Organic carbon deliveries and their flow related dynamics in the Fitzroy estuary
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Transcript of Organic carbon deliveries and their flow related dynamics in the Fitzroy estuary
www.elsevier.com/locate/marpolbul
Marine Pollution Bulletin 51 (2005) 119–127
Organic carbon deliveries and their flow related dynamicsin the Fitzroy estuary
Phillip Ford a,b,*, Pei Tillman a,b, Barbara Robson a,b, Ian T. Webster a,b
a CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Australiab Cooperative Research Centre for Coastal Zone, Estuary and Waterway Management, 80 Meiers Road, Indooroopilly, QLD 4068, Australia
Abstract
The Fitzroy estuary (Queensland, Australia) receives large, but highly episodic, river flows from a catchment (144,000km2) which
has undergone major land clearing. Large quantities of suspended sediments, and particulate and dissolved organic carbon are deliv-
ered. At peak flows, d13C (�21.7 ± 0.8&) and C/N (14.8 ± 1.3) of the suspended solids indicate that the particulate organic material
entering the estuary is principally soil organic carbon. At the lower beginning flows the particulate organic matter comes from in-
stream producers (d13C = �26&). The DOC load is about 10 times the POC load. Using the inverse method, budgets for POC and
DOC were constructed for high and low flows. Under high flows, only a small portion of the POC and DOC load is lost in the
estuary. Under dry season (low flow) conditions the estuary is a sink for DOC, but remains a source of POC to the coastal waters.
Crown Copyright � 2004 Published by Elsevier Ltd. All rights reserved.
Keywords: Dissolved; Particulate; Carbon; Soil; Budgets; Estuaries
1. Introduction
The Fitzroy river, Queensland, Australia (Fig. 1) is a
major source of sediments, and both particulate and dis-
solved nutrients to the southern lagoon of the Great
Barrier Reef. At Rockhampton, 60km upstream from
the mouth (and the last gauging point before the river
enters the estuary) the total sediment flux since 1950
has been estimated to be 260 million tonnes (Kelly and
Wong, 1996). The large catchment (144,000km2) wascovered with Brigalow scrub (Leguminosae: Acacia
harpophylla F. Muell) before European settlement in
the mid 19th century. Efforts to remove the extensive
vegetation cover were largely ineffectual until the
1960s. Since then there has been major land clearing
0025-326X/$ - see front matter Crown Copyright � 2004 Published by Else
doi:10.1016/j.marpolbul.2004.10.019
* Corresponding author. Address: CSIRO Land and Water, GPO
Box 1666, Canberra, ACT 2601, Australia. Tel.: +61 2 6246 5559; fax:
+61 2 6246 5560.
E-mail address: [email protected] (P. Ford).
with much of the woodland replaced by wooded grass-
lands which are now mainly used for extensive cattlegrazing. Smaller areas are used for dry land agriculture
and irrigated horticulture. The sedimentary material
delivered to the head of the estuary consists mainly of
fine particles (�90% < 1lm) (Bormans et al., 2004) com-
posed of montmorillonite and other smectites. For the
adjacent Burdekin catchment, which has a similar
monsoonal climate, geology, and pattern of vegetation
clearing and landuse, sediment delivery post-Europeansettlement, is estimated to have increased 5- to 10-fold
(McCulloch et al., 2003). It is suggested that the sedi-
ment input from all Great Barrier Reef catchments has
increased, on average, by about a factor of 4 (Furnas,
2003) relative to pre-European times.
Rivers deliver both particulate and dissolved organic
matter to coastal regions via estuaries. An accurate
characterization of the nature and quantities of organicmaterial delivered, as well as the transformations of the
organic matter in the estuary, is central to understanding
vier Ltd. All rights reserved.
Fig. 1. Location map of the Fitzroy catchment, major contributing rivers, and Fitzroy estuary.
120 P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127
the functioning of these systems in their present form.
Such knowledge is needed also to be able to make
well-founded predictions of the consequences for estua-
rine functioning, of changes to land use, or reductions inflows due to dam construction. The particulate organic
carbon (POC) arises from organic coatings on clay par-
ticles, and as well as fragments of vegetation, freshwater
phytoplankton and other forms of organic carbon. The
transported sediments will thus be a vector for the trans-
port of heavy metals and pesticides adsorbed to the or-
ganic layers (Karickhoff et al., 1979; Voice and Webber,
1983). When the terrigenous organic material enters theestuary it is subject to further heterotrophic consump-
tion by bacteria, as well as serving as food for some
higher organisms. While the sediment may decrease
the available light for photosynthetic organisms, the
associated organic carbon may be utilized by some sym-
biotic corals (Anthony and Fabricius, 2000) and com-
pensate for the energy losses due to the decreased light
and increased cleaning required. On a global scale thetransport of terrigenous organic carbon is the largest
flux of carbon from the land to the sea (Meybeck,
1982) and the rate of riverine discharge of POC is com-
parable with the global rate of accumulation of organic
carbon in all marine sediments (Berner, 1989). For all
estuaries, depending on the size of the riverine flows rel-
ative to the estuary volume and their tidal characteris-
tics, the residence time for organic matter in the
estuary can vary enormously. Even when the nominalestuarine transit time is short and microbially mediated
changes to terrigenous carbon are slight, physicochemi-
cal processes such as flocculation of fine particles at rel-
atively low salinities (S < 3&) (Millman et al., 1975) can
create more favourable conditions for breakdown and
greatly change the dynamics of the organic matter with-
in the estuary.
For sourcing and tracing offshore sediments it isimportant to be cognizant of the isotopic characteristics
of the organic material entering such coastal areas. On
occasion it has proved difficult to distinguish between
marine plankton remains and terrigenous organic mat-
ter delivered by nearby major rivers (Onstad et al.,
2000). The concept of dissolved organic carbon (DOC)
as encountered in the context of estuarine and marine
research needs to be carefully defined and related to cur-rent perspectives on the dissolved organic carbon gener-
ated in the upstream areas and delivered to the estuary
(where it becomes estuarine DOC). Riverine DOC is
produced primarily by leaching of leaf litter within the
stream, and by groundwater inflows which have infil-
Fitzroy River Discharge
1991 1993 1995 1997 1999 2001 20030
1000
2000
3000
4000
Dis
char
ge (m
3 s-1)
Fig. 2. Daily discharge showing highly episodic character of flows.
P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127 121
trated through organic rich areas of the soil (reviewed in
Boulton et al., 1998). It is composed primarily of humicsubstances (Ertel et al., 1986) with lesser amounts of
polysaccharide carbohydrates and amino acids (Volk
et al., 1997). Part of this material can be taken up by
bacteria (O�Connell et al., 2000) and up to about 10%
of riverine DOC can be respired on passing through
the estuary (Moran et al., 1999). Riverine DOC has a
major impact on coastal DOC dynamics and forms part
of the microbial food web there (Zweifel et al., 1995).There are seemingly large, and unexplained differences
in the DOC degradation rates between nearby estuaries,
or within the same estuary.
In this study we report on the measurements made on
the delivery of particulate organic matter from the
catchment and make inferences as to how this has chan-
ged as a consequence of the human-generated changes in
vegetation and land use. In addition we quantify the re-moval and transformation processes for POC and DOC
in the estuary under high (i.e. flood conditions) and low
flow (very limited freshwater discharge).
The Fitzroy catchment lies at the boundary of the
tropical and temperate convergence zones. Both rainfall,
and runoff from the catchment, are highly seasonal
occurring mainly in the Antipodian summer leading to
large, but short-lived flows, which rapidly flush the estu-ary clean of salt water (Fig. 2). For the rest of the year
freshwater flows into the estuary are small. They arise
from limited discharges over the fish ladder at the Rock-
hampton barrage, and the release of treated waste water
(18Mld�1) at Rockhampton.
2. Experimental methods
2.1. Field site and sediment sample collection
One litre samples were collected daily during flow
events from the raw water inlet (1m beneath surface)
of Fitzroy River Water Corporation�s drinking water
treatment plant 6km above the barrage at Rockhamp-
ton, Queensland Australia. The polyethylene samplebottles had been previously acid washed and rinsed
twice with MilliQ water. The samples were preserved
by freezing at �20 �C. The samples were passed through
a <64lm sieve and the suspended sediment was recov-
ered by centrifugation (3000g, 15min) and dried to con-
stant weight at 105 �C. Carbon and nitrogen content and
the stable isotopic concentrations (d13C and d15N) were
determined simultaneously using an automatic C and Nanalyser interfaced to a Europa 20–20 mass spectro-
meter. The samples had been stood overnight over con-
centrated HCl to remove carbonate minerals. Stable
isotope ratios are expressed as parts per mil (&) devia-
tion from recognised world standards (PDB and air).
Measurement precision is ±0.2&.
2.2. Hydrology
Discharge was measured at the official gauging sta-
tion at the Gap (AWM 130005) about 80km upstream
of the barrage and then corrected for the transit time be-
tween the Gap and the downstream sampling station.
2.3. Estuarine water samples
Water samples for TOC (total organic carbon) and
DOC (dissolved organic carbon) were collected from a
depth of 0.25m, at 12 stations spaced along the estuary
downstream of the barrage. Sample collection was done
as closely as possible to the same time of the lunar cycle
each month. The DOC sample was filtered through a
0.45lm cellulose nitrate syringe filter (Sartorius), and
both the TOC and DOC samples were then preservedby freezing at �20 �C. On return to the laboratory the
samples were melted, thoroughly mixed, and, after acid-
ification, analysed using an Analytical 1010 TOC Ana-
lyser. Water column physical parameters (temperature,
salinity, pH, dissolved oxygen, and turbidity) were
measured at the surface, and then 1m intervals to the
bottom at each site, using a Hydrolab Minsonde 4a
water quality probe.
2.4. Calculation of POC and DOC budgets
Monthly budgets of TOC and DOC were calculated
using the box-model method of Webster et al. (2004).
Briefly, sample positions were corrected to position at
mid-tide based on a 1-D tidal model of the estuary cal-
ibrated against tidal observations at Port Alma near themouth, and 1/3 (Thompson�s Point) and 2/3 (Nerrim-
bera) of the length of the estuary. The changes in the ob-
served salinity distribution at the 12 stations was used to
estimate the spatially varying tidal dispersion coefficient.
The 1-D advection dispersion model was then used to
calculate the anticipated DOC concentration in the var-
ious boxes based on the concentrations of the preceding
month and modeled advection and dispersion. The dif-ferences between the actual concentration and the pre-
dicted represents the gain or loss of the material
POC vs TSS
5
6
122 P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127
attributable to all causes such as sedimentation and
resuspension, microbial metabolism including sediment
diagenesis, and uptake by phytoplankton.
0
1
2
3
4
0 200 400 600 800 1000 1200TSS (mg l-1)
%C
Fig. 4. Variation in organic carbon content (%) of suspended solids as
function of suspended solids concentration (mgl�1) for 1998 event.
3. Results and discussion
3.1. Discharge and particulate organic carbon
concentration and sources
In the Fitzroy River, the concentration of suspended
solids (TSS) is highly dependent on the discharge (Fig.
3) rising from pre-flood levels of �20mgl�1 to more
than 1000mgl�1 at maximum discharge, and then slowlydeclines. The discharge increases by 4 orders of magni-
tude, and the suspended sediment concentration by 2 or-
ders of magnitude. At these TSS levels the river is highly
turbid (1500 NTU) and autochthonous production is
negligible during the flood flow. Thus the carbon con-
tent and isotopic signature of the sediment reflects the
characteristics of the sediment sources and the changes
(if any) which the material has undergone in passagefrom the catchment to the estuary. Suspended sediment
concentrations remain elevated (relative to pre-flood
values) well after the discharge has declined. This is seen
in the 1998 observations (Fig. 3), where one month after
the passage of the initial small flood peak (37Mld�1) the
TSS has only decreased from 360mgl�1 to 200mgl�1. A
further month later it sinks to 170mgl�1 when the major
flood flow arrives. This is due to the slow settling of thevery fine particles delivered by the flood and leads to
hysteretic effects between TSS and discharge which pre-
clude the construction of realistic empirically based dis-
charge-load relationships. For instance, in the major
event in April–May, 1998 the TSS concentration be-
tween the passage of the two major flood peaks (Fig.
3), is twice the maximum observed during the initial
maximum discharge, despite the discharge then beingabout 1/3 that in February.
1998 Flood TSS and
0
20000
40000
60000
80000
100000
120000
140000
26-Dec-97 23-Jan-98 20-Feb-98 20-Mardate
Dis
char
ge (M
l d-1
)
Fig. 3. Discharge (continuous line) and suspended sediment
At high TSS concentrations (Fig. 4) the organic car-
bon content in the 1998 event is remarkably uniform(1.54% (±0.2%; n = 23)). Similarly, the POC concentra-
tion inferred from measurement of TOC and DOC,
and TSS in the barrage (Fig. 3), shortly after the passage
of the flood peaks (S < 1&) gave an average organic
carbon content of the suspended solids of 1.87%. As
TSS declines below 200mgl�1 the carbon content in-
creases reaching an approximate asymptote of 5.5% C
at nominal 0mgl�1 TSS.The d13C signature of the suspended sediments shows
(Fig. 5) an analogous constancy at TSS concentrations
above 200mgl�1, staying in the range of �21 to
�22& (average 21.7 ± 0.8&; n = 23). While at lower
TSS concentrations, d13C becomes appreciably lighter
and asymptotic to approximately �27& at nominal
0mgl�1 TSS. Table 1 summarises these observations to-
gether with similar data for samples collected intermit-tently during flood events from 2000 to 2003. Samples
were collected approximately 500km upstream when
the water left the sub-catchment, and again when the
same water mass reached the barrage downstream.
Table 1 also shows the d13C and d15N values for para
grass (Poaceae: Urochloa mutica (Forsk) Stapf.), an
Discharge
-98 17-Apr-98 15-May-98 12-Jun-980
200
400
600
800
1000
1200
TSS
( mg
l-1)
concentration (filled squares) for flood event in 1998.
Table 1
Summary of characteristics of suspended sediments in Fitzroy River
during floods at concentrations greater than, and less than, 200mgl�1
and soil parameters
Suspended sediment property TSS > 200mgl�1 TSS < 200mgl�1
1998 Barrage suspended
sediment d13C
�21.7 ± 0.8& Decreasing towards
�27& as TSS
declines
1998 Barrage suspended
sediment d15N
5.1 ± 1.5& 3.0 ± 2.0&
% Organic C 1.57 ± 0.45% Increasing
towards 5.5%
C/N (atomic) 14.8 ± 1.3 10.8
d13C-upstream suspended
sediment 2001–2003
�22.5 ± 1.1& N/A
d13C-downstream suspended
sediment 2001–2003
�21.3 ± 1.3& N/A
Surface soils cropped areas
Comet catchment
�15.9& N/A
d13C Para grass (leaves) �12.4& N/A
(C/N = 40.2)
d13C Para grass (roots) �13.5& N/A
(C/N = 20.8)
d15N Para grass (leaves) 9.4 N/A
d15N Para grass (roots) 5.5 N/A
y = 1.064Ln(x) - 28.434R2 = 0.6033
-28
-26
-24
-22
-20
0 200 400 600 800 1000 1200
TSS ( mg l-1 )
del 13
C (0
/00 )
Fig. 5. Variation in carbon isotopic signature (d13C) of suspended
solids as function of suspended solids concentration (mgl�1) for 1998
event.
P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127 123
introduced pasture plant which grows profusely in theriparian zone of the lower reaches of the Fitzroy and
is dislodged as floating rafts of vegetation during floods.
No mixture of para grass and sediments can account for
the observed d13C signal of the organic carbon at low
TSS.
We suggest that the organic carbon delivered to the
estuary at high suspended sediment concentrations is
soil organic carbon derived from a savanna environmentwith a mixture of C3 trees and C4 grasses. The observed
d13C (�21.7&) is close to the value calculated from %C
using the relationship of Bird and Pousai (1997). Fur-
thermore, the observed d13C is in the middle of the range
(�18 to �24&) found for fine particles collected during
a flood event in arid temperate Australia (Olley, 2002)
and for both the top soils and subsoils in the sediment
source region. It also matches the d13C value appropri-
ate for a wet (treed) or a dry (grassed) savanna predicted
from the particle size, based on an extensive survey of
tropical and subtropical biomes in northern Australia.
Further support for our identification of the suspendedsediment carbon as soil organic carbon comes from
the work of Masiello and Druffel (2001) on the Santa
Clara River—a highly episodic stream which, like the
Fitzroy, exports sediment in a few high flux events. They
showed that the exported sediment had an organic car-
bon d13C signature of �22.2 ± 0.8& and that the trans-
ported sediment was derived from old and deeply eroded
soils. The C/N was greater than 10 in both catchments(Fitzroy C/N: 14.8 ± 1.3; Santa Clara: 11.2 ± 3.3). C/N �10 is the global average (Meybeck, 1982) and values
greater than this are often indicative of soil organic
matter.
In contrast, the material, transported at low TSS con-
centrations at the beginning of the flow, has a much
lower d13C (�26&), lower C/N (10.8), and a much higher
%C, than the material at the flood peak. While the d13Cis close to the value characteristic of C3 trees and could
arise from vegetation growing immediately adjacent to
the river and mobilized in the first stages of the flood
(Bird et al., 1998), the low C/N precludes such an expla-
nation. The d13C and C/N sugget that the material trans-
ported in the initial stages of the flow is FSPOM
produced in situ by primary producers, including bio-
films (Burns and Walker, 2000), using terrestrial materi-als (Olley, 2002).
As has been noted previously (Onstad et al., 2000;
Masiello and Druffel, 2001) terrestrial material with a
d13C of �22& can be confused with marine detritus
which has a similar signature. This is a potentially con-
founding issue in the Fitzroy as we attempt to develop
more accurate estimates of changes in sediment delivery
due to human-induced land-use changes. If we assumethat the greatest impact of altered land use has been to
change the delivery of soil and associated soil carbon
disproportionately relative to in-stream debris, then we
can make three inferences from these results which
may help in unravelling the sedimentary history from
near-shore cores. Moving down core, the following ef-
fects should be seen crossing the boundary reflecting
pre- and post-European land-uses. They are:
1. d13C becomes more negative.
2. The C/N decreases.
3. The 14C age becomes younger (a counter intuitive
result!).
3.2. Dissolved organic carbon
Dissolved organic carbon (DOC) was measured in
samples collected at the regular sampling stations along
Table 2
DOC concentrations, flow volumes, and DOC loads (Fitzroy estuary
2000–2003)
Date of flow DOC concentration
(mgCl�1)
Flow
volume (Mm3)
DOC load
(Tonnes C)
November 2000 9.78 377 3687
December 2000 7.11 1616 11,490
January 2001 6.09 365 2223
February 2001 7.48 593 4435
April 2001* 4.27 115 491
February 2002* 2.88 101 291
March 2002* 4.87 29 141
Jun 2002* 3.71 95 352
February 2003 7.9 1809 14,291
March 2003 7.83 901 7054
* Total discharge less than two barrage volumes.
124 P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127
the estuary, within 3weeks of the start of a flood event.
The data, when plotted against salinity (Mantoura and
Woodward, 1983) shows straight line behaviour with a
high correlation coefficient (R2 = 0.96). Table 2 brings
together the results from a total of 10 flood events ob-
served over 3years. This includes four events wherethe total discharge (integrated over the flow period)
was comparable to the volume of the barrage
(60Mm3). These small flows are not sufficiently large
to completely fill the estuary with water from the
upstream region, merely displacing the barrage contents
into the estuary. The larger flows (>2 barrage volumes)
have higher DOC concentrations than the smaller flows
(7.7mgl�1 and 3.7mgl�1 respectively). The linear rela-tionship between DOC and salinity is observed in many
estuaries (Laane and Koole, 1982; Mantoura and
Woodward, 1983; Berger et al., 1984) and is taken to
indicate that the riverine DOC is refractory on the time
scale of its passage through the estuary. A more recent
comparative study (Hopkinson et al., 1998) indicates
that estuarine bacteria could consume the DOC, but
the bacterial growth rate was low and of the order of1.5lgmg�1 d�1 DOC. Thus under short residence times
any DOC metabolism would not be detected. In the
Fitzroy, the waters are highly turbid post-flood and pho-
tochemical activation of DOC (Kieber et al., 1989)
Fig. 6. Fitzroy estuary showing sampling sites (filled circles
which can increase DOC removal rates several-fold
(Miller and Moran, 1997) is prevented. The budget
method, discussed later, quantifies the DOC behaviour
in the low-flow period when the water column clears
and photochemical effects are potentially possible.
The total load of DOC delivered by the river into theestuary in the 20months from April 2001 to January
2003 is 1275tonnes. This is very small compared to the
preceding 4months (November 2000 to February
2001) when 21,835 tonnes were delivered, or the subse-
quent episode covering only 2months (February to
March 2003) when a further 21,245 tonnes came down-
stream. These results exemplify the highly episodic char-
acter of tropical rivers, such as the Fitzroy, draining drysavannas, and the intermittent delivery of large loads of
dissolved nutrients and organic carbon, both dissolved
and particle-attached.
For comparison, we have calculated also the annual
DOC load from the City of Rockhampton�s three waste
water treatment plants using the weighted mean DOC
concentration and the annual discharge of the treated
effluent. This amounts to �50tonnes and is dwarfedby the inputs of even the smallest flow (indicated by *
in Table 2).
3.3. DOC and POC budgets by the inverse method
We applied the Inverse method of Webster et al.
(2004) to develop budgets of DOC and POC, and to esti-
mate fluxes of DOC and POC in the Fitzroy estuary.The estuary was divided into six sections (Fig. 6) with
each section containing at least one sampling station.
Budgets for DOC and TOC (the two carbon species ana-
lysed for) were constructed, and then the POC budget
was calculated by difference from the TOC and DOC.
Because of the highly episodic flows separate budgets
were calculated for the wet season (1 November 2000–
30 April 2001) and the dry season (1 May 2001–30November 2001). The total discharges were 2951Mm3
and 115Mm3 respectively—a factor of 25 different.
For ease of presentation the budgets of individual cells
have been consolidated into ‘‘up-stream’’ (cells 1–3)
) in relation to the sectors used for flux calculations.
P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127 125
and ‘‘down-stream’’ (cells 4–6) units (Fig. 7a and b).
Each unit covers approximately the same length of the
estuary. The downstream unit includes the large ‘‘cut-
through’’ loop (Fig. 6), and ends at the station 2.5km
from the actual estuary mouth.
The wet season delivery of DOC calculated by thismethod (21,136tonnes) is in good agreement with the
same parameter inferred from the riverine DOC concen-
tration and the total discharge (21,835tonnes). The con-
cordance between the results of the two methods is not
as good for the dry season delivery (Inverse method:
49tonnes; concentration · discharge: 491tonnes). The
disparity probably arises from two factors. The small
flow delivers only a relatively small load of DOC whichproduces only small concentration differences at all sta-
tions pre- and post-flood. The budget depends on these
differences and thus has a proportionally larger error.
The assumption of a constant DOC concentration
(a)
(b)
Fig. 7. Annual budget (tonnes) for (a) DOC, (b) POC under high flow
(wet season) and low flow (dry season) conditions in upstream and
downstream sectors (Fig. 6). Arrows indicate direction of net flux.
through out the flood may be poorly founded. The re-
sults above show that there is discharge-dependent dif-
ferences in DOC concentration, with the smaller flows
having lower DOC concentrations. The initiation and
concluding phases of these smaller flows would have
even lower concentrations also and thus taking the max-imum concentration will overestimate the load.
Under wet season (high flow) conditions there is a
slight loss of DOC (�3%), but the bulk of the incoming
material passes straight through the estuary. The inverse
method deals with net changes in DOC, so this loss in-
cludes adsorption of DOC by particles, diffusive ex-
change between the sediments and the water column,
as well as microbial metabolism. The small loss explainsthe apparent contradiction between the view, discussed
in Section 1, of riverine DOC being refractory because
of its conservative character demonstrated by straight
line behavior in DOC vs salinity plots; and the in vitro
measurements showing measurable consumption of
DOC in estuarine waters. Our results show that the
amount of DOC removed is too small to produce a
noticeable deflection in the DOC vs salinity plots.In the dry season (low flow) conditions the Fitzroy
estuary is a net source of DOC from both the upstream
and downstream sectors. The total flux of 587tonnes of
DOC into the estuary waters is very nearly balanced by
the wet season loss of 679tonnes. Examination of the
fluxes (Fig. 8) shows that different parts of the estuary
have quite different characteristics for DOC production
and removal. The most downstream station (cell 6) is asource of DOC under both high and low flow conditions
(�20mmolm�2 d�1). Under the low flow conditions the
contribution from this cell is sufficient to offset the losses
of DOC (40mmolm�2 d�1) occurring in cells 4 and 5
immediately upstream, and make the contribution for
this sector positive i.e. flux into the water column. In
contrast, upstream cells 2 and 3 are net contributors of
DOC at approximately 8mmolm�2 d�1 under dry sea-son conditions. While under wet season conditions these
cells are sinks for DOC reflecting losses by microbial
metabolism and agglomeration to form particles.
DOC Fluxes
-50
-40
-30
-20
-10
0
10
20
30
0 10 20 30 40 50 60
Distance downstream from barrage (km)
Flux
( m
mol
m-2
d-1)
Wet SeasonDry Season
Fig. 8. Fluxes of DOC (mmolm�2d�1) measured along the estuary
under high flow (filled squares) and low flow (filled circles).
126 P. Ford et al. / Marine Pollution Bulletin 51 (2005) 119–127
Under both wet and dry season conditions our bud-
get calculations show (Fig. 7b) that the Fitzroy is a
major source of POC to the coastal seas. As would be
anticipated, the contribution is much greater in the
wet season with the amount of POC mobilized within
the estuary being comparable to the amount totalamount delivered by the flood. The downstream sector
is the dominant source of POC in the wet season (down-
stream:upstream 16:1). This dominance extends to the
dry season though the disparity is reduced (down-
stream:upstream 4:1). As the POC is associated with
the fine sediments the POC budget indicates that the
wet season flow moves a significant amount of fine sed-
iment out of the estuary. If we take the POC content ofthe incoming riverine suspended solids (1.57% see Table
1) as typical of the POC content of the mobilised sedi-
ments, then the estuary contributes an additional
151,000tonnes of sediment in the wet season and a fur-
ther 34,000 tonnes in the dry season over and above the
amount that is carried through the estuary by the flood
flow. This may come from bank erosion during the high
flow periods (suggestion of anonymous referee). Theestuary as a whole, is however prograding (R. Packett,
pers. commun.) and there must be a long term source
of this additional material. We suggest that it is material
deposited in previous floods in the near shore coastal
area. Our tidal data used in the model calibration shows
that tides in the Fitzroy estuary are quite asymmetric,
with the ‘‘ins’’ being of shorter duration, and conse-
quently of higher velocity than the ‘‘outs’’. Thus therewill be a net movement of sediment into the estuary
on each tidal cycle, but this is too small to be detected
by our monthly sampling.
4. Conclusion
In the Fitzroy POC is delivered in the episodic floodsand is primarily composed of soil organic carbon. It
makes up about 1.5% of the total suspended solids.
Under low flows the proportion of POC derived from
in-stream primary production rises, but the load of
POC decreases. Given that the sediment load has been
considerably elevated, relative to pre-European settle-
ment, the character of the POC entering Keppel Bay
has changed. Under pristine conditions, the vegetationcomponent would have been higher and less diluted with
relatively refractory SOC. The ecological consequences
are that land clearing has led to conditions in the receiv-
ing waters which are more favorable to organisms able
to metabolise relatively refractory organic matter. The
DOC concentrations in large flood events are remarka-
bly uniform (7mgl�1), but smaller (insufficient to flood
the estuary) events are approximately half the DOC con-centration of the larger events. Budgets for the estuary
constructed using the Inverse method show that an addi-
tional amount of POC, comparable to that transmitted
through the estuary, is removed from the estuary in
flood events. We hypothesize that that much of this
material is ‘‘pumped’’ back into the estuary by tidal
processes which are not fully captured by the budget
method. Under flood flows DOC is removed in the estu-ary, but in the dry season, there is a flux of DOC into the
water column from the estuary. The mouth of the estu-
ary, where macrotidal effects are largest is always a
source of DOC to the water column. The upstream por-
tion is a source of DOC under low flows and a sink
under high flows, while the lower portion of the estuary
is always a sink though it removes more DOC in flood
events than during the dry season.
Acknowledgments
We gratefully acknowledge the assistance of the
Queensland EPA, especially Andrew Moss and John
Ferris, in the collection of the samples, and Bob Noble
and Bob Packett of the CRC Coastal Zone, Estuary,and Waterway Management (CRCCZEWM) for logis-
tic support in Rockhampton and the collection of tidal
data used in calibrating the tidal model. Dr Lynda
Radke and Dr Jon Olley (and 2 anonymous referees)
provided penetrating and helpful comments on an ear-
lier draft. We thank the management and staff of Fitzroy
River Water Corporation for assistance with the flood
sampling program. This work was conducted underthe auspices of the CRCCZEWM and we gratefully
acknowledge their support.
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