Stable isotope analysis confirms substantial differences between subtropical and temperate shallow lake food webs

Abstract

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.

Full text

PRIMARY RESEARCH PAPER
Stable isotope analysis confirms substantial differences
between subtropical and temperate shallow lake food webs
Carlos Iglesias .Mariana Meerhoff .Liselotte S. Johansson .
Ivan Gonza
´lez-Bergonzoni .Ne
´stor Mazzeo .Juan Pablo Pacheco .
Franco Teixeira-de Mello .Guillermo Goyenola .Torben L. Lauridsen .
Martin Søndergaard .Thomas A. Davidson .Erik Jeppesen
Received: 14 December 2015 / Revised: 4 June 2016 / Accepted: 6 June 2016
ÓSpringer International Publishing Switzerland 2016
Abstract 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 func-
tioning. Based on nitrogen and carbon stable isotope
analysis, we compared the food-web structure in a
series of subtropical (Uruguay, 30–35°S) and temper-
ate (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 connec-
tion with ambient temperature as a major underlying
factor, are, however, still incipiently known.
Keywords Food-web structure Food-web length 
Omnivory Ecosystem functioning
Introduction
The height and shape of trophic webs may potentially
affect the entire ecosystem functioning (Post et al.,
Handling editor: Katya E. Kovalenko
C. Iglesias (&)M. Meerhoff 
I. Gonza
´lez-Bergonzoni N. Mazzeo 
J. P. Pacheco F. T. Mello G. Goyenola
Grupo de Ecologı
´a y Rehabilitacio
´n de Sistemas
Acua
´ticos, Departamento de Ecologı
´a Teo
´rica y Aplicada,
Centro Universitario de la Regio
´n Este-Facultad de
Ciencias, Universidad de la Repu
´blica, Tacuarembo
´s/n
CP 20000, Maldonado, Uruguay
e-mail: caif@cure.edu.uy
C. Iglesias M. Meerhoff L. S. Johansson 
I. Gonza
´lez-Bergonzoni T. L. Lauridsen 
M. Søndergaard T. A. Davidson E. Jeppesen
Department of Bioscience, Aarhus University,
Vejlsøvej 25, 8600 Silkeborg, Denmark
I. Gonza
´lez-Bergonzoni
Departamento de Ecologı
´a y Evolucio
´n, Facultad de
Ciencias Universidad de la Repu
´blica, Montevideo,
Uruguay
T. L. Lauridsen T. A. Davidson E. Jeppesen
Arctic Research Centre, Aarhus University, Building
1110, C. F. Møllers Alle 8, 8000 Aarhus, Denmark
T. L. Lauridsen M. Søndergaard E. Jeppesen
Sino-Danish Education and Research Centre, Beijing,
China
123
Hydrobiologia
DOI 10.1007/s10750-016-2861-0

2000; Woodward, 2009; Emmerson, 2012). Particu-
larly, the number of steps involved in the transfer of
energy from primary producers to top predators, i.e.,
the food-web length (hereafter FWL), seems at least
partly determined by ecosystem productivity and size,
ambient temperature, habitat heterogeneity, and
changes in species richness (including arrival and loss
of species). These variations may occur among, but
also within, ecosystems across temporal and spatial
scales [as described in the pioneer works by Elton
(1927) and Lindeman (1942) and reviews by Doi et al.
(2012), Pimm (1991), and Post (2002a)]. However, a
global-scale analysis showed that lake and stream
FWL exhibited no direct or, at most, a weak relation-
ship with ecosystem size, mean annual air tempera-
ture, or latitude; however, there was a tendency for
FWL to be longer at high latitudes than in the tropics
(Vander Zanden & Fetzer, 2007).
Theoretical analyses (Arim et al., 2007a,b; Post &
Takimoto, 2007) and modeling exercises (Takimoto
et al., 2012) suggest that the length and also the
connection strength within a food web may be
explained, at least in part, by the degree of omnivory
of intraguild predators (IGP) (Post & Takimoto, 2007;
Takimoto et al., 2012). Widespread feeding on lower
trophic positions would result in shorter FWL (Lay-
man et al., 2005; Post & Takimoto, 2007), a
phenomenon termed the ‘‘omnivory mechanism’’
(Post & Takimoto, 2007). In contrast, addition of
species with potentially different diets, as expected in
subtropical regions where fish richness and specific
and functional diversity are far higher than in similar
temperate shallow lakes (Teixeira-de Mello et al.,
2009), could result in longer FWL [the ‘‘addition
mechanism’’ sensu Post & Takimoto (2007)]. Thus,
contrasting scenarios, indirectly linked to the climate
regime, could emerge depending on the predominance
of each mechanism. However, empirical evidence of
the relationships between omnivory and specific
richness and FWL, and the relative importance of
the underlying mechanisms, is still scarce (Glazier,
2012).
The shape of food webs also responds to the
occurrence of different types of primary producers.
When several resources co-occur, more complex
pathways for the transfer of energy and matter may
exist (Polis & Strong, 1996; Vadeboncoeur et al.,
2005). Such multiple pathways may occur in shallow
lakes where both pelagic primary production (by
phytoplankton) and littoral or benthic primary pro-
duction (by periphyton) can be important sources of
energy (Vadeboncoeur et al., 2003; Vander Zanden
et al., 2011). However, the extent to which the
different basal resources are exploited might be
indirectly linked to differences in ambient temperature
(Meerhoff et al., 2007). Changes in the width of the
carbon resources (carbon range, sensu Layman et al.
(2007)), together with the occurrence of complex
pathways, could thus be found in different lake types
and under different climate regimes.
To elucidate variations in the length and shape of
trophic webs from shallow lakes in two regions with
distinct climates, we analyzed stable isotopes (d
15
N,
d
13
C) of key biological communities of the food web
in five subtropical (Uruguay, 30–35°S) and four
temperate (Denmark, 55–57°N) shallow lakes. Con-
sidering the greater fish species richness of subtropical
systems, we expected that longer FWL and a wider use
of carbon sources would occur, if the ‘‘addition
mechanism’’ prevails. Alternatively, FWL might be
shorter in the subtropics, if the expected higher degree
of omnivory and its consequent effects at the com-
munity level predominate in the set of studied lakes.
Methods
Study sites
We selected five shallow lakes located along the east
coast of Uruguay covering a wide range in trophic
states and water transparency (Table 1). The lakes
represent the typical variability in the trophic state of
Uruguayan shallow lakes (Kruk et al., 2009; Pacheco
et al., 2010). For the comparison, we used data from
four lakes representative of shallow lakes in Denmark,
selected to ensure, to as high an extent as possible,
comparability with the Uruguayan lakes regarding
trophic state and key limnological characteristics, an
exception being size that was somewhat smaller in the
temperate region.
Field sampling
The samples were collected at the end of the growing
season (late summer) in both countries (March in
Uruguay, August in Denmark). A similar sampling
protocol was used in both countries and included an
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123

intensive sampling of the pelagic and littoral habitats
to obtain taxa representing all trophic levels and
carbon sources. For stable isotope analyses (SIA), we
collected samples of the principal consumers in both
pelagic and littoral areas. To ensure a sufficient
amount of organisms for the analysis, lake water was
pumped through conical plankton nets (20 and 65 lm
for phytoplankton and zooplankton, respectively),
macrophyte-associated macroinvertebrates and ben-
thic macroinvertebrates were sampled by intensively
swiping a hand net and by integrating several dredges
covering the entire bottom of each lake, respectively.
Fish were captured with multimesh-size gillnets and
electrofishing; the sampling effort used included the
deployment of several gillnets which were set over-
night. Electrofishing was conducted in the littoral
areas at sunset to capture small specimens and littoral
sit-and-wait predators. This combined sampling
method can appropriately capture the structure of the
target community in both studied regions (Teixeira-de
Mello et al., 2009), fact confirmed as we found species
a priori unknown to be in the studied systems. All
samples were rapidly frozen and transported to the
laboratory.
Following recommendations by Post (2002b),
principal carbon source signals from the pelagic and
littoral areas were indirectly estimated from the two
well-known primary consumers (as substitution for
primary producers), namely filter-feeding bivalves
and grazing snails (Post, 2002b). The selection of the
right baseline individuals is essential for the estima-
tion of an ‘‘average’’ food-web length in the commu-
nity, and the selection of gastropods and bivalves
seems the best strategy as they are long-living and low
dispersion organisms representing two contrasting
energy uptake pathways (Post, 2002b; Jardine et al.,
2014). Macrophyte leaves and periphyton washed
from the predominant macrophytes were also
sampled.
Sample processing for isotopic analysis and data
analysis
In the laboratory, samples of plants, periphyton,
phytoplankton, zooplankton, macroinvertebrates, fish
flank muscle, and snail and bivalve soft tissue were
freeze dried and ground to a fine powder for
stable isotope analysis (SIA). Each sample (1–3 mg,
weighed to 0.01 mg precision) was transferred to tin
capsules and analyzed at the UC Davis Stable Isotope
Facility (University of California, USA) for carbon
and nitrogen stable isotopes. The food-web structure
of each lake was visualized by plotting the trophic
position (based on d
15
N isotopic signature values)
against d
13
C values for all available organisms (Fry,
1991).
We estimated the trophic position of each individ-
ual according to Post (2002b):
Table 1 Main limnological features of the subtropical and temperate study lakes measured simultaneously with the stable isotope
sampling or according to the already published results (Pacheco et al., 2010)
Vaeng Tranevig Gammelmose Denderup Cisne Diario Garcia Clotilde Blanca
Area 15 2.7 1.3 4.6 127 101 13.5 29 60
Z
max
1.7 0.9 1.6 1.9 3.2 1.7 2 3.1 2.5
SD 0.9 0.75 1.3 1.8 0.4 1.05 1.7 1.8 0.64
Temp 17 16.6 16.4 16.3 13.1 19.2 16.3 17.6 19.6
pH 7.9 7.6 8.1 8.1 7.1 7.3 6.32 7 7.4
Cond 268 71 595 171 210 348 142 360 320
Chl-a 65.8 37.6 78.5 7.2 6 10 2 2.3 38.6
PVI 0 44 3 27 0 40 5 28 13
TP 113 60.5 157 54 413 75.8 29.8 27.7 51.9
TN 1018 1040 2212 664 1048 825 332 451 1017
Lake area (ha), maximum depth, Zmax (m), Secchi depth, SD (m), summer values of temperature, Temp (°C), conductivity, Cond
(lScm
-1
), percentage of lake volume inhabited by submerged plants, PVI (%), phytoplankton biomass as chlorophyll-a
concentration, Chl-a (lgl
-1
), and water concentrations of total phosphorus, TP (lgl
-1
), and total nitrogen, TN (lgl
-1
). Lakes are
ordered by decreasing fish richness in both regions (Table 2)
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Trophic position TrPoðÞ¼½ðd15Nconsumer
d15Nbase Þ=2:98þ2;
where d
15
N
consumer
is the isotopic signature of each
individual analyzed and d
15
N
base
is the averaged
baseline organisms (bivalves and snails), 2.98 is the
expected d
15
N fractionation per trophic level (Van-
derklift & Ponsard, 2003), and 2 is the theoretical
trophic level of baseline organisms (Post, 2002b). We
estimated the FWL as the maximum trophic position
for each lake.
In addition to FWL, we also calculated community-
wide metrics (Layman et al., 2007; Jackson et al.,
2011) to identify key features of the specific food
webs: (i) carbon range (CR), which is the difference
between the most d
13
C-enriched and the most d
13
C-
depleted values, for both the total consumer food web
(excluding basal resources) and per trophic level
(CR2, CR3); (ii) total area of the web (TA), measured
as the convex hull area given by all species in the d
13
C-
TrPo biplot and by the adjustment of the standard
ellipse areas (SEA) in the biplot; and (iii) the mean
nearest neighbor distance (NND), as the mean of the
Euclidean distances to the nearest neighbor of each
species in the biplot. CR indicates the amplitude of the
carbon resources being used; TA and SEA represent a
measure of the total amount of niche space occupied
by the trophic web, whereas smaller NND values
indicate redundancy of species with similar trophic
ecology. Although both TA and SEA represent the
trophic niche space occupied by communities, ellipse-
based SEA are developed in a Bayesian framework,
rendering this method unbiased with respect to sample
size and thus more robust than the convex hull area-
based TA metrics (Jackson et al., 2011). Despite that
the estimation of these metrics is usually made using
raw d
15
N (Layman et al., 2007), they have also been
estimated by standardizing d
15
N to trophic web length.
We used the latter method in our study as it shows
reduced variability in d
15
N due to factors other than
trophic fractionation (e.g., Gonza
´lez-Bergonzoni
et al., 2014).
We calculated these parameters using SIAR and
SIBER packages in R software and PAST software
(Hammer et al., 2001) and tested for differences
between climate zones in the measured trophic web
attributes (i.e., FWL, CR, TA, SEAb, and NND) using
the Mann–Whitney nonparametric test. Spearman
correlations among FWL, fish richness, CR,
ecosystem size (i.e., lake surface area), and lake
pelagic productivity (using phytoplankton Chl-a con-
centration as a proxy) were also calculated.
Results
FWL was, on average, one trophic position shorter in
the subtropical lakes than in the temperate lakes
(Table 2; Fig. 1). There was no significant correlation
between FWL and ecosystem size and pelagic pro-
ductivity (inferred using Chl-a concentration as
proxy). Mean fish richness was greater in the subtrop-
ical than in the temperate lakes (9.5 ±1.5 and
5.3 ±1.1 SE species per lake, respectively) and was
significantly correlated with both ecosystem size
(r
2
=0.82) and pelagic productivity (r
2
=0.67).
The subtropical fish assemblages included several
relatively small-sized omnivorous species of which
Jenynsia multidentata Jenyns, 1842 and Cnesterodon
decemmaculatus Jenyns, 1842 were the most abundant
(Table 3). Several potentially piscivorous species
(Teixeira-de Mello et al., 2009) like Australoheros
facetus Jenyns, 1842, Hoplias malabaricus Bloch,
1794, Oligosarcus jenynsii Gu
¨nther, 1864, Rhamdia
quelen Quoy & Gaymard, 1824, and Synbranchus
marmoratus Bloch, 1795 were also frequently
observed (Table 3). Among the piscivores, H. mal-
abaricus did not reach the top of the food web but held
the same trophic position as small-sized omnivores
(Table 3). In contrast, O. jenynsii always occurred at
the highest trophic level. Remarkably, the small-sized
J. multidentata, usually classified as omni-planktivore
(Goyenola et al., 2011), exhibited high mean d
15
N
values in all systems (Table 3). Shrimps, in particular
Palaemonetes argentinus Nobili, 1901, occurred in all
the subtropical lakes and, was abundant in four of the
lakes where they occupied the 3
rd
trophic position
along with predatory macroinvertebrates and several
omnivorous fish species.
Notwithstanding their relative paucity of species,
temperate fish assemblages (Table 4) consisted
roughly of the same trophic groups that characterized
the subtropical communities caught during this study.
Potential piscivores were abundant, including Esox
lucius L., Perca fluviatilis L., and Anguilla anguilla L.
(Table 4). Several fish species held higher trophic
positions (around 4th trophic position) than observed
in the subtropical lakes. Esox lucius was the apical
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123

species in the food web in two out of the four lakes,
with values close to the 6th trophic position (corre-
sponding to one individual; Table 4), and was not
lower than the 4th position in any of the lakes. In one
lake, both P. fluviatilis and Tinca tinca L. occupied
higher trophic positions than E. lucius, likely reflect-
ing the overall small body size of the latter (Table 4).
Taking the food webs as a whole, the overall mean
d
13
C carbon range (CR) was slightly (though not
statistically significant) wider in the warmer lakes,
being 8.7 in the subtropical and 7.5 in the temperate
lakes (Table 2). The carbon range per distinct trophic
level showed some variation between regions: no
significant differences appeared at trophic position 2
(primary consumers), but CR was twice as wide at
position 3 (secondary consumers) in the subtropical
lakes (Z=1.96, P=0.05; Table 2).
Temperate trophic webs typically had a triangular
convex hull area, whereas the subtropical webs were
typically trapezoid shaped (Figs. 1,2), being shorter
and wider at the 3rd trophic position (Table 3; Fig. 2).
Also SEAb captured the differences in trophic niche
space between the regional food webs, the minor and
major axes being more similar in the temperate lakes,
whereas a larger area towards the major axis was
occupied in the subtropical systems (Fig. 3). Surpris-
ingly, TA did not show statistically significant differ-
ences between climate zones, in contrast to SEAb
(Z=2.32, P=0.02; Table 2; Fig. 2). The nearest
neighbor distance (NND) was significantly shorter in
the subtropical lakes (Z=1.98, P=0.05; Table 2).
Discussion
Results based on SIA showed large differences in
trophic web structure between comparable shallow
lakes from regions with contrasting climates. In
particular, food-web length was shorter in the sub-
tropical than in temperate shallow lakes, supporting
the second of the contrasting hypotheses. This finding
cannot be ascribed to differently sized top predators in
each climate region as large fish specimens, usually
classified as piscivores or omni-benthi-piscivores
(Teixeira-de Mello et al., 2009; Gelo
´s et al., 2010),
occurred in both the regions. However, in the
subtropical lakes, the trophic position of larger fish
was similar to that of smaller-sized species and in
Table 2 Fish species richness (FR) and community-wide metrics from subtropical (above) and temperate (bottom) lakes, calculated
based on the distribution of species in the d
13
C-d
15
N biplots (Fig. 1)
Lake FR* FWL* CR CR 2 CR 3* SEAb* TA NND*
Cisne 13 4.0 7.7 4.5 3.5 5.2 35.2 0.4
Diario 11 3.4 9.7 9.7 5.9 4.2 27.6 0.4
Garcia 11 3.9 9.9 9.4 7.6 5.8 29.7 0.6
Clotilde 9 4.1 8.5 7.7 5.1 5.4 30.1 0.6
Blanca 4 4.4 7 7 3.4 5.2 17.2 0.7
UY Median 11 4.0 8.5 7.7 5.1 5.2 29.7 0.6
Range 4–13 3.4–4.4 7.0–9.9 4.5–9.7 3.4–7.6 4.2–5.8 17.2–35-2 0.4–0.7
Vaeng 8 5.1 4.9 4.3 2.6 6.8 58.7 0.6
Tranevig 6 5.8 10.5 7.3 0.6 6.8 45.7 0.9
Gammelmose 4 4.6 4.6 4.4 3.0 6.7 18.3 1.2
Denderup 3 4.4 9.9 9.9 4.1 6.6 36.7 0.6
DK Median 5 4.8 7.4 5.9 2.8 6.75 41.2 0.8
Range 3–8 4.4–5.8 4.6–10.5 4.3–9.9 0.6–4.1 6.6–6.8 18.3–58.7 0.6–1.2
Z
value
1.85 2.2 0.1 0.7 1.96 2.32 1.35 1.98
P0.06 0.02 0.9 0.5 0.05 0.02 0.18 0.05
Food web length (FWL), maximum trophic position for each lake, carbon range (CR), standard ellipse areas (SEAb), total area (TA),
convex hull area encompassed by all species, and mean nearest neighbor distance (NND) values are shown. Median and range for
each climate area are provided together with the results of statistical analyses, indicating significant (P\0.05) or marginally
significant (0.05 \P\0.10) differences (*) between locations (Mann–Whitney non parametric tests). Lakes are ordered by
decreasing FR in both the regions
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some cases similar to that of predatory macroinverte-
brates. Such an apparent mismatch between measured
and expected trophic positions for large predatory fish
has previously been reported for some tropical rivers
(Layman et al., 2005), where the wide variation in
trophic position of tropical predatory fish was sug-
gested to be due to multiple feeding strategies, which
typically occur in low latitude species-rich systems
(Layman et al., 2005). Large-sized tropical piscivores
usually feed on the most abundant items of prey,
typically detritivorous species, which gives them a
short trophic position, only two trophic steps away
from basal resources such as detritus (e.g., Watson
et al., 2013; Jardine, 2016).
Concerns may arise regarding the application of
nitrogen stable isotopes for estimation of trophic web
length using a single average trophic fractionation
value, as trophic fractionation is not truly constant
throughout the whole food web (Bunn et al., 2013). In
streams and rivers, it has been shown that trophic steps
between algae and grazing macroinvertebrates can
produce average trophic enrichment values as low as
0.6%, and 1.6%enrichment between grazing and
predator macroinvertebrates, whereas the trophic
enrichment between invertebrate and fish compart-
ments can range from approximately 2.2–3.9%.By
using an average trophic enrichment of 2.98%from a
meta-analysis specifically arrayed for lake systems
(Post, 2002b), we assumed that there were no differ-
ences in trophic enrichment created by climate regions
and that the number of trophic steps between inver-
tebrate and fish compartments was the same in both.
This seems reasonable as there is no evidence for
differential trophic fractionation in different regions of
the world (e.g., Bunn et al., 2013) and as we found
both grazing and predatory macroinvertebrates in both
the regions. Thus, we have no reason to suppose that
the observed differences can be caused by factors
other than the higher average number of trophic steps
in the temperate than in the subtropical lakes. In fact,
our study might overestimate the maximum trophic
position in some Uruguayan lakes as herbivorous and
omnivorous fish usually enrich their N signature by
4%with respect to algae (Bunn et al., 2013). This
probably explains the surprisingly elevated trophic
position observed here when using the lower average
trophic enrichment value of Post,(2002b). Another
potential methodological limitation in the use of
stable isotopes in trophic position estimates is the fact
that stable isotopes reflect dietary assimilation in the
last sampling weeks/months (Heady & Moore, 2012),
whereas there are well-known seasonal changes in
feeding strategies of fish in both subtropical and
temperate regions, for example, towards higher veg-
etal consumption by several omnivores in summer
(Persson, 1986; Gonza
´lez-Bergonzoni et al., 2016).
We aimed to avoid the bias of different time frames in
the fish stable isotopes by conducting the sampling
during the same season (the end of the growing/
reproductive season) in both the regions.
As expected from earlier studies (e.g., Lazzaro
et al., 2003; Meerhoff et al., 2007; Gonza
´lez-Ber-
gonzoni et al., 2012), we also observed higher fish
species richness in the subtropical lakes. Fish richness
was positively correlated with both lake surface area
and pelagic primary producer biomass as expected
from the richness–productivity and richness–ecosys-
tem size relationships (Rosenzweig, 1995; Lawton,
1999; Dodson et al., 2000). According to the proposed
insertion and addition mechanisms (Post, 2002a),
additional (including higher) trophic levels might be
expected as more fish species occur in the subtropical
food webs. However, we observed shorter FWLs in the
subtropical lakes, suggesting that other mechanisms
prevailed. One such mechanism could be a different
degree of omnivory, which is a predominant charac-
teristic of subtropical and tropical fish assemblages
(e.g., Jepsen & Winemiller, 2002; Lazzaro et al., 2003;
Meerhoff et al., 2007). An increase in the proportion of
herbivorous fish species has been observed with the
decreasing latitude and increasing water temperature
in a variety of aquatic ecosystems worldwide (Gonza
´-
lez-Bergonzoni et al., 2012), concurring at community
level with predictions of the Metabolic Theory of
Ecology (Brown et al., 2004) suggesting that energy
limitation may lead to enhanced omnivory to satisfy
the boosted metabolic needs (Brown et al., 2004; Arim
bFig. 1 Stable isotope-based biplots showing the convex hull
areas encompassing all fish species. Left temperate lakes, right
subtropical lakes. The diagrams show trophic position (inferred
from d
15
N) against d
13
C signals. For fish, each point represents
the mean value of 2–20 individuals of different sizes. Herb.
Invert. and Carn. Inv. are the averages of all invertebrate
specimens as assigned to each particular trophic group
according to the literature (herbivorous, invertivorous, or
carnivorous). Gaster and Bival are the averages of Gastropoda
and Bivalvia in each lake (baseline signals of littoral and pelagic
food webs in the calculations). Error bars represent ±1SE.
Lakes are ordered by decreasing fish richness in both the regions
(Table 2)
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Table 3 Fish species from the Uruguayan (subtropical) lakes used for the stable isotope analyses
Species Cisne Diario Garcı
´a Clotilde Blanca
nMean TrPo nMean TrPo nMean TrPo nMean TrPo nMean TrPo
Australoheros facetus* Jenyns, 1842 1 3.1 3.69 10 5.8 3.62 1 2.6 3.34
Hoplias malabaricus* Bloch, 1794 3 5.2 3.23 5 30.2 4.31
Oligosarcus jenynsii* Gu
¨nther, 1864 2 12 3.66 10 12.7 3.55 10 8.6 4.02 9 16.9 4.65
Rhamdia quelen* Baird & Girard, 1854 4 4 3.59 6 3.9 3.69
Synbranchus marmoratus* Bloch, 1795 6 24.3 3.85
Characidium rachovii Regan, 1913 6 2.7 3.24
Corydoras paleatus Jenyns, 1842 1 3.5 4.03
Astyanax sp. Baird & Girard, 1854 8 5.8 3.30 3 2 3.4 10 3.5 3.63
Cheirodon interruptus Jenyns, 1842 1 2.76 10 4 3.16 6 4.7 3.53
Cnesterodon decemmaculatus Jenyns, 1842 10 2.6 3..47 3 2.5 3.66 10 2.4 3.57 1 1.8 3.79
Diapoma terofali Ge
´ry, 1964 10 4.6 3.25 10 5.6 3.26
Gymnogeophagus cf. meridionalis 8 3.4 3.37 15 3.7 3.78
Hyphessobrycon anisitsi Eigenmann, 1907 2 3.7 3.53
Hyphessobrycon boulengeri Eigenmann, 1907 1 3.7 3.76
Heptapterus mustelinus Valenciennes, 1835 1 2.6 3.9
Heterocheirodon yatai Casciotta,
Miquelarena & Protogino, 1992
2 4.1 3.22
Hyphessobrycon luetkenii Boulenger, 1887 8 5 3.51
Jenynsia multidentata Jenyns, 1842 10 2.8 3.49 10 4.3 3.78 9 3.6 4.06 11 3.6 4.38
Phalloceros caudimaculatus Hensel, 1868 3 2.2 3.21 2 2.2 3.42
Pimelodella australis Eigenmann, 1917 2 4 3.89
Pseudocorynopoma doriae Perugia, 1891 6 4.5 3.29
Steindachnerina biornata Braga & Azpelicueta, 1987 10 9.9 2.67 9 11.7 2.55 6 10.1 3.13
Hypostomus commersoni Valenciennes, 1836 2 4.1 4.69
Hisonotus sp. Eigenmann, 1889 5 5.2 3.2
Parapimelodus valenciennis Lu
¨tken, 1874 1 15.1 2.95
The number of analyzed specimens (n), mean body length, and estimated trophic position (TrPo) are shown. * Potentially piscivorous species. Lakes are ordered by decreasing
fish richness (Table 2)
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123

et al., 2007a). A diverse diet that incorporates a higher
amount of different items (Arim et al., 2007a), and
enhanced feeding on lower trophic positions (Beisner
et al., 1997; Petchey et al., 1999), could potentially
satisfy the greater energy demands of organisms at a
given trophic position under higher ambient
temperatures.
Regarding the carbon range, we found similar
values at the base of the trophic web in the two
climatic zones, indicating a similar use of carbon
sources (i.e., phytoplankton and periphyton) by pri-
mary consumers. However, at the secondary consumer
level (CR3), the carbon range was significantly
broader in the subtropical lakes, pointing to a mixture
of simultaneously occurring strategies where some
taxa have a lower integration of carbon sources, while
other co-occurring taxa integrate several carbon
sources (Fig. 3). Fish reliance on periphyton as a
major carbon source has previously been demon-
strated in shallow temperate lakes; its importance
depends, however, on water clarity and the consequent
relative importance of benthic primary and secondary
production (Vander Zanden & Vadeboncoeur, 2002;
Jones & Waldron, 2003). In our subtropical systems,
many species occupied an intermediate position
(secondary consumers) in the food web (i.e., several
fish species and shrimps) and may act as additional
pathways for the different carbon sources (Post &
Takimoto, 2007). Therefore, intermediate consumers
could enhance the transfer of basal carbon to higher
trophic positions without adding more trophic links to
the web. In addition, higher functional redundancy in
warmer lakes was evidenced here by a closer nearest
neighbor distance (NDD), meaning that more species
occupied similar trophic web positions in the subtrop-
ical compared to the temperate lakes.
As a consequence of the shorter FWL in the
subtropical lakes and the suggested differences in
energy pathways in the different climate zones, the
shapes of the food webs (depicted by the convex hull
shapes and community-wide metrics) differed
between the two climatic regions studied (see
Fig. 2). Our results suggest that temperate trophic
webs are characterized by multichain omnivory [IGP
module, sensu Vadeboncoeur et al. (2005)], with one
top predator integrating the different carbon sources
fuelling the web (mainly represented here by phyto-
plankton and periphyton and with an intermediate
d
13
C value). Conversely, in the subtropical lakes, the
Table 4 Fish from the Danish (temperate) lakes used for the stable isotope analyses
Species Vaeng Tranevig Gammelmose Denderup
nMean TrPo nMean TrPo n Mean TrPo nMean TrPo
nAbramis brama L. 28 9.2 3.68 11 24.2 4.47
Esox lucius* L. 7 33.8 4.87 1 50.8 5.82 1 38 4.59 4 14.4 3.37
Perca fluviatilis* L. 43 12.7 4.36 17 13.5 4.35 32 16.2 3.95
Rutilus rutilus L. 60 15 4.38 12 14.7 4.34
Scardinius erythrophthalmus L. 31 9 3.33 21 13.5 4.13 6 12.9 4.33
Carassius carassius L. 1 15 3.61
Gymnocephalus cernua L. 1 7 3.26 4 34.6 4.23
Tinca tinca L. 2 41 4.36 8 45.4 4.76 7 45.4 3.52
Anguilla anguilla* L. 9 44.3 4.3
The number of analyzed specimens (n), mean body length, and estimated trophic position (Tr Po) are shown. * Potentially piscivorous species. Lakes are ordered by decreasing
fish richness (Table 2)
Hydrobiologia
123

Fig. 2 Trophic diversity for the set of shallow lakes in
Denmark (triangles) and Uruguay (circles), depicted by Total
Convex Hull area (full lines) and Standard Bayesian Ellipses
(SEA; dotted lines). Both representations graphically captured
the higher trophic positions in the temperate systems. However,
only the SEA analysis was able to statistically express the
differences (Table 2)
Fig. 3 Conceptual models of trophic web functioning in the
temperate (left) and subtropical (right) lakes inferred from d
13
C-
TP biplots and community-wide metrics. The arrows above the
model indicate lowering of one trophic position occurring
concomitantly with a widening of the carbon range at the third
level of the chain. R1 and R2, phytoplankton and periphyton,
respectively; PC, primary consumers; SC, secondary con-
sumers; IC, intermediate consumers; TP, top predators; CR,
total carbon range; CR3 and CR2, the carbon range that reaches
the trophic positions of primary and secondary consumers;
FWL, food-web length
Hydrobiologia
123

occurrence of a combination of multichain and single-
chain omnivory, and the resultant more complex
energy transfer pathways, might explain the commu-
nity metrics (particularly CR3) and the shapes
observed. The higher strength of the IGP module,
together with a more reticulated topology of the
trophic web in the warmer lakes (Meerhoff et al.,
2007), may account for both the lower realized trophic
web length and the wider CR in higher trophic
positions (and the same basal range) in such lakes.
Our results should be interpreted with caution due
to possible limitations of the applied methodology (for
instance, the assumption of a constant fractionation
rate or the appropriateness of baseline value calcula-
tions) or by excluding effects of fish foraging behavior
(Lazzaro et al., 2009) and fish-induced stoichiometry
alterations as those described by Danger et al. (2009).
Nevertheless, they provide empirical evidence for
previously raised hypotheses suggesting that the
structure and interactions of the trophic webs in
subtropical lakes are more complex than those in cold
temperate ones (Lazzaro et al., 2003; Meerhoff et al.,
2007; Jeppesen et al., 2012).
We are also proposing here a conceptual model
rising the principal differences between trophic webs
in both the regions and the underlying forcing
mechanisms occurring (Fig. 3); however, we still lack
complete understanding of how such differences in
food-web shape affect, for instance, the biomass of
particular communities and biotic interactions at given
trophic levels as well as how lake ecosystem func-
tioning, resilience, and stability (Post & Takimoto,
2007) are affected.
Acknowledgments We are grateful to Anne Mette Poulsen
for manuscript editing and to Tinna Christensen for improving
the figures. We also thank Frank Landkildehus, Kirsten
Landkildehus Thomsen, and Mette E. Bramm in Denmark;
and Juan M. Clemente, Claudia Fosalba, Soledad Garcı
´a,
Nicolas Vidal, Natalia Barbera
´n, Malvina Masdeu, Mariana
Vianna, and Alejandra Kroger in Uruguay, for valuable field
assistance. The project was supported by the Ministry of
Science, Technology and Innovation of Denmark. EU-WISER
and EU-REFRESH, ‘‘CLEAR’’ (a Villum Kann Rasmussen
Centre of Excellence project), CRES, CIRCE, and The Research
Council for Nature and Universe (272-08-0406 and FNU
16-7745) supported EJ. CI was supported by a PhD
Scholarship from Aarhus University-Danish Research Agency.
NM was supported by Maestrı
´a en Ciencias Ambientales, and
NM, MM, and CI were supported by PEDECIBA. NM, MM,
FTM, and CI were supported by SNI (ANII) and MM also by
ANII-FCE 2009-2749 and the L
´Ore
´al-UNESCO (supported by
DICYT) for Women in Science national award. We deeply
acknowledge the constructive comments of two anonymous
reviewers and the handling editor Katya Kovalenko.
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