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Organic matter sources and transport in an agriculturally dominated temperate watershed
Applied Geochemistry 19 (2004) 1111–1121 www.elsevier.com/locate/apgeochem
Organic matter sources and transport in an agriculturally dominated temperate watershed Shelly A. Munsona, Anne E. Careyb,* a
Environmental Science Graduate Program, The Ohio State University, Columbus, OH, USA Department of Geological Sciences, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, OH 43210-1398, USA
b
Abstract The quality, quantity, and origin of suspended organic matter were studied in the highly agricultural Upper Scioto River in Central Ohio. Late summer baseflow conditions were compared to late autumn high flow conditions. Variables examined in the suspended matter were the total suspended solids concentration, semi-quantitative concentrations of lignin, carbohydrate concentrations, total organic C, total and organic P, and d-13C. Also examined were ratios of C to N, organic C to organic P ratios and fluxes of total organic C. The primary hypothesis of this research was that the quality (or biodegradability) and quantity of organic matter in the Upper Scioto River would increase during autumn stormflow conditions due to inputs of fresh terrestrial organic matter. The autumn suspended matter was also expected to reflect C4 plant contributions from corn organic matter. Results show that the quality and quantity of organic matter were greater during summer, as reflected in low molar ratios (178:1) of organic C to organic P, and higher organic C content of the suspended matter in summer. Summer suspended matter was 3.6% organic C and autumn suspended matter was 2.3% organic C. Carbon to N molar ratios in both seasons were very close to the Redfield ratio (6.6:1 in summer and 6.7:1 in autumn). Total suspended matter and total organic C concentrations were lower in autumn (8.7 mg/l1 TOC and 17.7 mg/l1 TSS) than in summer (17.5 mg/l1 TOC and 39.0 mg/l1 TSS), but the fluxes were greater in autumn due to greater stream flow. Stable isotope analyses suggested a phytoplankton or C3 plant source (most likely corn) for summer organic C (mean d13C of 24.8%) and a phytoplankton or C4 plant source for autumn organic matter (d13C=21.5%). # 2004 Published by Elsevier Ltd.
1. Introduction 1.1. Background Human activity has greatly altered natural hydrological systems and nutrient cycles in the recent past. In Ohio, this is manifested in artificial drainage of the naturally hydric soils for agriculture and the alteration of stream hydrology. The dominant causes of impairment for many streams in agricultural parts of Ohio are nutrient enrichment and habitat alteration. Nutrient enrichment occurs as a result of fertilizer application to soils and its loss to the receiving waters. Commercial
* Corresponding author. Fax: +1-614-292-7688. E-mail address: [email protected] (A.E. Carey). 0883-2927/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2004.01.010
urea fertilizers decompose through nitrification to NH+ 4 , and it oxidizes under aerobic conditions to NO 3 (Vitousek et al., 1997; Clark and Fritz, 1997). Nitrate is water-soluble and is lost through leaching. It is also toxic to humans, especially young children, at levels greater than 10 mg NO3–N l1, by decreasing the O2 carrying capacity of the hemoglobin in blood. A recent study has also shown the possibility of increased cancer risk for older women drinking NO 3 -contaminated water (Weyer et al., 2001). Often a limiting nutrient, P is present in fertilizers and can cause water quality problems. Phosphorus is more transport limited than NO 3 because it adsorbs to sediments and as a result is lost through runoff and erosion (Heathwaite et al., 2000). The restoration of riparian areas has been promoted as a best management practice to control runoff and the resulting loss of N and P (Clausen et al., 2000).
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The ability of a stream to store organic matter (OM) is another change caused by physically altering the stream channel. Straight channels cannot store as much OM as natural, meandering channels and act more as conduits for the OM (Paul, 1999). This has implications for food web dynamics because storage is important for the next year’s community productivity. Food webs will also be affected by the nutrient enrichment that will increase OM quality. Bacteria and other detritivores will potentially have an overabundance of labile OM available, rapidly multiply, and cause anoxic conditions. Therefore, studying the biodegradability of OM from an agriculturally dominated system is important. Sources and sinks of OM are also an important consideration. Anthropogenically affected systems will have different OM sources (e.g., petroleum hydrocarbons and polycyclic aromatic hydrocarbons and other combustion products from fuels, herbicides and pesticides) than will natural systems whose organic matter sources are primarily soil organic matter, terrestrial plant litter and phytoplankton. 1.2. Objectives The overall goal of this research was to examine a stream within an agriculturally dominated system and determine how the quality, quantity and sources of OM varied under different hydrological conditions. Two sampling events were conducted at 3 locations along the Upper Scioto River main stem in order to compare baseflow to high flow (summer to autumn) conditions. The 3 locations were treated as replicates. The variables investigated were major ions, lignin content, C to N ratios (C:N), organic C to organic P ratios (OC:OP), total organic C (TOC), TOC fluxes, total suspended solids (TSS), and d-13C. The anticipated results of this study were: Because of higher runoff, major ions such as + 3 NO 3 , NO2 , NH4 , and PO4 , that are associated with agricultural activities, will increase in concentration in the autumn. Quality of the OM will be higher in the autumn and lower in the summer as a result of autumn inputs of fresh OM from the runoff of decomposed, agriculturally produced debris. Therefore, there will be lower C:N ratios and lower lignin content in the autumn OM. In the summer, OM sources will be mostly autochthonous, or in-stream, but in autumn sources will be more allochthonous, or terrestrial. This will be shown through d-13C values from the riverine TSS indicating mostly C3 plant inputs (23 to 34%) from leaf litter in the autumn, but possibly some C4 plant (e.g., corn) contribution (6 to 23%) as a result of nearby
corn cultivation. In the summer, phytoplankton (18 to 24%) should be the dominant source of OM in the river. Increased runoff and overland flow will cause an increase in particulate matter as well as nutrients associated with them. This will mean higher TSS and TOC fluxes. OC:OP ratios will either be lower or remain unchanged due to a concurrent increase in OC.
2. Physical setting The Upper Scioto watershed of west-central Ohio lies in the Eastern Cornbelt Plains and encompasses 1860 km2 (Fig. 1). The 3 sampling locations at La Rue, Prospect, and the Mink Street Bridge (south of Prospect) are in areas whose land use is primarily agricultural and were chosen because of their accessibility and their locations upstream of the major reservoir on the river, the O’Shaughnessy Reservoir in southwestern Delaware County (Fig. 1). The reservoir has a no-flow (lentic) environment which is undesirable as this study is of a flowing system. The sampling locations were also chosen because of their proximity to the USGS gauging station #03219500 at 40 250 N, 83 120 W in Prospect. The geologic setting of the Upper Scioto River is the glacial till plains (Wisconsin glacial drift) which is underlain by Silurian limestone and dolostone bedrock. Approximately 32% of the area has more than 3% OM in the upper 25 cm as well as more than 27% clay in the topsoil (Soil Regions of Ohio Soil Characteristics, 2003). Approximately 31% of the area has a seasonal high water table at less than 40 cm below the surface. Much of the land has been drained for successful cultivation of corn and soybeans. The watershed has little relief and just 6% of the area has a slope of more than 8%. The soils can be characterized as basic, clayey and productive on a flat landscape (Soil Regions of Ohio Soil Characteristics, 2003).
3. Methods 3.1. Sampling Samples were taken at the 3 locations during late summer baseflow conditions and during late autumn event flow conditions. The late summer samples were collected on 4 September 2002 when the discharge was 0.40 m3 s1 and the high (or event) flow samples were collected on 16 November 2002 when the discharge was 6.94 m3 s1, on the falling limb of a hydrograph whose peak discharge of 36.82 m3 s1 occurred on 11 November (Fig. 2). All samples were grab samples collected in 18 M deionised water cleaned polyethylene bottles
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Fig. 1. Map of the study area showing its location in west-central Ohio. The 3 sampling locations on the Upper Scioto River are indicated by the arrows. The location of the US Geological Survey’s stream gauging station #03219500 at 40 250 N, 83 120 W in Prospect, Ohio, is noted by the open circle.
(Welch et al., 1996), with the exception of the TOC samples, which were collected in combusted (450 C for 3 h) amber glass bottles. 3.2. Analytical Major ions were determined by ion chromatography using a Dionex1 DX-120 Ion Chromatograph instrument (2.3% precision). Samples were filtered through Nuclepore1 0.4 mm pore size polycarbonate membrane filters using dedicated polycarbonate filter funnels and bell jars so that samples were filtered directly into sample bottles. Samples were refrigerated at 4 C until analysis. Samples for lignin analysis were dried at 105 C for 24 h. The semi-quantitative method of Pocklington and Hardstaff (1974) was used to estimate mg lignin g1 sediment (50% precision). Total suspended solids (TSS) were determined gravimetrically on samples which were filtered with weighed Nuclepore1 0.4 mm pore size membrane filters. The filters were then oven dried (105 C for 24 h) and reweighed to determine TSS. Particulate organic C (POC) was determined by pooling all
% total C (%TC) data from the CN analyzer, a Thermoquest CE instrument model NC 2100 Soil (also used for the C:N ratios), and from the d-13C determinations and multiplying by TSS to get POC in mg l1. Samples were filtered through Millipore1 47 mm diameter glass fiber filters (0.7 mm pore size), acidified to a pH of 2 using concentrated HCl and refrigerated in the dark. Dissolved organic C (DOC) of these filtrates was determined using a Shimadzu1 TOC-5000 TOC analyzer (2.4% precision). TOC was calculated as the sum of POC and DOC. TOC and particulate P loadings were calculated using USGS discharge data for each specific sampling date. C:N ratios were determined by pooling all available %TC data determined by a CN analyzer and the d-13C analysis. The % total N (%TN) data were taken from the available CN analyzer data. La Rue and Mink Street high flow samples did not contain enough suspended matter to determine %TN. Therefore, the Prospect %TN was used for these C:N ratios. Particulate P and organic P (TP and OP) was determined colorimetrically by adapting the methods of Aspila et al. (1976) and Strickland and Parsons (1972). Carbon
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Fig. 2. Scioto River discharge measured at Prospect, Ohio (US G.S. stream gauging station #03219500) for late summer and autumn 2002. The arrows show the days on which sampling was conducted for this study.
isotope analyses were performed by Isotope Services, Inc., Los Alamos, New Mexico. The samples for isotope analysis were oven-dried (105 C for 24 h) before they were sent to Isotope Services where d-13C was determined using a VG Isomass Sira Series II Mass Spectrometer that is fed from a N/A 1500 Carlo-Erba Elemental Analyzer (0.11% precision).
4. Results 4.1. Hydrology The stream gauging station at Prospect, Ohio (Hydrologic Unit Code 05060001, USGS station #03219500) has been maintained since July 1925 and 70 a of data are available. Due to little rainfall during the summer of 2002, the river experienced baseflow conditions for the most of the summer (Fig. 2). Precipitation data from the Ohio State University airport in Columbus, Ohio (approximately 72 km south of Prospect, Ohio) were obtained through the National Climatic Data Center’s Website (www.ncdc.noaa.gov/servlets/ ULCD which was accessed on 24 June 2003). Rainfall for August 2002 was 54.1 mm and 22 days of 31 were without rain. For September, prior to the baseflow sampling, there was a trace of rain h1 on 3 September 2002, from 3.00 to 5.00 pm. Therefore it can be said that
there was no intense precipitation event preceding the sampling date. The average instantaneous daily discharge on 4 September 2002 (during the baseflow sampling event) was 0.4 m3/s, well below the average daily discharge of 2.0 m3/s for the 70 a of record for that date. However, August is probably more representative of the conditions for the river in early September. Streamflow in August 2002 is in the lowest quintile for all Augusts for the period of record (1926–present). The 16 November 2002 average instantaneous daily discharge was 6.9 m3 s1, close to the mean November instantaneous daily discharges since 1926 of 7.2 m3 s1. Rainfall for November 2002 totaled 76.2 mm and 18 days of November had rainfall. For the period of 10–16 November, the 10th, 11th, 15th and 16th had precipitation. 10 November received the most: it rained for 14 h of 24 and intensity ranged from a trace h1 to 14 mm h1. On the 11 November it rained for 2 h with intensity ranging from a trace h1 to 0.3 mm h1. On the 15th it rained for 13 h and intensity ranged from a trace h1 to 1 mm h1. 16 November received less: it rained for 7 h and intensity ranged from a trace h1 to 0.3 mm h1. Therefore, there were heavy precipitation events on the 10 November and light events on the 11th, 15th and 16th. During the September sampling, the mean temperature of the river was 26.6 C and the pH mean was 8.48. In November, the mean water temperature was 5.27 C and the mean pH was 7.72.
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4.2. TSS, TOC, and TOC loading The TSS mean was higher during baseflow (39.0 mg l1, Table 1) compared with its concentration at high flow (17.7 mg l1, Table 2). Use of the nonparametric Tukey’s quick test is appropriate to analyze small sample sizes as were obtained in this study. Tukey’s quick test compares the number of values in one group which are smaller than all the values in the second group and the number of values in the second group which are larger than all values in the first group. The T statistic is the sum of those values. If T56, then the null hypothesis of no difference between groups is rejected (P=0.10 and is not dependent upon the number of samples). By the Tukey quick test, one rejects the null hypothesis for the TSS concentrations. Tukey’s quick test was similarly applied to all the data (Tables 1–3). The POC concentrations (Tables 1 and 2) were not significantly different between baseflow
and high flow (POC means were 1.46 and 0.42 mg l1, respectively). The DOC concentrations (Table 3) were greater during baseflow (mean of 16.1 mg l1) than at high flow (mean of 8.3 mg l1). POC concentrations in the suspended matter was not significantly different between baseflow and high flow. During baseflow, the mean POC was 36.0 mg g1 and during high flow POC was 23 mg g1. The TOC concentrations, however, were significantly different. At baseflow, the TOC mean was 17.5 mg l1 and at high flow it was 8.7 mg l1. The TOC loads were different by Tukey’s quick test, but because of the high discharge in November the TOC loads were greater, even though the concentrations of TOC were lower at high flow. The mean daily TOC loading for baseflow was 686 and 5200 kg C day1 for high flow. The POC content of the suspended matter was not significantly different. POC made up an average of 3.6% of the suspended matter during baseflow and 2.3% of the suspended matter during high flow.
Table 1 Baseflow particulate sample values for all locations and their means (values in parentheses calculated using LOD for OP method) Sample
TSS (mg/l) TOC (mg/l) POC (mg/g) POC (mg/l) TOC loading (kg/day) TP loading (kg/day) Lignin (mg/g) C:N (molar) OC:OP (molar) d-13C
La Rue
Prospect
Mink St.
Mean
52.2 12.9 46.0 2.40 505 2.5 1.1 10.6 351:1 23.83
30.4 23.8 35.5 1.08 933 3.5 0.94 5.20 43:1 25.17
34.3 15.9 26.5 0.909 620 0.67 1.2 4.12 140:1 ( <610:1) 25.41
39.0 11.6 17.5 5.63 36.0 9.76 1.46 0.816 686 222 2.2 1.44 1.1 0.092 6.62 3.47 178:1 (335:1)157:1 (284:1) 24.800.851
Table 2 High flow particulate sample values for all locations and their means (values in parentheses: used LOD in calculation, values with asterisks: used Prospect %TN see Section 3.2) Sample
TSS (mg/l) TOC (mg/l) POC (mg/g) POC (mg/l) TOC loading (kg/day) TP loading (kg/day) Lignin (mg/g) C:N (molar) OC:OP (molar) d-13C
La Rue
Prospect
Mink St.
Mean
9.0 10 20 0.18 6000 0 (0.44) 1.2 5.84:1* (2200:1) 22.43
27.5 9.6 22.9 0.630 5750 0 (0.30) 1.6 6.62:1 (20,000:1) 20.57
16.5 6.60 26.5 0.437 3960 0 (0.33) 1.6 7.65:1* (12,000:1) 21.35
17.79.31 8.71.9 233.3 0.420.23 52001100 0 (0.360.074) 1.50.23 6.700.908 (11,000:18900:1) 21.400.934
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Table 3 Dissolved species for both events, all locations and their means. All values in mg l1. B=baseflow, H=high flow (means with asterisks calculated using the LOD for at least one value) La Rue-B NO 3 NO 2 NH+ 4 PO3 4
0.279
Prospect-B Mink Street-B Mean-B
La Rue-H Prospect-H
Mink Street-H Mean-H
0.157
12.6 0.363 0.264 0.703 9.78
6.37
0.317
0.2510.0836
8.98 0.295 0.259
4.3. Dissolved N and P
5. Discussion
Nitrate-N was greater (Table 3), under high flow conditions (high flow: 9.3 mg N l1, baseflow: 0.25 mg N l1). The other dissolved N species and soluble reactive phosphate were detected in small numbers of samples. Statistical tests have not been applied to these data.
5.1. TSS, POC, and TOC loading
4.4. C:N and OC:OP ratios The C:N molar ratios for the two flow conditions were similar (Tables 1 and 2) and using Tukey’s quick test, the null hypothesis of no difference is accepted (baseflow mean: 6.63:1, high flow mean: 6.70:1). The OC:OP ratios were higher during baseflow conditions (Tables 1 and 2). However, the OC:OP ratios for the high flow samples were calculated using the detection limit for the OP concentrations, because no organic P was detected in the high flow suspended matter samples. This may have been due to the possible loss of sample during analysis which resulted in no detectable organic phosphate in the suspended matter in all the high flow samples. 4.5. Lignin and d-13C Lignin content of the suspended matter samples (Tables 1 and 2) was not significantly different by Tukey’s quick test. The mean lignin concentration was 1.5 mg g1 under high flow conditions and 1.1 mg g1 under baseflow conditions. The semi-quantitative lignin analysis yielded few observable lignin-stained particles in any of the samples, but there were larger sized and more numerous stained patches in the high flow samples. Carbon-13 values were significantly different for the organic matter in the samples for the two events (Tables 1 and 2). During baseflow, the mean value for the suspended organic matter was d-13C of 24.80% and during high flow the mean d-13C was 21.45%.
9.33 3.13 0.235*0.166 0.175*0.149 0.318*0.334 8.30 1.90
The greater suspended solids (mean TSS of 39 mg l1) during the baseflow sampling (Table 1), compared to a mean TSS of 18 mg l1 in high flow conditions (Table 2), suggests that there was a dilution from the higher river discharge in November. This observation is different from what many previous studies (e.g., Cronan et al., 1999, DePetris and Kempe, 1993, Buffam et al., 2001) have found for high flow conditions, which is an increase of both TSS and DOC. This discrepancy of the results with what many others have observed could be because the November precipitation was not intense enough to cause a significant amount of runoff. Collection of TSS samples at the Prospect station from July 2000 to June 2001 showed a general increase of TSS with river discharge, but also with a dependence upon the local stage of the stream hydrograph. The highest concentration of TSS observed at Prospect was 180 mg l1 on 12 April 2001 when the stream discharge was 55 m3 s1. That sampling was conducted on the rising limb of a flood hydrograph and at the highest river stage observed during a year of monthly sampling at Prospect. On 20 June 2001, on the falling limb of the stream hydrograph, a TSS concentration of 86 mg l1 was measured at a much lower stream discharge (3.2 m3 s1). The November 2002 sample was collected when the discharge was 6.9 m3 s1, 4 days after the peak discharge of 37 m3 s1 for the local hydrograph. Thus, even though the stream discharge was far greater in the autumn sample than in the summer sample, much of the readily transported TSS may have been washed off earlier in the local hydrograph, on one of the several days before sampling in the autumn, and that would account for the higher value of TSS measured at baseflow conditions. Another possible explanation is that because of the very low stream stage during the baseflow sampling, the sampling procedure disturbed the fine bedload and part of what was determined as suspended solids had actually been fine bedload prior to the
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disturbance by sampling. Great care was taken during sampling, but water depth was only slightly greater than the size of the sample bottles and there may have been inadvertent disturbance. Summer baseflow POC values had high variability, and this was the reason that the values for the two sampling conditions were not significantly different. The cause for these values’ variability appears to be the La Rue summer sample. POC at La Rue was 2.40 mg l1 while Prospect and Mink Street POC were 1.08 and 0.909 mg l1 (Table 1), respectively. It may be possible that the large POC value at La Rue is the result of chicken waste from the large chicken egg production facility located within the village of La Rue. However, as a fraction of TSS, baseflow conditions did not have a significantly higher POC concentration than did the high flow condition. The POC content of the Scioto River suspended matter collected in this study is similar to the results of the SCOPE project on major world rivers of Ittekkot and Laane (1991). Overall, Ittekkot and Laane (1991) found that the POC content of TSS (range of 1.3–8.4% or 0.6–14.2 mg C l1) showed an inverse relationship between the content and concentration of POC, and POC decreased with the TSS, as the authors have observed in the Scioto River. The Scioto data fall within the range observed for world rivers (Fig. 3a). Dissolved organic C content of the stream samples ranged from 6.15 mg l1 for the Mink Street bridge sample at high flow to 22.7 mg l1 for the Prospect sample at baseflow. The means of the two sampling events were 16.1 mg l1 at baseflow and 8.3 mg l1 at high flow (Table 3). As others (Ittekkot and Laane, 1991) have observed, the Scioto River DOC concentrations declined with increasing stream discharge. For world rivers, typically the DOC/POC ratio is 10.8 at TSS concentrations less than 15 mg l1 and 5.8 at TSS concentrations of 15–50 mg l1 (Ittekkot and Laane, 1991). However, the DOC/POC ratios in our Scioto River samples were higher, approximately 11–20 mg l1 (Fig. 3b). Although Ittekkot and Laane (1991) found the primary difference among world rivers to be high DOC/POC ratios in lowland rivers, the authors’ low gradient, lowland river has higher DOC/POC ratios than typical of the rivers studied by the SCOPE project (Fig. 3b). During high flow conditions, the DOC concentrations at all 3 sampling sites were similar, with a mean DOC of 8.3 mg l1. However, for the baseflow samples, the Prospect DOC concentration of 22.7 mg l1 was larger than at the other two locations. At La Rue the DOC was 10.5 mg l1 and at Mink Street bridge was 15.0 mg l1 (Table 3). Possible causes for this difference among the stations could be contamination of the Prospect DOC sample or a high level of groundwater contribution to the baseflow there. The TOC loading was sig-
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nificantly different between the two flow conditions and was greater during high flow. Even though the TOC concentration was greater during baseflow, the TOC loading was greater for high flow, as a result of the 18fold increase in discharge (0.4 m3 s1 vs. 6.9 m3 s1) during the high flow conditions sampled in November. Compared with other world rivers (Aitkenhead and McDowell, 2000), the calculated DOC yields for the Scioto River are low, but not unlike other cultivated sites, which they report to have among the lowest DOC yields of the biomes they studied. For the baseflow sampling, the DOC yield was 0.0038 kg ha1 day1. November DOC yields averaged 0.034 kg ha1 day1. These values bracket the average of 0.011 kg ha1 day1 for cool grasslands and 0.015 kg ha1 day1 for average cultivated lands which Aitkenhead and McDowell (2000) estimate. The hypothesis that POC, TOC and TSS concentrations would increase under high flow conditions was incorrect, probably as a result of high flow sampling on the falling limb of the hydrograph or possibly (but less likely) due to an ineffectual amount of precipitation on the day of the November sampling event. TOC loading did increase under high flow because of the large increase in stream discharge then. 5.2. N and P dissolved species Dissolved nutrients transported in the river can serve to stimulate the growth of phytoplankton and other aquatic vegetation which can then be sources of organic matter transported in the river. The significantly higher concentration of NO3–N measured during the high flow conditions is the most notable result for the dissolved species, similar to results found in other agricultural streams and rivers (Castillo et al., 2000; Jarvie et al., 1997; Magner and Alexander, 2002). In November, the La Rue NO3–N concentration of 12.6 mg l1 (Table 3) was greater than the 10 mg N l1 limit for drinking water. Nitrate yield for the Upper Scioto basin during baseflow was low, 6105 kg ha1 day1 which reflects both the low NO3–N concentration then (0.25 mg l1) and the very low discharge (0.4 m3 s1). Nitrate yield for the high flow sampling was 0.04 kg ha1 day1, a reflection of both the greater concentrations measured in November (mean of 9.3 mg NO3–N l1) and the higher stream discharge (6.9 m3 s1). The yield calculated for the November sampling is similar to that of 0.038 kg ha1 day1 determined by Goolsby and Battaglin (2001) for the Scioto River basin at Higby, Ohio. The basin area at Higby covers 13,284 km2 and includes all of the Upper Scioto and part of the Lower Scioto River basin. Similar to the results of Carey et al. (2001), Goolsby and Battaglin (2001) found less N yield in dry years than in wet years. Those studies considered annual budgets and
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Fig. 3. (a) Plot of particulate organic C content (POC) and total suspended matter (TSS) of world rivers (data from Ittekkot and Laane, 1991), indicated by circles and the data from this study, indicated by inverted traingles. (b) Plot of the ratio of dissolved organic C to particulate organic C (DOC/POC) and total suspended matter (TSS) for world rivers (data from Ittekkot and Laane, 1991), and the data from this study, indicated by inverted triangles.
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yields, but the seasonal sampling bears out the overall finding that during dry periods there is little rainfall to transport soluble species like NO3 to the streams, and that during dry periods N is stored on the landscape, to be transported during another season or another year when rainfall, runoff and groundwater discharge are all greater. Nitrate can be removed through denitrification, storage in groundwater or sequestration by organic matter. In studies of the Upper Mississippi River basin and of the Mobile-Alabama River basin, the largest and most important inputs of N to agricultural watersheds were found to be fertilizer and manure application (Carey et al., 2001, 2003). Agricultural areas have been found to have larger N retention than either urban or forested watersheds (Carey et al., 2001), but retention of applied fertilizer was found to be far greater during years of low rainfall and low river discharge, compared to high flow years in both those studies of N-budgets in large river basins (Carey et al., 2001, 2003). Summer 2002 was very dry in Ohio, and the higher NO3 measured in the autumn high-flow sampling may be the result of transport of N applied during the summer which was retained on the landscape during the dry summer. In contrast to what occurs with dissolved N species, dissolved PO3 4 was diluted at high flow. This has been observed in many other studies (Boar et al., 1995; Jarvie et al., 1997; McDowell et al., 2001). Phosphorus transfer from agricultural soils to receiving waters is highly variable in form and magnitude. Factors controlling spatial distrubution include clay content of soil, which gives soils a higher sorption capacity (McDowell et al., 2001), erosion rates (McDowell and Sharpley, 2003), topography, proximity to the drainage network, hydrology, and location of fertilizer application (Fraser et al., 1999). Other factors controlling dissolved phosphate include those such as rainfall intensity and duration control temporal variations (Fraser et al., 1999). Water year 2002 was a dry year for the Upper Scioto River watershed. This suggests that both N and P were being stored in the soil and biomass instead of transported by the receiving waters. The soils’ large clay content also provides a large storage capacity. Wetter years may certainly show different results than those from this study. For example, there could be considerably higher NO3 and dissolved PO4 concentrations under high flow conditions and possibly lower values under baseflow. 5.3. C:N, OC:OP and TP loading The C:N molar ratios in the particulate matter were similar. The mean values of 6.6 and 6.7 suggest a major autochthonous component for the participate matter during both summer baseflow and autumn high flow
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conditions. Low C:N ratios, close to the Redfield C:N ratio of 6.6:1, are indicative of algal-derived material (Redfield et al., 1963). The high nutrient content of the Upper Scioto River appears to support a larger than normal proportion of phytoplankton, evidenced by the low C:N ratios for both seasons. The Susquehanna River in Pennsylvania was also found to have N-rich POM indicative of algal derived material (Hopkinson et al., 1998). Due to the small sample volume available, analysis of the N content of autumn samples was impossible to determine with any precision. This introduces some uncertainty in the autumn La Rue and Mink Street bridge C:N ratios. The %TC values for the autumn samples were all similar, around 2%. Perhaps %TN followed the same pattern, and one %TN value in the calculation of C:N for these may be a good approximation. The results therefore do not support the hypothesis that C:N ratios would be lower in the autumn. Summer OC:OP molar ratios were reliably determined for this study. The autumn OC:OP ratios were not determined due to what appears to be experimental error in the autumn OP analysis. There was virtually no P extracted from the samples. Using the limit of detection as the concentration of organic P in the autumn OP analyses, the authors estimated the P concentrations. The summer OC:OP mean was 178:1 and the estimated autumn OC:OP mean was 11,000:1 (Tables 1 and 2). The Redfield C:P ratio of 106:1 is applicable to marine phytoplankton (Redfield et al., 1963). Freshwater species tend to be more nutrient limited and stream C:P ratios will be < 350:1 (Ruttenberg and Gon˜i, 1997). All summer samples had C:P ratios of 4350:1. This suggests a dominant phytoplankton component for all locations. Woody tissue has the highest C:P ratios and are > 1300:1 (Ruttenberg and Gon˜i, 1997). Therefore, the estimated autumn ratio of > 11,000 seems unlikely. The hypothesis that the ratios would be lower in autumn cannot be evaluated, but it seems that they should actually be greater in the autumn as a result of the higher terrestrial inputs. TP loading was 2.2 1.44 kg day1 for summer baseflow and undetermined for autumn. The majority of TP reaching streams (from overland flow) in agricultural catchments is in particulate form, such as OP (McDowell et al., 2001). Therefore, most of the P reaching the channel is probably from the saturated soils adjacent (within 30 m) to the stream. McDowell et al. (2001) also states that in addition to dilution, P concentrations are controlled by resuspension, sorption– desorption, landscape processes such as soil transport in overland and subsurface flow, and soil P concentrations. It does seem likely that there was some sort of experimental error in the autumn OP analysis since there should have been some present. It could also be speculated that the OP concentrations would have followed
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the behavior of the other suspended particles (that of TSS and POC) and diluted in the autumn, but the extent to which may have happened is unknown. 5.4. Lignin Lignins are exclusively terrestrial in origin and are the most refractory component of terrestrial plant cell wells. Thus lignins are good indicators of land-derived POM. The semi-quantitative method of Pocklington and Hardstaff (1974) is a simple and rapid way to get a general indication of the amount of lignin in a sediment sample. The trade-off is that it has a precision of only about 50%. The baseflow samples contained 1.22–1.93 g lignin per gram of suspended matter and the high flow samples contained 0.94–1.22 mg lignin per gram of suspended matter. This slightly larger amount of lignin in autumn samples compared with summer was not significantly different by Tukey’s quick test. Ittekkot and Laane (1991) found that there is generally a decrease in labile C (or conversely an increase in refractory C) during high concentrations of suspended matter. They suggested that during times of high suspended matter in streams primary production is reduced because of the decreased light penetration. Ittekkot and Laane found that the temperate rivers studied had a higher content of labile particulate C compared to tropical rivers where in-stream processes are more likely to degrade the C before it is transported to the oceans. 5.5. d-13C The 13C results were the most definitive in this study. By Tukey’s quick test the d-13C values were determined to be significantly different. The mean summer d-13C value was 24.80% and the mean autumn d-13C value was 21.45% (Tables 1 and 2). Given the ranges of d-13C values for various types of organic C sources (Ruttenberg and Gon˜i, 1997), summer POM in the Upper Scioto River contained a possible mixture of C3 plant material (within the range: 23 to 34%) and phytoplankton material (on the edge of the range: 18 to 24%). Soybeans, a C3 plant, are a major crop in the counties (Delaware, Marion and Hardin Counties) where the samples were collected. Autumn POM contained a mixture of C4 plant material (values fall in the range: 6 to 23%) and phytoplankton (values fall in the range: 18 to 24%). The fact that the autumn d-13C value was significantly lower than the C3 range of values in very interesting. The intensive cultivation of corn in the Upper Scioto River watershed appears to contribute significantly to the suspended POM in the November samples, which were collected after the corn harvest. These results are in contrast to those of Weiguo et al. (2003) who found in rivers draining non-agricultural areas of the Chinese Loess Plain that summer
d-13C values were more positive (average of 24.1%) than their November d-13C values (average of 26.4%) and reflected the greater abundance of C4 plants during summer, rather than in November, as found in the present study. The hypothesis that one of the POM sources would be corn seems to be correct.
6. Conclusions The quality, quantity, and sources of suspended organic matter in the highly agricultural Upper Scioto River was dominated by land use practice within the watershed. Superimposed upon land use, daily to seasonal variations in hydrology also have a major influence on the organic geochemical composition of the suspended material. Spatial variations in the suspended matter caused some of the results to have unexpected outcomes. TOC loading increased from 700 kg day1 in summer to 5200 kg day1 in the autumn, but concentrations of other constituents determined decreased during autumn high flow. POC mean concentrations were 1.5 mg l1 at baseflow and 23 mg l1 at high flow. DOC mean concentration was 16 mg l1 during baseflow and 8.3 mg l1 during high flow. Mean TOC concentration was 17.5 mg l1 during baseflow and 8.7 mg l1 during high flow. Mean TSS concentration was 39 mg l1 at baseflow and 17.7 mg l1 at high flow. Dissolved N species all increased in the higher flows but dissolved PO4 decreased. Mean NO3–N was 0.25 mg l1 during baseflow and 9.33 mg l1 during high flow. Nitrite–N was only detected in two of the high flow samples (mean of 0.24 mg N l1) and in none of the baseflow samples. C:N molar ratios at both sampling times were found to be very close to the Redfield ratio and suggest the existence of a high-N, algal component for both summer and autumn suspended organic matter. OC:OP ratios also pointed to a high quality algal component for the summer but an unsure component for autumn samples due to possible experimental error. Summer TP loading was 2.2 1.44 kg/day; autumn TP loading was undeterminable. Lignin concentrations in the suspended matter may have been slightly greater in the autumn, indicating more terrestrial inputs at that time but the low precision of the semi-quantitative method prevented determining that with any confidence. Stable C isotope analysis showed a mixture of phytoplankton and C3 plant sources for summer samples (mean d13C of 24.8) and a phytoplankton and C4 plant (corn) sources for the autumn samples (mean d13C of 21.4). The results of the study show that the quality of the organic matter transported in the river is dominated by the agricultural land use practice within the watershed and the major crops of the C3 plant, soybeans, and the C4 plant, corn.
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Acknowledgements This research was performed as part of SAM’s master’s thesis and she would like to thank her committee members, Virginie Bouchard and W. Berry Lyons, for their support. Partial funding for this study was received from the Department of Geological Sciences at the Ohio State University. References Aitkenhead, J.A., McDowell, W.H., 2000. Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales. Global Biogeochem. Cycles 14, 127–138. Aspila, K.I., Agemian, H., Chau, A.S.Y., 1976. A semi-automated method for the determination of inorganic, organic, and total phosphate in sediments. Analyst 101, 187–197. Boar, R.R., Lister, D.H., Clough, W.T., 1995. Phosphorus loads in a small groundwater-fed river during the 1989–1992 East Anglian drought. Water Res. 29, 2167–2173. Buffam, I., Galloway, J.N., Blum, L.K., McGlathery, K.J., 2001. A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream. Biogeochem 53, 269–306. Carey, A.E., Lyons, W.B., Bonzongo, J., Lehrter, J.C., 2001. Nitrogen budget in the Upper Mississippi watershed. Environ. Engineer. Geosci. 7, 251–265. Carey, A.E., Nezat, C.A., Pennock, J.R., Jones, T., Lyons, W.B., 2003. Nitrogen budget of the Mobile-Alabama River system watershed. Geochem.: Explor., Environ., Anal. 3, 1– 6. Castillo, M.M., Allan, J.D., Brunzell, S., 2000. Nutrient concentrations and discharges in a Midwestern agricultural catchment. J. Environ. Qual. 29, 1142–1151. Clark, I., Fritz, P., 1997. Environmental Isotopes in Hydrogeology. CRC Press LLC, Boca Raton, Florida. Clausen, J.C., Guillard, K., Sigmund, C.M., Dors, K.M., 2000. Water quality changes from riparian buffer restoration in Connecticut. J. Environ. Qual. 29, 1751–1761. Cronan, C.S., Piampiano, J.T., Patterson, H.H., 1999. Influence of land use and hydrology on exports of carbon and nitrogen in a Maine river basin. J. Environ. Qual. 28, 953–961. DePetris, P.J., Kempe, S., 1993. Carbon dynamics and sources in the Parana´ River. Limnol. Oceanog. 38, 382–395. Fraser, A.I., Harrod, T.R., Haygarth, P.M., 1999. The effect of rainfall intensity on soil erosion and particulate phosphorus transfer from arable soils. Water Sci. Technol. 39, 41–45. Goolsby, D.L., Battaglin, W.A., 2001. Long term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA. Hydrol. Proc. 15, 1209–1226. Heathwaite, L., Sharply, A.S., Gburek, W., 2000. A conceptual approach for integrating phosphorus and nitrogen management at watershed scales. J. Environ. Qual. 29, 158–166. Hopkinson, C.S., Buffam, I., Hobbie, J., Vallino, J., Perdue, M., Eversmeyer, B., Prahl, F., Covert, J., Hodson, R., Moran, M.A., Smith, E., Baross, J., Crump, B., Findlay, S.,
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Foreman, K., 1998. Terrestrial inputs of organic matter to coastal ecosystems: An intercomparison of chemical characteristics and bioavailability. Biogeochem. 43, 211–234. Ittekkot, V., Laane, R.W.P.M., 1991. Fate of riverine particulate organic matter. In: Degens, E.T., Kempe, S., Richey, J.E. (Eds.), Biogeochemistry of Major World Rivers, SCOPE 42. John Wiley & Sons, Chichester, pp. 233–243. Jarvie, H.P., Neal, C., Leach, D.V., Ryland, G.P., House, W.A., Robson, A.J., 1997. Major ion concentrations and the inorganic carbon chemistry of the Humber rivers. Sci. Total Environ., 194/195, 285–302. Magner, J.A., Alexander, S.C., 2002. Geochemical and isotopic tracing of water in nested southern Minnesota corn-belt watersheds. Water Sci. Technol. 45, 37–42. McDowell, R., Sharpley, A., 2003. Uptake and release of phosphorus from overland flow in a stream environment. J. Environ. Qual. 32, 937–948. McDowell, R., Sharpley, A., Folmar, G., 2001. Phosphorus export from an agricultural watershed: linking source and transport mechanisms. J. Environ. Qual. 30, 1587–1595. Paul M.J., 1999. Stream Ecosystem Function Along a LandUse Gradient. PhD Thesis, Univ. Georgia, Athens. Pocklington, R., Hardstaff, W.R., 1974. A rapid semi-quantitative screening procedure for lignin in marine sediments. J. Cons. Int. Explor. Mer. 36, 92–94. Redfield, A.C., Ketchum, B.H., Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: Hill, M.N. (Ed.), The Sea. Wiley, Chichester, pp. 26–77. Ruttenberg, K.C., Gon˜i, M.A., 1997. Phosphorus distribution, C:N:P ratios, and d-13Coc in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary organic matter. Mar. Geol. 139, 123–145. Soil Regions of Ohio Soil Characteristics. Retrieved 25 June 2003, from the Ohio Department of Natural Resources, Division of Soil and Water Conservation Website: http:// www.dnr.state.oh.us/soilandwater/soilreg6.htm. Strickland J.D.H., Parsons J.R., 1972. Determination of reactive phosphorus. In: A Practical Handbook of Seawater Analysis, second ed. Bulletin 167. Fisheries Board of Canada, Ottawa. Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H., Tilman, D.G., 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecol. Applications 7, 737–750. Weiguo, L., Zisheng, A., Weijian, Z., Head, M.J., Delin, C., 2003. Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of season change in C4/C3 vegetation. Appl. Geochem. 18, 1241–1249. Welch, K.A., Lyons, W.B., Graham, E., Neumann, K., Thomas, J.M., Mikesell, D., 1996. Determination of major element chemistry in terrestrial waters from Antarctica by ion chromatography. J. Chromatog. A 739, 257–263. Weyer, P.J., Cerhan, J.R., Kross, B.C., Hallberg, G.R., Kantamneni, J., Breuer, G., Jones, M.P., Zheng, W., Lynch, C.F., 2001. Municipal drinking water nitrate level and cancer risk in older women: The Iowa women’s health study. Epidemiol. 11, 327–338.
Organic matter sources and transport in an agriculturally dominated temperate watershed Shelly A. Munsona, Anne E. Careyb,* a
Environmental Science Graduate Program, The Ohio State University, Columbus, OH, USA Department of Geological Sciences, 275 Mendenhall Laboratory, 125 S. Oval Mall, Columbus, OH 43210-1398, USA
b
Abstract The quality, quantity, and origin of suspended organic matter were studied in the highly agricultural Upper Scioto River in Central Ohio. Late summer baseflow conditions were compared to late autumn high flow conditions. Variables examined in the suspended matter were the total suspended solids concentration, semi-quantitative concentrations of lignin, carbohydrate concentrations, total organic C, total and organic P, and d-13C. Also examined were ratios of C to N, organic C to organic P ratios and fluxes of total organic C. The primary hypothesis of this research was that the quality (or biodegradability) and quantity of organic matter in the Upper Scioto River would increase during autumn stormflow conditions due to inputs of fresh terrestrial organic matter. The autumn suspended matter was also expected to reflect C4 plant contributions from corn organic matter. Results show that the quality and quantity of organic matter were greater during summer, as reflected in low molar ratios (178:1) of organic C to organic P, and higher organic C content of the suspended matter in summer. Summer suspended matter was 3.6% organic C and autumn suspended matter was 2.3% organic C. Carbon to N molar ratios in both seasons were very close to the Redfield ratio (6.6:1 in summer and 6.7:1 in autumn). Total suspended matter and total organic C concentrations were lower in autumn (8.7 mg/l1 TOC and 17.7 mg/l1 TSS) than in summer (17.5 mg/l1 TOC and 39.0 mg/l1 TSS), but the fluxes were greater in autumn due to greater stream flow. Stable isotope analyses suggested a phytoplankton or C3 plant source (most likely corn) for summer organic C (mean d13C of 24.8%) and a phytoplankton or C4 plant source for autumn organic matter (d13C=21.5%). # 2004 Published by Elsevier Ltd.
1. Introduction 1.1. Background Human activity has greatly altered natural hydrological systems and nutrient cycles in the recent past. In Ohio, this is manifested in artificial drainage of the naturally hydric soils for agriculture and the alteration of stream hydrology. The dominant causes of impairment for many streams in agricultural parts of Ohio are nutrient enrichment and habitat alteration. Nutrient enrichment occurs as a result of fertilizer application to soils and its loss to the receiving waters. Commercial
* Corresponding author. Fax: +1-614-292-7688. E-mail address: [email protected] (A.E. Carey). 0883-2927/$ - see front matter # 2004 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2004.01.010
urea fertilizers decompose through nitrification to NH+ 4 , and it oxidizes under aerobic conditions to NO 3 (Vitousek et al., 1997; Clark and Fritz, 1997). Nitrate is water-soluble and is lost through leaching. It is also toxic to humans, especially young children, at levels greater than 10 mg NO3–N l1, by decreasing the O2 carrying capacity of the hemoglobin in blood. A recent study has also shown the possibility of increased cancer risk for older women drinking NO 3 -contaminated water (Weyer et al., 2001). Often a limiting nutrient, P is present in fertilizers and can cause water quality problems. Phosphorus is more transport limited than NO 3 because it adsorbs to sediments and as a result is lost through runoff and erosion (Heathwaite et al., 2000). The restoration of riparian areas has been promoted as a best management practice to control runoff and the resulting loss of N and P (Clausen et al., 2000).
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The ability of a stream to store organic matter (OM) is another change caused by physically altering the stream channel. Straight channels cannot store as much OM as natural, meandering channels and act more as conduits for the OM (Paul, 1999). This has implications for food web dynamics because storage is important for the next year’s community productivity. Food webs will also be affected by the nutrient enrichment that will increase OM quality. Bacteria and other detritivores will potentially have an overabundance of labile OM available, rapidly multiply, and cause anoxic conditions. Therefore, studying the biodegradability of OM from an agriculturally dominated system is important. Sources and sinks of OM are also an important consideration. Anthropogenically affected systems will have different OM sources (e.g., petroleum hydrocarbons and polycyclic aromatic hydrocarbons and other combustion products from fuels, herbicides and pesticides) than will natural systems whose organic matter sources are primarily soil organic matter, terrestrial plant litter and phytoplankton. 1.2. Objectives The overall goal of this research was to examine a stream within an agriculturally dominated system and determine how the quality, quantity and sources of OM varied under different hydrological conditions. Two sampling events were conducted at 3 locations along the Upper Scioto River main stem in order to compare baseflow to high flow (summer to autumn) conditions. The 3 locations were treated as replicates. The variables investigated were major ions, lignin content, C to N ratios (C:N), organic C to organic P ratios (OC:OP), total organic C (TOC), TOC fluxes, total suspended solids (TSS), and d-13C. The anticipated results of this study were: Because of higher runoff, major ions such as + 3 NO 3 , NO2 , NH4 , and PO4 , that are associated with agricultural activities, will increase in concentration in the autumn. Quality of the OM will be higher in the autumn and lower in the summer as a result of autumn inputs of fresh OM from the runoff of decomposed, agriculturally produced debris. Therefore, there will be lower C:N ratios and lower lignin content in the autumn OM. In the summer, OM sources will be mostly autochthonous, or in-stream, but in autumn sources will be more allochthonous, or terrestrial. This will be shown through d-13C values from the riverine TSS indicating mostly C3 plant inputs (23 to 34%) from leaf litter in the autumn, but possibly some C4 plant (e.g., corn) contribution (6 to 23%) as a result of nearby
corn cultivation. In the summer, phytoplankton (18 to 24%) should be the dominant source of OM in the river. Increased runoff and overland flow will cause an increase in particulate matter as well as nutrients associated with them. This will mean higher TSS and TOC fluxes. OC:OP ratios will either be lower or remain unchanged due to a concurrent increase in OC.
2. Physical setting The Upper Scioto watershed of west-central Ohio lies in the Eastern Cornbelt Plains and encompasses 1860 km2 (Fig. 1). The 3 sampling locations at La Rue, Prospect, and the Mink Street Bridge (south of Prospect) are in areas whose land use is primarily agricultural and were chosen because of their accessibility and their locations upstream of the major reservoir on the river, the O’Shaughnessy Reservoir in southwestern Delaware County (Fig. 1). The reservoir has a no-flow (lentic) environment which is undesirable as this study is of a flowing system. The sampling locations were also chosen because of their proximity to the USGS gauging station #03219500 at 40 250 N, 83 120 W in Prospect. The geologic setting of the Upper Scioto River is the glacial till plains (Wisconsin glacial drift) which is underlain by Silurian limestone and dolostone bedrock. Approximately 32% of the area has more than 3% OM in the upper 25 cm as well as more than 27% clay in the topsoil (Soil Regions of Ohio Soil Characteristics, 2003). Approximately 31% of the area has a seasonal high water table at less than 40 cm below the surface. Much of the land has been drained for successful cultivation of corn and soybeans. The watershed has little relief and just 6% of the area has a slope of more than 8%. The soils can be characterized as basic, clayey and productive on a flat landscape (Soil Regions of Ohio Soil Characteristics, 2003).
3. Methods 3.1. Sampling Samples were taken at the 3 locations during late summer baseflow conditions and during late autumn event flow conditions. The late summer samples were collected on 4 September 2002 when the discharge was 0.40 m3 s1 and the high (or event) flow samples were collected on 16 November 2002 when the discharge was 6.94 m3 s1, on the falling limb of a hydrograph whose peak discharge of 36.82 m3 s1 occurred on 11 November (Fig. 2). All samples were grab samples collected in 18 M deionised water cleaned polyethylene bottles
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Fig. 1. Map of the study area showing its location in west-central Ohio. The 3 sampling locations on the Upper Scioto River are indicated by the arrows. The location of the US Geological Survey’s stream gauging station #03219500 at 40 250 N, 83 120 W in Prospect, Ohio, is noted by the open circle.
(Welch et al., 1996), with the exception of the TOC samples, which were collected in combusted (450 C for 3 h) amber glass bottles. 3.2. Analytical Major ions were determined by ion chromatography using a Dionex1 DX-120 Ion Chromatograph instrument (2.3% precision). Samples were filtered through Nuclepore1 0.4 mm pore size polycarbonate membrane filters using dedicated polycarbonate filter funnels and bell jars so that samples were filtered directly into sample bottles. Samples were refrigerated at 4 C until analysis. Samples for lignin analysis were dried at 105 C for 24 h. The semi-quantitative method of Pocklington and Hardstaff (1974) was used to estimate mg lignin g1 sediment (50% precision). Total suspended solids (TSS) were determined gravimetrically on samples which were filtered with weighed Nuclepore1 0.4 mm pore size membrane filters. The filters were then oven dried (105 C for 24 h) and reweighed to determine TSS. Particulate organic C (POC) was determined by pooling all
% total C (%TC) data from the CN analyzer, a Thermoquest CE instrument model NC 2100 Soil (also used for the C:N ratios), and from the d-13C determinations and multiplying by TSS to get POC in mg l1. Samples were filtered through Millipore1 47 mm diameter glass fiber filters (0.7 mm pore size), acidified to a pH of 2 using concentrated HCl and refrigerated in the dark. Dissolved organic C (DOC) of these filtrates was determined using a Shimadzu1 TOC-5000 TOC analyzer (2.4% precision). TOC was calculated as the sum of POC and DOC. TOC and particulate P loadings were calculated using USGS discharge data for each specific sampling date. C:N ratios were determined by pooling all available %TC data determined by a CN analyzer and the d-13C analysis. The % total N (%TN) data were taken from the available CN analyzer data. La Rue and Mink Street high flow samples did not contain enough suspended matter to determine %TN. Therefore, the Prospect %TN was used for these C:N ratios. Particulate P and organic P (TP and OP) was determined colorimetrically by adapting the methods of Aspila et al. (1976) and Strickland and Parsons (1972). Carbon
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Fig. 2. Scioto River discharge measured at Prospect, Ohio (US G.S. stream gauging station #03219500) for late summer and autumn 2002. The arrows show the days on which sampling was conducted for this study.
isotope analyses were performed by Isotope Services, Inc., Los Alamos, New Mexico. The samples for isotope analysis were oven-dried (105 C for 24 h) before they were sent to Isotope Services where d-13C was determined using a VG Isomass Sira Series II Mass Spectrometer that is fed from a N/A 1500 Carlo-Erba Elemental Analyzer (0.11% precision).
4. Results 4.1. Hydrology The stream gauging station at Prospect, Ohio (Hydrologic Unit Code 05060001, USGS station #03219500) has been maintained since July 1925 and 70 a of data are available. Due to little rainfall during the summer of 2002, the river experienced baseflow conditions for the most of the summer (Fig. 2). Precipitation data from the Ohio State University airport in Columbus, Ohio (approximately 72 km south of Prospect, Ohio) were obtained through the National Climatic Data Center’s Website (www.ncdc.noaa.gov/servlets/ ULCD which was accessed on 24 June 2003). Rainfall for August 2002 was 54.1 mm and 22 days of 31 were without rain. For September, prior to the baseflow sampling, there was a trace of rain h1 on 3 September 2002, from 3.00 to 5.00 pm. Therefore it can be said that
there was no intense precipitation event preceding the sampling date. The average instantaneous daily discharge on 4 September 2002 (during the baseflow sampling event) was 0.4 m3/s, well below the average daily discharge of 2.0 m3/s for the 70 a of record for that date. However, August is probably more representative of the conditions for the river in early September. Streamflow in August 2002 is in the lowest quintile for all Augusts for the period of record (1926–present). The 16 November 2002 average instantaneous daily discharge was 6.9 m3 s1, close to the mean November instantaneous daily discharges since 1926 of 7.2 m3 s1. Rainfall for November 2002 totaled 76.2 mm and 18 days of November had rainfall. For the period of 10–16 November, the 10th, 11th, 15th and 16th had precipitation. 10 November received the most: it rained for 14 h of 24 and intensity ranged from a trace h1 to 14 mm h1. On the 11 November it rained for 2 h with intensity ranging from a trace h1 to 0.3 mm h1. On the 15th it rained for 13 h and intensity ranged from a trace h1 to 1 mm h1. 16 November received less: it rained for 7 h and intensity ranged from a trace h1 to 0.3 mm h1. Therefore, there were heavy precipitation events on the 10 November and light events on the 11th, 15th and 16th. During the September sampling, the mean temperature of the river was 26.6 C and the pH mean was 8.48. In November, the mean water temperature was 5.27 C and the mean pH was 7.72.
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4.2. TSS, TOC, and TOC loading The TSS mean was higher during baseflow (39.0 mg l1, Table 1) compared with its concentration at high flow (17.7 mg l1, Table 2). Use of the nonparametric Tukey’s quick test is appropriate to analyze small sample sizes as were obtained in this study. Tukey’s quick test compares the number of values in one group which are smaller than all the values in the second group and the number of values in the second group which are larger than all values in the first group. The T statistic is the sum of those values. If T56, then the null hypothesis of no difference between groups is rejected (P=0.10 and is not dependent upon the number of samples). By the Tukey quick test, one rejects the null hypothesis for the TSS concentrations. Tukey’s quick test was similarly applied to all the data (Tables 1–3). The POC concentrations (Tables 1 and 2) were not significantly different between baseflow
and high flow (POC means were 1.46 and 0.42 mg l1, respectively). The DOC concentrations (Table 3) were greater during baseflow (mean of 16.1 mg l1) than at high flow (mean of 8.3 mg l1). POC concentrations in the suspended matter was not significantly different between baseflow and high flow. During baseflow, the mean POC was 36.0 mg g1 and during high flow POC was 23 mg g1. The TOC concentrations, however, were significantly different. At baseflow, the TOC mean was 17.5 mg l1 and at high flow it was 8.7 mg l1. The TOC loads were different by Tukey’s quick test, but because of the high discharge in November the TOC loads were greater, even though the concentrations of TOC were lower at high flow. The mean daily TOC loading for baseflow was 686 and 5200 kg C day1 for high flow. The POC content of the suspended matter was not significantly different. POC made up an average of 3.6% of the suspended matter during baseflow and 2.3% of the suspended matter during high flow.
Table 1 Baseflow particulate sample values for all locations and their means (values in parentheses calculated using LOD for OP method) Sample
TSS (mg/l) TOC (mg/l) POC (mg/g) POC (mg/l) TOC loading (kg/day) TP loading (kg/day) Lignin (mg/g) C:N (molar) OC:OP (molar) d-13C
La Rue
Prospect
Mink St.
Mean
52.2 12.9 46.0 2.40 505 2.5 1.1 10.6 351:1 23.83
30.4 23.8 35.5 1.08 933 3.5 0.94 5.20 43:1 25.17
34.3 15.9 26.5 0.909 620 0.67 1.2 4.12 140:1 ( <610:1) 25.41
39.0 11.6 17.5 5.63 36.0 9.76 1.46 0.816 686 222 2.2 1.44 1.1 0.092 6.62 3.47 178:1 (335:1)157:1 (284:1) 24.800.851
Table 2 High flow particulate sample values for all locations and their means (values in parentheses: used LOD in calculation, values with asterisks: used Prospect %TN see Section 3.2) Sample
TSS (mg/l) TOC (mg/l) POC (mg/g) POC (mg/l) TOC loading (kg/day) TP loading (kg/day) Lignin (mg/g) C:N (molar) OC:OP (molar) d-13C
La Rue
Prospect
Mink St.
Mean
9.0 10 20 0.18 6000 0 (0.44) 1.2 5.84:1* (2200:1) 22.43
27.5 9.6 22.9 0.630 5750 0 (0.30) 1.6 6.62:1 (20,000:1) 20.57
16.5 6.60 26.5 0.437 3960 0 (0.33) 1.6 7.65:1* (12,000:1) 21.35
17.79.31 8.71.9 233.3 0.420.23 52001100 0 (0.360.074) 1.50.23 6.700.908 (11,000:18900:1) 21.400.934
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Table 3 Dissolved species for both events, all locations and their means. All values in mg l1. B=baseflow, H=high flow (means with asterisks calculated using the LOD for at least one value) La Rue-B NO 3 NO 2 NH+ 4 PO3 4
0.279
Prospect-B Mink Street-B Mean-B
La Rue-H Prospect-H
Mink Street-H Mean-H
0.157
12.6 0.363 0.264 0.703 9.78
6.37
0.317
0.2510.0836
8.98 0.295 0.259
4.3. Dissolved N and P
5. Discussion
Nitrate-N was greater (Table 3), under high flow conditions (high flow: 9.3 mg N l1, baseflow: 0.25 mg N l1). The other dissolved N species and soluble reactive phosphate were detected in small numbers of samples. Statistical tests have not been applied to these data.
5.1. TSS, POC, and TOC loading
4.4. C:N and OC:OP ratios The C:N molar ratios for the two flow conditions were similar (Tables 1 and 2) and using Tukey’s quick test, the null hypothesis of no difference is accepted (baseflow mean: 6.63:1, high flow mean: 6.70:1). The OC:OP ratios were higher during baseflow conditions (Tables 1 and 2). However, the OC:OP ratios for the high flow samples were calculated using the detection limit for the OP concentrations, because no organic P was detected in the high flow suspended matter samples. This may have been due to the possible loss of sample during analysis which resulted in no detectable organic phosphate in the suspended matter in all the high flow samples. 4.5. Lignin and d-13C Lignin content of the suspended matter samples (Tables 1 and 2) was not significantly different by Tukey’s quick test. The mean lignin concentration was 1.5 mg g1 under high flow conditions and 1.1 mg g1 under baseflow conditions. The semi-quantitative lignin analysis yielded few observable lignin-stained particles in any of the samples, but there were larger sized and more numerous stained patches in the high flow samples. Carbon-13 values were significantly different for the organic matter in the samples for the two events (Tables 1 and 2). During baseflow, the mean value for the suspended organic matter was d-13C of 24.80% and during high flow the mean d-13C was 21.45%.
9.33 3.13 0.235*0.166 0.175*0.149 0.318*0.334 8.30 1.90
The greater suspended solids (mean TSS of 39 mg l1) during the baseflow sampling (Table 1), compared to a mean TSS of 18 mg l1 in high flow conditions (Table 2), suggests that there was a dilution from the higher river discharge in November. This observation is different from what many previous studies (e.g., Cronan et al., 1999, DePetris and Kempe, 1993, Buffam et al., 2001) have found for high flow conditions, which is an increase of both TSS and DOC. This discrepancy of the results with what many others have observed could be because the November precipitation was not intense enough to cause a significant amount of runoff. Collection of TSS samples at the Prospect station from July 2000 to June 2001 showed a general increase of TSS with river discharge, but also with a dependence upon the local stage of the stream hydrograph. The highest concentration of TSS observed at Prospect was 180 mg l1 on 12 April 2001 when the stream discharge was 55 m3 s1. That sampling was conducted on the rising limb of a flood hydrograph and at the highest river stage observed during a year of monthly sampling at Prospect. On 20 June 2001, on the falling limb of the stream hydrograph, a TSS concentration of 86 mg l1 was measured at a much lower stream discharge (3.2 m3 s1). The November 2002 sample was collected when the discharge was 6.9 m3 s1, 4 days after the peak discharge of 37 m3 s1 for the local hydrograph. Thus, even though the stream discharge was far greater in the autumn sample than in the summer sample, much of the readily transported TSS may have been washed off earlier in the local hydrograph, on one of the several days before sampling in the autumn, and that would account for the higher value of TSS measured at baseflow conditions. Another possible explanation is that because of the very low stream stage during the baseflow sampling, the sampling procedure disturbed the fine bedload and part of what was determined as suspended solids had actually been fine bedload prior to the
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disturbance by sampling. Great care was taken during sampling, but water depth was only slightly greater than the size of the sample bottles and there may have been inadvertent disturbance. Summer baseflow POC values had high variability, and this was the reason that the values for the two sampling conditions were not significantly different. The cause for these values’ variability appears to be the La Rue summer sample. POC at La Rue was 2.40 mg l1 while Prospect and Mink Street POC were 1.08 and 0.909 mg l1 (Table 1), respectively. It may be possible that the large POC value at La Rue is the result of chicken waste from the large chicken egg production facility located within the village of La Rue. However, as a fraction of TSS, baseflow conditions did not have a significantly higher POC concentration than did the high flow condition. The POC content of the Scioto River suspended matter collected in this study is similar to the results of the SCOPE project on major world rivers of Ittekkot and Laane (1991). Overall, Ittekkot and Laane (1991) found that the POC content of TSS (range of 1.3–8.4% or 0.6–14.2 mg C l1) showed an inverse relationship between the content and concentration of POC, and POC decreased with the TSS, as the authors have observed in the Scioto River. The Scioto data fall within the range observed for world rivers (Fig. 3a). Dissolved organic C content of the stream samples ranged from 6.15 mg l1 for the Mink Street bridge sample at high flow to 22.7 mg l1 for the Prospect sample at baseflow. The means of the two sampling events were 16.1 mg l1 at baseflow and 8.3 mg l1 at high flow (Table 3). As others (Ittekkot and Laane, 1991) have observed, the Scioto River DOC concentrations declined with increasing stream discharge. For world rivers, typically the DOC/POC ratio is 10.8 at TSS concentrations less than 15 mg l1 and 5.8 at TSS concentrations of 15–50 mg l1 (Ittekkot and Laane, 1991). However, the DOC/POC ratios in our Scioto River samples were higher, approximately 11–20 mg l1 (Fig. 3b). Although Ittekkot and Laane (1991) found the primary difference among world rivers to be high DOC/POC ratios in lowland rivers, the authors’ low gradient, lowland river has higher DOC/POC ratios than typical of the rivers studied by the SCOPE project (Fig. 3b). During high flow conditions, the DOC concentrations at all 3 sampling sites were similar, with a mean DOC of 8.3 mg l1. However, for the baseflow samples, the Prospect DOC concentration of 22.7 mg l1 was larger than at the other two locations. At La Rue the DOC was 10.5 mg l1 and at Mink Street bridge was 15.0 mg l1 (Table 3). Possible causes for this difference among the stations could be contamination of the Prospect DOC sample or a high level of groundwater contribution to the baseflow there. The TOC loading was sig-
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nificantly different between the two flow conditions and was greater during high flow. Even though the TOC concentration was greater during baseflow, the TOC loading was greater for high flow, as a result of the 18fold increase in discharge (0.4 m3 s1 vs. 6.9 m3 s1) during the high flow conditions sampled in November. Compared with other world rivers (Aitkenhead and McDowell, 2000), the calculated DOC yields for the Scioto River are low, but not unlike other cultivated sites, which they report to have among the lowest DOC yields of the biomes they studied. For the baseflow sampling, the DOC yield was 0.0038 kg ha1 day1. November DOC yields averaged 0.034 kg ha1 day1. These values bracket the average of 0.011 kg ha1 day1 for cool grasslands and 0.015 kg ha1 day1 for average cultivated lands which Aitkenhead and McDowell (2000) estimate. The hypothesis that POC, TOC and TSS concentrations would increase under high flow conditions was incorrect, probably as a result of high flow sampling on the falling limb of the hydrograph or possibly (but less likely) due to an ineffectual amount of precipitation on the day of the November sampling event. TOC loading did increase under high flow because of the large increase in stream discharge then. 5.2. N and P dissolved species Dissolved nutrients transported in the river can serve to stimulate the growth of phytoplankton and other aquatic vegetation which can then be sources of organic matter transported in the river. The significantly higher concentration of NO3–N measured during the high flow conditions is the most notable result for the dissolved species, similar to results found in other agricultural streams and rivers (Castillo et al., 2000; Jarvie et al., 1997; Magner and Alexander, 2002). In November, the La Rue NO3–N concentration of 12.6 mg l1 (Table 3) was greater than the 10 mg N l1 limit for drinking water. Nitrate yield for the Upper Scioto basin during baseflow was low, 6105 kg ha1 day1 which reflects both the low NO3–N concentration then (0.25 mg l1) and the very low discharge (0.4 m3 s1). Nitrate yield for the high flow sampling was 0.04 kg ha1 day1, a reflection of both the greater concentrations measured in November (mean of 9.3 mg NO3–N l1) and the higher stream discharge (6.9 m3 s1). The yield calculated for the November sampling is similar to that of 0.038 kg ha1 day1 determined by Goolsby and Battaglin (2001) for the Scioto River basin at Higby, Ohio. The basin area at Higby covers 13,284 km2 and includes all of the Upper Scioto and part of the Lower Scioto River basin. Similar to the results of Carey et al. (2001), Goolsby and Battaglin (2001) found less N yield in dry years than in wet years. Those studies considered annual budgets and
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Fig. 3. (a) Plot of particulate organic C content (POC) and total suspended matter (TSS) of world rivers (data from Ittekkot and Laane, 1991), indicated by circles and the data from this study, indicated by inverted traingles. (b) Plot of the ratio of dissolved organic C to particulate organic C (DOC/POC) and total suspended matter (TSS) for world rivers (data from Ittekkot and Laane, 1991), and the data from this study, indicated by inverted triangles.
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yields, but the seasonal sampling bears out the overall finding that during dry periods there is little rainfall to transport soluble species like NO3 to the streams, and that during dry periods N is stored on the landscape, to be transported during another season or another year when rainfall, runoff and groundwater discharge are all greater. Nitrate can be removed through denitrification, storage in groundwater or sequestration by organic matter. In studies of the Upper Mississippi River basin and of the Mobile-Alabama River basin, the largest and most important inputs of N to agricultural watersheds were found to be fertilizer and manure application (Carey et al., 2001, 2003). Agricultural areas have been found to have larger N retention than either urban or forested watersheds (Carey et al., 2001), but retention of applied fertilizer was found to be far greater during years of low rainfall and low river discharge, compared to high flow years in both those studies of N-budgets in large river basins (Carey et al., 2001, 2003). Summer 2002 was very dry in Ohio, and the higher NO3 measured in the autumn high-flow sampling may be the result of transport of N applied during the summer which was retained on the landscape during the dry summer. In contrast to what occurs with dissolved N species, dissolved PO3 4 was diluted at high flow. This has been observed in many other studies (Boar et al., 1995; Jarvie et al., 1997; McDowell et al., 2001). Phosphorus transfer from agricultural soils to receiving waters is highly variable in form and magnitude. Factors controlling spatial distrubution include clay content of soil, which gives soils a higher sorption capacity (McDowell et al., 2001), erosion rates (McDowell and Sharpley, 2003), topography, proximity to the drainage network, hydrology, and location of fertilizer application (Fraser et al., 1999). Other factors controlling dissolved phosphate include those such as rainfall intensity and duration control temporal variations (Fraser et al., 1999). Water year 2002 was a dry year for the Upper Scioto River watershed. This suggests that both N and P were being stored in the soil and biomass instead of transported by the receiving waters. The soils’ large clay content also provides a large storage capacity. Wetter years may certainly show different results than those from this study. For example, there could be considerably higher NO3 and dissolved PO4 concentrations under high flow conditions and possibly lower values under baseflow. 5.3. C:N, OC:OP and TP loading The C:N molar ratios in the particulate matter were similar. The mean values of 6.6 and 6.7 suggest a major autochthonous component for the participate matter during both summer baseflow and autumn high flow
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conditions. Low C:N ratios, close to the Redfield C:N ratio of 6.6:1, are indicative of algal-derived material (Redfield et al., 1963). The high nutrient content of the Upper Scioto River appears to support a larger than normal proportion of phytoplankton, evidenced by the low C:N ratios for both seasons. The Susquehanna River in Pennsylvania was also found to have N-rich POM indicative of algal derived material (Hopkinson et al., 1998). Due to the small sample volume available, analysis of the N content of autumn samples was impossible to determine with any precision. This introduces some uncertainty in the autumn La Rue and Mink Street bridge C:N ratios. The %TC values for the autumn samples were all similar, around 2%. Perhaps %TN followed the same pattern, and one %TN value in the calculation of C:N for these may be a good approximation. The results therefore do not support the hypothesis that C:N ratios would be lower in the autumn. Summer OC:OP molar ratios were reliably determined for this study. The autumn OC:OP ratios were not determined due to what appears to be experimental error in the autumn OP analysis. There was virtually no P extracted from the samples. Using the limit of detection as the concentration of organic P in the autumn OP analyses, the authors estimated the P concentrations. The summer OC:OP mean was 178:1 and the estimated autumn OC:OP mean was 11,000:1 (Tables 1 and 2). The Redfield C:P ratio of 106:1 is applicable to marine phytoplankton (Redfield et al., 1963). Freshwater species tend to be more nutrient limited and stream C:P ratios will be < 350:1 (Ruttenberg and Gon˜i, 1997). All summer samples had C:P ratios of 4350:1. This suggests a dominant phytoplankton component for all locations. Woody tissue has the highest C:P ratios and are > 1300:1 (Ruttenberg and Gon˜i, 1997). Therefore, the estimated autumn ratio of > 11,000 seems unlikely. The hypothesis that the ratios would be lower in autumn cannot be evaluated, but it seems that they should actually be greater in the autumn as a result of the higher terrestrial inputs. TP loading was 2.2 1.44 kg day1 for summer baseflow and undetermined for autumn. The majority of TP reaching streams (from overland flow) in agricultural catchments is in particulate form, such as OP (McDowell et al., 2001). Therefore, most of the P reaching the channel is probably from the saturated soils adjacent (within 30 m) to the stream. McDowell et al. (2001) also states that in addition to dilution, P concentrations are controlled by resuspension, sorption– desorption, landscape processes such as soil transport in overland and subsurface flow, and soil P concentrations. It does seem likely that there was some sort of experimental error in the autumn OP analysis since there should have been some present. It could also be speculated that the OP concentrations would have followed
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the behavior of the other suspended particles (that of TSS and POC) and diluted in the autumn, but the extent to which may have happened is unknown. 5.4. Lignin Lignins are exclusively terrestrial in origin and are the most refractory component of terrestrial plant cell wells. Thus lignins are good indicators of land-derived POM. The semi-quantitative method of Pocklington and Hardstaff (1974) is a simple and rapid way to get a general indication of the amount of lignin in a sediment sample. The trade-off is that it has a precision of only about 50%. The baseflow samples contained 1.22–1.93 g lignin per gram of suspended matter and the high flow samples contained 0.94–1.22 mg lignin per gram of suspended matter. This slightly larger amount of lignin in autumn samples compared with summer was not significantly different by Tukey’s quick test. Ittekkot and Laane (1991) found that there is generally a decrease in labile C (or conversely an increase in refractory C) during high concentrations of suspended matter. They suggested that during times of high suspended matter in streams primary production is reduced because of the decreased light penetration. Ittekkot and Laane found that the temperate rivers studied had a higher content of labile particulate C compared to tropical rivers where in-stream processes are more likely to degrade the C before it is transported to the oceans. 5.5. d-13C The 13C results were the most definitive in this study. By Tukey’s quick test the d-13C values were determined to be significantly different. The mean summer d-13C value was 24.80% and the mean autumn d-13C value was 21.45% (Tables 1 and 2). Given the ranges of d-13C values for various types of organic C sources (Ruttenberg and Gon˜i, 1997), summer POM in the Upper Scioto River contained a possible mixture of C3 plant material (within the range: 23 to 34%) and phytoplankton material (on the edge of the range: 18 to 24%). Soybeans, a C3 plant, are a major crop in the counties (Delaware, Marion and Hardin Counties) where the samples were collected. Autumn POM contained a mixture of C4 plant material (values fall in the range: 6 to 23%) and phytoplankton (values fall in the range: 18 to 24%). The fact that the autumn d-13C value was significantly lower than the C3 range of values in very interesting. The intensive cultivation of corn in the Upper Scioto River watershed appears to contribute significantly to the suspended POM in the November samples, which were collected after the corn harvest. These results are in contrast to those of Weiguo et al. (2003) who found in rivers draining non-agricultural areas of the Chinese Loess Plain that summer
d-13C values were more positive (average of 24.1%) than their November d-13C values (average of 26.4%) and reflected the greater abundance of C4 plants during summer, rather than in November, as found in the present study. The hypothesis that one of the POM sources would be corn seems to be correct.
6. Conclusions The quality, quantity, and sources of suspended organic matter in the highly agricultural Upper Scioto River was dominated by land use practice within the watershed. Superimposed upon land use, daily to seasonal variations in hydrology also have a major influence on the organic geochemical composition of the suspended material. Spatial variations in the suspended matter caused some of the results to have unexpected outcomes. TOC loading increased from 700 kg day1 in summer to 5200 kg day1 in the autumn, but concentrations of other constituents determined decreased during autumn high flow. POC mean concentrations were 1.5 mg l1 at baseflow and 23 mg l1 at high flow. DOC mean concentration was 16 mg l1 during baseflow and 8.3 mg l1 during high flow. Mean TOC concentration was 17.5 mg l1 during baseflow and 8.7 mg l1 during high flow. Mean TSS concentration was 39 mg l1 at baseflow and 17.7 mg l1 at high flow. Dissolved N species all increased in the higher flows but dissolved PO4 decreased. Mean NO3–N was 0.25 mg l1 during baseflow and 9.33 mg l1 during high flow. Nitrite–N was only detected in two of the high flow samples (mean of 0.24 mg N l1) and in none of the baseflow samples. C:N molar ratios at both sampling times were found to be very close to the Redfield ratio and suggest the existence of a high-N, algal component for both summer and autumn suspended organic matter. OC:OP ratios also pointed to a high quality algal component for the summer but an unsure component for autumn samples due to possible experimental error. Summer TP loading was 2.2 1.44 kg/day; autumn TP loading was undeterminable. Lignin concentrations in the suspended matter may have been slightly greater in the autumn, indicating more terrestrial inputs at that time but the low precision of the semi-quantitative method prevented determining that with any confidence. Stable C isotope analysis showed a mixture of phytoplankton and C3 plant sources for summer samples (mean d13C of 24.8) and a phytoplankton and C4 plant (corn) sources for the autumn samples (mean d13C of 21.4). The results of the study show that the quality of the organic matter transported in the river is dominated by the agricultural land use practice within the watershed and the major crops of the C3 plant, soybeans, and the C4 plant, corn.
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Acknowledgements This research was performed as part of SAM’s master’s thesis and she would like to thank her committee members, Virginie Bouchard and W. Berry Lyons, for their support. Partial funding for this study was received from the Department of Geological Sciences at the Ohio State University. References Aitkenhead, J.A., McDowell, W.H., 2000. Soil C:N ratio as a predictor of annual riverine DOC flux at local and global scales. Global Biogeochem. Cycles 14, 127–138. Aspila, K.I., Agemian, H., Chau, A.S.Y., 1976. A semi-automated method for the determination of inorganic, organic, and total phosphate in sediments. Analyst 101, 187–197. Boar, R.R., Lister, D.H., Clough, W.T., 1995. Phosphorus loads in a small groundwater-fed river during the 1989–1992 East Anglian drought. Water Res. 29, 2167–2173. Buffam, I., Galloway, J.N., Blum, L.K., McGlathery, K.J., 2001. A stormflow/baseflow comparison of dissolved organic matter concentrations and bioavailability in an Appalachian stream. Biogeochem 53, 269–306. Carey, A.E., Lyons, W.B., Bonzongo, J., Lehrter, J.C., 2001. Nitrogen budget in the Upper Mississippi watershed. Environ. Engineer. Geosci. 7, 251–265. Carey, A.E., Nezat, C.A., Pennock, J.R., Jones, T., Lyons, W.B., 2003. Nitrogen budget of the Mobile-Alabama River system watershed. Geochem.: Explor., Environ., Anal. 3, 1– 6. Castillo, M.M., Allan, J.D., Brunzell, S., 2000. Nutrient concentrations and discharges in a Midwestern agricultural catchment. J. Environ. Qual. 29, 1142–1151. Clark, I., Fritz, P., 1997. Environmental Isotopes in Hydrogeology. CRC Press LLC, Boca Raton, Florida. Clausen, J.C., Guillard, K., Sigmund, C.M., Dors, K.M., 2000. Water quality changes from riparian buffer restoration in Connecticut. J. Environ. Qual. 29, 1751–1761. Cronan, C.S., Piampiano, J.T., Patterson, H.H., 1999. Influence of land use and hydrology on exports of carbon and nitrogen in a Maine river basin. J. Environ. Qual. 28, 953–961. DePetris, P.J., Kempe, S., 1993. Carbon dynamics and sources in the Parana´ River. Limnol. Oceanog. 38, 382–395. Fraser, A.I., Harrod, T.R., Haygarth, P.M., 1999. The effect of rainfall intensity on soil erosion and particulate phosphorus transfer from arable soils. Water Sci. Technol. 39, 41–45. Goolsby, D.L., Battaglin, W.A., 2001. Long term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA. Hydrol. Proc. 15, 1209–1226. Heathwaite, L., Sharply, A.S., Gburek, W., 2000. A conceptual approach for integrating phosphorus and nitrogen management at watershed scales. J. Environ. Qual. 29, 158–166. Hopkinson, C.S., Buffam, I., Hobbie, J., Vallino, J., Perdue, M., Eversmeyer, B., Prahl, F., Covert, J., Hodson, R., Moran, M.A., Smith, E., Baross, J., Crump, B., Findlay, S.,
1121
Foreman, K., 1998. Terrestrial inputs of organic matter to coastal ecosystems: An intercomparison of chemical characteristics and bioavailability. Biogeochem. 43, 211–234. Ittekkot, V., Laane, R.W.P.M., 1991. Fate of riverine particulate organic matter. In: Degens, E.T., Kempe, S., Richey, J.E. (Eds.), Biogeochemistry of Major World Rivers, SCOPE 42. John Wiley & Sons, Chichester, pp. 233–243. Jarvie, H.P., Neal, C., Leach, D.V., Ryland, G.P., House, W.A., Robson, A.J., 1997. Major ion concentrations and the inorganic carbon chemistry of the Humber rivers. Sci. Total Environ., 194/195, 285–302. Magner, J.A., Alexander, S.C., 2002. Geochemical and isotopic tracing of water in nested southern Minnesota corn-belt watersheds. Water Sci. Technol. 45, 37–42. McDowell, R., Sharpley, A., 2003. Uptake and release of phosphorus from overland flow in a stream environment. J. Environ. Qual. 32, 937–948. McDowell, R., Sharpley, A., Folmar, G., 2001. Phosphorus export from an agricultural watershed: linking source and transport mechanisms. J. Environ. Qual. 30, 1587–1595. Paul M.J., 1999. Stream Ecosystem Function Along a LandUse Gradient. PhD Thesis, Univ. Georgia, Athens. Pocklington, R., Hardstaff, W.R., 1974. A rapid semi-quantitative screening procedure for lignin in marine sediments. J. Cons. Int. Explor. Mer. 36, 92–94. Redfield, A.C., Ketchum, B.H., Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: Hill, M.N. (Ed.), The Sea. Wiley, Chichester, pp. 26–77. Ruttenberg, K.C., Gon˜i, M.A., 1997. Phosphorus distribution, C:N:P ratios, and d-13Coc in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary organic matter. Mar. Geol. 139, 123–145. Soil Regions of Ohio Soil Characteristics. Retrieved 25 June 2003, from the Ohio Department of Natural Resources, Division of Soil and Water Conservation Website: http:// www.dnr.state.oh.us/soilandwater/soilreg6.htm. Strickland J.D.H., Parsons J.R., 1972. Determination of reactive phosphorus. In: A Practical Handbook of Seawater Analysis, second ed. Bulletin 167. Fisheries Board of Canada, Ottawa. Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H., Tilman, D.G., 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecol. Applications 7, 737–750. Weiguo, L., Zisheng, A., Weijian, Z., Head, M.J., Delin, C., 2003. Carbon isotope and C/N ratios of suspended matter in rivers: an indicator of season change in C4/C3 vegetation. Appl. Geochem. 18, 1241–1249. Welch, K.A., Lyons, W.B., Graham, E., Neumann, K., Thomas, J.M., Mikesell, D., 1996. Determination of major element chemistry in terrestrial waters from Antarctica by ion chromatography. J. Chromatog. A 739, 257–263. Weyer, P.J., Cerhan, J.R., Kross, B.C., Hallberg, G.R., Kantamneni, J., Breuer, G., Jones, M.P., Zheng, W., Lynch, C.F., 2001. Municipal drinking water nitrate level and cancer risk in older women: The Iowa women’s health study. Epidemiol. 11, 327–338.