Riparian vegetation removal alters consumer]resourcestoichiometry in an Australian lowland stream

Darren P. GilingA,C, Paul ReichA,B and Ross M. ThompsonA

AAustralian Centre for Biodiversity and School of Biological Sciences, Monash University,

Clayton, Vic. 3800, Australia.BArthur Rylah Institute for Environmental Research, Department of Sustainability

and Environment, Heidelberg, Vic. 3084, Australia.CCorresponding author. Email: darren.giling@monash.edu

Abstract. Anthropogenic impacts on stream ecosystems generate changes in nutrient and carbon availability which actas stoichiometric challenges to consumers. We tested the hypothesis that removal of Eucalyptus riparian vegetation altersin-stream resource stoichiometrywith flow-on effects for a benthic consumer (the freshwater crayfish,Cherax destructor).

Sites with high and low riparian canopy cover were selected on a lowland stream in south-eastern Australia. A reduction inriparian vegetation canopy cover was associated with decreased terrestrial detritus (low nutritional quality; high carbon tonitrogen (C :N) ratio) and increased cover of macrophytes and filamentous algae (high quality; low C :N ratio). Thisresource-quality shift was associated with a small but significant decrease in C. destructor C :N ratio (molar ratio of

muscle tissue). This suggests that the animals are deviating from homeostasis and may be in better condition in the streampools dominated by in-stream productivity. A significant negative relationship between C. destructor length and C :Nratiowas observed, suggesting that resource-quality impactsmay differwith age. The present study has shown that riparian

loss alters stoichiometric interactions in stream benthic ecosystems, with potential consequences for stream processes suchas nutrient cycling. Ecological stoichiometric theory should therefore be further utilised to make predictions of ecologicalimpacts in freshwater systems.

Additional keywords: carbon, C :N ratio, crayfish, crustacean, detritus, elemental homeostasis, nitrogen.

Received 20 April 2011, accepted 28 July 2011, published online 2 November 2011

Introduction

Ecological stoichiometry examines the flow and balance ofenergy and nutrients in ecological interactions (Frost and Elser2001). It uses common elements required by organisms to

examine trophic interactions across all levels of biologicalorganisation (Elser et al. 2000b; Frost et al. 2002). Ecologicalstoichiometry primarily investigates the balance of carbon (C)relative to the nutrients nitrogen (N) and phosphorus (P), which

are often limiting for consumer growth. Autotrophic producershave relatively plastic elemental ratios and typically possesshigh and variable C :N and C : P ratios that respond to abiotic

resource availability (Elser et al. 2000a; Cross et al. 2003;DeMott 2003). This contrasts with heterotrophic consumers thathave constrained and low C :N and C : P ratios, creating an

elemental mismatch between autotrophs and heterotrophs(Cross et al. 2003). Heterotrophs exhibit a more strict elementalhomeostasis than do producers, meaning that their elementalcomposition does not fluctuate when consuming resources

of differing quality (Cross et al. 2003; Persson et al. 2010),although there is now strong evidence that the degree of elementalhomeostasis within heterotrophs varies among taxa and envir-

onments (DeMott 2003; Persson et al. 2010; Vrede et al. 2011).

There are a range of factors and mechanisms that determine

consumer C : nutrient ratios, both within and among species.These include phylogenetics (Liess and Hillebrand 2005;Martinson et al. 2008), body size (Woods et al. 2004; Hamback

et al. 2009), feeding mode or trophic level (Fagan et al. 2002;Hamback et al. 2009), differences in resource allocation (Vredeet al. 2011), ontogenetic diet shifts (Laspoumaderes et al. 2010)and specific growth rate (Elser et al. 1996). In relation to diet,

heterotroph elemental ratios can shift when consuming a foodsource with a very high C : nutrient ratio compared with theirbody composition (DeMott et al. 1998). This represents a

deviation from strict elemental homeostasis. In this case,C assimilation efficiency and growth rate decline, and biomassC : nutrient ratio increases only slowly (DeMott et al. 1998;

Adams and Sterner 2000). For example, Small and Pringle(2010) described elevated P content in leaf litter and epilithonacross streams with a natural P gradient, which produced anincrease in P content in a range of invertebrate consumers. The

degree of elemental mismatch between producers and primaryconsumers also can influence diet selection and hence food-webstructure and dynamics (Elser and Urabe 1999; Cruz-Rivera and

Hay 2000; Anderson et al. 2004).

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Marine and Freshwater Research, 2012, 63, 1–8

http://dx.doi.org/10.1071/MF11092

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In many stream and lake ecosystems, anthropogenic impactscan alter the elemental ratio and consumer assimilation of

available basal resources. Basal resource quality (C : nutrientratio) is affected by changes in the level of autochthonousproduction (Deegan and Ganf 2008), alteration of riparian zones

(Schulze and Walker 1997) and waste inputs from treatmentplants in urban systems (Dang et al. 2009; O’Brien andWehr 2010). Increases in atmospheric CO2 concentration are

predicted to alter C : N : P ratios in terrestrial leaf-litter inputsand algae (Hargrave et al. 2009; Woodward et al. 2010). Thesechanges can alter stoichiometric interactions between producersand consumers, affecting rates of secondary production

(deBruyn et al. 2003; Gucker et al. 2011), favouring particularmacroinvertebrate taxa (Singer and Battin 2007; Dang et al.

2009; Evans-White et al. 2009) and altering nutrient fluxes

(Singer and Battin 2007). Lower C :N and C : P ratios in basalresources increase benthic invertebrate consumer growth rates(Stelzer and Lamberti 2002; Back et al. 2008; Giling et al.

2009). In urban systems affected by sewage, changes in produc-er resource quality have also been shown to alter the C : nutrientratios of consumers that deviate from strict homeostasis (Singerand Battin 2007). The removal of native riparian vegetation

and/or replacement with exotic tree species can also have largeimpacts on consumer stoichiometry. Where riparian trees areremoved, there can be a shift in basal productivity towards

in-stream autotrophs such as macrophytes and algae. Wherenative trees are replaced with exotic species, this may alterC :N : P ratios in leaf litter, affecting the degree of stoichio-

metric imbalance and litter-breakdown rates (Janssen andWalker 1999; Hladyz et al. 2009).

Stoichiometric interactions in detritus-based benthic food

webs in streams have received relatively little attention com-pared with lake pelagic zones (Cross et al. 2005; Frost et al.2005). This is despite.80% of plant primary production beingrecycled through detritus (Wetzel and Ward 1992). In streams,

terrestrial detritus is a low-quality food source with a highC : nutrient ratio compared with autochthonous aquatic foodsources such as algae, periphyton and macrophyte material

(Cross et al. 2003; Deegan and Ganf 2008; Lau et al. 2009).Detritus therefore places important stoichiometric growth andreproduction constraints on benthic consumers (Frost et al.

2002). The effect of changes in detritus quantity or quality instreams has been shown to affect consumer C : nutrient ratios(particularly with respect to P) in some instances (Cross et al.2003; Small and Pringle 2010); however, this is often not the

case with N (Cross et al. 2003; Deegan and Ganf 2008). This isimportant for stream-ecosystem processes occurring in thebenthos because changes in the body nutrient content of benthic

consumers or their food resources affects nutrient excretion andcycling (Balseiro and Albarino 2006; Alves et al. 2010). On thebasis of past research in detrital-based stream ecosystems, we

expect that disturbances affecting the link between the streamand riparian zone will reduce the amount of low-quality (highC : nutrient ratio) detrital resources available for consumers.

We investigated changes in the basal resource stoichiometryof a lowland south-eastern Australian stream with contrastingamounts of riparian canopy cover. In some locations, the nativeriparian canopy species had been completely removed, resulting

in significant reductions of allochthonous material and a

concomitant increase in autochthonous production, presumablyin response to higher light availability (Giling et al. 2009). The

stream has been previously shown to be primarily supportedby benthic detrital material from the surrounding vegetation(Reid et al. 2008). We hypothesised large perturbations in the

stoichiometric composition of available basal resources withcanopy cover loss. However, despite any shifts in nutrientavailability, past research suggests consumer C :N ratios are

often static (Cross et al. 2003; Deegan and Ganf 2008). There-fore, we expected relatively strict C :N homeostasis to beexhibited by an omnivorous benthic invertebrate, the freshwatercrayfish, Cherax destructor.

Materials and methods

Study site and organism

Six study sites were located along an 8-km section of JoycesCreek, a lowland agricultural stream near Castlemaine, Victoria,

Australia (upstream, 37.178S, 143.968E; downstream 37.118S,143.968E). Joyces Creek is a third-order tributary of the LoddonRiver, and the major land use surrounding the study stream is

mixed grazing and cropping. At the time of sampling (April andMay 2008), each study site comprised an isolated pool that hadbeen connected by stream flow only for three brief periods after

January 2005 (totalling 77 days, with discharge lower thanhistorical baseflow). Site selection was based on the degree ofcanopy cover of the dominant riparian tree species, Eucalyptuscamaldulensis (river red gum). Three ‘closed’ sites were chosen

with remnant trees (high canopy cover; Sites H1, H2 and H3)and three ‘open’ sites with little or no surrounding vegetation(low canopy cover; Sites L1, L2 and L3). The site categories

were interspersed longitudinally to avoid bias. The closed siteswere characterised by large amounts of terrestrial detritalmaterial and little aquatic plant biomass, whereas open sites

contained far less detritus and large quantities of filamentousalgae and aquatic vegetation (see Giling et al. 2009). Cheraxdestructor, an omnivorous crayfish, was chosen to investigateconsumer elemental homeostasis. This species is important

because it shreds and consumes large amounts of detrital andliving organicmatter, and freshwater crayfish are known to exerta strong influence on aquatic food webs (Nystrom and Strand

1996; Whitledge and Rabeni 1997).

Sample collection and analysis

Percentage drymass of C andNwas obtained fromC. destructor

from all six study sites, whereas basal resources and otherinvertebrates were collected from two sites (H1 and L1).C. destructor was collected using unbaited fyke nets (6-mm

mesh, 80 cm� 55 cm opening) and unbaited minnow traps(3-mm mesh, 45 cm� 25 cm� 25 cm box). All C. destructorindividuals were removed and euthanised before being frozen

(a total of seven, two and eight from sites H1, H2, H3, and 12, sixand seven from sites L1, L2, L3, respectively). The gender ofeach animal was determined, length measured as the orbital

carapace length (OCL, mm) and a section of the tail muscledissected from each of the 42 crayfish individuals trapped.Invertebrates were collected with a dip net, and includedchironomid larvae (collectors), gastropods (grazers), corixids

(omnivores), notonectids (predators) and larval odonates

2 Marine and Freshwater Research D. P. Giling et al.

(predators). The invertebrates were kept alive for at least 24 h tovoid their digestive tracts. Basal resources collected included all

of the dominant macrophytes (Typha sp., Potamogeton tricar-

inatus and Triglochin procera) as well as coarse particulateorganic matter (benthic CPOM), E. camaldulensis leaves, green

filamentous algae (Cladophora), and biofilm (scraped off tilesplaced in study pools for 6 weeks for biofilm growth), whichwere then frozen. Benthic CPOM differed from E. camaldu-

lensis leaves in that it contained large debris from a range ofplant species, includingmacrophytes, whereas E. camaldulensissamples contained only river red gum leaves and fragments. Wecollected both fresh (FR) and conditioned (CO) Typha sp.,

CPOM and E. camaldulensis leaves. All crayfish muscle, andother invertebrate and basal resource material were washed andoven-dried (608C, 1 week) before being crushed into a fine

powder with a ball-bearing grinder or mortar and pestle forchemical analysis. Individual crayfish were used as replicates ateach site, whereas resources and other invertebrate species were

pooled to create a composite sample for each of L1 and H1.Percentage elemental analysis was carried out at the StableIsotopes Analysis Laboratory at Griffith University, Brisbane,with an elemental analyser (Eurovector EA 3000, Milan, Italy).

Data analysis

All C and N results are presented as molar ratios. Comparison ofC. destructor C :N ratio and length (OCL) among sites was

made with a single factor analysis of variance (ANOVA), with aplanned comparison between open and closed sites. Analysis ofcovariance (ANCOVA) with OCL as a linear covariate was also

used to analyse the effect of site on C :N when corrected foranimal length. All ANOVA and ANCOVA results were highlysignificant and likely to be statistically robust despite an un-

balanced design resulting from an unequal number of crayfishbeing trapped. Linear regression was used to investigate therelationship between C. destructor OCL and C :N ratio, andunequal variances Student’s t-tests were used to analyse dif-

ferences in C. destructor C :N ratio or OCL and gender. Datawere untransformed and met the assumptions of all statisticalanalyses. We conducted all analyses in the statistical software R

(version 2.7.0).

Results

Effect of riparian vegetation on consumer and resource C :N

The C :N ratios of C. destructor tail muscle were significantly

higher at closed sites than at open sites (single-factor ANOVAwith planned comparison, F1,36¼ 18.39, P, 0.001, Fig. 1). Thepotential influence of length (OCL) was analysed by including

C. destructor OCL as a linear covariate to site in an ANCOVAmodel of C :N ratio. There was no significant interactionbetween site category (open or closed) and OCL (single-factor

ANCOVA, F1,39¼ 0.386, P¼ 0.538), and C. destructor C :Nratio was significantly greater at closed sites when corrected forOCL (single-factor ANCOVA, F1,40¼ 11.415, P¼ 0.002).

Percentage composition of C andN, as well as the C :N ratio,revealed CPOM as a relatively low-quality food source with alarge proportion of C (Fig. 2, mean C :N¼ 84). High C :N ratioswere also found in E. camaldulensis leaves (mean C :N¼ 41)

and the macrophyte Typha sp. (mean C :N¼ 43), whereas

macrophytes P. tricarinatus (C :N¼ 17) and T. procera

(C :N¼ 13) were relatively higher-quality food sources, withhigher N relative to C content (Fig. 2). Algae (C :N¼ 28) andbiofilm (mean C :N¼ 7) also had relatively low C :N ratios. All

macroinvertebrate samples had lower C :N ratios than didproducer and detrital resources.

Relationship among consumer C :N, size and gender

There was a significant negative linear relationship between

OCL and the C :N ratio of C. destructor at both the closed sites(simple linear regression, R2¼ 0.38, F1,15¼ 9.20, P¼ 0.008,Fig. 3) and open sites (simple linear regression, R2¼ 0.26,

F1,23¼ 8.08, P¼ 0.009, Fig. 3). There was also a significantnegative linear relationship when analysed across all 42 animals(simple linear regression, R2¼ 0.37, F1,40¼ 23.29, P, 0.001).No significant effect ofC. destructor sex on C :N ratio (separate

variances Student’s t-test, t20.07¼ 0.021, P¼ 0.983) or OCL(separate variances Student’s t-test, t17.77¼�0.414, P¼ 0.888)was detected.

Discussion

Effect of riparian vegetation on consumer and resource C :N

The loss of riparian canopy cover shifted the C :N ratio ofavailable basal resources by increasing the availability of low

C :N resources such asmacrophytes and algae. As hypothesised,high-quality autochthonous resources were more common instream pools when canopy cover was reduced. Across all sites,

a large elemental imbalance between consumers and potentialfood sources was identified. Benthic CPOM and terrestrialinputs (E. camaldulensis) had lower N content than did

3.0

3.1

3.2

3.3

3.4

3.5

H1 H2 H3 L1 L2 L3

C :

N (

�s.

e.)

ratio

Study site

Closed Open

b

a

Fig. 1. Mean (�s.e.) Cherax destructor carbon to nitrogen (C :N) ratio at

H1 (n¼ 7), H2 (n¼ 2), H3 (n¼ 8), L1 (n¼ 12), L2 (n¼ 6) and L3 (n¼ 7).

Different letters indicate a significant (P, 0.001) difference between the

groups (single-factor ANOVA with planned comparison).

Vegetation removal alters in-stream stoichiometric balance Marine and Freshwater Research 3

macrophytes, algae and biofilm. On average, aquatic inverte-brates had C :N ratios four times lower than those of aquaticautotrophs (macrophytes, biofilm and Cladophora), and 14

times lower than those of terrestrial and aquatic detritus (CPOM,E. camaldulensis, conditioned Typha sp.). The imbalancesbetween resources and consumers observed in the current studywere similar to, or in the case of CPOM, evenmore extreme than

those from other studies of streams with high terrestrial-detritusinputs (e.g. Lau et al. 2009; Gucker et al. 2011). At locationsalong a forest-to-pasture gradient in South Australia, Deegan

and Ganf (2008) reported mean C :N ratios ranging from 23.3 to59.4 for CPOM, from 7.1 to 9.1 for T. procera, from 7.7 to 22.5for filamentous algae and from 4.5 to 5.7 for invertebrates. In

two headwater streams in the USA, Cross et al. (2003) alsofound considerable elemental imbalances whereby leaf detritushad very high C :N ratios (mean C :N¼ 73) compared with

invertebrate taxa (mean C :N ranges 5.1–6.7 between trophicgroups). Thus, as expected, detrital food sources contain smallamounts of N and P relative to C, placing higher stoichiometric

constraints on benthic consumers, potentially reducing fitness(Cross et al. 2003).

Our data suggest that C. destructor from isolated streampools may deviate from strict elemental homeostasis. The tailmuscle of crayfish from the open sites possessed significantly

lower C :N ratios than did that of the closed-site crayfish,suggesting they had access to higher-quality (low C :N) foodsources. This was not expected on the basis of the lack ofdeviation from C :N homeostasis observed in previous studies

(Cross et al. 2003; Deegan and Ganf 2008). However, it isconsistent with the large amount of macrophyte material of lowC :N ratio (and, to a smaller degree, filamentous algae) available

in the open-site pools. Consumer diet or trophic-mode catego-ries have been successfully shown to control body compositionamong species (Hamback et al. 2009; Alves et al. 2010). For

example, Fagan et al. (2002) found that insect predators hadhigher body N content than did herbivorous insects. However,intraspecific variation in C : nutrient ratios in response to dietquality is less common and previous studies have found

inconsistent results with respect to the potential for aquaticmacroinvertebrate species to exhibit stoichiometric plasticity.Most aquatic studies have reported no differences in inverte-

brate C :N ratios within taxa with varying diets (Cross et al.

2003; Liess and Hillebrand 2006; Ventura et al. 2008). DeeganandGanf (2008) showed that C : N ratios in two orders of benthic

invertebrates were relatively uniform across stream sites,despite large fluctuations in resource C :N ratios, and Finket al. (2006) reported that herbivorous macroinvertebrates had

a narrow C :N range from lake sites with contrasting periphytonquality. In contrast to these studies, Fink and Von Elert (2006)found that gastropod C :N ratio increased when fed on a

3.0

3.1

3.2

3.3

3.4

3.5

3.6

0 10 20 30 40 50C

: N

rat

io

OCL (mm)

Fig. 3. Scatterplot displaying the significant negative linear relationships

between Cherax destructor orbital carapace length (OCL, mm) and carbon

to nitrogen (C :N) ratio at the closed (R2¼ 0.38, n¼ 17, P¼ 0.008, solid

circles and solid line) and open (R2¼ 0.26, n¼ 25, P¼ 0.009, open circles

and dashed line) canopy sites.

0

20

40

60

80

100

CP

OM

(C

O)

CP

OM

(F

R)

Typ

ha s

p. (

CO

)

E. c

amal

dule

nsis

(F

R)

E. c

amal

dule

nsis

(C

O)

Cla

doph

ora

Typ

ha s

p. (

FR

)

P. t

ricar

inat

us

T. p

roce

ra

Bio

film

Div

ing

beet

le

Cor

ixid

Odo

nata

Not

onec

tidae

Chi

rono

mid

s

C. d

estr

ucto

r

Mea

n (�

s.e.

) C

: N

rat

io

Basal resources and consumers

Gas

trop

ods

Fig. 2. Mean (�s.e.) carbon to nitrogen (C :N) ratio for each basal resource

(white) and consumer (grey), averaged across sitesH1 andL1.Because these

samples (apart from Cherax destructor) were collected at one or two sites

and pooled for a composite sample, n¼ 1 (Cladophora, P. tricarinatus,

T. procera, diving beetle, Odonata, chironomids and gastropods) or n¼ 2

(CPOM, Typha sp., E. camaldulensis, biofilm, corixid and notonectid). The

value forC. destructor is amean (�s.e.) of all 42 individuals. Basal resources

labelled CO and FR indicate conditioned (from the wet area of the pool) and

fresh (outside the wet area) material, respectively.

4 Marine and Freshwater Research D. P. Giling et al.

low-quality algae diet, compared with a high-quality algae diet.Additionally, multiple studies have found a deviance from C : P

homeostasis in consumers with a diet enriched in P (Cross et al.2003; Back et al. 2008; Small and Pringle 2010).

We found evidence of a stoichiometric shift in muscle

composition of C. destructor. Because this was relatively small,we need to be cautious in inferring large impacts on crayfishgrowth or crayfish impacts on the aquatic community. However,

the shift was consistent across sites and, coupled with thelarge and pervasive change in resource quality and quantity,provides evidence of a human-induced change in stoichiometricinteractions. Riparian alteration affected the in-stream producer–

consumer stoichiometric balance, meaning that invertebrateconsumers could face differing stoichiometric constraints underaltered riparian inputs, because macrophytes and filamentous

algae with low C :N ratios were present only at sites with littlecanopy cover.

Relationship between consumer C :N ratio, size and gender

Compared with interspecific size class or body mass(e.g. Hamback et al. 2009; Alves et al. 2010), there is lesspublished information on the effect of intraspecific size or age

on C : nutrient ratios. There was a significant negative effect oflength on the C :N stoichiometric ratios of crayfish, whichcontrasts with the results of other studies on benthic macro-

invertebrates. Back et al. (2008) found no significant relation-ship between mayfly nymph-development stage and C :N ratio.Alves et al. (2010) reported no significant trend between body

mass and N content in three benthic macroinvertebrate species,although two of these species showed significant positive trendsbetween body size and P content. In pelagic systems, differences

in the C :N ratio among ontogenetic stages in copepods fromlake ecosystems have been observed (Villar-Argaiz et al. 2002).There remains significant uncertainty about interactionsbetween body size and stoichiometry. This relationship is likely

to be further complicatedwhen species exhibit ontogenic dietaryshifts, as has been observed in freshwater crayfish (e.g. Guanand Wiles 1998; Parkyn et al. 2001).

There was evidence of a size- or age-based shift in stoichio-metric ratios in the current study. The observation that juvenilespossessed tail muscle with higher C :N ratios compared to

adults was unexpected for several reasons. Juvenile crayfishare typically thought to consume more invertebrate prey thando adult crayfish (Guan and Wiles 1998; Parkyn et al. 2001).Invertebrate prey have a considerably lower C :N ratio than do

producers and should contribute to juveniles having a low C :Nratio. Additionally, C : N ratios generally decrease with increas-ing growth rates (Vrede et al. 2004), and juvenile crayfish grow

faster than do adults (Whitledge and Rabeni 1997). Otherstudies have shown changes in body stoichiometry as a resultof ontogenic dietary shifts. Laspoumaderes et al. (2010)

observed a decrease in the C :N ratio of body tissue in later lifestages of the copepod Parabroteas sarsi, which exhibits anontogenic shift towards a more predatory diet. However, in the

current study, the more predatory life stage had a higher C :Nratio – the opposite of what would be predicted.

Alves et al. (2010) hypothesised that such a trend could beexplained by a non-limiting element accumulating while the

organism meets another stoichiometric requirement. Using this

theory, it may be that adult C. destructor individuals areingesting C or P as a limiting element and accumulating the

non-limiting nutrient N incidentally. Excretion data wouldallow determination of which nutrient is in excess (Sterner1990; McManamay et al. 2011); however, that data were not

collected in the current study. An alternate hypothesis for theobserved result is that the prolonged period of drought leadingup to sampling may have altered availability of resources. In

smaller habitats (e.g. drying stream pools), crayfish may beexposed to increased interspecific and intraspecific competitionand spend more time in torpor. Lower C :N ratios evident inlarger crayfish may reflect a legacy of conditions before our

sampling period, because C. destructor can live for up to8 years (Lake and Sokol 1986). Further work should examinethe mechanisms controlling allometric differences in the

C : nutrient ratios within benthic invertebrate taxa.It appears unlikely that the difference in the C :N ratios was

attributable to reproductive investment. Investment in reproduc-

tion by adults of any species generally demands large amountsof N and P (Færøvig and Hessen 2003). Therefore, it may beexpected that adult crayfish would have higher C :N ratios thando juveniles, if they are allocating additional nutrients towards

reproductive material. There was no evidence of gender-baseddifferences in stoichiometry; hence, the observed changes in theC :N ratio cannot be explained by differential investment in

reproductive tissues by male and female C. destructor indivi-duals. However, the present results are based on muscletissue only, and results from whole-body measurement of

stoichiometry may have differed.

Potential stoichiometric impacts on stream ecosystems

Degradation and loss of riparian vegetation is a commonanthropogenic impact worldwide (e.g. Naiman and Decamps1997; Pozo et al. 1997; Bunn et al. 1998). Past studies havefound that riparian clearing can increase autochthonous pro-

duction and the importance of autochthonous resources forstream foodwebs (Bunn et al. 1997;Hicks 1997). On the basis ofprevious results and our study, this change is likely to substan-

tially alter resource stoichiometry. Increased nutrient content asa result of a shift to autochthonous basal resources is predictedby ecological stoichiometric theory to reduce stoichiometric

constraints and increase consumer growth rates (Stelzer andLamberti 2002; Back et al. 2008). In turn, this effect couldinfluence stream community and food-web structure. Thisinfluence is expected to be large for crayfish because they are

known to have pervasive effects as grazers (Brooks 1997) andinvertebrate predators (Lodge et al. 1994; Momot 1995).Evidence from urban systems shows that altered basal resource

quality also can affect taxa distribution (Dang et al. 2009;Evans-White et al. 2009) and secondary production (deBruynet al. 2003) by aquatic macroinvertebrates. Taxa identity, body

composition and size within individual taxa can influencenutrient recycling and excretion rates (Small et al. 2009; Alveset al. 2010;McManamay et al. 2011). Therefore, we predict that

riparian degradation resulting in changes to resource andconsumer stoichiometry could have important implications forstream processes such as nutrient recycling. Furthermore,changes to leaf quality affect other stream functions, including

rates of litter breakdown and resource depletion (Hladyz

Vegetation removal alters in-stream stoichiometric balance Marine and Freshwater Research 5

et al. 2009). These effects are likely to be similar whenlow-quality eucalypt detritus is compared with higher-quality

macrophyte and algal biomass.Although our workwas undertaken in a single stream, Joyces

Creek is typical of many small lowland streams in the southern

Murray Basin, and there is no a priori reason to assume that theresults found here are not generalisable to crayfish populationselsewhere in southern Australia. Furthermore, the processes

hypothesised to underpin the effect should be widely geograph-ically applicable. The data reported here were collected only in asingle season (autumn). Stoichiometric ratios are known to varytemporally (Matthews and Mazumder 2005), and the impor-

tance of different basal resources to freshwater crayfish can shiftseasonally (Reid et al. 2008). The studywas also limited in that itconsidered the stoichiometric composition of muscle tissue

only, and included only N. Additional insights would have beenpossible if whole-body data had been available, or if concentra-tions of P had also been measured.

Our data showed that riparian habitat degradation as a resultof agricultural activity can alter the stoichiometric balance ofin-stream producers and consumers and potentially affect theconsumer C :N ratio in non-homeostatic taxa. Detrital-based

stream systems are likely to be highly vulnerable to alterations inresource stoichiometry because detrital supply depends onfunctioning lateral and longitudinal linkages between the

stream, its floodplain and the riparian zone. Detritus also is oflow quality compared with other basal resources. There are agrowing number of stoichiometric studies demonstrating the

effects of human-induced change across a wide range ofecosystems and spatial scales (e.g. Humborg et al. 2000;Tuchman et al. 2002; Singer and Battin 2007). The present

study further extends the evidence that ecological stoichiometrycan be utilised effectively to study how anthropogenic activitiesare altering C and nutrient availability, and the consequences ofthis for communities and ecosystem function. Ecological

stoichiometric theory should therefore be further utilised topredict ecological impacts in freshwater systems and showssome promise for inclusion as an indicator for monitoring the

ecological response to riparian restoration.

Acknowledgements

Department of Sustainability and Environment permits and Monash

University animal ethics approval (DSCI/2008/06) were gained before

undertaking this study. Financial support was provided by the Murray–

Darling Basin Authority and by the Australian Research Council via

LP0990038 to R. M. Thompson et al. Comments on previous versions by

BjornGucker, SusieHo, Samantha Imberger,MardianaAli andAlissaMonk

and anonymous reviewers contributed to the quality of the manuscript. We

thank TomDaniel, Matthew Johnson, Eddy Giling and Andrew Fry for field

assistance, and landholders for access to the streams. Chemical analyses

were performed by Rene Diocares at Griffith University, Brisbane. This

research and publication were supported by the Australian Centre for Bio-

diversity at Monash University.

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