Chemical Geology 351 (2013) 229–244
Contents lists available at SciVerse ScienceDirect
Chemical Geology
journal homepage: www.elsevier.com/locate/chemgeo
Dynamic of particulate and dissolved organic carbon in small volcanic
mountainous tropical watersheds
E. Lloret a,⁎, C. Dessert b,c, L. Pastor a, E. Lajeunesse d, O. Crispi c, J. Gaillardet b, M.F. Benedetti a,⁎⁎
a
Equipe Géochimie des Eaux, Université Paris Diderot, Sorbonne Paris Cité, IPGP, UMR 7154, CNRS, 75205 Paris, France
Equipe de Géochimie et Cosmochimie, IPGP, Sorbonne Paris Cité, Université Paris-Diderot, UMR 7154, CNRS, 75205 Paris, France
Observatoire Volcanologique et Sismologique de Guadeloupe (OVSG) IPGP, UMR 7154, CNRS, Le Houëlmont, FWI, 97113 Gourbeyre, Guadeloupe
d
Equipe de Dynamique des Fluides Géologiques, IPGP, Sorbonne Paris Cité, Université Paris-Diderot, UMR 7154, CNRS, 75205 Paris, France
b
c
a r t i c l e
i n f o
Article history:
Received 20 March 2013
Received in revised form 8 May 2013
Accepted 14 May 2013
Available online 31 May 2013
Editor: David R. Hilton
Keywords:
Carbon fluxes
DOC
POC
DIC
Small tropical rivers
Watersheds
a b s t r a c t
In the tropical zone, small watersheds are affected by intense meteorological events. These events play an
important role in the erosion of soils and therefore on the associated organic carbon fluxes to the ocean.
We studied the geochemistry of three small watersheds around the Basse-Terre volcanic Island (French
West Indies, FWI) during a four years period, by measuring dissolved organic carbon (DOC), particulate
organic carbon (POC) and dissolved inorganic carbon (DIC) concentrations. The mean annual yields ranged
between 8.1–15.8 tC km−2 yr−1, 1.9–8.6 tC km−2 yr−1 and 8.1–25.5 tC km−2 yr−1 for DIC, DOC and POC,
respectively. Floods and extreme floods (i.e., extremely high discharge associated to extreme meteorological
events such as cyclones or tropical storms) account for 42.6% of the yearly water flux and represent 54.5% of
the annual DOC flux, and more than 85% of the annual POC flux. The DIC flux occurs essentially during low
water levels with 75% of the annual flux. During low water levels and floods, the dissolved carbon is exported
in majority under the inorganic form (DIC/DOC = 2.6 ± 2.1), while during extreme floods, the dissolved
carbon transported is mostly organic (DIC/DOC = 0.7 ± 0.2). The partial “residence time” taking into
account only the physical processes (erosion and transport) in Guadeloupean soils has been estimated
between 381 and 1000 years. These relatively short times could be linked to the intensity of meteorological
events rather than the frequency of meteorological events. The total export of organic carbon coming
from small tropical and volcanic mountainous rivers is estimated at 2.4 ± 0.6 MtC yr−1 for DOC and at
5.9 ± 2.4 MtC yr−1 for POC, emphasizing that these carbon fluxes are significant and should be included
in global carbon budgets. In addition, the quality of terrestrial organic matter (POC/DOC, and C/N ratios)
arriving to the ocean is different from the one of large river origin. These inputs are responsible for a fast
transport of terrestrial organic matter to the ocean but their effect on regional and global carbon budget is
still a matter of debate.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Soil organic matter contains 1400 to 1500 Gt of carbon and is one
of the major pools of carbon at the Earth surface (Schlesinger, 1977;
Gregory et al., 1999). Soil erosion represents a major input of organic
carbon in aquatic ecosystems. During meteorological events, soil
organic carbon (SOC) can be intensively either leached or eroded
and transferred to aquatic ecosystems in dissolved and particulate
forms, respectively (Lal, 2004). During its transport from rivers
to the oceans, terrestrial organic carbon can be mineralized and/or
exported into the ocean (DOC and POC), or deposited and stored
(POC) in aquatic ecosystems under low discharge (i.e.: alluvial plains,
⁎ Correspondence to: E. Lloret, Department of Renewable Resources, University of Alberta,
442 Earth Sciences Building, Edmonton, AB T6G 2E3, Canada.
⁎⁎ Corresponding author.
E-mail addresses: [email protected] (E. Lloret), [email protected] (M.F. Benedetti).
0009-2541/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.chemgeo.2013.05.023
mangroves; Meybeck, 1993; Hedges et al., 1997; Lal, 2003, 2004).
Once in the ocean, this terrestrial organic matter is then mineralized,
and/or buried in sediments, and/or transported offshore (Hedges
et al., 1997; Hansell and Carlson, 2002; Benner, 2004; Burns et al.,
2008). Having been subjected to microbial degradation in soils
(Oades, 1988; Hedges et al., 1994) and aquifers (e.g. Nelson et al.,
1993), riverine DOC and POC arriving in estuaries and coastal oceans
can be recalcitrant and resistant to the degradation in these ecosystems. Terrestrial organic matter thus represents approximately
one third of the organic matter buried in all marine sediments and
is stored over geological timescales leading to atmospheric carbon
dioxide sequestration (Berner, 1989; Hedges and Keil, 1995; Hedges
et al., 1997; Schlünz and Schneider, 2000; Gordon and Goñi, 2004;
Burdige, 2005; Galy et al., 2007).
Organic carbon transport from continents to oceans represents
around 40% of the global continental carbon flux (DOC, DIC, POC and
particulate inorganic carbon), varying between 400 and 900 Mt yr−1
230
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
(Hedges et al., 1997; Schlünz and Schneider, 2000; AitkenheadPeterson et al., 2003). For large rivers, the portion of DOC exported is
around 60% of the total organic carbon export. Uncertainties on the
estimation of the global organic carbon transfer are partly due to the
lack of holistic quantitative studies taking into account all sizes of watersheds as well as all type of climatic regimes. Indeed studies generally
focus on large river systems like the Mississippi (Bianchi et al., 2007;
Duan et al., 2007), the Ganga–Brahmaputra (Galy et al., 2008), tributaries of the Amazon River (Moreira-Turcq et al., 2003; Johnson et al.,
2006; Aufdenkampe et al., 2007; Bouchez et al., 2010) and, large Arctic
rivers (Yenisei, Ob, Lena Rivers; Ludwig et al., 1996a; Dittmar and
Kattner, 2003; Gebhardt et al., 2004; Raymond et al., 2007), which integrate differences in lithology, vegetation, soils and climate. Small mountainous rivers directly connected to the oceans are less studied than
large rivers, although they play an important role in transport in organic
matter, their yields and runoff being inversely proportional to the
watershed area (Milliman and Meade, 1983; Walling, 1983; Degens
and Ittekkot, 1985; Milliman and Syvitski, 1992). Recent works have
demonstrated that these small rivers are major sources of POC (Kao
and Liu, 1996; Schlünz and Schneider, 2000; Lyons et al., 2002; Carey
et al., 2005; Hilton et al., 2008b), DOC (Lloret et al., 2011) and dissolved
major elements to the oceans (Louvat and Allègre, 1997; Dessert et al.,
2009; Goldsmith et al., 2010; Calmels et al., 2011; Lloret et al., 2011).
Due to their location in the tropical zone, numerous small mountainous
rivers are affected by aperiodic intense precipitation events such as
cyclones or tropical storms that can play an important role on soil
erosion and can potentially increase total organic carbon fluxes released
by these systems (Waterloo et al., 2006; Dawson et al., 2008; Goldsmith
et al., 2008; Hilton et al., 2008a; Bass et al., 2011; Lloret et al., 2011;
Jeong et al., 2012; Wohl et al., 2012).
The Guadeloupe Island (French West Indies) provides an unique
opportunity to study the yield and the flux of carbon from such
small watersheds as well as the impact of these meteorological events
on inorganic and organic carbon fluxes. Its monolithologic volcanic
composition, combined with its lack of fossil organic carbon, helps
to constrain the influence of other factors such as climate, soil composition, and age of the bedrock. In addition, high rates of chemical
weathering and mechanical denudation are reported for volcanic lithology (Louvat and Allègre, 1997; Dessert et al., 2001, 2003; Goldsmith
et al., 2010; Gaillardet et al., 2012). Moreover, Guadeloupean soils
(Andosol and ferralitic soils; Colmet-Daage and Bernard, 1979) present
surface horizons enriched in organic matter (10–15 %; Colmet-Daage
and Lagache, 1965; Duchaufour, 2001; Lloret, 2010). Guadeloupean
coasts are impacted by numerous meteorological events like tropical
storms and cyclones (Zahibo et al., 2007), that can accentuate the soil
erosion (Waterloo et al., 2006; Dawson et al., 2008; Hilton et al.,
2008a; Lloret et al., 2011). Lloret et al. (2011) focused on spatial and
temporal distribution of dissolved inorganic carbon (DIC) and dissolved
organic carbon (DOC) concentrations in the Guadeloupean rivers. This
study underlined the importance of floods on carbon export, more
than 50% of the annual DOC export being transported during these
hydrologic events. During floods, rivers are fed by surface solutions
enriched in DOC resulting from leaching of freshly deposited organic
matter. Under low water conditions, the ground flow path is the
major source of organic carbon and is characterized by low DOC concentrations in rivers. Lloret et al. (2011) also estimated that DOC yields by
small volcanic and mountainous islands under tropical climate range
between 2.5 and 5.7 t km−2 yr−1 and are similar to the DOC yields
calculated for large tropical rivers like the Amazon (5.8 t km−2 yr−1;
Moreira-Turcq et al., 2003), the Orinoco and the Parana (4.8 and
1.4 t km−2 yr−1, respectively; Ludwig et al., 1996b and references
therein). However, the DOC yields calculated in Lloret et al. (2011)
were based on a discrete sampling (2 low water and 2 flood levels
only) and probably are an underestimation of these yields. Indeed
Bass et al. (2011) showed that the DOC and the POC fluxes could be
underestimated by 49 to 78%, respectively if high temporal resolution
sampling is not performed. Moreover, Lloret et al. (2011) did not report
POC fluxes and did not compare carbon with other particulate macronutrients, like nitrogen which, when it is combined with C/N ratio,
can be a good indicator of organic matter origin.
The aim of this paper is to supplement existing knowledge on small
mountainous rivers and further demonstrate their important role in
global carbon budget. To achieve this goal, we used new data (with notably POC and particulate nitrogen (PN) concentration data) acquired at
high temporal resolution to (1) calculate the annual yields of different
carbon fractions and PN, (2) determine processes controlling dissolved
carbon and particulate fraction distributions, (3) estimate the impact of
meteorological events on the annual carbon fluxes, (4) calculate the
carbon mass balance at Guadeloupean watershed scale, (5) estimate
the carbon organic export by small and tropical mountainous islands
to the world ocean. To address these issues we have selected three
watersheds hydrologically monitored for several decades with various
size, elevation, ages, slope, and exposure to rainfall. One river was monitored intensively with an automatic water sampler equipped with
pressure sensors activated during flood events, increasing significantly
the time resolution of sampling.
2. Study area
Guadeloupe is part of the Lesser Antilles volcanic arc generated by
the subduction of the North American plate beneath the Caribbean
plate. The volcanic island of Basse-Terre, part of the Guadeloupe
archipelago, belongs to the central segment of the arc (e.g., Feuillet
et al., 2011) (Fig. 1). The main characteristics of the studied watersheds are given in Table 1 and summarized in Fig. 2. The three watersheds are studied as part of the ObsErA (INSU-CNRS) observatory
devoted to the study of weathering and erosion in the French West
Indies. The observatory belongs to the French network of monitored
watersheds (RBV supported by INSU-CNRS and AllEnvi).
2.1. Vegetation cover, geology and soils
The three watersheds are located in the National Park of Guadeloupe
in the central part of the volcanic Basse-Terre Island. The vegetation
is mainly dominated by tropical rainforest and by altitude forest type
at the head of watersheds (Rousteau et al., 1994; Rousteau, 1996).
The Bras-David watershed located in the center of the Basse-Terre
Island is essentially composed of Pleistocene andesitic and dacitic
formations (Samper et al., 2007), covered by a very thick ferralitic
soil (>15 m; Colmet-Daage and Bernard, 1979). These soils were
previously studied (Buss et al., 2010; Lloret, 2010; Sak et al., 2010) and
consist of highly weathered volcanoclastic debris flows containing
rocky clasts at various stages of weathering. Clays, dominantly halloysite,
represent about 75 wt.% of the mineralogy and nonclays are almost
entirely Fe(III)-hydroxides and quartz/cristobalite. The average C/N
ratio (expressed as %wt:%wt) is 12.9 ± 3.4 (Lloret, 2010). The slopes
observed for this watershed are essentially included between 25 and
48% (Plaisir et al., 2003). The Capesterre and the Vieux-Habitants watersheds, located respectively in the southeast and southwest parts of the
Basse-Terre Island, are underlain by andesitic rocks linked to late Pleistocene volcanism (Samper et al., 2007) and covered with thin Andosol
(b1 m; Colmet-Daage and Bernard, 1979; Cattan et al., 2007), related
to the steep slopes of the young volcanic rocks. The average C/N ratio is
11.8 ± 1.6 (Lloret, 2010). The slopes observed for these two watersheds
are higher than 49% (Plaisir et al., 2003).
2.2. Climate and hydrology
The Basse-Terre Island is characterized by a wet tropical climate,
with a mean annual temperature around 23 °C and 75% humidity
(Plaisir et al., 2003). The average annual precipitation for the last
20 years ranges from 1200 to 8000 mm yr− 1, depending on the
231
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
Fig. 1. Lesser Antilles map with the location of the Guadeloupe Island.
have the same basin head, where they receive the same annual cumulated rainfall (about 4000 mm yr−1).
The Guadeloupean runoff represents approximately 60% of the precipitation and the discharges of the studied rivers are monitored by the
DEAL (French Water Survey agency; http://www.hydro.eaufrance.fr).
For the 2007–2010 period, the instantaneous discharge of the BrasDavid River ranges from 0.2 to 25.7 m3 s−1, those of the Capesterre
River from 0.5 to 119.7 m3 s−1, and those of the Vieux-Habitants
River from 0.5 to 44.9 m3 s−1 (Fig. 2). Flood events occurred throughout the years, but in majority during the wet season. Significantly high
discharges occurred during strong rainy events and tropical storms
like the cyclone Dean (17 August 2007), and two storms that occurred
on the 05 May 2009 and the 19 June 2010. Based on the DEAL discharge
dataset and according to the frequency of discharges, we determined
that studied rivers are in flood during 9.9% of the year, and in extreme
flood during 0.1% of the year. Rivers become flooded when discharge exceeds 3.5 m3 s− 1, 1.3 m3 s− 1, and 2.6 m3 s− 1 and, are extremely flooded when discharge exceeds 32.5 m3 s− 1, 9.0 m3 s− 1,
and 18.0 m3 s− 1 for the Capesterre River, the Bras-David River and
topography. There are two seasons: a dry season from January to
June and a wet rainy season from July to December (60 to 90% of
the average annual precipitation). During the wet season, hurricanes
and tropical depressions produce individual rainfall events, which play
a major role on the erosion of Guadeloupean soils and weathering
products. The spatial distribution of precipitation is strongly influenced
by easterly winds and topography (Fig. 2). We collected precipitation
data from three Météo-France pluviometers close to our sampling points
(Petit-Bourg Duclos, Capesterre-Belle-Eau Neufchâteau and VieuxHabitants Gendarmerie Beausoleil) and from the OVSG-IPGP meteorological station at the summit of La Soufrière volcano (1463 m, the highest
point of the Lesser Antilles). The highest precipitation is recorded at the
top of the volcano, with annual cumulated rainfall varying between 4100
and 5100 mm during the 2007–2010 period. The Bras-David and the
Capesterre watersheds are located on the windward coast influenced
by easterly winds and also high annual precipitation, between 2000
and 4300 mm yr−1.The Vieux-Habitants watershed, located on the
leeward coast, receives on average1100 mm yr−1of rainfall near the
sampling location. The Capesterre and the Vieux-Habitants watersheds
Table 1
Watershed characteristics with the number of flooding and “extreme” flooding days.
Sites
Lat.
Long.
Area Elevation
Age of
Slopesb
bedrocka
km2
Myrs
m
Vegetationc
0– 25– 49– >99% Thickets Altimountain Rainforest Evergreen 2007 2008 2009 2010
24% 48% 99%
forest
forest
%
%
Bras David
N16°10′33.6″ W61°41′34.8″ 11.3
228–1088 1.460
38
48
14
0
Capesterre
N16°04′18.0″ W61°36′34.1″ 16.6
200–1342 0.554
18
32
45
5
Vieux
N16°05′11.8″ W61°43′31.3″ 19.3
Habitants
250–1354 0.435
13
32
51
4
a
b
c
(Samper et al., 2007).
(Plaisir et al., 2003).
(Rousteau et al., 1994; Rousteau, 1996).
Days in flood
(extreme floods)
35
65
33
39
29
9
34
53
4
44
(2)
83
(3)
57
(4)
79
(2)
125
(6)
97
(4)
103
(15)
141
(12)
50
(3)
64
(3)
99
(12)
232
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
Fig. 2. Map of the Basse-Terre Island with the location of the three studied watersheds. Instantaneous water discharge and monthly cumulated rainfall are presented for each river
from January2007 to December 2010. The absence of bars or lines corresponds to an absence of measurements. Long dashes represent the minimum discharge for the flood level
and short dashes represent the minimum discharge for the extreme flood level. The rainfall at the Soufrière volcano has been corrected to the wind effect.
the Vieux-Habitants River, respectively. The number of days when
the rivers were in flood and in extreme flood is listed in Table 1.
3. Methodology
3.1. Sample collection
Pristine water samples were collected upstream of any anthropogenic activities (Fig. 2). Surface water was sampled manually
from 2007 to 2010 at different hydrological stages corresponding to
low water levels and floods. An automatic water sampler, ISCO6712, was set up on the Capesterre River. It allowed the sampling of
27 flood events including 5 extreme events during which up to 24
samples were taken (from every 15 min to 2 h), as changes can
occur over intervals as small as dozen of minutes (e.g., Bass et al.,
2013).
An important aspect of our investigation was to assess whether
our river sampling covers a representative range of flow rates.
This was done by comparing the distribution of flow discharges
to the distribution of sampled discharges for each river (Fig. 3).
233
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
The distributions of flow discharges were calculated using the flow rate
chronicles of the “Direction Regionale de l'Environnement”. Fig. 3 shows
that the use of an automatic water sampler allowed us to cover a representative range of flow rates in the case of the Capesterre River. This is
not the case for the Bras-David and the Vieux-Habitants rivers as the
difficulty of performing manual sampling during floods prevented us
from sampling high discharges.
Samples used for the measurement of dissolved organic carbon concentrations were filtered through glass fibre filters (GF/F Whatman® by
Schleicher & Schuell cut off 0.7 μm), acidified with concentrated H3PO4
(85%) in pre-cleaned and pre-combusted glass bottles and stored at 4 °C
in the dark. Filters with the total suspended matter (TSM) were dried
and then stored in Petri dish.
3.2. Analytical methods
Temperature and pH were measured in situ, with field probes.
Alkalinity values were measured at the laboratory with an automatic
acid–base titration stand (Radiometer TIM840with Schott probe)
by the Gran method with a precision of 1%.
Total Suspended Matter (TSM) concentrations were measured
on pre-weighted GF/F filters and dried at 60 °C before and after the
filtration. POC and PN were assessed directly from GF/F filters. One
eighth of each filter was weighed (±0.001 mg DW) and analyzed
in a Thermo Scientific Flash 2000 organic elemental analyzer, after
overnight (12 h) acidification under concentrated HCl (37%) acid
vapor prior to the determination of organic carbon. The precision
was b 5%. Dissolved organic carbon (DOC) concentrations were measured using a Shimadzu TOC-VCSH analyzer (Sugimura and Suzuki,
1988). The detection limit was 0.24 mg l−1 and the precision was 2%.
3.3. DIC calculations
As carbon transported in rivers provides a major link between
land and atmosphere, it is important to have an accurate measure
of the alkalinity that can be used to calculate CO2 out-gassing from
rivers. The contribution of the organic matter in the measure of
alkalinity (i.e. the titration of organic matter acido-basic moieties)
will induce an overestimation of CO2 fluxes and weathering rates
(Hunt et al., 2011). A correction is then needed to remove its contribution to the measured alkalinity when high DOC concentrations
are measured. The charge of organic matter was estimated using
the NICA-Donnan model (Benedetti et al., 1996). Assuming that
the organic matter is mainly composed of fulvic acid as in natural ecosystem and that 70% of this organic matter is reactive (Jouvin et al.,
2009), then the amount of protons consumed during the titration
by the organic matter is calculated and the alkalinity can be corrected.
Calculations of DIC were made using in-situ pH, temperature,
corrected alkalinity and ionic strength measurements performed in
the laboratory. The pH values for Guadeloupean rivers range between
6.03 and 8.63, and the temperature values vary between 20.0 °C and
26.5 °C. The following simplified formula (neglecting concentrations
of OH− and H+) was then used:
DIC ¼
1
γ
1þγ1 #10ðpKA1 −pHÞ þγ1 #10ðpH−pKA2 Þ
2
Alkcor
þ γ2
2
1þγ #10ðpKA2 −pHÞ þγ2 #10ðpHA1 þpKA2 −2pHÞ
1
with Alkcor = alkalinity corrected of organic moieties, γ1 and γ2 = activity
coefficients of ions mono-charged or bi-charged, KA1 and KA2 = first
and second acidity constants of carbonate system, respectively.
4. Results
4.1. Carbon distribution for the three watersheds
The solid transport associated to the soil erosion mainly occurs
during floods. TSM concentrations for the three rivers range between
6 and 476 mg l−1 (Table 2). For comparison, values from other small
mountainous rivers from subtropical mountainous rivers in Taiwan
(TSM = 2 to 10 000 mg l−1; Kao and Liu, 1996; Kao et al., 2005;
Dadson et al., 2005; Hilton et al., 2012), or tropical mountainous
rivers of Puerto Rico (TSM = 6.96 to 61.8 mg l−1; McDowell and
Asbury, 1994). POC concentrations in the three rivers range between
0.3 and 75 mgC l−1 (Table 2) and represent between 3.5 and 23.5%
of the TSM (Fig. A1). For comparison, they fall in the same range
as non-fossil POC values in Taiwan (0.1–100 mg l−1; Kao and Liu,
1996; Hilton et al., 2008a, 2012), and are significantly higher than values
reported in Puerto Rico (0.19–1.73 mg l−1; McDowell and Asbury,
1994). PN concentrations range between 0.02 and 4.9 mg l−1and are significantly higher than data obtained for Puerto Rico (0.02–0.08 mg l−1;
McDowell and Asbury, 1994). The mean annual C/N ratio varies between
13.7 ± 0.8 and 16.4 ± 4.3 and is higher than C/N ratios of local soils
(on average 12.5; Lloret, 2010).
All rivers have a similar range of DOC concentrations, between 0.34
and 5.75 mg l−1 (Table 2). DOC concentrations are similar to those measured in tropical mountainous rivers of Puerto Rico (≈1.32–2.16 mg l−1;
Fig. 3. Comparison between the distribution of discharges (black line) and the distribution of sampled discharges for the Capesterre, the Bras-David and the Vieux-Habitants rivers.
234
Table 2
Range of particulate concentrations (TSM, POC, PN) and dissolved concentrations (DIC, DOC) for the three studied watersheds and incidentally the average value balanced to the discharge. N represents the number of samples.
2009
2010
2007
2008
2009
2010
2007
2008
2009
2010
2007
2008
2009
2010
2007
2008
2009
2010
N = 15
0.51–
5.31
(2.23)
N = 143
0.56–
4.73
(2.25)
N=4
0.92–
1.99
(1.50)
N=5
0.53–
1.21
(0.77)
N = 10
0.47–
2.27
(1.31)
N=3
0.46–
0.93
N=8
0.61–
2.11
(1.52)
N = 71
0.65–
5.75
(1.95)
N=2
0.72–
1.28
N = 24
2.92–
8.38
(5.63)
N = 155
0.78–
7.62
(2.30)
N = 16
2.42–
6.89
(3.24)
N = 14
2.12–
6.97
(4.13)
N = 142
1.43–
8.72
(3.53)
N=4
4.14–
6.19
(4.99)
N=5
3.71–
5.57
(4.55)
N = 10
1.51–
5.39
(3.00)
N=3
2.31–
6.01
N=2
1.96–
2.42
N=2
7.5–
30.9
N=0
N=0
N=0
N=2
0.26–
2.36
N=0
N=0
N=0
N=1
0.11
N=0
N=0
N=0
N = 68
1.20–
4.97
(2.21)
N=2
4.72–
4.82
N = 33
5.8–
153.6
(56.2)
N=2
11.6–
11.9
N = 18
20.2–
72.0
(51.8)
N=1
45.3
N=1
21.5
N=6
60.9–
476
(240)
N=0
N = 33
0.40–
19.65
(6.70)
N=2
0.41–
1.54
N = 18
1.58–
7.91
(5.17)
N=1
4.30
N=1
1.58
N=6
9.0–
74.8
(33.4)
N=0
N = 33
0.02–
1.37
(0.45)
N=1
0.10
N = 18
0.12–
0.60
(0.35)
N=1
0.28
N=1
0.13
N=6
0.66–
4.92
(2.31)
N=0
N=0
N=0
N=0
Vieux
Habitants
n
X
i¼1
Qi
CiQi
n
X
i¼1
Following previous authors (Hilton et al., 2008a; Liu et al., 2011),
we choose to estimate the annual fluxes of DOC, DIC, TSM, POC and
PN using the rating curve method which appeared more appropriate
than the averaging method (Walling and Webb, 1981; Ferguson,
4.4. Calculation of dissolved and particulate yields
In the specific hydrological context of mountainous and tropical
Guadeloupean rivers, the timing of sample collection is critical. As
an illustration of the temporal variations, the extreme flood from 17
August 2007, associated with Cyclone Dean is shown on Fig. 5 for
the Capesterre River. The overall global sampling time is one day and
the discharge increases quickly from 1.5 to 30.0 m3 s−1 in 90 min,
reaching a maximum discharge of 50.8 m3 s−1 after 150 min. This
rapid increase in flow leads to significant modifications in carbon
export: DIC concentrations decrease from 4.01 to 1.39 mg l−1 when
DOC concentrations increase from 0.87 to 4.52 mg l−1, leading to an
increase of the DOC/DIC ratio. These temporal variations in a single
hydrological event illustrate the importance of an accurate sampling
of the entire flood when we attempt to characterize the bio-hydrodynamic of all carbon forms within the context of small tropical mountainous watersheds.
4.3. Concentration–discharge relationship
Monthly DIC concentrations (Fig. 4a) vary between 1.51 and
5.82 mg l−1, and conversely to discharge, indicating a partial dilution
effect. Monthly DOC concentrations (Fig. 4a) reveal a seasonal pattern,
with minimum values measured for the driest months (January, February
or March; median = 0.96 mg l−1), and maximum values calculated
for the rainiest months (August, September and October; median =
2.16 mg l−1). Monthly particulate concentrations (TSM and POC) and
C/N ratio (Fig. 4b) in the river do not reveal a seasonal trend, with little
variations from one month to another. Monthly TSM, POC and C/N are
about 40.0 mg l−1, 4.0 mg l−1 and 15.0, respectively. Further analysis
on suspended materials would confirm these observations.
Cm ¼
The large number of measurements obtained on the Capesterre
River from the automatic sampler allows a better resolution of temporal variations and in chemical flux calculations. Monthly average
concentrations (Cm) of TSM, POC, PN, DIC and DOC were calculated
from instantaneous concentrations (Ci) and instantaneous discharge
(Qi), using the formula:
4.2. Monthly variations for the Capesterre River
McDowell and Asbury, 1994), or for steep tropical rainforest river of
Australia (1.60–5.90 mg l−1; Bass et al., 2011).
For the Capesterre River, the Bras-David River and the VieuxHabitants River, the contribution of organic matter represents
respectively 0.3 to 16.1%, 0.2 to 10.6%, and 0.2 to 4.9% of the measured
alkalinity. Calculated DIC concentrations range between 0.78 and
8.72 mg l−1 for the Capesterre River, 1.96 and 8.38 mg l−1 for the
Bras-David River, 2.31 and 6.89 mg l−1 for the Vieux-Habitants River
(Table 2). They do not significantly vary annually and are similar to
results obtained in other parts of Guadeloupe by Lloret et al. (2011),
for other volcanic islands as Mont Serrat and the Dominica (Goldsmith
et al., 2010; Gaillardet et al., 2012), and to the lowest values measured
in Mount Cameroun rivers (Benedetti et al., 2003) and the Réunion
rivers (Louvat and Allègre, 1997), where the alkalinities are not corrected for the DOC contribution.
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
TSM concentrations correspond to the difference between mass of filter before and after filtration normalized to the filtered water volume.
a
2008
N = 26
0.46–
2.07
(0.95)
N = 194
0.48–
4.06
(2.28)
N = 18
0.34–
2.39
(1.89)
Capesterre
2007
Bras David
PN (mg.L−1)
POC (mg l−1)
TSM (mg l−1)a
DIC (mg l−1)
DOC (mg l−1)
Sites
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
235
Fig. 4. Variations of average monthly concentrations in dissolved fraction DIC and DOC (a), and in particulate fraction (TSM, POC and C/N) (b) for the Capesterre River. Monthly
cumulated liquid discharges (grey bars) are represented on each graph to allow comparison between river discharge and particulate and dissolved concentrations.
1987; Letcher et al., 1999). Chemical fluxes were therefore computed
as follows. Concentrations of DOC, DIC, TSM, POC and PN are plotted as
a function of discharge (Q) in the Capesterre River in Fig. 6a and b
(Similar rating curves are for the Bras-David and the Vieux-Habitants
Rivers are shown in Fig. A2). Despite a lot of scatter, the concentration
data follow linear trends in these log–log plots, suggesting power law
dependencies. The concentration–discharge relationships for each element x were all fitted by a power-law relationship:
Cx ¼ αQ
β
where α and β are proportionality constant and exponent, respectively.
Discharges of the three studied rivers are monitored by the
DEAL with a time step varying typically from one measurement every
15 min during floods to one measurement every hour at low flow. To
avoid a bias towards large flow rates, we interpolated these discharge
data on a regular time step equal to 15 min. The instantaneous fluxes
(Finstx) of each element x were then calculated with the same time
resolution using the rating concentration–discharge curves:
Finstx ¼ CxQ ¼ αQ
βþ1
Integrating the instantaneous fluxes with respect to time allowed
us to compute the annual averaged fluxes (Fx) of each element x for
the years 2007, 2008, 2009 and 2010:
t2
Fx ¼ α∫ Q
βþ1
dt
t1
Assuming the uncertainty on the flow rate measurement is negligible,
the uncertainty on the annual flux is given by:
"
t2
ΔFx ¼ Δα ∫ Q
t1
βþ1
#
"
t2
dt þ Δβ ∫ lnðQ ÞQ
t1
"
#
t2
Δα
βþ1
Fx þ Δβ α∫ lnðQ ÞQ
¼
dt
α
t1
βþ1
#
dt
Leading to:
ΔFx ¼
"
#
βþ2
Δα
Δβ
Q
Δα
Δβ
Fx þ
Fx þ αΔβ
Fx þ
Fx
lnQ ≈
α
βþ2
α
βþ2
βþ2
Note that any interruption of the discharge record on a period
longer than 24 h was considered a gap in the data. Gaps were not
taken into account in the calculation of the annual averaged fluxes
of DIC, DOC, POC and PN. The cumulated duration of gaps varies typically between 0% and 17% so that their influence on calculated annual
fluxes is moderate (see Table 3). Only the Vieux-Habitants River may
be problematic as the cumulated duration of gaps is 60% for 2009
and 94% in 2010. The corresponding values of fluxes should therefore
be considered with caution. The yields corresponding to the ratio of
these fluxes on the surface area of the watershed are displayed in
Table 3. As pointed out by several authors (Ferguson, 1986, 1987;
Preston et al., 1989; Singh and Durgunoglu, 1989), the rating curve
method used in this investigation is likely to underestimate the
chemical load. Several methods have been proposed to correct for
this bias (Ferguson, 1986; Cohn et al., 1989). However these methods,
based on the use of a correction factor, rely on strong assumptions about
the statistical distribution of the data and are still debated (Letcher et al.,
1999). The values presented in Table 3 should therefore be considered as
a lower bound to the actual yields. Yields calculated for the three studied
rivers range between 1.9 and 8.6 t km−2 yr−1 for DOC, between 8.1 and
15.8 t km−2 yr−1 for DIC, between 8.1 and 25.5 t km−2 yr−1 for POC,
between 67 and 213 t km−2 yr−1 for TSM and between 0.6 and
1.7 t km−2 yr−1 for PN (Table 3). Yields are systematically higher for
the Capesterre River than for the Bras-David and the Vieux-Habitants
rivers. For all three rivers, yields are higher in 2009 and 2010, corresponding to high precipitation rates.
5. Discussion
The following discussion will first focus on the comparison between
concentrations and discharge and the annual yields of dissolved and
particulate fractions for the three rivers from January 2007 to December
236
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
Fig. 5. Hourly follow up of an extreme event in the Capesterre River: 17 August 2007, associated to Dean Cyclone. Variations of DOC and DIC concentrations, and DOC/DIC ratios
during the event. Long dashes represent the minimum discharge for the flood level and short dashes represent the minimum discharge for the extreme flood level.
2010. Then the discussion will address the question of the impact of
floods on carbon export and the distribution of carbon forms according
to the type of meteorological events (tropical storms, cyclones). Finally
we will present the carbon dynamic at the scale of small tropical watershed and compare carbon fluxes from small tropical mountainous rivers
with those from large rivers.
5.1. Carbon and nitrogen yields
Fig. 6. DOC and DIC concentrations (a), TSM, POC and PN concentrations (b) plotted as
a function of the discharge (Q) for the Capesterre River. Long dashes represent the
minimum discharge for the flood level and short dashes represent the minimum
discharge for the extreme flood level. Lines correspond to the best fit of the data by a
power law: concentration = αQβ (see Supplementary, Table A1 and A2).
DIC concentrations decrease with discharge increasing (Figs. 6
and A2). This indicates a partial dilution effect during floods due to
significant water inputs from rainfall. This partial dilution is also
monthly observed (Fig. 4a). DIC yields for the Capesterre River
(11.3–15.8 t km− 2 yr− 1, Table 3) are slightly higher than for the
Bras-David River (9.8–12.7 t km− 2 yr− 1) and the Vieux-Habitants
River (8.1–11.9 t km−2 yr−1). These new yields, determined from
temporal survey, are consistent with those estimated in our previous
study (Lloret et al., 2011). As these new estimates take into account
the high-discharge events, they are more accurate and slightly lower
than previous ones, reflecting the partial dilution effect during floods.
Although computed differently, these DIC yields are in agreement
with those determined for rivers flowing through volcanic lithology
under tropical climate (Louvat and Allègre, 1997; Dessert et al., 2001,
2003; Goldsmith et al., 2010; Gaillardet et al., 2012). Moreover, the
highest yield observed for the Capesterre River is likely due to the steeper
slopes (>49 %; Table 1) related to the young age of rock formation and
high precipitation in this watershed located in the south-eastern of the
Basse-Terre island (Lloret et al., 2011). DIC yields calculated for these
three Guadeloupean rivers are significantly higher than the yields determined for the large rivers under wet tropical climate, like the Amazon
(5.0 t km−2 yr−1) or the Orinoco (5.5 t km−2 yr−1; Cai et al., 2008).
The results show a positive correlation between concentrations
of DOC and particulate fractions (TSM, POC and PN) and discharge
(Figs. 6 and A2). DOC and POC contents have already been shown to
increase during high-discharge events in tropical and subtropical
watersheds (Hilton et al., 2008a; Bass et al., 2011; Hilton et al., 2012;
Jeong et al., 2012). The positive DOC/discharge correlation suggests
that mobilization of DOC is more significant than the dilution effect, in
contrast to observation for DIC. Different factors play a role in organic
material mobilization as explained by Lloret et al. (2011) for a smaller
dataset of DOC concentrations. During low water level periods, the
Table 3
DIC, DOC, POC, TSM and PN yields (respectively YDIC, YDOC, YPOC, YTSM, YPN) in t km-2 yr-1 calculated with power law method (see Section 4.4 for calculation description) for the Capesterre River, the Bras-David River and the Vieux-Habitants
River.
Q (m3 yr−1)
YDOC
YDIC
89.1%
88.4%
39.7%
6.6%
3.8 ∗ 107
5.1 ∗ 107
3.6 ∗ 107
1.9 ± 0.3
2.7 ± 0.5
1.9 ± 0.3
9.6 ± 1.0
11.9 ± 1.2
8.1 ± 0.8
4.1 ∗ 107
2.2
9.9
YDIC
Duration of
the record
Q (m yr
11.5 ± 1.0
11.8 ± 1.1
12.7 ± 1.1
9.6 ± 0.9
11.4
96.1%
95.2%
97.1%
83.3%
4.8
6.6
7.6
7.4
6.6
4.9 ±
7.4 ±
8.6 ±
8.6 ±
7.4
0.2
0.3
0.4
0.4
11.3 ±
14.2 ±
15.8 ±
15.2 ±
14.1
0.3
0.4
0.4
0.4
67 ±
153 ±
177 ±
213 ±
153
27
62
72
86
8.1 ±
18.4 ±
21.2 ±
25.5 ±
18.3
4.1
9.3
10.7
12.9
0.6 ±
1.3 ±
1.5 ±
1.7 ±
1.2
0.3
0.6
0.7
0.9
Duration of
the record
YPN
YPOC
YTSM
YDIC
YDOC
)
−1
)
YDOC
107
107
107
107
107
∗
∗
∗
∗
∗
0.4
0.5
0.7
0.4
±
±
±
±
Years
Duration of
the record
Q (m yr
2007
2008
2009
2010
4-year Average
89.0%
94.2%
100%
100%
2.5
2.7
3.2
2.0
2.6
2.6
2.9
4.2
2.4
3.0
3
−1
3
107
107
107
107
107
∗
∗
∗
∗
∗
Vieux-Habitants
Capesterre
Bras-David
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
237
rivers are fed by ground water flow from the saprolite. These deep
waters are depleted in DOC. The organic matter molecules are larger
(molecular weight), more aromatic and less hydrophilic. Conversely,
during flood periods, the rivers are fed by surface runoff DOC enriched
solutions. The organic matter molecules are smaller, less aromatic and
more hydrophilic. Additional factors could also play a role in the case
of the particulate fraction (POC and PN). Guadeloupean soil surface
layers are enriched in organic matter (10–15% of carbon; ColmetDaage and Lagache, 1965; Lloret, 2010); and during meteorological
events these layers are leached enriching water runoff in DOC and
POC. Moreover, the rate at which geomorphic processes erode the landscape depend on the steepness of the topography (Roering et al., 2001;
Scharrón et al., 2012), and high rates of physical erosion by land sliding
and water runoff are therefore expected to occur in Guadeloupean
watersheds (slope > 25%; Plaisir et al., 2003). Water runoff preferentially mobilizes loose material from soil surface layers (Gomi et al.,
2008), like POC and PN. This is consistent with previous studies linking
non-fossil organic carbon increase with discharge (Hilton et al., 2012;
Smith et al., 2013). These processes influence the organic carbon quality
and reactivity exported from river to ocean. Moreover this positive
trend between POC concentrations and discharge indicates that the
POC is not sequestered in rivers and is directly transported in the ocean.
DOC yields calculated for the Capesterre River (4.9–8.6 t km−2 yr−1,
Table 3) are higher than yields calculated for the Bras-David
River (2.4–4.2 t km−2 yr−1) and the Vieux-Habitants River (1.9−
2.7 t km−2 yr−1). The Capesterre watershed is more exposed to rainfall, because of its windward localization, and presents steep slopes
(Fig. 2) leading to intense soil erosion processes, and thus high yields.
The relative low DOC yields for the Vieux-Habitants River are coupled
to the leeward coast localization of the catchment. Its rainfall gradient
is important, ranging from 5100 mm yr−1 in mountainous headwater
to 1100 mm yr−1in the downstream forested area, near the sampling
point. The head of the basin, that is mostly watered, is characterized
by very thin soils which limit the amount of stored organic carbon available to mobilization into the river. The higher DOC yields found for the
Bras-David River could be due to large amount of organic rich soils
found in this catchment and to slightly different above ground biomass
(the watershed is represented by ferralitic soils and 65% of rainforest;
Table 1). Finally, for all three rivers, DOC yields are higher in 2009 and
2010 than in 2007 and 2008, as 2009 and 2010 were the rainiest years
with significant extreme flood events (Table 1). We will show in the
following discussion that these DOC yields are significant and of the
same order of magnitude as those determined for some other small
mountainous catchments under wet tropical climate. The organic
carbon yield (DOC + POC) calculated for the Capesterre River varies
Fig. 7. DOC versus DIC fluxes (respectively FDOC and FDIC), and their distribution according to
the water level: low water level, floods (maximum instantaneous discharge ≥ 3.5 m3 s−1)
and extreme floods (maximum instantaneous discharge ≥ 32.5 m3 s−1).
238
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
between 13.0 and 34.1 t km−2 yr−1 and represents more than 50% of
the total carbon yield (DIC + DOC + POC) that varies between 24.3
and 49.3 t km−2 yr−1. This shows the importance of organic carbon
export in these small volcanic and tropical mountainous watersheds.
PN yields calculated for Guadeloupean rivers (0.6–1.7 t km−2 yr−1,
Table 3) are higher than yields determined for small mountainous rivers
and large rivers under tropical climate (0.00–0.39 t km−2 yr−1; Lewis
et al., 1999 and references therein). This is probably due to very steep
watershed slopes in Guadeloupe (>25%, Table 1), inducing higher soil
erosion in the Capesterre watershed than in these other tropical watersheds. These high PN yields may be important for nutrient budget as PN
constitutes one of the major forms of nutrient transport (e.g., Meybeck,
1982).
5.2. Impact of extreme meteorological events on carbon dynamic
The instantaneous DOC and DIC fluxes acquired between 2007 and
2010 for the Capesterre River (Fig. 7) allowed the determination of
different DOC/DIC ratio (between 0.1 and 2.3) related to the flood
intensity. Then, extreme flood samples (EXT) are clearly differentiated
from the others (LWL + FL: low water level and floods).Three extreme
floods resulting from rains related to the Dean cyclone (17th of August,
2007), a heavy localized rainfall (13th of April, 2008) and the Otto
cyclone (7th of October, 2010) were sampled. During each event, a
minimum of 140 mm of rain was recorded at the volcano's summit
(OVSG-IPGP data), illustrating the intensity of these events. The event
intensity strongly impacts the transport and dynamic of carbon
(Fig. 7). Apart from these three events, the transport of dissolved carbon
is mainly inorganic, with DIC fluxes generally higher than DOC fluxes
(Fig. 7). This trend is reversed during extreme floods, resulting in
unusual high DOC/DIC ratios around 2 for the Dean Cyclone samples.
During extreme meteorological events, high DOC exports have typically
been associated with near surface hydrologic flow paths that intersect
DOC rich forest floor and superficial soil layers (Hornberger et al.,
1994; Boyer et al., 1997; Hagedorn et al., 2000). For LWL and FL events,
low DOC exports are associated to a vertical flow in soil and a short
contact time with the organic rich top soil layer (Johnson et al., 2006;
Lloret et al., 2011). This interpretation is supported for LWL events by
δ13C isotopic composition of DOC as well as the organic matter characteristics. Lloret et al. (2011) have shown a trend between δ13CDOC versus
1/DOC and between δ13CDOC versus organic matter characteristics
(aromaticity, molecular weight) in Guadeloupean Rivers which
correspond to the mixing between two end-members. The first
end-members represents the low water level with the lowest concentrations in DOC associated to the largest, the most aromatic,
Fig. 8. The relative abundance (in %) of LWL, FL and EXT for Annual discharge, DIC, DOC
and POC fluxes.
the less hydrophilic, the less mobile organic matter molecules and
C depleted DOC. Conversely, the second end-member represents
the floods with highest concentrations in DOC associated to the
smallest, the less aromatic, the most hydrophilic, the most mobile
organic matter molecules and 13C most enriched.
The contribution of floods and extreme floods on the total carbon
export, during the 2007–2010 period and for the Capesterre River
is shown in Fig. 8. The DIC fluxes occur essentially during the low
water level, representing 75% of the annual flux. Organic carbon export occurs essentially during floods and extreme floods. Around
54.5% of the DOC is exported during floods and extreme floods. Our
results show that more than 85% of the annual POC flux occurs during
floods and that extreme events account for 28% of this annual flux
alone. This carbon export partitioning is observed for all years of the
study, except for 2007, an unusual “dry” year mainly characterized
by low water level. It emphasizes once again the great interest to
monitor rivers over several years to estimate carbon exports as representative as possible in tropical context. Bass et al. (2011) showed
that DOC and POC fluxes are largely underestimated if high temporal
resolution sampling is not performed. It also suggests that there is
“room” for increased carbon export if the frequency and the intensity
of aperiodic intense precipitation events such as cyclones or tropical
storms increase (Knutson and Tuleya, 2004; Emanuel, 2005; Trenberth,
2005; Goldsmith et al., 2008; Hilton et al., 2008a).
The most common pattern observed in many rivers in the world
associates a variation in POC concentration to discharge and TSM concentration (Ittekkot et al., 1985). Then an increase in liquid discharge
associated with an increase in TSM generally causes a decrease in
the percentage POC in TSM (Meybeck, 1982). This pattern has been
shown to be a result of the dilution of organic matter by mineral
matter and has been reported in the different tropical region like
in Southeastern Asian Rivers (Ittekkot et al., 1985) and in South
American Rivers (Depetris and Lenardón, 1982; Hedges et al., 1986;
Moreira-Turcq et al., 2003). This trend was not clearly observed for
Capesterre River's suspended materials that have a mean annual
POC/TSM ratio relatively constant over the years (ca. 0.12 ± 0.04).
In our study, the C/N ratio varies between 9.0 and 27.2 but does
not depend on discharge. These variations suggest that the POC in
the river likely comes from two sources. The higher value of C/N
ratio compared to soil (C/N = 12.5 on average; Lloret, 2010) suggests
some inputs of less degraded organic matter, and could come from
litter which exhibits higher C/N ratios (25.8 ± 0.7; Lloret, 2010).
This is in agreement with observations done for small mountainous
rivers in Taiwan (Kao and Liu, 2000; Hilton et al., 2010) and Australia
(Bass et al., 2011). The appropriate and exhaustive sampling of FL and
EXT type of events is therefore a key issue to estimate correctly DOC
and POC fluxes and this is not always done in watershed studies
limiting the impact of the obtained results in terms of global carbon
mass balance.
The distribution of POC/DOC ratio is controlled by erosion over
watershed, including climate and vegetation, geomorphology and
tectonic, and even chemical and mineral compositions as previously
demonstrated by Ludwig et al. (1996b). The POC/DOC ratio is sometime characteristic of the type of event, for instance Malcolm and
Durum (1976) have observed in the Mississippi River a variation
of POC/DOC ratios depending on the type of meteorological events,
with an increasing ratio when discharge increases. We observe the
same trend in Guadeloupean rivers, indicating an increase of POC export higher than DOC export. The Capesterre River, with annual ratio
varying between 1.35 and 1.67, is different from trend observed in the
large tropical rivers like the Parana, the Orinoco and the Amazon and
its major tributaries (Milliman and Meade, 1983; Richey et al., 1990;
Moreira-Turcq et al., 2003), where the DOC flux is the major part
of the total organic carbon flux (POC/DOC b 1). However, Wu et al.
(2007) reported POC/DOC higher than those in Asian rivers affected
by the monsoon. An explanation would be that these rivers have
13
239
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
Fig. 9. Summary of the carbon mass balance at the Guadeloupean watershed. The yields are in tC km−2 yr−1. Underlined values were obtained for our watershed. Other values are
from other tropical watersheds (Bray and Gorham, 1964; Madge, 1965; Hopkins, 1966; Rodin et al., 1967; Blanchart and Bernoux, 2005; Silver et al., 2005). Details of each flux and
stock in supplementary.
extensive areas where organic matter would be generated by primary
production and exported in particulate form (Burns et al., 2008) or
flood-plains allowing “older geogenic” POC remobilization that will
modify the POC/DOC ratio. We suppose that high annual POC/DOC
ratios (1.35–1.67) observed for the Capesterre River are likely due
to the mountainous topography which may lead to fast flushing of
sediments (Burns et al., 2008). Unlike the large rivers, the small rivers
of Guadeloupe are directly connected to the oceans and present steep
slopes and no flood-plains. Erosion of soils and the strength of the
event are the major drivers of the POC/DOC ratio observed. A larger
data set of POC/DOC ratios corresponding to a larger number and
typology of more intense precipitation events is needed to confirm
these hypotheses.
5.3. Carbon stocks, fluxes and partial “residence time” of organic carbon
in soils
The incorporation of the carbon dynamic of small rivers into carbon
models is a complex multistep long-term goal. A first step to reach this
goal is to document carbon stocks and fluxes at the scale of high order
stream watersheds. The two main carbon stocks in a watershed are
aboveground biomass and soils. The fluxes represent links between
atmosphere, vegetation, soils and river at the watershed scale. These
fluxes and stocks are reported on Fig. 9 (measured or calculated; See
Supplementary for details) for the Capesterre watershed, where DIC,
DOC and POC were measured intensively. We can observe that the
carbon mass balance for Guadeloupean soils is not at the equilibrium.
Table 4
Calculations of the partial “residence time” (TimeSOC) for different large tropical watersheds and small mountainous watershed including the Capesterre watershed (see Section 5.3
for calculation description), with DOC and POC yields (respectively YDOC and YPOC) and carbon stock in soils for the 30 first cm.
Site
Large tropical rivers
Small mountainous rivers
a
b
c
d
e
f
g
h
Amazona
Orinocob
Paranab
Mengongc
New Zealandd
Puerto Ricoe
Papua New Guineaf
Taiwang
Guadeloupeh
Climate
2007
2008
2009
2010
4-year Average
Tropical
Tropical
Tropical
Tropical
Temperate
Tropical
Tropical
Tropical
Tropical
YPOC
YDOC
C stock in soil
TimeSOC
tC km−2 yr−1
tC km−2 yr−1
tC km−2
yrs
1.09
1.64
0.26
0.58
43.4
6.95
17.16
15.44
8.1
18.4
21.2
25.5
18.31
5.84
4.81
1.43
5.70
2.44
6.62
4.99
4.10
4.9
7.4
8.6
8.6
7.38
10,300
10,300
10,300
15,340
28,000
10,000
8400
6950
13,000
1486
1560
6059
2445
610
736
380
355
1000
504
436
381
580 ± 280
Lewis et al. (1999) and references therein; Moreira-Turcq et al. (2003); Beusen et al. (2005); Baudin et al. (2007).
Ludwig et al. (1996a, 1996b); Baudin et al. (2007).
Boeglin et al. (2005).
Coomes et al. (2002); Lyons et al. (2002); Whitehead et al. (2002); Carey et al. (2005); Hilton et al. (2008b).
McDowell and Asbury (1994); Baudin et al. (2007); Stallard (2012).
Hartemink (2004); Burns et al. (2008).
Kao and Liu (1996, 1997, 2000); Hilton et al. (2012).
This study; Blanchart and Bernoux (2005).
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E. Lloret et al. / Chemical Geology 351 (2013) 229–244
The outputs (≈1700 tC km−2 yr−1; total soil respiration and carbon
exports) are three times higher than inputs (=555 tC km−2 yr−1;
litterfall + root production) (Fig. 9). Assuming that the values used
for this budget are representative of the Guadeloupean Island, this
could indicate that Guadeloupean soils are a source of CO2 to the
atmosphere. With an average pCO2 higher than the atmospheric pCO2
(cf. Supplementary), the Guadeloupean rivers seem to be also a source
of CO2 (e.g., Richey et al., 2002; Rasera et al., 2008; Butman and
Raymond, 2011). This statement relies on the absence of mineralization
of DOC and POC in rivers.
If we assume that the budget presented in this paper is representative of the Basse-Terre Island, we can roughly estimate a partial
“residence time” for organic carbon in soil (TimeSOC), this partial
“residence time” taking into account only the physical processes
as erosion transport (see formula below).
TimeSOC ¼
StockSOC
YDOC þ YPOC
with StockSOC the organic carbon stock in soils and YDOC and YPOC the
organic carbon yields (Table 4), and can be compared to an analogous
method by Hilton et al. (2012), although here we consider DOC too.
This estimation does not take into account any biogeochemical
reaction as mineralization, dissolution, or respiration. This partial
“residence time” will then be representative of the watershed ability
to export carbon to the ocean and is an important parameter to a better
understanding of setting ecosystem age and preventing retrogression.
The soil organic carbon in the Capesterre watershed exhibits a
partial “residence time” of 580 ± 280 years (Table 4), which is in the
range of those calculated for other small tropical watersheds of Puerto
Rico (736 years), Papua New Guinea (380 years), Taiwan (355 years)
and small temperate mountainous watersheds of New Zealand
(610 years) but indeed shorter than those calculated for large tropical
watersheds like the Mengong (2445 years) and the South American
tropical large watershed (1486–6059 years). The New Zealand watersheds export large amounts of modern and geological particulate
organic carbon, due to their localization on an active mountain belt
inducing rapid erosion and short TimeSOC (Hilton et al., 2008b). For
the Guadeloupean and other small tropical watersheds, the shorter
TimeSOC are due to the large exports of DOC and POC, related to the
leaching of soil surface layers and litters and the enrichment in DOC of
the top soil hydrologic flow paths that intersect organic matter rich forest litter and soil layers during meteorological events. In addition, these
steep tropical high order stream watersheds do not host floodplains or
deposition areas that could lead to a decrease of the POC concentrations
by labile organic carbon mineralization like within larger river systems
(Leithold et al., 2006; Battin et al., 2008) and would generate smaller
POC fluxes inducing higher TimeSOC values as found in the larger watersheds (Mengong, South American large watersheds; Table 4).
5.4. Small tropical and volcanic mountainous islands: a pool of organic
carbon for oceans
For Guadeloupean rivers, the major part of organic carbon export
occurs during the extreme meteorological events and is linked to
three important parameters: the high intensity of precipitation leading
to high runoff, the high slopes, and the high organic matter contents in
Andosols and ferralitic soils. Moreover the organic carbon export occurs
essentially in particulate form. To address the relative importance
of this organic carbon export to coastal oceans, we compare yields
Fig. 10. Map of POC, DOC and POC/DOC ratios for large rivers and small tropical-temperate and volcanic mountainous rivers (only watersheds directly connected to oceans are
represented). Data are based on Moreira-Turcq et al. (2003), Ludwig et al. (1996b), Dittmar and Kattner (2003), Gebhardt et al. (2004), Holmes et al. (2002); Rachold et al.
(2004) for DOC and POC fluxes of large rivers, Lyons et al. (2002), Carey et al. (2005) and Hilton et al. (2008b) for DOC and POC in New Zealand, McDowell and Asbury (1994)
and Stallard (2012) for Puerto Rico, Burns et al. (2008) for Papua New Guinea, Kao and Liu (1996, 1997, 2000) and Hilton et al. (2012) for Taiwan, Bass et al. (2011) for Australia.
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
determined for the Capesterre watershed (Table 3) with those of other
rivers. We compiled data from the literature and reported the mean
POC and DOC yields of small and large rivers on the map in Fig. 10.
Only yields of small watersheds directly connected to the oceans have
been considered in our study. DOC yield of Guadeloupean rivers are
close to yields determined for Puerto-Rico watersheds (McDowell and
Asbury, 1994; Stallard, 2012) and slightly higher than yields estimated
for small mountainous rivers under wet tropical (Taiwan rivers,
the Sepik River from Papua New Guinea and the Thompson Creek
River in Australia; 2.3–5.0 t km−2 yr−1; Kao and Liu, 1997; Burns
et al., 2008; Bass et al., 2011) and temperate climate (New Zealand;
0.6–5.2 t km−2 yr−1, Carey et al., 2005). If we integrate our results in
the overall context of continental DOC flux, we find that these flows
are higher than major rivers yields, including the Amazon and the
Orinoco. The Guadeloupean POC yield is similar to high yields calculated
for tropical mountainous rivers of Taiwan and Papua New Guinea (Kao
and Liu, 1996, 1997, 2000; Burns et al., 2008; Hilton et al., 2012). Bass
et al. (2011) presented data showing an average monthly POC flux of
68 kg km−2 yr−1, about 0.8 t km−2 yr−1, from their Australian watersheds, which is at least 10 times lower than our estimates. POC yields
are often higher than yields determined for the large rivers under wet
tropical climate, like the Amazon or the Orinoco (Beusen et al., 2005).
This is likely due to the strong erosive power of rainfall and the steep
slopes of watersheds allowing erosion of enriched organic matter soil
layers and leading to important export of DOC and POC (slopes of
Guadeloupean catchments higher than 49 %; Table 1).
Even if the surfaces of volcanic and mountainous tropical islands are
low compared to continental area, organic carbon yields are so important that these surfaces can represent a significant proportion of the
global annual carbon export. To extrapolate the overall flow of organic
carbon from small volcanic and mountainous tropical watersheds we
consider that average DOC and POC fluxes of Guadeloupean rivers are
representative of this carbon export. The surface of volcanic rocks in
oceanic islands and island arcs under wet tropical climate represents
about 0.32 106 km2 (without Indonesia and Papua New Guinea which
are partially volcanic; Dessert et al., 2003; Allègre et al., 2010). The
resulting global carbon fluxes of DOC and POC are 2.4 ± 0.6 MtC yr−1
and 5.9 ± 2.4 MtC yr−1, respectively. These carbon exports would be
much higher if we considered all the tropical mountainous islands and
not only volcanic islands. With an area of 5.85 106 km2, the Amazon
Basin is the largest DOC contributor to global ocean with approximately
34.2 MtC yr−1. However, the Amazonian POC flow of 6.4 MtC yr−1 is
quite comparable to that estimated for volcanic islands. The average
PN flux (0.2 ± 0.1 MtN yr−1) is also in the range of particulate nitrogen
flux reported for the Solimoes and the Amazon rivers by Moreira-Turcq
et al. (2003). We conclude that small tropical and volcanic mountainous
islands are major contributor of water and organic matter to coastal
waters. At the view of these first order results which can be refined,
we show that the nutrient mass budget to ocean is currently underestimated if we do not consider these small mountainous watersheds.
In addition the role of these small islands could become more important
with time as a diminishing fraction of world's rivers remains unaffected
by humans. Dams are a leading cause of disruption by leaving few major
free-flowing river systems (Finer and Jenkins, 2012). They could induce
significant modification downstream and upstream of the dam site
for organic matter quality and total carbon flux to the ocean.
In addition the quality of the organic matter transported by
Guadeloupean rivers is different from the one of the continental origin.
In Fig. 10, we have also reported the POC/DOC ratio for large rivers and
small tropical mountainous rivers. We can observe that this ratio varies
from 2.5 to 3.5 for small rivers of Guadeloupe, Taiwan and Papua New
Guinea and are significantly higher than large rivers values, except for
the Huang He River which presents the most extreme mechanical
erosion rates and POC/DOC ratio around 30. This indicates that the
organic matter exported by small islands is different, in term of quality
and reactivity, from the organic matter exported by the largest rivers.
241
A different ultimate fate for particulate and dissolved organic matter
may result more from separate physical pathways than contrasting
chemical compositions (Hedges et al., 1997). In the case of tropical
islands organic matter, dominated by particulate organic matter inputs,
could settle rapidly through the marine water column and typically
accumulates to suboxic depths of coastal marine sediments favoring
a long term storage compared to a more rapidly degradable dissolved
organic matter (Hedges et al., 1997; Burdige, 2005; Galy et al., 2007).
In the northern New Guinea coastal ocean, Burns et al. (2008) estimate
that 47% of the annual river organic carbon load (Sepik River) is
retained on the narrow coastal shelf or is degraded in the water column
and that the remaining 53% is discharged to the deep water in the
Bismarck Sea, through incised canyons on the shelf and slope. This
exported organic material is available for long distance transport into
ocean and for long term deposition into marine sediments. The efficiency
of fluvial POC burial for the Sepik River is quite important compared to
large tropical rivers entering wide coastal shelves, where the majority
of fluvial carbon load is retained, as shown for example in the Amazon
River estuary (e.g., Richey et al., 1990; Keil et al., 1997). Sedimentation
processes occurring in the Lesser Antilles island arc are well illustrated
(Deplus et al., 2001; Picard et al., 2006). The Grenada back-arc basin,
located west of the volcanic arc (Fig. 1), is around 2900 m depth and is
filled by at least 9000 m of sediments. The Grenada basin constitutes a
highly efficient trap for sediments provided mainly by the volcanic arc
activity and could also be an efficient trap for organic materials coming
from the 4000 km2 of the volcanic arc islands covered with rainforest.
Our results show that tropical volcanic islands are significant pools
of organic carbon for oceans. Moreover, because mechanical erosion
rates are very efficient in these particular contexts, the resulting
terrestrial sediment flows are important and could be favorable to
organic matter burial. However, their effect on carbon budget is still
a matter of debate and should be take into account in further carbon
cycle modelling.
6. Conclusions
This study provides insight on temporal variations in particulate
and dissolved organic carbon and dissolved inorganic carbon in small
volcanic and mountainous rivers under tropical climate.
The floods and extreme floods, which represent 42.6% of the annual
water flux, represent on average 54% of the annual DOC flux, and 85%
of the annual POC flux. The DIC fluxes occur essentially during the
low water level, with about 75% of the annual flux. The repartition of
dissolved carbon fraction between inorganic and organic form seem to
depend on the hydrodynamic. Then, during low water level and floods,
the dissolved carbon is essentially exported under inorganic form, while
during extreme floods, the organic carbon is the dominant form.
Guadeloupean rivers present very high DOC and POC yields. The DOC
yields range from 1.9 to 8.6 t km−2 yr−1 and are close to the DOC yields
of the large tropical rivers, like the Amazon (5.84 t km−2 yr−1) or the
Orinoco (4.81 t km−2 yr−1) Rivers. The POC yields vary between 8.1
and 25.5 t km−2 yr−1 and are higher than POC yields from large tropical
rivers (Amazon and Orinoco, about 1.5 t km−2 yr−1). Guadeloupean
rivers are directly connected to the oceans and present steep slopes
and no floodplains. Erosion of soils and strength of the event are thus
probably the major drivers of the POC/DOC ratio (i.e., POC/DOC > 1)
and the high POC yields observed for these rivers.
Guadeloupean soils exhibit partial “residence times”, represented by the ratio of stock organic carbon in soil on erosion transport
(YDOC + YPOC), from 580 ± 280 years, depending on the overall
strength of the meteorological events and not only their number.
A first order globalization of DOC and POC fluxes for all volcanic arc
islands and oceanic island under tropical climate indicates that the
organic carbon export from these islands is non negligible in the worldwide organic carbon export and the fate of this organic matter is probably different from the one delivered by the largest rivers of the world
242
E. Lloret et al. / Chemical Geology 351 (2013) 229–244
since its speciation (i.e. DOC vs. POC) is significantly different. The fate
of this type of organic matter and its effect on the carbon budget of
the oceanic system should be clarified with more studies on similar
types of islands using high temporal resolution sampling to obtain a
larger data set of POC/DOC ratios corresponding to a larger number
and typology of more intense precipitation events.
Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.chemgeo.2013.05.023.
Acknowledgments
We gratefully acknowledge two anonymous reviewers and the
editor David R. Hilton for their helpful and critical comments on the
original manuscript. This work could not be done without logistical
support from the OVSG. We thank Guadeloupean DEAL, and especially
M. M. Pellegrinelli-Verdier, for hydrologic data and Météo France
Guadeloupe for rainfall data; Professor G. Sarazin for discussion about
alkalinity and Miss H. Lazard for her help on POC analysis. This work
has been financially supported by the French program funded by the
INSU-CNRS (PPF ObsErA).
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