Fill - invertebrates and brook trout Salvelinus fontinalis Determination of

AQUATIC BIOLOGY

Aquat Biol

Vol. 17: 107–117, 2012

doi: 10.3354/ab00465

Published online November 6

INTRODUCTION

Organic matter that supports consumers in lakes

can be produced within the ecosystem (autochtho-

nous matter) or imported into the systems from adja-

cent terrestrial habitats (allochthonous matter). In

low-productivity lakes that dominate in the boreal

ecoregion, the relative availability of these resources

is controlled by allochthonous terrestrial inputs (i.e.

leaf litter falling from riparian vegetation; Ask et al.

2009, Karlsson et al. 2009). Thus, lakes are coupled to

watershed inputs of organic matter as movements of

organic matter connect terrestrial ecosystems with

aquatic food webs (Pace et al. 2004). Moreover, be -

cause the vast majority of lakes in this region are

small and shallow and their perimeter to area ratios

are relatively high (Schindler & Scheuerell 2002),

they can be highly influenced by allochthonous

organic matter of terrestrial origin (Weidel et al.

2008, Premke et al. 2010, Solomon et al. 2011).

Littoral zones are known to receive large inputs of

allochthonous matter from riparian vegetation

(Schindler & Scheuerell 2002). Most current studies

have focused on the pelagic food webs. However, lit-

Determination of food sources for benthic

invertebrates and brook trout Salvelinus fontinalis

in Canadian Boreal Shield lakes using stable

isotope analysis

Patricia Glaz

1,

*

, Pascal Sirois

2

, Christian Nozais

1

Département de biologie et centre d’études nordiques, Université du Québec à Rimouski, Rimouski, Québec G5L 3A1, Canada

2

Laboratoire des sciences aquatiques, Département des sciences fondamentales, Université du Québec à Chicoutimi,

Chicoutimi, Québec G7H 2B1, Canada

ABSTRACT: Allochthonous inputs can be an important contribution of organic matter in lake eco-

systems. Yet, our understanding of the patterns of energy dependence of littoral invertebrates and

fish is poor. We measured carbon (δ

13

C) and nitrogen (δ

15

N) stable isotope values for primary pro-

ducers, terrestrial detritus, benthic macroinvertebrates, zooplankton and brook trout Salvelinus

fontinalis in 8 oligotrophic Canadian Boreal Shield lakes to determine food sources that support

benthic consumers and brook trout. Mixing models used to determine animal diets from stable iso-

tope analysis showed leaf litter to be the principal food source for benthic primary consumers in 6

out of 8 lakes. Brook trout derived its carbon mainly from benthic predatory macroinvertebrates in

all lakes, with a contribution ranging from 60 to 90%. Zooplankton also contributed to brook trout

diet in 3 of 8 lakes, ranging from 28 to 37%. δ

15

N increased from primary producers to consumers

at different trophic positions in all lakes. Nitrogen isotopic signatures of brook trout became more

positive with increasing fish length, indicating a change in fish foraging strategy related to size.

Our study (1) suggests that carbon from terrestrial habitat (leaf litter) may contribute significantly

to the food web of oligotrophic Canadian Boreal Shield lakes and (2) highlights the importance of

carbon originating from benthic habitats in supporting brook trout in these lakes.

KEY WORDS: Allochthonous subsidies · Benthic macroinvertebrates · Zooplankton · Brook trout ·

Diet breadth · Size-related diet shift · Stable isotopes · Boreal lakes

Resale or republication not permitted without written consent of the publisher

Aquat Biol 17: 107–117, 2012

108

toral benthic pathways often play a key role in lake

production and food web dynamics (Vadeboncoeur

et al. 2002). Indeed, littoral benthic macroinverte-

brates are an important heterotrophic component of

lake ecosystems and constitute a link between basal

resources and upper trophic levels in the food web

such as fishes (Vander Zanden & Vadeboncoeur

2002, Vander Zanden et al. 2006). Knowledge of the

relative importance of allochthonous matter sources

supporting benthic macroinvertebrates and ulti-

mately fish is still poor (Carpenter et al. 2005, Solo -

mon et al. 2008). However, Cole et al. (2006) showed

that in oligotrophic lakes between 60 and 85% of

benthic macroinvertebrates used allochtonous

organic matter to meet their energy requirements.

A variety of processes at multiple trophic levels cre-

ate linkages among terrestrial, pelagic and benthic

energy pathways in lakes (Schindler & Scheue rell

2002, Vadeboncoeur et al. 2002). For instance, benthic

consumers may utilize dissolved or particulate detritus

of terrestrial origin, and fishes may consume benthic,

pelagic, or terrestrial prey (Solomon et al. 2011).

Many fishes use both benthic and pelagic habitats or

undergo ontogenic habitats shifts, integrating benthic

and pelagic food webs (Vadeboncoeur et al. 2002).

Brook trout Salvelinus fontinalis is a widely distributed

and economically important fish species of eastern

Canadian Boreal Shield lakes (Power 1980). It has

been identified as a generalist carnivore fish that

feeds mainly on benthic macroinvertebrates (Trem-

blay & Magnan 1991, Lacasse & Magnan 1992), but

also on zooplankton, terrestrial insects (Power et al.

2002, Tremblay-Rivard 2007) and fish, including

brook trout (Magnan 1988, La casse & Magnan 1992).

Recent studies have shown that stable isotope signa-

tures could be used to identify intraspecific shifts in

feeding strategies of brook trout (Power et al. 2002,

Tremblay-Rivard 2007). However, little is known

about brook trout diet and size-related diet shifts in

lakes of the Canadian Boreal Shield.

In this study, we measured naturally occurring car-

bon and nitrogen stable isotope ratios of basal re -

sources (terrestrial detritus, macrophytes, periphy-

ton, sediment organic matter), benthic organisms,

zooplankton and brook trout in 8 oligotrophic Cana-

dian Boreal Shield lakes. Using stable isotope analy-

sis we examined feeding relationships among con-

sumers. Our objectives were to: (1) determine the

ultimate sources from which benthic primary con-

sumers derive their energy in order to evaluate the

relative importance of allochthonous organic matter

for these consumers and (2) identify brook trout diet

breadth and size-related diet shifts.

MATERIALS AND METHODS

Study area

The study was conducted in the province of Que-

bec, on the Boreal Shield, in the Mistassibi River

drainage basin. The stu dy area is dominated by vir-

gin black spruce Picea mariana forests. Eight oligo-

trophic lakes with similar morphometric characteris-

tics were selected in a territory covered by pristine

boreal forest that was recently accessible by a net-

work of forestry roads (Table 1). In all lakes, the only

fish species found was brook trout, except for Lake 8,

in which brook trout coexists with white sucker

Catostomus commersoni.

Sampling

Eight lakes were sampled in July 2008. Water sam-

ples and pH measurements were taken at 5 littoral

sampling stations set uniformly around each lake. At

each sampling station, pH was measured at the sub-

surface using a YSI 556 MPS probe. Water trans-

parency was estimated with a Secchi disk at the

deepest part in each lake. Water samples (4 l) were

collected at each littoral station at 50 cm below the

surface with an Alpha bottle for the determination

of chlorophyll a (chl a), dissolved organic carbon

(DOC), total phosphorus (TP) concentrations and iso-

topic analysis of particulate organic matter (POM).

Terrestrial detritus (leaf litter, woody debris), macro-

phytes, periphyton (epilithon and epixylon), surface

sediment organic matter (SOM), benthic macro -

invertebrates, zooplankton and brook trout were

sampled for stable isotope analysis. Terrestrial detri-

tus, macro phytes and SOM were hand-collected in

the littoral zone of lakes and placed in scintillation

vials. Periphyton was collected by scraping from

rocks and woody debris in the littoral zone and

placed in scintillation vials. Benthic invertebrates

were collected in the littoral zone using a Turtox D-

net with a mesh size of 500 µm (n = 3 replicates per

station) and placed in cooler boxes. Zooplankton

samples were obtained by towing a plankton net

(mouth opening 20 cm, mesh 53 µm) mounted along-

side a boat for 5 to 10 min. Zooplankton were then

placed in Ziploc

®

bags containing water. Fish were

captured with 6 experimental gill nets, according to

the Quebec Ministry of Natural Resources and

Wildlife standard protocol (mesh sizes of 1, 1.5, 2, 2.5,

3, and 3.5 inches [25 to 90 mm]) set simultaneously

around the lake, perpendicular to the shore with the

Glaz et al.: Food web structure in Boreal Shield lakes

small mesh always set toward the shore. Gill nets

were left overnight for 12 h. All fish samples were

placed in cooler boxes and transported to the labora-

tory for processing.

Sample preparation and analyses

Water samples for the determination of chl a were

filtered (200 ml or more) onto Whatman GF/F filters.

Pigments were extracted for 24 h in 90% acetone, at

5°C in the dark without grinding. Chl a was deter-

mined using the Welschmeyer et al. (1993) method.

DOC was determined by filtering subsamples of

water through precombusted (500°C, 5 h) Whatman

GF/F filters. The filtrate was placed in glass vials with

teflon-lined caps and acidified with 25% v/v H

3

PO

4

(10 µl ml

−1

). DOC values were obtained using a TOC-

5000A analyzer (Shimadzu), following the protocol of

Whitehead et al. (2000). DOC reference standards

were produced by the Hansell’s Certified Reference

Materials (CRM) program. Total phosphorus (TP) was

measured using the molybdenum blue method (Stain-

ton et al. 1977) after autoclaving 50 ml samples with

0.5 g of potassium persulfate for 1 h at 120°C. For the

determination of isotopic signatures of POM, subsam-

ples of water were filtered through precombusted

(500°C, 5 h) Whatman GF/F filters and stored frozen

at −20°C until analysis. Terrestrial detritus, macro-

phytes, periphyton and SOM samples were cleared

and separated with the aid of a dissecting microscope.

Benthic macroinvertebrates and zooplankton samples

were identified at the order or family level using Mer-

ritt & Cummins (1996) and Edmond son (1959) respec-

tively. After identification, benthic macroinvertebrates

were frozen separately at −20°C while zooplankton

was frozen as a pool of organisms at −20°C. The total

length of each brook trout was recorded. A 2 cm

square and 1 cm deep block of dorsal white muscle

without skin sample was taken from 25 brook trout in

each lake. All muscle samples were kept frozen at

−20°C in scintillation vials until analysis.

Stable isotope analyses

POM filters, terrestrial detritus, macrophytes, peri-

phyton, SOM, whole benthic invertebrates, zoo-

plankton, and brook trout samples were dried at

60°C for 48 h. Mortar and pestle were thereafter used

to grind samples into a fine powder (except for POM

filters). Powder and filter samples were then encap-

sulated in pressed tin capsules (5 × 9 mm) and tin foil

cups (Costech Analytical Technology), respectively.

Encapsulated dry mass was approximately 1 mg for

terrestrial detritus, macrophytes, periphyton, benthic

invertebrates, zooplankton, and brook trout muscle

tissue and 3 mg for SOM. Lipid correction techniques

were not considered in our analyses because of low

carbon to nitrogen (C:N) ratio values (Vander Zan-

den & Rasmussen 1999). Previous studies showed

that lipids are lower in animals analyzed as muscle

tissue, as in our study for fish, than animals analyzed

as whole (McCutchan et al. 2003). C:N ratio values

for fish varied between 3.9 (Lake 7) and 4.7 (Lake 6)

in our study.

Analyses of stable isotopes ratios of C (δ

13

C) and N

(δ

15

N) were carried out at the Institut des sciences de

la mer (ISMER, Rimouski, Quebec, Canada) using a

COSTECH ECS 4010 Elemental Analyser coupled

with a DeltaPlus XP Isotope Ratio Mass Spectrometer

(IRMS, Thermo Electron). System control as well as

acquisition and treatment of the data were carried

out using the Isodat 2 software. Stable isotope ratios

were expressed in δ notation as parts per thousand

(‰) according to the equation:

δX = [(R

sample

兾R

standard

) − 1] × 1000 (1)

where X is

13

C or

15

N and R is the corresponding

13

C:

12

C or

15

N:

14

N ratios.

International standards used for the measurement

were Vienna Pee Dee Belemite (VPDB) limestone for

13

C and atmospheric nitrogen for

15

N. Regional stan-

dards for in-lab normalization regressions to deter-

mine sample δ values were anhydrous caffeine

(Sigma Chemical), Mueller Hinton Broth (Becton

Dickinson) and Nannochloropsis, respectively. These

homemade standards were calibrated once a year

using standards from the National Institute of Stan-

dards and Technology (NIST). Replicate analyses of

standards gave analytical errors (SD) of ±0.30‰ for

C and ±0.18‰ for N.

Data analyses

To estimate the relative contribution of the differ-

ent food sources to the diet of benthic invertebrates

and brook trout, we adopted a Bayesian multi-source

stable isotope mixing model, available as an open

source R package (SIAR, Parnell et al. 2010). SIAR

takes data on organism isotopes and fits a Bayesian

model to their dietary habits based on a Gaussian

likelihood with a dirichlet distribution prior mixture

on the mean. SIAR offers a number of advantages

over earlier mixing models; it can incorporate trophic

109

Aquat Biol 17: 107–117, 2012

enrichment factors within the model and known C

and N concentrations dependence, assuming that for

each element, a source’s contribution is proportional

to the contributed mass times the elemental concen-

tration in that source. We assumed a trophic enrich-

ment of +1‰ for

13

C (De Niro & Epstein 1978) and

+3.4‰ for

15

N (Minagawa & Wada 1984). To assess

these trophic enrichment assumptions, we combined

organisms into main groups of consumers: zooplank-

ton, benthic primary consumers and predatory

macroinvertebrates and calculated average δ

13

C and

δ

15

N for each group.

Using Daphnia spp. as the baseline trophic level,

trophic position of consumers was calculated using

the formula given in Vander Zanden & Rasmussen

(2001):

Trophic position

consumer

=

(δ

15

N

consumer

− δ

15

N

baseline

)兾f + 2

(2)

where δ

15

N

consumer

is the δ

15

N value of the consumer

for which the trophic position is estimated, δ

15

N

baseline

is the δ

15

N of the baseline organism, 2 is the expected

trophic position of the organism used to estimate

baseline δ

15

N, and f is the fractionation factor

between a predator and its prey which corresponds

to 1 trophic level. We used the fractionation factor of

3.4‰ as proposed by Minagawa & Wada (1984) and

generally used in studies estimating trophic position

of aquatic consumers (Vander Zanden et al. 1997).

Stable isotopes of carbon and nitrogen were also

individually plotted against fish length in each lake.

Simple linear regressions were then calculated for

each lake.

RESULTS

Physico-chemical data for the 8 lakes are outlined

in Table 1. The lakes are small (0.031 to 0.288 km

2

),

as are their catchments (0.202 to 2.895 km

2

). All the

lakes are relatively shallow (maximum depth < 9 m)

and exhibited low levels of phytoplankton biomass

(chl a) but high DOC concentrations (Table 1). This is

typical of oligotrophic Canadian Shield lakes, which

are slightly acidic, with high concentrations of humic

materials (Power et al. 2002, Glaz et al. unpubl.).

Benthic consumers and potential food sources

Mayflies (Ephemeroptera), corixids (Corixidae),

am phi pods (Amphipoda) and caddisflies (Tricho -

ptera), all of them non-predatory macroinverte-

110

Lake 1 Lake 2 Lake 3 Lake 4 Lake 5 Lake 6 Lake 7 Lake 8

Latitude (° N) 50° 25’ 44” 50° 29’ 22” 50° 23’ 13” 50° 28’ 34” 50° 30’ 9” 50° 31’ 25” 50° 30’ 40” 50° 28’ 11”

Longitude (° W) 71° 57’ 28” 71° 57’ 32” 72° 1’ 24” 71° 57’ 15” 71° 47’ 1” 71° 56’ 26” 71° 56’ 5” 71° 46’ 51”

Lake area (km

2

) 0.170 0.169 0.063 0.031 0.288 0.090 0.277 0.043

Catchment area (km

2

) 0.916 2.799 0.586 0.202 2.895 1.761 2.416 0.339

Drainage area (km

2

) 0.746 2.630 0.523 0.171 2.606 1.671 2.138 0.296

Maximum depth (m) 5 2 0.5 2 9 4.5 7.5 2

Secchi depth (m) 1.25 1.50 na

a

1.75 1.50 1.65 1.40 1.40

pH 5.92 (0.10) 5.75 (0.02) 5.94 (0.05) 5.87 (0.07) 5.92 (0.06) 5.02 (0.05) 5.62 (0.15) 5.38 (0.28)

Chl a (mg m

−3

) 0.427 (0.065) 0.390 (0.061) 0.617 (0.200) 0.363 (0.050) 0.982 (0.212) 0.546 (0.060) 0.681 (0.161) 0.486 (0.072)

DOC (mg l

−1

) 10.78 (0.57) 12.06 (0.58) 12.56 (1.01) 12.33 (0.48) 11.91 (0.73) 9.82 (0.40) 8.98 (0.43) 13.73 (1.04)

TP (µg l

−1

) 5.05 (0.26) 4.95 (0.52) 5.77 (0.50) 5.13 (1.01) 5.09 (0.70) 4.69 (0.47) 5.26 (0.70) 4.65 (0.55)

a

Not measured because lake was too shallow

Table 1. Lake characteristics of the 8 studied Canadian Boreal Shield lakes. pH, chl a, dissolved organic carbon (DOC) and total phosphorus (TP) are reported as

means (SD) over the 5 sampling stations in the photic zone. na: not available

Glaz et al.: Food web structure in Boreal Shield lakes

111

brates, were grouped as benthic primary consumers.

Leeches (Hirudinea), water mites (Hydracarina),

dragonflies (Anisoptera), damselflies (Zygoptera),

dysticids (Dysticidae) and alderflies (Sialidae), all of

them active predators, and Empidids (Empididae)

and Chironomids (Chironomidae), which are mainly

predators, were enriched in

15

N relative to primary

consumers in all lakes (Fig. 1). Consequently, these

organisms were pooled as predatory macroinverte-

brates. A mixing model was performed to evaluate

basal resource contribution to benthic primary con-

sumers. Benthic primary consumers derive their car-

bon mainly from leaf litter in all lakes (Table 2). How-

ever, in Lakes 1 and 2 epilithon and epixylon also

contributed to their diet, with mean contributions of

43 and 46% respectively (Table 2).

Brook trout diet breath

Brook trout mean carbon isotope ratios ranged

from −31.1 (Lake 8) to −26.2‰ (Lake 3) and mean

nitrogen isotopes ratios ranged from 5.6 (Lake 1) to

6.4‰ (Lake 4). Both δ

13

C and δ

15

N signatures for

brook trout were well separated from their potential

prey items in all lakes. Brook trout were enriched in

15

N relative to all other sources in all lakes (Fig. 1).

The mixing model confirmed that brook trout derive

their carbon biomass mainly from predatory benthic

macroinvertebrates in all lakes, with feasible mean

contributions ranging from 60 (Lake 2) to 90%

(Lake 3) (Table 3). However, in Lakes 1, 2 and 8, zoo-

plankton also appeared as a potentially important

food resource with feasible mean contributions of 28,

37 and 32% respectively. The mixing model also

indicated that brook trout do not depend on benthic

primary consumers for food (Table 3).

Trophic position

δ

15

N increased from primary producers to con-

sumers at different trophic positions in all lakes (Fig.

1), indicating that the heavy nitrogen isotope (

15

N)

was preferentially retained during nutrient assimila-

tion and incorporation into animal tissues. Brook

trout were enriched in

15

N by 3.77‰ relative to

predatory macroinvertebrates when all lakes were

pooled (Table 4). However, predatory macroinverte-

brates were only enriched by 2.60‰ in

15

N relative to

benthic primary consumers (Table 4).

Mean trophic position of consumers was signifi-

cantly different among the 3 main trophic groups

exa mined in all lakes (Table 5). The a posteriori

Tukey test confirmed the existence of significant dif-

ferences for all paired combinations in all lakes.

Predatory macroinvertebrates had a mean trophic

position that was significantly higher than that of

benthic primary consumers and fish had a signifi-

cantly higher trophic position than both benthic pri-

mary consumers and predatory macroinvertebrates

in all lakes (Table 5).

Size-related diet shift of brook trout

Brook trout mean length ranged from 192 (Lake 8)

to 245 mm (Lake 2). Brook trout showed great vari-

ability in δ

13

C values and they became significantly

more depleted in

13

C with increasing length in

Lakes 2, 4 and 8 (although in Lake 4 sampling size

was low, n = 8, in comparison to the other lakes, n =

25). Lakes 1, 3, 5, 6 and 7 did not show any particular

pattern in relation to increasing fish length (Fig. 2a,

Table 6). Brook trout became significantly more en-

riched in

15

N with increasing body length in all lakes

but Lake 4, and this relationship was also significant

when all lakes were pooled (Fig. 2b, Table 7).

DISCUSSION

Leaf litter appeared to be the principal carbon

source for benthic primary consumers in 7 out of our

8 lakes, suggesting a significant contribution of allo -

chthonous matter to the littoral food web. Brook trout

seemed to derive their food mainly from benthic

predatory macroinvertebrates in all lakes, although

zooplankton also contributed to brook trout diet in 3

out of 8 lakes. A size-related diet shift was also ob -

served for brook trout as they became

15

N-enriched

with increasing body length. However, no consistent

pattern was observed for δ

13

C values.

Carbon flow in lake food webs

In oligotrophic lakes, humic matter originating in

the catchment can be an important contribution of

allochthonous matter for aquatic organisms (France

1997, Glaz et al. unpubl.). Moreover, the loading of

allochthonous matter can greatly exceed autochtho-

nous primary production (Carpenter et al. 2005).

High DOC concentrations strongly influence light

penetration in lakes (Carignan et al. 2000, Karlsson

et al. 2009) and present an unfavorable environment

Aquat Biol 17: 107–117, 2012

112

Fig. 1. Stable isotope signatures of carbon (δ

13

C) and nitrogen (δ

15

N) for potential food sources and brook trout for 8 lakes: (a)

Lake 1; (b) Lake 2; (c) Lake 3; (d) Lake 4; (e) Lake 5; (f) Lake 6; (g) Lake 7 and (h) Lake 8. Error bars represent the mean ± SE.

POM: particulate organic matter

Glaz et al.: Food web structure in Boreal Shield lakes

for photosynthetic fixation of carbon

in situ by autotrophic phytoplankton

(Jones 1992), limiting autochthonous

production. This is consistent with the

low chl a and high DOC values en -

countered in our studied lakes.

This study demonstrates that allo -

chthonous matter (leaf litter) is the

main food source contributor of ben-

thic primary consumers, although

benthic algae also contributed to their

diet in Lakes 1 and 2. Also, since pri-

mary consumers are prey for preda-

tory invertebrates (Merritt & Cum-

mins 1996) that are, in turn, principal

dietary items for fish, allochthonous

matter assimilated by primary con-

sumers is transferred up the food

web. Benthic invertebrates would be

then the primary link be tween terres-

trial carbon and fish. Our results sup-

port an emerging consensus that ter-

restrial and benthic support of lake

food webs can be substantial (Vade -

boncoeur et al. 2002, Premke et al.

2010, Solomon et al. 2011).

Trophic position

Our results enabled the description

of the vertical structure of the food

web using δ

15

N. Benthic primary con-

sumers, predatory macroinvertebrates

and fish had mean trophic positions of

2.25, 2.99 and 4.08 respectively. If we

consider primary producers as Tro -

phic level 1, then trophic levels of

these groups would be 1.25, 1.99 and

3.08 respectively. These results are

113

Lake Leaf litter Epilithon Epixylon Macrophytes POM

Mean Percentile Mean Percentile Mean Percentile Mean Percentile Mean Percentile

1 43 26–59 abs 43 29−56 7 0−20 6 0−19

2 36 10–58 10 0−20 abs 46 28−66 7 0−19

3 44 25–63 3 0−9 abs 27 0−49 26 0−49

4 52 40–66 17 0−35 10 0−26 13 0−31 7 0−20

5 95 90–99 2 0−5 abs 1 0−4 1 0−3

6 78 68–87 abs 4 0−11 6 0−17 8 0−17

7 47 23–71 7 0−20 22 0−44 11 0−27 11 0−31

8 57 45–70 7 0−20 10 0−25 12 0−28 13 0−28

Table 2. Bayesian mixing model (SIAR) results for benthic primary consumers in 8 lakes. Percentiles show the 1st to 99th range

of potential contribution. Sources with a necessary contribution (minimal potential contribution > 0%) in bold. abs: absent

Lake Benthic primary Predatory Fish F p

consumers macroinvertebrates

1 2.31 (0.34) 2.87 (0.41) 4.09 (0.13) 240.71 <0.001

2 2.21 (0.43) 2.97 (0.52) 3.97 (0.12) 168.10 <0.001

3 2.63 (0.33) 3.23 (0.38) 4.41 (0.14) 172.34 <0.001

4 2.25 (0.21) 3.18 (0.53) 4.22 (0.12) 90.04 <0.001

5 2.40 (0.24) 3.11 (0.30) 4.09 (0.18) 446.22 <0.001

6 2.23 (0.25) 2.91 (0.34) 4.03 (0.16) 269.27 <0.001

7 1.90 (0.26) 2.99 (0.44) 4.00 (0.16) 323.14 <0.001

8 2.08 (0.32) 2.73 (0.25) 3.85 (0.14) 324.13 <0.001

Pooled mean 2.25 (0.30) 2.99 (0.40) 4.08 (0.14)

Table 5. Mean (± SD) trophic-position estimates in all lakes and analysis of

variance (ANOVA) results comparing estimates between benthic primary con-

sumers, predatory macroinvertebrates and fish in 8 lakes. The a posteriori

Tukey test confirmed the existence of significant differences for all paired

combinations in all lakes. Significance level α = 0.05

Lake Zooplankton Benthic primary Predatory

consumers macroinvertebrates

Mean Percentile Mean Percentile Mean Percentile

1 28 20−36 3 0−9 68 58−79

2 37 30−45 2 0−7 60 51−68

3 2 0−5 8 0−17 90 80−99

4 13 0−24 14 0−35 73 48−93

5 5 0−13 8 0−19 86 74−97

6 6 0−14 7 0−17 86 76−97

7 13 0−23 6 0−15 80 70−89

8 32 23−42 3 0−8 64 55−73

Table 3. Salvelinus fontinalis. Bayesian mixing model (SIAR) results for brook trout

in 8 lakes. Percentiles show the 1st to 99th range of potential contribution. Sources

with a necessary contribution (minimal potential contribution > 0%) in bold

δ

15

N (‰) δ

13

C (‰)

Fish 5.93 (0.48) −28.70 (1.02)

Predatory macroinvertebrates 2.16 (1.20) −28.52 (1.40)

Benthic primary consumers −0.44 (0.93) −29.33 (1.45)

Zooplankton −0.57 (0.64) −32.97 (0.91)

Table 4. Mean (± SD) nitrogen (δ

15

N) and carbon (δ

13

C) stable isotope signatures

of principal groups of consumers. All lakes were pooled

Aquat Biol 17: 107–117, 2012

very consistent with traditional food-chain models,

which attribute trophic positions of 1, 2 and 3 relative

to primary consumers, predatory macroinvertebrates

and fish (Vander Zanden & Rasmussen 1999). Our re-

sults are also consistent with previous isotopic studies

in which predatory macroinvertebrates had higher

15

N ratios than primary consumers and fish were en-

riched in δ

15

N relative to invertebrates (Vander Zan-

den & Rasmussen 1999, Herwig et al. 2004, Anderson

& Cabana 2007). Brook trout exhibited slightly higher

δ

15

N enrichment (3.77‰) than the typical assumed

shift of 3.4‰ per trophic level (DeNiro & Epstein 1978,

Minagawa & Wada 1984). However, Post (2002) calcu-

lated a standard deviation associated with this typical

value of ±1‰. For predatory macroinvertebrates, the

δ

15

N fractionation value was lower than expected

(2.60‰). Minagawa & Wada (1984) reported a wide

range in δ

15

N trophic enrichment values across taxa

(0 to 9‰). Moreover, δ

15

N trophic enrichment was

shown to differ between herbivores (mean = 2.5‰)

and carnivores (mean = 3.4‰) (Vander Zanden & Ras-

mussen 2001), suggesting that in our study omnivory

(feeding at more than one trophic level) may be im-

portant for predatory macroinvertebrates as well.

Brook trout, however, is corroborated as carnivorous.

Brook trout diet breadth

Stable isotopes analyses and mixing models

showed that brook trout fed primarily on benthic

predatory macroinvertebrates in the 8 Canadian

Boral Shield lakes, with a contribution ranging from

60 to 90% of fish diet. While zooplankton contributed

28 to 37% in 3 of 8 lakes, it does not appear to be a

main source of energy for fish in our lakes. Brook

trout, as with other salmonids, are visual predators

and are known to feed on the most abundant and vis-

ible food, selecting the largest prey items (Allan

114

–24

Lake 1

Lake 2

Lake 3

Lake 4

Lake 5

Lake 6

Lake 7

Lake 8

–26

–28

–30

–32

–34

–36

100 150

a

b

200 250

δ

13

C (‰)δ

15

N (‰)

300 350 400

4.5

5.0

5.5

6.0

6.5

7.0

7.5

100 150 200 250

Brook trout length (mm)

300 350 400

Fig. 2. Salvelinus fontinalis. Changes in brook trout (a) car-

bon (δ

13

C) and (b) nitrogen (δ

15

N) stable isotope signatures

with increasing fish length in 8 lakes

Lake n Slope r

2

F p

1 25 0.0008 0.0003 0.0061 0.9383

2 24 −0.0081 0.4058 15.0264 0.0008**

3 25 0.0104 0.1508 4.0847 0.0551

4 8 −0.0173 0.5643 7.7710 0.0317*

5 25 0.0125 0.1436 3.8573 0.0617

6 24 −0.0072 0.0911 2.2056 0.1517

7 25 −0.0125 0.1176 3.0645 0.0934

8 24 −0.0185 0.1899 5.3916 0.0294*

All lakes 180 −0.0054 0.0150 2.7473 0.0992

Table 6. Salvelinus fontinalis. Simple regression results of

carbon stable isotope ratio (δ

13

C) against brook trout length

in 8 lakes. **p < 0.001, *p < 0.05

Lake n Slope r

2

F p

1 25 0.0117 0.3302 11.3376 0.0027**

2 24 0.0045 0.5807 30.4728 <0.0001**

3 25 0.0095 0.2344 7.0408 0.0142*

4 8 0.0086 0.3880 3.8047 0.0990

5 25 0.0152 0.7891 86.0649 <0.0001**

6 24 0.0054 0.3904 14.0915 0.0011*

7 25 0.0114 0.6383 40.5921 <0.0001**

8 24 0.0075 30.2704 8.1535 0.0092*

All lakes 180 0.0064 0.2490 59.3456 <0.0001**

Table 7. Salvelinus fontinalis. Simple regression results of

δ

15

N vs. brook trout length in 8 lakes. **p < 0.001, *p < 0.05

Glaz et al.: Food web structure in Boreal Shield lakes

1978). The larger body size of benthic invertebrates

relative to pelagic prey makes feeding on benthos

more energetically attractive to fish, increasing the

efficiency of this trophic link (Vander Zanden et al.

2006). This may explain the greater proportion of

benthic macroinvertebrates in fish diet compared to

zooplankton in our study, as also shown by other

studies (Lacasse & Magnan 1992, Power et al. 2002,

Herwig et al. 2004). Brook trout are known to coexist

in many lakes with other species, especially the

white sucker Catostomus commersonii, a specialist

benthivore (Scott & Crossman 1973). When living in

sympatry with white sucker, brook trout shift their

feeding niche from benthic macroinvertebrates to

zooplankton in response to interspecific competition

for resources (Magnan 1988, Lacasse & Magnan

1992). In this study, only Lake 8 contained both brook

trout and white sucker. In this lake, zooplankton rep-

resents 32% of the diet of brook trout according to

mixing models. This is consistent to a certain extent

with the brook trout niche shift when living in sym-

patry with white sucker. However, this may not be

the only explanation, as in Lakes 1 and 2 zooplankton

also contributed to brook trout diet (28 and 37%

respectively), but in these 2 lakes there was no pres-

ence of white sucker. Other lake characteristics may

influence brook trout planktonic feeding behavior.

In addition, other studies on brook trout have esti-

mated that they consume between 10 (Tremblay-

Rivard 2007) and up to 30% (Vander Zanden et al.

2006) of terrestrial prey. We have not taken this food

source into account and as a result may have under-

estimated the allochthonous matter source contribu-

tion to brook trout diet coming in the form of terres-

trial insects. Future studies on brook trout feeding

should consider sampling terrestrial prey as a poten-

tial food source. Invertebrate isotopic signatures vary

in the short term, possibly confounding diet compar-

isons with fish that have much slower tissue turn-

over rates. As our sampling was conducted in July,

stable isotope results would be limited to the under-

standing of the feeding behavior of brook trout dur-

ing spring or early summer.

Brook trout size-related diet shifts

Ontogenetic diet shifts and size-selective predation

are common among fish (Werner & Gilliam 1984). In

our study, δ

13

C signatures exhibited a complex pat-

tern. δ

13

C values became significantly more depleted

with increasing fish length in 3 lakes, likely indica-

ting a diet shift related to size. Other studies have

shown a significant

13

C-enrichment with increasing

fish length (Grey 2001, Xu et al. 2007), but they con-

sidered both alevin and juvenile stages, which was

not the case in our study. When all trout stages were

considered, Grey (2001) did not find a particular pat-

tern of

13

C values in relation to increasing fish length.

Furthermore, a change in δ

13

C values is indicative of

a shift in benthic or pelagic feeding (Vander Zanden

et al. 1998) and expecting that a change in diet (to a

different trophic level) can occur without a signifi-

cant change in δ

13

C may be reasonable. In contrast,

in our study, δ

15

N ratios of brook trout became more

positive with increasing fish length in 7 out of 8 lakes.

This was also shown in other studies on brook trout

(Power et al. 2002, Tremblay-Rivard 2007) and other

fish species (Grey 2001, Xu et al. 2007) and is typical

of an ontogenetic dietary shift (Power et al. 2002). As

a fish increases in length and age, it is capable of

handling larger prey items and ingesting later instars

of macroinvertebrates and terrestrial insects that fall

into the lake (Power et al. 2002).

In conclusion, our study suggests than carbon orig-

inating from terrestrial habitat, particularly leaf litter,

may significantly contribute to the food web in east-

ern Canadian Boreal Shield lakes. This study also

highlights the importance of carbon produced in

benthic habitats to support brook trout in small oligo-

trophic boreal lakes. Our stable isotope study indi-

cates that benthic macroinvertebrates are important

contributors to fish diet. It supports a growing body of

literature suggesting that benthic rather than pelagic

habitats are the primary energy pathway contribut-

ing to fish production (Vadeboncoeur et al. 2002,

Herwig et al. 2004, Weidel et al. 2008).

Acknowledgements. This is a contribution to the scientific

program of the Chaire de recherche sur les espèces aqua-

tiques exploitées. This research project was supported by a

grant from the Fonds de la recherche forestière du

Saguenay− Lac-Saint-Jean and Fonds Québécois de la

Recherche sur la Nature et les Technologies (FQRNT) to

C.N. and P.S. P.G. was supported by a PhD fellowship from

FQRNT. P. Bérubé from the Ministère des Ressources

naturelles et de la Faune du Québec also contributed to this

project. We are grateful to people who contributed to the

sampling in the field: M. Bergeron, Y. Bhérer, G. Diab and

D. Gauthier. We also thank AbitibiBowater Inc. for provid-

ing land use information and field facilities. Two anonymous

reviewers provided helpful comments on an earlier version

of the manuscript.

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Editorial responsibility: Asbjørn Vøllestad,

Oslo, Norway

Submitted: March 9, 2011; Accepted: July 30, 2012

Proofs received from author(s): October 16, 2012

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