HYDROLOGICAL PROCESSESHydrol. Process. 28, 1835–1843 (2014)Published online 27 February 2013 in Wiley Online Library(wileyonlinelibrary.com) DOI: 10.1002/hyp.9716

Contrasting carbon export dynamics of human impacted andpristine tropical catchments in response to a short-lived

discharge event

Adrian M. Bass,1,2* N.C. Munksgaard,1,3 M. Leblanc,2 S. Tweed2 and M.I. Bird11 School of Earth and Environmental Science, James Cook University, Cairns Campus, Queensland, Australia

2 National Centre for Groundwater Research and Training, School of Earth and Environmental Sciences, James Cook University, Cairns, Australia3 Research Institute for the Environment and Livelihoods, Charles Darwin University, Darwin, Australia

*CJamE-m

Co

Abstract:

Utilising newly available instrumentation, the carbon balance in two small tropical catchments was measured during twodischarge events at high temporal resolution. Catchments share similar climatic conditions, but differ in land use with onedraining a pristine rainforest catchment, the other a fully cleared and cultivated catchment. The necessity of high resolutionsampling in small catchments was illustrated in each catchment, where significant chemical changes occurred in the space of afew hours or less. Dissolved and particulate carbon transport dominated carbon export from the rainforest catchment during highflow, but was surpassed by degassing of CO2 less than 4 h after the discharge peak. In contrast, particulate organic carbondominated export from the cleared catchment, in all flow conditions with CO2 evasion accounting for 5–23% of total carbon flux.Stable isotopes of dissolved inorganic carbon (DIC) in the ephemeral rainforest catchment decreased quickly from ~1.5 %to ~�16 % in 5 h from the flood beginning. A two-point mixing model revealed that in the initial pulse, over 90% of the DICwas of rainwater origin, decreasing to below 30% in low flow. In the cultivated catchment, d13CDIC values varied significantlyless (�11.0 to �12.2 %) but revealed a complex interaction between surface runoff and groundwater sources, with groundwaterDIC becoming proportionally more important in high flow, due to activation of macropores downstream. This work adds to anincreasing body of work that recognises the importance of rapid, short-lived hydrological events in low-order catchments toglobal carbon dynamics. Copyright © 2013 John Wiley & Sons, Ltd.

KEY WORDS dissolved inorganic carbon; dissolved organic carbon; land use; stable isotopes; flood; river

Received 13 March 2012; Accepted 11 January 2013

INTRODUCTION

First-order streams are a major interface between theterrestrial and aquatic environments, and can representa significant pathway for the transport of terrestrial carbonaway from its source. Headwater catchments comprise70–80% of the terrestrial area (Gomi et al., 2002). Carbontransported downstream has been recognised as theprimary source of energy to aquatic food webs inheadwater catchments (Cummins et al., 1983; Johnsonet al., 2006a), as well as providing a significant linkbetween the land and atmosphere via the heterotrophicbreakdown of organic material and weathering processesthat result in outgassing of CO2 from fluvial systems(Richey et al., 2002; Mayorga et al., 2005).Dissolved and particulate organic carbon (D/POC) are

significant fractions of the aquatic carbon pool andimportant components of the carbon cycle as a whole.DOC is the largest reduced pool of carbon in aquaticecosystems (Volk et al., 1997), is a significant source of

orrespondence to: Adrian Bass, Earth and Environmental Science,es Cook University, Cairns, Australia.ail: adrian.bass@jcu.edu.au

pyright © 2013 John Wiley & Sons, Ltd.

energy in streams (e.g. Maurice and Leff, 2002) andrepresents a significant flux of carbon from terrestrialecosystems (e.g. Cole et al., 2007, Bird et al., 2008).Dissolved inorganic carbon (DIC) also comprises asignificant fraction of the aquatic carbon pool andrepresents a substantial lateral flux from inland watersystems as well as a vertical flux through CO2 outgassing.In the Amazon basin, larger river systems were found toproduce CO2 emissions equating to over ten times thedissolved carbon lateral flux (e.g. Mayorga et al., 2005).Discharge is a primary driver controlling the export of

carbon in fluvial systems. DOC and POC export has beenshown to increase during flood events in temperatecatchments, accounting for between 36 and >50% ofannual DOC export (e.g. Hope et al., 1994; Hinton et al.,1997, 1998; Clark et al., 2007). Recent work in Australianrainforests has also reported a strong dependence of DOCand POC export on catchment discharge (e.g. Bass et al.,2011). Measurements in the Daintree rainforest in tropicalnorthern Australia showed 85% of DOC and POC exportoccurred during flood events that occurred only 9% of thetime over an annual cycle.As more studies elucidate the dynamism of short-term

discharge events (e.g. Waldron et al., 2007), the need forhigh-resolution data collection in lower order fluvialsystems is becoming apparent, and progress in this area

Figure 1. The Atika Creek (AC) and Scheu Creek (SC) catchmentsshowing the respective equipment deployment locations

1836 A. M. BASS ET AL.

is beginning to be made. Instrumentation now existsthat can autonomously record total and dissolved organiccarbon (DOC) (e.g. Waterloo et al., 2006, Bass et al.,2011). Johnson et al. (2010) developed a system thatcontinuously records pCO2, and instruments to measureDIC in both ocean (Sayles and Eck., 2009) and fluvialenvironments (Bass et al., 2012a, b) have been described.These advances in measurement capability, coupled toexisting automated data loggers for various hydrologicaland hydrochemical parameters, are enabling high-resolution carbon budgets to be calculated in small,hydrologically responsive systems.The links between carbon transfers and the dynamic

hydrologic cycle are accentuated in catchments exposedto environmental change. Land use change is a significantprocess affecting many terrestrial aquatic environments.Previous studies have shown that land use change, fromforest to agriculture for example, causes significantalterations in the input and export dynamics of particulateand dissolved nutrients to and from low-order catchments(e.g. Attiwill, 1991; Correll et al., 1991; Cooke andPrepas, 1998). However, the majority of research to datehas taken place in the temperate zone, with relatively fewstudies addressing this issue in the tropics. The work thathas been reported suggests that land use change in thetropics will first be reflected in low-order catchments(Richey et al., 1997) and can have a significant effecton the fluxes of carbon from these systems (e.g. Thomaset al., 2004).In this study, we utilised newly available analytical

equipment to quantify the carbon fluxes during a singledischarge event in two small tropical catchments withhigh temporal resolution. We compiled complete carbonbudgets for a pristine rainforest catchment and a heavilymodified cultivated sugarcane catchment throughout tworelatively small discharge events. This study aimed toexamine how the balance between lateral and verticalcarbon fluxes changed during a discharge event at finetemporal scale in each catchment and how this balancewas potentially impacted by land use alteration.

METHODS

Study area

Our study sites were situated in the wet tropical regionof Queensland, Australia (Figure 1). Atika Creek (AC)(16o 49.064’ S, 145o 41.212 E) drains a pristine rainforestcatchment with a steep gradient; descending from 508 mto 13 m over 6.3 km. Soils in the AC catchment aredominated by brown and red dermosols; the geology iscomposed of metamorphic rocks (mudstone, siltstoneand conglomerates) from the Devonian, belonging to theHodgkinson formation (Murtha, 1986). The local climateis tropical humid with a pronounced wet season betweenDecember and May. Average annual rainfall is 1992 mmwith a mean maximum and minimum temperature of29.0 �C and 20.8 �C respectively. AC is an ephemeralcatchment with little to no flow during the dry season.

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Scheu Creek (SC) (17o 34.847 S, 146o 0.202 E) drainsan entirely cleared catchment with a relatively shallowgradient; descending from 78 m to 10 m over 8.3 km. Soilsin the SC catchment are dominated by red ferrosols sittingon the Atherton basalt formation from the Quaternaryera (Murtha et al., 1996). Over 90% of the SC catchmentis used for agricultural practices, mainly sugar cane(Saccharum sp) cultivation. The local climate is similarto that of AC, though the average annual precipitationtends to be higher at 3575 mm. Mean maximum andminimum air temperatures are similar to AC at 27.9 �C and19.3 �C, respectively. Significant groundwater contributionmeans SC flows year-round.

Data collection

In December 2011, both SC and AC were instrumentedfor approximately 24 h, for the duration of a singledischarge event each. The Queensland GovernmentDepartment of Environmental Research Management hasa continually operating gauging station on SC allowingaccess to discharge data. In AC, water depth and velocitywere measured at 15-min intervals using an ultrasonicDoppler sensor (Starflow model 6526; UNIDATA). TheStarflow system measures the Doppler shift of suspendedparticles and microscopic air bubbles to calculate velocity.Velocity measurements were converted into dischargevalues by integrating the channel cross-sectional area.Rainfall was measured with a HOBO RG3-M logging raingauge. In SC, genuine low flow was not measured as ourexperiment began on the tail of a significant flow event theday before. Thus, while we are unable to present a genuinelow-flow high-flow assessment, we were still able todeduce significant flow-related impacts.At each site, water was pumped approximately 10 m

from the creek site into a polyethylene flume. Flow throughthe flume was approximately 6 l min�1.The flume wasinstrumented with two data loggers, a TROLL 9500(In-Situ) water quality probe which recorded watertemperature, pH and conductivity at 15-min intervals.pH was calibrated against pH 4, 7 and 10 buffers;

Hydrol. Process. 28, 1835–1843 (2014)

Figure 2. Precipitation, discharge, water temperature, pH, electrical conductivity (EC) and turbidity recorded over the two study periods. Data from ACis shown on the left-hand side (A), data from SC on the right (B). Data collected at 15-min intervals. Truncation of the SC turbidity data was caused by

excessive sediment blocking the measurement path. The magnitude of these events is average for each catchment

1837TROPICAL FLUVIAL CARBON EXPORT

conductivity and turbidity were calibrated against100 mS cm�1 and 100 NTU solutions, respectively. AS::can Spectrolyser was used to record DOC, total organiccarbon, nitrate and turbidity. Details on the S::canSpectrolyser calibration can be found in Bass et al. (2011).DIC and d13CDIC was measured using the Isotopic

Continuous Dissolved Inorganic Carbon Analyser(ISO-CADICA). A full description of the ISO-CADICA system has been presented by Bass et al.(2012b). The ISO-CADICA records DIC concentrationto� 0.12 mg l�1 and d13CDIC to �0.2 %. DIC valuesfor groundwater and overland flow at SC were measuredusing manual samples collections in acid washed, pre-evacuated sealed glass vials, containing 100 ml ofdegassed phosphoric acid. Samples were then analysedby isotope ratio mass spectrometry (e.g. Torres et al.,2005). Groundwater samples were collected after purging

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two government piezometers, both screened in thequaternary basalt aquifer at 5-m and 15.7-m depthbelow ground. Surface runoff collectors were placed onthe hillslope to sample overland flow.

CO2 flux calculations

pH and DIC concentration were each recorded at 15-minintervals. Using these two parameters, we calculated thetotal alkalinity and pCO2 using the software CO2SYS.Flux of CO2 from the stream surfaces was estimated withthe widely used one-dimensional stagnant film model:

CO2 flux mol m�2 d�1� � ¼ k � pCO2w � pCO2að Þ

where k is the gas transfer velocity (m day�1), pCO2w isthe partial pressure of CO2 in water and pCO2a is thepartial pressure of CO2 in the atmosphere. Significant

Hydrol. Process. 28, 1835–1843 (2014)

1838 A. M. BASS ET AL.

variation in flux estimates can be caused by the estimatedvalue of k which has been shown to be a function ofturbulence at the air–water interface (Zappa et al., 2007).Further, Butman and Raymond (2011) reported a relation-ship where k can be predicted for individual catchmentsbased on the slope and velocity:

k m day�1� � ¼ S� V � 2841:6þ 2:03ð Þ

Where S is the slope of the stream segment, and V isthe water velocity. Utilising this equation, we calculated kvalues for each site at 15-min intervals which varied from3.8 to 4.9 and 2.7 to 3.6 at AC and SC, respectively.Total export of carbon as DOC, POC and DIC was

estimated by integrating the spot measurement concentra-tions across the cross-sectional area of each stream.Vertical flux as CO2 was estimated by multiplying thecalculated vertical flux per m2 by the estimated area of thestream surface, estimated as the length of the streammultiplied by an average width of 1 m.DIC mixing models were carried out for both AC

and SC, describing the balance between rainwater andC3-derived DIC, and the balance between surface runoffand groundwater DIC, respectively.

d13CT �MT ¼ d13CR=RO �MR=RO

� �

þ d13CC3=GW �MC3=GW

� �

Where d13CT is the measured isotope value of withincreek DIC, MT is the fractional mass of within creek DIC(equal to 1), R/RO is values of rainwater / surface runoff forAC and SC, respectively, and C3/GW values for C3vegetation-derived DIC and groundwater DIC, respectively.Rainwater d13CDIC was assumed to be between �7 %,

typical for rainwater in the pH range of 4.3–6.5 (Clark andFritz, 1997), and 0 %, indicative of atmosphericequilibrated rainfall (Mook et al., 1974, Zhang et al.,1995; Shulte et al., 2011). Surface runoff was not collectedduring the AC experiment due to lack of appropriatesampling equipment; therefore, C3-derived soil waterDIC was estimated by taking average values for tropicalC3 vegetation of between �27.7 % and �29.1 % (Birdand Pousai, 1997), and applying a fractionation factor of+4.4 % due to diffusion processes (Cerling et al., 1991).The C3 value was further corrected for creek pH accordingto Schulte et al. (2011). Both surface runoff (�10.4 %)and groundwater (�14.4 % to �12.7 %) d13CDIC weremeasured during the SC experiment and used in ourmixing model.

Figure 3. The relative concentrations of DOC, POC, DIC and the rate ofcarbon evasion from A) AC and B) SC over the discharge events (shaded

regions)

RESULTSEvent-induced variation in stream chemistry. Water

temperature was similar between catchments, though eachshowed some variation over the sample periods (Figure 2). Inboth AC and SC, the temperature seems to be predominantlyinfluenced by time of day, both declining in the early parts ofthe study during darkness and rising in the day.

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Compared to SC, AC was characterised by higher pHand conductivity. Average pH and EC in AC was6.49� 0.15 and 86.7� 16.8 mS cm�1, respectively,compared to 5.82� 0.08 and 45.6� 2.5 mS cm�1 in SC.Initial pH in AC was significantly below the majority ofthe data, rising rapidly from ~5.9 to ~6.6 in 90 min fromthe event onset. The significant rise in pH was accompan-ied by a rise in EC, increasing from~ 20 mS cm�1 to 70 mScm�1 in the same period. In comparison, both pH and ECvaried significantly less in SC, varying by only 0.2 and8 mS cm�1, respectively. Both pH and EC decreasedconcurrently with increasing discharge in SC, the oppositeto AC where peak pH and EC were measured during low-flow conditions.Turbidity increased in response to discharge in both

AC and SC, though absolute values showed a significantdifference. Baseline turbidity in AC was below 20 NTUincreasing to ~70 NTU in peak flow, significantly lessthan in SC where the maximum turbidity measured was~220 NTU. Peak turbidity in SC was missed due to loggerfailure, and the actual maximum turbidity was likely over250 NTU. Turbidity in both AC and SC was primarily afunction of catchment discharge.

Carbon concentration and export dynamics. Discharge-related response of lateral carbon export varied dependingon the fraction of carbon being considered. In both AC andSC, POC flux is the most significant fraction during high-flow periods (Figure 3), reaching a maximum instantan-eous concentration of 18.4 mg l�1 and 30.9 mg l�1 inAC and SC, respectively. Truncation of the SC data, dueto blocking of the measurement path by excessive

Hydrol. Process. 28, 1835–1843 (2014)

Figure 4. Horizontal and vertical fluxes of carbon from the a) AC and b)SC catchments. Total bar height represents the total C export, divided intothe lateral flux of DOC, POC and DIC and the vertical flux of C in the

form of CO2 evasion

Figure 5. Variation in DIC concentration and d13CDIC over dischargeevents in a) AC and b) SC. Shaded regions show the discharge patterns

over the same period

1839TROPICAL FLUVIAL CARBON EXPORT

particulates, likely means the maximum concentrationof POC was missed, but is estimated to be approximately40–45 mg l�1.In contrast to POC which responded similarly to

discharge at each site, both DOC and DIC showedcontrasting discharge responses between AC and SC.DOC and DIC responded in opposite direction in AC, withDOC rising rapidly from an approximate low-flow value of6 mg l�1 to over 18 mg l�1 in high flow. DIC, however,was observed to increase from a minimum value of 2 mgl�1 to approximately 5 mg l�1 as the discharge graduallydropped. DOC was generally the dominant form of carbonin AC, except for a brief period during peak flow wherePOC dominated. The balance between DIC and POCchanges over the discharge regime with POC dominant inflows over 0.01 m3 s�1, and DIC dominant in flows belowthis value.DOC was significantly lower in SC than in AC, with

an average of only 0.9� 0.4 mg l�1 compared to11.9� 3.1 mg l�1. DOC rose during times of low flowand dropped as discharge increased, the opposite patternto that observed in AC. DIC concentration was lowest inlow-flow periods in SC, rising by approximately 2 mg l�1

during high discharge periods.Differences in the inorganic carbon dynamics of each

system were further illustrated by estimated carbon evasionfrom the stream surfaces (Figure 3). In AC, carbon evasiongenerally increased steadily from 0.05 mg C m�2 s�1 to0.07 mg C m�2 s�1 as the discharge event passed and low-flow conditions returned. Carbon evasion from SC wasapproximately an order of magnitude higher than AC,reaching a peak of nearly 0.17 mg C m�2 s�1, but this peakcorresponded to the highest discharge level,Carbon exported from the catchment was divided

into four components, the lateral transport of DOC, POCand DIC, and the vertical evasion flux of carbon from thestream surface. The balance between the different formsof export changed significantly throughout the dischargeevents at each site (Figure 4). DOC and POC dominatedcarbon export from AC during the discharge event(Figure 4a). At peak discharge, POC alone accountedfor over 55% of the carbon being exported from AC. Theproportional significance of vertical carbon evasion fromAC increased as the discharge peak receded, increasingfrom less than 5% initially to effectively 100% duringbase-flow conditions.Total carbon export from SC was highest during high

discharge periods, as in AC, but export was dominated byPOC and DIC, rather than DOC which contributed, atmost, 5% of the total carbon exported from the catchment.During high flow, POC accounted for as much as 77% ofthe carbon exported from SC, and was at least 28% of totalexport even during low flow. When compared to AC,vertical evasion of carbon from SC was small in terms oftotal carbon exported, accounting for between 5% and 23%during high and low flow, respectively.

DIC concentration and d13CDIC dynamics. Boththe concentration of DIC and the d13CDIC values varied

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with catchment discharge, but the response patternsdiffered between AC and SC (Figures 5 and 6). In AC,DIC concentration rose from a minimum of approximately2.3 mg l�1 to levelling out between 4.5 and 5.0 mg l�1 asdischarge decreased. The increase in DIC concentrationwas split into two distinct periods. The initial rise of~2 mg l�1 occurred rapidly, in less 2.5 h, with thesubsequent increase of ~1 mg l�1 taking an additional 16 h.

Hydrol. Process. 28, 1835–1843 (2014)

Figure 6. DIC concentration plotted against d13CDIC in A) AC and B) SCillustrating the contrasting responses of within creek inorganic carbon

dynamics to discharge regime

1840 A. M. BASS ET AL.

The rise inDIC concentration in ACwas accompanied bya concurrent decrease in the d13CDIC, from an initial high of�1 % to a relatively stable �16 % during low flow. Thedecline in d13CDIC was rapid, taking approximately 4 h,from which point it varied little. Variation in d13CDIC overthe last 10 h of sampling in AC was less than 0.4%.Changes in both DIC concentration and d13CDIC in

SC were relatively small. DIC concentration varied by only1.5 mg l�1 over the study period and, in contrast to AC,increased concurrently with discharge. Baseline DICconcentration in SC was ~5 mg l�1 and peaked at 6.3 mgl�1, an hour after peak discharge was measured. As in AC,d13CDIC in SC responded in the opposite way to DICconcentration and declined with increasing discharge. Themagnitude of d13CDIC change was less in SC than ACdeclining by ~1% as discharge increased. SC d13CDIC variedbetween �11.0 % and �12.2 % over the discharge event.Source of DIC for each catchment was estimated

using two component mixing models. In AC, there is nosignificant contribution from carbonate sources, and nomeasurable C4 vegetation areas. Initial flow was between93 and 100% rainwater, dropping as the flood peaksubsided until C3-derived DIC accounted for the majorityof DIC present. Three hours after peak flow, on averagesoil-derived DIC accounted for between 69 and 83% oftotal DIC export.In SC, measurements of d13CDIC in surface runoff

(�10.4%) and groundwater (�12.7 to – 14.4%) enabled

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delineation between the main carbon source. Thecontribution of DIC from groundwater increased rapidlyafter the peak event, increasing to between 50% and 80%during the discharge peak.

DISCUSSION

While the duration of each discharge event was similarbetween catchments, it must be noted that initial conditionswere different. In SC, there was flow prior to the event, andthe measured event was preceded by an event of equal sizethe previous day, suggesting base-flow conditions were notcaptured at t0. In AC, conversely, the site was completelydry before the measured event. The initially dry conditionsin AC likely provide an explanation for initially extremeresults in various hydrochemical parameters. Conductivityand pH for example were initially low and increasedsignificantly over the first hour as soil water began tocontribute to the initial pulse of rainwater. In addition,d18O and dD values of rainwater during the AC eventshowed gradually declining d values over the 3-h rainperiod (d18O=�10 to �14 %; dD=�70 to �100 %;Munksgaard et al., 2012). A mass balance calculationbased on the rainfall isotope composition and measured Oand H isotope compositions of simultaneous manual watersamples from AC at 0.5 – 1 h intervals (unpublished data)indicates that up to 90% of the creek flow was derivedfrom surface runoff of rainwater during the initial 3-hperiod of intense rainfall. However, as rainfall intensitydeclined, water isotope values gradually rose and stabilisedat d18O=�7.8 % and dD=�48% signifying a change toa sub-surface source for the creek flow as rainfall subsided.Independent modelling using water and DIC isotopes werein close agreement.

Organic carbon dynamics. DOC and POC concentra-tions were slightly higher than other rainforest studieswith similar catchment properties (e.g. McDowell andAsbury, 1994; Johnson et al., 2006b; Bass et al., 2011).The high initial concentrations of DOC and POC are afunction of a prolonged dry period preceding the study,allowing organic carbon build up and degradation in thecatchment. A similar initial organic carbon peak wasobserved in a rainforest catchment in tropical north-eastAustralia, where the highest DOC and POC concentrationswere measured directly after a dry period, despite thefact the associated discharge event was relatively small(Bass et al., 2011). This initial ‘flushing’ peak is likely afeature of catchments prone to prolonged dry spells, asthis phenomenon has also been observed in temperature aswell as tropical catchments (Boyer et al., 1997; Hintonet al., 1997).The classical understanding of carbon export dynam-

ics from low-order catchments is that increasingdischarge is accompanied by an increase in surfacerunoff and shallow sub-surface flow, which in turnmobilises stores of DOC and POC, leading to ameasured increase in the export waters. This has beenobserved in other tropical catchments in the Amazon

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1841TROPICAL FLUVIAL CARBON EXPORT

(Johnson et al., 2006b) and in Australia (Bass et al.,2011). Indeed, data from the Amazon (Johnson et al.,2006b) showed that overland flow, which dominates inhigh discharge events, could have up to 25 mg l�1 ofDOC, compared to a maximum of only 0.5 mg l�1 inemergent ground water.POC concentration reached higher levels in SC than in

AC. Previous studies have observed cleared catchmentshaving higher concentrations of particulates than forestedareas, due in part to soils becoming less stable and higherdensities of aquatic plants (e.g. Niell et al., 2001, Thomaset al., 2004). The density of aquatic plants in SC is low;suggesting mobilisation of catchment soils may beresponsible for the higher particulate concentrationsobserved than in AC. In SC, the response of POCappeared to follow the previously described pattern ofincreasing with higher discharge, but DOC showed littlesignificant change with increasing discharge, and ifanything recorded a slight decline in higher flow. Webelieve this is related to the accompanying peak inturbidity and particulate material. Much of the SCcatchment has high clay content, capable of adsorptionof DOC. Thus it is possible that even though DOCconcentration may increase with discharge, the increasedconcentration of particulates acts to remove it from thedissolved phase.

Inorganic carbon dynamics. The response of DIC tothe discharge events varied between AC and SC. In AC,DIC concentration was initially low, reflecting the highproportion of rainwater in the creek water at this time.DIC concentration increased rapidly as the dischargeevent peaked and likely reflects the flushing of pore waterin surface soil layers, as well as a potential piston effectactivating a ground water influx. Ground water classicallyhas high concentrations of DIC as a result of dissolutionfrom the soil zone (Richey et al., 2002) and water–rockinteractions. The increase in stream DIC concentration,recorded at around midnight, approximately 2 h after peakdischarge, may represent the period where groundwater orsoil water provides more of the channel DIC.Stable isotope values of DIC support the source

component mixing model for DIC in AC. Initial DICin AC was ~�1 %, suggesting a high proportion ofrainwater, close to atmospheric equilibrium, and very rapidsurface runoff. The decline in d13CDIC is significant as thedischarge peak ends, dropping to ~�16 %. The base-flowvalue of d13CDIC will reflect a combination of C3 organicmatter-derived carbon, groundwater and residual surfacerunoff. Our modelling suggests rainwater DIC initiallydominates inorganic carbon transport in AC during adischarge event, but the contribution declines rapidly untilC3-derived DIC dominates (over 90%) less than 6 h later.DIC dynamics in SC illustrate the complexity

associated with up scaling local findings as DICresponds to discharge in the opposite fashion to AC;with DIC concentrations increasing concurrently withdischarge. The magnitude of the change in SC is lessthan AC, likely a result of the initially dry stream

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conditions at AC. DIC concentration increased by lessthan 1.5 mg l�1 in SC, compared to ~3 mg l�1 in AC.The response of DIC is likely a function of interestinghydrological processes present in SC. A detailed studyand breakdown of the SC hydrograph will be presentedelsewhere, but we will present an overview of the DICcontrolling factors here.Like AC, the DIC concentration in SC is believed to

represent a combination of different source components.High flow and turbidity, coupled to low macrophyte andbiofilm cover mean in-channel biological processes, areunlikely to have a significant effect. The significantdifference between SC and AC in terms of hydrology isthat in high-flow, SC can be fed by groundwater flow viamacropores (pipe flow) which force groundwater andunsaturated zone water into the creek channel. Directmeasurements of pipe flow and surface runoff DICconcentration (approx. 20.8 mg l�1 and 2 mg l�1,respectively), along with observations of pipe flowactivation during high precipitation, suggest that there isa flux of DIC-rich water from pipe flow into the systemwhich causes the observed rise in creek water DIC.The bulk of the SC catchment comprises C4 vegeta-

tion. In general, DIC derived from C4 plants is around�10 % (Shulte et al., 2011). The measured range in SCd13CDIC (�10.8 % to �12.2 %) suggests the dominantsource of DIC is from C4 sources. Using measured valuesfor surface runoff (�10.4 %) and groundwater (�12.7 %to �14.4 %), a two-source mixing model suggestsgroundwater DIC becomes more significant after thedischarge event peaks. The changes in DIC concentra-tions and d13CDIC values are rapid, taking less than anhour, illustrating the pressing need for high temporalresolution sampling in systems such as this for effectivecarbon cycle delineation.Ribolzi et al. (2000) found that the bulk of water

generated from a storm event was from overland flow in anagricultural catchment in the Mediterranean. The increasein POC observed in SC may support a similar conclusionin SC, but evidence from DIC concentrations and d13CDIC

suggest a more complicated system, potentially fed bydifferent sources in the duration of a single event.

Vertical and lateral carbon fluxes. Calculated rates ofvertical carbon flux per stream unit area were significantlydifferent between catchments (Figure 5). When extrapo-lating over the entire experimental period, average verticalcarbon flux from AC was equivalent to 5.4� 0.9 g C m�2

d�1, compared to 11.1� 1.7 g C m�2 d�1 in SC. AC iswithin the range reported for other tropical rainforestsystems such as the Amazon with measured fluxes between0.6 and 13.1 g C m�2 d�1 (Richey et al., 2002). Recentwork estimated fluxes over 40 600 km2 of US streams andfound fluxes ranging from 2.4 to 11.0 g C m�2 d�1

(Butman and Raymond, 2011). While values for SC are onthe high end of the reported measurements, we are unawareof other estimates from heavily impacted, low-ordertropical catchments such as this with which to make adirect comparison.

Hydrol. Process. 28, 1835–1843 (2014)

1842 A. M. BASS ET AL.

Our results clearly show that in both catchments, thetime of day flux measurements taken will have a significantimpact on extrapolating long-term estimates. Changesrelated to discharge can be both rapid and substantial. Foraccurate carbon budgets variability in flow and pool size/flux, over small temporal ranges must be taken intoaccount, as has also been reported in temperate catchments(Waldron et al., 2007).Fluxes of POC dominate the heavily impacted

catchment of SC. The transformation of forest land toagricultural land acts to destabilise the sediment andpromote the mobilization of particulates during floods.Due to potential DOC adsorption onto particulatematerial, it is difficult to accurately assess the importanceof DOC export, but we can say with confidence thatorganic carbon is the dominant form of carbon exportfrom SC. The significant quantities of POC exported fromSC suggest that the shift from forest to agricultural landmay have significant effects on the quantities and form ofcarbon exported downstream and potentially into thecoastal oceans. While AC too is dominated by organiccarbon fluxes during peak flow, DOC tends to dominateon average, consistent with results for other rainforeststudies in the area (Bass et al., 2011).This study illustrates the significance of vertical

evasion of carbon from low-order rainforests catchmentsalso. In AC, over 85% of the carbon exported duringbaseflow is through vertical evasion to the atmosphere.Given that low-order streams drain 70–80% of theterrestrial environment (Gomi et al., 2002), this is apotentially significant flux of carbon. However, whetherthis represents a new source of carbon or a simple re-distribution of terrestrial carbon that would otherwise beprocessed on land is unclear.

CONCLUSIONS

Studies of carbon export dynamics in low-order tropicalcatchments are still relatively rare (e.g. Cummins et al.,1983; Johnson et al., 2006b; Bass et al., 2011) generally asa result of logistic difficulties. Due to the often rapid natureof hydrological events in these small catchments, detailsof discharge-related changes can be easily missed.Previous work has revealed the significant effect dischargeevents have on carbon export (Hope et al., 1994; Hintonet al., 1998; Clark et al., 2007), but few have examinedthese events at fine temporal resolution. The observationsin this study provide a clearer understanding of howcarbon dynamics change over a discrete discharge eventin two contrasting small, tropical catchments. We haveobserved contrasting responses that appear to be based onsignificant differences in the hydrological processesbetween catchments, and which are inevitably linked toland cover differences.In our rainforest catchment, we observed the generally

accepted discharge response, where mobilisation oforganic carbon dominates flood export. However, wealso measured the significant vertical flux of carbon from

Copyright © 2013 John Wiley & Sons, Ltd.

the stream surface. More work, including a significantlylonger temporal duration, would be required to assess theannual significance of this vertical flux. In contrast, wefound evidence that, while organic carbon exportdominates, discharge events in SC caused the mobilisa-tion of DIC-rich unsaturated zone water and groundwatervia pipe flow pathways, leading to elevated rates of DIClateral flux. In addition, the significantly higher propor-tion of organic carbon exported from SC, as POC,compared to the forested AC catchment suggests futureland use in the tropics can have significant effects oncarbon export dynamics and associated processes.

ACKNOWLEDGEMENTS

We thank Joel Bailey and Kelly Rae for field assistance.This work was funded by an Australian Research CouncilLIEF (Linkage Infrastructure, Equipment and Facilities)grant LE110100144 MIMES: Mobile Isotope Monitoringfor Environmental Studies’ to MIB. Funding was partlysupported by the ARC Discovery project ‘Greenhouse gasemissions from tropical floodplains’ (DP110103364), andthe National Centre for Groundwater Research andTraining, an Australian Government initiative, supportedby the Australian Research Council and the National WaterCommission. This manuscript was greatly improved bycomments from an anonymous reviewer.

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