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Riparian forest modifies fuelling sources for stream food webs but not food-chain length in lowland streams of Denmark


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Several studies have shown that the origin of carbon fuelling food webs in streams depends on riparian cover type. In forested stream sites allochthonous resources fuel food webs, whereas autochthonous resources support biomass in grassland (open-canopy) stream sites. However, some studies suggest that autochthonous carbon (of highest quality) is preferentially assimilated regardless of riparian cover and that the food-chain length (FCL) may be larger in grassland than in forested sites. We used stable isotopes of carbon and nitrogen in adjacent grassland and forested reaches to compare the contribution of autochthonous vs. allochthonous resources to the biomass of the whole macroinvertebrate assemblage and to the most abundant taxa. Moreover, we compared the FCL between forested and grassland sites by estimating the trophic position of brown trout, Salmo trutta. Autochthonous support to macroinvertebrate biomass was higher in grassland than in forested sites, often changing from a dominantly autochthonous to an allochthonous-generated biomass from grassland to forested. This held true for the whole macroinvertebrate assemblage and for specific species. FCL remained similar between reach types. Our study suggests that autochthonous resources are assimilated to a higher extent when their availability increases with canopy openness but allochthonous carbon sustain macroinvertebrate biomass in forested reaches.
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Riparian forest modifies fuelling sources for stream food
webs but not food-chain length in lowland streams
of Denmark
I. Gonza
´lez-Bergonzoni .P. B. Kristensen .A. Baattrup-Pedersen .
E. A. Kristensen .A. B. Alnoee .T. Riis
Received: 19 November 2015 / Revised: 7 July 2017 / Accepted: 18 July 2017
ÓSpringer International Publishing AG 2017
Abstract Several studies have shown that the origin
of carbon fuelling food webs in streams depends on
riparian cover type. In forested stream sites allochtho-
nous resources fuel food webs, whereas autochthonous
resources support biomass in grassland (open-canopy)
stream sites. However, some studies suggest that
autochthonous carbon (of highest quality) is prefer-
entially assimilated regardless of riparian cover and
that the food-chain length (FCL) may be larger in
grassland than in forested sites. We used stable iso-
topes of carbon and nitrogen in adjacent grassland and
forested reaches to compare the contribution of
autochthonous vs. allochthonous resources to the
biomass of the whole macroinvertebrate assemblage
and to the most abundant taxa. Moreover, we
compared the FCL between forested and grassland
sites by estimating the trophic position of brown trout,
Salmo trutta. Autochthonous support to macroinver-
tebrate biomass was higher in grassland than in
forested sites, often changing from a dominantly
autochthonous to an allochthonous-generated biomass
from grassland to forested. This held true for the whole
macroinvertebrate assemblage and for specific spe-
cies. FCL remained similar between reach types. Our
study suggests that autochthonous resources are
assimilated to a higher extent when their availability
increases with canopy openness but allochthonous
carbon sustain macroinvertebrate biomass in forested
Handling editor: David J. Hoeinghaus
Electronic supplementary material The online version of
this article (doi:10.1007/s10750-017-3313-1) contains supple-
mentary material, which is available to authorized users.
I. Gonza
´lez-Bergonzoni A. Baattrup-Pedersen
A. B. Alnoee
Department of Bioscience, Aarhus University, Vejlsøvej
25, 8600 Silkeborg, Denmark
I. Gonza
Laboratorio de Etologı
´a, Ecologı
´a y Evolucio
´n, Instituto
de Investigaciones Biolo
´gicas Clemente Estable,
Montevideo, Uruguay
I. Gonza
´lez-Bergonzoni (&)
Departamento de Ecologı
´a y Evolucio
´n, Facultad de
Ciencias, Universidad de la Repu
´blica, Igua
11400 Montevideo, Uruguay
P. B. Kristensen A. B. Alnoee T. Riis
Department of Bioscience, Aarhus University, Ole Worms
´, Building 1135, 8000 Aarhus, Denmark
E. A. Kristensen
EnviDan, Vejlsøvej 23, 8600 Silkeborg, Denmark
DOI 10.1007/s10750-017-3313-1
Keywords Resource subsidy Allochthonous
detritus Stable isotopes Bayesian mixing models
Trophic position Carbon subsidies
The relationship between resource availability and
energy flow suggested since the River Continuum
Concept (Vannote et al., 1980) indicates that the
energetic sources for riverine food webs shift along the
stream longitudinal gradient. Small headwater
forested stream reaches are fuelled primarily by
allochthonous sources, whereas autochthonous
sources from in-stream production, such as periphytic
algae, increase in importance as energetic sources for
food webs towards larger open-canopy reaches (Van-
note et al., 1980; Collins et al., 2016). The higher
importance of allochthonous sources in small-forested
streams may reflect the intimate contact between the
water and the riparian surroundings, representing a
high cross-ecosystem resource exchange (e.g. Gregory
et al., 1991; Moldenke & Ver Linden, 2007; Sullivan
2013). Towards larger open-canopy sites (e.g. grass-
land riparian area) this contact is reduced and an
increase in light irradiation produces an increase of in-
stream productivity, probably favouring the use of
autochthonous resources (e.g. Vannote et al., 1980;
Sullivan, 2013; Collins et al., 2016).
While the River Continuum Concept has several
limitations and it is not universal, being based on and
supported by studies conducted in north temperate
mountain forested basins (e.g. Minshall, 1967; Cum-
mins, 1974; Dekar et al., 2012), its broader ideas about
the role of riparian forest cover in determining the
energy fuelling food webs (but not only at a commu-
nity-structure level as originally stated) have been
supported by studies within the patch dynamics
concepts (e.g. Spencer et al., 2003; Thompson &
Townsend, 2003; Winemiller et al., 2010; Junker &
Cross, 2014; Collins et al., 2016). For example recent
evidence comparing small forested and larger grass-
land reaches in both tropical and temperate streams
suggest that the role of riparian forest in increasing
allochthonous contribution to biomass is consistent
across climate regions (Collins et al., 2016). The
origin of food webs has mostly been studied by
comparing stream reaches located in contrasting land
use and riparian cover types (e.g. Rounick et al., 1982;
Thompson & Townsend, 2003; Whiting et al., 2011).
However there are also many studies under semi-
experimental conditions such as before and after forest
clear-cut (e.g. Rounick et al., 1982; Noel et al., 1986;
Thompson et al., 2009) or in areas affected by
wildfires that have removed riparian forests (e.g.
Spencer et al., 2003). Whatever the riparian forest
reduction has caused, in all studies where canopy
openness increases an increase in stream primary and
secondary production has been observed (Noel et al.,
1986; Thompson & Townsend, 2003; Thompson et al.,
2009; Whiting et al., 2011). This increase in canopy
openness and productivity has been related to an
increase in the reliance of primary consumers on
autochthonous resources such as benthic algae (Rou-
nick et al., 1982; Spencer et al., 2003; Thompson &
Townsend, 2003; Junker & Cross, 2014).
However, the relationship between riparian forest
and the allochthonous subsidies to food webs has not
been supported universally. For example, stable iso-
tope analyses of carbon (C) and nitrogen (N) from a
large variety of stream types (from headwater to
lowland streams) and across biogeographic regions
(e.g. Fu
¨reder et al., 2003; Brito et al., 2006; McNeely
et al., 2006; Lau et al., 2009a,b; Jardine et al., 2013)
suggest that autochthonous resources may be the main
energetic source for food webs, also in-streams with
significant riparian forest cover in both some temper-
ate (e.g. Fu
¨reder et al., 2003; McNeely et al., 2006) and
tropical climates (e.g. Brito et al., 2006; Lau et al.,
2009a,b). Based on similar evidence for larger rivers,
the revised Riverine Productivity Model (Thorp &
Delong, 2002) postulates that most of the biomass in
all riverine food webs is fuelled by autochthonous
sources independently of the presence of riparian
forest and/or high availability of allochthonous
organic matter. This would reflect the fact that
autochthonous carbon is more easily incorporated into
the biota as it is more labile than the allochthonous
carbon (Thorp & Delong, 1994,2002 and references
therein) and more nutritious (as suggested by its lower
C:N ratio than detrital sources, Lau et al., 2009b).
The ideas in the River Continuum Concept and the
Riverine Productivity Model should not be considered
as opposite, since they target different systems and
organisation levels (whole river longitudinal gradient
and communitary changes in macroinvertebrates con-
tra lowland streams and rivers and whole metazoan
biomass), but there is some debate on whether the
ideas of the Riverine Productivity Model might apply
also to headwater streams, where River Continuum
Concept predicted a dominance of allochthonous-
feeders in the community (Finlay, 2001; Brito et al.,
2006; Moulton, 2006 Lau et al., 2009b). This may
challenge the generality of the role of riparian forests
in determining the main energetic subsidy of stream
food webs (Fu
¨reder et al., 2003; McNeely et al., 2006;
Moulton, 2006; Li & Dudgeon, 2008; Lau et al.,
In addition several studies have reported positive
relationships between system productivity and food-
chain length (from here on ‘‘FCL’’) (e.g. Pimm &
Kitching, 1987; Post et al., 2000; Thompson &
Townsend, 2005), among other explanatory variables
for variation in FCL such as for example the relation-
ship between hydrological variability and ecosystem
area (McHugh et al., 2010; Sabo et al., 2010; Sullivan
et al., 2015). At the reach scale, unshaded reaches are
usually more productive than forest reaches where
shading limits algal growth (Thompson et al., 2009;
Whiting et al., 2011; Junker & Cross, 2014), and Lau
et al. (2009b) revealed a longer FCL in open canopy
than in forested reaches, possibly as a consequence of
increased availability of high quality food of auto-
chthonous origin and a higher transfer efficiency to
primary consumers of autochthonous resources rela-
tive to allochthonous resources. Alongside, questions
emerge on the role of riparian forest in determining
energy pathways in stream food webs and how
forested riparian areas may regulate maximum FCL.
Here, using C and N stable isotopes we studied the
role of riparian forest in driving stream food webs in
four Danish headwater streams sampled during sum-
mer. Each of the studied streams had a forested
(closed canopy) and a grassland (open canopy) section
(c.a. 100 m) separated c.a. 500 m from each other,
allowing us to directly compare the differences in the
food webs of adjacent forest and grassland streams.
This sharp transition was caused by deforestation by
agricultural practices in the past. First, we aimed to
assess the contribution of allochthonous (i.e. coarse,
suspended, and fine benthic particulate organic mat-
ter: abbreviated as CPOM, SPOM, and FBOM,
respectively, from hereon) vs. autochthonous (i.e.
epibenthic algae biomass) energy sources to primary
consumers between forest and grassland stream
reaches considering the whole community (all
macroinvertebrates pooled) and specific species
(those common between reach types). Specifically,
based on previous works in temperate forest streams
(i.e. Junker & Cross 2014; Collins et al., 2016), we
hypothesised that the fuelling resources for food webs
depend on riparian cover type at the reach scale.
Autochthonous material would be the dominant
fuelling source for food webs (i.e. contributes [50%
to consumer biomass) in grassland sites where algal
biomass availability is greater than in forest streams,
whereas allochthonous material is the dominant
fuelling source in the forested streams. Moreover we
expect that along the entire studied stream sections the
proportion of biomass derived from allochthonous
resources increase with increasing canopy coverage.
Alternatively stream food webs could be mainly based
on autochthonous resources independently of riparian
forest presence as postulated by the Riverine Produc-
tivity Model for larger riverine systems. Second, we
aimed to assess FCL in forest and grassland sections
within the same system. FCL was estimated as the
maximum trophic position of a same top predator in
each site, and we hypothesised that FCL is longer in
grassland sites, reflecting the higher availability of
high quality autochthonous C (following the produc-
tivity-FCL relationship hypothesis, e.g. Thompson &
Townsend, 2005), resulting in a higher trophic
position of top predators. Similarily we expect that
FCL increase with increasing benthic algae biomass
and decreasing canopy coverage.
Materials and methods
Study streams and timing of study
All four streams were located in Jutland, Denmark,
and selected to represent an open and a forested reach
of approximately at least 100 m, being separated from
each other in about 500 m distance, with a sharp
transition between each reach type (Fig. 1). This study
design (eight sites in total, four forested and four
grassland, one forested and grassland site located in
each river) was chosen to try to keep other conditions
than the differences caused by the riparian settings, as
similar as possible. The direction of the flow varied
between study sites, from forest to grassland in two of
the study streams (River Alsted, a tributary to River
Gudenaa, and River Skader, a tributary to River
Alling) and from grassland to forest in the other two
sites (River Granslev, tributary to River Gudenaa, and
Storkesig Brook, tributary to River Aarhus). These
different flow orientations were chosen to try to avoid
biases produced by the potential effect of flow
direction. However, the effect of river flow was not
tested explicitly due to lack of replicates (two streams
with each type of flow) Samplings for food-web
reconstruction using stable isotopes were conducted
from mid- to late June 2013.
Stream physical and chemical characteristics
A 100 m long sample reach was delineated in the
downstream end of both the open and the forested
canopy reach (Fig. 1). Stream physical conditions
were characterised according to the Danish National
Monitoring Programme for the Aquatic Environment
(Friberg et al., 2005). Measurements were performed
along 20 transects positioned perpendicular to the
stream flow every 5 m within the 100 m stream reach.
Width was measured along each transect, which was
furthermore divided into five quadrates used for depth
measurements and characterisation of substrate and
macrophyte coverage. Substrates covering C25% of
the quadrate were quantified according to six cate-
gories: rock ([60 mm), coarse gravel (10–60 mm),
fine gravel (3–10 mm), sand (0.25–3 mm), mud and
clay (\0.25 mm) and debris. Macrophyte coverage
was determined from cover estimates within each of
the quadrates applying the following five categories:
\5, 5–25, 25–50, 50–75, and[75%. This information
was used to calculate mean reach-scale macrophyte
coverage. Measurements of discharge were conducted
at the transition point between the forested and
grassland reach using the velocity-area method
according to Jensen & Frost (1992). Furthermore, we
recorded water temperature and pH in the field and
took an integrated water sample which was trans-
ported refrigerated to the lab for analysis of alkalinity
and nutrients. Stream water temperature was regis-
tered with a Hobo
Pendant temperature logger, and
pH was measured with checkkit
micro pH-WP2 0–14
pH. Total N (TN) was analysed on a Shimadzu TOC-
N, whereas NH
and PO
were analysed on
a Lachat (LACHAT instruments, USA). TP was
determined based on the methodology described in
Brix & Schierup (2001). Alkalinity was estimated
from the same samples by Gran titration using
0.01 mM HCl.
Canopy cover was measured on each bank in each
transect with a LAI-2000 plant canopy analyzer (LI-
COR, Lincoln, Nebraska) determining the Leaf Area
Index (LAI) by recording light received at a horizontal
Fig. 1 Map showing the
studied sites and a sampling
diagram within each site.
The geographical
coordinates are shown for
each locality in which
adjacent forested and
grassland reaches were
sampled. On the right panel
a sampling diagram shows
the areas and distances
sampled in detail for each
site, using River Skader as
an example
plane (e.g. a stream water surface) expressed as a
proportion of the light received under unshaded
conditions (e.g. no canopy). LAI characterises plant
foliage cover which is defined by the one-sided green
leaf area per unit ground surface area (Chen & Black,
1992; Monteith & Unsworth, 1973). LAI values range
from 0 to 1, 1 representing a completely unshaded
stream (Welles & Norman, 1991). Based on LAI, we
then estimated the proportion of canopy cover follow-
ing the same procedure as Kristensen et al. (2014).
Biotic measurements
Epibenthic algae biomass was estimated at reach scale
as the mean chlorophyll a (Chl-a; lgm
) content of
five samples taken on each of the dominant benthic
substrate types present (i.e. [25% of substrate cover-
age) within each reach using a modified (cut through)
syringe of known area (6.74 cm
) on soft bottom
substratum, a kayak tube on gravel (22.23 cm
) and
scraping off a surface area (6.74 cm
) on stones. The
samples were transferred to vials and then filtrated
onto glass microfiber filters (GFC) in the laboratory. A
total of 197 samples were analysed. Samples were
extracted in ethanol (96%) and analysed in a spec-
trophotometer (Shimadzu, UV21 160/Shimadzu, UV-
1700) following standardised methods (Pedersen,
Collection of food sources and consumers
for stable isotope analysis
Sources and consumers for stable isotope analysis
were sampled to represent major basal autochthonous
and allochthonous sources, and all samples were
collected integrating samples located along the same
100 m reach used for reach characterisations. For
basal sources, we sampled filamentous algae by hand
picking and periphyton by scraping stones and wash-
ing sand. The samples were filtered through GFC
filters to remove the water and small invertebrates and
obtain epilithic and epipsammic periphyton isotopic
signatures. For CPOM, allochthonous materials such
as wood, leaves, and parts of terrestrial plants were
collected manually in the stream bottom in proportion
to its prevalence in the environment, integrating
different tree and plant species in decomposition
along all microhabitats present within the 100-m-
study reach, and an integrative water sample from the
water column was collected for SPOM (filtered on
GFC filters). FBOM was collected using an open-
ended PVC cylinder (16 cm in diameter) that was
pushed into the sediment in five locations along the
100-m reach. The sediments were lightly disturbed by
hand down to about 1-cm depth, the water in the
cylinder was mixed and a water sample taken. The
samples were stored refrigerated and transported to the
lab where they were filtered through a GFC filter and
prepared for SIA according to standard procedures
(Levin & Currin, 2012). Leaves of macrophyte species
present were also collected for SIA.
Fish were caught by single-pass electrofishing in
the study reach and macroinvertebrates by sweep
netting along transects and hand picking from rocks
and hard substratum along the 100-m reach. Despite
that it was a qualitative sampling, the effort was
similar in all stream reaches, and invertebrate taxa
present in abundances permitting stable isotope anal-
ysis (i.e. potentially representing [1 mg dry weight
after processing) were collected. Fish (a total of 43
individuals collected) were euthanized with an over-
dose of 2-phenoxy-ethanol and a portion of dorsal
muscle was extracted. All samples were refrigerated
and transported to the lab where they were immedi-
ately frozen. In the lab, invertebrates were identified
using taxonomic keys (e.g. Mey 1997; Dobson et al.,
2012) with bulk samples of macroinvertebrates
grouped per taxa (most frequently to genus level),
and source samples were identified to the lowest
possible taxonomic level and prepared for isotopic
analysis following standard procedures (including
cleaning and removal of shells and hard parts in
macroinvertebrates) (Levin & Currin, 2012). Samples
were freeze-dried, weighed (0.5–1.5 mg for animal
tissues, 2–3 mg for mosses and periphyton) and sent
for analysis at UC Davis stable isotope facility,
California, USA, using a continuous flow isotope
ratio mass spectrometer (IRMS). The natural abun-
dance of heavy and light C and N stable isotopes
C and
N), relative to the proportions of
these isotopes of a standard, Pee Dee Belemnite rock
and N of air, respectively, was determined. Propor-
tions are given as delta values (d
C and d
Energy source contribution modelling
The potential contribution of main energetic sources to
food webs was estimated by Bayesian mixing models
that use d
C and d
N of food sources and consumers
and their fractionation coefficients to estimate the
most probable proportion of the biomass generated by
each food item for an individual consumer (Parnell
et al., 2010,2013b; Phillips, 2012). We therefore used
the main food sources well known from the literature
as potential components of the diet of primary
consumer macroinvertebrates, as this prevents inclu-
sion of unlikely sources improving model accuracy
(Parnell et al., 2010). The allochthonous source input
was the mean and standard deviation of isotopic
signatures of CPOM, SPOM, and FBOM, as well as
terrestrial riparian vegetation. As autochthonous
source input, we use mean and standard deviation of
isotopic signatures of filamentous algae and periphy-
ton from stone scrapes and sand wash. This aggrega-
tion of sources into a two-source model prevents the
under-determination of mixing models and has been
suggested as a standard methodology to follow in
these cases (Fry, 2013). Macrophytes were excluded
from the analysis as they very rarely constitute an
important food source for macroinvertebrate con-
sumers as has been observed in studies of macroin-
vertebrate gut content (Cummins & Klug, 1979;
Newman, 1991), stable isotopes (e.g. Hart & Lovvorn,
2003; Belicka et al., 2012), and fatty acid composition
(e.g. Belicka et al., 2012). The mean and standard
deviations of fractionation values used in the models
were taken from a meta-analysis (Post, 2002). Addi-
tionally it should be noted that calculation results
regarding the autochthonous contribution to food webs
might be slightly underestimated as we considered
FBOM and SPOM as an allochthonous source, while
Li & Dudgeon (2008) found that a minor fraction of
this resource might, in fact, be of autochthonous
origin. On a similar way the isotopic signature of
epilithic algae may also contain traces of allochtho-
nous material deposited with the benthic biofilm
matrix. However, we chose to disregard any con-
founding effect of this, as ‘‘allochthonous’’ is doubt-
less constituted by a vast majority of allochthonous
material and ‘‘autochthonous’’ by a majority of algal
matter; furthermore, the modelling set-up was consis-
tent between sties.
We built two Bayesian mixing models per stream,
one for the grassland and one for the forested site
based on mean and SD of isotopic signatures of
autochthonous vs. allochthonous resources and sepa-
rate bulk samples for each macroinvertebrate taxon
and samples of each fish individual. The modelling
was performed for all macroinvertebrate data pooled,
followed by estimation of the autochthonous and
allochthonous contribution for each taxon sample
separately in the SIAR package (Parnell et al.,
2013a,b) in R software (R core team, 2014). This
permitted us to test for differences in the autochtho-
nous contribution for all macroinvertebrates pooled
and also for similar species present in different reach
types using all samples of the same taxa as replicates.
Trophic position estimation
To test for changes in FCL between open canopy and
forested reaches, we estimated the trophic position of
the most abundant and frequent top predator found in
each reach type, in all cases brown trout (Salmo trutta
Linnaeus, 1758). The use of fish as an indicator of
trophic position in stream reaches can be problematic
because fish may migrate locally, thereby integrating
the isotopic signatures of both reach types, which may
hamper the estimation of reach type-specific food-
chain length. However, in our study brown trout was
chosen as a model secondary consumer because it is
ubiquitous in most Danish streams, easy to collect and
exhibits a sedentary behaviour with a restricted home
range (e.g. Bachmann, 1984; Ovidio et al. 2002).
Predatory invertebrates were not used because same
taxa were not present along all our study sites. To
validate its use as model for FCL in this study, we
tested if d
N isotopic signatures differed between
reach types, expecting that the isotopic signatures
would differ between adjacent forested and open-
canopy reaches within the same streams. FCL was
estimated as maximum trophic position in each site,
which was calculated according to standard proce-
dures (Vander Zanden et al., 1997; Post, 2002):
Trophic position ¼d15Nconsumer d15 Nbase
where 3.4 is the fractionation factor representing mean
enrichment of heavy to light isotopes of 1-trophic level
in d
N(%) of freshwater biota (Post, 2002), and 2 is
added as it represents the theoretical primary con-
sumer level of 2. We used the average d
N of all
primary consumers as baseline for trophic position
estimation. This is commonly used as a representative
average baseline value for a given food web (Jardine
et al., 2014). Therefore, trophic position was calcu-
lated separately with a distinct baseline of mean
primary consumers for each forested and grassland
reach. Different food-web compartments have slightly
different fractionation values (ranging from 1.6
enrichment in d
N when basal invertebrates are
assimilated by predatory invertebrates and from 2.2 to
3.9 when macroinvertebrates are assimilated by fish
(e.g. Bunn et al., 2013; Hussey et al., 2014); however,
the use of a single average fractionation value is
commonly used to estimate an ‘‘average’’ FCL and
more appropriate in this case because we do not know
the exact number of trophic steps within each food-
web compartment. We consider that the study is
unbiased in this aspect as the calculation was made
consistently along all sampling sites and similar (and
thus, comparable) to most FCL studies using stable iso-
topes (e.g. Gonza
´lez-Bergonzoni et al., 2014; Iglesias
et al., 2016; Kirstensen et al., 2016). However, we
need to note that the real FCL could be underestimated
here if more trophic steps occur within the macroin-
vertebrate assemblage (e.g. Hussey et al., 2014).
Statistical analysis
In order to identify environmental differences that
characterise forest and grassland sites, we first used all
variables known in literature to differ between grass-
land and forest stream reaches (canopy coverage, fine
substrate coverage, stream width, macrophyte cover-
age, benthic algae biomass, and total nitrogen; Teix-
eira de Mello et al., 2016, and references therein) in a
PERMANOVA test (Anderson, 2001). Following this,
and in case of finding differences in the multivariate
test we compared each physical, chemical, and
biological characteristics of the two reach types using
one-way ANOVA after checking that the assumptions
regarding homoscedasticity were met. The compar-
isons were made by grouping the recordings from all
forest and grassland reaches of all streams, as well as
separately for forest vs. grassland reaches within each
stream using each square plot for which physical
characteristics were described (i.e. in the case of
substrate coverage) or a sample was taken (i.e. in the
case of Chl-a) as a replicate. We used mixed-effects
linear regression (using stream identity as a random
factor) to test the relationship between the percentage
of riparian coverage and epibenthic algae biomass
given as Chl-a (lgm
Because grassland and forested reaches are indeed
determined by a series of environmental characteristics
more than only canopy coverage (e.g. Teixeira de Mello
et al., 2016), we compared the autochthonous contribu-
tion to invertebrate biomass between ‘‘grassland’’ and
‘forest’ reaches within each stream using one-way
mixed-effects ANOVA, using stream identity as a
random factor as the assumptions for its application
were met. For the comparison using all pooled macroin-
vertebrates, each bulk sample by taxon was used as
replicate indistinctively of the taxonomic identity.
Whenever the same species or taxon was present in
both reach types of the same stream, we tested for
intraspecific differences in autochthonous contribution
to its biomass between the grassland and forested
reaches using bulk samples of that species as replicates
([3 bulk samples in each reach type). The species tested
were Ephemera danica Mu
¨ller, 1764 and Gammarus
pulex (Linnaeus, 1758) in River Granslev and Storkesig
Brook, Simuliidae spp. in Storkesig Brook and Lim-
nephilus sp. inRiver Skader. In River Alsted, intraspeci-
fic differences in autochthonous contribution could not
be tested due to lack of replicates. To test differences
between d
C values in sources and top
predators of forest vs. grassland, we used one-way
ANOVA applying each sample as a replicate. The tests
were conducted for each stream separately and by
pooling data from all streams to compare forested and
open-canopy reaches using all sample individuals. The
same procedure was applied to test for differences in
FCL between forest and grassland sites using the trophic
position estimates of each trout individual.
Finally, to detect which of the environmental
variables characterising forested vs grassland reaches
has more influence on determining the origin of
biomass and FCL we performed mixed-effects linear
regression models using stream identity as a random
effect factor. These models tested the relationships
between mean autochthonous support to macroinver-
tebrate biomass (using all pooled macroinvertebrate
samples) and FCL with riparian coverage, stream
width and depth, proportion of fine substrates (sand
coverage), macrophyte coverage, benthic algae bio-
mass, and nutrient concentration (TN and TP). The
accomplishments of modelling assumptions were
tested following inspection of residual patterns (Zuur
et al., 2009). The marginal r
value, describing the
proportion of variance explained by the fixed-effect
factor in mixed-effects regressions were estimated
using a function developed by (Nakagawa & Schiel-
zeth, 2013) in R software and it is here reported as ‘r
’’ .
Physical, chemical, and biological characteristics
of the study reach
PERMANOVA test showed clear differences between
forested and grassland reaches in the group of
variables tested (F=2.7; df =6, P=0.02) suggest-
ing that forested reaches are characterised by a
combination of higher canopy coverage, stream width
and dominance of fine substrates, and lower macro-
phyte coverage and biomass of benthic algae than the
grassland sections (Table 1). When each environmen-
tal factor was analysed individually we corroborated
that forested reaches had a higher riparian canopy
cover (one-way ANOVA, F=7.3; df
P\0.05, Table 1) and lower epibenthic algae
biomass (Chl-a) than grassland reaches (forest sites
had approximately half of the biomass of the grassland
sites), both when comparing an average for all reaches
(one-way ANOVA, F=4.4, df
=28; P\0.05)
and for the individual reaches (one-way ANOVA,
P\0.05 for all, Table 1). Moreover, algal biomass
decreased with increasing riparian canopy cover
(Mixed-effect linear regression: df =133, F=87.8,
P\0.0001, r
=0.24), suggesting that algal biomass
was, indeed, limited by light in the forested reaches.
The forested reaches also had lower macrophyte cover
than the grassland sections (one-way ANOVA,
P\0.05), except for River Skader where no signif-
icant difference in macrophyte cover between reach
types was found (one-way ANOVA, P[0.05)
(Table 1). Storkesig Brook and River Granslev were
about 1 m wider towards their forest reaches. These
are typical changes observed in Danish lowland
stream morphology associated with the presence of
riparian forests (e.g. Teixeira de Mello et al., 2016).
Otherwise, no significant differences were found
between reach types (Table 1).
Allochthonous and autochthonous food resources
in forest and grassland reaches
Overall, autochthonous and allochthonous food
sources did not overlap between stream types in either
N values (Supplementary Appendix,
Table 1), and the elaboration of the stable isotope
mixing models could therefore be considered optimal
(Fry, 2006) (Supplementary Appendix, Table 1;
Fig. 2).
Macroinvertebrate taxa from the most representa-
tive feeding and functional groups were collected in all
stream reaches including filter feeders, shredders,
collector-gatherers, and predatory invertebrates (Sup-
plementary Appendix, Table 2). Macroinvertebrate
samples collected in each stream are shown in the
supplementary appendix in Table 2. SIAR Bayesian
mixing models using all primary consumers pooled
within each stream (n=188 in total) showed that the
autochthonous contribution to total biomass was
always higher in the grassland than in the forested
reaches, which meets our expectations according to
our first hypothesis (Fig. 3). In three of the streams
(River Granslev, River Alsted and Storekesig Brook),
the autochthonous contribution was higher in the
grassland, and in all four streams (River Granslev,
Alsted, Storkesig Brook and River Skader) the
allochthonous contribution was higher in the forested
compared to the grassland reaches (although only
slightly for River Skader) (Fig. 3). In River Granslev,
most of the primary consumer biomass was supported
by autochthonous resources, both in forest and grass-
land reaches (means of 62 and 85% of primary
consumer biomass being generated from autochtho-
nous sources in forested and grassland reaches,
respectively, Fig. 3). In contrast, River Skader seemed
to be fuelled mostly by allochthonous sources in both
reach types as autochthonous sources never dominated
over allochthonous sources (means of 10 and 28% of
autochthonous support in forested and grassland
reaches, respectively, Fig. 3). In River Alsted (flowing
from forest to grassland) and Storkesig Brook (flowing
from grassland to forest) a strong change in main
energy support to food webs occurred, shifting from a
mainly allochthonous-supported biomass in forested
to an autochthonous-supported biomass in grassland
reaches (from mean values of 37 to 60% of
autochthonous support to biomass in River Alsted
and from 6 to 55% of autochthonous contribution to
biomass in Storkesig Brook; Fig. 3).
The pattern of increasing mean autochthonous
contribution to biomass towards grassland reaches
became significant for all study sites (mixed-effects
=3; F=16.2; P=0.02). This was
Table 1 Physical, chemical, and biological characteristics (means with min–max range in parentheses) of closed-canopy forested and open-canopy grassland reaches measured
at base flow at the time of sampling in each stream
Grassland ?forest Forest ?Grassland
Storkesig Granslev Alsted Skader
Grassland Forest Grassland Forest Grassland Forest Grassland Forest
Median July water temperature
13.9 (10.7–17.3) 14.5 (10.9–18.5) 11.4 (4.5–11.5) 11.2 (4.7–11.2) 16.3
Discharge (l s
) 18 18 136 136 297 297 106 106
Canopy cover (%) 27 93 78.7 88.4 54.8 84 55 73
Velocity (m/s) 0.30 0.29 0.26 0.30 0.50 0.40 Nd Nd
Width (m) 1.2 (0.8–1.5) 2.3 (1.4–4.9) 2,2 (1.4–3.7) 3.1 (2.1–4) 3.0 (2.1–4.1) 4.4 (3.8–5.3) 2.1 (1.6–2.5) 3.4 (1.8–3.9)
Depth (cm) 9.0 (3–30) 8.0 (4–30) 30.0 (5–70) 30.0 (3–80) 27.1 (20.4–40) 17.3 (0–45) 22.8 (15–30) 15.0 (6–25)
Boulders (%) 12.6 (0–31) 10.9 (0–25) 6.1 (0–30) 1.5 (0–11) 13.4 (0–27.3) 16.8 (0–41.7) 2.4 (0–20) 16.2 (0–33.3)
Coarse gravel (%) 16.1 (0–33) 25.1 (733) 14.1 (0–36) 0.4 (0–8) 25.3 (7.7–38.5) 22.2 (6.3–40) 11 (0–26.7) 20 (0–33.3)
Fine gravel (%) 17.9 (5–29) 23.4 (13–29) 2.3 (0–30) 7.8 (0–23) 24.6 (9.1–35.7) 20.4 (6.7–31.3) 20.2
17.2 (6.7–25)
Sand (%) 21.6 (5–38) 24.0 (12–38) 13.7 (0–30) 36.3 (10–55) 27.2 (14–45.5) 21.3 (6.3–33.3) 32.2
22.1 (0–33.3)
Debris (%) 9.4 (0–33) 13.9 (0–38) 12.0 (0–55) 9.5 (0–33) 10.0 (0–23.1) 14.5 (0–33.3) 17.9 (0–30.8) 16.0 (0–41.7)
Macrophyte cover (%) 10.5
0.4 (0.0–3.0) 25.1 (4.0–48.0) 3.7 (0.0–32.5) 47.1 (16.5–98.0) 6.5 (0.0–27.5) 11.7 (0.0–31) 5.9 (0.0–
Benthic chl.a (mg m
)43 (8–128) 32 (6–82) 34 (3–93) 12 (6–45) 34 (13–90) 8 (2–47) 28 (2–56) 8 (2–18)
TN (mg l
) 2.4 (1.7–3.1) 2.7 (2.3–3) 0.8 (0.8–0.9) 0.9 (0.9–1) 2.9 (1.2–5.4) 2.9 (1.4–5.1) 4.9 (2.3–6.4) 5.8 (5.4–6.3)
(mg l
) 1.9 (1.4–2.4) 2.02 (1.4–2.4) 0.6 (0.5–0.7) 0.5 (0.4–0.5) 2.7 (1.1–4.9) 2.8 (1.3–4.9) 5.5 (4.8–6.4) 5.7 (4.3–6.4)
(mg l
) 0.06
0.2 (0.05–0.5) 0.05 (0.04–0.05) 0.04 (0.03–0.06) 0.02 (0.01–0.04) 0.04 (0.03–0.04) 0.04
TP (mg l
) 0.19
0.11 (0.06–0.16) 0.11 (0.06–0.15) 0.08 (0.07–0.09) 0.08 (0.08–0.09) 0.14
(mg l
) 0.07
Alkalinity (meqv l
) 2.8 (2.6–3.1) 2.5 (2.1–3.1) 2.3 (2.2–2.3) 2.1 (2.1–2.2) 2.0 (2.0–2.1) 2.0 (2.0–2.1) 2.8 (2.6–3) 2.7 (2.1–3.2)
Temperature is given as median values, with min–max range in parentheses. Bold indicates significant difference (at 0.05 alevel) between forested and open reaches within the
same stream
Nd not determined
also evident separately for River Granslev (df
F=8.0; P\0.01), River Alsted (df
F=8.0; P\0.01) and Storkesig Brook (df
F=37; P\0.0001), when tested with one-way
ANOVA for each stream using the results from the
estimation of the autochthonous contribution to each
taxon sample. However, the autochthonous contribu-
tion was not significantly different between grassland
and forested reaches in River Skader using this
statistical approach (df
=29; F=0.47; P[0.05).
At the intraspecific level, the higher autochthonous
contribution in grassland reaches remained evident for
the different taxa tested (Fig. 4). The mayfly E. danica
had a higher autochthonous contribution to its biomass
in grassland than in forest reaches of Storkesig Brook
=6; F=50; P\0.0001), and River Granslev
=8; F=20.7; P\0.0001) Simuliidae sp. and
G. pulex had a higher autochthonous contribution to
their biomass in grassland than in forested reaches in
Storkesig Brook (ANOVA: df
=8; F=162;
Fig. 2 Biplot showing
isotopic signatures of
N of main-grouped
resources (autochthonous:
benthic algae) and
allochthonous (CPOM,
invertebrates and fish
(brown trout) in grassland
and forested stream reaches
of four streams. Grey
symbols indicate signature
of each bulk sample
[allochthonous (x);
autochthonous (?);
invertebrates (o); and fish
(d)] and dark circles
represent mean values with
standard deviation of
isotopic signatures of each
of these groups. For details
of invertebrates collected
see Supplementary
Appendix Tables 1 and 2
P\0.0001 for Simulidae and ANOVA: df
F=84.9; P\0.0001 for G. pulex), and G. pulex also
had a higher autochthonous contribution in grassland
than in forested reaches in River Granslev despite a
marginal Pvalue (df
=6; F=4.8; P=0.07;
Fig. 4). However this did not occur for Limnephilidae
caddisflies in River Skader (df
=4; F=4.0;
P[0.05; Fig. 4).
Riparian coverage was the main environmental
driver of changes in the energetic support to food
webs, being the only factor strongly and negatively
correlated to the mean autochthonous support to
macroinvertebrate biomass (Mixed-effects linear
regression df =3, F=39.1, P=0.01 r
Table 2). Additionally we found a tendency (with
marginal Pvalues) to decrease autochthonous support
to biomass with increasing stream width (df =3,
F=7.7, P=0.06 r
=0.52) and nitrogen concen-
trations (df =3, F=9.7, P=0.05 r
Table 2). The proportion of autochthonous generated
Fig. 3 Results of SIAR
Bayesian mixing models for
the estimation of
autochthonous and
allochthonous contribution
to biomass (95, 75 and 50%
credibility intervals, in
increasing colour tone
respectively) of primary
consumer invertebrates in
forested and grassland
reaches of four streams. Left
panels forested reaches;
right panels grassland
reaches for each stream.
From above to below, the
streams studied: River
Granslev, River Alsted,
Storkesig Brook, and River
Skader. The autochthonous
contribution is higher at
grassland reaches within the
same stream (although in
River Skader 95%
credibility intervals of
autochthonous contribution
overlapped greatly between
the forested and open reach)
biomass was not related to macrophyte cover, benthic
algae biomass, stream depth, substrate cover, or total
phosphorous concentrations in our dataset (Table 2).
Food-chain length
Brown trout d
N from each reach type was signifi-
cantly different (one-way ANOVA, Pvalues
always \0.001, Table 3), being lower in forest than
in grassland reaches, probably reflecting the
differences in d
N of basal resources (Figs. 2,5;
Table 3). As expected, we thus validate the use of this
species in FCL estimation. However, opposite to our
expectations and despite the different contribution of
autochthonous and allochthonous resources in forest
and grassland reaches food webs, the FCL did not
differ between forest and grassland reaches neither
when all sites were analysed together (mixed-effects
=3; F=0.02; P=0.8) nor when
differences were compared within each stream (one-
Fig. 4 The autochthonous contribution to the biomass of the
same species present in forested and grassland reaches of each
stream based on SIAR Bayesian mixing model estimations for
each individual. The autochthonous contribution was highest in
the grassland compared to the forested reach for E. danica
=6; F=50; P\0.0001), G. pulex (df
F=84.9; P\0.0001), and Simuliidae sp. (df
F=162; P\0.0001) in Storkesig Brook, E. danica (df
F=20.7; P\0.0001) and marginally for G. pulex in River
Granslev (df
=6; F=4.8; P=0.07), but not significant for
Limnephilidae sp. (df
=4; F=4.0; P[0.05) in River
Skader. Significant differences between forested and grassland
reaches are indicated by asterisk
way ANOVA, P\0.05) (Table 3; Fig. 5). The only
environmental parameter found as a potential deter-
minant of FCL in our dataset was the proportion of fine
sediments in substrate, significantly decreasing FCL
(Mixed-effects linear regression df =3, F=33.4,
P=0.01 r
=0.36, Table 2).
The autochthonous contribution to the biomass of
macroinvertebrates was higher in grassland than in
forest sections of the same stream as suggested in our
first hypothesis, and it was also the dominant fuelling
source in the grassland reach of three of the four
streams (generating slightly [50% of the biomass of
macroinvertebrates). Furthermore, the proportion of
autochthonous contribution to biomass of macroin-
vertebrates decreased with increasing canopy cover-
age. These findings contrast with the Riverine
Productivity Model postulates about metazoan food
webs being based on autochthonous sources indepen-
dent of its availability or canopy coverage, and
supports the previous theories and empirical studies
evidencing the patchy nature of carbon assimilation
dynamics in streams (Spencer et al., 2003; Junker &
Cross, 2014; Collins et al., 2016). In this sense, the
presence of riparian forest at the reach level promotes
an enhanced use of allochthonous resources by
primary consumers (Vannote et al., 1980; Spencer
et al., 2003; Junker & Cross, 2014; Collins et al.,
2016). Our findings also match with those of Leberfin-
ger et al. (2011) who compared the contribution of
autochthonous and allochthonous food resources with
Table 2 Changes in autochthonous contribution to macroinvertebrate biomass and food-chain length with varying environmental
parameters in grassland and forested stream reaches
Parameters Mixed-effects linear regression test parameters
Autochthonous contribution Mean trophic position
F;Pvalue; df r
F; df; Pvalue r
Canopy cover (%) 39.1; 0.01; 3 0.23 (-) 0.07; 0.79; 3
Width (m) 9.7; 0.05; 3 0.26 (-) 3.7; 0.14; 3
Depth (cm) 4.6; 0.12; 3 0.2; 0.6; 3
Sand (%) 0.2; 0.68; 3 33.4; 0.01; 3 0.36 (-)
Macrophyte cover (%) 3.1; 0.15; 3 0.6; 0.5; 3
Benthic chl.a (mg m
) 4.5; 0.12; 3 0.6; 0.47; 3
TN (mg l
)7.7; 0.06; 3 0.52 (-) 0.5; 0.54; 3
TP (mg l
) 1.2; 0.35; 3 2.6; 0.2; 3
Relationships are tested as mixed-effects linear models including stream identity as a random factor, significant relationships are
marked in bold and marginal Pvalues remarked in italics, the coefficient of determination (r
) is given for significant relationships
Table 3 Comparison of isotopic signatures and trophic position of brown trout (Salmo trutta)
Stream nd
N Trophic position
Alsted 10 0.01; 8; [0.1 17.9; 8; <0.01 1.5; 8; [0.1
Granslev 7 20.2; 5; <0.01 11.3; 5; <0.01 3.1; 5; [0.1
Skader 9 1.8; 7; [0.1 6.9; 7; <0.05 3.8; 7; [0.1
Storkesig 10 17.1; 8; <0.01 51.2; 8; <0.0001 0; 8; [0.1
All streams 36 2.6; 34; [0.1 22.5; 34; <0.0001 0.3; 34; [0.1
Details on statistical test parameters are shown as follows: number of individuals analysed (n), ANOVA F, degrees of freedom of
residuals and Pvalue (F;df
;Pvalue) for tests within d
C, d
N, and trophic position in each stream when all forested and
grassland reaches are pooled. Significant differences are given in bold. For more information see Fig. 5
shredder macroinvertebrate biomass in closed- and
open-canopy reaches in Sweden. These authors found
that although allochthonous resources were always the
most important food resource for macroinvertebrates
in both closed and open-canopy sections, the auto-
chthonous contribution to shredder biomass was
higher in open than in closed-canopy conditions
(Leberfinger et al., 2011).
The period before sampling (spring) was typical in
its temperature and precipitation being within the
average range of most previous years (1961–1990)
(Cappelen, 2011); however. we need to remark that
this was a one-season and one-year study, so the
generalisation about its application to other seasons
and regions should be avoided without further evi-
dence. Our results are representative of the time period
reflected by stable isotopes in consumer tissues, and
for many invertebrates this may be the period
2–3 weeks before sampling and about one month for
fish during summer in Danish streams (at least for
N) (Riis et al., 2012). Therefore, these results are
valid for the early summer season and not the whole
year. Based on a previous seasonal study of a forested
temperate stream in North America, we expect a
predominantly autochthonous-based primary con-
sumer biomass towards summer with rising temper-
atures and allochthonous predominance during
autumn, winter, and spring (Junker & Cross, 2014).
Despite the autochthonous contribution to inverte-
brates probably represented the maximum annual
level in our study, our results together with that of
related studies (e.g. Leberfinger et al., 2011; Junker &
Fig. 5 Variations in
isotopic signatures and
trophic position of brown
trout (S. trutta) in forested
and grassland reaches of the
studied streams. Forested
stream reaches are marked
in grey and grassland
reaches in white, significant
differences (ANOVA
P\0.05) are marked with
asterisk.AMean and
standard deviation of d
values of trout individuals;
C values are lower in
grassland than in forested
reaches in River Granslev
and Storkesig Brook.
BMean and standard
deviation of d
N values of
trout individuals; d
always higher in grassland
than in forested reaches.
CMean and standard
deviations of estimations of
trophic position of trout;
trophic positions do not
differ between forested and
grassland reaches. When all
forested and grassland
reaches are pooled in the
analysis, d
N is higher in
grassland than in forested
reaches. Details on statistics
can be found in Table 3
Cross, 2014; Collins et al., 2016) do not support that
the hypothesis based on Riverine Productivity Model
(i.e. the largest fraction of riverine biomass being of
autochthonous origin) also apply to small-forested
streams (e.g. Brito et al., 2006; Mc Neely et al., 2006;
Li & Dudgeon, 2008; Lau et al., 2009b). Allochtho-
nous resources are frequently found to be prevalent
over autochthonous foods (e.g. in half of the scenarios
in our study), at least for macroinvertebrate assem-
blages in temperate forested streams, which contrasts
with the hypotheses of the Riverine Productivity
Model made for larger riverine systems (Thorp &
Delong, 2002).
Changes in energy assimilation between forested
and grassland sections occurred even for the same
species, as observed for G. pulex, E. danica and
Simuliidae sp. Therefore, the increase in the auto-
chthonous contribution to primary consumers with
increasing canopy openness does not necessarily arise
from a change in assemblage-specific composition (as
studied in the River Continuum Concept) but from
dietary changes within the same species. Our findings
of a plastic dietary assimilation of these invertebrate
species are in agreement with those of previous dietary
(e.g. Lo
´guez et al., 2009) and stable isotope
studies (e.g. Collins et al., 2016). Particularly, dietary
studies have shown that G. pulex is a well-known
generalist herbivore that is able to consume allochtho-
nous carbon from fungi and bacteria and also
autochthonous carbon sources, such as periphytic
algae, depending on food type availability (Moore,
1975; Grac¸a et al., 1993). The same applies to the filter
feeder E. danica (e.g. Austin & Baker, 1988;Lo
´guez et al., 2009) and Simulidae (e.g. Burton,
1973; Moore, 1977) that filter particulate organic
matter from both allochthonous and autochthonous
sources located upstream present in the water column
(Moore, 1977; Wallace & Merritt, 1980).
The patterns of autochthonous and allochthonous
carbon assimilation in the biota of riverine systems
probably reflect a combination of changes in food
availability and quality. Thus, whenever the epiben-
thic algae biomass increased (towards the open-
canopy grassland sites) the proportion assimilated by
the invertebrates may have increased, probably due to
their higher nutritional quality compared to terrestrial
sources (Thorp & Delong, 2002; Lau et al., 2009b;
Marcarelli et al., 2011). We base this argument upon
the fact that terrestrial sources were readily available
at all the stream reaches regardless of type (reflected in
the similar % of debris in the stream bottom, Table 1),
but that epibenthic algae biomass was higher in
grassland reaches where the autochthonous consump-
tion increased with decreasing canopy coverage. This
interpretation of our findings also agrees with exper-
imental evidence demonstrating a preferential assim-
ilation of autochthonous C at the top of a food web
when both allochthonous and autochthonous C are
equally available (Lau et al., 2009c).
Food type availability driven by forest canopy
cover can explain a large part of the variability
observed regarding autochthonous or allochthonous
dominance of stream food webs (Finlay, 2001; Bunn
et al., 2003; Brito et al., 2006; Lau et al., 2009b).
However, despite that within the same river grassland
reaches have higher autochthonous contribution than
forest reaches, this relationship does not seem to be as
straightforward across all the study sites. For example,
the open section of River Granslev has [70% of
canopy coverage and the macroinvertebrate biomass is
mostly autochthonous fuelled, while the open-canopy
section of River Skader has \60% canopy coverage
and its biomass is largely allochthonous fuelled. This
suggests that the carbon dynamics in streams can be
highly patchy, and other factors than the presence of
riparian forest (e.g. stream reach morphology) may
drive the carbon assimilation pathways at a larger
scale (Sullivan, 2013). In this sense it is worth noting
that within our dataset, despite that canopy coverage
was the strongest predictor of the autochthonous
support to food webs, we also found weak tendencies
to decrease autochthonous contribution to inverte-
brates with increasing stream width and nitrogen
concentration. This could be attributed to the role of
local reach characteristics affecting this pattern. For
example the dominance of pool and/or riffle stream
habitats may determine whether allochthonous mate-
rial can be deposited being available for consumers or
if it is only being transported downstream (Sullivan,
2013). In this sense, the upstream characteristics can
also influence locally, and even though we tried to
control for this in our study, we could still have an
effect of stream flow direction in our results. For
example, the only forest reach with dominant auto-
chthonous contribution is River Granslev, where the
stream flows from the grassland to forest and high
availability of autochthonous matter drifting to the
forest section could explain this. The opposite is true
for River Skader, where the streamflow comes from
forest to grassland direction and both grassland and
forest are mostly allochthonous dominated. Given this
high cross-system variability we argue that large scale
factors such as stream morphology and upstream
riparian patch size affect locally at a lower scale on the
physical environment (i.e. light irradiation and pres-
ence of different micro habitats) and food availability,
which may ultimately determine the patchy patterns of
carbon assimilation in small temperate streams.
Trophic position of brown trout was found to be
around three, corresponding well to an invertivorous
species and matching previous results for this species
in streams (e.g. Cucherousset et al., 2007; Kristensen
et al., 2016). Contrary to our expectations, the
contrasting isotopic signature of trout between forest
and grassland reaches did not represent a change in
FCL between stream reach types, and our second
hypothesis was therefore rejected. It needs to be
remarked, however, that the contrasting isotopic
signatures in brown trout of same-stream forested
and grassland sections support their sedentary beha-
viour having a home range of probably few-hundred
metres with the streams (at least in the last month
before sampling: time reflected by the isotopic
Some of the main determinants of stream FCL are
known to be ecosystem size, productivity and distur-
bance regime (Pimm & Kitching, 1987; Post et al.,
2000; Thompson & Townsend, 2005; McHugh et al.,
2010; Sabo et al., 2010). Data from streams reviewed
in Warfe et al. (2013) show that a positive relationship
between FCL and ecosystem size is found very often
(observed in eight out of ten reviewed studies), and a
negative relationship between disturbance regime and
FCL is also common (found in seven out of eleven
study cases), but positive relationships between pro-
ductivity and FCL are less frequently found (in seven
out of sixteen studies reviewed there). This also
matches the observed for rivers, where the increase in
FCL with ecosystem area depends on decreasing
disturbance intensity with increasing area, but a
relationship with food resource availability is not
evident (Sabo et al., 2010). In our study (comparing
streams with similar ecosystem area and disturbance
regimes), the availability of detritus was similar in
both reach types, while the availability of benthic
algae was greater in grassland than in the forested
reaches (probably reflecting higher all together local
productivity in grassland streams), but this did not
translate into a difference in FCL between reach types.
A lack of a relationship between productivity and
FCL was reported several times before and sometimes
discussed as a consequence of the presence of mobile
species that translocate energy form the most produc-
tive patch to the less productive ones (e.g. Warfe et al.,
2013). This could apply to our study, especially
considering that the spatial extent of the study was of
few-hundred metres, and most of the studied species
could move such distances (in a longer time-scale than
reflected in stable isotope analysis), distributing
energy across the ecosystem patches.
Our result contrasts the findings of Lau et al.
(2009a) that tropical forest streams were at least one
trophic position shorter than similar open-canopy
streams. However, two main differences between the
methods applied to estimate trophic positions in Lau
et al. (2009a) and our study might partly account for
these differences. Firstly, they only used the organism
with the lowest d
N value regardless of its taxonomic
identity in each stream reach as base for estimating
trophic position, whereas we used the average of
primary consumers in each site. Secondly, Lau et al.
(2009a) collected and used different top predators
between stream reach types, while we always found
and used the same species (S. trutta). Thus, the
enlarged FCL revealed in Lau et al. (2009a) may be the
result of the inclusion of species consuming fish or
feeding selectively on predatory invertebrates in open-
canopy reaches and not necessarily a change in trophic
strategy of the same species, as tested here.
In our study site, independently of riparian
cover, FCL was found related to substrate cover-
age, decreasing as the sand coverage in the stream
bottom increased. Sand-substrates could be consid-
ered as more homogeneous than coarser substrates,
which create more micro-scale heterogeneity of
flow conditions and represent high refuge and food
resource availability increasing diversity of
macroinvertebrates (e.g. Kovalenko et al., 2012).
Moreover, a higher dominance of this substrate
type implies a lower diversity of substrate types (a
well-known proxy for habitat heterogeneity in
streams, e.g. Palmer et al., 2009; Kovalenko
et al., 2012). Higher habitat heterogeneity usually
promotes a higher functional and taxonomic diver-
sity creating more trophic niches, partly due to
attenuation of flow disturbances in streams (e.g.
Warfe et al., 2008; Kovalenko et al., 2012). Thus,
it seems reasonable that an increasing dominance of
sand in the stream-bottom decrease habitat hetero-
geneity producing shorter FCL because more
exposure to disturbances and less trophic diversity
usually means simpler (and more likely shorter)
food-chain lengths (Kovalenko et al., 2012).
In our study, we used paired grassland and forest
stream reaches to demonstrate that closed-canopy
coverage in forest reaches can alter energy pathways
in streams. We have shown that these changes in
subsidies to the whole macroinvertebrate biomass
occur also at intraspecific levels (within same species),
suggesting that the change in energetic sources for
macroinvertebrate biomass can be independent of a
change in species composition. The study also high-
lighted the need for continued field and experimental
research into mechanisms driving resource subsidies
in streams to unveil ecosystem level consequences of
changed subsidies at the food-web base. This becomes
particularly important in streams where riparian forest
planting is used as a measure to mitigate agricultural
and climate-induced impacts on the ecosystems (e.g.
Broadmeadow et al., 2011) or when testing the effect
of clear-cutting or wildfire removing riparian areas.
Our study shows that riparian forest may not only
change stream community structure, as reported
before in previous studies (e.g. Gregory et al., 1991;
Moldenke & Ver Linden, 2007; Teixeira-de Mello
et al., 2016), but may also affect the resource subsidies
and food-web structure with potential ecosystem
consequences. A greater understanding of this subject
will improve the implementation of riparian buffer
corridors, thus avoiding adverse consequences for
stream biodiversity and functioning. For example,
even though invertebrates can exploit a wide diversity
of autochthonous and allochthonous foods, autochtho-
nous-derived carbon seems essential during the crit-
ical life stages of many invertebrates in spring and
summer (Junker & Cross, 2014).
Acknowledgements The authors are grateful for financial
support from The Danish Natural Science Research Council (T.
Riis Grant #272-09-0012), the Carlsberg Foundation (T. Riis
Grant #2013_01_0258) and the European Union 7th Framework
projects REFRESH under Contract No. 244121 and MARS
under Contract No. 603378 (A. Baattrup-Pedersen).
I. Gonza
´lez-Bergonzoni received support from SNI (Agencia
Nacional de Investigacio
´n e Innovacio
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... Also, ongoing global changes have resulted in noticeable increases in nutrient concentrations, including nitrogen and phosphorus, in the stream ecosystems over the past several decades (Butman et al., 2015;Vörösmarty et al., 2010). There is widespread recognition that such changes have a profound impact on the structure and functioning of the stream ecosystems (Gonzalez-Bergonzoni et al., 2018;Machado-Silva et al., 2022;Warry et al., 2016). ...
... These changes in primary producers can result in increased trophic diversity and alterations in food web structure within streams (Ardón et al., 2021;García et al., 2017). Although several studies have examined the isolated effects of nutrient pollution or riparian canopy cover on stream food webs (Bergfur et al., 2009;de Guzman, 2022;Gonzalez-Bergonzoni et al., 2018), the joint effects of these stressors on resource flow and trophic structure in stream ecosystems are not yet fully understood. ...
Riparian deforestation, which leads to increase in light intensity and excessive nutrient loading in waterways, are two pervasive environmental stressors in the stream ecosystems. Both have been found to alter basal resource availability and consequently stream food webs. However, their interactive effects on trophic structure in stream food webs are unclear. Here, we manipulated light intensity and nutrient availability in three headwater streams to evaluate their effects on consumer diet composition and food web characteristics (i.e., trophic diversity and redundancy) with stable isotope analysis. Dietary analysis revealed that the relative contribution of stream periphyton to the diets of macroinvertebrates increased, while that of allochthonous resources, specifically leaf litter from the terrestrial ecosystems in the catchment, decreased in response to open canopy and nutrient enrichment in the streams. The trophic diversity also increased with the elevated light intensity and nutrient availability, while the trophic redundancy decreased, suggesting a reduced ability of the stream ecosystems to resist environmental changes. Nutrient enrichment also increased the δ15N ratios of periphyton and macroinvertebrates, indicating potential δ15N enrichment of stream benthos by nitrogen pollution. Our results suggested that an increase in light intensity due to riparian canopy openness and stream water nutrient enrichment primarily from human activities have interactive effects on resource flow and trophic structure in stream food webs.
... In tropical forested streams, some studies have found support for the general framework of the RCC (Greathouse & Pringle, 2006;Tomanova et al., 2007), but Neres-Lima et al. (2016) clearly demonstrated the importance of autochthonous resources even in shaded reaches. Because few streams located outside north temperate forested watersheds have received sufficient research attention despite their widespread global occurrence (González-Bergonzoni et al., 2018), broad patterns of resource use occurring in streams located outside of temperate forested regions are generally not well understood. While the authors of the RCC noted that autochthonous resources may be more important than allochthonous resources in non-forested headwater streams with open canopies (Minshall et al., 1983(Minshall et al., , 1985Vannote et al., 1980), this idea was proposed as a contextual qualification of a concept developed for forested systems. ...
... Our results for temperate steppe streams, together with studies from temperate forested watersheds (Hayden et al., 2016;Mayer & Likens, 1987;McNeely et al., 2007), streams in subarctic and alpine regions Fujibayashi et al., 2019), tropical stream ecosystems (Hayden et al., 2021;Neres-Lima et al., 2016), large rivers (Bowes et al., 2020;Thorp & Bowes, 2017), desert floodplains (Bunn et al., 2003) and a cave river ecosystem (Liew et al. 2019b) provide united evidence that while some consumers may assimilate allochthonous F I G U R E 5 Bayesian stable isotope mixing model (MixSIAR) posteriors (mean ± SD) indicating the relative proportional contribution of each basal resource category (aquatic, bacterial, fungal and terrestrial) assimilated by fishes collected in six ecoregions: Mongolia grassland (a), Mongolia montane (b), Mongolia semi-arid terminal basin (c), US grassland (d), US montane (e) and US semi-arid terminal basin (f; n = 356). ...
Full-text available
Quantifying the trophic basis of production for freshwater metazoa at broad spatial scales is key to understanding ecosystem function and has been a research priority for decades. However, previous lotic food web studies have been limited by geographic coverage or methodological constraints. We used compound‐specific stable carbon isotope analysis of amino acids (AAs) to estimate basal resource contributions to fish consumers in streams spanning grassland, montane and semi‐arid ecoregions of the temperate steppe biome on two continents. Across a range of stream sizes and light regimes, we found consistent trophic importance of aquatic resources. Essential AAs of heterotrophic microbial origin generally provided secondary support for fishes, while terrestrial carbon did not seem to provide significant, direct support. These findings provide strong evidence for the dominant contribution of carbon to higher‐order consumers by aquatic autochthonous resources (primarily) and heterotrophic microbial communities (secondarily) in temperate steppe streams.
... By intercepting and utilising the nutrients from agricultural areas, they protect freshwater ecosystems against eutrophication [53]. A riparian forest modifies the fuelling sources for stream food in which allochthonous carbon sustains the macroinvertebrate biomass [54], retains fine sediments, nutrients and pesticides and controls water temperature and primary production [55]. The smaller cover of a riparian forest is associated with a significantly greater percentage of silt and very fine organics in the substratum [56]. ...
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The objectives of our survey were to determine the most important environmental factors within buffer zones that influenced mollusc communities and to evaluate the ecological conservation value of natural aquatic habitats (NAHs) that support mollusc species. Analysis of the spatial structure of buffer zones and catchments was based on a set of landscape metrics. Land cover classes were determined, and buffer zones within a radius of 500 m from a sampling point were marked out. Mollusc samples were collected from each NAHs. Our results showed that the number of patches and mean patch size were most associated with the distribution of mollusc species. Within patches of buffer zones, the length of the catchment boundaries with low-density housing, an increasing area of forest and pH of the water were also significant. Our results proved that landscape metrics provide essential information about catchment anthropogenic transformation. Therefore, landscape metrics and the designated buffer zones should be included in restoration plans for the river, water bodies and adjacent habitats as elements of modern, sustainable water management. NAHs located along a valley of a lowland river provide refuges for molluscs, play an essential role in the dispersal of IAS, create important protective biogeochemical barriers for rivers, constitute necessary sources of moisture and water and support microhabitats for distinct mollusc communities, especially in the context of global warming.
... It is predicted that salmonid fishes in the lake will have  13 C values that are indicative of the heterotrophic pathway in littoral regions, distinguishable from the pelagic autotrophic pathway. Detritus deriving from terrestrial sources has been shown to have distinct  13 C values compared to autotrophs found in lotic ecosystems (Rounick et al., 1982, González-Bergonzoni et al., 2018, Lau et al., 2009). ...
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Salmonid fisheries in the Snowy Mountains of Australia are an important recreational, social and economic asset. Despite their high value, there is a lack of fundamental ecological understanding of what factors are most important in determining the persistence of a high-quality recreational fishery in this region. This is particularly important given that there is emerging evidence of declining catches and size of fish in some Snowy Mountain lakes, most notably Lake Jindabyne. To address this issue, the current study assessed the major energy pathways supporting Salmo trutta, Oncorhynchus mykiss and Salmo salar in Lake Jindabyne and its tributaries. Stable isotope and gut content analysis techniques were employed to address two hypotheses; (1) The littoral zones of Lake Jindabyne are the dominant zone for foraging for all salmonid size classes, and (2) energy derived from tributaries feeding Lake Jindabyne (Snowy River, Little Thredbo River and Sawpit Creek) will contribute to salmonid biomass in the lake. Littoral margins of Lake Jindabyne were the dominant foraging habitat for salmonid species of all sampled size classes, however there was some evidence for utilisation of the energy pathway functioning in the pelagic zone by Oncorhynchus mykiss. Some coupling between lentic and lotic systems was established, based on similarities in 13C between salmonids in the Snowy River and Oncorhynchus mykiss in Lake Jindabyne. These findings will assist future management decisions aimed to improve the Lake Jindabyne salmonid fishery, by highlighting critical zones on which to focus management efforts. This study also creates a solid foundation for future research to fully understand the energy pathways contributing to salmonid biomass in Lake Jindabyne.
... It has been acknowledged that specific environmental elements determine the FCL only under specific conditions [7,18], e.g., available resources determine the FCL only when resources are limited [5,28,29] or habitat heterogeneity and the complexity of the food web could buffer the influence of disturbance on the FCL [25]. Besides, FCL is also affected by overfishing [8], ecosystem structure [30], and riparian vegetation cover characteristics [31]. Therefore, it is necessary to independently study the FCL in specific ecosystems and the controlling factors for those ecosystems rather than simply inferring from the results of other ecosystems. ...
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Food chain length (FCL) is a critical measure of food web complexity that influences the community structure and ecosystem function. The FCL of large subtropical rivers affected by dams and the decisive factors are far beyond clear. In this study, we used stable isotope technology to estimate the FCL of fish in different reaches of the main stream in the Yangtze River and explored the key factors that determined the FCL. The results showed that FCL varied widely among the studied areas with a mean of 4.09 (ranging from 3.69 to 4.31). The variation of FCL among river sections in the upstream of the dam was greater than that in the downstream. Regression analysis and model selection results revealed that the FCL had a significant positive correlation with ecosystem size as well as resource availability, and FCL variation was largely explained by ecosystem size, which represented 72% of the model weight. In summary, our results suggested that ecosystem size plays a key role in determining the FCL in large subtropical rivers and large ecosystems tend to have a longer food chain. Additionally, the construction of the Three Gorges Dam has been speculated to increase the FCL in the impoundment river sections.
... Stable isotope analysis has been increasingly used, in ecological studies of freshwaters as a powerful tool to determine the flow of energy and elements across food webs (e.g. González-Bergonzoni et al., 2018Post, 2002;Xu & Xie, 2004). ...
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Riparian forest may represent a key subsidy to aquatic food webs. While most research has been done in running waters, the origin of resources subsidising biomass and the role of riparian coverage in food webs remains largely unstudied in reservoirs. This research evaluated the role of forest riparian zones as food web subsidies in a recently formed reservoir of Ijuí River, Brazil. The diet of fish and the origin of carbon and nitrogen fuelling fish biomass was compared between littoral habitats with riparian forests and of open canopy. Sampling for stable isotopes of fish and basal resources and the application of bayesian mixing models showed that most of the fish production originates from terrestrial carbon in this recently formed reservoir. Moreover, an increasing terrestrial support to the biomass was found towards the riparian forest areas (subsidising c.a. 70% vs. around 57% of fish biomass in littoral areas of riparian forest and open canopy, respectively).This study remarked the role of riparian forest areas as energetic subsidises to aquatic ecosystems. Management actions in newly formed reservoirs should include preserving riparian forests to avoid losing natural ecosystem subsidies.
... In addition, a measure of organism biomass sustained by each food resource is needed to estimate the contribution of different food resources to food webs. Although organism biomass is considered when addressing the resource importance from gut content analyses, biomass values are lacking in many studies using assimilation estimates from stable isotopes (Fellman et al., 2015;González-Bergonzoni et al., 2017;Ishikawa, Uchida, Shibata, & Tayasu, 2014; was reflected in food-web structure. There was an increase in detritivore biomass and conservation of omnivore biomass with increasing forest cover, leading to a more equal distribution of community biomass among macroinvertebrates comprising individual food webs. ...
Understanding how different food resources sustain stream food webs is fundamental towards increasing our knowledge on trophic structure and energy flow pathways in fluvial ecosystems. Food webs in small mountain streams are sustained by autochthonous (instream primary production) and allochthonous (inputs from the terrestrial ecosystem) organic resources, with their relative importance highly dependent on catchment land cover. This study aimed to understand how catchment land cover determines food resource type (autochthonous, allochthonous) and quantity in mountain streams, and how this affects energy flow pathways and food web structure. We hypothesised that food resource type and quantity would reflect catchment land cover. Thus, changes in food resources would lead to shifts in macroinvertebrate assimilation of autochthonous and allochthonous food resources and consequently in dominant energy flow pathways. We further hypothesised that changes in food resources will have strong effects on dominant feeding groups and community biomass distribution among taxa in food webs. Energy flow pathways were quantified by combining macroinvertebrate biomass measures and assimilation of food resources estimated from δ ² H and δ ¹⁵ N in 10 streams along a forest cover gradient, located in the Cantabrian Mountains (northern Spain). Results showed that grassland/shrub dominated streams had a higher proportion autochthonous food resources and a lower proportion of allochthonous food resources, whereas forested streams showed the opposite pattern. Changes in food resources with forest cover resulted in shifts in food resource assimilation and dominant energy flow pathways. Forested streams were mainly sustained by allochthonous resources, while streams flowing through grassland/shrub landscapes were mostly sustained by autochthonous resources. Food resource assimilation differed between feeding groups. Detritivores showed a fixed assimilation of allochthonous resources independent of resource quantity, while omnivore assimilation was determined by the dominant food resource. This was reflected in food‐web structure. There was an increase in detritivore biomass and conservation of omnivore biomass with increasing forest cover, leading to a more equal distribution of community biomass among macroinvertebrates comprising individual food webs. The dependence of stream food webs on dominant food resources highlights the importance of catchment land cover in determining energy flow pathways and food web structure in low order mountain streams. These findings will improve our predictions on the effects of land cover change on the functioning of mountain stream ecosystems.
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The integrated assessment of stream networks and terrestrial land use contributes a critical foundation for understanding and mitigating potential impacts on stream ecology. Riparian zone delineation and management is a key component for regulating water quality, particularly in agricultural watersheds. We present a national assessment of riparian zone land uses according to stream order for the entire hydrological network in the Uruguayan landscape in Southeastern South America. We classified over 82,500 km of streams and rivers in Uruguay into seven Strahler order classes and delineated riparian buffers of 100 and 500 m, depending on stream order, covering a total of 13% of the terrestrial land area in Uruguay. Natural vegetation cover in riparian zones averaged 77% among basins, whereby natural grassland dominated first and second order stream buffers at 58% and 49%, respectively. This highlighted the importance of grasslands in headwater regions of the country. Riparian forests formed corridors along larger streams, representing a mere 9% of buffers in first order streams but reaching 46% of buffers of 6th order streams. Among the six major basins of Uruguay, we found differences in the relative importance of riparian forests and crop cover in headwater stream riparian zones, as well as differences in relative crop cover within riparian zones. Results show that streams in subtropical grassland landscapes originate in open grassland environments, which has major implications for thermal regimes, carbon inputs, and stream biodiversity. Riparian buffer management should consider geographic differences among different basins and ecoregions within Uruguay.
Studies have shown that food chain length is governed by interactions between species richness, ecosystem size and resource availability. While redundant trophic links may buffer impacts of species loss on food chain length, higher extinction risks associated with predators may result in bottom‐heavy food webs with shorter food chains. The lack of consensus in earlier empirical studies relating species richness and food chain length reflects the need to account robustly for the factors described above. In response to this, we conducted an empirical study to elucidate impacts of land‐use change on food chain length in tropical forest streams of Southeast Asia. Despite species losses associated with forest loss at our study areas, results from amino acid isotope analyses showed that food chain length was not linked to land use, ecosystem size or resource availability. Correspondingly, species losses did not have a significant effect on occurrence likelihoods of all trophic guilds except herbivores. Impacts of species losses were likely buffered by initial high levels of trophic redundancy, which declined with canopy cover. Declines in trophic redundancy were most drastic amongst invertivorous fishes. Declines in redundancy across trophic guilds were also more pronounced in wider and more resource‐rich streams. While our study found limited evidence for immediate land‐use impacts on stream food chains, the potential loss of trophic redundancy in the longer term implies increasing vulnerability of streams to future perturbations, as long as land conversion continues unabated.
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The origin of resources supporting metazoan biomass in rivers has long been a subject of debate. The river wave concept (RWC) postulates that the energetic basis of food webs varies along its spatial–temporal location with respect to flow pulses. According to the RWC, river flow determines carbon assimilation in food webs, but this may also depend on river geomorphology. However, studies testing this theory are scarce, particularly those from large subtropical rivers. To analyse the origin of fish biomass in areas of differing geomorphology, we combined stable isotope analysis with standardised measurements of biomass of a diverse fish assemblage along the lower Uruguay River. Furthermore, using 14 years of monitoring data, we tested for relationships between the biomass of species dominantly fuelled by allochthonic resources and the river flow. Fish biomass was dominantly allochthonous-derived along most of the studied sites. At all trophic levels, autochthonous-derived fish biomass was the highest in an upstream anabranch functional process zone (FPZ) (fuelling 54% of the total biomass), while allochthonous-derived biomass prevailed downstream, in the widest sections of an unconstrained lowland FPZ (fuelling 64–72% of the total biomass). Moreover, the dominant species that derived most of its biomass from allochthonous resources (Prochilodus lineatus) increased its biomass following flood pulses. This study supports the RWC statements that, at a spatial scale, local river geomorphology affects fuelling sources for food webs (probably by determining contrasting resource availability scenarios) and, at a temporal scale, increases in the allochthonous fraction of biomass are driven by flood pulses.
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Differences in trophic web structure in otherwise similar ecosystems as a consequence of direct or indirect effects of ambient temperature differences can lead to changes in ecosystem functioning. Based on nitrogen and carbon stable isotope analysis, we compared the food-web structure in a series of subtropical (Uruguay, 30–35°S) and temperate (Denmark, 55–57°N) shallow lakes. The food-web length was on average one trophic position shorter in the subtropical shallow lakes compared with their temperate counterparts. This may reflect the fact that the large majority of subtropical fish species are omnivores (i.e., feed on more than one trophic level) and have a strong degree of feeding niche overlap. The shapes of the food webs of the subtropical lakes (truncated and trapezoidal) suggest that they are fuelled by a combination of different energy pathways. In contrast, temperate lake food webs tended to be more triangular, likely as a result of more simple pathways with a top predator integrating different carbon sources. The effects of such differences on ecosystem functioning and stability, and the connection with ambient temperature as a major underlying factor, are, however, still incipiently known.
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Stable-isotope analysis is widely used in aquatic ecosystem studies to evaluate trophic structure and resource dynamics. Because δ 15 N values vary in freshwater systems, e.g., reflecting variations in land use, suitable baseline indicators must be specified. Few investigators have identified specific baseline organisms based on thorough and methodical screening. We screened for baseline organisms in temperate lotic waters based on 4 criteria: 1) baseline organisms should be easy to collect, 2) within-site variation in δ 15 N levels should be low, 3) δ 15 N should reflect land use, and 4) trophic position (TP) of consumers calculated from the baseline should be independent of system-specific δ 15 N variability as long as no systematic change in food consumption occurred. We investigated individual taxa and bulked groups representing different feeding modes as baselines. We found that Simuliidae, a sestonic filter feeder, fulfilled all criteria. Furthermore, TP estimates of 2 common fishes that were based on the Simuliidae or grouped filterers as baselines were the only estimates in our study that were independent of landuse changes. In addition, the diet of these fishes did not change across land use as based on stable-isotope mixing-model analysis. Simuliidae also had the lowest within-site variation, i.e., the lowest trophic level range, probably a result of uniform feeding behavior. Therefore, Simuliidae and grouped filterers could be suitable baseline indicators in future studies. We recommend minimizing δ 15 N variability in and among systems because the precise, complex choice, timing, or proportions of food sources consumed cannot be mimicked. We also promote combining TP estimation and mixing-model analyses as a strong tool in studies of stream food webs.
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The characteristics of riparian vegetation along streams vary with natural and anthropogenic factors. Deforestation for agricultural purposes has consequences for the physical in-stream structure and function, such as the predominance of autotrophic or heterotrophic stream metabolism. Open canopy lowland streams are often dominated by macrophytes, with potential direct and indirect effects on the fish community. We tested for possible differences in the structure (relative abundance of species, mean body size, and density) and composition (species richness, species identity, and different trophic groups) of fish assemblages between open canopy streams (OCS) and riparian forest streams (RFS), including pool and riffle habitats, in temperate lowland Denmark. OCS reaches exhibited higher alpha and beta diversity and frequently hosted rare species. Almost 50 % of the recorded species appeared only in OCS. OCS also had smaller mean body size of fish and tended to have higher fish densities. The relative abundance of the different trophic groups did not differ between the two streams types, but the RFS had a higher abundance and occurrence frequency of intolerant salmonids. Our results suggest that modification of riparian habitats can affect richness patterns and that strong functional changes may occur as a consequence of forest clearance through changes in the relative importance of a keystone species, trout (Salmo trutta).
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Environmental determinants of fluvial food-chain length (FCL) remain unresolved, with predominant hypotheses pointing to productivity, disturbance, and/or ecosystem size. However, drainage configuration (for example, drainage density, and stream length)—in spite of recent advances demonstrating the significance of catchment structure to habitat and biodiversity of fluvial systems—has yet to be explored in relation to FCL. In this study, we quantified the relative influences of ecosystem size and structure on FCL for linked stream–riparian food webs. At 19 stream reaches distributed within three mountain catchments of northern Idaho, USA, we sampled aquatic and riparian consumers and determined FCL using the naturally abundant stable isotopes 13C and 15N. Food-chain length was then related to reach measures of size and structure using an information-theoretic model selection approach. Model selection was followed by exploratory linear regression of FCL with purported mechanistic factors (that is, resource availability and disturbance regime). FCL ranged from 2.6 to 4.4 across study reaches and was best explained by catchment structure such as number of tributary junctions and distance to nearest downstream confluence. Regression analyses suggested that disturbance regime may mechanistically link number of tributary junctions and FCL, as well as drainage area and FCL. Our results introduce novel evidence that ecosystem structure may integrate the effects of several mechanistic factors and thus be an important predictor of food-web structure.
Cross system subsidies of energy and materials can be a substantial fraction of food web fluxes in ecosystems, especially when autochthonous production is strongly limited by light or nutrients. We explored whether assimilation of terrestrial energy varied in specific consumer taxa collected from streams of different sizes and resource availabilities. Since headwater streams are often unproductive, we expected that inputs from surrounding terrestrial systems (i.e. leaf litter, terrestrial invertebrates) would be a more important food source for consumers than in mid-size rivers that have more open canopies and higher amounts of primary production available for consumers. We collected basal resources, invertebrates, and fish along a gradient in stream size in the Adirondack Mountains (NY, USA) and in Trinidad and Tobago and analyzed all samples for hydrogen isotopes as a means of differentiating biomass derived from allochthonous versus autochthonous sources. We found significant differences in allochthonous energy use within individual consumer taxa, showing that some taxa range from being entirely allochthonous to entirely autochthonous depending on where they were collected on the stream size gradient (grazers and collector–gatherer functional feeding groups), while other taxa are relatively fixed in the source of energy they assimilate (shredder and predator functional feeding groups). Consistent with expectations, allochthonous energy use was positively correlated with canopy cover in both regions for most feeding groups, with individuals from small, shaded streams having a more pronounced allochthonous signal than individuals collected from larger streams with less canopy cover. However, consumers in the shredder/detritivore feeding group did not vary among sites in their allochthonous energy use, and had a mostly allochthonous signal regardless of canopy cover and algal biomass. Our results demonstrate that the importance of energy from terrestrial subsidies can vary markedly but are similar in both temperate and tropical streams, suggesting a widely consistent pattern.This article is protected by copyright. All rights reserved.