leaky savannas: Significance of lateral carbon fluxes in seasonal tropics

Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/280875023

Leakysavannas:thesignificanceoflateralcarbonfluxesintheseasonaltropics

ARTICLEinHYDROLOGICALPROCESSES·AUGUST2015

ImpactFactor:2.7·DOI:10.1002/hyp.10640

6AUTHORS,INCLUDING:

AdrianMBass

UniversityofWesternSydney

23PUBLICATIONS126CITATIONS

SEEPROFILE

NielsCMunksgaard

CharlesDarwinUniversity

55PUBLICATIONS616CITATIONS

SEEPROFILE

MichaelIBird

JamesCookUniversity

243PUBLICATIONS5,642CITATIONS

SEEPROFILE

Availablefrom:MichaelIBird

Retrievedon:23August2015

http://www.researchgate.net/publication/280875023_Leaky_savannas_the_significance_of_lateral_carbon_fluxes_in_the_seasonal_tropics?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_2

http://www.researchgate.net/publication/280875023_Leaky_savannas_the_significance_of_lateral_carbon_fluxes_in_the_seasonal_tropics?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_3

http://www.researchgate.net/?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_1

http://www.researchgate.net/profile/Adrian_Bass?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_4


http://www.researchgate.net/institution/University_of_Western_Sydney?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_6


http://www.researchgate.net/profile/Niels_Munksgaard?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_4


http://www.researchgate.net/institution/Charles_Darwin_University?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_6


http://www.researchgate.net/profile/Michael_Bird6?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_4


http://www.researchgate.net/institution/James_Cook_University?enrichId=rgreq-e2a2dda6-bd42-4796-9f12-79522a9f8eff&enrichSource=Y292ZXJQYWdlOzI4MDg3NTAyMztBUzoyNjI5NDEzNDU4NDExNTJAMTQzOTcwMTUxMDE3Nw%3D%3D&el=1_x_6


This article is protected by copyright. All rights reserved.

Leaky savannas: the significance of lateral carbon fluxes in the seasonal tropics

Tweed S.1,2,3*

, Leblanc M.1,3,4

, Bass A.2,5

, Harrington G.A.1,6

, Munksgaard N.2,7

, Bird M.I.2

1National Centre for Groundwater Research and Training, Australia.

2Centre for Tropical Environmental and Sustainability Science, College of Science,

Technology and Engineering, James Cook University, Cairns, Australia 3IRD, UMR G-eau, Montpellier, France

4Hydrogeology Department, University of Avignon, Avignon, France

5Hawkesbury Institute for the Environment, University of Western Sydney, Science Road,

Richmond, Australia. 6Innovative Groundwater Solutions, Adelaide, Australia

7Research Institute for the Environment and Livelihoods, Charles Darwin University,

Australia

*Corresponding author's email address: [email protected]

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/hyp.10640


Abstract

Globally, Dissolved Inorganic Carbon (DIC) accounts for more than half the annual flux of

carbon exported from terrestrial ecosystems via rivers. Here we assess the relative influences

of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river

catchment characterized by distinct land cover, climate and geology transition from the wet

tropical mountains to the low lying savanna plains. Processes controlling changes in river

DIC were investigated using dissolved organic carbon (DOC), particulate organic carbon

(POC) and DIC concentrations and stable isotope ratios of DIC (13

CDIC) at two time scales;

seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon

Analyser (ISO-CADICA) was used to measure diel DIC concentration and 13

CDIC changes at

a 15 minute temporal resolution. Results highlight the predominance of biologically mediated

processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in

DIC concentrations varying between 3.55-3.82 mg/L, and 13

CDIC values ranging from -

19.7±0.31 to -17.1±0.08 ‰. In contrast, at the seasonal scale we observe wet season DIC

variations predominantly from mixing processes, and dry season DIC variations due to both

mixing processes and biological processes. The observed wet season increases in DIC

concentrations (by 6.81 mg/L) and 13

CDIC values of river water (by 5.4 ‰) largely result

from proportional increases in subsurface inflows from the savanna plains (C4 vegetation)

region relative to inflows from the rainforest (C3 vegetation) highlands. The high DIC river

load during the wet season results in the transfer of 97% of the annual river carbon load.

Therefore, in this gaining river there are significant seasonal variations in both the

hydrological and carbon cycles, and there is evidence of substantial coupling between the

carbon cycles of the terrestrial and the fluvial environments. Recent identification of a

substantial savanna carbon sink in wetter years in the recent past does not take into account

the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the

atmosphere.

Key words: dissolved inorganic carbon, diel, seasonal, carbon cycle, tropics, savanna.


1. Introduction

Rivers are the major conduits through which a proportion of terrestrial net ecosystem

productivity is transferred to the ocean as dissolved organic carbon (DOC), particulate

organic carbon (POC) and dissolved inorganic carbon (DIC). Aufdenkampe et al. (2011)

estimated that of the 2.7 Pg C yr-1

exported from the land to inland waters, 1.2 Pg is returned

to the atmosphere by CO2 evasion, 0.6PgC is stored as sediments in inland water bodies and

0.9 Pg is exported to the ocean. Around half of the carbon is exported to the ocean as organic

carbon, with the other half exported as DIC either by rivers or from direct groundwater

discharge (Cole et al., 2007). DIC is responsible for more than half of the annual flux of

carbon from terrestrial ecosystems, via inland waters to the atmosphere by evasion, or to the

global ocean.

Riverine DIC is therefore an important and dynamic component of the global carbon cycle

(e.g. Telmer and Veizer, 1999; Cartwright, 2010; Bass et al., 2014; Tobias and Böhlke, 2011;

Brunet et al., 2005; Aufdenkampe et al., 2011), and the changes in river DIC loads are

affected by a range of biogeochemical processes (e.g. Bouillon et al., 2014). For example, in

tropical rainforests, the high loading of river water with particulate and dissolved organic

matter can result in high CO2 concentrations, depleted 13

CDIC values and reduced pH due to

biotic respiration (Wetzel, 2001). In comparison, during photosynthesis the CO2 is consumed

and 13

CDIC values increase (Waldron et al., 2007), with reported fractionation factors of

13

CDIC during photosynthesis ranging from 0.971-0.983 (Falkowski and Raven, 1997;

Gammons et al., 2011; Tobias and Bohlke, 2011). In the absence of elevated concentrations

of ammonium, or a highly eutrophic environment (Gammons et al., 2011), rapid

photosynthesis can also increase the pH of water during the consumption of CO2 (Wetzel,

2001).

The precipitation of carbonate minerals can play a dominant role in controlling the river DIC

concentrations (e.g. Tobias and Böhlke, 2011), and within the pH range 5.0 to 7.0 the

speciation between CO2(aq) and HCO3- may play a role in fractionating

13CDIC values (e.g.

Waldron et al., 2007). In the headwaters of catchments, there is also a high potential for the

degassing of CO2 from rivers to the atmosphere (e.g. Doctor et al., 2008). In addition, in

gaining surface waters the mixing dynamics of different waters (rainfall, overland flow, soil

water and groundwater) can also affect DIC concentrations. This results from the distinct DIC

concentrations (and potentially 13

CDIC values) of source waters. An example would be

mixing between atmospherically equilibrated low-DIC rainwater with soil water or

groundwater containing dissolved soil CO2 that has been respired from plants that use the C3

or C4 photosynthetic pathway, and/or dissolution of carbonates (e.g. Cane and Clark, 1999;

Doctor et al., 2008; Schulte et al., 2011).

The relative role of each of these processes varies both spatially (Waldron et al., 2007;

Dawson et al., 2001) and temporally (Tobias and Böhlke, 2011). There have been many

recent studies that have highlighted the importance of these processes influencing surface

water chemistry over the diel-scale (e.g. Hayashi et al., 2012), and in particular the changes in

carbon concentrations (e.g. de Montety et al., 2011; Gammons et al., 2011). Some studies

have found that diel-scale processes can significantly influence the DIC cycling in large river

systems (e.g. Tobias and Böhlke, 2011).

The majority of riverine carbon cycle studies to date have focused on rivers in the mid- to

high latitudes (e.g. Cai, 2003,), or major rivers in the ever wet tropics such as the Amazon


(e.g. Mayorga et al., 2005). Less attention has been paid to rivers draining the seasonally wet

savanna-dominated tropics. Tropical savannas (mixed tree-grass systems) represent a globally

significant biome that covers ~12% of the earth’s surface but account for 30% of global

primary productivity (Grace et al., 2006). Primary productivity in savannas is closely linked

to a distinct ‘wet season’ with high terrestrial productivity (and respiration) coincident with a

seasonal peak in discharge from rivers draining savanna regions. During the intervening dry

season, terrestrial productivity is low and river discharge either ceases, or decreases

dramatically with groundwater inflows becoming more important (King et al., 2015). It is

therefore reasonable to expect that there may be tighter coupling between the terrestrial and

riverine carbon cycles than is the case in less seasonal environments.

Tropical savannas cover 25% of Australia, and represent the dominant biome of the seasonal

tropics of northern Australia. More than 60% of river discharge from the Australian continent

occurs along the northern coast from catchments dominated by savanna vegetation, with this

discharge concentrated over a three to five month ‘wet season’ (Petherham et al., 2008;

Warfe et al., 2011).

In this study we sought to assess the relative significance of forest and savanna ecosystems to

the riverine carbon cycle using a river that traverses distinct geographical provinces; wet

tropical rainforest mountains and a dry tropical savanna plain. The river also has distinct

variations in discharge between the wet and dry seasons, and is a gaining river system.

Therefore, the study site is well suited to addressing the following questions on carbon

cycling in tropical rivers: (1) how significant are DIC exports from wet tropical versus

seasonal tropical savanna regions; (2) what is the role of sub surface waters in the transfers of

DIC; and (3) what are the seasonal versus diel variations in DIC exports.

2. Study Site

The Mitchell River Basin covers an area of ~72,000 km2 in the north-eastern tropical region

of Australia (Fig. 1). The basin is bounded in the east by the Great Dividing Range, which

forms a steep hillslope terrain along the basin margin (surface elevations range between 200

to 1500 mAHD, Fig. 1). The remaining part of the basin is comprised of low relief plains

(elevations ranging from 0 to 500 mAHD), and is bounded in the west by the Gulf of

Carpentaria. The area for this study is located in the north-eastern upper catchment of the

Basin (Fig. 1). The drainage area for the Mitchell River sample site is approximately ~ 2500

km2, and the McLeod River (a tributary of the Mitchell) drainage area is ~500 km

2. Within

the study area there are steep surface topographic (0.04 m/m) and average annual rainfall

(2.7×10-5

mm/km) gradients from east to west, which create a distinct land cover boundary

(Fig. 1). The high rainfall region in the east is predominantly covered by tropical rainforest

that forms part of the World Heritage Wet Tropic rainforest area. To the west of these

mountains are the dry tropics plains that are covered by savanna vegetation.

Across the Mitchell River Basin, the mean annual rainfall is ~1000 mm and the mean annual

areal potential evapotranspiration (APET) is ~1900 mm. The APET exceeds rainfall during

most months except January to March (CSIRO, 2009). The annual monsoon wet season in the

region is during December to March, and high rainfall during this period creates a strong

seasonal signature in the hydrologic cycle (CSIRO, 2009). Within the study area, a rainfall

station located in the rainforest (Julatten station; Fig. 1) has a long term mean of ~1600


mm/yr (1927-2011; Bureau of Meteorology, 2012). 86% of rainfall at this station occurs

during the wet season. Located only 25 km west is the Mount Carbine station (Fig. 1) which

has a long term mean of ~920 mm/yr (1988-2011; Bureau of Meterology, 2012), of which

94% falls during the wet season. Mount Carbine station is located on the boundary of the

savanna plains that form part of the dry tropics region. During the study period, the area

experienced a relatively wet year during 2011, with an annual total of 1426 mm in the

savanna (Mount Carbine station) and 2247 mm in the rainforest (Julatten station). However,

during the dry season from 1-April to Mid-October 2011 conditions were dry; the savanna

and rainforest regions received rainfall totals of only 8 and 81 mm respectively (1 and 4 % of

total annual rainfall respectively).

High rainfall in the wet season supports an extensive surface drainage network that is fed by

groundwater inflows, and drains water from the mountain ranges in the east to the sea in the

west over ~750 km. Typical of wet and dry tropical rivers, the Mitchell River has high wet

season flows and low dry season flows. During the wet season of 2011 the average discharge

value was 153 m3/s in the upper Mitchell River, and during the dry season the average

discharge was 8 m3/s (Fig. 2). The river water depth ranged from 0.77 to 6.04 m. During the

wet season, river discharge fluctuations reached up to changes in values of ~700 m3/s over 18

days (January 2011). In comparison to other tropical rivers in this NE Australian region

(Archer, Copperfield, Flinders, Leichardt, and Staten), the Mitchell River has proportionally

high dry season discharge (median value is over 4 times greater than any of the other rivers).

Continuous dry season discharge of the Mitchell River indicates a continuous source of

inflowing subsurface waters, and these have been found to vary spatially, both in terms of

flux and water chemistry contributions (Batlle Aguilar et al., 2014).

In this study, we analyse the groundwater carbon contributions to the river from aquifers in

the rainforest and savanna regions. The changes in land cover approximately correspond to

changes in the aquifer geology. The savanna plains comprise predominantly a Late Devonian

sedimentary sequence, which is bordered by an igneous felsic intrusive unit in the rainforest

highlands (Fig. 1). Groundwater in these regional sedimentary and igneous aquifers is

recharged diffusely by seasonal rainfall. In addition, there are heterogeneously distributed

alluvial/colluvial deposits along the river banks (Fig. 1). The sand and clay content of these

alluvial deposits and are locally important in controlling groundwater recharge via bank

storage following large monsoonal flood events in the Mitchell River (Welch et al., 2014).

The mean floodplain widths in the upper catchment of the Mitchell River Basin is smaller (<

2km) compared with the lower catchment (up to 9km; Rustomji et al., 2010). In addition to

river bank inundation, during intense storm events the study area has significant lateral

contributions to river discharge from overland flow and groundwater pathways.

3. Methods

Time series of river hydrochemical data (13

CDIC, 18

O, 2H, DIC, DOC, POC, electrical

conductivity (EC) and pH) were used in conjunction with river discharge data to analyse the

seasonal changes in DIC. Groundwater data was also incorporated to elucidate the potential

controls on DIC in the upper Mitchell River.

3.1 Seasonal Sampling Sites

The locations of the sampling sites in the Mitchell River Basin are presented in figure 1, and

consist of two river and 7 groundwater sites. The main river analysed in this study is the


Mitchell River, however sampling results from the McLeod River are also presented to

determine inflows from the McLeod tributary influencing Mitchell River DIC. The sampling

sites for these rivers are located in the plains, and are 10 km (McLeod River site) and 40 km

(Mitchell River site) west of the mountain ranges (Fig. 1). Using the land use map from the

Queensland Land Use Mapping Program (1999), the land cover for the contributing areas

above the sampling sites were broadly classed as savanna or rainforest. At the Mitchell River

site the contributing areas are ~30% rainforest and ~70% savanna, compared with equal areas

of rainforest and savanna for the McLeod River site.

At these two river sites there are surface water gauging stations monitored by the State of

Queensland’s Department of Environment and Resource Management. The McLeod River

and Mitchell River were sampled in this study 25 and 27 times respectively from Dec-2010 to

Nov-2011. The sampling intervals varied during the wet and dry seasons; from December

2010 through to April 2011 (wet season) the river water was sampled at weekly intervals,

from April to June 2011 the sampling interval became fortnightly, and from June to

November 2011 the sampling interval became less frequent (Fig. 2).

Within the study area there are no groundwater bores regularly monitored by government

agencies. The groundwater sampled for this study includes 7 bores and 1 spring located in the

savanna region, and 1 spring located in the rainforest area. Groundwater samples from three

shallow bores (screened to depths < 20 m below ground level) located in the savanna region

were collected during the wet season (February-March) and dry season (October) of 2011,

and samples from the 2 springs from the savanna and rainforest areas were collected during

the dry season (August) of 2012.

3.2 Chemical Analysis

The river water and groundwater samples were analysed for 13

CDIC, DIC, 18

O, 2H, pH,

EC, and temperature. All of the surface water samples were collected in the morning to limit

diel cycling influences on parameters. However, as discussed further below a diel experiment

was also undertaken to determine processes at this scale.

The river water samples collected for 13

CDIC analysis had HgCl2 added for preservation.

Stable isotope values (18

O, 2H, and

13CDIC) of river water and groundwater were analysed

using the Finnigan MAT 252 mass spectrometer at Monash University, Melbourne. The

precision is 18

O = ±0.1‰, 2H = ±1‰, and

13CDIC = ±0.1‰.

18O values were measured

via equilibration with CO2 at 25ºC for 24 to 48 hours, and 2H were measured via reaction

with Cr (850ºC). Data were normalised following Coplen (1988) and 18

O and 2H values are

expressed relative to V-SMOW. CO2 from DIC was liberated by acidification using H3PO4 in

a He atmosphere and analysed by continuous flow. 13

CDIC values are expressed relative to

V-PDB.

Groundwater samples from boreholes were obtained using a 12V submersible pump. EC was

monitored during pumping; once stabilized, samples were collected and pH and temperature

were measured immediately at surface. For rivers, all samples were taken from flowing

waters, and EC, pH and temperature were recorded in the field. HCO3 and dissolved CO2

concentrations were also measured in the field using Hach digital titrator kits. The total

alkalinity was determined by titration to a pH between 3.7 and 5.1, and includes all

carbonate, bicarbonate and hydroxide species. Replicates of CO2 titrations revealed a poor

reproducibility; therefore DIC concentrations were calculated using the field measurements


of pH, temperature and total alkalinity (precision of ~ ±10% and detection limits was 1.64

mM) in the software CO2SYS. Since the pH range of river water is 5.93-7.51 and 6.19-7.20

for groundwater; [CO32-

] was assumed to be negligible so that DIC concentrations were

calculated as the sum of [CO2] and [HCO3-].

For analysis of DIC, in the results section we discuss the excess pCO2. This was calculated

after the method by Doctor et al. (2008), where the excess pCO2 is the ratio of the pCO2 in

water compared with the atmospheric pCO2 (10-3.5

mol/L). During degassing there is an

enrichment of 13

CDIC values in the residual DIC, and an overall reduction in DIC i.e., when

excess pCO2>1 (that is when free CO2 is saturated with respect to atmospheric CO2).

3.3 Diel experiment

Diel scale changes in the Mitchell River carbon species were analysed to determine whether

the diel differences in temperature, biological and geochemical processes were significantly

affecting DIC, DOC and POC concentrations and 13

CDIC values. To assess the significance

of these diel scale influences a sampling site in the Mitchell River was selected upstream of

the McLeod River confluence. This allowed the analysis of diel processes without the effects

of mixing between the two river waters.

During the dry season of 2012 (3-4 October; Fig. 2) the diel experiment was undertaken for

two night time and one day time period (over 36 hours). During this time, water from the

Mitchell River was continuously measured for EC, depth, dissolved oxygen, pH, and

temperature at a 15 minute resolution using a HydrolabTM

DS5X sonde. Air temperature and

solar radiation were analysed at the same interval using Hobo U30 weather stationTM

. In

addition, at 15 minute intervals DOC of the river water was measured using a S::can

Spectrolyser with a precision of ±0.15 mg/L. The instrument was originally calibrated against

manual spot samples (e.g. Bass et al. 2011), and for this study a suite of DOC standards using

sodium hydrogen phthalate were employed. Calibration was in the range of 0.1 mg/L up to 20

mg/L.

In addition, at 15 minute intervals, the DIC concentration and 13

CDIC were measured

continuously using the Isotopic Continuous Dissolved Inorganic Carbon Analyser (ISO-

CADICA), which involves an automated acidification interface coupled to a Picarro G2101-i

Cavity Ring-down Spectrometer and is described in detail in Bass et al. (2012). The precision

of the DIC concentration and 13

CDIC ISO-CADICA data is ±0.12 mg/L and ±0.2 ‰

respectively. To calibrate this data, river samples for DIC and 13

CDIC were collected during

the experiment and analysed using a Gas-Bench III headspace analyser coupled to a Delta

VPlus mass spectrometer at the James Cook University Advanced Analytical Centre.

3.4 Hydrology and water chemistry ancillary data

Climate data was acquired from the Bureau of Meteorology (2012). River discharge data for

the Mitchell and McLeod River sites are from the State of Queensland’s Department of

Environment and Resource Management (DERM, 2011). Major ion concentration data for

the Mitchell River is from DERM and Batlle-Aguilar et al. (2014). The Mitchell River major

ion data are for the period 1999-2011 and include five dry seasons and two wet seasons

sampled from the same site as the seasonal sampling in this study. This data was used to

calculate the saturation indices for calcite, aragonite, dolomite and CO2(g) in PHREEQC

(Parkhurst and Appelo, 1999).


4. Results and Interpretation

4.1 Seasonal variations in Mitchell and McLeod River carbon

Corresponding to changes in rainfall in both the rainforest and savanna regions (Fig. 3a),

during the wet season (Dec-Mar) river discharge reaches up to 757 and 255 m3/sec

respectively in the Mitchell and McLeod Rivers (Fig. 3b). In comparison, dry season (Apr-

Nov) river discharges remain 25 m3/sec in both rivers. The rivers pH and EC values show

no significant seasonal trends (Fig. 3d; Table 1). However, between the wet and the dry

seasons, there are noticeable seasonal trends observed in the 13

CDIC and DIC data. The

McLeod River13

CDIC peaks during the wet season up to -9.2 ‰, and has a linear slope of

0.04 (R2 0.45). Similarly, the maximum value for the Mitchell River is during the wet season,

where values increase from -15.5 to -10.1 ‰, with a linear slope of 0.03 (R2 0.52) (Fig. 3b).

During the first 3 months of the dry season the 13

CDIC values remain relatively high, and

then decrease during the end of the dry season to minimum values of -16.0 ‰ and -17.5 ‰

for the Mitchell and McLeod Rivers respectively (Fig. 3b). These changes in 13

CDIC values

result in dry season linear slopes of -0.20 and -0.02 for the Mitchell and McLeod Rivers

respectively (R2 0.54 and 0.63 respectively).

The increases in wet season river discharges are also accompanied by increases in DIC

concentrations for both the Mitchell and McLeod Rivers, with values peaking at to 9.19 and

6.69 mg/L respectively (Fig. 3c). The corresponding positive linear slopes are 0.04 and 0.02

for the Mitchell and McLeod Rivers respectively (R2 0.51 and 0.20 respectively). In contrast,

during the dry season, both rivers exhibit relatively constant and low DIC concentrations. The

dry season declines in DIC concentrations result in low linear slopes of -0.01 for both the

Mitchell and McLeod Rivers (R2 0.19 and 0.32 respectively). The seasonal changes in DIC

concentrations results in significant differences between wet and dry season DIC loads

exported by the rivers; particularly the Mitchell River (Fig. 4). The calculated Mitchell River

DIC exports were up to 192 tonnes/day during the wet season, whereas during the dry season

the maximum value recorded was 21 tonnes/day at the sampling sites. During 2011, the

McLeod River contributed ~15% to the total DIC load of the Mitchell River. Other sources of

DIC to the Mitchell River and controls on the large seasonal variations are discussed further

below.

DOC and POC concentrations in the Mitchell River were analysed at six intervals starting in

March and finishing in November 2011. Results show relatively low concentrations of DOC

(2.75-3.12 mg/L) and POC (2.36-2.39 mg/L) relative to DIC. DOC and POC concentrations

also remain relatively stable during the transition from the wet to dry season compared with

the decrease in DIC concentrations with river discharge changes (Fig. 3c). For example, from

March to June the slope of the linear regression trendline for DIC is -0.060, whereas for DOC

and POC the slope is -0.005 and -0.007 respectively. Towards the end of the dry season,

concentrations of DOC and POC increase in both the Mitchell and McLeod Rivers (Fig. 3c).

4.2 Diel-scale variations in Mitchell River

Changes in Mitchell River chemistry were observed over a period of two nights and one day

to assess the diel-scale processes affecting changes in DIC. During the experiment river

levels and EC values remained relatively stable (Fig. 5). The increase in solar radiation values

during daylight are followed by minor increases in the river pH, dissolved oxygen (DO), and

water temperatures, and after a small decrease in turbidity, levels recover again after 10am

(Fig. 5).


Diel changes in DIC, 13

CDIC, DOC and POC values were also observed (Fig. 6). The mean

DIC concentrations during the first and second nights differ (3.76±0.04 and 3.60±0.05 mg/L

respectively). However, nights 1 and 2 both have positive linear slopes (values of 0.004; R2

0.58 and 0.64 respectively) with DIC concentrations increasing by 0.17 and 0.21 mg/L

respectively (between 9pm and 3am). The DIC trends in nights 1 and 2 are positively

correlated (R2 = 0.59). In comparison, during daylight the DIC concentrations decrease by

0.24 mg/L (between 9am-3pm), with a negative linear slope of -0.001 (R2 0.06).

For 13

CDIC the mean values decrease over the experiment from -18.29±0.61‰ during night 1

to -18.91±0.39‰ during night 2. The daytime and night time trends in 13

CDIC are inverse to

the DIC values. The daytime 13

CDIC increases by a maximum of 1.8 ‰ with a positive linear

slope of 0.02 (R2 0.11). During nights 1 and 2

13CDIC values decrease by 2.3 and 1.7 ‰

respectively, and both have negative linear slopes (values of -0.03 and -0.05; R2 0.26 and

0.56 respectively).

DOC also increases during the daylight (by maximum of 0.16 mg/L), and remains relatively

stable between 9pm and 3am during nights 1 and 2 (7.08±0.02 and 7.14±0.02 mg/L

respectively). These DOC trends result in little correlation between day and night time values

(R2 = 0.05-0.06), and between nights 1 and 2 (R

2 = 0.03). The POC concentration changes

closely follow the trend observed in turbidity (R2 = 0.83), with an increase after 10am during

the daylight hours (by maximum of 0.16 mg/L). POC values show no significant correlation

between day and night time values (R2 = 0.008-0.009), and between nights 1 and 2 (R

2 =

0.003).

Compared with the changes observed in the Mitchell River at a seasonal scale, from the wet

to dry seasons, the diel variations in DIC concentrations and 13

CDIC values are relatively

minor. The maximum diel differences in DIC and 13

CDIC values are 0.27 mg/L and 2.6 ‰

respectively (1 = 0.09 mg/L and 0.6 ‰ respectively) (Fig. 6). In contrast, the changes in

seasonal DIC and 13

CDIC values are 6.81 mg/L and 6.1 ‰ respectively (1 = 1.57 mg/L and

1.7 ‰ respectively) (Fig. 3b). Even for DOC and POC concentrations, which were not

sampled as frequently as DIC, the diel variations (1 = 0.04 and 0.10 mg/L respectively)

remain significantly lower compared with the seasonal scale variations in DOC and POC

concentrations (1 = 0.97 and 0.45 mg/L respectively).

4.3 Controls on seasonal variations in Mitchell River carbon

The seasonal trends observed in the DIC and 13

CDIC values in the Mitchell River may be

influenced by (1) in-stream processes: evaporation, respiration, photosynthesis, degassing,

speciation and geochemical reactions, and (2) mixing with subsurface inflows.

4.3.1 In-stream processes

Wet and dry season 18

O and 2H values for Mitchell River water range from -4.7 to -9.5 ‰

and -27.9 to -63.6 ‰ respectively (Table 1). The Mitchell River, McLeod River and

groundwater 18

O and 2H values all lie close to the global and local (Munksgaard et al.,

2012) meteoric water lines (Fig. 7). The Mitchell River and the groundwater 18

O and 2H

values have the same slope as the local meteoric water line (7.1), and McLeod River only

shows a small deviation from this slope (7.2). Since evaporation would cause a fractionation

of these stable isotope values to the right of the meteoric water lines, this data indicates that


evaporation is not a dominant process influencing the seasonal changes in DIC concentrations

(or other solutes) of the river waters.

The changes in river DIC concentrations with 13

CDIC values can indicate the influence of in-

stream respiration and photosynthesis processes; where biotic respiration produces increased

CO2 concentrations and reduced pH and 13

CDIC values, and in contrast photosynthesis

consumes CO2 and pH and 13

CDIC values increase. The seasonal results from the Mitchell

River show a positive correlation between DIC concentrations and 13

CDIC values during the

wet and dry seasons (Fig. 3b,c). In addition, there is little correlation observed between

changes in the river pH values with DIC concentrations or increases in 13

CDIC (R2 values are

0.1 and 0.02 for the Mitchell and McLeod Rivers respectively; Fig. 8). Therefore respiration

and photosynthesis are unlikely to be the dominant process controlling changes in DIC.

Photosynthesis may result in the production of DOC and POC towards the end of the dry

season (Fig. 3c). However, other competing processes are affecting 13

CDIC, which results in

a net declining effect of these values.

There is degassing of CO2 from the river to the atmosphere. Mitchell River water has excess

pCO2 ratios ranging from 1 to 33 in the wet season (average of 8), and 1 to 11 in the dry

season (average of 4). The results during the wet season indicate a corresponding increase in

excess pCO2 and 13

CDIC (Fig. 9). However, trends in DIC data do not show corresponding

declines in concentrations during the wet season (Fig. 3c). Therefore, although occurring,

degassing is unlikely to be the dominant processes controlling seasonal changes in DIC

concentrations.

The Mitchell River has pH values ranging from 5.90 to 7.50 during the wet season, and 6.19

to 7.26 during the dry season. Within this pH range the speciation between CO2(aq) and HCO3-

may play a role in fractionating 13

CDIC values. The trends in pH and 13

CDIC values during

the wet and dry seasons indicate there is a weak inverse relationship (Figs. 3b and d).

Therefore hydration or dehydration between CO2(aq) and HCO3- may influence

13CDIC river

values particularly during the wet season.

In-stream geochemical reactions can affect both DIC concentrations and 13

CDIC values. The

major ions required to analyse in-stream geochemical reactions were not sampled as part of

this study. However from historical data of the Mitchell River (which have a similar range in

EC, pH and DIC values as samples from our study) the calculated SIs of calcite, aragonite

and dolomite indicate that the rivers are undersaturated with respect to these minerals during

both the wet and dry seasons (Table 2). Therefore, the precipitation of carbonate minerals is

unlikely to play a dominant role in controlling the observed decrease in river DIC

concentrations during the dry season. These results do however indicate that the dissolution

of these minerals can potentially increase DIC concentrations during the wet season and

affect isotope values.

4.3.2 Mixing with subsurface inflows

Subsurface inflows to the Mitchell River can originate from both unsaturated zone interflow

and the groundwater system. Results for shallow groundwater are presented in Table 3.

During the wet season in 2011, the groundwater sampled has pH values within the same

range as river values, whereas the groundwater EC values (135 to 21690 S/cm) and DIC

concentrations (16.47 to 347.40 mg/L) are significantly higher compared with river values

(Fig. 3b and c). The median groundwater value for 13

CDIC during the wet season (-12.0 ‰)


was close to that of the river value (-11.0 ‰), and during the dry season the median

groundwater 13

CDIC value (-12.8 ‰) was higher than the corresponding river value (-15.3

‰). For the groundwater samples, the higher range in 13

CDIC values during the wet season

compared with lower groundwater values in the dry season corresponds to the trends

observed in river water values.

13

CDIC compositions and DIC concentrations of groundwater can reflect mixing between

different sources (Fig. 10). There are both savanna C4 grasses and rainforest C3 detritus at the

Mitchell River study site (e.g. Hunt et al., 2011). 13

CDIC values of groundwaters recharged

via these two major land use types are therefore distinct. In addition, for groundwater the

dominant source of carbon is often from both soil CO2gas and dissolved carbon derived from

water-rock interactions. In this study area, carbon can be sourced from the dissolution of

freshwater carbonates. Following the modeling presented by Clark and Fritz (1997), where

groundwater interacts with freshwater carbonates in an open system and where there is C4

vegetation, the modeled groundwater 13

CDIC values range from -9.6 to -2.0 ‰. In

comparison, where there is C3 vegetation, the modeled groundwater values are lower (-23.5

to -16.0 ‰). All groundwater 13

CDIC values in this study are located in the savanna plains

(C4) and have measured 13

CDIC values ranging from -15.5 to -7.0 ‰. The 13

CDIC values for

river water measured in this study (-17.5 to -10.2 ‰) range in between the modeled values

for C3 and C4 systems. The seasonal 13

CDIC trends in this gaining river system may therefore

reflect temporal variations in subsurface inflows from the rainforest highlands (C3) and

savanna plains (C4) regions.

5. Discussion

5.1 Controls on DIC at seasonal and diel scales

In this study, the main processes driving changes in Mitchell River water DIC have been

considered. However, DIC in river systems is often influenced by many counter balancing

effects of multiple processes at varying spatial and time scales. During the wet season, the

changes in DIC in the Mitchell River cannot be entirely accounted for by in-stream biological

controls, but can reflect changes in mixing processes. The observed increase in 13

CDIC values

of river water samples during the wet season can result from proportional increases in inflows

from the savanna plains (C4) region. The corresponding increase in DIC concentrations

during the wet season may also reflect changes in DIC concentrations of inflowing

groundwater and increases in groundwater inflows due to increased groundwater to river

hydraulic gradients. The increased wet season DIC concentrations indicate proportionally

greater inflows of the more mineralised groundwater compared with overland flows from the

savanna region. In-stream mineral dissolution processes may also contribute to increases in

DIC concentrations during the wet season.

During the dry season, the results highlighted a decrease in 13

CDIC values and DIC

concentrations of river water that may also be controlled by mixing processes, and result

from relative increases in inflows from less mineralized waters from the rainforest highlands

(C3 vegetated) region. However, river DOC and POC data also indicates influences from

photosynthesis. A study by Hunt et al. (2011) at the Mitchell River found that in the dry

season the river had favourable conditions for aquatic primary production, but during the wet

season primary production was limited. In our study we see this effect on the DOC and POC

concentrations increases during the dry season. These increases are not likely the result of


subsurface inflows since groundwater generally has lower DOC and POC compared with

surface waters, nor evaporation effects because we see no fractionation in the stream 18

O

and 2H isotopes. Instead in-stream photosynthesis may increase the DOC and POC

concentrations during the dry season. The stream 13

CDIC values have a greater net effect

from mixing with waters from the rainforest area resulting in the observed dry season

declines in values.

In this study, seasonal-scale changes in 13

CDIC values and DIC concentrations of the

Mitchell River were far more pronounced than changes observed at the diel scale; which is

contradictory to other studies that have found diel-scale processes to play an important role in

the DIC cycling in river systems (e.g. Tobias and Böhlke, 2011). In addition to the

differences in the magnitude of change, the controlling processes also differed between the

two scales. Over the diel scale, riverine DIC concentrations can be affected by biologically

mediated processes (photosynthesis and respiration), geochemical processes (dissolution and

precipitation of carbonate minerals), and temperature controlled changes in the solubility

coefficients affecting dissolved gas concentrations (e.g. de Montety et al., 2011; Bass et al.,

2013). Concentrations of major ions at the diel-scale were not collected and therefore we

cannot assess the role of carbonate mineral reaction controls on diel DIC. The inverse trends

between DO and DIC values (Fig. 5 and 6) are indicative of biological controls (daytime

photosynthesis and night-time respiration). During aquatic photosynthesis there are also

associated increases in both DOC and 13

CDIC values, which corresponds with the observed

day time changes in the Mitchell River (Fig. 6). The diel temperature change (0.8°C),

13

CDIC value change (1.8 ‰), DIC and DOC concentration changes (0.24 and 0.16 mg/L

respectively) were small for a tropical environment. For example Bass et al. (2013) reported a

temperature change of 11°C, a 13

CDIC value change of 7.0 ‰, and DIC and DOC changes of

10.0 and 1.4 mg/L respectively in a tropical floodplain. However this was conducted during

the wet season when daytime temperatures reach annual maximum values, whereas our diel

experiment was during dry season conditions. Regardless of the differences in the magnitude

of diel changes, both studies in tropical waters highlight the predominance of biologically

mediated processes controlling diel changes in DIC, 13

CDIC, and DOC values. In this study,

these biologically mediated diel processes are in contrast to the seasonal-scale processes

controlling DIC changes in the Mitchell River.

5.2 Seasonal DIC cycling in tropical rivers

The terrestrial carbon exported via surface waters is the net result of a continuum of

terrestrial DIC sources, gas transfers, and sediment storage (Cole et al., 2007). However, the

role of groundwater in sequestering and releasing carbon in this dynamic system model

remains relatively unexplored. In many rivers, previous studies have demonstrated an inverse

relationship between DIC concentrations and discharge rates in both temperate rivers (e.g.

Atekwana and Krishnamurthy, 1998; Finlay 2003; Hagedorn and Cartwright, 2010) and

tropical rivers (e.g. Probst et al., 1994; Ferguson et al., 2011). More limited are the studies

that have demonstrated a corresponding seasonal increase in DIC loads with river discharge.

For example, Huang et al. (2012) reviewed the fluvial carbon export from 175 tropical rivers.

Few of these tropical rivers reviewed present seasonal time series DIC data, and none of these

show seasonally positive trends in DIC concentrations with discharge. Examples of previous

works where DIC data is positively correlated with discharge include catchments exposed to

an acceleration of permafrost thawing (Walvoord and Striegl, 2007), and land cover change

(e.g. Raymond et al., 2008). Although these studies are not in tropical systems, the similarity


with the Mitchell River is that the augmentation in DIC with discharge corresponds to an

increase in subsurface contributions to the rivers.

In the Mitchell River, the seasonal increase in the river DIC load during periods of high

rainfall and river discharge due to seasonal fluctuations in subsurface inflows. A recent study

in a separate catchment of the wet tropics in northern Australia presents an example of

increased river DIC loads during a rainfall event due to increased groundwater inflow,

however this study was conducted at the event-scale (Bass et al., 2014). In the Mitchell River,

during some of the early wet season peak river floods there is may be an inverse relationship

between DIC concentrations and discharge at this event-scale (data sampled at a higher

resolution would be required to confirm this). However, during the late wet season, when

subsurface inflow from the savanna dominates, DIC concentrations increase during periods of

high river discharge. This high component of subsurface flow to the Mitchell River from the

savanna region during the wet season supplies 97% of the annual carbon load over a 4 month

period. Therefore, in the Mitchell River, there is significant seasonal variation in both the

hydrological and carbon cycles, and the role of wet season subsurface flows is the main

driver for lateral transport of DIC from this tropical system.

This result is of significance for better understanding the carbon budget of savannas. Battin et

al. (2009) have pointed out that terrestrial carbon cycle studies rarely account for lateral

export of carbon in determining whether an ecosystem is a net source or sink of carbon,

noting that a significant proportion of carbon ‘sequestered’ by terrestrial ecosystems may

actually be returned to the atmosphere by riverine CO2 evasion. Savannas, and particularly

the savannas of northern Australia, have recently been suggested as significant carbon sinks

during wetter periods (Poulter et al., 2014; Liu et al., 2015). While it is possible that much of

this sink is returned to the atmosphere by fires in subsequent drier years, it is also possible

that a substantial proportion of any carbon ‘gained’ by savannas in wet years is lost through

‘leakage’ into rivers particularly during periods of high discharge in the wet season.

6. Conclusions

In this seasonal tropical Mitchell River system, the hydrological contrast between the

monsoon and dry seasons results in significant shifts in DIC cycling and export. Wet season

increases in river discharge and subsurface inflows result in 97% of annual DIC loads being

transferred from this terrestrial system over four months. River DIC during the wet season is

increasingly derived from mixing with subsurface savanna waters and was also influenced by

in-stream mineral dissolution processes. In comparison, during the dry season river DIC is

influenced by mixing with inflows from the rainforest region and in-stream biologically

mediated processes. At the diel scale during the dry season, the use of the ISO-CADICA to

monitor DIC concentrations and 13

CDIC values at a resolution of 15 minutes highlighted day-

time photosynthesis and night-time respiration controls on DIC. The distinctions between wet

and dry season DIC processes illustrates the significant role hydrological changes, and more

specifically subsurface transfers, can have on river carbon cycling in a seasonal tropical river

system. Given (i) the importance of the savanna biome for carbon cycling globally (Grace et

al., 2006; Poulter et al., 2014; Liu et al., 2015) and (ii) the tight positive coupling between

rainfall, terrestrial productivity/respiration, river discharge and DIC export identified in this

study, further research on the significance of lateral carbon fluxes to seasonal tropical rivers

is warranted. Poulter et al. (2014) suggested that the tropical savanna carbon sink identified

during wetter years in the recent past, may be lost to the atmosphere in subsequent drier years


by fire and/or rapid decomposition. However, it is also possible that a substantial fraction of

any ‘sink’ is rapidly cycled from savanna to river and back to the atmosphere, and so does not

constitute a sink at all.

Acknowledgements

This study was funded by the National Centre for Groundwater Research and Training

(NCGRT). The authors also wish to acknowledge help from Adam Fakes, Jordi Battle

Aguilar, Chani Welch, Jack Koci, Joel Bailey and Nicolas Rockett. The authors have no

conflict of interest to declare.

References

Atekwana, E.A., Krishnamurthy, R.V., 1998. Seasonal variations of dissolved inorganic

carbon and 13

C of surface waters: application of a modified gas evolution technique.

Journal of Hydrology, 205, 265-278.

Aufdenkampe, A. K., Mayorga, E., Raymond, P. A., Melack, J. M., Doney, S. C., Alin, S. R.,

... & Yoo, K. (2011). Riverine coupling of biogeochemical cycles between land,

oceans, and atmosphere. Frontiers in Ecology and the Environment, 9(1), 53-60.

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. Limnology and Oceanography. 56, 2282-2292.

Bass A.M., Bird M.I., Munksgaard N.C., Wurster C.M. 2012. ISO-CADICA: Isotopic -

Continuous Dissolved Inorganic Carbon Analyser. Rapid Communications in Mass

Spectrometry 26: 639–644.

Bass. A. M., Munksgaard, N., Leblanc, M., Tweed, S., Bird, M 2014. Contrasting carbon

export dynamics of human impacted and non-impacted tropical catchments in response

to a short-lived discharge event. Hydrological Processes. 28, 1835–1843.

Bass, A., O’Grady, D., Berkin, C., Leblanc, M., Tweed, S., Nelson, P., and Bird, M. 2013.

High diurnal variation in dissolved inorganic C, δ13C values and surface efflux of CO2

in a seasonal tropical floodplain, Environmental Chemistry Letters, 11(4), 399 - 405.

Batlle-Aguilar J., Harrington G.A., Leblanc M., Welch C., Cook P.G. 2014. Chemistry of

groundwater discharge inferred from longitudinal river sampling. Water Resources

Research, Vol. 50 (2), 1550-1568.

Bouillon S., Yambélé A., Gillikin D.P., Teodoru C., Darchambeau F., Lambert T., Borges

A.V. 2014. 2014. Contrasting biogeochemical characterisitics of the Oubangui River

and tributaries (Congo River basin). Nature, Scientific Reports 4: 5402.

Brunet, F., Gaiero, D., Probst, J., Depetris, P., Lafaye, G., Stille, P., 2005. 13

C tracing of

dissolved inorganic carbon sources in Patagonian rivers (Argentina). Hydrological

Processes, 19, 3321-3344.

Bureau of Meterology, 2012. Climate Data Online http://www.bom.gov.au/climate/data/

Cai, W.-J., 2003. Riverine inorganic carbon flux and rate of biological uptake in the

Mississippi River plume, Geophysical Research Letters, 30(2), 1032,

doi:10.1029/2002GL016312.

Cane, G., Clark, I.D., 1999. Tracing Ground Water recharge in an agricultural watershed with

isotopes. Ground Water, 37(1), 133-139.

Cartwright, I., 2010. The origins and behaviour of carbon in a major semi-arid river, the

Murray River, Australia, as constrained by carbon isotopes and hydrochemistry.

Applied Geochemistry. 25, 1734–1745.


Clark, I., Fritz, P., 1997. Environmental isotopes in hydrogeology. Lewis Publishers.

Cole J.J., Prairie Y.T., Caraco N.F. McDowell W.H., Tranvik L.J., Striegl R.G., Duarte C.M.,

Kortelainen P., Downing J.A., Middelburg J.J., Melack J. 2007. Plumbing the global

carbon cycle: Integrating inland waters into the terrestrial carbon budget. Ecosystems,

10, 171-184.

Coplen, T.B., 1988. Normalization of oxygen and hydrogen isotope data. Chemical Geology.

72, 293–297.

CSIRO., 2009. Water in the Mitchell region, pp 347-416 in CSIRO. Water in the Gulf of

Carpentaria Drainage Division. A report to the Australian Government from the CSIRO

Northern Australia Sustainable Yields Project. CSIRO Water for a Healthy Country

Flagship, Australia. 479pp

Dawson J.J.C, Bakewell C., Billett M.F., 2001. Is in-stream processing an important control

on spatial changes in carbon fluxes in headwater catchments? The Science of the Total

Environment 265, 153-167.

de Montety V., Martin J.B., Cohen M.J., Foster C., Kurz M.J., 2011. Influence of diel

biogeochemical cycles on carbonate equilibrium in a karst river. Chemical Geology

283, 31–43.

DERM, 2011. Water Monitoring Data Portal.

http://watermonitoring.derm.qld.gov.au/host.htm

Doctor, D.H., Kendall, C., Sebestyen, S.D., Shanley, J.B., Ohte, N., Boyer, E.W., 2008.

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of

carbon dioxide from a headwater stream. Hydrological Processes. 22, 2410-2423.

Ferguson, P.R., Dubois, K.D., Veizer, J., 2011. Fluvial carbon fluxes under extreme rainfall

conditions: Inferences from the Fly River, Papua New Guinea. Chemical Geology. 281,

283–292.

Finlay, J.C., 2003. Controls of streamwater dissolved inorganic carbon dynamics in a forested

watershed. Biogeochemistry. 62, 231-252.

Falkowski, P.G., Raven, J.A., 1997. Aquatic Photosynthesis. Blackwell Sciences, Malden,

Mass.

Gammons, C.H., Babcock, J.N., Parker, S.R., Poulson, S.R., 2011. Diel cycling and stable

isotopes of dissolved oxygen, dissolved inorganic carbon, and nitrogenous species in a

stream receiving treated municipal sewage. Chemical Geology. 283, 44–55.

Grace, J., José, J. S., Meir, P., Miranda, H. S., & Montes, R. A. (2006). Productivity and

carbon fluxes of tropical savannas. Journal of Biogeography, 33(3), 387-400.

Hagedorn, B., Cartwright I., 2010. The CO2 system in rivers of the Australian Victorian Alps:

CO2 evasion in relation to system metabolism and rock weathering on multi-annual

time scales. Applied Geochemistry. 25, 881–899.

Hayashi M., Vogt T., Mächler L., Schirmer M., 2012. Diurnal fluctuations of electrical

conductivity in a pre-alpine river: effects of photosynthesis and groundwater exchange.

Journal of Hydrology 450-451, 93-104.

Huang T-H., Fu Y-H., Pan P-Y., Chen C-T A., 2012. Fluvial carbon fluxes in tropical rivers.

Current Opinion in Environmental Sustainability, 4, 162-169.

Hunt, R.J., Jardine, T.D., Hamilton, S.K., and Bunn, S.E., 2011. Temporal and spatial

variation in ecosystem metabolism and food web carbon transfer in a wet-dry tropical

river. Freshwater Biology, doi:10.1111/j.1365-2427.2011.02708.x.

King, A. J., Townsend, S. A., Douglas, M. M., & Kennard, M. J. (2015). Implications of

water extraction on the low-flow hydrology and ecology of tropical savanna rivers: an

appraisal for northern Australia. Freshwater Science, 34(2), 741-758.


Liu, Y. Y., van Dijk, A. I., de Jeu, R. A., Canadell, J. G., McCabe, M. F., Evans, J. P., &

Wang, G. (2015). Recent reversal in loss of global terrestrial biomass. Nature Climate

Change.

Mook, W.G., Bommerson, J.C., and Staverman, W.H., 1974. Carbon isotope fractionation

between dissolved bicarbonate and gaseous carbon dioxide. Earth and Planetary

Science Letters. 22, 169-176.

Munksgaard N.C., Wurster, C.M., Bass, A., Bird, M.I., 2012. Extreme short-term stable

isotope variability revealed by continuous rainwater analysis. Hydrological Processes.

Vol. 26 (23), pp 3630–3634.

Parkhurst, D.L., Appelo, C.A.J., 1999. User’s guide to PHREEQC (version 2): a computer

program for speciation, batch-reaction, one-dimensional transport, and inverse

geochemical calculations. US Geological Survey Water-Resources Investigations

Report 99–4259, US Geol Surv, Reston, Va.

Petheram, C., McMahon, T. A., & Peel, M. C. (2008). Flow characteristics of rivers in

northern Australia: implications for development. Journal of Hydrology, 357(1), 93-

111.

Poulter, B., Frank, D., Ciais, P., Myneni, R. B., Andela, N., Bi, J., ... & van der Werf, G. R.

(2014). Contribution of semi-arid ecosystems to interannual variability of the global

carbon cycle. Nature, 509(7502), 600-603.

Probst, J.L., Mortatti, J., Tardy, Y., 1994. Carbon river fluxes and weathering C02

consumption in the Congo and Amazon River basins. Applied Geochemistry. 9, 1-13.

Raymond, P.A., Oh, N-H., Turner, R.E., Broussard, W., 2008. Anthropogenically enhanced

fluxes of water and carbon from the Mississippi River. Nature Letters. 451(24), 449-

452, doi:10.1038/nature06505.

Rustomji, P., Shellberg, J., Brooks, A., Spencer, J. and Caitcheon, G. (2010) A catchment

sediment and nutrient budget for the Mitchell River, Queensland. A report to the

Tropical Rivers and Coastal Knowledge (TRaCK) Research Program. CSIRO Water for

a Healthy CountryNational Research Flagship. 119p.

Schulte, P., van Geldern, R., Freitag, H., Karim, A., Négrel, P., Petelet-Giraud, E., Probst, A.,

Probst, J-L., Telmer, K., Veizer, J., Barth, J.A.C., 2011. Applications of stable water

and carbon isotopes in watershed research: Weathering, carbon cycling, and water

balances. Earth-Science Reviews. 109, 20–31.

Telmer, K., Veizer, J., 1999. Carbon Fluxes, pCO2 and substrate weathering in a large

northern river basin, Canada: carbon isotope perspectives. Chemical Geology. 159, 61-

86.

Tobias, C., Böhlke, J., 2011. Biological and geochemical controls on diel dissolved inorganic

carbon cycling in a low-order agricultural stream: Implications for reach scales and

beyond. Chemical Geology. 283, 18-30.

Waldron, S., Scott, E.M., Soulsby, C., 2007. Stable Isotope Analysis Reveals Lower-Order

River Dissolved Inorganic Carbon Pools Are Highly Dynamic. Environmental Science

and Technology. 41, 6156-6162.

Walvoord, M. A., Striegl, R. G., 2007. Increased groundwater to stream discharge from

permafrost thawing in the Yukon River basin: Potential impacts on lateral export of

carbon and nitrogen, Geophysical Research Letters. 34, L12402,

doi:10.1029/2007GL030216.

Warfe, D. M., Pettit, N. E., Davies, P. M., Pusey, B. J., Hamilton, S. K., Kennard, M. J., ... &

Halliday, I. A. (2011). The ‘wet–dry’in the wet–dry tropics drives river ecosystem

structure and processes in northern Australia. Freshwater Biology, 56(11), 2169-2195.


Welch C., Harrington G. A., Leblanc M., Batlle-Aguilar J., Cook P. G. 2014. Relative rates

of solute and pressure propagation into heterogeneous alluvial aquifers following river

flow events. Journal of Hydrology, 511, 891-903.

Wetzel, R.G., 2001. Limnology, Lake and River ecosystems. Third edition.


Table 1. Mean, maximum and minimum values of chemical parameters sampled from the

Mitchell and McLeod River sites during wet (grey) and dry (white) seasons.

EC pH Temp DIC 13C DOC SD POC SD

18O 2H

(uS/cm) (°C) mg/L ‰ mg/L mg/L mg/L mg/L ‰ ‰

Mitchell River

Wet season

mean 67.8 6.7 25.3 4.89 -12.99 2.93 0.01 2.37 0.03 -6.9 -43.6

maximum 88.3 7.5 29.7 9.18 -10.12 3.12 0.02 2.39 0.05 -4.7 -27.9

minimum 46.6 5.9 23.7 2.38 -15.53 2.75 0.01 2.36 0.01 -9.5 -63.6

Dry Season

mean 76.7 6.8 20.0 3.76 -12.34 3.46 0.02 2.50 0.02 -6.4 -39.1

maximum 82.9 7.3 29.0 5.41 -9.91 5.15 0.03 3.13 0.04 -5.2 -31.3

minimum 56.0 6.2 15.2 2.76 -15.98 2.53 0.01 1.78 0.00 -7.4 -46.5

McLeod River

Wet season

mean 54.9 6.5 23.1 4.28 -12.40 - - - - -7.2 -43.5

maximum 58.2 7.5 24.4 6.69 -9.24 - - - - -5.8 -33.5

minimum 49.4 5.9 22.3 2.61 -16.28 - - - - -8.4 -52.9

Dry Season

mean 50.7 6.8 18.1 2.19 -12.35 2.52 0.02 2.65 0.06 -6.7 -40.7

maximum 57.9 7.3 23.5 3.93 -10.24 4.50 0.03 3.21 0.09 -6.0 -35.2

minimum 38.7 6.2 13.0 1.54 -17.45 1.89 0.01 2.28 0.04 -7.4 -44.4


Table 2. EC, pH, DIC concentrations, and saturation indices of CO2g and carbonate minerals

and in the Mitchell River sampled between Aug-99 and May-08 (data from DERM, 2011),

and Oct-11 (data from Batlle-Aguilar et al., 2014).

Season EC pH DIC Saturation Indices

(S/cm) mg/L CO2(g) Calcite Aragonite Dolomite

17-Aug-99 dry 65.1 7.18 2.97 -3.66 -2.22 -2.37 -4.10

19-Nov-99 dry 64.8 6.51 4.12 -2.83 -2.64 -2.78 -5.26

23-Feb-00 wet 58.0 6.90 4.13 -2.59 -3.19 -3.33 -6.12

20-Oct-00 dry 65.3 7.13 3.10 -3.64 -2.42 -2.57 -4.26

27-Feb-01 wet 49.9 7.08 3.24 -3.75 -1.97 -2.11 -3.73

13-May-08 dry 74.0 7.15 3.53 -3.53 -2.17 -2.32 -4.04

17-Oct-11 dry 79.0 7.00 17.31 -4.72 -0.74 -0.89 -1.42


Table 3. Groundwater EC, pH, temperature dissolved oxygen, DIC and 13

C values sampled

during wet (grey) and dry (white) seasons.

Site Date screen EC pH Temp DO DIC 13C

18O 2H

depth (m) (uS/cm) (°C) mg/L mg/L ‰ ‰ ‰

Bore 2A 27-Feb-11 14.4-14.6 21120 6.70 27.0 5.0 337.49 -12.5 -5.9 -39.9

Bore 2A 14.4-14.6 -13.5

Bore 2B 01-Mar-11 9.8-10.8 21690 6.63 27.2 347.39 -12.0 -6.1 -43.3

Bore 2B 15-Oct-11 9.8-10.8 -12.7

Bore 3A 01-Mar-11 14.9-15.1 1419 6.48 27.9 162.87 -13.0 -7.2 -50.2

Bore 3A 15-Oct-11 14.9-15.1 -13.7

Bore 3B 26-Feb-11 8.3-9.3 18080 6.30 27.0 1.2 153.53 -12.2 -6.7 -47.1

Bore 3B 15-Oct-11 8.3-9.3 -13.6

Bore 1A 27-Feb-11 7.9-8.1 135 6.19 26.4 3.0 19.29 -9.7 -8.6 -56.8

Bore 1A 15-Oct-11 7.9-8.1 -15.5

Bore 1B 26-Feb-11 5.9-6.1 148 6.30 26.4 3.5 17.63 -7.0 -8.7 -57.0

Bore 1B 15-Oct-11 5.9-6.1 -11.0

Bore 4A 26-Feb-11 3.6-4.6 176 7.20 25.7 4.0 16.47 -8.6 -8.1 -51.0

Bore 4A 15-Oct-11 3.6-4.6 -9.6

spring 1 1-Aug-12 -7.6 -49.6

spring 2 2-Aug-12 -6.1 -33.5


Figure 1. Location of Mitchell River Basin in northeastern Australia, locations of surface

water and groundwater sampling sites, rainfall station locations, land use, elevation ranges in

meters Australian Height Datum (mAHD) and geology of the study area.


Figure 2. Timing of samples collected relative to the wet (grey) and dry (white) seasons, and

Mitchell River discharge. Discharge values in mm/day were calculated using the 3000 km2

drainage area of the upper Mitchell River Basin. The river seasonal samples were collected

during December 2010 to November 2011, groundwater was sampled twice during the wet

(February-March) and dry (October) seasons of 2011, and river diel sampling was undertaken

in the dry season of 2012 (3-4 October 2012).


Figure 3. Data for the McLeod and Mitchell Rivers during the wet (grey) and dry (white)

seasons. (a) Daily rainfall data from the savanna (Mount Carbine station) and the rainforest

(Julatten station) regions; (b) River discharge, river 13

C values, and groundwater 13

C

values; (c) River DIC, DOC and POC concentrations; and (d) River pH, EC and temperature

values.


Figure 4. Seasonal variations in river DIC loads exported by the Mitchell and McLeod

Rivers. Error bars shown are based on the combined uncertainty of river discharge values

(±10%) and the field HCO3 measurements (±10%) at the sampling sites.

0

50

100

150

200

250

1-D

ec-

10

1-J

an-1

1

1-F

eb-1

1

1-M

ar-1

1

1-A

pr-

11

1-M

ay-1

1

1-J

un

-11

1-J

ul-

11

1-A

ug-

11

1-S

ep-1

1

1-O

ct-1

1

1-N

ov-

11

DIC

load

to

nn

es/d

ay

Mitchell

McLeod


Figure 5. Diel changes in EC, depth, water temperature, LDO, pH, turbidity, solar radiation

and air temperature for the Mitchell River.


Figure 6. Diel changes in DIC, 13

C, DOC and POC for the Mitchell River. The box and

whisker plots for each graph show the difference between the median and first quartile (lower

box), the difference between the median and the third quartile (upper box), the up whisker is

is the maximum minus the third quartile, and the down whisker is the first quartile minus the

minimum.


Figure 7. Results of river and groundwater 18

O and 2H values compared with the global and

local meteoric water lines (GMWL and LMWL respectively).

-65

-60

-55

-50

-45

-40

-35

-30

-25

-10 -9 -8 -7 -6 -5 -4

2H

‰

18O ‰

GMWL

LMWL

groundwater

McLeod River

Mitchell River dry season

Mitchell River wet season


Figure 8. 13

C and pH of seasonal river water samples shows little correlation between these

parameters.

-18.0

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

5.5 6 6.5 7 7.5 8

1

3 CD

IC ‰

pH

Mitchell River McLeod River


Figure 9. Excess pCO2 in river water relative to the atmosphere and 13

C of rivers during the

wet season (grey shade) and dry season.

-18

-17

-16

-15

-14

-13

-12

-11

-10

-9

-8

0

5

10

15

20

25

30

35

1-D

ec-1

0

1-J

an-1

1

1-F

eb-1

1

1-M

ar-1

1

1-A

pr-

11

1-M

ay-1

1

1-J

un

-11

1-J

ul-

11

1-A

ug-

11

1-S

ep-1

1

1-O

ct-1

1

1-N

ov-

11

1-D

ec-1

1

1

3C

‰

Exce

ss p

CO

2

Excess pCO2

13C


Figure 10. Variations in 13

C values with DIC concentrations for seasonal river samples.

Boxes highlight the range in values for groundwater sampled in this study, rainfall values

(Schulte et al., 2011), waters where there is freshwater carbonate or silicate dissolution (Clark

and Fritz, 1997), and waters where the vegetation is C3 or C4 (Clark and Fritz 1997; Waldron

et al., 2007). The inset schematic above the graph presents potential processes that can

influence DIC concentrations and/or 13

CDIC values in river water and the expected changes

in values during each process (e.g. change in dissolved carbon species between CO2(aq) and

HCO3-; Mook, 2006).

leaky savannas: Significance of lateral carbon fluxes in seasonal tropics

Documents

Transcript of leaky savannas: Significance of lateral carbon fluxes in seasonal tropics