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Dissolved organic matter and its optical properties in a blackwater tributary of the upper Orinoco river, Venezuela
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Org. Geochem. Vol. 28, No. 9/10, pp. 561±569, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00028-X 0146-6380/98 $19.00 + 0.00
Dissolved organic matter and its optical properties in a blackwater tributary of the upper Orinoco river, Venezuela TOM J. BATTIN* Department of Ecology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria (Received 21 August 1997; returned to author for revision 20 November 1997; accepted 12 March 1998) AbstractÐColored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) concentrations were investigated in a blackwater tributary (river Surumoni) of the upper river Orinoco (South Venezuela) during November and December 1996. DOC concentrations were high (010 mg C Lÿ1), relatively invariant in time and largely decoupled from discharge. The Surumoni has the highest CDOM levels reported hitherto with slopes of ln-linearized absorption spectra ranging from 0.0085± 0.0152 nmÿ1, a mass speci®c absorption coecient at l = 300 nm of 6.45 L mgÿ1 mÿ1. Optical signatures indicate that the CDOM is highly aromatic in nature and of terrestrial origin in the Surumoni, whereas autochthonous sources are also likely to contribute signi®cantly to the Orinoco CDOM pool. Hydrologic connectivity of the active channel with fringing ¯oodplains largely determined the spatiotemporal variation of CDOM in the Surumoni. The upriver channel is straight, braided in some reaches, and CDOM optical properties remained largely invariant along its course. Downriver, the meandering channel receives substantial amounts of highly degraded, aromatic material from extensive ¯oodplains. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐcolored dissolved organic matter, absorbance and ¯uorescence of CDOM, Orinoco river, Surumoni river
NOTATION l Al al al S fl
Orinoco, the carbon biogeochemistry of Carribean waters (Bidigare et al., 1993; Blough et al., 1993). For instance, its microbial processing largely determines the carbon cycling in freshwaters. In addition, its humic fraction in¯uences the chemophysical characteristics of an ecosystem through light attenuation, pH buering and complexation of trace metals (e.g. McKnight and Aiken, 1998). In streams and rivers, allochthonous carbon from the watershed often prevails over autochthonous carbon derived from primary producers and is thought to constitute the primary energy source to heterotrophic bacteria. However, carbon release from algae is often more available to heterotrophic bacteria (e.g. Kaplan and Bott, 1989). Knowing the contribution of these sources and their dynamics is fundamental for the understanding of riverine ecosystems. Many precursors of humic substances, such as phenolics, aniline derivatives or polycyclic aromatic hydrocarbons, give dierential absorbance signatures in the UV range (e.g. Chin et al., 1994) and as recently demonstrated in ¯uorescence (McKnight et al., 1998). Absorbance and ¯uorescence characteristics can thus yield information regarding the degree of aromaticity and structural properties of CDOM, and hence oer valuable techniques to trace its sources in aquatic ecosystems. In fact, CDOM compounds derived from terrestrial vegetation and soils contain a signi®cant content of
Excitation and emission wavelength of light (nm) CDOM absorbance at l (cmÿ1) CDOM absorption coecient at l (mÿ1) CDOM mass-speci®c absorption at l (L mgÿ1 mÿ1) Spectral slope of a ln-linearized regression for the dependence of CDOM al of l (nmÿ1) CDOM ¯uorescence at emission l produced by 370 nm excitation (relative units)
INTRODUCTION
The transport of riverine terrestrial carbon into the oceans is a prominent link in the global carbon cycle. Lewis and Saunders (1990) put the relative contribution of the Orinoco river to the global carbon ¯ux of the world's oceans at 1.6%. This corresponds to an annual yield of 6.8 106 t yrÿ1 organic carbon from the entire Orinoco system. The bulk part of this carbon is dissolved (DOC) and consists of highly degraded material primarily derived from soil humic and fulvic acids (Ertel et al., 1986; Hedges et al., 1994). Humic substances also constitute the chief fraction of the colored dissolved organic matter (CDOM) which is thought to represent the largest reservoir of aquatic dissolved organic matter (Blough and Green, 1995). CDOM regulates the functioning of freshwater ecosystems on several levels and, as for the *E-mail: tomba@p¯aphy.pph.univie.ac.at. 561
562
T. J. Battin
In particular, the research objectives were to answer the questions: (1) what is the potential of optical signatures to characterize CDOM sources in tropical rivers? and (2) what are the spatial and temporal variations of DOC concentration and CDOM along the longitudinal continuum of the river Surumoni? SITE DESCRIPTION
Fig. 1. Map showing the location of the river Surumoni and sampling sites.
aromatic carbon, whereas microbially derived CDOM is lower in aromaticity (e.g. McKnight et al., 1994). In this paper, data are presented on DOC concentrations and the optical properties of CDOM from the blackwater Surumoni river, a minor tributary of the upper river Orinoco, South Venezuela. Research on tropical CDOM largely focuses on river's plumes and coastal zones (e.g. Blough et al., 1993), whereas carbon biogeochemistry from the upper end of the tropical riverine continuum still remains poorly investigated (cf. Lewis et al., 1995).
The river Surumoni is a minor tributary of the upper Orinoco river in the Amazonian plains with their con¯uent (03810'100N 65840'220W, 90 m a.s.l.) ca. 12 km downriver of the village La Esmeralda (Fig. 1). The Surumoni drains a pristine, tropical lowland rainforest watershed of unknown area. Soils are typically areno-latosoles and notably plinthi-humi-geric ferrasols (FAO-UNESCO, 1988). Ferrasols are characterized by an advanced stage of weathering resulting in low activity of clays, low cation exchange capacity and the absence of weatherable minerals in coarser soil fractions. Channel geomorphology divides the Surumoni into two distinct stretches (Fig. 1, Table 1). The upper Surumoni is torrential and has a relatively straight channel with some reaches braided; water is relatively shallow and velocity high. The lower Surumoni channel is meandering and accompanied
Table 1. Geomorphological characterization of the river Surumoni channel (03.12.1996, gauge = 1.52 m) and mean 2S.E. (n) values for water temperature, conductance and total suspended solids (TSS) along the Surumoni and in the Orinoco rivers Site
Downriver distance (km)
Water depth (m)
Channel width/ water depth
SU1
0
0.47
75.8
SU2
0.7
0.38
81.6
SU3
1.3
0.63
26.9
SU4
1.6
1.15
13.0
SU5
2.0
1.47
8.2
SU6
2.2
1.67
Mean upper Surumoni
0.96 20.22
14.4 a
36.2 213.3b
SU7
3.7
3.45
7.0
SU8
4.5
2.37
12.2
SU9
5.1
4.80
5.4
SU10
6.3
3.20
14.1
d
SU11
7.1
n. d.
SU12
7.3
n. d.
SU13
8.1
5.75
Mean lower Surumoni R. Orinoco a
3.91 20.60 8.6
n. d.
n. d. n. d. 7.0 a
9.1 21.7b n. d.
Signi®cantly dierent at p < 0.01, Kruskal±Wallis one-way ANOVA. Signi®cantly dierent at p < 0.05, Kruskal±Wallis one-way ANOVA. c Signi®cantly dierent at p < 0.05, Kruskal±Wallis one-way ANOVA. d Not determined. b
Conductance (mS cmÿ1)
TSS (mg Lÿ1)
25.70 2 0.10 (2) 25.60 2 0.30 (2) 25.50 2 0.10 (2) 25.70 2 0.11 (2) 25.78 2 0.60 (6) 25.84 2 0.19 (7) 25.74 2 0.09
10.23 20.06 (2) 9.77 20.30 (2) 9.79 20.30 (2) 9.71 20.69 (2) 10.10 20.84 (6) 9.96 20.74 (7) 9.96 20.33c
4.12 22.88 (2) 1.67 20.03 (2) 5.71 21.89 (2) 1.53 20.14 (3) 1.53 20.60 (3) 1.84 20.39 (5) 2.23 20.44
25.53 2 0.15 (7) 25.53 2 0.22 (7) 25.63 2 0.21 (7) 25.78 2 0.29 (7) 27.66 2 0.75 (7) 25.95 2 0.30 (15) 26.95 2 0.38 (7) 26.10 2 0.16
10.45 20.70 (7) 10.55 20.70 (7) 10.99 20.60 (7) 11.01 20.49 (7) 10.88 20.47 (7) 11.08 20.38 (15) 10.33 20.49 (7) 10.81 20.20c
4.24 22.79 (5) 1.81 20.34 (5) 2.29 20.43 (4) 2.78 20.38 (5) 1.32 20.21 (4) 1.88 20.33 (10) 4.07 21.45 (7) 2.49 20.42
27.33 2 0.13 (7)
15.67 20.20 (7)
15.33 21.96 (4)
Temperature (8C)
Dissolved organic matter in a tropical blackwater river
by extensive fringing ¯oodplains and diuse tributary inputs. Water has the typical dark appearance of blackwater rivers, depths reach almost 6 m, and velocity is low. Conductance was signi®cantly lower in the upper Surumoni, no dierences were detected in water temperature or total suspended solids (Table 1). In 1996, total precipitation amounted 3700 mm, and the Orinoco hydrograph was characterized by an exceptionally high ¯ood [Fig. 2(a)]. During the study period, precipitation amounted 292 mm [Fig. 2(c)]. The Surumoni hydrograph [Fig. 2(b)] was largely shaped by the Orinoco which determined drainage through back¯ooding. An increase in water level, still during the receding limb of the Orinoco, reconnected the main channel water of the Surumoni with fringing water bodies. Daily precipitation and sta gauge data from the Orinoco were recorded by the SADA Amazonas, La Esmeralda. The Surumoni hydrograph was produced from mean (n = 3) daily readings of a sta gauge (arbitrary datum) installed ca. 1.3 km (SU11) upstream of the con¯uent. SAMPLING AND ANALYTICAL METHODS
Depth-integrated samples were collected from the mid-channel at 6 upriver and 7 downriver sites in the upper and lower Surumoni, respectively
563
(Table 1), and in the Orinoco ca. 300 m upriver of the con¯uent during November and December 1996. Additional samples were collected at the Surumoni gauging station (SU11). Samples were pre-processed in the Surumoni ®eld station (Austrian Academy of Sciences) within 3 h after collection and subsequently stored in the dark (48C) pending laboratory analysis at the University of Vienna. Samples were ®ltered (Whatman GF/F) and aliquots were preserved with sodium azide (14 mM ®nal concentration) (Kaplan, 1994) for further DOC analysis. Aliquots were ®ltered for absorbance and ¯uorescence measurements. All samples were preserved in borosilicate vials with Te¯on-lined caps. Filters and glassware were precombusted (4508C, 6 h). Samples were shipped on dry ice and analyzed within 5 weeks after collection. CDOM exhibits broad and unstructured absorption spectra that decrease nearly exponentially throughout the near UV and visible wavelength ranges (Fig. 3). Previous studies (e.g. Bricaud et al., 1981; Zepp and Schlotzhauer, 1981; Green and Blough, 1994) have shown that DOM absorption spectra can be ®tted to an exponential equation: al al0 ebÿS lÿl0 c
1
where al and al0 are the absorption coecients at wavelength l and reference wavelength l0. The values of al and al were calculated by converting the absorbance, Al, using the equations: al 2:303
Al r
2
Al Cr
3
al * 2:303
where l is the wavelength, r is the cuvette path length in meters and C is the DOC concentration in mg C Lÿ1. Values of the slope parameter, S, were obtained from linear least squares regressions of plots of ln-transformed (Green and Blough, 1994; Blough and Green, 1995) absorption coecients vs wavelength. In addition to full absorbance spectra (250± 500 nm) measured on a limited set of samples, absorbance at 254, 300 and 436 nm was routinely measured on all samples. The ratio of a254/a436 is related to S through the equation S Fig. 2. (a) 1996 hydrograph of the Orinoco river as recorded in La Esmeralda. The framed part of the curve designates the study period. (b) Hydrograph of the Surumoni during the November±December 1996 ®eld period; open circles denote dates where the river was sampled along its longitudinal continuum. The framed part of the hydrograph designates the storm of November 21st. (c) Daily precipitation during the ®eld period.
ln a254 =a436 182
4
which is obtained from equation 1. Both values describe the absorption spectra and have been used independently by various photochemists (e.g. Strome and Miller, 1978; Abbt-Braun, 1992; Green and Blough, 1994; McKnight et al., 1997) to describe CDOM of dierent type and sources. For comparative purposes, a300 was adopted as a rela-
564
T. J. Battin
trolled according to Eaton (1995) and the coecient of variation was generally <4%. McKnight et al. (1998) have introduced a ¯uorescence index that allows the discrimination between CDOM from terrestrial/soil and aquatic/ microbial sources. This index is the ratio of the emission intensity at 450 nm to the emission intensity at 500 nm with an excitation of 370 nm. In order to reference the values from the Surumoni and Orinoco rivers along the ¯uorescence continuum, the f450/f500 ratio of two end-members was measured. Fulvic acids from the Suwannee river (standard of the International Humic Substances Society), U.S. southeastern coastal plain, were employed as the terrestrial/soil end-member. Fulvic acids from the antarctic Lake Fryxell were used as the aquatic/microbial end-member source (e.g. McKnight et al., 1994, 1998). Measurements were performed in 1 cm quartz cuvettes with a Jasco 820FP ¯uorometer and MilliQ water was used as blank. DOC concentration was measured by the Pt-catalyzed high-temperature combustion method with a Shimadzu TOC 5000 total organic carbon analyzer. Inorganic carbon was purged from acidi®ed samples (HCl, pH 2) with CO2 free air. For each sample, three replicate injections (150 ml) were performed that resulted in a coecient of variation <2%. Standards were prepared with potassium hydrogen phthalate in doubly distilled MilliQ water. Calibration was performed by running four standards over an appropriate range and one laboratory blank (doubly MilliQ water). DOC concentrations were calculated from the slope of the best ®t regression line. Total suspended solids (TSS) were measured from 1 L water samples ®ltered through ashed and preweighed glass-micro®ber ®lters (Whatman GF/ F) that were dried to constant weight at ca. 608C. RESULTS AND DISCUSSION
Fig. 3. Representative absorption spectra, plotted as the absorption coecient (a) and the ln-transformed absorption coecient (b) vs wavelength. Samples were collected along the Surumoni on 24.11.1996 (1.61 m sta gauge). Linear least squares ®ts are shown in (b). Corresponding S (nmÿ1) are Orinoco (0.0105), SU5 (0.0124), SU11 (0.0110), SU6 (0.0116), SU10 (0.0121), SU8 (0.0112), SU12 (0.0130) and SU13 (0.0101).
tive index of the CDOM concentration as done by Blough et al. (1993) for the Orinoco out¯ow. Absorbance measurements were made with a Hitachi U-2000 spectrophotometer with a 1 cm quartz cuvette using MilliQ water as reference. Average absorbance values from 700 to 800 nm were set to zero to correct the spectra for refractive index eects (Green and Blough, 1994). The quality of the spectrophotometric measurements was con-
DOC concentrations and CDOM levels DOC concentrations ranged from 4.50 to 20.39 mg C Lÿ1 in the river Surumoni and from 4.48±16.89 mg C Lÿ1 in the Orinoco river. Richey et al. (1990) measured a maximum concentration of 012 mg C Lÿ1 in the river Negro, and DePetris and Paolini (1991) measured 14.7 mg C Lÿ1 in the river Atabapo. Tributaries draining from the Guyana shield are generally higher in DOC than those from the Andean montane and alluvial zones (DePetris and Paolini, 1991). Still, the Surumoni DOC concentrations apparently exceed the values summarized in DePetris and Paolini (1991). Figure 3 shows representative absorption spectra from water samples collected along the Surumoni and from the Orinoco on 24.11.1996. The coecient of determination (r2) for the ®t of S was consist-
1.242 0.04b (1.08±1.36) (7)
0.0143 2 0.0003a (0.0087±0.0143) (24)
0.0112 2 0.0007a (0.0086±0.0139) (6)
10.51 20.27a (4.50±20.39) (71)
10.51 22.10a (4.68±16.89) (7)
Lower R. Surumoni
R. Orinoco
Data are represented as the mean 2SE, (min±max) and (n). Sites having the same superscript letter are not statistically dierent (Kruskal±Wallis one-way ANOVA, a = 0.05).
8.532 0.53a (5.57±9.74) (7) 28.56 2 2.62b (24.74±41.38) (6) 4.24 20.85a (1.46±6.53) (6) 4.67 21.35b (1.24±99.02) (6) 4.342 0.89b (2.00±8.06) (6) 32.42 2 6.46b (22.6±57.3) (5)
0.0124 2 0.0003 (0.0085±0.0152) (21) 10.62 20.26 (6.94±13.11) (17) Upper R. Surumoni
33.7 24.31b (18.6±44.9) (6)
1.16 20.01a (1.06±1.51) (70) 9.582 0.19a (4.37±11.30) (70) 24.77 20.88a (3.00±52.69) (72) 6.36 20.31a (1.83±8.18) (22) 11.18 2 0.50a (2.87±27.68) (72) 12.12 20.62a (2.56±36.25) (72) 69.16 23.65a (14.7±99.5) (25)
a
108.2 2 2.3a (23.2±181.6) (72)
1.15 20.02a (1.06±1.31) (23) 9.702 0.25 (6.42±11.34) (23) 24.26 20.81 (20.30±35.87) (23) 6.54 20.51 (1.24±15.04) (21)
565
a
a
103.1 2 2.9 (79.9±133.2) (23)
71.09 24.95 (20.5±151.1) (21)
a
10.85 20.50 (9.14±19.92) (23)
a
9.922 0.44 (7.96±18.43) (23)
a254/a436
a a a a
a436 (L mgÿ1 mÿ1) a300 (L mgÿ1 mÿ1) a254 (L mgÿ1 mÿ1) a436 (mÿ1) a300 (mÿ1) a254 (mÿ1) S (nmÿ1) DOC (mg Lÿ1)
The ¯uorescence index of Orinoco CDOM averaged 1.24 2 0.04 and was signi®cantly higher than in the Surumoni (01.15, see Table 2). Considering the ¯uorescence index of Lake Fryxell and the Suwannee river of 1.562 0.03 and 1.112 0.03, respectively, CDOM from the Surumoni is unequivocally of terrestrial origin. Whereas, considering the observed CDOM ¯uorescence in the Orinoco as a mixing product of both end-members, implies a qualitative contribution of 030% from autochthonous sources. For all data pooled, the ¯uorescence index, f450/f500, and the absorbance ratio a254/a436, were inversely related (r2=0.86, p < 0.0001, n = 100). This relation translates into a higher content of aromatic carbon associated with terrestrial rather than with autochthonous CDOM, and thus supports recent ®ndings by McKnight et al. (1997, 1998). It should be noted, however, that the ¯uorescence index has higher discriminatory power than the absorbance ratio (see Table 2). This pattern is further con®rmed by a signi®cantly higher a254 in the Surumoni yet a signi®cantly higher a436 in the Orinoco (see Table 2).
Site
Sources of CDOM
Table 2. DOC concentration, absorbance and ¯uorescence parameters of CDOM from the upper and lower Surumoni and in the Orinoco
ently higher than 0.99 despite the departure from linearity for wavelengths below 330 nm. This shoulder was more apparent in samples from the lower Surumoni than from the upper Surumoni and the Orinoco. A similar slope discontinuity was reported by Green and Blough (1994) for highly absorbing coastal and estuarine waters, and is probably attributable to degradation products of lignin and tannins (Lawrence, 1980). S averaged 0.0121 and 0.0112 nmÿ1 for the Surumoni and the Orinoco, respectively (Table 2). Both rivers had S values as low as 00.008 nmÿ1 which were associated with high ¯ow (see below); the highest value was computed from an upper R. Surumoni sample (0.0151 nmÿ1). These values are within the lower range or even lower than those published by Zepp and Schlotzhauer (1981) from blackwater rivers in the southern United States, yet, they are closely bracketed by values for terrestrial humic acids (00.012 nmÿ1) (see Blough and Green, 1995). Blough et al. (1993) reported S values from the lower end of the riverine continuum that average 0.0141 nmÿ1 in the Orinoco out¯ow and which are virtually identical with 00.014 nmÿ1 from the Amazon river (Green and Blough, 1994). Continuing the ¯ow path of CDOM into the ocean, S ranges from 00.013±0.018 nmÿ1 in ``brown'' coastal waters to r0.02 nmÿ1 in oligotrophic ``blue'' waters (cf. Blough and Green, 1995). Similar patterns emerge from the comparison of a300 that averaged 070 and 32 mÿ1 for the Surumoni and the Orinoco, respectively, and is thus roughly 3.5 and 1.6 times higher than in the out¯ow of the Orinoco (15±25 mÿ1, Blough et al., 1993).
f450/f500
Dissolved organic matter in a tropical blackwater river
566
T. J. Battin
Aromatic moieties absorbing in the UV range are mainly associated with lignin phenolics that, in fact, are known to largely contribute to the humic pool of tropical blackwater rivers (e.g. Ertel et al., 1986). On the other hand, absorbance at 436 nm is close to the peak (440 nm) of the blue absorption band of chlorophyll a and, according to Davies-Colley and Vant (1987), has the potential to relate to algal biomass. Lewis (1988) found algal biomass to be 5fold higher in the Orinoco than in two blackwater tributaries. Further, Jae et al. (1995, 1996) reported signi®cant contributions from allochthonous and autochthonous sources to the total lipid pool in rivers from the Orinoco basin. Spatial and temporal patterns of DOC and CDOM No signi®cant dierences were found in DOC concentrations between the upper and lower sites of the Surumoni and Orinoco (Table 2). DOC concentrations ¯uctuated within a relatively narrow window (9.79±11.73 mg C Lÿ1) in the upper sites, whereas downriver DOC concentrations varied between 4.50 and 16.89 mg C Lÿ1. A similar distribution pattern with stable values in upriver sites and ¯uctuating values in downriver reaches was found for CDOM optical properties (Fig. 4). This was particularly true for the concentration level of CDOM, as indicated by a300 . The increase in aromaticity, as indicated by low S, and the CDOM concentration level, as indicated by high a300 in SU11, for instance, clearly hints at inputs from near channel water bodies. This is furthermore supported by continuously elevated temperature and lower conductance in SU11 (see Table 1). CDOM characteristics returned to upriver values shortly after the input which makes simple dilution more likely than processing. Also, there is a slight increase in the upriver ¯uorescence index, and again strong downriver signals pointing at autochthonous sources of CDOM. No signi®cant variation of S values were measured along the Surumoni (Table 2, Figs 3 and 4) which suggests that channel CDOM remains chemically unaltered during its transport into the Orinoco. This observation is consistent with previous studies (Ertel et al., 1986; Hedges et al., 1994) that demonstrated the organic matter composition being almost invariant along the river Amazon main stem. The storm of November 21st. (Fig. 5) caused a rapid but moderate increase of the water level. As a result, SU11 conductance and total suspended solids increased immediately, and concurrently reached extreme values at the hydrograph peak. DOC concentration, however, remained relatively invariant and increased only by a factor of 01.3 during the ®rst 15 h (Fig. 5). The absence of a clear relationship between DOC concentration and water level was further emphasized when pooling all data over the study period. The river Surumoni hydro-
Fig. 4. Longitudinal patterns of conductance, DOC concentration and CDOM parameters along the upper and lower Surumoni and in the Orinoco (designated by arrow). Symbols refer to sample sites as described in Table 1. Data from 3 ®eld trips are represented: squares 24.11.1996, circles 01.12.1996, triangles 08.12.1996.
graph explained in fact only 26% (p < 0.05, n = 13) of the variance in downriver DOC concentration. No relationship was found in the upriver sites. This poor relationship supports emerging evidence that the DOC concentration in rivers draining lowland tropical areas of high rainfall is largely decoupled from discharge or is in a certain homeostasis (see Lewis et al., 1986). Despite large ¯uctu-
Dissolved organic matter in a tropical blackwater river
567
However, the underlying mechanisms are yet to be understood. By contrast, optical signatures clearly show a dierential response to hydrologic ¯uctuations. As illustrated by the storm event of November 21st (Fig. 5), CDOM aromaticity responded very sensitively with a distinct peak at the hydrograph peak. The good relationships between gauge and S and distinct al in the lower Surumoni river and to some extent in the Orinoco, yet not in the upper Surumoni (Fig. 6), further substantiate this pattern. Mass speci®c absorptions at 254, 300 and 436 nm correlated only with water level in the downriver Surumoni (Fig. 6), and slopes of the respective linear regressions were 0.0635, 0.0535 and 0.0102. CDOM absorbing in the UV range thus responded more sensitively to rising water level than those molecules absorbing in the visible range.
Fig. 5. Response of conductance, total suspended solids, DOC concentration CDOM aromaticity (a254/a436) to the storm of November 21st in the lower river Surumoni site SU11. Circles refer to parameters measured, the solid line to the gauge.
ations in discharge, Lewis et al. (1986) were unable to relate DOC concentration to discharge in the Caura river, Guyana shield, Venezuela. Likewise, Guyot and Wasson (1994) did not ®nd any relation between DOC concentration and discharge in the whitewater river Beni, upper Amazon basin, and reported limited variation (1±3 fold) in DOC concentration over a broad temporal scale. Commonly, an increased DOC concentration during high discharge results, at least partially, from shifts in ¯ow paths from routing through deep and carbon poor soil layers to shallow and organic rich layers. Lewis et al. (1986) postulated the buer capacity of the soil system as a possible explanation for the homeostasis of DOC concentration in some tropical rivers.
Fig. 6. Relationship between the ln-linearized slopes, S (left panels) and water level for the upper R. Surumoni SU6, lower R. Surumoni SU12 and the R. Orinoco. Right panels show the relationship between a254 (solid square), a300 (®lled circle) and a436 (open triangle) and river gauge. Indicated are only signi®cant coecients of determination for least squares linear regressions.
568
T. J. Battin SUMMARY
This study represents the ®rst evaluation of optical CDOM properties to trace sources of organic matter and to understand its dynamics in a tropical riverine landscape. CDOM signatures point at a tangible contribution of autochthonous sources in the upper Orinoco river yet not in the blackwater tributary, the river Surumoni. Certainly, along with the recent ®ndings from lipid biogeochemistry (Jae et al., 1995, 1996) these emerging patterns are largely based on qualitative ®gures which still make a quantitative balance on sources premature. However, they bear the potential to expand our view on DOM in tropical rivers that has been presumed to be essentially terrestrial in origin. Optical signatures that are related to high ¯ow point at terrestrial sources that supply substantial amounts of highly aromatic CDOM, and to some extent at autochthonous sources. The most likely sources, rather than storm runo, are fringing ¯oodplains that accumulate both dead organic matter and algal biomass and that are highly productive in DOC. Carbon from fresh leaf litter and primary producers in ¯oodplains is microbially reactive and likely respired within these systems. By contrast, residual molecules are routed as highly degraded material into the downriver channel. Conversely, surface runo and possibly subsurface routing are likely the major input vectors in the upriver Surumoni. More work needs to be done now to quantify ¯uxes from distinct sources to the riverine carbon pool. Achieving this goal ultimately requires knowledge on hydrologic pathways on a watershed scale. Associate EditorÐS. G. Wakeham AcknowledgementsÐThe paper bene®tted from comments by William M. Lewis, Diane McKnight, Robert Chen and Matt McCarthy. Diane McKnight also provided standard fulvic acids and JoÈrg Szarzynski gauge and precipitation data. Thanks to Pia Grubbauer, Erwin Nemeth and Birgit SchroÈder for ®eld assistance. Hans Winkler made my trip to the R. Surumoni ®eld station of the Austrian Academy of Sciences possible. This work was partially supported by KUMU-8101. REFERENCES
Abbt-Braun, G. (1992) Spectroscopic characterization of humic substances in the ultraviolet and visible region and by infrared spectroscopy. In Progress in Hydrogeochemistry, eds G. Matthess and F. Frimmel. Springer, Berlin. Bidigare, R. R., Ondrusek, M. and Brooks, J. M. (1993) In¯uences of the Orinoco river out¯ow on distribution of algal pigments in the Carribean Sea. Journal of Geophysical Research 98, 2245±2257. Blough, N. V. and Green, S. A. (1995) Spectroscopic characterization and remote sensing of nonliving organic matter. In Role of Nonliving Organic Matter in the Earth's Carbon Cycle, eds R. G. Zepp and C. Sonntag, pp. 23±45. John Wiley and Sons Ltd, London.
Blough, N. V., Za®riou, O. C. and Bonilla, J. (1993) Optical absorption spectra of waters from the Orinoco river out¯ow: Terrestrial input of colored organic matter to the Carribean. Journal of Geophysical Research 98, 2271±2278. Bricaud, A., Morel, A. and Prieur, A. (1981) Absorption by dissolved organic matter of the sea (yellow substance) in the UV and visible domains. Limnology and Oceanography 26, 43±53. Chin, Y. P., Aiken, G. and O'Loughlin, E. (1994) Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environmental and Science Technology 28, 1853±1858. Davies-Colley, R. J. and Vant, W. N. (1987) Absorption of light by yellow substance in freshwater lakes. Limnology and Oceanography 32, 416±425. DePetris, P. J. and Paolini, J. E. (1991) Biogeochemical aspects of South American rivers: The Parana and the Orinoco. In Biogeochemistry of Major World Rivers, eds E. I. Degens, S. Kempe and J. E. Richey, pp. 105±125. Wiley, New York. Eaton, A. (1995) Measuring UV-absorbing organics: a standard method. Journal of the American Water Works Association Feb., 86±90. Ertel, J. R., Hedges, J. I., Devol, A. H., Richey, J. E. and Nazare GoeÂs Ribero, M. (1986) Dissolved humic substances of the Amazon River system. Limnology and Oceanography 31, 739±754. FAO-UNESCO (1988) Soil Map of The World. Revised Legend. Food and Agriculture Organization, Rome, Italy. Green, S. A. and Blough, N. V. (1994) Optical absorption and ¯uorescence properties of chromophoric dissolved organic matter in natural waters. Limnology and Oceanography 39, 1903±1916. Guyot, J. L. and Wasson, J. G. (1994) Regional pattern of riverine dissolved organic carbon in the Amazon drainage basin of Bolivia. Limnology and Oceanography 39, 452±458. Hedges, J. I., Cowie, G. L., Richey, J. E., Quay, P. D., Benner, R., Strom, M. and Forsberg, B. R. (1994) Origins and processing of organic matter in the Amazon River as indicated by carbohydrates and amino acids. Limnology and Oceanography 39, 743±761. JaeÂ, R., Wol, G. A., Cabrera, A. C. and Chitty, H. C. (1995) The biogeochemistry of lipids in rivers of the Orinoco basin. Geochimica Cosmochimica Acta 59, 4507±4522. JaeÂ, R., ElismeÂ, T. and Cabrera, A. C. (1996) Organic geochemistry of seasonally ¯ooded rain forest soils: molecular composition and early diagenesis of lipid components. Organic Geochemistry 25, 9±17. Kaplan, L. A. (1994) A ®eld and laboratory procedure to collect, process, and preserve freshwater samples for dissolved organic carbon analysis. Limnology and Oceanography 39, 1470±1476. Kaplan, L. A. and Bott, T. L. (1989) Diel ¯uctuations in bacterial activity on streambed substrata during vernal algal blooms: eects of temperature, water chemistry, and habitat. Limnology and Oceanography 34, 718±733. Lawrence, J. (1980) Semi-quantitative determination of fulvic acid, tannin and lignin in natural waters. Water Research 14, 373±377. Lewis, W. M. Jr. (1988) Primary production in the Orinoco River. Ecology 69, 679±692. Lewis, W. M., Jr. and Saunders, J. F., III (1990) Chemistry and element transport by the Orinoco main stem and lower tributaries. In The Orinoco River as an ecosystem, eds F. H. Weibezahn, H. Alvarez and M. L. William Jr., pp. 211±239. Impresos Rubel, Caracas. Lewis, W. M. Jr., Saunder, J. F. III, Levine, S. N. and Weibezahn, F. H. (1986) Organic carbon in the Caura
Dissolved organic matter in a tropical blackwater river River, Venezuela. Limnology and Oceanography 31, 653± 656. Lewis, W. M., Jr, Hamilton, S. K. and Saunders, J. F., III (1995) Rivers of northern South America. In River and Stream Ecosystems of the World, eds. C. E. Cushing and G. W. Minshall, pp. 219±256. Elsevier, Amsterdam. McKnight, D. M. and Aiken, G. R. (1998) Sources and age of aquatic humus. In Aquatic humic substances: Ecology and Biogeochemistry, eds. D. O. Hessen and L. Tranvik. Springer Verlag, Berlin (in press). McKnight, D. M., Andrews, E. D., Spaulding, S. A. and Aiken, G. R. (1994) Aquatic fulvic acids in algal-rich antarctic ponds. Limnology and Oceanography 39, 1972± 1979. McKnight, D. M., Harnish, R., Wershaw, R. L., Baron, J. S. and Schi, S. (1997) Chemical characteristics of particulate, colloidal, and dissolved organic material in Loch Vale Watershed, Rocky Mountains National Park. Biogeochemistry 36, 99±124.
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McKnight, D. M., Boyer, E. W., Doran, P., Westerho, P. K., Kulbe, T. and Andersen, D. T. (1998) Spectro¯uorometric characterization of aquatic fulvic acid for the determination of precursor organic material and general structural properties. Limnology and Oceanography (in press). Richey, J. E., Hedges, J. H., Devol, A. H., Quay, P. D., Victoria, R., Martinelli, L. and Forsberg, B. R. (1990) Biogeochemistry of carbon in the Amazon River. Limnology and Oceanography 35, 352±371. Strome, D. J. and Miller, M. C. (1978) Photolytic changes in dissolved humic substances. Internationaler Verein fuÈr Theoretische und Angewande Limnologie Verhandlungen 20, 1248±1254. Zepp, R. G. and Schlotzhauer, P. F. (1981) Comparison of photochemical behavior of various humic substances in water: III Spectroscopic properties of humic substances. Chemosphere 10, 479±486.
Org. Geochem. Vol. 28, No. 9/10, pp. 561±569, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0146-6380(98)00028-X 0146-6380/98 $19.00 + 0.00
Dissolved organic matter and its optical properties in a blackwater tributary of the upper Orinoco river, Venezuela TOM J. BATTIN* Department of Ecology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria (Received 21 August 1997; returned to author for revision 20 November 1997; accepted 12 March 1998) AbstractÐColored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) concentrations were investigated in a blackwater tributary (river Surumoni) of the upper river Orinoco (South Venezuela) during November and December 1996. DOC concentrations were high (010 mg C Lÿ1), relatively invariant in time and largely decoupled from discharge. The Surumoni has the highest CDOM levels reported hitherto with slopes of ln-linearized absorption spectra ranging from 0.0085± 0.0152 nmÿ1, a mass speci®c absorption coecient at l = 300 nm of 6.45 L mgÿ1 mÿ1. Optical signatures indicate that the CDOM is highly aromatic in nature and of terrestrial origin in the Surumoni, whereas autochthonous sources are also likely to contribute signi®cantly to the Orinoco CDOM pool. Hydrologic connectivity of the active channel with fringing ¯oodplains largely determined the spatiotemporal variation of CDOM in the Surumoni. The upriver channel is straight, braided in some reaches, and CDOM optical properties remained largely invariant along its course. Downriver, the meandering channel receives substantial amounts of highly degraded, aromatic material from extensive ¯oodplains. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐcolored dissolved organic matter, absorbance and ¯uorescence of CDOM, Orinoco river, Surumoni river
NOTATION l Al al al S fl
Orinoco, the carbon biogeochemistry of Carribean waters (Bidigare et al., 1993; Blough et al., 1993). For instance, its microbial processing largely determines the carbon cycling in freshwaters. In addition, its humic fraction in¯uences the chemophysical characteristics of an ecosystem through light attenuation, pH buering and complexation of trace metals (e.g. McKnight and Aiken, 1998). In streams and rivers, allochthonous carbon from the watershed often prevails over autochthonous carbon derived from primary producers and is thought to constitute the primary energy source to heterotrophic bacteria. However, carbon release from algae is often more available to heterotrophic bacteria (e.g. Kaplan and Bott, 1989). Knowing the contribution of these sources and their dynamics is fundamental for the understanding of riverine ecosystems. Many precursors of humic substances, such as phenolics, aniline derivatives or polycyclic aromatic hydrocarbons, give dierential absorbance signatures in the UV range (e.g. Chin et al., 1994) and as recently demonstrated in ¯uorescence (McKnight et al., 1998). Absorbance and ¯uorescence characteristics can thus yield information regarding the degree of aromaticity and structural properties of CDOM, and hence oer valuable techniques to trace its sources in aquatic ecosystems. In fact, CDOM compounds derived from terrestrial vegetation and soils contain a signi®cant content of
Excitation and emission wavelength of light (nm) CDOM absorbance at l (cmÿ1) CDOM absorption coecient at l (mÿ1) CDOM mass-speci®c absorption at l (L mgÿ1 mÿ1) Spectral slope of a ln-linearized regression for the dependence of CDOM al of l (nmÿ1) CDOM ¯uorescence at emission l produced by 370 nm excitation (relative units)
INTRODUCTION
The transport of riverine terrestrial carbon into the oceans is a prominent link in the global carbon cycle. Lewis and Saunders (1990) put the relative contribution of the Orinoco river to the global carbon ¯ux of the world's oceans at 1.6%. This corresponds to an annual yield of 6.8 106 t yrÿ1 organic carbon from the entire Orinoco system. The bulk part of this carbon is dissolved (DOC) and consists of highly degraded material primarily derived from soil humic and fulvic acids (Ertel et al., 1986; Hedges et al., 1994). Humic substances also constitute the chief fraction of the colored dissolved organic matter (CDOM) which is thought to represent the largest reservoir of aquatic dissolved organic matter (Blough and Green, 1995). CDOM regulates the functioning of freshwater ecosystems on several levels and, as for the *E-mail: tomba@p¯aphy.pph.univie.ac.at. 561
562
T. J. Battin
In particular, the research objectives were to answer the questions: (1) what is the potential of optical signatures to characterize CDOM sources in tropical rivers? and (2) what are the spatial and temporal variations of DOC concentration and CDOM along the longitudinal continuum of the river Surumoni? SITE DESCRIPTION
Fig. 1. Map showing the location of the river Surumoni and sampling sites.
aromatic carbon, whereas microbially derived CDOM is lower in aromaticity (e.g. McKnight et al., 1994). In this paper, data are presented on DOC concentrations and the optical properties of CDOM from the blackwater Surumoni river, a minor tributary of the upper river Orinoco, South Venezuela. Research on tropical CDOM largely focuses on river's plumes and coastal zones (e.g. Blough et al., 1993), whereas carbon biogeochemistry from the upper end of the tropical riverine continuum still remains poorly investigated (cf. Lewis et al., 1995).
The river Surumoni is a minor tributary of the upper Orinoco river in the Amazonian plains with their con¯uent (03810'100N 65840'220W, 90 m a.s.l.) ca. 12 km downriver of the village La Esmeralda (Fig. 1). The Surumoni drains a pristine, tropical lowland rainforest watershed of unknown area. Soils are typically areno-latosoles and notably plinthi-humi-geric ferrasols (FAO-UNESCO, 1988). Ferrasols are characterized by an advanced stage of weathering resulting in low activity of clays, low cation exchange capacity and the absence of weatherable minerals in coarser soil fractions. Channel geomorphology divides the Surumoni into two distinct stretches (Fig. 1, Table 1). The upper Surumoni is torrential and has a relatively straight channel with some reaches braided; water is relatively shallow and velocity high. The lower Surumoni channel is meandering and accompanied
Table 1. Geomorphological characterization of the river Surumoni channel (03.12.1996, gauge = 1.52 m) and mean 2S.E. (n) values for water temperature, conductance and total suspended solids (TSS) along the Surumoni and in the Orinoco rivers Site
Downriver distance (km)
Water depth (m)
Channel width/ water depth
SU1
0
0.47
75.8
SU2
0.7
0.38
81.6
SU3
1.3
0.63
26.9
SU4
1.6
1.15
13.0
SU5
2.0
1.47
8.2
SU6
2.2
1.67
Mean upper Surumoni
0.96 20.22
14.4 a
36.2 213.3b
SU7
3.7
3.45
7.0
SU8
4.5
2.37
12.2
SU9
5.1
4.80
5.4
SU10
6.3
3.20
14.1
d
SU11
7.1
n. d.
SU12
7.3
n. d.
SU13
8.1
5.75
Mean lower Surumoni R. Orinoco a
3.91 20.60 8.6
n. d.
n. d. n. d. 7.0 a
9.1 21.7b n. d.
Signi®cantly dierent at p < 0.01, Kruskal±Wallis one-way ANOVA. Signi®cantly dierent at p < 0.05, Kruskal±Wallis one-way ANOVA. c Signi®cantly dierent at p < 0.05, Kruskal±Wallis one-way ANOVA. d Not determined. b
Conductance (mS cmÿ1)
TSS (mg Lÿ1)
25.70 2 0.10 (2) 25.60 2 0.30 (2) 25.50 2 0.10 (2) 25.70 2 0.11 (2) 25.78 2 0.60 (6) 25.84 2 0.19 (7) 25.74 2 0.09
10.23 20.06 (2) 9.77 20.30 (2) 9.79 20.30 (2) 9.71 20.69 (2) 10.10 20.84 (6) 9.96 20.74 (7) 9.96 20.33c
4.12 22.88 (2) 1.67 20.03 (2) 5.71 21.89 (2) 1.53 20.14 (3) 1.53 20.60 (3) 1.84 20.39 (5) 2.23 20.44
25.53 2 0.15 (7) 25.53 2 0.22 (7) 25.63 2 0.21 (7) 25.78 2 0.29 (7) 27.66 2 0.75 (7) 25.95 2 0.30 (15) 26.95 2 0.38 (7) 26.10 2 0.16
10.45 20.70 (7) 10.55 20.70 (7) 10.99 20.60 (7) 11.01 20.49 (7) 10.88 20.47 (7) 11.08 20.38 (15) 10.33 20.49 (7) 10.81 20.20c
4.24 22.79 (5) 1.81 20.34 (5) 2.29 20.43 (4) 2.78 20.38 (5) 1.32 20.21 (4) 1.88 20.33 (10) 4.07 21.45 (7) 2.49 20.42
27.33 2 0.13 (7)
15.67 20.20 (7)
15.33 21.96 (4)
Temperature (8C)
Dissolved organic matter in a tropical blackwater river
by extensive fringing ¯oodplains and diuse tributary inputs. Water has the typical dark appearance of blackwater rivers, depths reach almost 6 m, and velocity is low. Conductance was signi®cantly lower in the upper Surumoni, no dierences were detected in water temperature or total suspended solids (Table 1). In 1996, total precipitation amounted 3700 mm, and the Orinoco hydrograph was characterized by an exceptionally high ¯ood [Fig. 2(a)]. During the study period, precipitation amounted 292 mm [Fig. 2(c)]. The Surumoni hydrograph [Fig. 2(b)] was largely shaped by the Orinoco which determined drainage through back¯ooding. An increase in water level, still during the receding limb of the Orinoco, reconnected the main channel water of the Surumoni with fringing water bodies. Daily precipitation and sta gauge data from the Orinoco were recorded by the SADA Amazonas, La Esmeralda. The Surumoni hydrograph was produced from mean (n = 3) daily readings of a sta gauge (arbitrary datum) installed ca. 1.3 km (SU11) upstream of the con¯uent. SAMPLING AND ANALYTICAL METHODS
Depth-integrated samples were collected from the mid-channel at 6 upriver and 7 downriver sites in the upper and lower Surumoni, respectively
563
(Table 1), and in the Orinoco ca. 300 m upriver of the con¯uent during November and December 1996. Additional samples were collected at the Surumoni gauging station (SU11). Samples were pre-processed in the Surumoni ®eld station (Austrian Academy of Sciences) within 3 h after collection and subsequently stored in the dark (48C) pending laboratory analysis at the University of Vienna. Samples were ®ltered (Whatman GF/F) and aliquots were preserved with sodium azide (14 mM ®nal concentration) (Kaplan, 1994) for further DOC analysis. Aliquots were ®ltered for absorbance and ¯uorescence measurements. All samples were preserved in borosilicate vials with Te¯on-lined caps. Filters and glassware were precombusted (4508C, 6 h). Samples were shipped on dry ice and analyzed within 5 weeks after collection. CDOM exhibits broad and unstructured absorption spectra that decrease nearly exponentially throughout the near UV and visible wavelength ranges (Fig. 3). Previous studies (e.g. Bricaud et al., 1981; Zepp and Schlotzhauer, 1981; Green and Blough, 1994) have shown that DOM absorption spectra can be ®tted to an exponential equation: al al0 ebÿS lÿl0 c
1
where al and al0 are the absorption coecients at wavelength l and reference wavelength l0. The values of al and al were calculated by converting the absorbance, Al, using the equations: al 2:303
Al r
2
Al Cr
3
al * 2:303
where l is the wavelength, r is the cuvette path length in meters and C is the DOC concentration in mg C Lÿ1. Values of the slope parameter, S, were obtained from linear least squares regressions of plots of ln-transformed (Green and Blough, 1994; Blough and Green, 1995) absorption coecients vs wavelength. In addition to full absorbance spectra (250± 500 nm) measured on a limited set of samples, absorbance at 254, 300 and 436 nm was routinely measured on all samples. The ratio of a254/a436 is related to S through the equation S Fig. 2. (a) 1996 hydrograph of the Orinoco river as recorded in La Esmeralda. The framed part of the curve designates the study period. (b) Hydrograph of the Surumoni during the November±December 1996 ®eld period; open circles denote dates where the river was sampled along its longitudinal continuum. The framed part of the hydrograph designates the storm of November 21st. (c) Daily precipitation during the ®eld period.
ln a254 =a436 182
4
which is obtained from equation 1. Both values describe the absorption spectra and have been used independently by various photochemists (e.g. Strome and Miller, 1978; Abbt-Braun, 1992; Green and Blough, 1994; McKnight et al., 1997) to describe CDOM of dierent type and sources. For comparative purposes, a300 was adopted as a rela-
564
T. J. Battin
trolled according to Eaton (1995) and the coecient of variation was generally <4%. McKnight et al. (1998) have introduced a ¯uorescence index that allows the discrimination between CDOM from terrestrial/soil and aquatic/ microbial sources. This index is the ratio of the emission intensity at 450 nm to the emission intensity at 500 nm with an excitation of 370 nm. In order to reference the values from the Surumoni and Orinoco rivers along the ¯uorescence continuum, the f450/f500 ratio of two end-members was measured. Fulvic acids from the Suwannee river (standard of the International Humic Substances Society), U.S. southeastern coastal plain, were employed as the terrestrial/soil end-member. Fulvic acids from the antarctic Lake Fryxell were used as the aquatic/microbial end-member source (e.g. McKnight et al., 1994, 1998). Measurements were performed in 1 cm quartz cuvettes with a Jasco 820FP ¯uorometer and MilliQ water was used as blank. DOC concentration was measured by the Pt-catalyzed high-temperature combustion method with a Shimadzu TOC 5000 total organic carbon analyzer. Inorganic carbon was purged from acidi®ed samples (HCl, pH 2) with CO2 free air. For each sample, three replicate injections (150 ml) were performed that resulted in a coecient of variation <2%. Standards were prepared with potassium hydrogen phthalate in doubly distilled MilliQ water. Calibration was performed by running four standards over an appropriate range and one laboratory blank (doubly MilliQ water). DOC concentrations were calculated from the slope of the best ®t regression line. Total suspended solids (TSS) were measured from 1 L water samples ®ltered through ashed and preweighed glass-micro®ber ®lters (Whatman GF/ F) that were dried to constant weight at ca. 608C. RESULTS AND DISCUSSION
Fig. 3. Representative absorption spectra, plotted as the absorption coecient (a) and the ln-transformed absorption coecient (b) vs wavelength. Samples were collected along the Surumoni on 24.11.1996 (1.61 m sta gauge). Linear least squares ®ts are shown in (b). Corresponding S (nmÿ1) are Orinoco (0.0105), SU5 (0.0124), SU11 (0.0110), SU6 (0.0116), SU10 (0.0121), SU8 (0.0112), SU12 (0.0130) and SU13 (0.0101).
tive index of the CDOM concentration as done by Blough et al. (1993) for the Orinoco out¯ow. Absorbance measurements were made with a Hitachi U-2000 spectrophotometer with a 1 cm quartz cuvette using MilliQ water as reference. Average absorbance values from 700 to 800 nm were set to zero to correct the spectra for refractive index eects (Green and Blough, 1994). The quality of the spectrophotometric measurements was con-
DOC concentrations and CDOM levels DOC concentrations ranged from 4.50 to 20.39 mg C Lÿ1 in the river Surumoni and from 4.48±16.89 mg C Lÿ1 in the Orinoco river. Richey et al. (1990) measured a maximum concentration of 012 mg C Lÿ1 in the river Negro, and DePetris and Paolini (1991) measured 14.7 mg C Lÿ1 in the river Atabapo. Tributaries draining from the Guyana shield are generally higher in DOC than those from the Andean montane and alluvial zones (DePetris and Paolini, 1991). Still, the Surumoni DOC concentrations apparently exceed the values summarized in DePetris and Paolini (1991). Figure 3 shows representative absorption spectra from water samples collected along the Surumoni and from the Orinoco on 24.11.1996. The coecient of determination (r2) for the ®t of S was consist-
1.242 0.04b (1.08±1.36) (7)
0.0143 2 0.0003a (0.0087±0.0143) (24)
0.0112 2 0.0007a (0.0086±0.0139) (6)
10.51 20.27a (4.50±20.39) (71)
10.51 22.10a (4.68±16.89) (7)
Lower R. Surumoni
R. Orinoco
Data are represented as the mean 2SE, (min±max) and (n). Sites having the same superscript letter are not statistically dierent (Kruskal±Wallis one-way ANOVA, a = 0.05).
8.532 0.53a (5.57±9.74) (7) 28.56 2 2.62b (24.74±41.38) (6) 4.24 20.85a (1.46±6.53) (6) 4.67 21.35b (1.24±99.02) (6) 4.342 0.89b (2.00±8.06) (6) 32.42 2 6.46b (22.6±57.3) (5)
0.0124 2 0.0003 (0.0085±0.0152) (21) 10.62 20.26 (6.94±13.11) (17) Upper R. Surumoni
33.7 24.31b (18.6±44.9) (6)
1.16 20.01a (1.06±1.51) (70) 9.582 0.19a (4.37±11.30) (70) 24.77 20.88a (3.00±52.69) (72) 6.36 20.31a (1.83±8.18) (22) 11.18 2 0.50a (2.87±27.68) (72) 12.12 20.62a (2.56±36.25) (72) 69.16 23.65a (14.7±99.5) (25)
a
108.2 2 2.3a (23.2±181.6) (72)
1.15 20.02a (1.06±1.31) (23) 9.702 0.25 (6.42±11.34) (23) 24.26 20.81 (20.30±35.87) (23) 6.54 20.51 (1.24±15.04) (21)
565
a
a
103.1 2 2.9 (79.9±133.2) (23)
71.09 24.95 (20.5±151.1) (21)
a
10.85 20.50 (9.14±19.92) (23)
a
9.922 0.44 (7.96±18.43) (23)
a254/a436
a a a a
a436 (L mgÿ1 mÿ1) a300 (L mgÿ1 mÿ1) a254 (L mgÿ1 mÿ1) a436 (mÿ1) a300 (mÿ1) a254 (mÿ1) S (nmÿ1) DOC (mg Lÿ1)
The ¯uorescence index of Orinoco CDOM averaged 1.24 2 0.04 and was signi®cantly higher than in the Surumoni (01.15, see Table 2). Considering the ¯uorescence index of Lake Fryxell and the Suwannee river of 1.562 0.03 and 1.112 0.03, respectively, CDOM from the Surumoni is unequivocally of terrestrial origin. Whereas, considering the observed CDOM ¯uorescence in the Orinoco as a mixing product of both end-members, implies a qualitative contribution of 030% from autochthonous sources. For all data pooled, the ¯uorescence index, f450/f500, and the absorbance ratio a254/a436, were inversely related (r2=0.86, p < 0.0001, n = 100). This relation translates into a higher content of aromatic carbon associated with terrestrial rather than with autochthonous CDOM, and thus supports recent ®ndings by McKnight et al. (1997, 1998). It should be noted, however, that the ¯uorescence index has higher discriminatory power than the absorbance ratio (see Table 2). This pattern is further con®rmed by a signi®cantly higher a254 in the Surumoni yet a signi®cantly higher a436 in the Orinoco (see Table 2).
Site
Sources of CDOM
Table 2. DOC concentration, absorbance and ¯uorescence parameters of CDOM from the upper and lower Surumoni and in the Orinoco
ently higher than 0.99 despite the departure from linearity for wavelengths below 330 nm. This shoulder was more apparent in samples from the lower Surumoni than from the upper Surumoni and the Orinoco. A similar slope discontinuity was reported by Green and Blough (1994) for highly absorbing coastal and estuarine waters, and is probably attributable to degradation products of lignin and tannins (Lawrence, 1980). S averaged 0.0121 and 0.0112 nmÿ1 for the Surumoni and the Orinoco, respectively (Table 2). Both rivers had S values as low as 00.008 nmÿ1 which were associated with high ¯ow (see below); the highest value was computed from an upper R. Surumoni sample (0.0151 nmÿ1). These values are within the lower range or even lower than those published by Zepp and Schlotzhauer (1981) from blackwater rivers in the southern United States, yet, they are closely bracketed by values for terrestrial humic acids (00.012 nmÿ1) (see Blough and Green, 1995). Blough et al. (1993) reported S values from the lower end of the riverine continuum that average 0.0141 nmÿ1 in the Orinoco out¯ow and which are virtually identical with 00.014 nmÿ1 from the Amazon river (Green and Blough, 1994). Continuing the ¯ow path of CDOM into the ocean, S ranges from 00.013±0.018 nmÿ1 in ``brown'' coastal waters to r0.02 nmÿ1 in oligotrophic ``blue'' waters (cf. Blough and Green, 1995). Similar patterns emerge from the comparison of a300 that averaged 070 and 32 mÿ1 for the Surumoni and the Orinoco, respectively, and is thus roughly 3.5 and 1.6 times higher than in the out¯ow of the Orinoco (15±25 mÿ1, Blough et al., 1993).
f450/f500
Dissolved organic matter in a tropical blackwater river
566
T. J. Battin
Aromatic moieties absorbing in the UV range are mainly associated with lignin phenolics that, in fact, are known to largely contribute to the humic pool of tropical blackwater rivers (e.g. Ertel et al., 1986). On the other hand, absorbance at 436 nm is close to the peak (440 nm) of the blue absorption band of chlorophyll a and, according to Davies-Colley and Vant (1987), has the potential to relate to algal biomass. Lewis (1988) found algal biomass to be 5fold higher in the Orinoco than in two blackwater tributaries. Further, Jae et al. (1995, 1996) reported signi®cant contributions from allochthonous and autochthonous sources to the total lipid pool in rivers from the Orinoco basin. Spatial and temporal patterns of DOC and CDOM No signi®cant dierences were found in DOC concentrations between the upper and lower sites of the Surumoni and Orinoco (Table 2). DOC concentrations ¯uctuated within a relatively narrow window (9.79±11.73 mg C Lÿ1) in the upper sites, whereas downriver DOC concentrations varied between 4.50 and 16.89 mg C Lÿ1. A similar distribution pattern with stable values in upriver sites and ¯uctuating values in downriver reaches was found for CDOM optical properties (Fig. 4). This was particularly true for the concentration level of CDOM, as indicated by a300 . The increase in aromaticity, as indicated by low S, and the CDOM concentration level, as indicated by high a300 in SU11, for instance, clearly hints at inputs from near channel water bodies. This is furthermore supported by continuously elevated temperature and lower conductance in SU11 (see Table 1). CDOM characteristics returned to upriver values shortly after the input which makes simple dilution more likely than processing. Also, there is a slight increase in the upriver ¯uorescence index, and again strong downriver signals pointing at autochthonous sources of CDOM. No signi®cant variation of S values were measured along the Surumoni (Table 2, Figs 3 and 4) which suggests that channel CDOM remains chemically unaltered during its transport into the Orinoco. This observation is consistent with previous studies (Ertel et al., 1986; Hedges et al., 1994) that demonstrated the organic matter composition being almost invariant along the river Amazon main stem. The storm of November 21st. (Fig. 5) caused a rapid but moderate increase of the water level. As a result, SU11 conductance and total suspended solids increased immediately, and concurrently reached extreme values at the hydrograph peak. DOC concentration, however, remained relatively invariant and increased only by a factor of 01.3 during the ®rst 15 h (Fig. 5). The absence of a clear relationship between DOC concentration and water level was further emphasized when pooling all data over the study period. The river Surumoni hydro-
Fig. 4. Longitudinal patterns of conductance, DOC concentration and CDOM parameters along the upper and lower Surumoni and in the Orinoco (designated by arrow). Symbols refer to sample sites as described in Table 1. Data from 3 ®eld trips are represented: squares 24.11.1996, circles 01.12.1996, triangles 08.12.1996.
graph explained in fact only 26% (p < 0.05, n = 13) of the variance in downriver DOC concentration. No relationship was found in the upriver sites. This poor relationship supports emerging evidence that the DOC concentration in rivers draining lowland tropical areas of high rainfall is largely decoupled from discharge or is in a certain homeostasis (see Lewis et al., 1986). Despite large ¯uctu-
Dissolved organic matter in a tropical blackwater river
567
However, the underlying mechanisms are yet to be understood. By contrast, optical signatures clearly show a dierential response to hydrologic ¯uctuations. As illustrated by the storm event of November 21st (Fig. 5), CDOM aromaticity responded very sensitively with a distinct peak at the hydrograph peak. The good relationships between gauge and S and distinct al in the lower Surumoni river and to some extent in the Orinoco, yet not in the upper Surumoni (Fig. 6), further substantiate this pattern. Mass speci®c absorptions at 254, 300 and 436 nm correlated only with water level in the downriver Surumoni (Fig. 6), and slopes of the respective linear regressions were 0.0635, 0.0535 and 0.0102. CDOM absorbing in the UV range thus responded more sensitively to rising water level than those molecules absorbing in the visible range.
Fig. 5. Response of conductance, total suspended solids, DOC concentration CDOM aromaticity (a254/a436) to the storm of November 21st in the lower river Surumoni site SU11. Circles refer to parameters measured, the solid line to the gauge.
ations in discharge, Lewis et al. (1986) were unable to relate DOC concentration to discharge in the Caura river, Guyana shield, Venezuela. Likewise, Guyot and Wasson (1994) did not ®nd any relation between DOC concentration and discharge in the whitewater river Beni, upper Amazon basin, and reported limited variation (1±3 fold) in DOC concentration over a broad temporal scale. Commonly, an increased DOC concentration during high discharge results, at least partially, from shifts in ¯ow paths from routing through deep and carbon poor soil layers to shallow and organic rich layers. Lewis et al. (1986) postulated the buer capacity of the soil system as a possible explanation for the homeostasis of DOC concentration in some tropical rivers.
Fig. 6. Relationship between the ln-linearized slopes, S (left panels) and water level for the upper R. Surumoni SU6, lower R. Surumoni SU12 and the R. Orinoco. Right panels show the relationship between a254 (solid square), a300 (®lled circle) and a436 (open triangle) and river gauge. Indicated are only signi®cant coecients of determination for least squares linear regressions.
568
T. J. Battin SUMMARY
This study represents the ®rst evaluation of optical CDOM properties to trace sources of organic matter and to understand its dynamics in a tropical riverine landscape. CDOM signatures point at a tangible contribution of autochthonous sources in the upper Orinoco river yet not in the blackwater tributary, the river Surumoni. Certainly, along with the recent ®ndings from lipid biogeochemistry (Jae et al., 1995, 1996) these emerging patterns are largely based on qualitative ®gures which still make a quantitative balance on sources premature. However, they bear the potential to expand our view on DOM in tropical rivers that has been presumed to be essentially terrestrial in origin. Optical signatures that are related to high ¯ow point at terrestrial sources that supply substantial amounts of highly aromatic CDOM, and to some extent at autochthonous sources. The most likely sources, rather than storm runo, are fringing ¯oodplains that accumulate both dead organic matter and algal biomass and that are highly productive in DOC. Carbon from fresh leaf litter and primary producers in ¯oodplains is microbially reactive and likely respired within these systems. By contrast, residual molecules are routed as highly degraded material into the downriver channel. Conversely, surface runo and possibly subsurface routing are likely the major input vectors in the upriver Surumoni. More work needs to be done now to quantify ¯uxes from distinct sources to the riverine carbon pool. Achieving this goal ultimately requires knowledge on hydrologic pathways on a watershed scale. Associate EditorÐS. G. Wakeham AcknowledgementsÐThe paper bene®tted from comments by William M. Lewis, Diane McKnight, Robert Chen and Matt McCarthy. Diane McKnight also provided standard fulvic acids and JoÈrg Szarzynski gauge and precipitation data. Thanks to Pia Grubbauer, Erwin Nemeth and Birgit SchroÈder for ®eld assistance. Hans Winkler made my trip to the R. Surumoni ®eld station of the Austrian Academy of Sciences possible. This work was partially supported by KUMU-8101. REFERENCES
Abbt-Braun, G. (1992) Spectroscopic characterization of humic substances in the ultraviolet and visible region and by infrared spectroscopy. In Progress in Hydrogeochemistry, eds G. Matthess and F. Frimmel. Springer, Berlin. Bidigare, R. R., Ondrusek, M. and Brooks, J. M. (1993) In¯uences of the Orinoco river out¯ow on distribution of algal pigments in the Carribean Sea. Journal of Geophysical Research 98, 2245±2257. Blough, N. V. and Green, S. A. (1995) Spectroscopic characterization and remote sensing of nonliving organic matter. In Role of Nonliving Organic Matter in the Earth's Carbon Cycle, eds R. G. Zepp and C. Sonntag, pp. 23±45. John Wiley and Sons Ltd, London.
Blough, N. V., Za®riou, O. C. and Bonilla, J. (1993) Optical absorption spectra of waters from the Orinoco river out¯ow: Terrestrial input of colored organic matter to the Carribean. Journal of Geophysical Research 98, 2271±2278. Bricaud, A., Morel, A. and Prieur, A. (1981) Absorption by dissolved organic matter of the sea (yellow substance) in the UV and visible domains. Limnology and Oceanography 26, 43±53. Chin, Y. P., Aiken, G. and O'Loughlin, E. (1994) Molecular weight, polydispersity, and spectroscopic properties of aquatic humic substances. Environmental and Science Technology 28, 1853±1858. Davies-Colley, R. J. and Vant, W. N. (1987) Absorption of light by yellow substance in freshwater lakes. Limnology and Oceanography 32, 416±425. DePetris, P. J. and Paolini, J. E. (1991) Biogeochemical aspects of South American rivers: The Parana and the Orinoco. In Biogeochemistry of Major World Rivers, eds E. I. Degens, S. Kempe and J. E. Richey, pp. 105±125. Wiley, New York. Eaton, A. (1995) Measuring UV-absorbing organics: a standard method. Journal of the American Water Works Association Feb., 86±90. Ertel, J. R., Hedges, J. I., Devol, A. H., Richey, J. E. and Nazare GoeÂs Ribero, M. (1986) Dissolved humic substances of the Amazon River system. Limnology and Oceanography 31, 739±754. FAO-UNESCO (1988) Soil Map of The World. Revised Legend. Food and Agriculture Organization, Rome, Italy. Green, S. A. and Blough, N. V. (1994) Optical absorption and ¯uorescence properties of chromophoric dissolved organic matter in natural waters. Limnology and Oceanography 39, 1903±1916. Guyot, J. L. and Wasson, J. G. (1994) Regional pattern of riverine dissolved organic carbon in the Amazon drainage basin of Bolivia. Limnology and Oceanography 39, 452±458. Hedges, J. I., Cowie, G. L., Richey, J. E., Quay, P. D., Benner, R., Strom, M. and Forsberg, B. R. (1994) Origins and processing of organic matter in the Amazon River as indicated by carbohydrates and amino acids. Limnology and Oceanography 39, 743±761. JaeÂ, R., Wol, G. A., Cabrera, A. C. and Chitty, H. C. (1995) The biogeochemistry of lipids in rivers of the Orinoco basin. Geochimica Cosmochimica Acta 59, 4507±4522. JaeÂ, R., ElismeÂ, T. and Cabrera, A. C. (1996) Organic geochemistry of seasonally ¯ooded rain forest soils: molecular composition and early diagenesis of lipid components. Organic Geochemistry 25, 9±17. Kaplan, L. A. (1994) A ®eld and laboratory procedure to collect, process, and preserve freshwater samples for dissolved organic carbon analysis. Limnology and Oceanography 39, 1470±1476. Kaplan, L. A. and Bott, T. L. (1989) Diel ¯uctuations in bacterial activity on streambed substrata during vernal algal blooms: eects of temperature, water chemistry, and habitat. Limnology and Oceanography 34, 718±733. Lawrence, J. (1980) Semi-quantitative determination of fulvic acid, tannin and lignin in natural waters. Water Research 14, 373±377. Lewis, W. M. Jr. (1988) Primary production in the Orinoco River. Ecology 69, 679±692. Lewis, W. M., Jr. and Saunders, J. F., III (1990) Chemistry and element transport by the Orinoco main stem and lower tributaries. In The Orinoco River as an ecosystem, eds F. H. Weibezahn, H. Alvarez and M. L. William Jr., pp. 211±239. Impresos Rubel, Caracas. Lewis, W. M. Jr., Saunder, J. F. III, Levine, S. N. and Weibezahn, F. H. (1986) Organic carbon in the Caura
Dissolved organic matter in a tropical blackwater river River, Venezuela. Limnology and Oceanography 31, 653± 656. Lewis, W. M., Jr, Hamilton, S. K. and Saunders, J. F., III (1995) Rivers of northern South America. In River and Stream Ecosystems of the World, eds. C. E. Cushing and G. W. Minshall, pp. 219±256. Elsevier, Amsterdam. McKnight, D. M. and Aiken, G. R. (1998) Sources and age of aquatic humus. In Aquatic humic substances: Ecology and Biogeochemistry, eds. D. O. Hessen and L. Tranvik. Springer Verlag, Berlin (in press). McKnight, D. M., Andrews, E. D., Spaulding, S. A. and Aiken, G. R. (1994) Aquatic fulvic acids in algal-rich antarctic ponds. Limnology and Oceanography 39, 1972± 1979. McKnight, D. M., Harnish, R., Wershaw, R. L., Baron, J. S. and Schi, S. (1997) Chemical characteristics of particulate, colloidal, and dissolved organic material in Loch Vale Watershed, Rocky Mountains National Park. Biogeochemistry 36, 99±124.
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McKnight, D. M., Boyer, E. W., Doran, P., Westerho, P. K., Kulbe, T. and Andersen, D. T. (1998) Spectro¯uorometric characterization of aquatic fulvic acid for the determination of precursor organic material and general structural properties. Limnology and Oceanography (in press). Richey, J. E., Hedges, J. H., Devol, A. H., Quay, P. D., Victoria, R., Martinelli, L. and Forsberg, B. R. (1990) Biogeochemistry of carbon in the Amazon River. Limnology and Oceanography 35, 352±371. Strome, D. J. and Miller, M. C. (1978) Photolytic changes in dissolved humic substances. Internationaler Verein fuÈr Theoretische und Angewande Limnologie Verhandlungen 20, 1248±1254. Zepp, R. G. and Schlotzhauer, P. F. (1981) Comparison of photochemical behavior of various humic substances in water: III Spectroscopic properties of humic substances. Chemosphere 10, 479±486.