regions, and breakdown rates were not related to concentrations of dissolved nutrients. However, microbial reproductive activity (sporulation rates) was significantly correlated with dissolved P concentration. Breakdown rates were explained better by presence and feeding activities of detritivores than by decomposer activity. Incorporation of breakdown rates in assessment schemes of stream ecological status will be difficult because leaf processing does not respond unequivocally to environmental factors when climatic regions are considered. Thus, regional adjustments of baseline standards in reference conditions will be required.
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Leaf-litter decomposition in headwater streams: a comparison of the
process among four climatic regions
Jesu
´sPozo
1,10
,Jesu
´s Casas
2,11
, Margarita Mene
´ndez
3,12
, Salvador Molla
´
4,13
,
Inmaculada Arostegui
5,14
, Ana Basaguren
1,15
, Carmen Casado
4,16
,
Enrique Descals
6,17
, Javier Garcı
´a-Avile
´s
7,18
, Jose
´M. Gonza
´lez
8,19
,
Aitor Larran
˜aga
1,20
, Enrique Lo
´pez
2,21
, Mirian Lusi
2,22
, Oscar Moya
6,23
,
Javier Pe
´rez
1,24
, Tecla Riera
3,25
,Neftalı
´Roblas
9,26
,AND M. Jacoba Salinas
2,27
1
Dpto. Biologı
´a Vegetal y Ecologı
´a, F. Ciencia y Tecnologı
´a, UPV/EHU, Apdo. 644, 48080 Bilbao, Spain
2
Dpto. Biologı
´aVegetalyEcologı
´a, Universidad de Almerı
´a, Ctra. Sacramento s/n, La Can
˜ada, 04120 Almerı
´a, Spain
3
Dpto. Ecologı
´a, F. Biologı
´a, Universidad de Barcelona, Avda. Diagonal 645, 08028 Barcelona, Spain
4
Dpto. Ecologı
´a, F. Ciencias, Universidad Auto
´noma de Madrid, Darwin 2, 28049 Madrid, Spain
5
Dpto. Matema
´tica Aplicada e Investigacio
´n Operativa, F. Ciencia y Tecnologı
´a, UPV/EHU, Apdo. 644,
48080 Bilbao, Spain
6
Instituto Mediterra
´neo de Estudios Avanzados, IMEDEA (CSIC), Miquel Marque
´s 21, 07190 Esporles, Mallorca, Spain
7
Dpto. de Ecologı
´a, F. de Ciencias Biolo
´gicas, Universidad Complutense de Madrid, C/ Jose
´Antonio
Novais, 2, 28040 Madrid, Spain
8
Dpto. Biologı
´a y Geologı
´a, Universidad Rey Juan Carlos, C/ Tulipa
´n, s/n, 28933 Mo
´stoles, Madrid, Spain
9
Centro de Investigaciones Ambientales de la Comunidad de Madrid, Ctra. M-607 km 20,
28760 Tres Cantos, Madrid, Spain
Abstract. The main purpose of our work was to elucidate factors responsible for the geographical
differences in leaf-litter decomposition rates in Spanish oligotrophic headwater streams. Decomposition
experiments with alder (Alnus glutinosa) leaf litter were carried out in 22 headwater streams in 4 different
climatic regions across the Iberian Peninsula (Cornisa Canta
´brica, Cordillera Litoral Catalana, Sierra de
Guadarrama, and Sierra Nevada). Streams that were similar in size, flowed mainly over siliceous substrate in
catchments with scarce human settlements and activities, and fell within a range of low nutrient
concentrations were chosen in each region. Breakdown rates were regionally variable and were low (0.109–
0.198%ash-free dry mass [AFDM]/degree day [dd]) in the Cornisa Canta
´brica, the most mesic and Atlantic
region, and high (0.302–0.639%AFDM/dd) in Sierra de Guadarrama, one of the coldest and most inland
areas. Temperature was not the determining factor affecting differences in breakdown rates among regions,
and breakdown rates were not related to concentrations of dissolved nutrients. However, microbial
reproductive activity (sporulation rates) was significantly correlated with dissolved P concentration.
Breakdown rates were explained better by presence and feeding activities of detritivores than by decomposer
activity. Incorporation of breakdown rates in assessment schemes of stream ecological status will be difficult
because leaf processing does not respond unequivocally to environmental factors when climatic regions are
considered. Thus, regional adjustments of baseline standards in reference conditions will be required.
Key words: leaf litter, decomposition, headwater streams, invertebrates, fungi, eutrophication, Spain.
19
jmgonzalez@escet.urjc.es
20
aitor.larranagaa@ehu.es
21
emlopez@ual.es
22
mirianlusi@hotmail.it
23
oscarmoyamesa@gmail.com
24
javier.perezv@ehu.es
25
triera@porthos.bio.ub.es
26
neftali.roblas@madrid.org
27
mjsalina@ual.es
10
E-mail addresses: jesus.pozo@ehu.es
11
jjcasas@ual.es
12
mmenendez@ub.edu
13
salvador.molla@uam.es
14
inmaculada.arostegui@ehu.es
15
ana.basaguren@ehu.es
16
c.casado@uam.es
17
ieaedc@uib.es
18
ciam03@bio.ucm.es
J. N. Am. Benthol. Soc., 2011, 30(4):935–950
2011 by The North American Benthological Society
DOI: 10.1899/10-153.1
Published online: 6 September 2011
935
Fluvial ecosystems have been impaired by and
continue to deteriorate because of a wide array of
human impacts of varying magnitude, ranging from
severe alterations with conspicuous effects to subtle and
cryptic modifications. Headwater streams, which rep-
resent .95%of the total number of stream segments
(Wallace and Eggert 2009), are less affected by humans
than other water bodies and are crucial reservoirs of
biodiversity. A critical step in preserving or improving
the integrity of a river (sensu Karr 1991) is to assess its
ecological status adequately with methods sensitive
enough to determine the consequences of human effects
or to guarantee the success of eventual restoration
actions. Traditional evaluations of river health rely on
physicochemical characteristics (Mu
¨ller et al. 2008, Fu et
al. 2009) or on structural properties of community
diversity and composition of several taxonomic groups,
mainly macroinvertebrates, algae, or macrophytes
(Barbour et al. 1999, De Jonge et al. 2008, Demars and
Edwards 2009).
Recently, ecologists have advocated use of func-
tional components of the ecosystem to evaluate river
health and have argued that, in some cases, stressors
might change function but not structure (Moulton
1999, Bunn and Davies 2000, Brooks et al. 2002, Gessner
and Chauvet 2002, Riipinen et al. 2009, Young et al.
2008). Moss (2008) pointed out that ecological quality is
measured accurately by paying attention primarily to
the intactness of several fundamental characteristics of
ecosystem function rather than to secondary character-
istics, such as particular concentrations of substances
or species composition.
This controversy of structural vs functional indica-
tors seems to be implicit in the European Water
Framework Directive (WFD) (2000/60/EC). An appar-
ent contradiction, noticed by Moss (2008), exists
between the definition of a high ecological status of
aquatic ecosystems and the instructions given in the
WFD on the way that ecological status is to be
determined or improved. A high ecological status
embraces fundamental characteristics (ecosystem func-
tion), but the instructions encourage focus on second-
ary details (mainly taxonomic structure) and, hence,
may undermine the fundamental improvement of
aquatic ecosystems that was intended (Moss 2008).
Leaf-litter decomposition in streams is a functional
ecosystem variable that integrates the activity of
several phylogenetic groups (Gessner and Chauvet
2002, Young et al. 2008). The rate of leaf-litter
decomposition depends on natural factors, such as
climate, geology, altitude, and latitude, and responds
strongly to changes in environmental variables (e.g.,
temperature, pH, dissolved O
2
, nutrients, sediments,
riparian vegetation) caused by anthropogenic distur-
bance (Webster et al. 1995, Molinero et al. 1996, Pozo
et al. 1998, Niyogi et al. 2003, Elosegi et al. 2006,
Sampaio et al. 2008).
Eutrophication is one of the most widespread
human effects on inland waters (Withers and Jarvie
2008). Studies on stream eutrophication generally
demonstrate that dissolved nutrients enhance decom-
position rates of leaf litter by increasing microbial
activity (e.g., Suberkropp and Chauvet 1995, Gulis and
Suberkropp 2003, Greenwood et al. 2007), at least
under moderate nutrient enrichment. However, the
effects on macroinvertebrate colonization and leaf-
litter consumption seem to be more variable, which
could be a result of the community variability between
regions or a possible response to other pollutants in
eutrophic streams (e.g., Pascoal et al. 2003). Further-
more, the effect of eutrophication on stream ecosystem
processes can depend on factors other than P or N
supplies, such as temperature and flow regimes,
substrate, and C supply (Dodds 2007, Withers and
Jarvie 2008), which may vary naturally within and
across regions (Casas et al. 2006). Therefore, natural
variation may hinder the application of functional
indices aimed at comparing the effect of eutrophication
across different geographical and climatic settings.
According to Karr and Chu (1999), an understanding of
the baseline of natural variation is the foundation for
precise assessment of change caused by humans.
Attempts have been made to evaluate ecosystem
functioning based on leaf decomposition across large
geographical areas (Irons et al. 1994, Young et al. 2004,
Lecerf et al. 2007, McKie et al. 2008, Woodward 2009,
Hladyz et al. 2010, Pe
´rez et al. 2011). The aim of our
study was to compare leaf-litter processing in small
headwater streams slightly affected by nutrient en-
richment among 4 different geographic and climatic
areas of the Iberian Peninsula (Cornisa Canta
´brica,
Cordillera Litoral Catalana, Sierra de Guadarrama, and
Sierra Nevada). We hypothesized that: 1) large-scale
abiotic conditions (especially temperature) would
influence biotic contributors to leaf breakdown and,
therefore, differences in decomposition rates among
regions, and 2) leaf-litter processing would respond
positively to dissolved nutrients through enhancement
of microbial activity.
Methods
Study sites
The study was conducted in 22 low-order streams
in the Iberian Peninsula: 6 in the north (Cornisa
Canta
´brica [CC]), 4 in the northeast (Cordillera Litoral
Catalana [CLC]), 7 in the center (Sierra de Guadar-
rama [SG]), and 5 in the south (Sierra Nevada [SN])
936 J. POZO ET AL. [Volume 30
(Fig. 1, Table 1). In each area, streams were similar in
size and flowed mainly over siliceous substrate
in catchments with scarce human settlements and
activities (Table 1). Annual precipitation and mean
temperature varied among regions from 310 to
923 mm and from 9.0 to 16.4uC, respectively, and
changed with altitude (Table 1). Streams from CC and
CLC were more mesic than those of SG and SN, which
were at higher altitudes. The Gorzynski Continental-
ity Index (GCI) was used as a measure of continen-
tality (i.e., climatic gradient) of the geographic areas
and for comparative purposes. It is calculated as GCI
=1.7(Mi – mi)/sin(Lat) – 2.4, where Mi and mi are the
highest and the lowest mean monthly temperatures
(uC), respectively, and Lat is latitude in degrees
(www.globalbioclimatics.org). Values were: 7.7 (CC),
17.1 (CLC), 23.6 (SG), and 32.0 (SN). Streams draining
larger catchments were chosen to avoid temporary
streams in the drier regions. Streams differed in
catchment area, but most streams had a mean channel
width ,5 m (Table 1).
Groups of streams spanned a range of low
dissolved nutrient concentrations (particularly P,
,50 mgPO
4
-P/L) in each area. Streams within this
low and narrow eutrophication gradient were diffi-
cult to locate in SG and SN because of the sharp
transition from oligotrophic to severe eutrophic
conditions caused by organic pollution (wastewater).
Therefore, in these areas, only streams with low
nutrient contents were sampled. As a consequence,
nutrient gradients in these areas were lower and
narrower than in the other 2 areas. Dispersed human
settlements and extensive farming in some areas in
northern Spain (CC, CLC) allowed us to meet the
required low nutrient-enrichment gradients.
Water variables
Water temperature was monitored continuously
with ACR Smart-Button (ACR Systems Inc., Surrey,
British Columbia) or HOBO Pendant (Onset Comput-
er Corporation, Bourne, Massachusetts) temperature
loggers throughout the study period (autumn–winter
2007–2008) in all streams. Conductivity, pH, and
dissolved O
2
(WTW multiparametric sensor) were
measured in situ, and water samples were taken for
nutrient analyses on each sampling date (n=6).
Nutrient analyses were done on water filtered
through precombusted glass-fiber filters (Whatman
GF/F). NO
3
2
concentration was determined by ion
chromatography (COMPACT IC1.1; Metrohm, Her-
isau, Switzerland) or with the sodium salicylate
method (Monteiro et al. 2003). NH
4+
was measured
with the manual salicylate method (Krom 1980),
NO
2
2
with the sulphanylamide method, soluble
reactive P (SRP) with the molybdate method, and
alkalinity through titration to an end pH of 4.5 (APHA
2005).
Litter bags and decomposition
Alder (Alnus glutinosa (L.) Gaertner) leaves were
used as a standard substrate to measure decomposi-
tion rates. All leaf litter used was collected in CC to
prevent local differences in the initial quality of
materials (Lecerf and Chauvet 2008b). Leaves were
collected from the forest soil just after abscission in
autumn 2007 and air-dried to constant mass. Five
grams (60.25) of alder leaves were weighed,
moistened (spray), and enclosed in mesh bags (15 3
20 cm, 5-mm mesh). Leaf bags (25 in each stream)
were tied with nylon lines to iron bars driven into the
stream bed along 50-m reaches. Extra sets of 5 bags
were immersed in the streams for 24 h and used to
correct the initial mass values for leaching. Such a
correction is made to better describe processing
dynamics once labile compounds have disappeared
from leaves (Suberkropp and Chauvet 1995, Ferreira
et al. 2006). Leaf incubation was initiated in late
autumn (November–December) 2007 to coincide with
the seasonal peak in leaf fall.
Five bags were retrieved after 7 d (t7) and at dates
that roughly corresponded to losses of 20 (t20), 35
(t35), 50 (t50), and 70%(t70) of the initial mass, as
estimated from exponential decomposition rates (k)
recalculated from previous data at each experimental
FIG. 1. Locations of the 4 study regions in the Iberian
Peninsula: Cordillera Canta
´brica (CC), Cordillera Litoral
Catalana (CLC), Sierra de Guadarrama (SG), and Sierra
Nevada (SN).
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 937
TABLE 1. Location, mean annual climate variables, catchment area, and landuse characterization of studied streams. Absolute minimum and maximum values
for each variable are in bold. Discharge values are ranges for the period of study. CC =Cornisa Canta
´brica, CLC =Cordillera Litoral Catalana, SG =Sierra de
Guadarrama, SN =Sierra Nevada.
Site Latitude Longitude
Altitude
(m asl)
Temperature
(uC)
Precipitation
(mm)
Catchment
area (ha)
Land use (%)
Channel
width
(m)
Riparian
tree
cover
(%)
Upstream
channel
slope (%)
Discharge
(L/s)
Native
vegetation
Affor-
ested
Agricul-
ture
CC1 42u59
9
59.64
0
N2u52
9
59.92
0
W415 11.1 923 400 96.0 4.0 0.0 4.2 97.5 11.7 20–54
CC2 43u10
9
12.72
0
N2u53
9
26.30
0
W100 14.0 873 280 9.4 81.6 9.0 3.3 95.3 8.8 14–47
CC3 43u07
9
09.84
0
N2u54
9
34.49
0
W150 12.5 850 504 4.8 89.5 5.8 3.7 98.2 14.5 11–110
CC4 43u06
9
16.92
0
N2u54
9
21.17
0
W165 12.5 850 285 0.6 83.5 15.9 3.6 94.3 11.0 4–57
CC5 43u08
9
52.8
0
N2u50
9
0.46
0
W145 14.0 873 534 1.1 83.3 15.6 3.9 89.3 9.0 23–164
CC6 43u08
9
59.28
0
N2u51
9
0.36
0
W125 14.0 873 237 1.2 88.3 10.5 3.6 88.1 11.9 4–71
CLC1 41u29
9
38.08
0
N2u16
9
15.06
0
E495 11.4 611 1165 71.5 26.8 1.7 5.1 97.3 18.0 18–55
CLC2 41u27
9
54.18
0
N2u16
9
41.27
0
E1122 11.4 611 241 91.3 5.5 3.2 7.4 92.0 16.5 13–25
CLC3 41u27
9
44.82
0
N2u09
9
48.49
0
E445 11.9 611 1420 88.2 3.2 8.6 6.8 91.6 8.1 212
CLC4 41u27
9
27.07
0
N2u09
9
40.25
0
E446 11.9 611 530 82.7 0.0 17.3 4.8 79.7 14.6 8–15
SG1 40u52
9
19.20
0
N3u06
9
38.91
0
W1380 9.0 772 178 48.9 36.7 12.3 2.5 81.1 27.5 2–72
SG2 40u29
9
13.92
0
N3u05
9
24.18
0
W1300 9.0 772 173 53.1 37.8 7.9 1.9 82.0 24.7 3–21
SG3 40u46
9
37.92
0
N4u0
9
40.58
0
W1390 9.0 772 175 19.7 80.5 0.0 2.3 66.6 29.2 13-26
SG4 40u54
9
59.04
0
N4u07
9
00
0
W1300 9.0 772 803 5.2 93.4 0.0 1.4 90.0 17.1 12–44
SG5 40u46
9
23.16
0
N3u90
9
66
0
W1220 9.0 772 1666 7.1 16.4 6.9 11.2 64.1 13.6 66406
SG6 40u47
9
15.00
0
N3u83
9
25
0
W1190 9.0 772 1129 26.1 57.0 0.9 9.4 83.4 20.0 24–208
SG7 40u34
9
58.80
0
N3u99
9
11
0
W1400 9.0 772 532 30.7 45.1 0.6 3.9 86.0 21.9 27–111
SN1 36u34
9
55.56
0
N3u11
9
08
0
W1130 16.4 456 4350 61.8 22.6 14.6 4.7 80.3 12.5 35–263
SN2 36u35
9
31.20
0
N3u09
9
00
0
W1120 16.4 456 4174 64.6 23.9 11.5 3.8 50.0 10.8 50–160
SN3 37u02
9
38.04
0
N3u00
9
40.58
9
W1680 16.4 456 1243 45.9 46.9 6.4 2.2 31.6 12.2 23–62
SN4 37u06
9
56.88
0
N3u09
9
03.93
0
W1316 12.9 310 1850 99.0 0.0 1.0 2.8 67.1 14.7 64–91
SN5 37u05
9
08.23
0
N3u02
9
32.89
0
W1460 12.9 310 1920 52.1 47.8 0.1 2.9 81.1 14.0 48–125
938 J. POZO ET AL. [Volume 30
site (Mendoza-Lera et al. 2010). t70 was achieved
between 46 (SG) and 113 (CLC) d. Initial mass refers
to initial ash-free dry mass (AFDM) corrected for
leaching. After retrieval, litter bags were placed in
individual plastic bags and transported in refriger-
ated containers to the laboratory where they were
processed immediately. Leaf material from each bag
was rinsed with filtered stream water, and the fauna
and mineral particles were separated from the leaf
litter on a 200-mm sieve. Only fauna from t50
samplings, which generally coincided with coloniza-
tion peaks in alder leaves (Hieber and Gessner 2002),
were preserved in 70%ethanol for later analysis.
Invertebrates were identified to family level under
a dissecting microscope, counted, and sorted into
functional feeding groups (the most representative for
the family) according to Merritt and Cummins (1996)
and Tachet et al. (2002). Fungal sporulation rate was
determined at t20 (2–3 wk after immersion), which
often coincides with the peak of conidial production
on alder leaves (Pascoal and Ca
´ssio 2004), from 1 set
of 5 leaf disks (12-mm diameter) punched from each
bag with a cork borer (see below). The remaining leaf
material was, as on the other sampling dates, oven-
dried (70uC, 72 h) and weighed. A portion was used
for nutrient analyses, and the rest was combusted
(550uC, 4 h) to determine AFDM.
Leaf material for nutrient analyses (C, N, and P)
was ground into fine powder (1-mm screen). C and
N were determined with a Perkin Elmer series II
CHNS/O elemental analyser (Perkin Elmer, Norwalk,
Connecticut). P was determined spectrophotometri-
cally after mixed acid digestion (molybdenum blue
method; Allen et al. 1974). Results were expressed as
%leaf-litter dry mass (DM) as it was analyzed and as
molecular elemental ratios (C:N, C:P, and N:P).
Sporulation of aquatic hyphomycetes (AH)
Leaf disks from each bag from t20 were incubated
in 100-mL Erlenmeyer flasks with 25 mL of filtered
stream water (Whatman GF/F) on a shaker (60 rpm)
for 48 h at 10uC. The resulting conidial suspensions
were transferred into 50-mL centrifuge tubes and
fixed with 2 mL of 37%formalin. An aliquot of the
suspension was filtered (Millipore SMWP 5-mm pore
size) for conidial identification and counting. Each
filter was stained with trypan blue in lactic acid
(0.05%), and conidia were identified (after Gulis et al.
2005 and species description protologues) and count-
ed under a microscope at 2503. Counting effort was
reduced with the assistance of voice recognition and
Excel data-entry generator software developed by
one of us (OM). Leaf-disk DM was determined as
described above for the bulk leaf material. Sporulation
rates were expressed as number of conidia produced
per mg leaf DM per day of in vitro incubation time.
Statistical analyses
The relationship between %AFDM remaining
(response variable) and elapsed time (predictor
variable) was fitted to linear (M
t
=M
0
2bt) and
exponential models (M
t
=M
0
e
2kt
), where M
0
is the
initial AFDM corrected for leaching, M
t
is the
remaining AFDM at time t, and bis the linear and k
the exponential decomposition rate. Streams differed
in temperature (Table 2), so breakdown rates were
calculated with each model in terms of time (d) and
accumulated heat, the sum of mean daily tempera-
tures accumulated by the sampling day (degree days
[dd]) (Stout 1989). The goodness of fit of the models
was evaluated by calculating the coefficient of
determination (R
2
) after expressing the values of the
response variable of both models in the same original
scale (Kva
˚lseth 1985, Quinn and Keough 2002). Slopes
(breakdown rates) were compared with nested anal-
ysis of covariance (ANCOVA; %AFDM as dependent
variable, streams nested within region and region as
factors, and dd as covariate). The ANCOVA model
was considered as a particular case of a linear mixed
model to account for the correlation resulting from the
clustered design of successive measurements at each
site (Verbeke and Molenberghs 2000). The model was
fitted using the method of restricted maximum
likelihood (REML), and the covariance structure used
was a 1
st
-order autoregressive (AR[1]). For other
variables (e.g., invertebrates, sporulation rates), com-
parisons were carried out by nested analysis of
variance (ANOVA; streams nested within region
and region as factors; sampling time was a factor
when leaf nutrient content was compared). Subse-
quent pairwise comparisons were made with Tukey’s
test (Zar 2010).
The influence of several variables that could be
potential predictors of the breakdown rate also was
tested. Simple linear regression was fitted indepen-
dently for all variables with the breakdown rate as
response variable. Region could have influenced the
relationships between the selected variables and the
breakdown rate, so these regressions were repeated
with region as a factor, and the significance of the
covariate in the resulting ANCOVA was examined.
Simple linear regression and correlation were used
when searching for relationships between variables
other than breakdown rate. The Shapiro–Wilk test
was used to assess normality, and transformation was
done when necessary. An arcsine(x) transformation
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 939
TABLE 2. Mean (6SE; n=5–6) values of physicochemical variables and daily mean temperature (range) during the experiments. Absolute minimum and
maximum values for each variable are in bold. SRP =soluble reactive P.
Site
Water temperature
(uC) SRP (mg P/L) NH
4+
(mg N/L) NO
2
2
(mg N/L) NO
3
2
(mg N/L) pH
Alkalinity
(meq/L)
Conductivity
(mS/cm)
CC1 7.5 (4.5–9.6) 13.7 63.2 15.5 64.1 2.3 60.7 147.3 644.4 7.61 60.06 0.74 60.02 131.4 65.8
CC2 8.8 (5.1–11.3) 24.7 63.3 20.6 65.6 1.6 60.5 293.5 640.1 7.94 60.07 1.83 60.06 316.8 626.1
CC3 8.2 (4.2–10.6) 25.5 67.5 27.8 64.4 5.8 61.6 414.1 6123.3 8.19 60.01 2.02 60.02 265.8 632.3
CC4 8.3 (4.9–10.6) 22.6 66.7 60.6 616.5 12.7 62.6 717.1 690.3 7.64 60.10 0.73 60.05 149.5 616.1
CC5 9.6 (4.6–11.4) 39.9 64.9 95.0 629.6 3.8 60.6 947.6 6123.2 8.14 60.06 2.49 60.12 351.4 616.9
CC6 10.1 (7.8–12.0) 51.7 68.8 91.5 629.4 19.3 69.6 1151.2 6143.9 8.06 60.04 2.28 60.11 323.6 631.8
CLC1 6.2 (3.8–8.2) 25.1 64.4 58.2 619.5 9.5 64.5 777.9 6130.3 7.26 60.02 2.09 60.05 210.6 61.4
CLC2 5.1 (3.2–6.8) 37.1 61.3 40.7 612.1 2.1 60.6 38.7 612.3 7.75 60.06 0.53 60.08 63.4 62.2
CLC3 4.7 (1.4–8.4) 31.2 63.5 187.4 644.1 15.7 63.3 243.0 624.2 7.72 60.07 3.23 60.07 330.8 65.1
CLC4 5.6 (4.4–7.9) 11.7 62.0 65.1 622.1 1.7 60.7 1135.2 6299.8 7.32 60.10 2.68 60.16 303.6 63.9
SG1 2.8 (0.9–6.0) 10.6 63.4 21.6 611.5 2.2 60.8 157.7 622.0 6.78 6,0.01 0.16 60.03 13.2 60.4
SG2 2.9 (0.5–6.6) 9.8 61.0 28.2 614.7 2.8 60.7 481.2 672.7 6.78 6,0.01 0.18 60.02 17.5 60.8
SG3 3.8 (1.4–6.9) 15.5 60.9 16.2 612.8 4.8 63.2 274.7 65.3 6.69 60.04 0.31 60.04 32.3 61.7
SG4 5.1 (3.8–7.9) 12.3 61.1 5.4 62.2 7.1 64.4 334.5 625.5 6.80 60.05 0.53 60.05 54.8 64.3
SG5 3.0 (0.5–6.3) 7.3 60.7 23.4 611.3 0.7 60.1 236.3 633.5 6.59 60.02 0.14 60.02 14.0 60.8
SG6 4.5 (2.1–7.3) 8.5 61.8 0.3 60.6 0.9 60.1 206.2 624.7 6.60 6,0.01 0.16 60.02 18.3 60.6
SG7 3.4 (1.1–6.4) 9.4 61.8 5.6 64.1 0.5 60.1 157.8 624.7 6.60 6,0.01 0.18 60.02 18.0 60.8
SN1 7.3 (5.0–9.1) 5.1 60.5 15.3 62.7 1.1 60.2 105.3 626.8 6.94 60.04 1.26 60.26 214.6 642.2
SN2 6.5 (4.6–8.3) 8.0 60.5 17.1 61.2 0.8 60.1 4.0 60.8 7.32 60.03 0.60 60.03 128.2 65.7
SN3 3.4 (1.8–5.7) 1.1 60.1 21.1 62.8 0.9 60.1 134.9 64.4 7.08 60.06 0.25 60.01 68.0 61.4
SN4 2.7 (0.7–5.1) 15.7 61.2 11.3 63.6 1.5 60.2 197.4 615.7 7.60 60.04 0.78 60.02 108.0 60.8
SN5 3.2 (1.2–5.0) 1.9 60.1 16.8 62.0 1.1 60.1 139.1 67.5 7.19 60.07 0.34 60.02 55.0 6,0.1
940 J. POZO ET AL. [Volume 30
was applied to percentage data, and !(x) or log(x)
transformations were used in the other cases. All
analyses were undertaken with SPSS 17.0 (SPSS Inc.,
Chicago, Illinois) and SAS 9.2 (SAS Institute Inc.,
Cary, North Carolina).
Results
Nutrient levels were relatively low (Table 2), but
water physicochemical variables differed noticeably
within and among regions (nested ANOVA, p,0.05;
Table 3). The highest values of physicochemical
variables were found in CC, the most oceanic region
according to the GCI (see Study sites), whereas the
lowest were registered in SG and SN, the most
continental ones.
Alder-leaf mass loss corrected for leaching (mean
leaching loss <18%) fit a linear model better than an
exponential model in terms of d (R
2
values higher in
all 22 cases) and dd (R
2
higher in 21 of 22 cases)
(Table 4). The linear rate based on dd, which
corrected for interregional temperature differences,
was used for analyses of spatial variation and
relationships to environmental variables. Breakdown
rates (%mass lost/dd) differed significantly among
and within regions (nested ANCOVA, p,0.001;
Table 5) and were high in SG, intermediate in SN, and
low in CLC and CC.
The initial quality of the leaf material was the same
for every stream (%composition 6SE before
leaching, n=5: C =47.1 61.3; N =2.48 60.14; P
=0.081 60.003). Percent C varied little throughout
the study (mean CV for all sites <5%). However, N
and P varied noticeably (mean CV <10%for N and
20%for P). As a consequence, variation observed for
C:nutrient ratios mainly depended on changes in N or
P in the leaf material. Furthermore, C:nutrient ratios
tended to decrease as the dissolved nutrient concen-
tration (both N and P) increased (Fig. 2A, B), but
differences among regions were not significant (nest-
ed ANOVA, p.0.05). At 19 of 22 sites, leaves lost P
relative to C, but this loss decreased as the dissolved P
increased (Fig. 2A). In contrast, at all sites, leaves
gained N relative to C, i.e., mean C:N decreased
during processing. This N enrichment increased as
dissolved N increased in streams (Fig. 2B).
Mean AH sporulation rates differed greatly among
and within regions (nested ANOVA, p,0.01, SG =
SN ,CLC =CC). Values ranged from ,0.01
conidium mg
21
d
21
(SG) to close to 6 (CC) (Table 6)
and were positively correlated with variables related
to dissolved solids, such as alkalinity (r=0.747, p,
0.01), conductivity (r=0.726, p,0.01) and SRP (r=
0.532, p,0.05), and temperature (r=0.558, p,0.01).
A total of 42 identifiable taxa of AH were found
(Table 6): 28 in CC, 25 in CLC, 23 in SN, and 19 in
SG. The 4 regions had 9 species in common, and
Flagellospora curvula was dominant.
Macroinvertebrate abundance differed among and
within regions (nested ANOVA, p,0.001, SG ,CLC
,SN =CC) and was represented by 49 families: 41 in
CC, 31 in SN, 27 in SG, and 19 in CLC (Table 7).
Shredders were represented by 11, 9, 9, and 7 families,
respectively, and were an important component of
macroinvertebrate assemblages in terms of abundance
in all regions (Table 7).
Linear regression analyses between breakdown rate
and associated variables showed that conductivity,
alkalinity, pH, sporulation rate (negative slope), and
channel slope (positive) were the most important
predictors of breakdown rate (Table 8). Several
variables had extreme values in SG, so regressions
were repeated with region as a factor. Conductivity
was the most significant variable explaining break-
down rate (Table 8), and the region factor was
not significant (p=0.205). Channel slope, shredder
density (positive), and alkalinity (negative) were the
other variables significantly related to the decay rate
(Table 8). Thus, the significant effect of shredders on
leaf processing appeared when the influence of region
was corrected. Furthermore, when sites from SG were
excluded from the regression analysis without region
as a factor, the significant effect of shredders also was
clear (Fig. 3). The negative highly significant relation-
ship between sporulation rate and breakdown rate
was clearly influenced by region and disappeared
when the regression was adjusted by region (Table 8).
No positive relationships were found between nutri-
ents and breakdown rate. However, the effect of
microbial activity on breakdown rate appeared to
depend on concentrations of dissolved nutrients (see
results of elemental ratios above).
Discussion
The main goal of our work was to elucidate factors
responsible for regional differences in leaf-litter
decomposition rates among oligotrophic headwater
streams. Headwater streams in our study were similar
and drained siliceous catchments within a narrow
low-to-moderate range of dissolved nutrient concen-
trations, particularly P. Despite the similarities among
streams, inter- and intraregional differences in water-
column physicochemical characteristics were appar-
ent. In general, nutrient concentrations and their
variability decreased with altitude.
Before identifying relationships between leaf pro-
cessing and environmental variables it was necessary
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 941
to calculate breakdown rates according to the model
that yielded the best fits. In many studies, leaf
breakdown is an exponential function of elapsed time
(Webster and Benfield 1986, Abelho 2001), but in our
study, the correction for leaching resulted in better fits
with single linear regressions. Therefore, we focused
on linear instead of exponential rates.Exponential rates
also are reported because they are frequent in the
literature (Irons et al. 1994, Lecerf and Chauvet 2008b).
As expected, breakdown rates were noticeably
variable within and among regions. Increases in
temperature are assumed to enhance biological
activities, leaf processing included (Webster and
Benfield 1986, Bergfur 2007). However, contrary to
our hypothesis, temperature was not a determining
factor of breakdown rate. The fastest rates, regardless
of the model used for their calculation, were found
where mean water temperature and accumulated heat
(dd) were the lowest (SG). The range of mean water
temperatures among sites (2.7–10.1) should have been
great enough to generate important differences in the
activity of detritivores and decomposers and in leaf-
litter processing rates (Friberg et al. 2009). However,
as has been noted in other studies (Fleituch and
Leichtfried 2007), other factors must have masked its
effects. Gonc¸alves et al. (2006) found faster decay rates
in a temperate stream than in a Mediterranean stream
(both on the Iberian Peninsula) or a Neotropical
stream. They suggested that the differences might
be related to consumer efficiency and proposed that
biological differences overrode the temperature effect.
If using degree-days eliminates the effect of
differing thermal regimes, rates should be similar
across latitudes, unless other factors are involved
(Irons et al. 1994). When we expressed breakdown
rates on a degree-day basis, differences between
regions with the warmest and the coldest streams
were even greater, as has been observed by others
(Hladyz et al. 2010). Thus, other factors in our study
were more important than temperature in determin-
ing breakdown rates. Similarly, rates (/dd) were
much faster in an Alaskan stream than in streams in
Costa Rica and Michigan (Irons et al. 1994), results
suggesting that interregional differences in litter
breakdown rates, as in our study, are not merely
consequences of shifts in water temperature.
The main factors significantly related to leaf
breakdown rate were conductivity, alkalinity (nega-
tively), and channel slope (positively). The negative
relationship between decay rate and conductivity (or
alkalinity) is difficult to explain, and positive rela-
tionships are more frequent in the literature (Young
et al. 2008). We did find opposite trends at some sites
(data not shown), but when the regression analysis
was adjusted by region the significant relationship
persisted. The negative relationship between decay
rate and conductivity might be, in part, an indirect
consequence of the effect of channel slope on decay
rate because both variables were highly correlated.
Channel slope affects water velocity and particle
transport, which contribute to physical abrasion on
leaves, accelerating leaf fragmentation (Paul et al.
2006) and masking the effect of moderate dissolved
nutrient concentration in headwaters (Spa
¨nhoff et al.
2007). The importance of physical abrasion on leaf
breakdown is context dependent, and some authors
have reported that the effect of physical abrasion is
trivial compared with the effects of biotic drivers
TABLE 3. Results of the nested analyses of variance for physicochemical variables during the experiments. SRP =soluble
reactive P.
Variable Source of variation df
1
df
2
Fp
Water temperature Region 3 18 20.87 ,0.001
Site(region) 18 95 8.26 ,0.001
SRP Region 3 18 10.45 ,0.001
Site(region) 18 95 8.07 ,0.001
NH
4+
Region 3 18 10.08 ,0.001
Site(region) 18 95 3.58 ,0.001
NO
2
Region 3 18 5.32 0.008
Site(region) 18 95 6.03 ,0.001
NO
3
Region 3 18 3.38 0.041
Site(region) 18 95 17.86 ,0.001
pH Region 3 18 41.47 ,0.001
Site(region) 18 95 17.52 ,0.001
Alkalinity Region 3 18 14.12 ,0.001
Site(region) 18 95 60.33 ,0.001
Conductivity Region 3 18 20.04 ,0.001
Site(region) 18 95 36.53 ,0.001
942 J. POZO ET AL. [Volume 30
(Hieber and Gessner 2002, Ferreira et al. 2006, Hladyz
et al. 2009). In our study, the significance of channel
slope diminished and that of shredders appeared
when the analysis was corrected by region (Table 8).
In contrast, particle sedimentation is a factor com-
monly suggested to slow leaf breakdown because
deposition of fine sediment on litterbags can limit
microbial and macroinvertebrate activity (Zweig and
Rabeni 2001, Niyogi et al. 2003, Rabeni et al. 2005,
Mesquita et al. 2007, Spa
¨nhoff et al. 2007) and, thus,
reduce processing rates. We only have indirect
measures of this effect (%ash content of leaf litter in
the bags), but the significant negative regression
between ash content and breakdown rate point to a
negative effect of fine sediment on leaf processing.
Shredder density positively influenced breakdown
rates when SG data were excluded from the regres-
sion analyses. The order of mean shredder densities
TABLE 4. Mean (SE) leaf-litter breakdown rates, linear band exponential k, of alder leaves in terms of time (d) and accumulated
heat (degree days [dd]). Bold indicates maximum site R
2
.
Site
Linear model, bExponential model, k
(%AFDM/d) (%AFDM/dd) (/d) (/dd)
Mean SE R
2
Mean SE R
2
Mean SE R
2
Mean SE R
2
CC1 1.45 0.09 0.922 0.198 0.013 0.918 0.036 0.004 0.829 0.0049 0.0005 0.846
CC2 1.11 0.06 0.940 0.128 0.007 0.939 0.022 0.002 0.889 0.0026 0.0002 0.891
CC3 1.44 0.11 0.879 0.184 0.015 0.881 0.035 0.004 0.774 0.0045 0.0005 0.777
CC4 1.46 0.09 0.913 0.179 0.012 0.909 0.032 0.003 0.905 0.0040 0.0003 0.902
CC5 1.03 0.06 0.934 0.109 0.006 0.942 0.024 0.003 0.844 0.0025 0.0003 0.856
CC6 1.34 0.07 0.953 0.135 0.008 0.954 0.026 0.002 0.945 0.0026 0.0002 0.944
CLC1 1.26 0.11 0.844 0.207 0.018 0.852 0.033 0.007 0.573 0.0055 0.0010 0.582
CLC2 1.57 0.10 0.911 0.321 0.021 0.918 0.044 0.007 0.639 0.0091 0.0014 0.656
CLC3 0.44 0.03 0.912 0.094 0.007 0.886 0.010 0.000 0.904 0.0014 0.0001 0.892
CLC4 0.53 0.09 0.621 0.090 0.016 0.607 0.011 0.004 0.339 0.0020 0.0006 0.362
SG1 1.65 0.13 0.842 0.639 0.053 0.842 0.053 0.007 0.700 0.0214 0.0024 0.746
SG2 1.38 0.13 0.796 0.496 0.048 0.794 0.031 0.006 0.535 0.0113 0.0020 0.552
SG3 1.62 0.14 0.827 0.432 0.040 0.810 0.053 0.011 0.482 0.0148 0.0028 0.508
SG4 1.85 0.15 0.851 0.369 0.030 0.852 0.074 0.012 0.581 0.0154 0.0024 0.610
SG5 1.41 0.15 0.768 0.496 0.047 0.803 0.035 0.007 0.487 0.0128 0.0022 0.549
SG6 1.31 0.11 0.833 0.302 0.027 0.823 0.029 0.005 0.569 0.0069 0.0012 0.584
SG7 1.62 0.13 0.844 0.482 0.039 0.851 0.037 0.007 0.579 0.0112 0.0018 0.599
SN1 1.05 0.07 0.904 0.143 0.010 0.906 0.023 0.004 0.624 0.0031 0.0005 0.633
SN2 1.30 0.06 0.946 0.198 0.011 0.944 0.027 0.003 0.838 0.0041 0.0004 0.841
SN3 0.74 0.04 0.936 0.226 0.013 0.934 0.013 0.002 0.844 0.0039 0.0004 0.843
SN4 0.96 0.06 0.922 0.359 0.023 0.921 0.019 0.003 0.738 0.0073 0.0010 0.731
SN5 0.83 0.04 0.946 0.265 0.014 0.941 0.014 0.002 0.873 0.0045 0.0004 0.865
FIG. 2. Relationship between alder C:nutrient ratio (molecular elemental ratio) and soluble reactive P (SRP) (A), dissolved
inorganic N (DIN) (B). Broken lines correspond to the value of each C:nutrient ratio in leaves before leaching.
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 943
among the 3 regions was opposite that of mean
conductivities, indicating that headwaters usually
had low conductivity concurrent with greater shred-
der density (relationship not statistically significant).
The highest breakdown rates in our study occurred
in SG and SN, the regions with the coldest temper-
atures and the lowest nutrient levels. Irons et al.
(1994) suggested the relative importance of inverte-
brates vs microorganisms changes along a latitudinal
gradient, with invertebrates more important in colder
waters at high latitudes (or high altitudes). If this
suggestion is correct, shredders should play a decisive
role on leaf breakdown in SG and SN, especially if, as
in our case, fungal activity were limited (indicated
by low sporulation rates). Some investigators have
shown that cold waters can favor some shredders like
stoneflies and caddisflies that are adapted to cooler
thermal regimes (Danks 2007). This situation could
exert a key role on leaf processing and would help
explain the faster breakdown rates in the colder areas.
Caddisflies and stoneflies were well represented in
SG and SN.
The degree of eutrophication of our streams was
low, but we expected leaf breakdown rates to respond
to increases in dissolved nutrients because of enhanced
microbial activity (Pozo 1993, Suberkropp and Chauvet
1995, Gulis and Suberkropp 2003). However, neither
dissolved nutrients (N and P) nor sporulation rate were
positively related to breakdown rate. Poor relation-
ships between leaf breakdown rates and water-column
nutrients have been found elsewhere. For instance, in a
study done along a gradient of water-column nutrient
enrichment in south-central Sweden, Bergfur (2007)
found little support for the conjecture that decompo-
sition rates were related to nutrient enrichment.
Perhaps, the potential effects of eutrophication in our
low-nutrient, low-variability system were overridden
by other factors with more important interregional
variation, such as density of shredders.
Sporulation rates were positively related to dis-
solved solids (alkalinity, conductivity, and SRP) but
not with breakdown rate (sporulation rates were
highest where breakdown rates were lowest). Sporu-
lation rates were measured only on one occasion for
each stream, but we assumed that the values could be
compared. The time elapsed from implantation to t20
differed among regions, but, in most cases (20 of 22),
it was 12 to 23 d, a period of high fungal spore
production (including peaks) by AH when a great
amount of the incubated leaf litter still remains
(Chauvet et al. 1997). This period seems to coincide
with the growth phase of mycelia (measured as
ergosterol) on leaf litter (Pozo et al. 1998). The
relationships between fungal activity, nutrient avail-
ability, and leaf decomposition in nutrient-poor
waters are probably complex, but the effects on
elemental ratios of leaf litter probably are related to
microbial activity. All leaves used in the experiments
came from the same location, so the variations in leaf
quality (N and P content) during breakdown were in
response to the local availability of dissolved nutri-
ents. Nevertheless, quality acquired (as C:N, C:P, and
N:P) and processing rates measured were not parallel.
According to Artigas et al. (2008), fungal N demands
for sporulation can be fulfilled at levels of dissolved
NO
3
2
,300 mg N/L, and no enhancement should be
expected with increased dissolved nutrients. How-
ever, in streams with low concentrations of dissolved
inorganic N (,40 mg/L) and P (,16 mg/L), leaf
decomposition and sporulation rates were stimulated
only when both nutrients were added together, which
suggests that these nutrients potentially colimited
fungal activity (Grattan and Suberkropp 2001). Grat-
tan and Suberkropp (2001) also reported that when N
concentrations were .65 mg/L, decomposition and
sporulation rates were stimulated by addition of P to
waters with P concentrations ,5mg/L. In our study,
dissolved NO
3
-N was .65 mg/L in most cases, and
mean dissolved PO
4
-P created a gradient from ,5mg/L
to 52 mg/L. These concentrations were high enough to
elicit a response in both decomposition and sporulation
rates according to Grattan and Suberkropp (2001), but
we observed a response only of sporulation rates. In
more eutrophic streams, microbial breakdown rate and
spore production are not predictable. Both positive and
negative effects have been reported in the literature, but
a reduction of species richness of AH involved in leaf
processing is often observed in eutrophic streams
(Lecerf and Chauvet 2008a). In our study, differences
in AH species richness might not be a consequence of
impairment but of natural forces because SG, a region
characterized by its nutrient-poor waters, circumneutral
pH, and low temperature, had the lowest richness.
On the other hand, the enhancement of breakdown
rates by increases in dissolved nutrients seems to
depend on leaf quality (Molinero et al. 1996), which
could explain why the decay rate of a high-quality
TABLE 5. Results of the nested analysis of covariance for
the linear rates in terms of degree days.
Source of
variation df
1
df
2
Fp
Degree days 1 528 3607.1 ,0.001
Region 3
degree days
3 528 198.9 ,0.001
Site(region) 3
degree days
18 528 19.4 ,0.001
944 J. POZO ET AL. [Volume 30
TABLE 6. Sporulation rates (minimum–maximum) for each aquatic hyphomycete taxon or form (no. mg
21
leaf dry mass d
21
).
Site abbreviations are given in Table 1.
Taxon CC CLC SG SN
Alatospora acuminata ‘‘pulchelloid’’
a
0.001–0.289 0–0.032 0–,0.001 0–,0.001
Alatospora acuminata sensu neotype
a
,0.001–0.005 0–,0.001 0–,0.001 0–0.001
Alatospora acuminata ‘‘subulate’’
a
0.011–0.35 ,0.001–0.552 0–,0.001 0–0.002
Alatospora flagellata (J. Go
¨nczo
¨l) Marvanova
´0–0.010
Alatospora pulchella Marvanova
´,0.001–0.007 0–0.001 0–,0.001
Anguillospora filiformis Greathead 0–0.009
Anguillospora furtiva Descals 0–,0.001
Anguillospora longissima (Sacc. & P. Syd.) Ingold 0.001–0.035 0–0.001 0–,0.001
Anguillospora rosea Descals & Marvanova
´0–,0.001
Articulospora tetracladia (Tubaki) Sv. Nilsson 0.001–0.007 0–0.005 0–0.024 0–0.001
Clavariopsis aquatica De Wild 0.001–0.145 0–0.020 0–0.001 0–0.008
Clavatospora longibrachiata Marvanova
´& Sv. Nilsson 0–0.052 0–0.019 ,0.001–0.008
Crucella subtilis Marvanova
´& Suberkr. 0–0.025
Culicidospora aquatica R. H. Petersen 0–0.081
Flagellospora curvula Ingold 0.003–5.966 0.011–2.377 ,0.001–0.330 0.119–1.836
Geniculospora grandis (Greathead) Sv. Nilsson Ex Nolan 0–0.061 0–,0.001
Geniculospora inflata Marvanova
´& Sv. Nilsson 0–0.010
Goniopila monticola/Margaritispora aquatica
b
0–0.001
Heliscella stellata (Ingold & Cox) Marvanova
´0–0.858 0–0.188 0–0.035
Heliscus lugdunensis Sacc. & The
´rry 0.001–0.004 0–0.013 ,0.001–0.014 ,0.001–0.003
Heliscus tentaculus Umphlett 0–0.003
Lemonniera alabamensis Sinclair & Morgan 0–0.006 0–0.084 0–0.009
Lemonniera aquatica De Wild 0–,0.001 0–0.004 0–0.001
Lemonniera centrosphaera Marvanova
´0–,0.001
Lemonniera cornuta Ranzoni 0–0.004 ,0.001–0.143 0–0.005
Lemonniera filiformis R. H. Petersen Ex Dyko 0–,0.001
Lemonniera terrestris Tubaki 0.002–0.088 0–0.002 ,0.001–0.028
Lunulospora curvula Ingold 0.004–0.145 0–0.003 0–,0.001
Stenocladiella neglecta Marvanova
´& Descals 0–1.073 0–0.098 0–0.010
Taeniospora gracilis var. enecta Marvanova
´0–0.009 0.001–0.006 0–,0.001
Tetrachaetum elegans Ingold 0.009–0.785 0.001–0.157 0–0.056 0.001–0.110
Tetracladium marchalianum De Wild 0.009–0.158 0–0.035 0–,0.001 0–0.002
Tetracladium setigerum (Grove) Ingold 0–,0.001
Trichocladium angelicum Rolda
´n0,0.001 0–,0.001
Tricladium angulatum Ingold 0–0.006 0–,0.001
Tricladium chaetocladium Ingold 0–0.022 0–0.003
Tricladium patulum Marvanova
´0–,0.001
Tricladium splendens Ingold 0–,0.001
Triscelophorus acuminatus Nawawi 0–,0.001
Triscelophorus monosporus Ingold 0–,0.001
Tumularia aquatica (Ingold) Descals & Marvanova
´0–,0.001
Tumularia tuberculata (Go
¨nczo
¨l) Descals & Marvanova
´0–,0.001
Total sporulation rate 0.560–6.208 0.766–3.633 0.003–0.517 0.222–1.924
Taxa number 28 25 19 23
a
Alatospora acuminata Ingold 1942 was described without a type. Marvanova
´and Descals (1985) later detected 2 strains in culture, which
have clearly distinguishable conidia. They included both in A. acuminata andreferredtothemas‘‘sensu stricto’’ (which the authors
designated as neotype) and ‘‘sensu lato’’. However, the bracketed terms above may cause confusion because, by definition, the ‘‘sensu stricto’’
concept should be included in ‘‘sensu lato’’ and this is not the case here, where conidial shapes of both strains do not overlap. We conclude
that A. acuminata sensu neotype should be kept as such, the category ‘‘sensu stricto’’ being redundant, and we propose to replace the term
‘‘sensu lato’’ by ‘‘pulchelloid’’, because its conidia strongly resemble those of A. pulchella.Werecognizea3
rd
conidial shape of what could
belong to A. acuminata. It is readily recognized by its strikingly subulate, unconstricted stalk, and we refer to it as ‘‘subulate’’.Thisshapeis
relatively abundant in our samples and at many other sites in the Iberian Peninsula and elsewhere, including Hungary (J. Go
¨nczo
¨l, Hungarian
National Museum, Budapest, personal communication) and possibly Australia. Pure culture and molecular studies underway will determine
whether these 2 forms, pulchelloid and subulate, may be the basis for erecting formal taxa, and whether they should be included in A.
acuminata. We included both forms as separate categories under A. acuminata to avoid losing potentially valuable ecological information.
b
Records of conidia of Goniopila monticola (Dyko) Marvanova
´& Descals and of typical conidia of Margaritispora aquatica Ingold
are lumped because the conidia are indistinguishable on nitrocellulose filters and overlap in size. Atypical forms of the latter
species have not been detected in our samples.
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 945
TABLE 7. Range of per site mean densities of invertebrate families collected from bags (no./g ash-free dry mass). Families are
ordered by mean density within each functional group (FG). CC =Cornisa Canta
´brica, CLC =Cordillera Litoral Catalana, SG =
Sierra de Guadarrama, SN =Sierra Nevada, Shr =shredder, Col =collector, Gat =gatherer, Filt =filterer, Scr =scraper,
Pred =predator.
Family Order FG CC CLC SG SN
Limnephilidae Trichoptera Shr 0–1.64 0–25.46 1.16–19.53 0.13–13.92
Leuctridae Plecoptera Shr 0–8.09 0–2.04 0–7.62 1.52–28.29
Gammaridae Crustacea Shr 0–49.89 0–2.15
Nemouridae Plecoptera Shr 0–2.38 0–17.38 0–4.78 0–13.16
Sericostomatidae Trichoptera Shr 0–0.84 0–27.06 0–10.09 0–0.10
Dryopidae Coleoptera Shr 0.52–15.93
Lepidostomatidae Trichoptera Shr 0–6.11 0–8.35
Capniidae Plecoptera Shr 0–4.81 0–1.88 0.12–3.07
Tipulidae Diptera Shr 0–0.46 0–1.54 0–1.21 0.12–0.86
Limoniidae Diptera Shr 0–0.86 0–0.48 0–0.41 0–1.22
Taeniopterygidae Plecoptera Shr 0–0.13 0–0.14
Odontoceridae Trichoptera Shr 0–0.14
Asellidae Crustacea Shr 0–0.11
Chironomidae Diptera Col–Gat 17.63–540.36 13.27–56.64 0–6.41 66.31–182.83
Oligochaeta Oligochaeta Col–Gat 1.41–80.48 0–1.65 0–0.63 0.46–27.77
Leptophlebiidae Ephemeroptera Col–Gat 0–4.32 0–4.05 0–3.68
Psychodidae Diptera Col–Gat 0–0.27 0–0.21 0–0.41 0.33–11.63
Ephemerellidae Ephemeroptera Col–Gat 0–3.53 0–0.16
Heptageniidae Ephemeroptera Col–Gat 0–0.63 0–1.61
Caenidae Ephemeroptera Col–Gat 0–1.58 0–0.51
Dixidae Diptera Col–Gat 0–0.16 0–0.12
Stratiomyidae Diptera Col–Gat 0–0.13
Hydropsychidae Trichoptera Col–Filt 0–1.65 0–2.55 0–0.82 2.09–24.85
Simuliidae Diptera Col–Filt 0–26.40 0–1.10 0–4.14
Brachycentridae Trichoptera Col–Filt 0–6.95
Philopotamidae Trichoptera Col–Filt 0–0.09 0–2.17
Hydrobiidae Mollusca Scr 0.13–56.86
Ancylidae Mollusca Scr 0–2.50
Scirtidae Coleoptera Scr 0–1.56 0–0.79 0–0.72
Goeridae Trichoptera Scr 0–0.48
Valvatidae Mollusca Scr 0–0.24 0–0.24
Glossosomatidae Trichoptera Scr 0–0.22
Baetidae Ephemeroptera Col–Gat–Scr 1.83–19.01 0–4.69 0–0.47 2.28–24.50
Elmidae Coleoptera Col–Gat–Scr 0–1.82 0–0.16 0–4.75
Hydraenidae Coleoptera Col–Gat–Scr 0–1.90 0–0.16
Planariidae Turbellaria Pred 0–2.32 0–4.29 0–17.25
Perlidae Plecoptera Pred 0–14.10 0–0.09
Empididae Diptera Pred 0–9.78 0–0.10 0.21–3.65
Polycentropodidae Trichoptera Pred 0–7.41 0–0.22
Athericidae Diptera Pred 0–1.13 0–0.70 0–0.09
Rhyacophilidae Trichoptera Pred 0–0.53 0–0.26 0–1.55
Ceratopogonidae Diptera Pred 0–0.47 0–0.58
Chloroperlidae Plecoptera Pred 0–0.39 0–0.40 0–0.65
Dytiscidae Coleoptera Pred 0–0.15 0–0.53 0–0.25
Hydrophilidae Coleoptera Pred 0–0.42
Cordulegasteridae Odonata Pred 0–0.46 0–0.11
Aeschnidae Odonata Pred 0–0.28
Calopterygidae Odonata Pred 0–0.17
Perlodidae Plecoptera Pred 0–0.16
Total shredders 0.75–50.68 0–46.38 7.93–29.10 3.86–73.36
Total invertebrates 57.92–683.30 21.67–121.56 10.85–38.30 114.47–271.29
Family number 41 19 27 31
946 J. POZO ET AL. [Volume 30
leaf species such as alder is less influenced than others
by dissolved nutrients (Pozo et al. 1998, Hladyz et al.
2010). However, alder leaf litter decay is sensitive to a
slight eutrophication when fine-mesh bags are used
for incubations and to changes in riparian vegetation
in nutrient-poor waters when coarse-mesh bags are
used (Elosegi et al. 2006), results suggesting that this
species is sensitive to the activity of both decomposers
and detritivores under different stressors. The re-
sponses of other species of poor quality (e.g., oak) to
moderate eutrophication tend to be higher but later
than responses of alder (Gulis et al. 2006), results
consistent with the slower decomposition rates of oak
leaves. Ferreira et al. (2006) showed that several
indicators of the decomposition process respond
faster in alder than in oak leaves (e.g., changes in
nutrient content, fungal biomass, and sporulation
peaks). Thus, alder leaf litter could be considered a
better candidate than leaves with slower decay for
assessing impacts on stream functioning because it
responds faster and its use reduces the risk of bag loss
caused by floods.
In conclusion: 1) temperature was not the deter-
mining factor for differences in breakdown rates
among regions nor did rates increase with dissolved
nutrients; 2) microbial activity (i.e., sporulation rates)
was related to dissolved P, but the effect of nutrients
on leaf breakdown rates was negligible; and 3)
variability in shredder density explained the geo-
graphical differences in breakdown rates, but their
role was masked by other factors (e.g., channel slope)
locally. Last, incorporation of breakdown rates into
assessment schemes of stream ecological status might
be hindered by the absence of unequivocal responses
of leaf processing to variations of environmental
factors among different geographical/climatic re-
gions. Precise use of this functional metric would
FIG. 3. Relationship between alder breakdown rate (b)
and shredder density. Data from Sierra de Guadarrama (SG)
were excluded from the regression. AFDM =ash-free dry
mass, dd =degree day.
TABLE 8. Summary of the regression analyses between the potential explanatory variables and the leaf-litter breakdown rate
not adjusted and adjusted by region (analysis of covariance) (right). Sign of the slope is indicated. Asterisks (*) highlight models in
which the region factor significantly affects the variable.
Variables
Unadjusted model Adjusted model
Slope R
2
p(F–test) Slope R
2
p(F–test)
Conductivity 20.786 ,0.001 20.837 0.004
Alkalinity 20.645 ,0.001 20.806 0.016*
Channel slope +0.644 ,0.001 +0.789 0.040*
pH 20.481 ,0.001 +0.743 0.277*
Sporulation rate 20.438 ,0.001 20.732 0.610*
Hyphomycete richness 20.286 0.010 20.760 0.150*
Ammonium 20.260 0.015 20.729 0.783*
Total invertebrate density 20.256 0.016 +0.754 0.194*
Shredder density +0.159 0.066 +0.797 0.028*
Shredder richness +0.148 0.077 +0.753 0.201
SRP 20.127 0.104 +0.734 0.543
Nitrate 20.112 0.127 20.747 0.265
Nitrite 20.104 0.144 20.729 0.780
Riparian cover 20.066 0.247 +0.729 0.757
Channel width 20.044 0.319 20.733 0.551
Total invertebrate richness 20.042 0.359 +0.736 0.461
2011] LEAF DECOMPOSITION IN HEADWATER STREAMS 947
require regional adjustments of baseline standards in
reference conditions.
Acknowledgements
This study was funded by the Spanish Ministry of
Education and Science (project CGL2007-66664-C04), by
the University of the Basque Country (Research grant
GIU05/38), and by the Basque Government (Research
grants IT-422-07 and IT-302-10). We are grateful to
Nadia Arkarazo, Aingeru Martı
´nez, Fernando Rodrı
´-
guez, Carolina Rozas, and Roberto Velilla for help with
field and laboratory work. We thank the ‘Cuenca Alta
del Manzanares’ Regional Park, the Pen
˜alara Natural
Park, the Gorbeia Natural Park, the Montseny Natural
Park, and the Sierra Nevada Natural-National Park for
sampling permits and assistance, and also acknowledge
the Spanish Meteorological Agency (AEMET) for
providing data on temperature and rainfall.
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950 J. POZO ET AL. [Volume 30
... The reference values could also be adapted. In particular, different reference values should probably be used across different regions exhibiting contrasting climate or geology (Pozo et al., 2011). It is well known that decomposition is faster in hard-than in soft-water streams (Suberkropp and Chauvet, 1995). ...
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There is increasing recognition that indicators of ecosystem functioning are needed to improve current stream monitoring schemes. However, to date, no attempt has been made to include functional metrics in large-scale routine monitoring programs under the European Water Framework Directive (WFD). One reason is the uncertainty if functional indicators really carry new and independent information about stream status or, to the contrary, remain broadly redundant with existing indicators of biological stream quality (based on stream communities). A second reason is that, despite increasing scientific knowledge on how anthropogenic pressures influence ecosystem process rates, no ‘ready for use’ tool is available to translate them into stream ecosystem status. Litter decomposition probably ranges among the most documented stream ecosystem processes, and its potential for bioindication has been repeatedly demonstrated during the past decade. Here we report an extensive comparison of routine French structural indicators (I2M2, IBGN, IBD) with alder litter (microbial and total) decomposition rates in 83 streams located in south-western France. Expectedly, microbial decomposition rates were positively correlated with fungal biomass and activity (conidial production rate), while total and invertebrate-driven decomposition rates increased together with detritivore density and diversity in litter bags. By contrast, correlations between litter decomposition rates and routine structural indicators were clearly weak (Spearman’s ρ
... The degradability of microbeads was examined in the Eramosa River in Guelph, ON (43.547,) using methods developed for particulate organic compounds such as leaf litter degradation (e.g., Gulis 19 mg; or~10,000 microbeads) of microbeads were placed into 100-μm nylon mesh bags (8 × 5 cm) to contain the beads and prevent feeding by macroinvertebrates. The mesh bags were fixed to the river bottom at two locations separated by~5 m, exposing them to natural water flow, sunlight, and microbial activity (Gulis and Suberkropp, 2002;Pozo et al., 2011). One mesh bag from each location was collected and brought back to the lab to be weighed and viewed under a dissecting microscope every 7 d for three consecutive weeks. ...
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The transport of particulate matter including the gametes, larvae and propagules of reproducing organisms and other organic matter involved in nutrient/contaminant transport are important processes, yet there are few environmentally friendly methods available to examine dispersal empirically. Herein we report on the development and application of a biodegradable and non-toxic physical model, based on alginate microbeads with modifiable size, density (ρ), and colour for use in dispersal studies. Specifically, the microbeads were designed to model the size and ρ of parasitic juvenile freshwater mussels (Unionidae; ρ = 1200 kg m⁻³), which undergo dispersal upon excystment from fish hosts. We released the juvenile-mussel and neutrally buoyant microbeads (ρ = 1000 kg m⁻³) in a local river and captured them in drift nets downstream. The concentration of microbeads declined with downstream distance, but neutrally buoyant microbeads were transported farther. Analysis of microbead capture rates could be described using the patterns of several mathematical models (negative exponential, power, and turbulent transport), which were consistent with the reported dispersal of mussel larvae and other taxa. These results support the use of alginate microbeads in dispersal studies, because their environmentally friendly and customizable properties offer improvements over non-biodegradable alternatives.
... Otros estudios han encontrado resultados variables, por ejemplo, para el caso particular del género Cecropia, donde han sido clasificadas como de degradación intermedia [41] y de degradación lenta [38,40]. Sin embargo, las especies pueden tener diferentes dinámicas de descomposición debido a las particularidades de las hojas, las concentraciones de diferentes compuestos químicos [58][59][60] y en una escala más amplia, por las condiciones climáticas, altitud, geología de las zonas de estudio; características propias de ambientes acuáticos e influencia del uso del suelo que pueden incidir en cambios de la composición de la fauna acuática, variaciones en la temperatura del agua, aumento de nutrientes, efecto abrasivo, entre otras [61][62][63][64]. ...
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Riparian forests provide high amounts of leaf litter to tropical headwater streams (1–3 order) and its decomposition is controlled by interactions between leaf quality and macroinvertebrate activity. However, few studies have been conducted in North Andean streams. We conducted a leaf litter decomposition experiment using three native tree species (Heliocarpus americanus, Nectandra sp., and Cecropia telealba) in two headwater streams in the Colombian Coffee-growing Eco-Region. The interactive roles of intrinsic factors (quality) and extrinsic factors (presence/absence of macroinvertebrates) on decomposition rates were tested. Three single-species treatments, a species-mixture treatment, and an artificial substrate treatment were incubated in either coarse-pore mesh or fine-pore mesh bags to allow or exclude macroinvertebrate colonization, respectively. Bags were removed from the streams 7, 14, 28 and 56 days after starting the study. Toughness and chemical quality of senescent leaves of each species were determined in order to test their effect on the decomposition rates. The k-values for the three single-species and the species-mixture treatments indicated that decomposition occurred at medium to fast rates (0.009–0.01 day⁻¹). H. americanus showed the greatest mass loss at the end of the trial, followed by C. telealba and Nectandra sp. Leaf toughness was positively correlated with carbon-to-nitrogen ratio (C: N) and carbon-to-phosphorus ratio (C: P) and, in turn, the three characteristics were negatively correlated with breakdown rate. A total of 3876 individuals from 13 orders, 35 families, and 47 genera colonized leaf-litter bags. Chironomidae, Lumbriculidae, and Hydropsychidae were the families with the highest abundance. The abundance and richness of macroinvertebrates in the leaf-litter bags showed no correlation with the descriptors of intrinsic characteristics (leaf quality). These results indicate that among the three tree-species in the study, the intrinsic characteristics of senescent leaves determine their mass loss rates, while the macroinvertebrate abundance or richness play a secondary role, probably due to the fact that shredders were not abundant.
... Degree-days (dd)-the sum of mean daily temperatures over the time period considered-were used rather than days so as to standardize the rates for the temperature differences across the zones (Fig. 1). This is a common procedure in order to standardize the incubation period in terms of accumulated heat (Pozo et al. 2011;Boyero et al. 2011). Decomposition rates were compared by a twoway analysis of co-variance (ANCOVA; fixed factors: incubation zone and leaf species; covariate: degreedays). ...
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Intermittent streams, dominant in arid and semi-arid regions, are considered to be more representative of global river networks than perennial rivers. The impacts of constant changes in hydrological regime on the functioning of these streams and associated riparian areas does, however, remain to be elucidated. In this study, litter derived from two deciduous tree species (chestnut and oak) was used to compare microbial-decomposition patterns between an intermittent stream channel and its riparian area over a 1-year period. The stream channel exhibited higher decomposition rates than the riparian area for litter from both species, and higher fungal biomass only for chestnut. Despite a prolonged absence of streambed surface water (254 days), differences in hydrological conditions in the wetter seasons (autumn and winter) shape the decomposition dynamics in both zones throughout the whole hydrological cycle. The results point out the importance of the ''hydrological imprint'' for the leaves' degradation; long-term studies are advisable over short-term ones to better understand the functioning of intermittent streams.
... Degree-days (dd)-the sum of mean daily temperatures over the time period considered-were used rather than days so as to standardize the rates for the temperature differences across the zones (Fig. 1). This is a common procedure in order to standardize the incubation period in terms of accumulated heat (Pozo et al. 2011;Boyero et al. 2011). Decomposition rates were compared by a twoway analysis of co-variance (ANCOVA; fixed factors: incubation zone and leaf species; covariate: degreedays). ...
Article
Full-text available
Intermittent streams, dominant in arid and semi-arid regions, are considered to be more representative of global river networks than perennial rivers. The impacts of constant changes in hydrological regime on the functioning of these streams and associated riparian areas does, however, remain to be elucidated. In this study, litter derived from two deciduous tree species (chestnut and oak) was used to compare microbial–decomposition patterns between an intermittent stream channel and its riparian area over a 1-year period. The stream channel exhibited higher decomposition rates than the riparian area for litter from both species, and higher fungal biomass only for chestnut. Despite a prolonged absence of streambed surface water (254 days), differences in hydrological conditions in the wetter seasons (autumn and winter) shape the decomposition dynamics in both zones throughout the whole hydrological cycle. The results point out the importance of the “hydrological imprint” for the leaves’ degradation; long-term studies are advisable over short-term ones to better understand the functioning of intermittent streams.
Article
Organic matter decomposition (OMD) is one of the important river ecosystem functions. Changes in land use and landscape pattern (LULP) have a serious influence on the OMD in neighboring river ecosystems. However, there is limited information on the influence paths of LULP on organic matter decomposition in river ecosystems. In this study, cotton strip (CS) as a substitute for investigating OMD, was introduced to the delineated catchments in Luanhe River Basin in China, meanwhile combining with remote sensing interpretation, water quality analysis, microbial sequencing, and redundancy analysis (RDA) to identify the dominant LULP metrics, water quality parameters, and microbial groups controlling the OMD. Then the structural equation models (SEMs) were used to connect these dominant controlling factors to track the influence paths of LULP on OMD in river ecosystems. RDA results indicated that construction land (CON), farmland (FAR) and landscape shape index (LSI) in LULP, total nitrogen (TN), chemical oxygen demand (COD) and pH in water quality, bacterial phyla Planctomycetes and Firmicutes, as well as fungal phyla Chytridiomycota and Basidiomycota were the dominant factors controlling the OMD (quantified by tensile strength loss (TSL) and respiration (RES)). These four microbial phyla contributed significantly to OMD. SEMs further proposed three paths to explain the mechanism of LULP influencing on OMD, which were CON - TN - Firmicutes - TSL, CON - TN - Chytridiomycota - RES, and FAR - COD - Chytridiomycota - TSL. CON promoted OMD mainly through enhancing TN content in river water to increase Firmicutes and Chytridiomycota. FAR increased Chytridiomycota by decreasing COD in river water, promoting OMD. These results will deepen our understanding of the influence of LULP on river ecosystem functions and provide valuable information for policymakers and managers to carry out watershed land planning and river management in the future.
Chapter
A key or keystone species is defined as a species with disproportionately large effects on the ecosystem relative to its abundance. In freshwater ecology it is often used with a bottom-up perspective, to refer to riparian plant species whose litter resources are of particular importance for invertebrate communities and ecosystem processes. This includes fast-decomposing species that represent an important litter supply in terms of nutrients (e.g., alder) and slow-decomposing species that last for long in the stream and are able to sustain communities in periods were preferred resources have disappeared (e.g., oak). This chapter will focus on the major role that litter of the genus Alnus (i.e., alder) plays in the decomposition process, a crucial component of stream ecosystem functioning. Alder litter often determines overall decomposition rates and how these are affected by factors such as plant diversity as well as rates of nutrient cycling or secondary production. We take advantage of the wide use of alder litter in multiple studies conducted at different spatial scales (from local to global) and with different approaches (from laboratory to field studies) to illustrate how the presence and abundance of a key riparian plant species can drive stream ecosystem functioning.
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Autotrophic respiration (AR), heterotrophic respiration (HR), and decomposition are important contributors to the carbon cycle in streams. It is important to understand how different environmental factors, such as canopy cover and dissolved organic carbon (DOC), influence these processes. DOC concentrations in northern forested streams are increasing, which may affect light and carbon availability. To examine the effects of DOC and canopy cover on these processes we measured gross primary production, ecosystem respiration and decomposition at 8 sites in 4 streams in the Upper Peninsula of Michigan and used quantile regression to estimate AR and HR. Among sites, AR and decomposition showed no relationship with canopy cover using Spearman’s correlation (p = 0.33), while neither respiration process nor decomposition showed a relationship with DOC concentrations (p = 0.75). The results do indicate potential regional and temporal variation in AR and HR; however the quantile regression approach is insufficient to examine this.
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Leaf litter of alder (Alnus glutinosa) is a key resource to detrital stream food webs. Due to its high quality and palatability, it is readily colonised by microorganisms and consumed by detritivores, contributing significantly to carbon and nutrient cycling and to ecosystem functioning. Given that this species has declined due to the spread of the pathogen Phytophthora alni, we investigated how its loss would alter leaf litter decomposition and associated stream assemblages of aquatic hyphomycetes and invertebrates, in a field experiment conducted in three streams. We compared litter mixtures containing alder plus three other species (Corylus avellana, Quercus robur and Salix atrocinerea; that is, 4-species treatments) with mixtures that excluded alder (3-species treatments) and all the monocultures (1-species treatments). The loss of alder reduced decomposition rates, despite the existence of an overall negative diversity effect after 3 weeks of exposure (that is, monocultures decomposed faster than mixtures) and no diversity effect after 6 weeks. Aquatic hyphomycete and detritivore assemblage structure in the mixture without alder differed from those of the mixture with alder and the monocultures, and the former had lower fungal sporulation rate and taxon richness. Our results suggest that alder loss from the riparian vegetation can significantly slow down the processing of organic matter in streams and produce shifts in stream assemblages, with potential consequences on overall ecosystem functioning. We highlight the importance of assessing the ecological consequences of losing single species, particularly those especially vulnerable to stressors, to complement the multiple studies that have assessed the effects of random species loss.
Chapter
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Monthly quantitative samples of benthic organisms were collected from,streams in four different watersheds,from,August 1968 through July 1969. Each of the watersheds,supports one of the follow- ing types of vegetation: old-field succession, hardwood forest, white pine forest with a few hardwoods, coppice forest. The kinds of or- ganisms,in the four streams were,generally similar but their relative importance,varied significantly. A Duncan's multiple-range test showed significant differences in the numbers,of most taxa among,the water- sheds. The old-field stream had the greatest abundance,while the cop- pice stream had,the greatest standing,crop biomass. The white,pine stream had,lowest standing crops of both numbers,and biomass. Most of the differences among,watersheds,were attributed to different inputs of allochthonous detritus.
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We examined the influence of stream water chemistry on relationships between fungal activity and breakdown rates of yellow poplar (Liriodendron tulipifera) leaves in eight streams that varied with respect to pH and nutrient (nitrate and phosphate) concentrations. We also performed a reciprocal exchange experiment of leaves that had been colonized by microorganisms in two streams with contrasting water chemistries. Decomposer activity varied greatly depending on the stream in which the leaves were placed. Variation in breakdown rates of yellow poplar leaves was over 9-fold maximum ATP concentrations associated with leaves varied as much as 8-fold, and maximum sporulation rates of fungi associated with leaves varied over 80-fold among streams. Among all streams, nitrate, phosphate, and temperature were positively correlated with one another and with decomposer biomass and activity. When hardwater streams were analyzed separately, nitrate concentration was the only variable that was significantly correlated with all measures of microbial activity and leaf breakdown. Consequently, nitrate concentration appeared to explain much of the variation we detected among streams. Responses to the reciprocal exchange experiment were rapid, with significant changes occurring within the first 5 d after the transfer. Leaves transferred from the hardwater stream containing relatively high concentrations of nitrate and phosphate to the softwater stream containing low concentrations of nutrients exhibited by large decreases in both ATP concentrations and sporulation rates, whereas ATP concentrations and sporulation rates increased when leaves received the reciprocal transfer. The fungi associated with decomposing leaves in streams appear to obtain a significant portion of their nutrients (e.g., nitrogen and phosphorus) from the water passing over the leaf surface. These results indicate that the chemistry of the water can be an important regulator of leaf breakdown in streams by affecting the activity of decomposer fungi.
Book
1. Introduction 2. Estimation 3. Hypothesis testing 4. Graphical exploration of data 5. Correlation and regression 6. Multiple regression and correlation 7. Design and power analysis 8. Comparing groups or treatments - analysis of variance 9. Multifactor analysis of variance 10. Randomized blocks and simple repeated measures: unreplicated two-factor designs 11. Split plot and repeated measures designs: partly nested anovas 12. Analysis of covariance 13. Generalized linear models and logistic regression 14. Analyzing frequencies 15. Introduction to multivariate analyses 16. Multivariate analysis of variance and discriminant analysis 17. Principal components and correspondence analysis 18. Multidimensional scaling and cluster analysis 19. Presentation of results.
Article
We examined the effect of nutrient addition on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation associated with yellow poplar (Liriodendron tulipifera L.) leaf disks in 3 streams that differed in water chemistry. We carried out these studies in flow-through channels that received additions of KH2PO4, NaNO3, both nutrients, or controls with no additions. When limiting nutrients were added to the water in all 3 streams, leaf-decaying fungi responded and decomposition rates increased. Two streams, Walker Branch and Payne Creek, contained low concentrations of both inorganic N (<40 μg/L) and P (<16 μg/L). In these streams, rates of leaf decomposition, concentrations of fungal biomass, and rates of sporulation were stimulated only when N and P were added together, indicating that these nutrients potentially colimited fungal activity. The other stream, Little Schultz Creek, contained low concentrations of P (<5 μg/L), but higher concentrations of N (65–200 μg/L) than Walker Branch and Payne Creek. Rates of leaf decomposition, fungal biomass, and sporulation were stimulated by P addition and when both nutrients were added together, indicating potential limition of fungal activity by P in this stream. Results from all 3 streams provide direct experimental evidence that leaf-decaying fungi can use nutrients dissolved in stream water and that, in some streams, rates of leaf decomposition are stimulated by the addition of these nutrients.
Article
The response of stream benthic invertebrates to surficially deposited fine sediment was investigated in 4 Missouri streams. Twenty to 24 sampling sites in each stream were selected based on similarities of substrate particle-size distributions, depths, and current velocities but for differences in amounts of deposited sediment, which ranged from 0 to 100% surface cover. Deposited sediment was quantified 2 ways: a visual estimate of % surface cover, and a measurement of substrate embeddedness, which were highly correlated with each other and with the amount of sand. Invertebrates were collected using a kicknet for a specified time in a 1-m2 area. Five commonly used biomonitoring metrics (taxa richness, density, Ephemeroptera, Plecoptera, and Trichoptera [EPT] richness, EPT density, and EPT/Chironomidae richness) were consistently significantly correlated across streams to deposited sediment. Shannon diversity index, Chironomidae richness, Chironomidae density, a biotic index, and % dominant taxon did not relate to increasing levels of deposited sediment. Tolerance values representing taxa responses to deposited sediment were developed for 30 taxa. Deposited-sediment tolerance values were not correlated with biotic index tolerance values, indicating a different response by taxa to deposited sediment than to organic enrichment. Deposited-sediment tolerance values were used to develop the Deposited Sediment Biotic Index (DSBI). The DSBI was calculated for all samples (n = 85) to characterize sediment impairment of the sampled streams. DSBI values for each site were highly correlated with measures of deposited sediment. Model validation by a resampling procedure confirmed that the DSBI is a potentially useful tool for assessing ecological effects of deposited sediment.