Carbon cycling and exports over diel and flood-recovery timescales in a subtropical rainforest headwater stream

Science of the Total Environment 550 (2016) 645–657

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Carbon cycling and exports over diel and flood-recovery timescales in a subtropical rainforest headwater stream Arún Looman a,b,⁎, Isaac R. Santos a,b, Douglas R. Tait a,b, Jackie R. Webb a,b, Caroline A. Sullivan b, Damien T. Maher b a b

National Marine Science Centre, Southern Cross University, Coffs Harbour, New South Wales, Australia School of Environment, Science, and Engineering, Southern Cross University, Lismore, New South Wales, Australia

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• First carbon export estimates of a pristine Australian subtropical headwater catchment. • Metabolism and hydrology were drivers of aquatic carbon flux. • Catchment exports ranged between 1.1 and 18.6 mmol C m− 2 d− 1 over the study period. • Diel variability changed according to flow regime.

a r t i c l e

i n f o

Article history: Received 19 August 2015 Received in revised form 14 January 2016 Accepted 14 January 2016 Available online xxxx Editor: D. Barcelo Keywords: Carbon budgets Greenhouse gas Carbon dioxide Metabolism Drought Flood

a b s t r a c t Catchment headwaters comprise the majority of all stream length globally, however, carbon (C) dynamics in these systems remains poorly understood. We combined continuous measurements of pCO2 and radon (222Rn, a natural groundwater tracer) with discrete sampling for particulate organic, dissolved organic and inorganic carbon (POC, DOC, and DIC) to assess the short-term carbon dynamics of a pristine subtropical headwater stream in Australia, over contrasting hydrologic regimes of drought, flash-flooding and recovery. Observations over 23 days revealed a shift from carbon losses dominated by CO2 outgassing under conditions of low flow (66.4 ± 0.4% of carbon export) to downstream exports of carbon during the flood (87.8 ± 9.7% of carbon export). DOC was the dominant form of downstream exports throughout the study (DOC:DIC:POC = 0.82:0.05:0.13). The broadest diel variability among variables occurred during the drought phase, with diel variability up to 662 μatm d−1 (or 27 μM [CO2*] d−1), 17 μM d−1 and 268 Bq m−3 d−1 for pCO2, dissolved oxygen and 222Rn, respectively. Diel dynamics indicated multiple interrelated drivers of stream water chemistry including groundwater seepage and instream metabolism. The catchment exported terrestrial carbon throughout the field campaign, with a mean net stream flux of 4.7 ± 7.8 mmol C m−2 (catchment area) d−1 which is equivalent to 1.4 ± 2.3% of the estimated local terrestrial net primary production. Our observations highlight the importance of accounting for hydrological

⁎ Corresponding author at: School of Environment, Science, and Engineering, Southern Cross University, Lismore, New South Wales, Australia. E-mail address: [email protected] (A. Looman).

http://dx.doi.org/10.1016/j.scitotenv.2016.01.082 0048-9697/© 2016 Elsevier B.V. All rights reserved.

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A. Looman et al. / Science of the Total Environment 550 (2016) 645–657

extremes when assessing the carbon budgets and ecosystem metabolism of headwater streams, and provide a first estimate of aquatic carbon exports from a pristine Australian subtropical rainforest. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Inland waters are a critical resource, forming a vital link in the cycling of carbon (C) between terrestrial and atmospheric pools (Cole et al., 2007; Cole, 2013; Raymond et al., 2013). Modern anthropogenic disturbance and climatic change is rapidly altering the structure and function of many inland waters (Vörösmarty and Sahagian, 2000; Milly et al., 2005; Regnier et al., 2013). In order to assess how inland waters will respond to climate change, the mechanisms of biogeochemical flux within and between terrestrial and aquatic systems must be adequately understood. However, the chemical and ecohydrological status of the majority of headwater ecosystems remains to be described (McGlynn et al., 2004; Bishop et al., 2008), particularly those draining warm temperate and subtropical forest biomes. Catchment headwaters comprise 65 to 90% of global drainage-basin area (Sidle et al., 2000; Gomi et al., 2002; Lowe and Likens, 2005; MacDonald and Coe, 2007). While larger rivers (Strahler order N3) comprise the greatest flowing freshwater expanse by surface area, first- and second-order streams including their perennial tributaries encompass the greatest extent by linear measure (Downing et al., 2012; Raymond et al., 2013). In contrast to larger aquatic systems in the lower catchment influenced by more regional biogeochemical factors (Poole, 2002), the structure and function of headwaters is largely dominated by local drivers (Nadeau and Rains, 2007). For example, the shallow water depths typical of low-order streams engenders a high surface area-to-volume ratio (Peterson et al., 2001), increasing the relative importance of carbon cycling within and between epilithic biofilms and hyporheic sediments (McClain et al., 2003; Romaní et al., 2004; Lyon and Ziegler, 2009). This constrained water volume also acts to maximise the direct and indirect influences of solar insolation on carbon spiralling dynamics (Hays, 2003; Battin et al., 2008; Swanson and Cardenas, 2010). Narrower channels maximise the interface between surface waters and the adjacent terrestrial landscape (Vannote et al., 1980; Gomi et al., 2002), resulting in systems that generally exhibit high levels of allochthonous carbon loading, mainly in the form of coarse particulate organic matter and dissolved humic compounds (França et al., 2009; Lyon et al., 2011). These external inputs to the water column subsidise aquatic food webs (Jansson et al., 2007; López et al., 2011; Guillemette et al., 2013). Headwater ecosystems are extremely efficient at metabolising this allochthonous organic carbon (Dawson et al., 2001), which leads to a general status of supersaturated partial pressure of carbon dioxide (pCO2) with respect to local atmospheric concentrations (Hope et al., 2004; Koprivnjak et al., 2010; Wallin et al., 2011; Billett and Harvey, 2013). High pCO2, coupled with high gas transfer velocities related to flowinduced turbulence, can produce greater net outgassing of CO2 from low-order streams than other inland waterbodies (Butman and Raymond, 2011; Wallin et al., 2013). Accordingly, the evasion of CO2 in first- through third-order streams can comprise the dominant component of net carbon flux from freshwater aquatic systems (Johnson et al., 2008; Humborg et al., 2010). Despite this fundamental importance, catchment headwaters have, until now, been overlooked in global carbon budgets (Cole et al., 2007; Battin et al., 2009; IPCC, 2013; Raymond et al., 2013). While the modelling of local- and regional- carbon fluxes using empirical (e.g. Humborg et al., 2010; Butman and Raymond, 2011; Rawlins et al., 2014) and direct quantitative methods (e.g. Hope et al., 2001; Johnson et al., 2006) has advanced in the last decade, these models are generally based upon data that does not account for diel variability. The associated uncertainties can result in significant discrepancies between modelled and observed fluxes (Parker et al., 2010; Worrall et al., 2013).

Previous research in upland catchments has contrasted the effects of stormflow versus baseflow on event-driven carbon exports (Hinton et al., 1997; Buffam et al., 2001; Johnson et al., 2006; Bass et al., 2011) and evaluated the role of these waters in the balance of local net ecosystem exchange (Shibata et al., 2005; Zhou et al., 2013). Further research has reported on the significance of diel cycling of oxygen and metals (Nimick et al., 2003; Riley and Dodds, 2013; Gammons et al., 2015), with these works revealing numerous complications arising from the unpredictable variability within and between diel oscillatory trends. Only a few studies, however, have combined investigations of how diel cycling and episodic discharge drive carbon dynamics in freshwater ecosystems (Dinsmore and Billett, 2008; Dyson et al., 2011; Dinsmore et al., 2013), with these studies focused in temperate systems. The aim of this study was to assess changes in carbon fluxes for an undisturbed subtropical rainforest headwater stream as the system shifted between hydrologic regimes of drought, flash-flooding and recovery. We hypothesised that (1) internal processes such as groundwater seepage and biological metabolism drive the flux of carbon during periods of low discharge, while external processes such as atmospheric forcing and erosion drive the flux of carbon during periods of high discharge; and, (2) that aquatic carbon exports are a minor component of subtropical rainforest carbon budgets and that these systems retain rather than export the majority of carbon sequestered locally via net primary production. To assess these hypotheses, we performed high-resolution measurements of radon, oxygen and carbon species concentrations (pCO2, DOC, DIC, POC and CDOM) before, during, and after a major rain event in a ‘pristine’ subtropical rainforest headwater stream in Australia. 2. Materials and methods 2.1. Study site Measurements were made at Rocky Creek in the Whian Whian State Conservation Area, northeast New South Wales Australia (28°36′S, 153°21′E; elevation 245 m asl). Sampling was conducted over 23 diel cycles in mid-Autumn, 2014. This consisted of an initial four-day field deployment during a period of brief drought [antecedent rainfall of between the 5th and 10th percentiles for three months preceding sampling, classifying this period as a drought of ‘serious’ rainfall deficiency; Bureau of Meteorology (BoM), 2014a], followed by an adjournment of 17 days before resuming fieldwork just prior to the onset of rainfall. The perennial second-order stream and its ephemeral tributaries drain the upper 11.4 km2 of a forested headwater catchment (Fig. 1), with hillslope gradients of less than 50% producing a relatively wide and low-relief valley bottom. Streambed morphology is a highly heterogeneous sequence of riffle-pool-step-pool microcosms. Average slope for the perennial stream reach derived from a 10 × 10 m DEM using ArcGIS 9 Spatial Analyst (elevation error ± 10 cm) is 0.011 m m− 1 over the 6.2 km study reach (stream density 0.55 km km−2). The study site is located on the southern rim of the Mt. Wollumbin Shield Volcano caldera. Catchment geology is derived from Late Palaeozoic (~245 mya) and Tertiary (23 mya) volcanics (Caprarelli and Leitch, 1998; Betts et al., 2002). The predominant lithologies are erosion-resistant rhyolite (N69% silicon), which weathers to form infertile and often acidic yellow Podosol soils of pH 5.0–6.5, and basaltic extrusions which weather to form red and brown Ferrosol soils (Morand, 1994; Isbell, 2002). Upon this, rainforest detritus builds well-structured organic-rich O and A1 horizons, particularly on the lower hillslopes and streamside riparian zone. The watercourse contains predominantly gravel and cobbles with little finer-fractioned soil alluvium present, except for on the outermost meanders and in the deeper (0.5 to 2 m) pools.

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Fig. 1. (a) Map of the upper Rocky Creek catchment showing its primary tributaries, Rocky Creek Dam (RCD) Gauging Station and the northern half of RCD itself. (b) Photograph of Rocky Creek looking upstream from the primary sampling site during the drought. (c) Peak discharge at the primary sampling site. The timeseries station is situated to the left (downstream) of the frame 3 m from the opposite riverbank.

The Rocky Creek watershed experiences a subtropical climate with maximum precipitation occurring from December to May (Austral summer). The mean annual temperature and precipitation measured at Rummery Park weather station 1.8 km away are 17.6 ± 4.9 °C and 2316 mm, respectively (1943–2004 period; BoM, 2014b). The subcatchment serves as a protected municipal supply; with its waters draining into Rocky Creek Dam located 2 km downstream from the sampling site (storage capacity 14,000 ML, secure yield 9600 ML per annum). There are five primary vegetation types within the catchment including wet- and dry-sclerophyll forests, subtropical- and warmtemperate rainforest alliances, and heathland. Dry sclerophyll forest dominated by tall Eucalyptus spp. (up to 80 m) occupies the ridges, while the subtropical rainforest and wet sclerophyll genera populate the sheltered slopes and valley floor (NSW NPWS, 2010; LPI, 2014). This study site was chosen to give account of a relatively undisturbed and ‘pristine’ ecosystem. 2.2. Discharge, morphometric and ancillary variables Water depth and velocity were measured at 10-min intervals using two Unidata Starflow current meters (model 6526) affixed inside the downstream outlet of two of the causeway's three culverts. These 10min measurements were made continuously over the 23 sampling days. Velocity measurements were also taken by use of manual floatation over the length of each culvert, twice daily during drought and baseflow and at greater frequencies during high-flow. During high flow conditions when the water level was above the causeway water velocity was measured at four points along the causeway length at 60min intervals, with stream width marked concurrently. This process was repeated for 32 h during floodwater recession until stream height fell to below the level of the causeway. Further discharge measurements were taken at Big Rocky Creek Gauging Station (28°37′S, 153°21′E), 1.5 km downstream from the primary sampling site. The station consists of a standard V-notch weir, coupled with an analogue 10″ circular chart recorder. Use of discharge data from this weir increases the catchment area of the study site by 3.1 km2, which was factored into all calculations using this data set. The final discharge was calculated from averaging all

three observation-types, and was standardised to 30-min intervals. Further velocity and discharge measurements were taken in the main sampling site pool using a third Starflow current meter. Rainfall, ambient temperature and wind speed data were collected on-site from a portable weather station (Davis Instruments Corp., USA). Due to damage to the weather station during the flood, catchment-specific rainfall data were unavailable for the 15 h period during peak-flow. Supplementary data used for this period was derived from the mean daily precipitation values taken from five surrounding BoM weather stations (upper Coopers Creek, Repentance Creek, Mullumbimby, Boat Harbour, and Uki; BoM, 2014c), with the average obtained from all stations of 243.8 ± 53.3 mm being in good agreement with the 233.0 mm recorded at the nearest rain gauge of upper Coopers Creek, 4.3 km away. Water temperature, dissolved oxygen (DO), pH, specific conductivity, turbidity and stream-water depth were measured using a calibrated Hydrolab DS5X multiparameter sonde, coupled with a LI-COR LI-192SA Underwater Quantum Sensor for measuring photosynthetically active radiation (PAR, resolution to 0.01 μmol s−1 m−2). pH was calibrated against pHNBS buffer standards 4.00, 7.00 and 10.01, and conductivity against a 1413 μS cm−1 solution every 7 to 10 days. Drift in pH between calibration intervals was b0.05 for each standard. Measurements for chlorophyll-a (Chl-a, a proxy for photosynthetic biomass) and chromophoric dissolved organic matter (CDOM) were taken with an ECO Triplet fluorometer (WETLabs, USA), which operates at excitation/emission wavelengths of 470/695 nm (sensitivity ± 0.016 μg L−1) and 370/ 460 nm [sensitivity ± 0.093 μg L− 1 Quinine Sulphate equivalents (QSE)], respectively. All data was recorded at 10-min intervals with sensor depths set at ~0.3 m from the surface during baseflow. 2.3. Coupled pCO2 and 222Rn timeseries Measurements of pCO2 and radon (222Rn, a natural groundwater tracer; t1/2 = 3.84 d) were taken at 10- and 30-min intervals, respectively, using the methodology described by Santos et al. (2012). Briefly, water was pumped continuously via bilge pump to a showerhead type gas equilibration device (GED) at a rate of 2–3 L min−1. A closed loop was

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established between the GED and both a CO2 gas analyser (LI-840A, LICOR Inc., USA) and radon-in-air monitor (RAD-7, Durridge Co. Inc., USA). The RAD-7 device measures the activity of 222Rn by collecting and counting its α-emitted daughter, 214Po+ (t1/2 = 164 μs; Dulaiova et al., 2005). The 30-min sampling period for 222Rn ensured counting uncertainties remained b 10%. The pump inlet was located 2 m from the Hydrolab station at ~20 cm below the water-air interface. The LI-840A gas analyser was calibrated using zero, 502, and 5141 μatm standards, and the RAD-7 factory calibrated one-month prior to deployment with analytical uncertainties b 5%. Temperature and pressure in the GED were measured with a CTD-Diver (Schlumberger Water Services, NL). For easy comparison of pCO2 with other carbon forms and dissolved oxygen we also present dissolved CO2 data (presented as [CO2*] which equals the sum of dissolved CO2 and carbonic acid), which is calculated as a function of solubility and pCO2. In addition to water column pCO2 (pCO2water), atmospheric pCO2 measurements were taken at both 20 cm above the water–air interface (pCO2air) directly over the in-stream bilge pump, and, in the adjacent riparian zone 15 m from the water edge at 5 m surface elevation (pCO2atm). Continuous monitoring occurred at 10-min intervals over three consecutive days during baseflow. These measurements also used LI-840A gas analysers calibrated to the same gas standards as described above. Hereafter, pCO2 alone refers solely to pCO2water, while pCO2air and pCO2atm are marked accordingly. 2.4. Discrete samples Discrete samples were taken centre-steam for POC, DOC, DIC and total suspended solids (TSS). Separation of dissolved and particulate fractions was achieved by filtering with a disposable 0.7 μm GF/F filter (Whatman) using a syringe; both filters and vials (40 mL borosilicate VOC vials) were pre-combusted at 450 °C for 12 h to remove any organic-C. Filters were frozen stored in sterile polycarbonate Petri dishes and analysed for TSS and POC within three weeks of sampling. DOC and DIC samples were treated with HgCl2, sealed with a Teflonlined rubber septa with no headspace and stored at 3 °C until analysis (within five-weeks of sampling). Discrete samples were taken at randomly determined intervals, once to twice daily during low-tomoderate flow and bi-hourly during peak discharge. DOC and DIC samples were analysed on an organic carbon analyser (TOC-VCPH; Shimadzu Corp., Japan) using acidification for DIC analysis and high-temperature catalytic oxidation for total dissolved carbon (TDC). DOC was then taken as the difference between TDC and DIC. Precision (SD of triplicates) was 1.7% for TDC. POC was measured concurrently with TSS by drying the filters in an oven overnight at 60 °C then recording each dry weight. Filters were then packed into tin capsules for POC analysis on a Thermo Fisher Flash elemental analyser (EA); coupled to a Thermo Fisher Delta-V Plus isotope ratio mass spectrometer (IRMS) and continuous flow system (ConFLo IV; Thermo Fisher Scientific, Inc., USA). Precision was ~1% (SD of triplicates). 2.5. Calculations Transfer of CO2 between surface waters and the atmosphere (evasion) is primarily a function of the concentration gradient at the water-air interface and the gas transfer velocity (k). CO2 evasion was calculated using the model described by Raymond et al. (2000) and Wanninkhof (2014): CO2 flux ¼ k  K 0  ðpCO2water –pCO2air Þ

ð1Þ

where, k is the gas transfer velocity (m d−1), K0 is the solubility of CO2, and pCO2water and pCO2air are the partial pressures of CO2 in streamwater and the overlying atmosphere, respectively. Typically, k has been modelled as a function of wind speed, depth, turbidity and surface water turbulence (Wanninkhof, 1992; Borges et al., 2004; Zappa et al., 2007; Abril et al., 2009). Recently, for inland waters, k has been

shown to correlate at the catchment scale with stream reach slope and velocity (Butman and Raymond, 2011; Wallin et al., 2011; Raymond et al., 2012). As the hydromorphology of Rocky Creek was largely bi-modal (i.e. riffle/pool), k was modelled individually for the faster- and slowerflowing segments of the stream reach. Outgassing rates for each riffle/ pool component were taken as the mean of six parameterizations given by Raymond et al. (2012): k600 ¼ ðVSÞ0:89  D0:54  5037

ð2Þ

k600 ¼ 1162  S0:77  V 0:85

ð3Þ

k600 ¼ ðVSÞ0:76  951:5

ð4Þ

k600 ¼ ðVS  2841 þ 2:02Þ

ð5Þ

k600 ¼ 929  ðVSÞ0:75  Q 0:011

ð6Þ

k600 ¼ 4275  ðVSÞ0:86  Q −0:14  D0:66

ð7Þ

where k600 is the gas transfer coefficient of a gas with a Schmidt number of 600 (m day−1), V is the water velocity (m s−1), S is the stream reach slope (unitless), D is the depth (m), and Q is the discharge (m3 s−1). We calculated depth, width and velocity for each riffle and pool sub-reach from the gauged culverts and primary sampling site pool morphometrics, respectively; with all spatiotemporal scaling between riffle and pool components satisfying the continuity relationship D = Q / (VW). Because the hydrodynamics of the Rocky Creek system varied over the study as a function of discharge, we applied a riffle-to-pool (R/P) ratio of 1:2 to the drought phase (i.e. pool surface area was two times higher than riffle); of 1:0 for peak discharge (i.e. 100% riffle area); and, of 1:1 for the remaining phases (i.e. of approximate equal surface area). The gas transfer coefficient, k, was calculated using temperature derived Schmidt numbers (ScT) suitable for fresh water (Jähne et al., 1987; Wanninkhof, 1992): k ¼ k600  ðScT =600Þ−n

ð8Þ

ScT ¼ 1911:1−118:11T þ 3:4527T 2 −0:04132T 3

ð9Þ

with the exponent n set at 2/3 for the quiescent pools with wind speeds b3.7 m s−1 and at 1/2 for the riffles exhibiting a more turbulent interface (Jähne et al., 1987; Guérin et al., 2007). Across the full range of hydrologic conditions, the Rocky Creek watercourse simultaneously reflected the dynamics of multiple fluvial systems. The gas transfer parameterizations of Raymond et al. (2012) used here accommodate this broad range in hydraulic variability, being derived from 563 direct tracer measurements in streams of varying Strahler order, hydraulic geometry and flow dynamics. In addition, the hydraulic geometries of the source drainage network (USA) are similar to those found in Australian inland waters (Harman et al., 2008). Due to the inherent limitations of the models containing the depth-term [Eqs. (2) and (7)], these were excluded from the analysis for discharge rates of N0.5 m3 s−1. For comparison with downstream exports of carbon, evasion of CO2 was estimated per m2 of stream area (SA) and then scaled to per catchment area (CA) for each 30-min time step. Correlations between variables were assessed using the Pearson least square regression coefficient (r2) for linear covariance, or Spearman correlation coefficient (rs) for variables displaying hyperbolic/curvilinear covariance such as exponential decay. Significance herein is implied at a confidence interval of 95% (α = 0.05). Where mean daily POC concentrations were unavailable due to high uncertainties resulting from low concentrations (i.e. where POC b15 μM), to complete the total carbon (TC) mass balance, substitute values were derived from linear interpolation of adjacent mean daily POC concentrations (n = 9 of 23 d). Data was

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time-standardised across all parameters to the nearest 30-min. All uncertainties reported below represent standard deviations of observations. 3. Results 3.1. Catchment hydrology Five contrasting flow regimes were observed across 23 consecutive diel cycles (Fig. 2). The initial deployment recorded 4 d of data following

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an antecedent phase of brief drought. The event hydrograph was further separated into the distinct phases of: first-flush (33.6 mm of precipitation over 2 d); flood (269.6 mm of rainfall over 4 d, with ~163 mm within a 5 h period leading to flash-flooding); recovery (7.1 mm of precipitation over 7 d); and baseflow (24.2 mm of intermittent precipitation over 6 d). Discharge ranged from a minimum of 0.04 m3 s−1 during the drought to a maximum of 22.7 m3 s−1 during the flood. Average daily discharge throughout the event was 87 ± 244 ML d−1 (±SD herein), or 17 ± 9 ML d−1 excluding the flood phase, representing average

Fig. 2. Time series of hydrochemical parameters sampled at the Rummery Rd causeway throughout each of the five given phases of pulse-flow (drought, first-flush, flood, recovery and baseflow). Sampling resolution is 30-min for dataloggers (pCO2/[CO2*], 222Rn, pH, discharge, DO and temperature) and at random intervals for discrete samples — except during the flood phase when samples were taken bi-hourly.

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baseflow conditions (NSW DoC, 2009). A total of 335 mm precipitation was received in the upper Rocky Creek catchment across the duration of the field campaign, of which 43.8% was observed as discharge. The mean lag time between rainfall and observed increase in discharge was ~4 h.

(679 ± 16 Bq m− 3), with a relatively large diel variability of 35.1% (238 ± 42 Bq m − 3 d − 1 ). The initial flushing of the soil profile on the 26 th March produced a sharp increase in 222 Rn from 412 to 630 Bq m − 3 , before near complete dilution of this groundwater proxy during peak-flow.

3.2. Ancillary parameters 3.3. DOC, DIC and POC Ambient atmospheric and streamwater temperatures ranged from 16.5 to 28.6 °C and 17.5 to 22.0 °C, with means of 21.0 ± 2.1 and 19.8 ± 1.4 °C, respectively (Fig. 2, Table 1). The mean wind speed was 0.1 ± 0.3 m s−1, reflecting the sheltered nature of the study site. Each diel cycle was characterised by a narrow window of high PAR, with the insolation maxima lasting for approximately 2–3 h each day. Streamwater remained acidic through all phases of the event with pH ranging from 4.82 to 6.57. The diel variability for streamwater pH fluctuated from 0.08 to 0.73 units d−1. Oscillations were greatest on days with appreciable rainfall. Specific conductivity ranged from a maximum of 66 μS cm−1 during the drought to a minimum of 39 μS cm−1 during peak-flow. Dissolved oxygen concentrations ranged from 220 to 276 μM (78 to 97% saturation), with an overall mean of 256 ± 12.2 μM. Lowest phaseaveraged DO concentrations (229 ± 0.3 μM) and broadest diel variability (±17.2 μM d−1) were observed during the drought. Instantaneous measurements of CDOM and Chl-a ranged from 17.5 to 44.3 μg L− 1 and 0.7 to 3.2 μg L−1, respectively. Diel oscillations were only apparent under conditions of baseflow, with diel variability at ~12% of the mean daily concentrations of 2.5 ± 1.0 μg L− 1 d− 1 for CDOM, and 0.13 ± 0.03 μg L− 1 d− 1 for Chl-a. Similar temporal trends in oscillation to those of CDOM were also evident for 222Rn and pCO2 (Fig. 3b). 222 Rn and pCO2 timeseries observations ranged from 53 to 830 Bq m−3 and 612 (24 μM [CO2*]) to 1750 μatm (69 μM [CO2*]), respectively (flood peak excluded; Fig. 2). Streamwater was consistently supersaturated in pCO2 with respect to local atmospheric concentrations (pCO2atm 425 ± 19 μatm), with daily means averaging ~ 3.1 times higher than atmospheric values throughout the drought phase at 1332 ± 39 μatm (52 ± 1.6 μM [CO2*]) and two times higher than pCO2atm for the remainder of the study period at 852 ± 104 μatm (35 ± 4.2 μM [CO2*]). Mean diel oscillation of pCO2, excluding the 864 μatm decline observed during the onset of flooding, was 609 ± 143 μatm d− 1 (23 ± 4.9 μM d−1 [CO2*]) and 214 ± 72 μatm d− 1 (8.2 ± 2.2 μM d− 1 [CO2*]) for the drought and remaining non-flood phases, respectively. The minimum pCO2 observation of 612 μatm occurred during the rising limb of the storm hydrograph just prior to pump failure (data gap in Fig. 2). The maximum of 1750 μatm occurred at ~ 12 pm midday on 6th March during the drought phase; coinciding with the highest rates of mean daily solar exposure (MDSE), PAR and streamwater temperature observed throughout the event (25.1 MJ m − 2 , 834 μE s− 1 m − 2 and 22.0 °C, respectively). Mean daily 222Rn concentrations were highest during the drought phase

The highest mean daily [DOC] occurred during the initial-flush (624 μM) while the lowest mean daily [DOC] occurring during the recovery phase (255 μM), with an overall average of 367 ± 90 μM. Mean daily [DIC] ranged from 29 to 138 μM, with an overall average of 69 ± 28 μM. Daily mean POC concentrations ranged from 20 to 93 μM, with an overall average of 38 ± 18 μM and a maximum instantaneous concentration of 118 μM. 3.4. Downstream exports of carbon DOC dominated the downstream exports of carbon, ranging from a mean daily minimum of 0.180 ± 0.016 mmol C m−2 [catchment area (CA)] d−1 during the drought to a maximum of 46.9 mmol C m−2 d−1 during the flood peak (Table 2), representing a 261-fold increase. Mean daily export of DOC under ‘normal’ flow conditions (i.e. excluding the flood phase) was 0.515 ± 0.359 mmol C m−2 d−1. Downstream DIC exports were much less variable than those of DOC, ranging from a consistent daily mean of 0.097 ± 0.038 mmol C m−2 CA d−1 during all phases other than flood, to a maximum of 0.889 mmol C m−2 d−1 during peakflow. Overall, POC exports displayed the greatest variability increasing from a mean daily minimum of 0.015 ± 0.002 mmol C m−2 CA d−1 under drought-flow to a maximum of 9.12 mmol C m−2 d−1 during the flood maxima, representing a 608-fold increase. 3.5. Atmospheric evasion of CO2 Site-specific estimates of k600 varied both spatially and temporally (Fig. 4). Modelled mean daily riffle k600 values were lowest during the drought (25.2 ± 0.3 m d− 1) and highest during peak-discharge [54.3 ± 5.2 m d−1; 1 in 2 yr Average Recurrence Interval (ARI) storm conditions; Table 2], with an overall mean of 41.1 ± 9.7 m d−1. In contrast, k600 pool flux ranged from 0.7 to 27.1 m d−1 (n.b. excluding the flood maxima when there were no pools present), with a mean daily transfer velocity of 1.9 ± 0.5 m d−1. Per stream area, mean daily riffle CO2–C fluxes were lowest during the flood event (501 ± 20 mmol C m− 2 [stream area (SA)] d− 1) and peaked during the initial-flush (1016 mmol C m−2 SA d−1), with a maximum instantaneous flux of 1711 mmol C m−2 SA d−1. Under ‘normal’ flow conditions, mean daily evasion of CO2–C from the stream surface ranged 19-fold from 34 ± 9.6 to 660 ± 127 mmol C m−2 SA d−1 for the pools and riffles, respectively. For comparison with downstream carbon exports CO2

Table 1 Mean daily values (±SD) for key parameters across each observed hydrographic phase. Where, DO is dissolved oxygen, pCO2 is the partial pressure of carbon dioxide in water, [CO2*] is the sum of sum of dissolved CO2 (CO2(aq)) and carbonic acid (free H2CO3), and POC, DOC and DIC are particulate organic, dissolved organic and inorganic carbon, respectively.

Rainfall (mm) Discharge (m3 s−1) Water temp. (°C) Sp. cond. (μs cm−1) Turbidity (NTU) 222 Rn (Bq m−3) DO (μM) pH pCO2 (μatm) [CO2*] (μM) DIC (μM) DOC (μM) POC (μM)

Drought

First flush

Flood

Recovery

Baseflow

0.0 0.06 ± 0.01 20.9 ± 0.02 65.4 ± 0.32 0.0 679 ± 16 229 ± 0.28 5.90 ± 0.04 1332 ± 38 52 ± 1.6 136 ± 2.4 432 ± 12 35 ± 1.2

33.6 0.36 20.3 60.8 0.0 381 251 5.86 1069 42 84 ± 10 598 ± 36 40 ± 11

269.6 4.07 ± 5.7 19.7 ± 0.22 49.9 ± 4.7 3.4 ± 4.5 143 ± 36 265 ± 3.8 5.61 ± 0.30 732 ± 48 29 ± 1.9 42 ± 15 407 ± 102 49 ± 29

7.1 0.23 ± 0.11 20.0 ± 0.30 52.6 ± 1.9 0.0 320 ± 66 259 ± 2.1 6.09 ± 0.05 853 ± 81 33 ± 3.2 57 ± 11 287 ± 29 24 ± 2.1

24.2 0.18 ± 0.01 18.9 ± 0.48 55.2 ± 0.40 0.0 370 ± 41 263 ± 3.1 6.30 ± 0.08 913 ± 54 35 ± 2.1 75 ± 1.0 334 ± 56 47 ± 12

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Fig. 3. Timeseries measurements for (a) chromophoric dissolved organic matter (CDOM) and chlorophyll-a (Chl-a) during the first-flush and flood; and (b) 222Rn, [CO2*] and CDOM during the final phase of baseflow.

evasion was also determined per catchment area (CA). Mean daily CO2– C evasion rates ranged from 0.304 to 1.31 mmol C m−2 CA d−1, with an average of 0.522 ± 0.230 mmol C m−2 CA d−1.

CO2 may constitute the dominant component of fluvial carbon flux from other freshwater ecosystems under conditions of mean annual flow (Humborg et al., 2010; Butman and Raymond, 2011), our results revealed a clear shift in the carbon balance from a system dominated by CO2 evasion during drought conditions to a system dominated by downstream advection of DOC during moderate-to-high flow. To better constrain the function and significance of headwater streams in the global carbon cycle it is important to refine our knowledge of instantaneous, diel, phase-specific and whole-event scale biogeochemical processes. As a step towards this we will discuss: (i) sensitivity analysis of the CO2 evasion model; (ii) the dominant driving forces of carbon flux throughout each phase of the event; and (iii) the carbon source/sink behaviour of subtropical headwater streams.

3.6. Stream carbon budget Based on daily averages, the total net flux from upper Rocky Creek across the 23 days of sampling totalled some 1.06 × 106 mol C. Of this, the majority was DOC (72.4%), followed by CO2 evasion (11.9%), POC (11.6%) and DIC (4.1%), respectively. Downstream carbon fluxes were highest during the flood phase, with the four days of storm-flow accounting for 87.1% of DOC, 92.2% of POC, and 56.4% of DIC total exports. In contrast, CO2–C evasion was highest during periods of low discharge with the flood phase accounting for only 29.2% of the total CO2–C flux. Under ‘normal’ flow conditions downstream exports as DOC dominated the stream carbon budget (43.1 ± 10.2%), while the outgassing of CO2– C was only the dominant component during the drought (66.4 ± 0.4%).

4.1. Sensitivity analysis of CO2 evasion estimates Due to the dynamic nature of event-driven hydrodynamics observed within upper Rocky Creek, the CO2 outgassing component represents the highest degree of uncertainty in local carbon budgets. The scaling of CO2–C fluxes during the flood phase is complicated by the limited number of higher-discharge measurements included in the source data sets used for the k parameterizations (median discharge, 0.54 m3 s−1; Raymond et al., 2012). By simply using mean stream

4. Discussion This study provided detailed observations of carbon dynamics within a pristine subtropical rainforest stream over contrasting hydrologic regimes of drought, flash-flooding and recovery. While degassing of

Table 2 Modelled mean daily k600 values and CO2–C evasion rates expressed per stream area (SA) and per catchment area (CA) for each of the five hydrographic phases (±SD; range of instantaneous fluxes in parentheses). Shown too are the mean daily lateral fluxes of POC, DOC and DIC as well as total carbon flux (TCF) for each phase. Phase

Drought First-flush Flood Recovery Baseflow

Riffles

Pools

k600 (m d−1)

CO2–C flux (mmol C m−2 SA d−1)

k600 (m d

25 ± 0.3 (21–29) 39 (34–45) 54 ± 5.2 (44–62) 43 ± 3.5 (38–47) 38 ± 1.9 (35–43)

879 ± 49 (483–1303) 1016 (474–1711) 501 ± 20 (300–786) 628 ± 47 (440–825) 633 ± 93 (341–1074)

1.1 ± 0.1 (0.7–2.3) 2.4 (1.9–3.2) – – 2.1 ± 0.5 (1.5–2.9) 1.9 ± 0.1 (1.4–2.7)

Lateral and vertical C Flux −1

)

CO2–C flux (mmol C m−2 SA d−1)

R/P ratio

CO2–C (mmol C m−2 CA d−1)

DIC (mmol C m−2 CA d−1)

DOC (mmol C m−2 CA d−1)

POC (mmol C m−2 CA d−1)

TCF (mmol C m−2 CA d−1)

39 ± 4.6 (14–122) 59 (36–104) – – 30 ± 1.9 (21–47) 32 ± 6.3 (17–52)

1:2

0.50 ± 0.06 (0.46–0.54) 0.74 (0.67–0.80) 0.80 ± 0.36 (0.49–1.3) 0.39 ± 0.02 (0.36–0.42) 0.43 ± 0.07 (0.30–0.55)

0.057 ± 0.005 (0.05–0.06) 0.15 (0.09–0.21) 0.54 ± 0.40 (0.18–0.89) 0.09 ± 0.03 (0.06–0.13) 0.10 ± 0.01 (0.09–0.11)

0.18 ± 0.02 (0.17–0.19) 1.1 (0.60–1.6) 15 ± 22 (1.3–47) 0.50 ± 0.30 (0.25–1.1) 0.45 ± 0.07 (0.37–0.54)

0.015 ± 0.002 (0.01–0.02) 0.082 (0.03–0.13) 2.5 ± 4.4 (0.1–9.1) 0.041 ± 0.02 (0.02–0.08) 0.063 ± 0.02 (0.05–0.08)

0.75 ± 0.06 (0.69–0.81) 2.1 (1.4–2.7) 19 ± 27 (1.9–58) 1.0 ± 0.35 (0.71–1.7) 1.1 ± 0.11 (0.88–1.1)

1:1 1:0 1:1 1:1

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Fig. 4. Timeseries trends of (a) the gas transfer velocity (k600) for the riffle and pool sub-reaches, with ranges depicted as broken lines and values used in this study shown in grey; and (b) instantaneous CO2–C evasion rates expressed per stream area, including flux dynamics at various riffle/pool ratios.

reach slope, hydraulic geometry and flow characteristics for the entire stream length across all five hydrologic phases and not accounting for temporal changes in stream reach width and R/P ratios, the specific flux dynamics for each unique flow-regime can be misrepresented (see Fig. 4). Furthermore, if changes in stream surface area associated with increased discharge are not factored into the modelling when scaling to per catchment area, these errors are compounded. Factored for this variability in flow via proportional scaling between riffle- and pool- sub-reaches, the refined modelling approach used here produces instantaneous k600 values ranging from 9.2 to 54.7 m d− 1, with a daily mean of 20.9 m d−1 under baseflow conditions; and instantaneous CO2–C evasion rates ranging from 177 to 905 mmol C m− 2SA d− 1, with a daily mean of 374 ± 83 mmol C m− 2 SA d− 1. For comparison, gas transfer velocities for low-order streams globally range from 1.2 to ~ 30 m d− 1, with a mean of 7.4 ± 4.5 m d− 1 (Wanninkhof et al., 1990; Richey et al., 2002; Jonsson et al., 2007; Humborg et al., 2010; Raymond et al., 2012; Peter et al., 2014; Crawford et al., 2015), and have been cited at up to 1848 m d−1 for rapids based on declines in O2 saturation deficit (Hall et al., 2012). Additionally, overall evasion (374 ± 83 mmol C m−2 SA d−1 in this study) is lower than rates reported for most tropical, subtropical, and boreal catchments which range from 541 ± 182 mmol C m−2 d−1 for streams and rivers of the USA (Butman and Raymond, 2011), 429 ± 78 mmol C m−2 d−1 in central Amazonia (Rasera et al., 2013), and 22 to 3820 mmol C m−2 d−1 for studies in lower-latitude climates (Hope et al., 2001; Humborg et al., 2010; Huotari et al., 2013; Wallin et al., 2013). Although the k600 values were relatively high, the low CO2 evasion rates observed in Rocky Creek throughout the study period may be attributed to low partial pressures of streamwater pCO2 (mean daily pCO2 of 1332 ± 39 and 852 ± 104 μatm d−1 for the drought and remaining phases, respectively) counterbalancing the high k600 values.

4.2. Shifting drivers of carbon biogeochemistry Three primary drivers of carbon cycling within aquatic systems are: (i) groundwater seepage (Johnson et al., 2008; Maher et al., 2013; Macklin et al., 2014); (ii) precipitation (Hinton et al., 1997; Jung et al., 2012; Bass et al., 2011, 2014; Gatland et al., 2014); and (iii) the balance between biological photosynthesis and respiration (Cole et al., 2000; Cole and Caraco, 2001; Lovett et al., 2006; Maher and Eyre, 2012). Event-driven shifts in these drivers and key trends are analysed here in each phase.

4.2.1. Drought Under conditions of low-flow there were no correlations evident between discharge and pCO2 (r2 = 0.01, n = 129, p N 0.05), yet there was a significant positive relationship between 222Rn and pCO2 (r2 = 0.44, n = 129, p b 0.05; Fig. 5a, b) indicating that groundwater seepage was a primary contributing source of CO2 to streamwaters during this phase as observed in other aquatic systems (Atkins et al., 2013; Call et al., 2015; Maher et al., 2015). Concurrently, DO and pCO2 exhibited a significant negative relationship (r2 = 0.60, n = 129, p b 0.05; normalised to 20 °C; Fig. 5c), with maximum pCO2 values coinciding with the diurnal solar maxima (Fig. 2) indicating that streamwater biology was a co-driver of CO2 fluxes at this time. Instantaneous specific conductivity and [DIC] also peaked during the drought (66 μS cm−1 and 138 μM, respectively), with DIC likely derived in part from the progressive buildup of HCO− 3 from weathering (Hagedorn and Cartwright, 2010; Liu et al., 2011; Moosdorf et al., 2011). The large diel oscillations among parameters during the drought phase may be due to lower water levels, engendering the system more susceptible to diurnal biophysical processes. 4.2.2. First-flush Correlations between discharge, 222Rn, DO, and pH vs. pCO2 were highest during this phase as the exogenous forcing from rainfall dominated streamwater hydrochemistry (Fig. 5a–d). The highest DOC was observed at this time, as a result of the flushing of catchment soils that had experienced a prolonged period of dry (Chittleborough et al., 1992; Boyer et al., 1997; Strohmeier et al., 2013). This may be a direct result of altered patterns of soil organic matter mineralisation (Butterly et al., 2009; Zhu and Cheng, 2013); altered C/N ratios (Birch, 1960); altered iron (Knorr, 2013) and sulphate chemistry (Worrall and Burt, 2008); as well as additions of DOC to soil porewaters from detrital microbial biomass and desiccated soil matrices (Navarro-García et al., 2012; Ouyang and Li, 2013; Pohlon et al., 2013; Shi and Marschne, 2014). This influx of DOC coincided with a large spike in 222 Rn, which is indicative of an initial flushing of shallow groundwater (Fig. 2). 4.2.3. Flood During the flood, 222Rn, pCO2 and DO approached near equilibrium with their respective atmospheric concentrations, indicating a ‘resetting’ of the hydrochemistry in each previously isolated pool microcosm. Accordingly, correlations between 222Rn, DO and pH vs. pCO2 became

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Fig. 5. Relationship between dissolved carbon dioxide ([CO2*]) and (a) discharge, (b) radon (222Rn), (c) dissolved oxygen (DO) and (d) pH. Concentration-discharge driven hysteretic loops trended clockwise for DO vs. pCO2 and anticlockwise for 222Rn- and pH- vs. pCO2. Pearson (r2) and Spearman (rs) correlations are also shown for each specific phase of the event-hydrograph; where n is equal to 129, 72, 163, 336 and 265 for the phases of drought, first-flush, flood, recovery and baseflow, respectively. All are significant (p b 0.05) except those marked with *.

weaker during this period (Fig. 5b–d). A series of storm surges (as indicated by Chl-a/CDOM timeseries trends; Fig. 3a) produced hillslope surface runoff, the sloughing of photosynthetic biofilms and the resuspension of sediments; generating the highest observed POC concentrations. At the same time, dilution lowered DIC and specific conductivity. Acidification of stream waters also occurred as the floodwaters drained from the catchment, with pH falling to below that of rainfall (rainwater 5.41, stream pH minima 4.82) leading to a shift in the carbonate equilibrium towards [CO2*]. This acidification suggests shallow subsurface flow paths contributed significantly to post-event drainage waters as observed elsewhere (e.g. Johnson et al., 2006, 2008; de Weys et al., 2011).

Fig. 6). The temporal sinuosity of diel oscillation for both DO and [CO2*] changed from a midday maxima to an early morning maxima pre- and post-flooding (Fig. 7a, c), with a significant decrease in the magnitude of oscillation for each parameter. The relationship between these two variables was relatively ‘balanced’ during the recovery phase (Fig. 7b), with the non-linear portion of the diel DO/CO2 relationship occurring at dawn and dusk (Fig. 6) describing a shift in dominance of autotrophic vs. heterotrophic metabolism for each time period, respectively. As the stream shifts towards greater rates of net heterotrophy over time, the diel oscillation amplitude may increase in a relatively predictable fashion as indicated in Fig. 7a and c. This could provide guidance when formulating discrete streamwater sampling strategies for DO and CO2.

4.2.4. Recovery The lower mean daily concentrations of DOC and POC observed during this phase were indicative of either the depletion of a finite source (Boyer et al., 1997), a change in the origin of organic-C within the catchment (Boyer et al., 2000; Ågren et al., 2008), or a shift in paths of contrasting transmissivity (Knorr, 2013). A gradual increase in the amplitude of diel oscillation was evident for DO and pCO2 (Fig. 2). This may be attributed to both increased rates of aquatic respiration resulting from the re-establishment of post-flood biotic populations (Barbosa and Chícharo, 2011; Décamps, 2011; Majdi et al., 2012) and the reactivation of biofilms that were periodically submerged during high-flow (Donato et al., 2014; Falasco et al., 2014). Diel hysteretic loops between [CO2*] and DO reflected these internal biological processes, with overall trends shifting from non-linear to linear as the event progressed through time (r2 = 0.46, n = 336, p b 0.05;

4.2.5. Baseflow A minor 14.3 mm precipitation event during the baseflow period resulted in erratic and less predictable diel oscillations for most parameters, with weakened relationships between discharge, 222Rn, DO and pH vs. pCO2. This suggests that external forcing from minor rain events may override the potential predictability of diel trends, which should also be factored for when sampling streamwater gases. 4.2.6. All phases Due to fewer instantaneous measurements of discrete vs. continuous sampling (n = 52 to 67 vs. 965), correlations of parameters measured via discrete samples can only be reliably drawn on a whole-event basis and exclude the influence of diel cycles. DOC did not correlate significantly with any other parameter. A lack of relationship between DOC with DIC suggests different sources and sinks within the catchment, with bulk-DOC

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Fig. 6. Relationship between dissolved oxygen (DO) and [CO2*] during the recovery phase shifted from (a) non-linear (red) to (b) linear as the event progressed through time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

most likely derived from more surficial flow paths within the soil profile (see Bishop et al., 2004; Seibert et al., 2009; Lyon et al., 2011; Grabs et al., 2012; Mei et al., 2012), and a DIC-only sink of atmospheric evasion also leading to a decoupling of the relationship between DIC and DOC. DIC on the other hand correlated positively and significantly with 222Rn (r2 = 0.81, n = 52, p b 0.05) suggesting its origins were from groundwater (e.g. Johnson et al., 2008; Savoy et al., 2011). At the whole-event scale, concentration-discharge driven hysteretic loops were evident for 222Rn, DO and pH vs. pCO2 (Fig. 5b–d). In addition, diurnal trends in pCO2 and DO coincide with a narrow 2 to 3 h window of PAR activity. This may be attributed to the site-specific aspect of the sampling site (i.e. north/ south stream orientation; Fig. 1a), which governs the shading patterns of riparian vegetation (to 60 m).

Fig. 7. Temporal changes in the amplitude and sinuosity of diel timeseries data for (a) the percent saturation of carbon dioxide in water (red) and (c) dissolved oxygen percent saturation (green) pre- and post-flood (6 March, drought; 6 April, recovery). During the recovery phase the diel oscillations for both variables are relatively balanced (b). It is expected that as catchment hydrology shifts towards drought net heterotrophy gradually predominates the balance, with predicted changes as indicated by the arrows. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.3. An ancient subtropical carbon sink? DOC concentrations within Rocky Creek remained moderate-tohigh throughout all hydrologic regimes relative to those reported for other similar streams (sampled range 255 to 624 μM, reported range 75 to 441 μM; Johnson et al., 2008; Bass et al., 2011, 2014; Jaffé et al., 2012; Huang et al., 2013); exhibiting moderate rates of DOC export under conditions of ‘normal’ flow (mean daily export 0.515 ± 0.359 mmol C m−2 CA d−1, reported range 0.066 to 1.03 mmol C m−2CA d−1; see Johnson et al., 2006). In contrast, the observed DIC concentrations of 29 to 138 μM were well below the reported range (191 to 741 μM; Zhou et al., 2013; Bass et al., 2014), with the mean daily DIC export rate of 0.097 ± 0.038 mmol C m−2 CA d−1 under ‘normal’ flow conditions reflecting this trend (cited range 0.043 to 0.657 mmol C m−2 CA d−1; Johnson et al., 2006). POC also remained relatively low throughout the sampling period (sampled range 20 to 93 μM, cited range 40 to 222 μM; Moreira-Turcq et al., 2003; Bass et al., 2011), and was extremely low during peak-flow when compared to other storm events of similar magnitude [maximum POC in this study 118 μM, cited POC concentration maxima in Bass et al. (2014) and Dhillon and Inamdar (2014) of 1532 and 20,980 μM, respectively]. Using these mean daily fluxes of carbon that encompass the range of conditions from low to high flow, first-order estimates can be made for the carbon source/sink capacity of upper Rocky Creek. Average regional terrestrial net primary productivity (NPP) has been calculated to be in the order of 330 mmol C m− 2 d− 1, which is 3.8 times the terrestrial global average NPP of 87 mmol C m−2 d−1, (Haverd et al., 2013a,b). This represents the highest reported rate of NPP on the Australian mainland. During the sampling period, mean net carbon flux from Rocky Creek was equivalent to 1.05 ± 0.107 mmol C m−2 CA d−1 under conditions of baseflow, 18.6 ± 26.5 mmol C m−2 CA d−1 during the flood, and 4.72 ± 7.81 mmol C m−2 CA d− 1 across all event phases (Table 2). These values place net aquatic carbon flux at 0.3 ± 0.1% of daily NPP during baseflow, 5.6 ± 7.6% under conditions of 1 in 2 yr ARI flooding (BoM, 2014b), and 1.4 ± 2.3% across all flow regimes. We highlight that these represent first-order estimates and do not factor for other considerations of net ecosystem exchange (NEE) such as the proportion of young- to old-growth forest within the catchment, inter-seasonal controls on carbon cycling dynamics, export of carbon via deep groundwater flow, or the influence of multiple smaller storm events on total carbon exports. For comparison, the aquatic carbon exports observed in Rocky Creek during this study were similar to those found in other forested headwater catchments, which were estimated to be 1.6% of local NEE in temperate and 2.9% of NEE in tropical watersheds (Shibata et al., 2005; Zhou et al., 2013).

5. Conclusions We report the first fluvial carbon budgets of a ‘pristine’ Australian subtropical rainforest headwater catchment. Our study quantifies the relative importance of lateral exports of individual carbon forms (DIC,

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DOC, POC) versus CO2 evasion across a drought–flood–baseflow hydrological cycle, and discusses the influence of biophysical processes (e.g. ecohydrology, environmental forcing, streambed morphology and groundwater flow) in controlling carbon dioxide evasion. Across the duration of the study CO2 outgassing accounted for between 58.8% and 80.3% (average 66.4%) of total daily aquatic net carbon flux during low flow (b0.07 m3 s−1), between 32.2% and 42.5% (average 39.1%) under conditions of moderate flow (0.07 to 0.4 m3 s− 1), and between 4.0% and 4.7% (average 4.2%) during times of high flow (N0.4 m3 s−1). In contrast, DOC export displayed a positive relationship with discharge at 24.1% of total net carbon flux during low flow, 45.6% under conditions of moderate flow, and 79.2% during times of high flow. The observed sinuosity and amplitude of diel trends changed considerably with flow regime. Days influenced by precipitation displayed more erratic oscillations, while days with higher insolation and under conditions of lowflow displayed broader and more predictable trends. This was shown to be the direct result of the complex interaction of multiple synergistic biotic- and abiotic-drivers shifting in importance according to the prevailing flow regime. Across all flow regimes, the stream carbon losses were equivalent to 1.4 ± 2.3% of the calculated regional net terrestrial primary productivity, implying that undisturbed Australian subtropical forests are effective in retaining carbon. Considering the mean 18-fold increase in total carbon exports between baseflow and flooding, further research is required to account for extremes of both low and high flows as well as the periodicity, intensity, and duration of episodic stormflow events when assessing the inter-annual carbon mass balance dynamics of low-order streams. In addition, to further refine estimates of greenhouse gas emissions from inland waters globally, the variability of diel oscillations of gaseous carbon fluxes should be incorporated into regional and local scale carbon flux models and monitoring programmes. Conflict of interest The authors declare no conflict of interest. Acknowledgments Special thanks are given to Rodney Holland, Anthony Acret, Paul Macklin, Mahmood Sadat-Noori and Andrew Faye for their assistance with fieldwork and logistics; and to Matheus Carvalho and Tyler Cyronak for their support in processing of samples. We acknowledge funding from the Australian Research Council (LP130100498, LE120100156, and DE150100581). References Abril, G., Commarieu, M.V., Sottolichio, A., Bretel, P., Guerin, F., 2009. Turbidity limits gas exchange in a large macrotidal estuary. Estuar. Coast. Shelf Sci. 83, 342–348. http:// dx.doi.org/10.1016/j.ecss.2009.03.006. Ågren, A., Buffam, I., Berggren, M., Bishop, K., Jansson, M., Laudon, H., 2008. Dissolved organic carbon characteristics in boreal streams in a forest–wetland gradient during the transition between winter and summer. J. Geophys. Res. 113, G03031. http://dx.doi. org/10.1029/2007JG000674. Atkins, M.L., Santos, I.R., Ruiz-Halpern, S., Maher, D.T., 2013. Carbon dioxide dynamics driven by groundwater discharge in a coastal floodplain creek. J. Hydrol. 493, 30–42. http://dx.doi.org/10.1016/j.jhydrol.2013.04.008. Barbosa, A.B., Chícharo, M.A., 2011. Hydrology and biota interactions as driving forces for ecosystem functioning. Treatise on Estuarine and Coastal Science. Elsevier Publishing, p. 9. Bass, A.M., Bird, M.I., Liddell, M.J., Nelson, P.N., 2011. Fluvial dynamics of dissolved and particulate organic carbon during periodic discharge events in a steep tropical rainforest catchment. Limnol. Oceanogr. 56, 2282–2292. http://dx.doi.org/10.4319/ lo.2011.56.6.2282. Bass, A.M., Munksgaard, N.C., Leblanc, M., Tweed, S., Bird, M.I., 2014. Contrasting carbon export dynamics of human impacted and pristine tropical catchments in response to a short-lived discharge event. Hydrol. Process. 28, 1835–1843. http://dx.doi.org/ 10.1002/hyp.9716. Battin, T.J., Kaplan, L.A., Findlay, S., et al., 2008. Biophysical controls on organic carbon fluxes in fluvial networks. Nat. Geosci. 2, 95–100. http://dx.doi.org/10.1038/ngeo101. Battin, T.J., Luyssaert, S., Kaplan, L.A., Aufdenkampe, A.K., Richter, A., Tranvik, L.J., 2009. The boundless carbon cycle. Nat. Geosci. 2, 598–600. http://dx.doi.org/10.1038/ngeo618.

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