tracing carbon sources in small urbanising streams: catchment-scale stormwater drainage overwhelms...

Tracing carbon sources in small urbanising streams:catchment-scale stormwater drainage overwhelms the effectsof reach-scale riparian vegetation

SAMANTHA J. IMBERGER* , † , ‡ , PERRAN L. M. COOK* ,† , MICHAEL R. GRACE* , † AND

ROSS M. THOMPSON†,§ , ¶

*Water Studies Centre, Monash University, Melbourne, Victoria, Australia†Australian Centre for Biodiversity, Monash University, Melbourne, Victoria, Australia‡Department of Resource Management and Geography, The University of Melbourne, Richmond, Victoria, Australia§School of Biological Sciences, Monash University, Melbourne, Victoria, Australia¶Institute for Applied Ecology, University of Canberra, Canberra, ACT, Australia

SUMMARY

1. Organic matter provides energy and nutrients to aquatic systems. Alterations to its sources and

processing have repercussions for water quality and food-web stability and structure. Despite

worldwide recognition of the impacts of urbanisation, there is limited understanding of the relative

importance of catchment-scale urban stormwater drainage connection and reach-scale riparian

vegetation on organic matter sources.

2. We investigated the effects of catchment-scale urban stormwater drainage connection and reach-

scale riparian vegetation cover on organic matter sources in small streams. Using stable isotopes and

elemental ratios (i.e. d13C, d15N and C : N), we traced the origin of microbially respired carbon and

standing stocks of dissolved organic carbon (DOC), suspended particulate organic matter (POM) and

benthic coarse particulate organic matter (CPOM).

3. Catchment-scale urban stormwater drainage connection significantly increased the contribution of

labile organic matter to POM and DOC standing stocks. Greater POM lability was a product of

increased inputs of autochthonous organic matter in more heavily urbanised streams, although the

origin of labile DOC was less clear.

4. While reach-scale riparian vegetation was the most likely source of the terrestrially dominated

CPOM observed across most sites, increasing cover had no significant effect on the origin of POM or

DOC standing stocks. We conclude that catchment-scale stormwater drainage impacts overwhelm

the effects of reach-scale riparian vegetation on the sources and lability of POM and DOC in small

streams.

5. Our results suggest that the protection or restoration of riparian vegetation, in the absence of

modifications to catchment-scale stormwater drainage connection, is insufficient to mitigate the

effects of urbanisation on organic matter sources, lability and processing in these small streams.

Keywords: Urbanisation, riparian vegetation, organic matter, stable isotopes, Keeling plots

Introduction

Organic matter represents a vital heterotrophic resource

in streams (Cummins, 1974; Vannote et al., 1980), and its

availability and processing has implications for the

supply of energy and nutrients to aquatic food webs. In

its many forms, organic matter also provides habitat and

channel structure (Ehrman & Lamberti, 1992; Jones,

1997) and influences ecosystem metabolism (Mulholland

et al., 2001), heavy metal toxicity (Choi, Cech &

Lagunas-Solar, 1998), nutrient cycling (Meyer, Paul &

Taulbee, 2005; Roberts, Mulholland & Houser, 2007) and

Correspondence: Samantha J. Imberger, Department of Resource Management and Geography, The University of Melbourne, Richmond,

Victoria 3121, Australia. E-mail: [email protected]

168 © 2013 John Wiley & Sons Ltd

Freshwater Biology (2014) 59, 168–186 doi:10.1111/fwb.12256

ultimately in-stream biodiversity and trophic structure

(Wallace et al., 1999). As a consequence, alterations to

organic matter supply and processing can have wide-

ranging impacts on stream ecosystem structure and

function.

Catchment urbanisation is a global phenomenon typi-

cally characterised by an increase in impervious surface

cover and a decrease in catchment and riparian vegeta-

tion abundance (Walsh et al., 2005b, 2007). The wide-

spread effects of this landuse change on stream

ecosystem structure are severe and complex and have

collectively been termed ‘the urban stream syndrome’

(Meyer et al., 2005; Walsh et al., 2005b). They include an

increased frequency and magnitude of high flows, chan-

nel incision, increased loads of nutrients and contami-

nants and reduced biotic diversity (Paul & Meyer, 2001;

Meyer et al., 2005; Walsh et al., 2005b). Despite these

well-characterised impacts, the effects on ecosystem pro-

cesses such as organic matter dynamics are less

frequently studied (Wenger et al., 2009).

Urbanisation can increase the breakdown rate of some

types of benthic coarse particulate organic matter

(CPOM) (see Chadwick et al., 2006; Paul, Meyer &

Couch, 2006; Imberger, Walsh & Grace, 2008) and reduce

the standing stock of benthic CPOM and small wood

(Bernhardt & Palmer, 2007; Carroll & Jackson, 2008; Im-

berger, Thompson & Grace, 2011); however, the effects

on organic matter sources and their processing are rarely

studied and poorly understood (but see Harbott &

Grace, 2005; G€ucker et al., 2011; Newcomer et al., 2012).

Newcomer et al. (2012) used the stoichiometry and

isotopic composition of suspended particulate organic

matter (POM) to demonstrate a shift towards higher-

quality (i.e. more labile) organic matter sources, such as

grass clippings, periphyton and wastewater, in more

heavily urbanised streams. Similarly, G€ucker et al. (2011)

observed more labile POM in a heavily urbanised

stream reach due to increased algal biomass and effluent

POM inputs. However, both these studies were under-

taken in streams receiving wastewater discharges, and

the extent to which such shifts would be observed in

urban streams affected by diffuse stormwater run-off

remains unclear.

While many of the impacts of urbanisation are driven

by stormwater run-off generated across highly

impervious catchments and piped directly to streams (i.e.

catchment-scale stormwater drainage connection) (Walsh,

2004; Konrad & Booth, 2005; Walsh, Fletcher & Ladson,

2005a), riparian vegetation loss is also an important factor

(Miller & Boulton, 2005; Walsh et al., 2005b). Riparian

zones play an important role in regulating the flow of

water and materials between terrestrial and aquatic eco-

systems (Gregory et al., 1991; Naiman & Decamps, 1997;

Groffman et al., 2003). Research in non-urban catchments

has shown that riparian vegetation can influence stream

ecosystem structure and function via the provision of

shade, organic matter and invertebrates and the regula-

tion of terrestrially derived run-off containing sediments,

nutrients and other pollutants (Sweeney, 1992; Groffman

et al., 2003; Sweeney et al., 2004). However, the degree to

which these functions continue to be performed in urban-

ised catchments, where stormwater pipes short-circuit

natural flow paths and the spatial extent and connectivity

of riparian vegetation is restricted, remains unclear. A

review of the limited literature to date shows conflicting

results. For example, both Hession et al. (2002) and Roy

et al. (2005) observed small but significant increases in

benthic leaf litter stocks in urban streams with increasing

canopy cover, while Carroll & Jackson (2008) found no

significant effect.

Organic matter in aquatic ecosystems is a heterogenous

mixture of particles and dissolved compounds in differ-

ing degrees of decomposition with varying chemical

composition and lability (Wetzel, 2001). This material can

ultimately be traced to a range of aquatic (autochtho-

nous) and terrestrial (allochthonous) sources, including

plant litter, soil, algae, macrophytes and biofilm (Barth,

Veizer & Mayer, 1998; Kendall, Silva & Kelly, 2001;

Wetzel, 2001). Alterations to the relative abundance of

these organic matter sources have the potential to affect

habitat, water quality, secondary productivity and ulti-

mately the composition and structure of aquatic food

webs (e.g. England & Rosemond, 2004; Thompson &

Townsend, 2004; G€ucker et al., 2011). We note, however,

that varying organic matter quality and consumer prefer-

ences mean that the relative abundance of differing basal

resources per se does not necessarily reflect their assimila-

tion by microbes or their incorporation higher in the

aquatic food web (see Bunn, Davies & Winning, 2003;

Clapcott & Bunn, 2003; Leigh et al., 2010). In order to

understand fully the processing of organic matter in

aquatic ecosystems, including urban streams, it is thus

important to identify the origins of both organic matter

standing stocks and that which is incorporated higher in

the food web (Bunn & Boon, 1993; Hicks, 1997; Leberfin-

ger, Bohman & Herrmann, 2011).

Stable isotopes are a powerful tool for investigating

energy flow through aquatic food webs (Lajtha &

Michener, 1994; Michener & Schell, 1994). The stable

isotope ratio of basal resources is driven by both physi-

cal- and enzymatic-based discrimination, which results

in a distinct signature that can be traced through aquatic

© 2013 John Wiley & Sons Ltd, Freshwater Biology, 59, 168–186

Tracing carbon sources along an urbanisation gradient 169

ecosystems (Peterson & Fry, 1987; Lajtha & Michener,

1994). These approaches have a long history and have

been especially useful for determining the origin of

organic matter which is often visually amorphous (Peter-

son & Fry, 1987; Andgradi, 1994; Rossi et al., 2010). More

recently, the use of stable isotopes and Keeling plots has

allowed the examination of microbially respired carbon

sources in aquatic environments (Waichman, 1996;

McCallister, Guillemette & del Giorgio, 2006; Karlsson,

Jansson & Jonsson, 2007).

Effective management of streams in urban and rapidly

urbanising environments relies on a clear understanding

of the mechanisms driving degradation. This research is

the first to investigate the simultaneous effects of catch-

ment-scale urban stormwater drainage connection and

reach-scale riparian vegetation cover on organic matter

sources. Previous work on the effects of urbanisation on

streams led to two hypotheses: (i) increasing drainage

connection would result in an increase in autochthonous

organic matter due to higher stream temperatures and

nutrient concentrations and (ii) increases in riparian

canopy cover would increase the contribution of

allochthonous-derived organic matter as a function of

greater terrestrial litter inputs and reduced light avail-

ability for in-stream production. If both hypotheses were

shown to be valid, then we were additionally interested

in the relative effects of drainage connection and ripar-

ian canopy cover on shifting organic matter sources.

Methods

Site selection and characterisation

We selected four small, perennial streams (first–second

order) on the fringe of metropolitan Melbourne, Austra-

lia (Table 1). The streams were located along a gradient

of catchment urbanisation, and we used effective imper-

viousness [i.e. the proportion of a catchment covered by

impervious surfaces connected directly to streams via

stormwater pipes; see Walsh et al. (2005a)] as our

measure of catchment-scale stormwater drainage connec-

tion. Within each stream, we selected paired reaches (i.e.

two reaches) ranging from 50 to 100 m in length, with a

contrasting extent of riparian cover. However, one

stream (Lyrebird Creek) was located within the Dande-

nong Ranges National Park, and thus, a paired ‘open’

canopied reach could not be obtained there.

We assessed canopy cover at all sites via two

parameters: daily photosynthetic active radiation (PAR)

flux and percentage canopy cover. We measured daily

PAR flux (mol m�2 day�1; terrestrial cosine recorder,

Dataflow Systems, Christchurch, New Zealand) at paired

reaches simultaneously, but equipment limitations

meant that individual streams were measured on differ-

ent days (see Table 1). These data were used to create a

nomenclature for classifying paired reaches as either rel-

atively ‘open’ or ‘closed’, since light transmission is

more strictly a measure of shading and not directly

equivalent to canopy cover. We used hemispherical digi-

tal photography to calculate percentage canopy cover at

five evenly spaced locations along each reach. Photo-

graphs were then processed using Gap Light Analyser

(version 2, Simon Fraser University, Burnaby, British

Columbia, and Cary Institute of Ecosystem Studies, Mill-

brook, New York, U.S.A.) to generate a continuous

predictor variable.

Major catchment landuses were either forest or urban.

Riparian corridors were up to 10 m in width and domi-

nated by evergreen Eucalyptus, Acacia and Melaleuca spp.

Additional site characteristics and selection criteria can

Table 1 Site characteristics, including location and key environmental variables. Values indicate mean � SE (where calculated).

Stream

Canopy

status

Below canopy

daily PAR flux

(mol m�2 day�1)

Location

TI*

(%)

EI†

(%)

Canopy

Cover (%)

Depth‡

(cm)

Width‡

(cm)

Velocity‡

(cm s�1)Latitude Longitude

Lyrebird§ ‘Closed’ 0.80 37° 49′ 48.0″ S 145° 23′ 49.2″ E 0.0 0.0 77 (3) 4.0 (0.5) 120 (10) 7.4

Monbulk ‘Open’ 26.40 37° 55′ 0.90′′ S 145° 21′ 25.9′′ E 4.1 0.8 55.0 (1.3) 34 (5) 220 (20) 2.3

Monbulk§ ‘Closed’ 10.80 37° 54′ 59.3″ S 145° 21′ 26.3′′ E 4.1 0.8 65.6 (0.5) 25 (3) 182 (19) 7.0

Mullum Mullum ‘Open’ 53.32 37° 48′ 39.1″ S 145° 13′ 39.8″ E 36.3 22.8 38 (2) 22 (2) 1780 (20) 0.6

Mullum Mullum§ ‘Closed’ 21.86 37° 48′ 24.9″ S 145° 12′ 27.3″ E 36.4 22.8 44.5 (0.7) 14 (4) 290 (40) 3.0

Blind ‘Open’ 28.47 37° 52′ 29.9″ S 145° 13′ 04.8″ E 36.1 30.0 46 (5) 18 (3) 280 (30) 1.1

Blind§ ‘Closed’ 24.90 37° 52′ 40.9″ S 145° 12′ 57.7″ E 36.0 29.7 42.9 (1.9) 10.8 (1.9) 190 (30) 3.1

*TI, total imperviousness: the percentage of a catchment covered by impervious surfaces.†EI, effective imperviousness: the percentage of a catchment covered by impervious surfaces connected directly to streams via stormwater

pipes.‡Indicates previously collected data, see Imberger et al. (2011).§Indicates the subset of sites used in organic matter respiration experiments.


170 S. J. Imberger et al.

be seen in Imberger, Thompson & Grace (2010). All

stream sampling was performed under baseflow condi-

tions during summer 2009/10 after 13 continuous years

of below-average annual rainfall.

Stable isotope analysis of organic matter sources and

standing stocks

Stable isotope and elemental analysis of dissolved and

particulate organic matter was used to examine the ori-

gin of otherwise indeterminate DOC, suspended POM

and benthic CPOM. At each study reach, we sampled all

visually abundant terrestrial organic matter sources

along a 100 m length of stream, including soil, grasses,

shrubs and trees. Material was collected from a number

of individual plants across varying size and age classes

and pooled by species. Sampling also included the

collection of distinct sources of autochthonous organic

matter, including macrophytes and filamentous algae.

We also collected epilithic biofilms, although given the

diverse range of primary organic matter sources within

biofilms and their potential to affect isotopic signatures

(Hladyz et al., 2011), we classified biofilms as a mixture

for the purpose of tracing primary organic matter

sources. Given minimal variation between species, ter-

restrial vegetation was later grouped as either C3 or C4

litter, thus leaving five key organic matter sources on

which to base our analysis (i.e. algae, macrophytes, soil,

C3 litter and C4 litter).

Detrital DOC and POM standing stocks were sampled

in duplicate and preserved following the methods of

Oakes et al. (2010) and Leigh et al. (2010), respectively.

Benthic CPOM was sampled in triplicate using a 25-cm-

diameter pipe corer to a depth of 10 cm. However,

wood >1 cm in diameter was discarded due to its

obvious origin and potential to overwhelm quantita-

tively the remaining material. All samples were trans-

ported on ice to the laboratory and processed following

the methods of Leigh et al. (2010). In the case of biofilms,

however, slurries (c. 120 mL) were centrifuged at 1445 g

for 5 min and the supernatant was discarded. Biofilm

residues and sieved soil samples were then oven-dried

at 60 °C to constant mass and ground using a mortar

and pestle.

All solid samples were analysed for elemental organic

C (%), organic N (%), d13C (&) and d15N (&) using a

continuous flow isotope ratio mass spectrometer (Micro-

mass Isoprime EuroVector EA300, Manchester, U.K.).

DOC samples were analysed for elemental organic C

(%) and d13C (&) using an O.I. Analytical TOC analyser

(Model 1010; O.I Analytical, College Station, TX, U.S.A.)

interfaced to a Finnigan continuous flow isotope ratio

mass spectrometer (DELTAplus XL; Thermo Fisher Scien-

tific, Waltham, MA, U.S.A.). Ratios of 13C to 12C and 15N

to 14N were expressed as the relative difference between

the sample and a conventional standard in per mil (&):

dXð&Þ ¼ Rsample

Rstandard� 1

� �� 1000

where X = 13C, or 15N and R = 13C/12C, or 15N/14N.

Stable isotope analysis of respired carbon

We used the concentration and stable isotope signature

of dissolved inorganic carbon (DIC) to identify the

source of respired carbon across varying pools. We com-

pleted short-term incubation experiments and monitored

the changes in the concentration and d13C of DIC

through time, based on the method of Karlsson et al.

(2007). We performed experiments based on three

carbon pools selected from a subset of four ‘closed’ can-

opy sites (see Table 1): (i) biofilm (i.e. colonised rock

surfaces), (ii) benthic coarse particulate organic carbon

(i.e. sieved benthic CPOM) and (iii) sediment (i.e. an

intact sediment core).

At each site, we collected two intact sediment cores

(6.7 cm diameter) to a depth of 10 cm, four rocks with

intact biofilm (c. 6 cm diameter) and two benthic CPOM

samples. In the laboratory, we added approximately

400 g (dry mass) of CPOM into each of two clean cores,

and two rocks (each) to another two clean cores, while

intact sediment cores were left undisturbed. Cores were

filled up with unfiltered site water and capped with an

airtight perspex lid, ensuring that all gas bubbles were

eliminated. Cores were then placed in an incubation tank,

where they were maintained at 15 °C (average summer

stream temperature) and continuously stirred for 48 h.

All cores were sampled in duplicate after 0, 24 and

48 h by filtering 40 mL of water through a pre-ashed

GF/F membrane. Samples were fixed and refrigerated

following Kaplan (1994) and CPSIL (2010). Cores were

then immediately topped up using refrigerated unfil-

tered site water, and the lids were resealed. All samples

were analysed for concentration and d13C of DIC by the

Colorado Plateau Stable Isotope Laboratory (Northern

Arizona University, North America), and ratios of 13C to12C were expressed as previously described.

We applied the Keeling plot method (see Keeling,

1958, 1961) to data from individual replicates of each

carbon pool. Given steady background DIC concentra-

tions and d13C signatures, there is a linear relationship

between the inverse DIC concentration and the DIC d13C



through time, where the intercept represents the d13Csignature of the respired carbon (Keeling, 1958; Pataki

et al., 2003; Karlsson et al., 2007). Following Pataki et al.

(2003), we used model 2 reduced major axis (RMA)

regression to calculate y intercept values and model 1

ordinary least-squares regression to estimate y intercept

standard errors (SE). This statistical approach allows the

incorporation of error in both DIC d13C and DIC concen-

trations when calculating regression intercepts and pro-

vides more accurate estimates of the intercept when

correlation coefficients are low (see Sokal & Rohlf, 1995;

Quinn & Keough, 2002; Pataki et al., 2003). The regres-

sion intercept was then adjusted for a �0.5& fraction-

ation during respiration (see Hullar et al., 1996; Karlsson

et al., 2007), and the final value was taken to indicate the

d13C signature of respired carbon. Note that DIC data

were considered unreliable and discarded unless all of

the following criteria were met: (i) the change in DIC

concentration over 48 hrs was > 0.3 ppm, (ii) the regres-

sion intercept SE was < 0.5 times the intercept, and (iii)

the regression r2 was > 0.5.

Assessing tracer viability and the varying contributions of

organic matter sources

To investigate the utility of individual tracers, we

assessed differences in d13C, d15N and elemental C : N

ratios of individual organic matter sources (e.g. algae,

macrophytes, C3 litter) and between a priori pooled

autochthonous and allochthonous sources using single-

factor analysis of variance (ANOVA) with source as a

fixed factor. Similarly, comparisons between the d13Csignature of organic matter respired across sites and

carbon pools were also investigated using single-factor

ANOVA. Where significant differences were observed,

post hoc pairwise comparisons were made using Tukey’s

honestly significant difference test (Tukey’s HSD).

To further investigate the sources of organic matter

contributing to detrital POM and benthic CPOM across

sites, we used a Bayesian mixing model in R based on

d13C and d15N data (stable isotope analysis in R, v4.1;

Parnell et al., 2008; R Development Core Team, 2009)

and qualitative visual analysis of tracer biplots. ‘Major’

organic matter sources contributing to detrital pools

were defined as those with a mean contribution >20% or

an upper 95% highest density region (HDR) of >35%.

Major sources were then further refined based on C : N,

where allochthonous contributions were excluded

where C : N < site-specific autochthonous C : N l+2rand autochthonous sources were excluded where

C : N > site-specific autochthonous C : N l+2r (i.e.

Lyrebird Creek = 8.09, Monbulk Creek = 7.57, Mullum

Mullum Creek = 14.75, Blind Creek = 9.81). However,

given the average C : N value of C4 litter across sites

was quite low (13.2 � 1.6), the C : N rule was not

applied in cases where the mixing model identified C4

litter as a ‘major’ source. When investigating the sources

of organic matter contributing to DOC and microbial

respiration, however, we relied solely on qualitative

comparisons with source d13C values (i.e. Table 2).

Ordinary least-squares (OLS) regression was used to

assess the changes in the composition of both detrital

organic matter (i.e. DOC, POM and benthic CPOM) and

carbon respired across pools (i.e. biofilm, benthic CPOM

and sediment), as a function of catchment-scale urbani-

sation (EI) and reach-scale riparian vegetation cover.

Relationships between the composition of organic matter

sources (e.g. algae, soil, C3 litter) and both EI and can-

opy cover were also assessed using OLS regression. In

all cases, variables were transformed where necessary to

meet assumptions of normality and homoscedascity, and

analyses were performed in R (R Development Core

Team, 2009) with a = 0.05.

Results

Using organic matter d13C, d15N and C : N values as

organic matter tracers

While we found a moderate separation between individ-

ual organic matter sources with respect to their C : N

ratio and both d13C and d15N signatures, we were unable

to detect statistically significant differences between

Table 2 Summary of average detrital organic matter source C : N,

d13C and d15N signatures across sites

Source

Composition

C : N d13C (&) d15N (&)

Algae 7.2a (0.3) �35.2a (1.6) 6.6b (0.8)

Macrophyte 10.2a,b (0.8) �30.9b,c (1.4) 7.1b (0.8)

Biofilm 10.5a (0.9) �28.8b,c (0.6) 5.1b (0.3)

C4 litter 13.2a,b,c (1.6) �14.2d (0.3) 7.13a,b (0.12)

Soil 20.6b,c (0.8) �28.00c (0.12) 1.3a (0.4)

C3 litter 43c (5) �29.8b (0.4) 2.3a (0.4)

Average

Autochthonous

8.7A (1.5) �33A (2) 6.8A (0.3)

Average

Allochthonous

26A (9) �24A (5) 3.5A (1.8)

Values represent mean � (SE, n = 2–63). Superscript letters denote

differences between individual sources (lower case) and pooled

sources (upper case), where contrasting letters indicate statistically

significant differences (Tukey’s HSD test).



grouped autochthonous and allochthonous sources (see

Table 2). The C : N ratio of individual sources ranged

from 7.2 � 0.3 to 43 � 5 and was similar across sites,

thus allowing moderate separation of individual sources

(Table 2). In the case of d13C signatures, we observed

comparably low intersite (i.e. intrasource) variability and

significant differences between several individual

organic matter sources with values ranging from

�14.2 � 0.3& to �35.2 � 1.6& (Table 2). The d15N sig-

nature of individual sources showed a largely similar

source separation profile to that of C : N ratio and also

displayed low intrasource variability. There were consid-

erable differences in the average C : N ratio and, to a

lesser extent, d13C and d15N of pooled autochthonous

and allochthonous sources across sites (except at Mul-

lum Mullum Creek; Figs 1–4). Overall, autochthonous

sources were generally characterised by lower d13C and

C : N ratios, but higher d15N signatures, than allochtho-

nous sources.

Detrital organic matter standing stock C : N ratios,

d13C and d15N values displayed low to moderate intra-

site variability, which facilitated accurate comparisons

with individual and pooled organic matter sources

(Figs 1–4). POM C : N values were quite low and ran-

ged from 8.56 to 13.52 across sites, while benthic CPOM

values ranged from 13.86 to 72.43. Overall d13C values

were less variable, with DOC signatures ranging from

�27.85 to �29.18& across sites and POM and benthic

CPOM signatures ranging from �27.37 to �32.19& and

�27.59 to �29.37&, respectively. By comparison, POM

and benthic CPOM d15N values ranged from 2.10 to

5.75& and �0.01 to 5.57& across sites, respectively.

These differences in elemental ratios and stable isotope

signatures across sites and carbon pools allowed an

effective investigation of shifting organic matter sources

as a function of varying catchment-scale stormwater

drainage connection and reach-scale riparian vegetation

cover.

Dominant organic matter sources across sites

Using stable isotope mixing models in conjunction

with C : N data (and confirmed via visual analysis of

tracer biplots), we showed that the dominant organic

matter sources contributing to detrital standing stocks

varied across sites and organic matter pools (see

Figs 1–4; Table 3). In the case of DOC, d13C values at

Blind, Lyrebird and, to a lesser extent, Monbulk Creek

were, regardless of canopy cover, most similar to allo-

chthonous organic matter sources such as soil. How-

ever, interpretation of isotopic signatures at Mullum

Mullum Creek was more difficult due to overlapping

sources on the d13C axis (see Fig. 3). With only one

tracer and greater than two organic matter sources,

however, it is difficult to draw conclusions on the

major sources contributing to DOC. Stable isotope mix-

ing models and C : N values showed that benthic

CPOM standing stocks were dominated by litter and

soil sources at all sites, except Mullum Mullum Creek

‘open’, where the isotopic composition and C : N ratio

were more akin to those of algae and macrophytes

(Figs 1–4; Table 3).

When POM and benthic CPOM d13C and d15N values

were compared, it appeared that the contribution of

autochthonous sources (including algae and macro-

phytes) to POM was considerably greater than that to

benthic CPOM. This was confirmed by the mixing

model that showed an average autochthonous contribu-

tion to POM and benthic CPOM of 39% and 33%,

respectively (Table 3). This trend, however, was most

clearly illustrated using elemental ratios, where the aver-

age POM C : N of 10.9 was significantly lower than that

of benthic CPOM (48.6; F1,33 = 74.60, P < 0.001).

The effects of catchment-scale urban stormwater drainage

connection

Our results showed evidence of a significant impact of

catchment-scale stormwater drainage connection on the

origin of carbon dominating organic matter standing

stocks in small streams. We observed a small but

significant increase in the d13C signature of DOC with

increasing stormwater drainage connection (r2 = 0.950,

P < 0.001; Fig. 5a). We also found a significant negative

relationship between effective imperviousness (EI) and

both the d13C signature and C : N ratio of POM

(r2 = 0.653, P = 0.028 and r2 = 0.876, P = 0.002, respec-

tively; Fig. 5c,d). This correlation between EI and POM

was also reflected in the output of the mixing model

when combined with C : N data. Despite this, we

observed no significant correlation between EI and

detrital benthic CPOM d13C or d15N signatures or

C : N ratios. Similarly, there was no significant effect

of stormwater drainage connection on the d13C signa-

ture of carbon respired within biofilm, benthic CPOM

or sediment. We found no significant relationship

between EI and the d13C, 15N or C : N value of

organic matter sources (including algae, macrophytes

or soil), thus suggesting that the observed correlations

between the C : N/d13C/d15N of detrital standing

stocks and EI were driven by shifting sources (not

shifting source signatures). While we did observe a



significant positive relationship between EI and the

d13C signature of C3 litter (r2 = 0.778, P = 0.020), the

regression coefficient was low (0.053 � 0.014) and unli-

kely to affect significantly the interpretation of detrital

and respired carbon correlations.

The effects of reach-scale riparian vegetation cover

We found modest evidence for the effects of reach-

scale riparian vegetation cover on the origin of detrital

standing stocks. There was a small but significant

δ15 N

(‰)

−2−1

01

23

45

67

Algae

BOM (closed)

Litter

POM (closed)

Soil

Biofilm

DOC (open)DOC (closed)POM and BOMSource

δ13C (‰)

C :

N

−40 −38 −36 −34 −32 −30 −28 −26

010

2030

4050

6070

Algae

BOM (closed)

Litter

POM (closed)

Soil

Biofilm

δ15N (‰)−2 −1 0 1 2 3 4 5 6 7

Algae

BOM (closed)

Litter

POM (closed)

Soil

Biofilm

Fig. 1 Tracer biplots showing mean (�SE, n = 2–13) d13C, d15N and C : N of suspended POM (POM), benthic CPOM (BOM) and potential

contributing sources at Lyrebird Creek. Vertical lines indicate the mean (�SE, n = 2) d13C signature of detrital DOC.



decline in the d13C signature of DOC with increasing

reach-scale riparian vegetation cover (r2 = 0.767,

P = 0.010; Fig. 5b). However, we detected no significant

effect of riparian cover on the d13C signature, d15N sig-

nature or C : N ratio of POM or benthic CPOM stand-

ing stocks. This result was also reflected in the output

of the mixing model when combined with the C : N

data. The validity of this interpretation was confirmed

by tests showing no significant correlations between

riparian canopy cover and the C : N/d13C/d15N

02

46

810

12

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

SoilBOM (open)

POM (open)

BiofilmDOC (open)DOC (closed)POM and BOMSource

C :

N

−40 −38 −36 −34 −32 −30 −28 −26

010

2030

4050

60

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

Soil

BOM (open)

POM (open)Biofilm

0 2 4 6 8 10 12

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

Soil

BOM (open)

POM (open)

Biofilm

δ15 N

(‰)

δ13C (‰) δ15N (‰)


contributing sources at open and closed reaches of Monbulk Creek. Vertical lines indicate the mean (�SE, n = 2) d13C signature of detrital

DOC.



signature of any dominant organic matter sources

including algae, macrophytes, C3 litter or soil.

Microbially respired carbon sources

Keeling plots provided a novel approach to investigate

the effects of site-specific environmental factors and

carbon pools on the dominant sources of microbially

respired carbon. Respired d13C values, however, were

often highly enriched in 13C relative to most of the

organic matter sources sampled and may indicate that

other biogeochemical processes, such as microbial frac-

tionation, carbonate dissolution or biogenic methane

production, were occurring during incubations. Keeling

−10

12

34

56

7

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

Soil

BOM (open)

POM (open)

Biofilm


C :

N

−32 −31 −30 −29 −28 −27 −26 −25

010

2030

4050

6070

Algae

BOM (closed)Litter

MacrophytePOM (closed)

Soil

BOM (open)

POM (open)Biofilm

−1 0 1 2 3 4 5 6 7

Algae

BOM (closed)Litter


Soil

BOM (open)

POM (open)

Biofilm

δ15 N

(‰)

δ13C (‰) δ15N (‰)


contributing sources at open and closed reaches of Mullum Mullum Creek. Vertical lines indicate the mean (�SE, n = 2) d13C signature of

detrital DOC.



plots that met the criteria for inclusion displayed strong

linear fits with r2 values ranging between 0.52 and 1.00

and an average of 0.81 � 0.04 (n = 18). Intercept SE

values ranged between 0.6 and 13&, with an average of

4.4 � 0.8& (n = 18); however, intrasite variation was

lower with SE values (based on replicate incubations)

ranging between 0.08 and 2&, with an average of 1.04&(see Table 4). We note, however, that small changes in

DIC concentration through time, and low regression r2

values, meant that several Keeling plots failed to meet

23

45

67

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

Soil

BOM (open)

POM (open)

Biofilm


C :

N

−38 −36 −34 −32 −30 −28 −26

020

4060

80

Algae

BOM (closed)

Litter

Macrophyte

POM (closed)

Soil

BOM (open)

POM (open)

Biofilm

2 3 4 5 6 7

Algae

BOM (closed)

Litter


Soil

BOM (open)

POM (open)

Biofilm

δ15 N

(‰)

δ13C (‰) δ15N (‰)


contributing sources at open and closed reaches of Blind Creek. Vertical lines indicate the mean (�SE, n = 2) d13C signature of detrital

DOC.



the criteria for inclusion and this prohibited statistically

robust comparisons of respired carbon across our land-

use gradient.

While replication was low, respired d13C values ran-

ged from �38 to �7.2& and varied significantly between

carbon pools when sites were grouped (F2,15 = 6.97,

P = 0.007). Biofilm signatures were significantly more

d13C depleted than benthic CPOM (P = 0.005), but not

significantly different to sediment (see Table 4). When

carbon pools were separated, we also observed a signifi-

cant difference in the respired d13C signature between

several sites. Overall, the d13C signature of biofilm

respired carbon was not significantly different between

Monbulk and Mullum Mullum Creeks, and while insuf-

ficient replicates prevented a statistical assessment, sig-

natures at Blind Creek did appear considerably more

depleted in 13C than at other sites (Table 4).

Discussion

Using organic matter d13C, d15N and C : N values as

organic matter tracers

When using d13C, d15N or C : N values alone to trace

organic matter sources, we were unable to separate sta-

tistically several sources (e.g. algal, macrophyte and C3

litter). When C : N, d13C and d15N values were consid-

ered in concert, however, clear differences between

algal, macrophyte, C4 litter, soil and C3 litter sources

were evident (Table 2). While macrophyte signatures

prevented significant differences between grouped

autochthonous and allochthonous sources based on indi-

vidual tracers, minimal macrophyte abundance in our

streams meant that this had little effect on our ecologi-

cal analysis. C : N ratios and d15N values conveyed lar-

gely similar information in our study, although the

smaller range observed between sources (d15N, 5.86&;

C : N, 36.38) meant that d15N was the least sensitive for

identifying shifts in the origin of organic matter. The

application of Bayesian mixing models to d13C and d15Ndata tended to produce wide-ranging solutions (i.e.

broad 95% HDRs); however, when the results were

combined with C : N ratios (and confirmed via visual

analysis of tracer biplots), we identified several patterns

of interest.

What carbon sources are contributing to detrital DOC,

POM and benthic CPOM?

The range in DOC d13C values was quite small, and all

values fell within the vicinity of soil sources. However,Table

3Percentagesourcecontributionto

POM

andben

thic

CPOM

based

onSIA

Rmixingmodel.Values

representmean(and95%

highestden

sity

region).

Site

Source(%

)

Lyrebird

Monbulk

Mullum

Mullum

Blind

‘Closed’

‘Closed’

‘Open

’‘Closed’

‘Open

’‘Closed’

‘Open

’

POM Algae

23.41(3.35–40

.59)

13.23(0.00–27

.96)

13.46(0.00–29

.79)

22.15*(0.00–42

.25)

21.97*(0.00–42

.8)

23.47*(0.24–44

.2)

19.24*(0.00–39

.25)

Macrophyte

—13

.83(0–29.67

)17

.33(0.13–33

.09)

25.42*(0.34–47

.3)

29.25*(1.59–53

.96)

27.2*(2.9–47.35

)21.22*(0.62–38

.51)

C4Litter

22(4.86–36

.63)

20.03(2.63–4.79)

14.12(1.42–28

.76)

9.64*(0.00–29

.02)

13.36*

(0.00–34

.62)

9.30

*(0.00–27

.36)

6.19*(0.00–17

.88)

C3Litter

27.84(1.97–49

.08)

24.35(0.48–45

.23)

26.06(1.52–47

.9)

23.09*

(0.00–44

.65)

20.73*

(0.00–40

.9)

21.5*(0.33–40

.77)

23.92*

(0.36–44

.49)

Soil

26.73(0.73–49

.69)

28.53(5.26–49

.97)

29.01(6.7–50.55

)19

.67*

(0.00–39

.89)

14.66*

(0.00–33

.8)

18.5*(0.07–36

.06)

29.41*

(4.00–53

.38)

Ben

thic

CPOM

Algae

4.23

(0.00–15

.94)

15.43(0.00–31

.75)

18.43*

(0.1–34.5)

17.73(0.00–35

.73)

23.1

(0.06–44

.16)

14.87(0.00–32

.11)

13.87(0–30.75

)

Macrophyte

—15

.89(0.00–35

.28)

17.93*

(0.00–36

.68)

18.15(0.00–37

.4)

36.76(6.81–68

.09)

18.95(0.3–3

6.53)

16.84(0.00–33

.79)

C4Litter

5.41

(0.00–17

.43)

13.93(0.49–26

.17)

14.27*

(0.99–27

.08)

10.12(1.22–17

.91)

14.61(0.1–33.79)

10.79(0.00–20

.63)

18.42(5.77–29

.76)

C3Litter

49.97(15.55–85.58)

24.18(0.28–45

.77)

23.42*(0.41–44

.7)

23.98(0.22–44

.62)

15.11(0.00–34

.76)

26.41(0.18–51

.54)

23.78(0.34–45

.63)

Soil

40.38(2.47–71

.45)

30.53(0.00–66

.12)

25.93*(0.5–51.56

)29.99(0.05–60

.6)

10.4

(0.00–29

.72)

28.96(1.48–55

.21)

27.06(0.04–54

.55)

Values

inboldface

indicate‘m

ajor’

contributingsources

based

onSIA

Routputan

dC

:N

values.

–Indicates

sourcenotfoundat

site.

*SIA

Rratestheproblemswiththesedataas

‘sev

ere’.



based on qualitative visual analysis, we were unable to

exclude definitively a contribution from other major

sources. While there is limited published riverine data

on which to compare DOC d13C signatures, our results

were lighter than those observed in both the Murray

River, Australia (Cartwright, 2010), and North Boulder

Creek, U.S.A. (Hood, Williams & McKnight, 2005), but

similar to results from Palmer et al. (2001) where the

DOC pool (�27.7&) was dominated by allochthonous

organic matter.

The sources of organic matter dominating POM varied

greatly across catchments. POM d13C values were more

variable across sites than either DOC or detrital benthic

CPOM, and the mixing model showed varying contribu-

tions of autochthonous (i.e. algae or macrophytes) and

allochthonous organic matter sources (i.e. C3 litter and

soil) across sites. However, POM C : N values were

quite low and in many cases approaching the Redfield

ratio of 6.6:1, thus indicating a considerable contribution

of algal- or macrophyte-derived carbon across sites

(especially at Mullum Mullum and Blind Creek). Many

studies suggest that C : N values <10–14 indicate

autochthonous material, while values >20–50 are consis-

tent with allochthonous-derived carbon (Kendall et al.,

2001; Wetzel, 2001; Leigh et al., 2010; Hunt et al., 2012).

While C4 litter C : N ratios were lower than expected,

when the results of the mixing model were combined

with C : N data, there appeared to be little contribution

of C4 material to POM.

Detrital benthic CPOM was dominated by allochtho-

nous organic matter sources at all but one site (Mullum

Mullum Creek ‘open’). Detrital benthic CPOM d13C sig-

natures were similar to those of soil and litter and con-

sistent with values and patterns observed in the

literature (Bunn et al., 2003; Fellows et al., 2006; Giling,

Reich & Thompson, 2009). Detrital benthic CPOM C : N

ratios were also similar to those of soil and litter sources,

although higher values at both Blind Creek ‘open’ and

Lyrebird Creek probably indicated a large contribution

of Eucalyptus bark, which displayed a very high C : N

value (i.e. 120 � 12, n = 6). In the case of Mullum Mul-

lum Creek ‘open’, however, significantly lower C : N

ratios, in association with the output of the mixing

model, indicated that CPOM was predominately autoch-

thonous in origin.

Our results suggest that while suspended POM repre-

sents a small fraction of total organic matter, it is con-

siderably more labile than the benthic CPOM detrital

pool during summer base flow. This may in turn indi-

cate that alterations to POM as a function of changing

landuse could have particularly significant impacts on

organic matter processing and the structure and produc-

tivity of aquatic food webs (for example, see G€ucker

et al., 2011).

0 15 30 EI (%)

(a) (b)

(c) (d)

Fig. 5 Water column DOC d13C (a-b), POM C : N ratio (c) and POM d13C (d) as a function of either effective imperviousness (EI) or riparian

canopy cover across sites. Point values represent mean � SE (n = 2) and circle diameters represent EI.



While stoichiometry alone does not always determine

incorporation higher in the food web (Bunn, Davies &

Mosisch, 1999; Clapcott & Bunn, 2003), alterations to

C : N ratios and microbial processing still have the

potential to impact ecosystem respiration, nutrient pro-

cessing and water quality.

Does catchment-scale urban stormwater drainage

connection alter the dominant sources of detrital carbon?

We found a significant effect of stormwater drainage

connection on the lability and origin of DOC and POM

in our streams. POM d13C and C : N varied with EI,

indicating greater autochthonous and reduced allochtho-

nous contributions to POM in more heavily urbanised

streams. Because macrophytes were only abundant at

one site, the increased autochthonous contribution in

more heavily urbanised streams was probably algal dri-

ven. The increase in labile, algal-derived POM could

have significant impacts on ecosystem respiration and

both food-web structure and productivity (G€ucker et al.,

2011). G€ucker et al. (2011) attributed a decrease in both

POM d13C and C : N ratios in a more heavily urbanised

stream reach (cf. agricultural headwaters) to shifting

organic matter sources including reduced allochthonous

inputs and increased algal biomass. In contrast to our

study, however, G€ucker et al.’s (2011) sites were affected

by sewage effluent. The correlation with EI suggests that

source shifts were being driven by diffuse impacts

associated with catchment-scale stormwater drainage

connection. Increases in algal biomass can be driven by

high nutrient concentrations, temperature and light

availability and by reduced grazing (Taylor et al., 2004;

Miller & Boulton, 2005; Bernhardt & Palmer, 2007), all of

which are associated with urban stormwater run-off

(Stepenuck, Crunkilton & Wang, 2002; Walsh et al.,

2005b). It is thus likely that changes to nutrients, temper-

ature and grazers, driven by diffuse stormwater run-off,

were the most significant factors contributing to shifts in

POM sources.

We observed a small but significant increase in DOC

d13C with increasing EI, although all values were within

the overlapping range of macrophytes and C3 litter

sources. Despite this uncertainty, it seems likely that

alterations to DOC d13C values in our streams were

associated with the shifting lability of DOC. Microbial

degradation of labile DOC leached from soil preferen-

tially removes small molecules and leaves the residual

pool enriched with DOC of high molecular weight and

lower d13C (Kalbitz et al., 2003). Previous research in our

streams has shown an increased percentage of bio-

available DOC (BDOC) and decreased DOC molecular

weight in more heavily urbanised environments (S.

Imberger, unpubl. data). Using these data, we also

observed a significant positive correlation between DOC

d13C and percentage BDOC and a significant negative

relationship between DOC d13C and DOC molecular

weight (see Fig. 6a,b). Our data may thus be a product

of greater concentrations of labile DOC in more

urbanised streams. This labile DOC could potentially

originate from algal exudates (see Petrone et al., 2011) or

C4 litter (e.g. lawn clippings) delivered via the

stormwater drainage network at high flow (Newcomer

et al., 2012). The interactions between landuse and

Table 4 Summary of Keeling plot regression statistics across sites and differing carbon pools.

Carbon Pool Replicate

Site

Lyrebird Monbulk Mullum Mullum Blind

d13C intercept

(&) r2d13C intercept



(&) r2

Biofilm 1 – – �23.0 (0.6) 1.00 �25 (9) 0.71 – –

2 – – �25.2 (1.9) 0.99 �28 (10) 0.65 �38 (13) 0.74

Mean – – �24.1 – �26.5 – �38 –

SE – – 1.1 – 1.0 – – –Benthic CPOM 1 �7.2 (0.7) 0.99 �18 (4) 0.78 – – �21 (5) 0.78

2 �11 (2) 0.55 �17 (6) 0.52 �17 (4) 0.79 �21 (3) 0.89

Mean �8.9 – �17.1 – �17 – �20.71 –

SE 1.7 – 0.5 – – – 0.08 –Sediment 1 – – �15 (4) 0.65 �16 (4) 0.74 �25 (4) 0.94

2 – – �19 (3) 0.87 �15.6 (1.5) 0.96 �29 (3) 0.98

Mean – – �16.9 – �15.70 – �27 –

SE – – 1.8 – 0.09 – 2 –

Individual replicate intercept values represent model 2 regression intercepts � (model 1 regression intercept SE), while summary values rep-

resent the mean of model 2 regression intercept �(SE of replicate model 2 regression intercepts, n = 2). Note: – denotes no available data.



dissolved organic carbon sources, quality and process-

ing, while critically important for ecosystem processes,

are infrequently documented and remain poorly under-

stood (but see Harbott & Grace, 2005; Petrone et al.,

2011).

Contrary to our hypothesis, we detected no significant

effect of urban stormwater drainage connection on detri-

tal benthic CPOM sources. This result may indicate that

benthic CPOM sources are resilient to catchment-scale

urbanisation, although it may be that the volume of

coarse allochthonous organic matter inputs simply over-

whelms any small increases in the contribution of either

algae (planktonic or benthic) or macrophytes.

Does reach-scale riparian vegetation cover alter the

dominant sources of detrital carbon?

In contrast to the limited published data, we were

unable to detect any significant effect of reach-scale

riparian canopy cover on the sources of POM or detrital

benthic CPOM in these streams (Roy et al., 2005; Rossi

et al., 2010). The extent of riparian vegetation cover had

no discernible effect on the composition of benthic

CPOM and was unable to mitigate the effects of urbani-

sation on increased autochthonous contributions to

POM. As previously discussed, it may be that benthic

CPOM is dominated by allochthonous material as a

function of input volumes, irrespective of small shifts in

canopy cover. In the absence of all riparian vegetation,

we would not expect this to be true, but canopy cover in

excess of 37.9% (i.e. the minimum value in this study)

may be too great to demonstrate an effect on benthic

CPOM standing stocks. In the case of POM, our findings

suggest that any effects of riparian canopy cover on

stream temperature or light were insufficient to

constrain the effects of urban stormwater drainage con-

nection on the contribution of algae to POM.

We found a small decrease in DOC d13C with increas-

ing canopy cover but, as the observed DOC d13C values

were within the range of macrophyte and litter sources,

these data were difficult to interpret. Previous research

in these streams has shown minor increases in long-term

CPOM storage in reaches with more ‘closed’ canopies

(Imberger et al., 2011). We consequently hypothesised a

small relative increase in the contribution of allochtho-

nous DOC sources, and thus an increase in DOC d13C,with increasing canopy cover. Our data were, however,

inconsistent with this hypothesis (Fig. 5b). Urban

streams in the current study tended to be relatively

more open (see circle widths in Fig. 5b), and it may be

that the strong positive relationship observed between

EI and DOC d13C generated a spurious weak negative

correlation between canopy cover and DOC d13C.Our results suggest that riparian vegetation cover and

associated inputs exert a greater influence on benthic

CPOM than on suspended POM. These findings further

highlight the importance of catchment-scale stressors on

POM sources and lability and thus have important

implications for prioritising management approaches

(i.e. catchment scale versus reach scale) in urban

streams.

What carbon sources are being respired within biofilm,

CPOM and sediment?

While Keeling plots provided novel insight into organic

matter processing, with only one tracer and five organic

matter sources, it was difficult to draw conclusions on

the major sources of respired carbon with any certainty.

In the case of biofilms, respired d13C values were highly

variable across sites and ranged from being most similar

to algal and macrophyte material (Blind Creek), to

reflecting a mixture of autochthonous and allochthonous

sources including soil, litter and macrophytes (Monbulk

0 15 30EI (%)

0 15 30EI (%)

(a) (b)

Fig. 6 Water column DOC d13C as a function of (a) biodegradable dissolved organic carbon content (BDOC) and effective imperviousness

(EI) and (b) dissolved organic carbon molecular weight (DOC MWT) and effective imperviousness (EI). Point values represent mean

d13C � SE (n = 2) and circle diameters represent EI.



and Mullum Mullum Creeks). In the case of Blind

Creek, this may indicate that biofilms were consuming

labile water column algal and macrophyte exudates

(Kaplan & Bott, 1982; Espeland, Francoeur & Wetzel,

2001) or cycling internally produced exudates (Jones &

Lock, 1993; Rier, Kuehn & Francoeur, 2007). The d13Csignature of carbon respired within biofilms at Monbulk

and Mullum Mullum Creeks was considerably more

enriched in 13C than those at Blind Creek, and this may

indicate a contribution from C4 sources. While we found

no evidence of C4 litter in detrital standing stocks, it

may be that microbial communities were preferentially

consuming this labile, low C : N material, thereby

reducing its residual abundance (see Waichman, 1996;

Clapcott & Bunn, 2003; Newcomer et al., 2012).

The respired d13C values observed in this study were

largely within the range reported in the limited pub-

lished literature (see Waichman, 1996; Karlsson et al.,

2007). Despite this, Keeling plots from benthic CPOM

and whole sediments (especially those from Lyrebird

Creek) revealed particularly 13C-enriched signatures

relative to the organic matter sources sampled. It is pos-

sible that many of our results were driven by significant

consumption of C4 material, although they may have

also been influenced by the preferential consumption of

polysaccharides (Benner et al., 1987). This study is the

only to have investigated benthic microbial processing

using the d13C of respired CO2 and Keeling plots

(cf. lake water column studies of Waichman, 1996; Karls-

son et al., 2007), and fractionating reactions occurring

within the benthic material may have been the cause of

our 13C-enriched values. Such reactions may have

included fungal respiration, carbonate dissolution or bio-

genic methane production via CO2 reduction. Fungal

respired CO2 can be enriched in 13C by 1.5–5.5& relative

to the C3 carbon source (Henn, Gleixner & Chapela,

2002), and fungal decomposition of CPOM and wood is

thus likely to have influenced our observations (Findlay

et al., 2002; Gulis & Suberkropp, 2003; Pascoal et al.,

2003). Alternately, dissolution of carbonate present in

the shells of bivalves, gastropods or the sediment (i.e.

CaCO3 + H2O + CO2 ? Ca2+ + 2HCO3�) can produce

enriched DIC d13C signatures in the order of �15 to

�5& (Andgradi, 1994; Kendall et al., 2001; Cartwright,

2010). Finally, under highly reducing conditions, bio-

genic methane production via CO2 reduction preferen-

tially uses 12C, producing 13C-depleted CH4 (�30 to

�106&; Botz et al., 1996; Hornibrook, Longstaffe & Fyfe,

2000; Gu, Schelske & Hodell, 2004), thus leaving the

residual DIC pool highly 13C enriched (for example, see

Deevey & Stuiver, 1964; Lojen, Ogrinc & Dolenec, 1997).

Management implications

The sources and processing of organic matter in urban

streams represent a major knowledge gap limiting our

ability to manage and restore these ecosystems. Our

research suggests that urbanisation increases the contri-

bution of algae and macrophytes to detrital POM and

increases concentrations of labile, low molecular weight

DOC, potentially as a result of algal, macrophyte or C4

litter exudates. Management of urban streams needs to

address the factors influencing C4 litter inputs and algal

and macrophyte biomass including drainage connection,

light availability, stream temperature, nutrient concen-

trations and consumer biomass (Taylor et al., 2004;

Walsh et al., 2005b). This could be achieved through the

application of stormwater retention and infiltration sys-

tems (e.g. ‘water-sensitive urban design’ or ‘low impact

development’: see Walsh et al., 2005a; Ladson, Walsh &

Fletcher, 2006). These measures reduce stream nutrient

loads and promote subsurface flows, thus playing an

important role in moderating stream temperatures and

reducing the transport of more distant litter sources

(Walsh et al., 2005a; Imberger et al., 2008). However, the

effective management of organic matter sources and

lability in both urban and rapidly urbanising environ-

ments also relies on the maintenance and restoration of

riparian vegetation. While we found no clear indepen-

dent effect of canopy cover on organic matter sources in

this study, riparian vegetation provides critical organic

matter inputs to urban streams and, at sufficient densi-

ties, may also moderate both light availability and

stream temperatures. Nevertheless, we conclude that the

protection or restoration of riparian vegetation, in the

absence of modifications to catchment-scale stormwater

drainage connection, is insufficient to mitigate the effects

of urbanisation on organic matter sources, lability and

processing in these small streams.

Acknowledgments

We thank Charlotte Hurry, Vera Eate and Anthony

Brown for their assistance in the field. We also gratefully

acknowledge Seth Wenger, Fran Sheldon, Alan Hildrew

and two anonymous reviewers for their constructive

comments on the manuscript.

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(Manuscript accepted 5 September 2013)



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