WATER RESOURCES RESEARCH, VOL. 39, NO. 6, 1161, doi:10.1029/2002WR001619, 2003
Production, storage, and output of particulate organic carbon: Waipaoa River basin, New Zealand Basil Gomez,1 N. A. Trustrum,2 D. M. Hicks,3 K. M. Rogers,4 M. J. Page,2 and K. R. Tate2 Received 30 July 2002; revised 29 January 2003; accepted 20 March 2003; published 19 June 2003.
[1] We compute the particulate organic carbon (POC) yield of the Waipaoa River, New
Zealand, using sediment rating curves in conjunction with measurements of the carbon content of the suspended sediment. To ascertain the source of the carbon and the extent to which the POC flux is tied to different erosion processes, we determined the stable isotopic carbon composition (d13C) and carbon to nitrogen (C/N) ratio of weathered bedrock, soil, and regolith. Most POC is derived from suspended sediment generated by gully erosion (incision into weathered bedrock), supplemented by landsliding during extreme events. The specific yield of POC from the headwaters (drainage area 1580 km2) is 55 g m2 yr1, which is very high by global standards and by comparison with other turbid steep-land rivers. The annual loss to floodplain storage is 4% (3.6 Kt) of the mean annual POC yield (86.7 Kt). Thus the Waipaoa River is a very effective conduit for INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 transporting POC to the ocean. Hydrology: Geomorphology (1625); 1625 Global Change: Geomorphology and weathering (1824, 1886); KEYWORDS: particulate organic carbon, suspended sediment, sediment rating curves, steep-land rivers Citation: Gomez, B., N. A. Trustrum, D. M. Hicks, K. M. Rogers, M. J. Page, and K. R. Tate, Production, storage, and output of particulate organic carbon: Waipaoa River basin, New Zealand, Water Resour. Res., 39(6), 1161, doi:10.1029/2002WR001619, 2003.
1. Background: Suspended Sediment and Carbon Fluxes [2] Rivers constitute the primary means by which particulate matter, including sediment and organic carbon, is transferred between the terrestrial and marine environments, and, though it is small in comparison with the transfers that occur between the atmosphere and biosphere or the atmosphere and oceans, the riverine carbon flux cannot be discounted [Sarmiento and Sundquist, 1992; Ludwig and Probst, 1999]. Particulate organic carbon (POC) yields are commonly estimated as a proportion of the fluvial suspended sediment discharge [Meybeck, 1982, 1993a; Ittekkot, 1988; Spitzy and Ittekkot, 1991; Ludwig et al., 1996; Stallard, 1998]. In most rivers the POC content of suspended sediment decreases as the suspended sediment concentration increases, though the effect may be offset because the flux of POC increases with increasing discharge [Meybeck, 1982, 1993a; Ittekkot and Laane, 1991]. Assuming an allochtonous source, the inverse relation between the POC content of suspended sediment and suspended sediment concentration may be explained by the dilution of organic-rich surficial soil, which is supplied to channels by sheet erosion, with sediment that has a lower POC concentration derived from lower down in the soil profile or the underlying regolith 1 Geomorphology Laboratory, Indiana State University, Terre Haute, Indiana, USA. 2 Landcare Research Ltd., Palmerston North, New Zealand. 3 National Institute of Water and Atmospheric Research, Christchurch, New Zealand. 4 Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand.
Copyright 2003 by the American Geophysical Union. 0043-1397/03/2002WR001619
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[Meybeck, 1981, 1982, 1993a]. However, like suspended sediment, the concentration of POC is not purely a function of discharge [Bilby and Likens, 1979; Naiman, 1982; Depetris and Kempe, 1993]. For example, the flux of POC is moderated by transfers into and out of storage, and may also be influenced by exchanges between rivers and their floodplains [Mulholland, 1981; Kempe, 1984; Ittekkot et al., 1986; Cuffney, 1988; Twilley et al., 1992; Billen et al., 1991; Wainright et al., 1992; Downing et al., 1993; Stallard, 1998]. [3] Distinguishing between the different sources of POC is important, not least because the oxidation of buried organic carbon may reduce the effectiveness of terrestrial sinks such as floodplains, and modern carbon derived from the upper portions of the soil profile is more reactive and has a higher potential for degradation within the fluvial system than ancient carbon derived from sedimentary rocks [Hedges et al., 1986; Richey et al., 1980; Ittekkot et al., 1985; Depetris and Kempe, 1993; Stallard, 1998]. Nevertheless, with the exception of a few large rivers [Milliman et al., 1984; Zhang et al., 1992], there is a dearth of data for turbid rivers, which it is presumed convey a significant proportion of particulate matter to the oceans [Meybeck, 1993a, 1993b; Kao and Liu, 1996]. Many of these rivers have small (95%) of the native vegetation cover was removed by European colonists during the nineteenth and early
twentieth centuries. The primary mainstem gauging site (Kanakanaia) is located 48 km from the coast (Figure 1). Upstream from Kanakanaia, 27% of the 1580 km2 basin is forested, and the remainder supports pasture or scrub. Downstream from Kanakanaia the river has a single thread, meandering channel and is bordered by a well-developed floodplain. [6] Different associations of rock types, structure and topography in the Waipaoa River basin give rise to distinct landform assemblages that exhibit varying degrees of stability and are affected by specific erosion processes [O’Byrne,1967; Gage and Black, 1979]. Crushed and fractured mudstone and argillite in the headwaters are predisposed to deep-seated complex mass movements (e.g., slumping) [Trotter, 1993; Zhang et al., 1993], and support numerous large gully complexes that deliver large amounts of fine-grained sediment directly to headwater channels [Pearce et al., 1981; DeRose et al., 1998; Marutani et al., 1999]. Established gullies are activated by small, frequent storms, and many are connected directly to river channels, though sediment storage in feeder channels and on alluvial fans moderates sediment delivery [Marutani et al., 1999; Kasai et al., 2001]. Shallow landsliding is the dominant erosion process on the highly erodible Tertiary siltstone and mudstone that crop out in the lower reaches of the Waipaoa River basin [Trustrum et al., 1999]. Large rainstorms intensify the erosion processes responsible for delivering sediment to the Waipaoa River, since sediment availability increases dramatically once the threshold for landsliding is exceeded and the effectiveness of individual storms becomes increasingly more pronounced [Page et al., 1999]. [7] The mean suspended sediment concentration of the Waipaoa River at Kanakanaia is 1700 mg L1, the suspended sediment load is 10.7 106 t yr1, and the specific suspended sediment yield is 6750 t km2 yr1 [Hicks et al., 2000]. Frequent runoff events are relatively more important than large storms to the long-term suspended sediment yield of the Waipaoa River [Hicks et al., 2000]; 50% of the load is transported by flows52 times the mean flow) shallow landslides typically generate 10 to 20% (and as much as 50%) of the suspended sediment transported by the Waipaoa River.
3. Samples and Analytical Approach [8] We analyzed suspended sediment samples, collected at gauging stations on the Mangatu and Waipaoa rivers,to
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Table 1. Summary Characteristics of Basins and Databases
Site name Drainage area Area under forest Bank-full dischargea Period of record used Duration of record use Sampling period Number of samples
Waipaoa River
Mangatu River
Kanakanaia 1580 km2 27% 1544 m3 s1 May 1979 through Sept. 1990 10.8 years June 1999 through April 2000 35
Omapere 183 km2 29% ... Aug. 1983 through Sept. 1990 6.9 years June 1999 through April 2000 40
a
Average for 44 km reach downstream from Kanakanaia.
characterize the modern POC flux (Figure, Table 1). The samples were obtained during the course of routine suspended sediment sampling and gauging activities, and collected in plastic and glass bottles using either Manning (pumping) or US-D49 (depth-integrated) samplers. Like Kao and Liu [1996], we rely on a small number of samples (collected in 1 year) that are representative of the range of in-channel flows that transport the bulk of the suspended sediment load (Figure 2). Because of the high suspended sediment concentrations that characteristically are encountered in the Waipaoa and its tributaries [Griffiths, 1982; Hicks et al., 2000], the suspended sediment concentration was determined by (dry) weighing after evaporation (at 40C), rather than filtration. Thus we did not adhere to the convention of distinguishing POC on the basis of its failure to pass through a 0.45 to 0.50 mm filter [Thurman, 1985], and percent POC was calculated from the %C(organic) in the sediment per liter sample volume. Nor were the dry samples frozen until laboratory analyses were performed. No large floods have occurred in the Waipaoa River basin since 1996. Thus we used samples from a compositionally similar (fine to medium silt; Table 2), dated sequence of overbank sediments deposited on McPhail’s Bend (Figure 1), during flows of known magnitude [Gomez et al., 1998, 1999], to support our assumptions about the percent POC present in suspended sediment transported at high flows and to assess the amount of POC sequestered on the floodplain. McPhail’s Bend has never been tilled, and the samples we analyzed were taken from the lower portion/center of undisturbed silt units that ranged between 0.1 and 0.5 m in thickness. To gain additional insight into the source of the carbon, we also analyzed samples of the suspended sediment discharged from two gullies, and a representative range of the earth materials (weathered bedrock, and soil and regolith on hillslopes prone to landsliding) present in the headwaters of the Waipaoa River basin (Table 2). Prior to analysis, these samples were air dried and stored in a cold room. [9] All our analyses were undertaken on bulk samples and were not focused on any particular size fraction. The C and N contents were determined using a Europa Geo 20/20 isotope ratio mass spectrometer, interfaced to an ANCA-SL elemental analyzer in continuous flow mode (EA-IRMS). Prior to placing 20 mg of powdered sediment in tin capsules for automated combustion, the samples were rinsed with deionized water and treated with dilute HCl to remove inorganic carbon, then rinsed with deionized water until neutral and dried in an oven at 30C. The carbon dioxide and nitrogen gases were resolved using gas chromatograph-
ic separation on a column at 85C, and analyzed simultaneously for isotopic abundance as well as total organic carbon and nitrogen. Standards and blanks were included during the run for internal calibration. The d13C value (given in %) is the ratio of 13C to 12C in a sample relative to the international Pee Dee belemnite (PDB) standard, where d13C = {[(13C/12C sample)/(13C/12C standard)] 1} 1000. The analytical precision of our measurements is 0.1%, and the reproducibility of our results is within 0.1% for carbon and 0.3% for nitrogen. The size
Figure 2. Suspended sediment concentration versus water discharge for the Waipaoa River at Kanakanaia and Mangatu River at Omapere.
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Table 2. Summary Data for Particulate Organic Carbon (POC) Loadings, C/N and Stable Isotope Ratios, and Particle Size Number of Samples a
Weathered bedrock Subsoil (>0.1 m deep)b Topsoil ( 5) Omapere (Q/Qmean < 5) Omapere (Q/Qmean > 5) Gullies Floodplain sediments
9 26 8 11 24 12 28 2 13
POC Content, %
C/N Ratio
0.3 ± 2.1 ± 5.0 ± 1.6 ± 0.6 ± 1.1 ± 0.5 ± 0.42 0.6 ±
6.8 ± 3.0 12.3 ± 2.5 9.5 ± 0.6 1.84 ± 5.9 NAc 19.1 ± 7.1 9.9d 8.5±3.7 9.4 ± 1.9
0.2 1.4 0.9 0.6 0.07 0.7 0.04 0.2
d13C 26.3 26.2 27.0 27.4 27.8 27.8 28.4 28.1 26.3
± ± ± ± ± ± ±
D50, mm 0.7 0.7 0.4 0.4 0.2 0.6 0.2
± 0.6
... ... ... NAc 8.5 ± 0.6 26.0 ± 7.7 23.4 ± 3.4 21.6 12.5 ± 6.2
a
Sandstone, siltstone, and argilite lithologies. Major soil types present on steep (30 – 40), pastoral, landslide-prone hillslopes. No data available. d Data available for 52), which are presumed to be representative of fluvial sediments in transport during extreme events, are plotted for comparative purposes. Crosses indicate ancillary samples collected at Kanakanaia for the purpose of assessing the accuracy of the methodology used to determine POC, and not incorporated in the regression model.
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Table 3. Suspended Sediment and POC Yield Estimates Mean Annual Suspended Sediment Yield, Mt yr1
Mean Annual POC Yield, Kt yr1
POC at Q/Qmean < 1, %
POC at Q/Qmean < 5, %
POC at Q > Bank-Full, %
2.21 12.42
13.19 86.75
4.9 8.2
41.0 26.8
... 21.5
Omapere Kanakanaia
Waipaoa River at Kanakanaia Q=Qmean < 5:0; percent POC ¼ 1:28ðQ=Qmean Þ0:426
ð1Þ
Q=Qmean > 5:0; percent POC ¼ 0:6
ð2Þ
Mangatu River at Omapere Q=Qmean < 5:0; percent POC ¼ 1:02ðQ=Qmean Þ0:420
ð3Þ
Q=Qmean > 5:0; percent POC ¼ 0:5
ð4Þ
where Q is the instantaneous discharge and Qmean is the mean flow (34.7 and 7.2 m3 s1 for the Waipaoa and Mangatu rivers, respectively). The regression models for the Waipaoa and Mangatu rivers have an R2 of 0.84 and 0.83, respectively, and for Q/Qmean > 5, where the percent POC is independent of discharge, the value we utilize is the arithmetic mean of the available data (Table 2). [12] We fitted a suspended sediment concentrationwater discharge rating curve to the log-transformed gauging data (open squares in Figure 2) using a modified version of the locally weighted scatter smoothing (LOWESS) technique [Cleveland, 1979]. The procedure we employed is the same as Hicks et al. [2000] used, except that, for ease of calculation, we reduced the LOWESS curve to a series of connected straight lines (power functions) and utilized only a portion of the available flow record. At Kanakanaia, the selected segment of the flow record was the same 11 year period that Gomez et al. [1999] utilized to assess the contribution that contemporary floodplain sequestration makes to the sediment budget of the Waipaoa River (Table 1). The segment for the Mangatu River was shorter because gaugings at Omapere did not begin until August 1983. We then combined the sediment rating with flow record and computed the suspended sediment yield across the flow distribution [cf. Hicks et al., 2000]. The POC yield was computed as a percentage of the suspended sediment discharge using equations (1) – (4). Our results are summarized in Table 3 and Figure 4. Our total suspended sediment load estimate for Kanakanaia is a refinement of the previously published estimate [Gomez et al., 1999] (the small, 8%, difference is due to an improved calculation routine and flow rating), and because we utilized only a portion of the available record, it differs from Hicks et al.’s [2000] estimate of the long-term suspended sediment yield. [13] A given percentage of the POC yield is transported by a slightly lower flow range, but, as expected, in most respects the cumulative plots of suspended sediment and POC yields versus Q/Qmean are very similar (Figure 4). Reference to Table 3 also indicates that the bulk of both the suspended sediment and POC are transported at flows greater than 5 times the mean flow. Thus the low percent-
age of POC associated with storm runoff has a controlling influence on the POC yield totaled over all flows, and we note that our load weighted estimates for percent POC (0.7 and 0.6%) are comparable to the mean values for high flows derived from measurements (equations (1) – (4)). Large rainstorms (such as the March 1988 storm) also impact the upper tail of the cumulative distribution of suspended sediment yield at Kanakanaia [Trustrum et al., 1999; Hicks et al., 2000], an effect that translates to the POC yield (Figure 4). Such storms also potentially impart nonstationarity to log Clog Q sediment rating relationships but have a minimal impact on the sediment ratings for Kanakanaia and Omapere [Hicks et al., 2000]. Our data do not permit us to evaluate effects due to hysteresis in the suspended sediment concentrationwater discharge relationship, but Hicks et al. [2000] found there was no significant difference (at the 5% level) between suspended sediment concentrations recorded during rising and falling stages at either Kanakanaia or Omapere.
5. Implications for the Origin and Fate of POC [14] Soil and rock necessarily make unequal contributions to a river’s suspended sediment load, but in the Waipaoa River basin the contribution made by deeply incised gully systems, in which the rate of incision can exceed 3 m yr1 [DeRose et al., 1998], far outweighs that made by spatially nonspecific processes such as surface erosion caused by overland flow and landsliding caused by large-magnitude storms [Trustrum et al., 1999; Hicks et al., 2000]. Gully erosion, which is associated with terrain underlain by
Figure 4. Cumulative distributions of suspended sediment and particulate organic carbon loads transported at discharges less than the indicated value.
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Figure 5. Carbon to nitrogen (C/N) ratio versus water discharge normalized by mean discharge during low flows in the Mangatu and Waipaoa rivers. Dashed line indicates mean C/N ratio of overbank sediment which is presumed to be representative of fluvial sediments in transport during extreme events (Q/Qmean > 52).
calcareous mudstone (argillite) rock of the Whangai Formation [Gage and Black, 1979; Gomez et al., 2003], not only sustains the catchment-averaged denudation rate of 1 to 2 mm yr1 that is implied by the Waipaoa River’s suspended sediment yield, but it also regulates suspended sediment and POC yields across the entire range of flows (i.e., there is no erosion threshold limitation on sediment supply), because the large gully complexes contribute sediment during base flow conditions, and all established gullies are activated by small, frequent rainstorms. Larger rainstorms merely intensify the fundamental process that is responsible for delivering sediment to the channel system, but during extreme events additional sediment is generated by earthflows and landslides. [15] In both the Mangatu and Waipaoa rivers, not only does the percent POC decline to a low, stable value during storm runoff (Figure 3b), but the C/N ratio also exhibits a similar trend over the same range of water discharge, Q/Qmean< 5 (Figure 5). The tendency toward stability implies either that the river system rapidly (in both space and time) homogenizes suspended particles that originate from diverse sources, or that they are derived from a particular source. High sediment delivery ratios [Marutani et al., 1999], short (of the order of hours or tens of hours) transport times and phase lags/leads between sediment concentration and flow peaks, and a lack of storage sites in low-order channels all militate against homogenization. Rather, the low, stable percent POC values and C/N ratios during storm runoff are consistent with the carbon loading of suspended sediment being influenced by gully erosion. [16] Gully erosion removes weathered bedrock and delivers it directly to stream channels. Sediment discharged from gullies has a low organic carbon loading, which is similar to that of suspended sediment in both the Mangatu and Waipaoa rivers when Q/Qmean > 5 (Table 2). The stable carbon isotopic ratios of these three categories of fluvial
sediment also are similar (Figure 6), but there is a significant (>1% level) difference between the ratios for gullies and weathered bedrock. We interpret the similarity between the stable carbon isotopic ratios of fluvial sediments transported during extreme events (as indicated by the overbank sediments), exposures of weathered bedrock, and regolith from the lower portions of soil profiles (B and C horizons) on steep hillslopes (Figure 6, Table 2), to be indicative of the contribution landsliding makes during extreme events [cf. Page et al., 1999; Hicks et al., 2000; Reid and Page, 2002]. The elevated organic carbon loadings and C/N ratios we observe at low discharges (Q/Qmean< 5) also likely represent contributions from sources other than gully erosion, such as C3 plants and humus. This material probably has a riparian origin, rather than being supplied by sheet erosion, because infiltrating rainwater that is returned to the channel through groundwater flow contributes to the base flow component of the hydrograph and the relation between infiltration capacity and runoff is such that overland flow is unlikely to be a widespread phenomenon during small rainstorms. Indeed, though the isotope ratios of suspended sediments transported during base flows and topsoils are somewhat similar, the organic carbon loadings and C/N ratios are significantly different (>1% level; Table 2 and Figure 6). Sheet erosion is an effective mechanism for removing sediment from unvegetated landslide scars and tails on pastoral hillslopes during, and for a 1 to 2 year period after extreme storms [Page et al., 1999; Hicks et al., 2000]. However, this has little impact on the carbon loading of fluvial sediment, because soil from the B and C horizons accounts for the bulk of the material mobilized and exposed by landsliding. Thus, as in other small, steep-land rivers, POC in the Waipaoa River appears to originate primarily from the erosion of sedimentary rocks, not from organic soil particles [cf. Kao and Liu, 1996; Leithold and Blair, 2001; Masiello and Druffel, 2001].
Figure 6. Stable carbon isotopic composition (d13C) and carbon to nitrogen (C/N) ratio of fluvial sediments and earth materials in the Waipaoa River basin. W, weathered bedrock; S, subsoil (>0.1 m deep); T, toposil (< 0.1 m deep); F, floodplain (overbank) sediments; K < 5, Kanakanaia – Q/Qmean < 5; K > 5, Kanakanaia – Q/Qmean > 5; O < 5, Omapere – Q/Qmean < 5; O > 5, Omapere – Q/Qmean > 5. Line through box indicates mean value and whisker caps the 10th and 90th percentiles.
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The specific yield of POC at Kanakanai is 55 g m2 yr . This is very high, both by global standards and by comparison with other turbid steep-land rivers [Degens and Ittekkot, 1985; Kao and Liu, 1996]. Taking into account the contribution to the suspended sediment yield made by the drainage area downstream from Kanakanaia (i.e., scaling up the yield to the full catchment area on a per area basis, and accounting for the increased contribution made by landsliding [Hicks et al., 2000]), we estimate that the 2205 km2 Waipaoa River basin annually delivers 130 Kt of POC to the Pacific Ocean. The transport of terrestrial carbon into rivers and eventually into the ocean constitutes an important link in the global carbon cycle, but the POC flux is likely tempered by losses to the floodplain [Ittekkot et al., 1986; Stallard, 1998]. However, in many small steep-land basins there is little opportunity for storage along the channel network and transport from hinterland to ocean may be accomplished in tens to hundreds of hours and, in contrast to the situation along large, low gradient rivers, there is little opportunity for exchange between the mainstem and floodplain [cf. Junk, 1985; Depretis and Kemp, 1993]. In the Waipaoa River, when Q/Qmean > 5, the load weighted POC concentration is 0.7%, and 73% of POC is transported by flows of equal or greater magnitude (Figures 3 and 4), but only 21.5% of POC is transported by flows in excess of the bank-full discharge (Q/Qmean = 52) (Table 3). Gomez et al. [1999] estimated that 6.5 Mt of sediment were deposited on the Waipaoa River floodplain in the period May 1979 to September 1990. Assuming an organic carbon loading of 0.6% is representative of the floodplain sediments (Table 2), we estimate that the total loss of POC to the floodplain within the same 44 km long reach was 19% (39 Kt) of the amount of POC transported by flows in excess of the bank-full discharge. This equates with an annual loss to storage of 4% (3.6 Kt) of the mean annual POC yield (86.7 Kt) at Kanakanaia. Thus the Waipaoa River is a very effective conduit for transporting POC to the ocean. [18] Our accumulated data also suggest that the relative role of different processes and their impact on suspended sediment and POC fluxes are distinguished by the distribution of erosion events in both space and time across the landscape [cf. Wolman and Gerson, 1978; Ludwig et al., 1996]. The implication is that in small, steep-land drainage basins, where hillslopes are often mantled by thin, mineral soils and coarse-textured regolith, the processes governing POC export will differ from those that operate in their larger, lowland counterparts [cf. Hedges et al., 1986; Richey et al., 1990]. In particular, the magnitude of soil loss due to accelerated erosion of surface horizons by overland flow may be small, compared with the amount of sediment produced by gully erosion or landsliding [cf. Hovius et al., 1997]. The net result is that the bulk of riverine POC exported from many steep-land drainage basins may consist of ancient organic matter derived from sedimentary rocks [cf. Kao and Liu, 1996; Leithold and Blair, 2001; Masiello and Druffel, 2001]. Note, however, that the pattern of sediment delivery from steep-land landscapes dominated by landsliding differs from that in basins where gully erosion is the dominant process responsible for delivering sediment to the channel system [cf. Hicks et al., 2000; Hovius et al., 2000; Masiello and Druffel, 2001]. In the [17] 1
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former case, there is a threshold limitation on sediment supply and so exceptional events may have more significance to the net export of POC. In the latter case, there is no threshold limitation on sediment supply and more frequent runoff events will have a greater impact on the POC yield.
6. Conclusion [19] Carbon isotopic and C/N ratios support our previous observation that gully erosion (fluvial incision into weathered bedrock) is the dominant process responsible for delivering sediment to the channel system [Hicks et al., 2000]. There is no threshold limitation on the supply of sediment (or POC) generated by gully erosion, and elevated (1 – 3%) carbon loadings observed at low discharges (Q/ Qmean< 1) likely reflect contributions from riparian sources rather than sheet erosion sensu stricto. The POC loading of suspended sediment in the Waipaoa River declines rapidly as discharge and suspended sediment concentration increase (Figure 3), and stabilizes at a low value (0.6%) that approaches the carbon content of the bedrock (Table 3) [cf. Kao and Lui, 1996]. For flows Q/Qmean > 5, which transport most (73%) of the POC (Figure 4), the load weighted value is 0.7%. However, only 21.5% of POC is transported by flows in excess of the bank-full discharge (Q/Qmean = 52). At Kanakanaia, the mean annual POC yield is 86.7 Kt yr1, the annual loss to storage on the flood plain downstream is 4% (3.6 Kt), and we estimate that 130 Kt of POC are delivered annually to the Pacific Ocean. [20] Acknowledgments. This paper is a contribution to Manaaki WhenuaLandcare Research’s Waipaoa Catchment Study. We are grateful to the New Zealand Foundation for Research, Science and Technology (contract 009306), the National Science Foundation (grants SBR-9807195 and BCS-0136375), and Indiana State University for supporting our work. We thank Dave Peacock (Gisborne District Council) and Bob Back (HydroTechnologies Ltd.) for the sediment gauging and flow data, Hannah Brackley (Landcare Research) for the suspended sediment load determinations, Yuko Sugita (ISU) for performing the particle size analyses, and Philip Warnes (IGNS) for isotope and C/N sample preparation.
References Bilby, R. E., and G. E. Likens, Effect of hydrologic fluctuations on the transport of fine particulate organic carbon in a small stream, Limnol. Oceanogr., 24, 69 – 75, 1979. Billen, G., C. Lancelot, and M. Maybeck, N, P, and Si retention along the aquatic continuum from land to ocean, in Ocean Margin Processes in Global Change, edited by R. F. C. Mantoura, J.-M. Martin, and R. Wollast, pp. 19 – 44, John Wiley, New York, 1991. Cleveland, W. S., Robust locally weighted regression and smoothing scatterplots, J. Am. Stat. Assoc., 74, 829 – 836, 1979. Cuffney, T. F., Input, movement and exchange of organic matter within a subtropical coastal blackwater river-floodplain system, Freshwater Biol., 19, 305 – 320, 1988. Degens, E.T., and V. Ittekkot, Particulate organic carbon: An overview, in Transport of Carbon and Minerals in Major World Rivers, part 3, edited by E. T. Degens, S. Kempe, and R. Herrera, Mitt. Geol. Pala¨ontol. Inst. Univ. Hamburg, 58, 7 – 27, 1985. Depetris, P. J., and S. Kempe, Carbon dynamics and sources in the Parana´ River, Limnol. Oceanogr., 38, 382 – 395, 1993. DeRose, R. C., B. Gomez, M. Marden, and N. A. Trustrum, Gully erosion in Mangatu Forest, New Zealand, estimated from digital elevation models, Earth Surf. Processes Landforms, 23, 1045 – 1053, 1998. Downing, J. P., M. Meybeck, J. C. Orr, R. R. Twilley, and H.-W. Scharpenseel, Land and water interface zones, Water Air Soil Pollut., 70, 123 – 137, 1993. Gage, M., and D. Black, Slope-stability and geological investigations at Mangatu State Forest, Tech. Pap. 66, 37 pp., N. Z. For. Serv., Wellington, N. Z, 1979.
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B. Gomez, Geomorphology Laboratory, Indiana State University, Terre Haute, IN 47809, USA. (
[email protected]) D. M. Hicks, National Institute of Water and Atmospheric Research, P.O. Box 8602, Christchurch, New Zealand. M. J. Page, K. R. Tate, and N. A. Trustrum, Landcare Research Ltd., Private Bag 11-052, Palmerston North, New Zealand. K. M. Rogers, Institute of Geological and Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand.
