stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

Stable isotope variability of meso-zooplankton along agradient of dissolved organic carbon

A. D. PERSAUD* , †, P . J . DILLON*, D. LASENBY* AND N. D. YAN †, ‡

*Department of Chemical Sciences, Trent University, Peterborough, ON, Canada†Department of Biology, York University, Toronto, ON, Canada‡Dorset Environmental Science Centre, Dorset, ON, Canada

SUMMARY

1. The d13C and d15N signatures of zooplankton vary with dissolved organic carbon

(DOC), but inconsistent and limited taxonomic resolution of previous studies have

masked differences that may exist among orders, genera or species and are attributable

to dietary and ⁄or habitat differences. Here we investigate differences among the

isotopic signatures of five zooplankton taxa (Daphnia, Holopedium, large Calanoida,

small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of

DOC concentration.

2. d13C signatures of Daphnia, small calanoids and large calanoids became more depleted

with increasing lake DOC, whereas Holopedium and cyclopoid d13C became enriched with

increasing DOC concentration.

3. The variability of d13C and d15N isotopic signatures among zooplankton groups was

reduced in high-DOC, compared to low-DOC lakes, especially for d13C. Differences in d13C

and POM-corrected d15N accounted for up to 33.7% and 19.5% of the variance,

respectively, among lakes of varying DOC concentration.

4. The narrow range of signatures found in higher DOC lakes suggests that different taxa

have similar food sources and ⁄or habitats. In contrast, the wide range of signatures in low-

DOC lakes suggests that different taxa are exploiting different food sources and ⁄or

habitats. Together with the variable trends in zooplankton isotopic signatures along our

DOC gradient, these results suggest that food web dynamics within the zooplankton

community of temperate lakes will change as climate and lake DOC concentrations

change.

Keywords: dissolved organic carbon, stable isotopes, taxonomic resolution, variability, zooplankton

Introduction

Dissolved organic carbon (DOC) is considered a key

integrator in small, temperate lakes because it controls

numerous chemical and physical properties of these

lakes, and in turn is affected by multiple anthropo-

genic stressors. The functions of DOC range from

direct fuelling of microbial activity (Azam et al., 1983;

Kritzberg et al., 2004) to control of the acid ⁄base

balance of lakes and complexation of metals, nutrients

and trace organics (Santschi, Lenhart & Honeyman,

1997; Williamson et al., 1999). These functions are

altered as the interaction of acid rain, stratospheric

ozone depletion and climate change alter DOC con-

centrations in temperate lakes (Hudson, Dillon &

Somers, 2003; Monteith et al., 2007). As the climate

warms, and droughts become more common in

certain areas, DOC can decline in response to reduced

Correspondence: A. D. Persaud, Department of Chemical

Sciences, Trent University, Peterborough, Ontario, Canada K9J

7B8. E-mail: [email protected]

Freshwater Biology (2009) 54, 1705–1719 doi:10.1111/j.1365-2427.2009.02224.x

� 2009 Blackwell Publishing Ltd 1705

DOC export from catchments and increased in-lake

photolysis (Schindler et al., 1996; Magnuson et al.,

1997). Additionally, stratospheric ozone depletion

increases ambient UVR levels, and directly contrib-

utes to DOC photo-degradation (Williamson et al.,

1999). On the other hand, in some lakes DOC

concentrations can increase due to changes in precip-

itation, acid deposition and release of organic carbon

from soils (Hongve, Riise & Kristiansen, 2004; Evans

et al., 2006; Monteith et al., 2007). Consequently, future

increases or decreases are both possible depending on

the stressor mix and specific catchment setting of the

lake.

Given the growing awareness of the significance of

DOC in lake dynamics, many studies have focussed

on effects of DOC concentration and source (allochth-

onous versus autochthonous) on phytoplankton and

bacteria (Jansson et al., 2000; Prairie, Bird & Cole,

2002), but fewer studies have explored the importance

of DOC for higher trophic levels. DOC can be

consumed directly through grazing or indirectly

transferred via the microbial food chain (Azam et al.,

1983; Hessen, Andersen & Lyche, 1990) and phyto-

plankton community (Cole et al., 2002; de Lange,

Morris & Williamson, 2003) to higher trophic levels.

Consequently, over 70% of zooplankton carbon can

originate from terrestrial carbon sources (Karlsson

et al., 2003; Carpenter et al., 2005), and it is important

to fully understand zooplankton trophic dynamics in

lakes of varying DOC concentration.

A general decrease in zooplankton d13C with

increasing DOC concentration and water colour has

been found. Bulk zooplankton, Copepoda and Clado-

cera d13C signatures are generally more depleted in

high-DOC humic and dystrophic lakes compared to

low-DOC clearwater lakes (France, del Giorgio &

Westcott, 1997; Lennon et al., 2006). This difference

has been attributed to heterotrophic activity and

fixation of recycled carbon by phytoplankton, and

their subsequent assimilation by herbivorous zoo-

plankton in higher-DOC lakes (France et al., 1997).

Jones et al. (1999) also found that zooplankton were

depleted in 13C compared to particulate organic

matter (POM) and attributed this to their assimilation

of methanotrophic bacteria. While these studies report

common trends in zooplankton isotopic signatures

along DOC and colour gradients, their limited and

inconsistent taxonomic resolution restricts examina-

tion of underlying mechanisms.

Lake productivity has also been related to zoo-

plankton d13C isotopic signatures (France et al., 1997;

Grey, Jones & Sleep, 2000). Zooplankton d13C becomes

increasingly enriched in 13C as lake total phosphorus

(TP) and productivity increase, especially if zooplank-

ton are feeding predominantly on phytoplankton. In

addition, variability in zooplankton d13C signatures

increases, and overlap between POM and zooplank-

ton d13C increases with productivity from oligotrophic

to eutrophic lakes (Grey et al., 2000).

In most zooplankton stable isotope studies, biomass

has been analysed in bulk, or coarsely separated into

cladocerans and copepods. Coarse resolution masks

differences that may exist between genera or at other

taxonomic levels. Yet differences in zooplankton d13C

and d15N can develop in response to selective feeding

(Meili et al., 1996; Matthews & Mazumder, 2003),

dietary lipids (Smyntek et al., 2007) and spatial habitat

specialisation (Matthews & Mazumder, 2006; Santer,

Sommerwerk & Grey, 2006). Therefore inconsistent

and sometimes limited taxonomic resolution in pre-

vious studies examining zooplankton isotopic signa-

tures along DOC gradients restrict any inferences

regarding dietary and feeding behaviour differences

among zooplankton orders, genera and species even

though such differences may exist.

To increase understanding of zooplankton trophic

dynamics, more detailed investigations must be done

to examine differences among zooplankton along a

DOC gradient. Here we used relative differences in

d13C and d15N isotopic signatures as an indication of

dietary and feeding behaviour differences among

zooplankton taxa. Our objective was to examine

trends in stable isotope signatures of five zooplankton

taxa among lakes with a DOC gradient and quantify

the proportions of variability in zooplankton d13C and

d15N attributed to differences in DOC concentration

among lakes. For this study zooplankton taxonomy

was (i) resolved to order, genera and species and (ii)

consistent among lakes. If different zooplankton taxa

have different diets, either because they feed at

different depths or on different organisms, there will

be substantial variation in their d13C and d15N signa-

tures. On the other hand, if different zooplankton taxa

have similar food and ⁄or occupy the same depths,

they will have similar isotopic signatures. Consider-

ing that different zooplankton taxa can feed on

different size fractions of POM, have diverse feeding

modes, and have different diurnal and nocturnal

1706 A. D. Persaud et al.

� 2009 Blackwell Publishing Ltd, Freshwater Biology, 54, 1705–1719

depth preferences (Cyr & Curtis, 1999; Wetzel, 2001),

we predicted that improved taxonomic resolution of

the zooplankton community would reveal variability

in isotopic trends among different zooplankton

orders, genera and species along a DOC gradient.

Methods

Study lakes

We selected sixteen study lakes on the Precambrian

Shield in the District of Muskoka and Haliburton

County, Central Ontario, Canada, with as large a DOC

range as possible. The study lakes had a sixfold range

of DOC concentration, and we sampled them for

DOC, DOM (dissolved organic matter), POM and

zooplankton (Table 1). Based on the OECD classifica-

tion (Vollenweider & Kerekes, 1980), the study lakes

were either oligotrophic or mesotrophic. They were

sampled in July and August of 2004 (15 lakes) and

2005 (10 lakes).

Stable isotope analyses

Isotope ratios and percent carbon and nitrogen of the

samples were determined using a Euro Elemental

Analyzer and a Micromass IsoPrime Continuous Flow

Isotope Ratio Mass Spectrometer in the Worsfold

Water Quality Centre at Trent University. Stable

isotope values were calculated as follows:

d& ¼ ½ðRsample=RreferenceÞ � 1� � 1000

where Rsample and Rreference are the sample and

reference isotope ratios (13C ⁄ 12C and 15N ⁄ 14N).

International and internal standards were used for

calibration of the isotopic data. The calibration stan-

dards for nitrogen were USGS 41 (d15N = 47.57&,

National Institute of Standards and Technology),

IAEA-N1 (d15N = 0.4&, International Atomic Energy

Agency) and LL-glutamic acid (d15N = )4.56 &,

Aldrich, St Louis, MO, U. S. A.). The calibration

standards for carbon were USGS 41 (d13C = 37.76&),

LL-glutamic acid (d13C = )29.05&) and caffeine (d13C,

)40.25&, Acros, Fair Lawn, NJ, U. S. A.). In addition

to calibration standards, DD-glutamic acid (Fisher, Fair

Lawn, NJ, U. S. A.) was used as a quality control

standard with d13C and d15N values of )14.12 & and

)2.71&. Analytical reproducibility was ±0.1& for C

and ±0.2& for N.

Zooplankton. Zooplankton were collected at the deep-

est point in each lake from vertical hauls through the

entire water column with a 150 lm mesh conical net.

Several hauls (6–12) were taken in each lake, the

contents placed in glass jars and kept cool during

transport to the laboratory. Animals were separated

into the following groups: Daphnia species,

Holopedium gibberum (Zaddack, 1955; referred to as

Holopedium), small Calanoida (total length <1 mm),

large Calanoida (total length >1 mm) and Cyclopoida

Table 1 Location and selected chemical

and physical characteristics of the study

lakes. Note that for the lakes sampled in

both 2004 and 2005, dissolved organic

carbon (DOC), total phosphorus (TP), and

Secchi depth represent a 2-year average

Lake Latitude Longitude

DOC

(mg L)1)

TP

(lg L)1)

Secchi

depth (m)

Axe† 45�23¢ 79�30¢ 10.3 8.3 1.2

Blue Chalk*† 45�12¢ 78�56¢ 2.6 3.9 7.2

Brandy*† 45�06¢ 79�31¢ 15.5 28.1 0.9

Chub*† 45�13¢ 78�59¢ 7.1 3.4 2.9

Crosson*† 45�05¢ 79�02¢ 6.4 6.9 3.9

Crown* 45�26¢ 78�40¢ 5.2 3.1 5.3

Devine* 45�11¢ 79�13¢ 7.3 10.7 2.3

Dickie*† 45�09¢ 79�05¢ 6.6 9.8 2.6

Fawn*† 45�10¢ 79�15¢ 11.6 13.9 1.7

Healey* 45�05¢ 79�11¢ 8.0 6.6 4.2

Heney* 45�08¢ 79�06¢ 4.4 6.4 2.6

Mckay*† 45�03¢ 79�10¢ 6.3 3.5 3.5

Moot* 45�09¢ 79�10¢ 8.4 13.1 2.1

Plastic* 45�11¢ 78�50¢ 2.9 4.2 7.9

Red Chalk*† 45�11¢ 78�56¢ 3.6 3.9 5.9

Saw*† 45�03¢ 79�02¢ 9.2 12.8 2.3

* and † denote lakes sampled in 2004 and 2005 respectively.

Stable isotopes in meso-zooplankton 1707


Table 2 Zooplankton groups, isotopic signatures and dominant zooplankton species for each group found in the study lakes

Lake DOC

category:

Lake

Zooplankton taxa

sample Dominant species

d13C d15N

2004 2005 2004 2005

Low-DOC lakes

Blue Chalk Daphnia D.d., D.g.m., D.p. )31.5 ± 0.5 )29.6 ± 1.1 4.4 ± 0.4 4.6 ± 0.5

Holopedium H.g. )38.3 ± 0.4 )36.7 ± 0.7 10.5 ± 1.0 9.9 ± 0.4

Large calanoids E.l., S.o. )27.9 ± 0.5 )28.4 ± 0.5 5.4 ± 0.4 5.1 ± 0.5

Small calanoids L.m. )31.3 ± 0.4 )31.8 ± 0.4 5.2 ± 0.8 5.2 ± 0.9

Cyclopoids C.b.t., C.s., E.a., M.e. )36.2 ± 0.4 )35.4 ± 0.2 6.4 ± 0.2 6.1 ± 0.5

Crown Daphnia D.a., D.c., D.d., D.p. )29.6 ± 0.3 2.1 ± 0.4

Holopedium H.g. )30.4 ± 0.5 12.3 ± 0.5

Large calanoids E.l. )27.2 ± 0.3 4.3 ± 0.6

Small calanoids L.m. )31.3 ± 0.6 8.9 ± 1.1

Cyclopoids A.v.c., C.b.t., C.s., M.e., T.e. )34.9 ± 0.5 4.6 ± 0.6

Heney Holopedium H.g. )33.1 ± 0.5 7.9 ± 1.0

Small calanoids L.m. )32.5 ± 0.4

Cyclopoids M.e., T.e. )30.5 ± 0.6 3.3 ± 0.3

Plastic Daphnia D.a., D.c. )30.2 ± 0.5 0.9 ± 0.4

Holopedium H.g. )32.2 ± 0.4 6.5 ± 1.1


Cyclopoids C.s., M.e. )32.6 ± 0.5 3.2 ± 0.6

Red Chalk Daphnia D.c., D.g.m., D.l., D.p. )30.5 ± 0.5 )32.9 ± 0.8 2.2 ± 0.3 5.0 ± 0.6

Holopedium H.g. )36.5 ± 0.4 )34.6 ± 0.7 7.6 ± 0.4 9.4 ± 0.6

Large calanoids S.o. )28.0 ± 0.2 4.2


Cyclopoids C.b.t, C.s., M.e., T.e. )33.6 ± 0.5 )35.3 ± 0.3 5.8 ± 0.9 9.1 ± 0.3

Intermediate-DOC lakes

Chub Daphnia D.a., D.c. )26.7 ± 0.4 )31.2 ± 0.2 2.9 ± 0.6 1.5 ± 0.7

Holopedium H.g. )29.7 ± 0.5 )32.3 ± 0.4 9.4 ± 0.8 6.6 ± 2.1



Cyclopoids C.s., M.e., T.e. )27.9 ± 0.8 )31.3 ± 0.2 5.4 ± 0.3 4.2 ± 0.3

Crosson Daphnia D.c. )28.9 ± 0.4 )34.5 ± 0.6 4.4 ± 0.2 2.5 ± 0.6

Holopedium H.g. )30.9 ± 0.6 )31.5 ± 0.8 10.7 ± 1.5 5.7 ± 1.0

Large calanoids S.o. )27.8 ± 0.4 3.9 ± 0.6


Cyclopoids C.b.t., M.e., T.e. )29.0 ± 0.4 )28.2 ± 0.5 4.3 ± 0.9 4.1 ± 0.4

Devine Holopedium H.g. )31.7 ± 0.3 11.2 ± 1.2

Large calanoids L.s. )30.4 ± 0.4 5.4 ± 0.6


Cyclopoids M.e. )33.6 ± 0.5 6.5 ± 0.9

Dickie Daphnia D.c. )27.4 ± 0.6 )29.5 ± 0.2 1.6 ± 0.5 1.9 ± 0.3

Holopedium H.g. )28.9 ± 0.3 )30.8 ± 0.9 9.1 ± 1.3 5.6 ± 0.4

Large calanoids E.l. )26.3 ± 0.2 )30.0 ± 1.0 4.7 ± 0.7 3.4 ± 0.7


Cyclopoids C.s., E.a., M.e., T.e. )29.5 ± 0.5 )33.9 ± 0.3 5.4 ± 0.5 6.1 ± 0.6

Mckay Daphnia D.a., D.c., D.d., D.l. )31.5 ± 0.2 )31.5 ± 0.5 4.7 ± 0.2 1.6 ± 0.3

Holopedium H.g. )30.5 ± 0.6 )30.8 ± 0.4 6.3 ± 2.0

Large calanoids S.o., E.l. )30.2 ± 0.4 )28.9 ± 0.2 5.8 4.7 ± 0.2

Small calanoids L.m. )30.5 ± 0.2

Cyclopoids C.b.t., M.e., T.e. )28.1 ± 0.6 )28.3 ± 0.6 5.5 ± 0.6 3.9 ± 0.6

High-DOC lakes

Axe Daphnia D.a., D.c., D.p. )32.7 ± 0.2 0.9 ± 0.4

Holopedium H.g. )32.2 ± 0.4 2.1 ± 0.6

Large calanoids S.o. )31.5 ± 0.3 2.4 ± 0.3


Cyclopoids C.b.t., M.e. )31.7 ± 0.3 3.1 ± 0.3



(Table 2). These groups were selected to account for

differences among (i) taxonomic groups with different

feeding strategies and (ii) different size groups within

taxonomic groups.

Zooplankton were kept in GF ⁄F filtered water for 2–

4 h periods to allow for any digestion, assimilation

and ⁄or gut evacuation. Following this period, zoo-

plankton were rinsed with Milli-Q water, dried at

50 �C for 48–72 h (Grey & Jones, 1999) and ground.

For each zooplankton group 2–4 replicate samples

were collected, with each sample consisting of 20–40,

20–40, 50–100, 75–150 and 75–150 adult individuals for

Daphnia, Holopedium, large calanoids, small calanoids

and cyclopoids respectively.

We sampled in mid-summer since the zooplankton

community is highly productive during this time and

consequently their isotopic signatures should amply

reflect their food source (Crumpton & Wetzel, 1982;

Grey et al., 2000). While zooplankton isotopic signa-

tures can vary seasonally (Matthews & Mazumder,

2005), during mid-summer (July and August) isotopic

signatures of cladoceran and copepod species can

vary synchronously in some lakes, with similar

patterns between years (Perga & Gerdeaux, 2006).

Hence, a single summer sampling can be adequate for

among-lake comparisons, if performed at the same

time (Grey et al., 2000).

The utilisation of stable isotopes for multiple lake

comparisons can be confounded by among-lake

differences in isotopic signatures of basal food

sources, namely bacteria, phytoplankton and detri-

tus in POM (Post, 2002). To address this issue and

correct for changes in basal d13C among lakes, we

performed a POM correction (using 0.7–33 lm

POM) on zooplankton d13C signatures (d13CZoop

POM-corrected) as follows:

d13CZoop POM�corrected ¼ d13CZooplankton � d13CPOM

Table 2 (Continued)

Lake DOC

category:

Lake

Zooplankton taxa

sample Dominant species

d13C d15N

2004 2005 2004 2005

Brandy Daphnia D.g.m., D.p., D.a., D.l. )33.7 ± 0.3 )30.7 ± 0.4 3.5 ± 0.2 4.9 ± 0.3

Large calanoids E.l. )33.6 ± 0.2 )32.7 ± 0.5 5.6 ± 0.2 7.8 ± 0.2


Cyclopoids A.v.c., C.b.t., M.e., T.e. )32.2 ± 0.5 )32.1 ± 0.3 5.2 ± 0.3 7.1 ± 2.0

Fawn Daphnia D.a., D.r. )34.1 ± 0.6 )33.2 ± 0.2 1.1 ± 0.2 1.8 ± 0.3

Holopedium H.g. )33.6 ± 0.3 2.3 ± 0.2

Large calanoids S.o. )32.9 ± 0.2 )31.9 ± 0.2 5.0 ± 0.4 5.1 ± 0.5


Cyclopoids A.v.c., C.b.t., M.e., T.e. )31.4 ± 0.3 )32.5 ± 0.7 4.1 ± 0.3 3.9 ± 0.2

Healey Daphnia D.c., D.l., D.p. )34.2 ± 0.2 3.1 ± 0.2

Holopedium H.g. )34.2 ± 0.4 5.1 ± 0.2


Small calanoids L.m. )35.6 ± 0.3 4.8

Cyclopoids M.e. )31.7 ± 0.2 4.4 ± 0.2

Moot Daphnia D.g.m, D.r. )32.5 ± 0.6

Cyclopoids A.v.c., C.b.t., M.e., T.e. )30.5 ± 0.4 5.1 ± 0.4

Saw Daphnia D.c., D.p. )30.8 ± 0.6 )28.8 ± 0.3 3.7 ± 0.2 2.8 ± 0.4

Holopedium H.g. )31.7 ± 0.1 )28.9 ± 0.2 7.2 ± 0.2 3.1 ± 0.2

Large calanoids S.o. )31.1 ± 0.4 )31.2 ± 0.5 5.7 3.6 ± 0.2

Cyclopoids M.e., T.e. )30.1 ± 0.3 )29.4 ± 0.3 4.2 ± 0.4 3.3 ± 0.3

Where more than one replicate was obtained for analysis, values shown represent the mean ± SD (for n = 2–4).

Species identifications were obtained from a survey performed by Michelle Palmer of York University. Each lake was sampled for SIA

and by M. Palmer within the same week, and sometimes the same day.

A.v.c., Acanthocyclops vernalis complex; D.a., Daphnia ambigua; D.c., Daphnia catawba; D.d., Daphnia dubia; D.g.m., Daphnia galeata men-

doate; D.l., Daphnia longiremis; D.p., Daphnia pulicaria; D.r.; Daphnia retrocurva; C.b.t., Diacyclops bicuspidatus thomasi; C.s., Cyclops scutifer;

E.a., Eucyclops agilis; E.l., Epischura lacustris; H.g., Holopedium gibberum; L.m., Leptodiaptomus minutus; L.s., Leptodiaptomus sicilis; M.e.,

Mesocyclops edax; S.o., Skistodiaptomus oregonensis; T.e., Tropocyclops extensus.



We assumed that the POM d13C isotopic signature is

integrating changes in isotopic signatures occurring in

the bacteria, phytoplankton and detritus that com-

prise POM.

POM d15N baseline corrections (0.7–33 lm POM)

were also performed on all zooplankton d15N signa-

tures (d15NZoop POM-corrected) as follows:

d15NZoop POM�corrected ¼ d15NZooplankton � d15NPOM

This was necessary to correct for differences in

nitrogen dynamics that can occur among lakes

(France, 1999). Further, POM d15N correction was

more appropriate than employing mussel d15N signa-

tures, a common choice (Post, 2002), since turnover

rates of zooplankton tissue are more comparable to

the rapid turnover rates of POM components versus

long-term integrators such as mussels.

POM and DOM. POM (0.7–33 lm size fraction) was

obtained by passing 33 lm-filtered lake water through

pre-combusted (450 �C for 4 h) Whatman GF ⁄F filters

(0.7 lm nominal pore size). The filters were dried at

50 �C. All 2004 and 2005 POM samples were com-

posite, volume-weighted epilimnion and metalimnion

samples. Epilimnion ⁄metalimnion sampling was cho-

sen for POM because demographic costs associated

with low temperatures and food limitations during

summer stratification restricts the time that most

zooplankton spend and ⁄or feed in the hypolimnion

(Williamson et al., 1996). Furthermore, we expected

that if zooplankton were not feeding in the epilimnion

and metalimnion their signatures would be dramat-

ically different from that of our sampled POM.

Water samples for DOM isotopic analysis (<0.7 lm)

were collected for each lake from filtrate of the POM

filtration process. DOM samples were dried in a

rotating evaporator and drying oven set at 50 �C. All

DOM samples were composite epilimnion ⁄metalim-

nion samples.

DOC and TP concentrations

DOC concentration samples (<0.7 lm) were from the

POM filtration process. DOC concentrations were

measured by oxidative combustion-infrared analysis

using a Shimadzu TOC Analyzer.

TP concentration samples were collected from the

33 lm-filtered lake water. TP concentrations were

measured colorimetrically using molybdate-ascorbic

acid after acid digestion (Ontario Ministry of the

Environment, 1983).

Statistical analyses

Our goal was to determine if zooplankton diet

and feeding behaviour changed along a DOC

gradient by estimating what proportion of

variability in d13C and d15N signatures among

taxonomic groups could be attributed to differences

in DOC concentration among lakes. A factorial

ANOVAANOVA design and the residual maximum-likeli-

hood method (REML) were used to estimate

variance components (Robinson, 1987). This design

was chosen because significant interactive effects

indicate if there were changes in carbon sources

utilised (d13C) and trophic status (d15N) among

different zooplankton groups.

Variance components analyses were performed

using d13C, d13CZoop POM-corrected, d15N and d15NZoop

POM-corrected for the five zooplankton groups.

First, we estimated variance components due to

lake, zooplankton taxa (taxa) and interactive lake

by zooplankton taxa (lake*taxa) effects. Then,

lakes were grouped according to lake DOC

concentration into low-DOC (c. <5.8 mg L)1, n = 5

for 2004 and n = 3 for 2005), intermediate-DOC

(5.9–7.4 mg L)1, n = 5 for 2004 and n = 3 for 2005)

and high-DOC (>7.5 mg L)1, n = 5 for 2004 and

n = 4 for 2005) categories, and we performed REML

to determine percent variances attributable to

lake DOC category (lake DOC category), zoo-

plankton groups (taxa) and interactive lake DOC

category by zooplankton taxa (lake DOC

category*taxa) effects.

In the absence of a suitable statistical technique that

could be used to clearly distinguish the groups, we

used the following details to divide lakes into three

DOC categories for REML analyses. (i) Trends in

zooplankton carbon and nitrogen isotopic signatures

along the DOC gradient were not unidirectional (see

Figs 1 & 2). (ii) Variances of zooplankton isotopic

signatures were not homogenous among the three

lake DOC categories (Levene’s test, P > 0.05). (iii) We

chose DOC categories that were either within or close

to ranges used by France et al. (1997) and Lennon et al.

(2006) in their studies of lakes with varying DOC

concentrations.



To examine the effect of taxonomic resolution,

average zooplankton isotopic signatures (uncor-

rected) were calculated for cladocerans and

copepods (coarse taxonomic resolution) and then

REML was performed using average d13C and d15N

values.

Lake DOC category

δ13C

(‰

)

–40

–35

–30

–25

–20Low Intermediate High

2004 Lake DOC category

–40

–35

–30

–25

–20

DOM POM

Low Intermediate High

2005 (a)

δ13C

(‰

)

–40

–35

–30

–25

–20Low Intermediate High Low Intermediate High

–40

–35

–30

–25

–20

DaphniaHolopedium Lg. CalanoidsSm. CalanoidsCyclopoids

(b)

Fig. 1 d13C of (a) DOM and POM, and

(b) Daphnia, Holopedium, cyclopoids, large

calanoids (Lg calanoids) and small

calanoids (Sm calanoids) versus lake DOC

categories (low, intermediate and high)

for midsummer 2004 and 2005.

δ15N

(‰

)

–4

–2

0

2

4

–4

–2

0

2

4 DOM POM

Low Intermediate High Low Intermediate High

2004 2005

δ15N

(‰

)

Lake DOC category

0

2

4

6

8

10

12

Low Intermediate High0

2

4

6

8

10

12DaphniaHolopediumLg. CalanoidsSm. CalanoidsCyclopoids

Lake DOC categoryLow Intermediate High

(a)

(b)

Fig. 2 d15N of (a) DOM and POM, and (b)

Daphnia, Holopedium, cyclopoids, large

calanoids (Lg calanoids), small calanoids

(Sm calanoids) and cyclopoids versus lake

DOC categories (low, intermediate and

high) for midsummer 2004 and 2005.



We determined trends in zooplankton isotopic

signatures along the DOC gradient, by calculating

Pearson product-moment correlation coefficients for

the different zooplankton orders, genera and species.

To assess the influence of DOC and TP concentra-

tions on d13C of different zooplankton groups, multi-

ple regression analyses were performed. Since TP and

DOC concentrations were correlated in our study

lakes (2004: r = 0.86 and P < 0.0001; 2005: r = 0.91 and

P < 0.0001), the backward stepwise method was

chosen because this method accounts for collinearity

among predictor variables (Zar, 1999). For these

analyses, our 2004 and 2005 datasets were combined

to increase the number of data points for the regres-

sion analyses.

The JMP 7 statistical program (SAS, Cary, NC,

U. S. A.) was used for all statistical analyses.

Results

Zooplankton isotopic signatures–DOC relationships

d13C signatures of Daphnia, small calanoids and

large calanoids declined with increasing DOC in

2004 and 2005, whereas Holopedium and cyclopoid

d13C increased with DOC concentration. Among

zooplankton groups, the trend was only significant

for large calanoids. When zooplankton d13C signa-

tures were corrected for among-lake POM d13C

differences, correlations were generally similar, with

the 2005 correlation between Holopedium d13CZoop

POM-corrected signatures and DOC being significant

(Table 3).

Trends between zooplankton d15N signatures and

DOC concentration varied among taxa and between

years (Table 3). When zooplankton d15N values were

corrected for among-lake differences in POM d15N,

most of the correlations were negative, with only the

2005 trend for Holopedium being significant.

Among taxa

Among zooplankton, large calanoids and Daphnia

generally had the most enriched d13C signatures,

ranging from )34.1& to )26.1& and )34.5& to

)26.8&, respectively, whereas Holopedium had some

of the most depleted d13C signatures, ranging from

)38.3& to )28.9& (Table 2). d13C signatures for small

calanoids and cyclopoids were intermediate with

ranges from )35.3& to )27.6& and )36.2& to

)27.9& respectively.

Among taxonomic groups, Daphnia had the lowest

d15N signatures, ranging from 0.9& to 5.0&, whereas

Holopedium had some of the most enriched signatures,

ranging from 2.1& to 12.3&. d15N was intermediate

for large calanoids, small calanoids and cyclopoids,

with ranges of 2.4& to 7.8&, 2.6& to 8.9& and 3.1&

to 9.1& respectively (Table 2).

Among-lake

Among lake variability of d13C and d15N signatures of

different zooplankton taxa generally declined with

increasing DOC concentration (Table 2, Figs 1 & 2).

For example, zooplankton isotopic signatures in 2004

ranged from )38.3& to )27.9& for d13C and 4.4 to

10.5& for d15N in Blue Chalk Lake with a DOC

concentration of 2.7 mg L)1. In contrast, d13C signa-

Table 3 Correlations between DOC concentration and d13C,

d13CZoop POM-corrected (d13CZPC), d15N and d15NZoop POM-corrected

(d15NZPC) for Daphnia, Holopedium, small calanoids, large

calanoids and cyclopoids. The correlation coefficients are from

Pearson product–moment correlations

Variables

2004 2005

r P r P

d13C and DOC concentration for:

Daphnia )0.46 0.11 )0.001 0.99

Holopedium 0.55 0.08 0.49 0.18

Small calanoids )0.38 0.19 )0.48 0.23

Large calanoids )0.66 0.02 )0.93 <0.01

Cyclopoids 0.29 0.28 0.29 0.41

d13CZPC and DOC concentration for:

Daphnia )0.22 0.47 0.37 0.29

Holopedium 0.56 0.07 0.78 0.01

Small calanoids )0.21 0.49 )0.11 0.79

Large calanoids )0.49 0.11 )0.39 0.38

Cyclopoids 0.36 0.18 0.43 0.22

d15N and DOC concentration for:

Daphnia 0.05 0.88 )0.08 0.83

Holopedium )0.14 0.69 0.97 <0.01

Small calanoids 0.16 0.64 0.39 0.33

Large calanoids 0.30 0.34 0.50 0.25

Cyclopoids )0.21 0.48 0.19 0.59

d15NZPC and DOC concentration for:

Daphnia )0.49 0.11 )0.37 0.29

Holopedium )0.36 0.28 )0.95 <0.01

Small calanoids )0.19 0.58 0.06 0.88

Large calanoids )0.22 0.48 0.53 0.22

Cyclopoids )0.39 0.16 )0.38 0.27



tures ranged from only )35.3& to )32.2&, and d15N

ranged from only 3.5& to 5.6&, in Brandy Lake with a

DOC of 14.1 mg L)1.

d13C of various zooplankton groups changed

among lakes since the interactive lake*taxa effect

accounted for the highest percent (45.3% and 74.9%

for 2004 and 2005 respectively) of d13C variability

(Table 4). When lakes were grouped according to

DOC concentration, the interactive lake DOC cate-

gory*taxa effect accounted for the highest percent of

variability in our 2005 data set (25.1%), but differ-

ences among lake DOC categories captured the most

variability in our 2004 data (33.7%). Nevertheless,

over 25% of variability in the 2004 data was due to the

interactive lake DOC category*taxa effect. Taxonomic

differences in zooplankton d13C signatures accounted

for a smaller portion of variability (range 2.7–13.7%

for ungrouped or grouped according to DOC concen-

tration, Table 4) in both years.

Correcting for changes in POM d13C among lakes

on zooplankton d13C did not dramatically change the

general pattern of REML outcomes for 2004 (Table 4).

In contrast for 2005, POM correction of d13C resulted

in lake and lake DOC categories being the main

contributors of variability (Table 3). Lipid correction

(using the mass balance equation by Smyntek et al.,

2007) did not dramatically change the outcome of any

REML analyses.

Unlike zooplankton d13C, taxonomic (62.8%) and

lake (24.1%) differences were prominent contributors

of zooplankton d15N variability in 2004 and 2005

respectively. In 2004, when lakes were grouped

according to DOC concentration, taxonomic differ-

ences remained the dominant factor (56.1%) of var-

iability. In 2005, in contrast, the interactive lake DOC

category*taxa effect was important when lakes were

grouped (27.4%) according to DOC concentration

(Table 4).

Overall, correcting zooplankton d15N for among-

lake POM d15N differences did not change the general

pattern of REML results (Table 4).

Taxonomic resolution

When zooplankton taxonomic groups were pooled

into cladoceran and copepod groups the percent

variance components of their d13C and d15N signa-

tures dramatically changed (Tables 4 & 5). All vari-

ability due to taxonomic and interactive effects

declined or was completely removed from the data

set. However, variability due to lake and lake DOC

categories effects were generally comparable.

Importance of DOC versus TP concentration for

zooplankton d13C

Overall, backward stepwise multiple regression anal-

yses indicated that zooplankton d13C signatures were

more dependent on DOC concentration than on TP

concentration (Table 6). DOC concentration was a

significant or marginally significant variable, albeit

explaining little variability in some cases, for four of

the five zooplankton groups: Holopedium, large cala-

noids, cyclopoids and small calanoids. In contrast, TP

concentration was only of secondary importance for

cyclopoid d13C signatures. Neither DOC nor TP

concentrations were predictors of Daphnia d13C

signatures.

Table 4 Percent variance results using the residual maximum-likelihood method on d13C, d13CZoop POM-corrected (d13CZPC), d15N, and

d d15NZoop POM-corrected (d15NZPC) for different zooplankton groups (Daphnia, Holopedium, large calanoids, small calanoids and

cyclopoids)

Factor d13C d13CZPC d15N d15NZPC Factor d13C d13CZPC d15N d15NZPC

2004 Lake 39.1 37.6 10.3 8.4 Lake DOC cat 33.7 22.8 5.0 13.9

Taxa 13.7 13.6 62.8 66.4 Taxa 5.4 6.1 56.1 54.3

Lake*Taxa 45.3 46.9 26.2 24.5 Lake DOC cat*Taxa 26.7 15.2 7.1 11.1

Residual 1.9 1.9 0.7 0.7 Residual 34.2 55.9 31.8 20.7

2005 Lake 19.7 50.0 46.4 44.6 Lake DOC cat 2.8 31.4 12.6 19.5

Taxa 2.7 0.1 24.1 25.0 Taxa 2.9 0.1 14.6 16.1

Lake*Taxa 74.9 47.1 25.9 26.7 Lake DOC cat*Taxa 25.1 2.3 27.4 27.5

Residual 2.7 2.8 3.6 3.7 Residual 69.2 66.2 45.4 36.9

Numbers represent the % variance attributed to Lake, Taxa, Lake DOC category (cat) and interactive effects.



Discussion

Our results are the first to illustrate variability in the

trajectories (zooplankton–DOC concentration trends)

followed by different zooplankton taxonomic groups

along a DOC gradient. Variable trends among zoo-

plankton along the DOC gradient show that different

zooplankton do not respond similarly to changes in

DOC concentration in temperate freshwater lakes. As

DOC concentrations increased, Daphnia, small cala-

noid and large calanoid species fed more on depleted

carbon sources, deeper in the water column and ⁄or on

phytoplankton which fixed more recycled carbon. In

contrast, with an increase in DOC concentrations,

Holopedium fed higher in the water column and ⁄or on

phytoplankton which fixed more enriched carbon

sources, while cyclopoids switched to feeding more

on enriched carbon sources (probably bacteriovores

feeding on bacteria fuelled by DOC) and ⁄or near

surface waters.

This study also goes beyond previous research to

quantify the differences in d13C and d15N signatures

among meso-zooplankton groups in lakes of varying

DOC concentration. Considerable d13C isotopic vari-

ability was attributed to the interactive lake DOC

categorisation*taxa effect and lake DOC categorisation

(total of 60.4% and 27.9% in 2004 and 2005 respec-

tively). This suggests there was substantial variability

in carbon sources being utilised by different zoo-

plankton groups, with a general reduction in zoo-

plankton d13C variability from low-DOC to high-DOC

lakes. This inference is further supported when

corrections are made for changes in isotopic signa-

tures that occurred in basal resources (POM) among

lakes (total of 38.0% and 33.7% variability of d13CZoop

POM-corrected for the lake DOC category and interactive

lake DOC category*taxa effects in 2004 and 2005

respectively).

The general reduction in d13C isotopic variability

among zooplankton groups with increasing DOC

suggests there are changes in their diet and ⁄or spatial

habitat specialisation among lakes. The wide range of

d13C signatures observed in low-DOC lakes suggests

that zooplankton were utilising different carbon

sources by feeding on different organisms and ⁄or at

different depths. In contrast, more closely related d13C

signatures among zooplankton groups in higher-DOC

lakes indicate that zooplankton were feeding on

similar carbon sources, either by consuming similar

organisms and ⁄or at similar depths.

Differences in isotopic variability in high-DOC and

low-DOC lakes indicate that DOC flow to zooplank-

ton will change as the climate changes. If lake DOC

concentrations increase with climate change, the

occurrence of overlapping zooplankton isotopic sig-

natures will become a common phenomenon in

temperate lakes. Indeed several studies have reported

that DOC concentrations are increasing in temperate

lakes (Hongve et al., 2004; Monteith et al., 2007).

Under such scenarios, flow of DOC to zooplankton

will occur via both the microbial loop and phyto-

plankton community (Steinberg, 2003). It is this DOC

Table 6 Multiple and linear regression models explaining

zooplankton d13C in the 15 study lakes

Factor r2 P

Daphnia None

Holopedium DOC 0.24 0.03

Small calanoids DOC 0.13 0.09

Large calanoids DOC 0.48 0.001

Cyclopoids DOC 0.10 0.02

TP 0.07 0.04

r2 represents the variability explained by TP and DOC concen-

trations.

Table 5 Percent variance results using the

residual maximum-likelihood method on

average d13C and d15N for cladocerans

and copepods

Factor d13C d15N Factor d13C d15N

2004 Lake 42.9 5.8 Lake DOC cat 38.1 5.1

Gross Taxa 1.9 0.4 Gross Taxa 0.7 0

Lake*Gross Taxa 2.4 8.1 Lake DOC cat*Gross Taxa 0 5.3

Residual 52.8 85.8 Residual 61.2 89.6

2005 Lake 24.5 46.3 Lake DOC cat 17.1 35.3

Gross Taxa 2.3 0.7 Gross Taxa 0 0

Lake*Gross Taxa 18.6 4.8 Lake DOC cat*Gross Taxa 5.2 3.6

Residual 54.6 48.2 Residual 77.7 61.1

Numbers represent the % variance attributed to Lake, Gross Taxa, Lake DOC category

(cat) and interactive effects.



‘‘swamping’’ effect that leads to less variability in

isotopic signatures among zooplankton consumers.

On the other hand, if DOC concentrations decline in

temperate lakes (Schindler, 1998) there will be less

flow of DOC to zooplankton and isotopic signatures

will be highly variable among taxa.

Other studies have also reported variability in

carbon sources used by zooplankton in some, but

not all lakes. Perga & Gerdeaux (2006) found that

similar carbon sources were utilised by zooplankton

in the higher-DOC Lake Geneva, but different carbon

sources were utilised by zooplankton found in the

lower-DOC Lake Annecy. Contrary to these results,

Karlsson et al. (2003) found variability in zooplankton

d13C in 15 Swedish lakes, but there was no consistent

trend in variability with lake DOC. Besides the fact

that the study lakes are different in their range of

DOC concentrations [12.9 mg L)1 for our study versus

7.1 mg L)1 for Karlsson et al. (2003)], a major differ-

ence between this study and Karlsson et al. (2003) is

that the zooplankton data were presented as gross

taxonomic groups: cladocerans and copepods. Our

analyses show that percent variance components of

zooplankton d13C and d15N taxonomic and interactive

effects dramatically changed when zooplankton tax-

onomic groups were pooled into cladoceran and

copepod groups. Consequently, it is likely that isoto-

pic trends were obscured by lower taxonomic resolu-

tion of the Karlsson et al. (2003) data.

Although changes in zooplankton d15N variability

with DOC concentration were weaker compared to

that of d13C, the d15N data provides additional

support for increasing dietary overlap with DOC

since 25% and 47% of POM-corrected d15N variability

can be attributed to the combined lake DOC category

and lake DOC category*taxa effects in 2004 and 2005

respectively. Since differences in nitrogen cycling

among lakes were accounted for with POM correction,

these results imply that the trophic level at which

various zooplankton groups were feeding is different

among lakes of varying DOC concentration. For

example, POM-corrected zooplankton d15N in 2004

ranged by 6.4& and 5.8& in low and intermediate

DOC lakes versus 2.7& in the high-DOC lakes.

An alternative to our hypothesis of reduction in

dietary differences with DOC is the possibility that

variability in zooplankton d13C signatures can be

confounded by overlap in d13C isotopic signatures of

different basal resources within POM. Therefore, even

though different zooplankton groups may have sim-

ilar overlapping d13C signatures, the POM component

they are feeding on can be different. In this case we

turn to our d15N data, where it is evident that the

range of d15NZoop POM-corrected signatures was also

smaller in the higher DOC lakes (2.7& for 2004). This

narrow range of 2.7& provides further evidence that

zooplankton may be feeding on similar food sources

since it is less than the commonly used fractionation

factor of 3.4& per trophic transfer.

Other studies have reported increasingly depleted

zooplankton d13C signatures in higher DOC systems,

but not the trend of reduced variability and overlap-

ping signatures (Fig. 3). Comparison of our data with

those from Jones et al. (1999) (includes species-specific

copepods, cladocerans and mixed copepod and cla-

doceran data), and Matthews & Mazumder (2003)

(Daphnia, Holopedium and calanoid data) reveals that

the general pattern of increasingly depleted zooplank-

ton d13C signatures with lake DOC concentration

holds over a wide range of DOC in small temperate

lakes. It also becomes evident that, with few excep-

tions, zooplankton isotopic signatures are generally

between )25& and )40&. What is not evident in this

compiled data set is the distinct trend of reduced

zooplankton isotopic variability as DOC concentration

increases. Absence of the reduced variability trend

probably results because (i) some of the Jones et al.

(1999) data represent gross taxonomic groups, (ii) both

studies presented data for a maximum of only three

taxa ⁄species and ⁄or (iii) no cyclopoid data were

Zoo

plan

kton

δ13

C

–50

–45

–40

–35

–30

–25

–20

Lake DOC concentration (mg L–1)

Jones et al. (1999)This study (2004 & 2005)

0 5 10 15 20 25 30

Matthews & Mazumder (2003)

Fig. 3 Compiled zooplankton d13C from our study, Jones et al.

(1999) and Matthews & Mazumder (2003). The shaded area

delineates the range of isotopic signatures for our 2004–05 data.



presented. Furthermore, our results are specifically for

mid-summer when thermal stratification is stable and

there are food and habitat gradients within the water

column of temperate lakes. Samples from Jones et al.

(1999) were collected at a comparable time of the year

(i.e. July–August), but Matthews & Mazumder (2003)

data were seasonal averages based on samples

collected from June to November. On the other hand,

it is possible that the trend we observed in our data is

region specific and may not exist across a wider

geographic area.

Other factors such as changes in species composi-

tion among lakes and selection of POM as a baseline

may have resulted in some isotopic variability in our

data. Species composition of Daphnia (1–4 species),

large calanoid (1–2 species) and cyclopoid (1–5 spe-

cies) taxa varied among lakes. Species-specific differ-

ences in diet and habitat can add to inter-specific

variability in isotopic signatures (Santer et al., 2006).

Significant differences have been found among sym-

patric cyclopoid species (Santer et al., 2006) and for

this study cyclopoid samples comprised up to five

different species. Interestingly, cyclopoid isotopic

signatures were not the most variable. Instead, the

most variable isotopic signatures were those of

Holopedium (9.4& and 7.9& ranges in 2004 and 2005

for d13C, 7.1& and 7.7 & ranges in 2004 and 2005 for

d15N), a taxonomic group that consisted of only one

species. Nevertheless, it is possible that species com-

position changes may have contributed to some

among-lake variability for multi-species zooplankton

samples.

To some extent our use of POM as the baseline

could have contributed to the variability estimates.

This may be true because POM is a highly variable

composite of phytoplankton, bacteria and detrital

material, and there can be selective feeding among

zooplankton groups. Consequently, a given size frac-

tion of POM may not be fully representative of the

d13C and d15N of primary food source(s) for individ-

ual zooplankton groups (Matthews & Mazumder,

2005). Additionally, differential fractionation of 15N

from a common baseline can occur depending on

nutritional stress (i.e. physiological status of con-

sumers) and food quality, and can lead to some

variation in isotopic signatures (Adams & Sterner,

2000; Vanderklift & Ponsard, 2003).

In addition to DOC concentration, lake productivity

can also influence trends in zooplankton d13C among

lakes (France et al., 1997; Grey et al., 2000). However

our data do not generally support trends previously

observed by France et al. (1997) and Grey et al. (2000)

for zooplankton. France et al. (1997) found that bulk

zooplankton d13C signatures become enriched as lake

productivity (TP) increases; while Grey et al. (2000)

reported that there is increasing variability in zoo-

plankton d13C (sorted as mixed grazing zooplankton

and predatory zooplankton) and overlap between

epilimnetic POM and zooplankton d13C as lake

trophic status increased from oligotrophic to eutro-

phic lakes. Neither consistent enrichment in zoo-

plankton d13C with increasing TP nor distinctive

trends in overlap between POM and zooplankton

d13C were evident (Fig. 4). Moreover, there was no

variability in our zooplankton d13C data that can be

attributed to TP differences among lakes (REML, 0%

variability among lakes of low, intermediate and high

TP concentrations for both years). Furthermore,

TP concentration (µg L–1)

–40

–35

–30

–25

–200 5 10 15 20 25 30

Mesotrophic >10–30

Oligotrophic 0–10

(a)

0 5 10 15 20 25 30

–40

–35

–30

–25

–20

DaphniaHolopedium Lg. CalanoidsSm. CalanoidsCyclopoids

Mesotrophic >10–30

Oligotrophic 0–10

(b)

Fig. 4 d13C of POM, Daphnia, Holopedium, cyclopoids, large

calanoids and small calanoids versus total dissolved phosphorus

(TP in lg L)1) for midsummer (a) 2004 and (b) 2005. OECD

classification of lakes based on TP concentrations is also

indicated.



multiple regression analyses showed that TP was only

of secondary importance for cyclopoid carbon signa-

tures. Here we show that when zooplankton samples

were sorted into different taxonomic groups, DOC

was of primary importance for four of the five

zooplankton groups. Nonetheless, it must be noted

that our study encompassed fewer lakes over a

smaller geographic area and with different ranges of

DOC and TP concentrations.

Improved taxonomic resolution of isotopic data for

these study lakes indicates that habitat specialisation

and ⁄or selective feeding varies along a DOC gradient

for the zooplankton groups examined. As demon-

strated, these differences that exist among groups are

obscured and ⁄or minimised when zooplankton com-

munities are coarsely grouped into copepods and

cladocerans. Furthermore, when bulk zooplankton

samples are analysed these differences are eliminated.

Hence stable isotope studies examining food web

patterns along key environmental gradients such as

DOC must be done with better and consistent taxo-

nomic resolution in order to capture and account for

these differences.

Changes in DOC concentration over time inevitably

alter basal resources, namely phytoplankton and

bacteria, available to zooplankton. Bacteria and phy-

toplankton communities are important connections

through which DOC is mineralised and mobilised to

higher trophic levels via bottom-up transfer (Cole

et al., 2002; Karlsson et al., 2003). Bacteria can directly

mineralise DOC, thereby providing a means of carbon

transfer to bacteriovores and some zooplankton graz-

ers (Blomqvist et al., 2001; Karlsson et al., 2004). This

DOC-bacteria-bacteriovores route will be of impor-

tance for zooplankton such as cyclopoids, which can

be predatory and appear to be feeding on more

enriched carbon sources in higher DOC lakes. Addi-

tionally, for species such as Holopedium, which can

filter feed smaller size particles, bacteria can poten-

tially contribute more to their diet in lakes with higher

DOC concentrations. In contrast to bacteria, phyto-

plankton can indirectly transfer carbon liberated from

the mineralisation of DOC even though their produc-

tivity is lower at higher DOC concentrations (Carpen-

ter et al., 1998; Cole et al., 2002). The importance of this

DOC-phytoplankton route will be of primary impor-

tance to zooplankton such as Daphnia species that

appear to be feeding largely on phytoplankton, even

in higher-DOC lakes.

It is possible that climatic change is altering DOC

levels in temperate lakes. Here we quantitatively

showed that variability in zooplankton isotopic sig-

natures can be attributed to differences in DOC

concentration among lakes. More importantly we

have shown that there is also variability in isotopic

trends of different zooplankton taxa along a DOC

gradient. This suggests that different zooplankton

groups, genera and species will ultimately respond

differently to changes in DOC concentrations in

temperate Precambrian Shield lakes.

Acknowledgments

Thanks to the staff of the Ontario Ministry of the

Environment in Dorset, Ontario, York University and

the Worsfold Water Quality Center at Trent Univer-

sity for their assistance. Jane Gowland, Thekla Hum-

mel, Heather Broadbent and Madalyn Budd assisted

in all data collection in the field. Bill Keller and

Michelle Palmer provided background zooplankton

information for the study lakes. Keith Somers pro-

vided statistical advice. Anurani D. Persaud was

supported by a Natural Science and Engineering

Research Council (NSERC) postgraduate scholarship

(CGS-D), an IODE War Memorial scholarship, a

Canadian Water Resources Association award and

an Ontario Graduate Scholarship in Science and

Technology.

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(Manuscript accepted 2 March 2009)



stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

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