Environ. Sci. Technol. 2009, 43, 7748–7753
Is the Composition of Dissolved Organic Carbon Changing in Upland Acidic Streams? J U L I A N J . C . D A W S O N , * ,† IAIN A. MALCOLM,‡ STUART J. MIDDLEMAS,‡ DOERTHE TETZLAFF,† AND CHRIS SOULSBY† Northern Rivers Institute, School of Geosciences, St. Mary’s Building, Elphinstone Road, University of Aberdeen, Aberdeen AB24 3UF, U.K., and The Freshwater Laboratory, Marine Scotland, Faskally, Pitlochry, Perthshire, PH16 5LB, U.K.
Received June 5, 2009. Revised manuscript received August 28, 2009. Accepted August 29, 2009.
The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.
Introduction The transport of organic carbon in upland streams is a key link between soil and ocean carbon pools (1). Organic carbon is an important source of energy substrate within stream ecosystems and its structure allows binding and transport of ions, nutrients, heavy metals, and organic pollutants as well as regulating pH (2, 3). Numerous long-term studies in northern temperate regions have shown rising dissolved organic carbon (DOC) concentrations in surface waters draining catchments dominated by organic-rich soils (4-7). However, little is known about whether the composition of these DOC exports is changing. The quality or composition * Corresponding author tel: +44 (0)1224 273728; e-mail: j.j.dawson@ abdn.ac.uk. † Northern Rivers Institute. ‡ The Freshwater Laboratory. 7748
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of DOC has both ecological significance, in terms of biodegradability and carbon cycling in freshwater ecosystems, as well as economic importance associated with treatment costs to meet environmental standards for color in potable waters (1, 5). In surface waters, DOC occurs in many forms, from simple hydrophilic molecules to the larger hydrophobic structures (such as humic and fulvic acids) that increase coloration of waters with associated higher levels of ultraviolet (UV) absorbance (8). The relationship between UV absorbance and DOC concentrations can be used as a proxy for inferring changes in quality or composition of DOC (8-11). Consequently, changes in this relationship may reflect fundamental alterations to decomposition processes occurring within the terrestrial or in-stream compartments of catchments. This relationship has previously been quantified using the specific UV absorbance per mass of carbon at particular wavelengths (SUVA), particularly between 250 and 280 nm. SUVA can be positively correlated with the aromaticity of organic matter and is a strong indicator of hydrophobic humic fractions within the DOC pool (8-12). Although similar SUVA values can result from a range of DOC compounds when taken from different aquatic environments (10), comparisons of UV absorbance draining the same catchment over time will be intrinsically related to DOC composition (compounds contributing to aromaticity) and reactivity, provided no major land-use changes have occurred (13). Furthermore, there is a negative relationship between SUVA and DOC biodegradability due to the proportional increase in hydrophobic structures when the SUVA is high, although this relationship is not unconditional as aliphatic organic compounds can also fluctuate to some extent in their biodegradability (8, 10). Despite the intense interest in changing DOC concentrations there are, to our knowledge, no long-term data sets (>20 years) published that provide an opportunity to concomitantly observe changes in DOC concentration and composition. Other relevant studies in the UK have been restricted to shorter periods of less than 5 years or long-term color changes alone (13-15). Competing hypotheses have been proposed to explain recent DOC concentration increases in upland streams. It is reasonable to assume that these mechanisms, which usually relate to increased mobilization of the stored soil carbon pool (16, 17), could also cause changes in DOC composition. Recently proposed mechanisms include reversal of soil acidification resulting from decreasing sulfur deposition; increased decomposition due to increasing air temperatures and/or increased drought frequency, increased runoff, land management changes, and CO2 fertilization (3, 5, 6). The aim of this study was to utilize long-term data sets of empirically derived DOC concentrations and UV absorbance to investigate whether the relationship between the two parameters has changed over time. Moreover, trends in depositional and hydrometerological factors were also assessed over the same time period to evaluate potential mechanisms that may drive any changing relationship apparent in the data sets.
Methodology Data Collection. The Allt a’Mharcaidh and Loch Ard (Burn 2) are two long-term upland organic-rich moorland monitoring sites (16) operated by Marine Scotland Freshwater Laboratory where regular measurements of both DOC concentrations and UV absorbance at a λ of 250 nm (Abs250) have been made between 1986 and 2007. In terms of the Scottish situation, the two catchments are situated in areas 10.1021/es901649b CCC: $40.75
2009 American Chemical Society
Published on Web 09/18/2009
TABLE 1. Annual Metrics for SUVA250 and the 13 Potential Explanatory Variables of the Long-Term Abs250 and DOC Relationship for (a) Allt a’Mharcaidh and (b) Loch Ard - Burn 2 from 1986 to 2007 Inclusivea mean mean load SUVA250 mean rain low flow median flow high flow mean alk median alk mean median SO4-S SO4-S year (L mg-1 m-1) T (°C) T10 (°C) T25 (°C) (mm) (L s-1 km-2) (L s-1 km-2) (L s-1 km-2) (µEq L-1) (µEq L-1) pH pH (mg L-1) (g m-2 yr-1) 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
5.35 5.47 6.74 5.07 5.72 5.85 5.34 4.75 5.11 3.74 3.72 3.96 4.22 3.86 3.79 3.84 3.81 3.51 4.53 4.07 3.39 3.47
5.42 4.84 5.47 6.12 6.02 5.84 6.25 5.46 5.85 6.64 5.10 6.47 5.79 6.13 5.95 5.58 6.20 6.82 6.30 6.33 6.63 6.25
13.0 11.2 11.4 12.7 12.5 12.6 12.7 10.8 12.6 15.3 12.3 13.3 12.5 13.1 12.8 12.1 13.3 14.5 13.5 13.5 14.1 13.4
9.71 8.45 9.37 9.26 9.25 9.39 9.83 8.66 9.25 10.2 9.26 10.3 9.34 9.60 9.46 9.17 9.66 10.2 9.74 9.76 10.0 9.70
1054 899 1046 911 1475 1075 1121 1189 1089 1186 723 963 1280 1299 1281 914 1254 700 1198 1186 1039 1317
9.41 12.5 11.3 4.50 10.7 9.32 7.90 8.87 6.10 4.76 6.26 8.47 13.2 8.19 9.06 9.09 10.4 6.48 9.94 6.86 7.30 nd
(a) Allt a’Mharcaidh 22.4 83.4 19.7 51.0 21.3 65.3 12.0 49.5 21.7 87.7 19.6 63.7 20.1 57.1 19.9 116 24.7 121 27.9 113 16.0 78.7 15.8 104 26.3 98.4 23.8 107 27.6 125 20.2 79.5 24.6 98.6 14.0 66.2 27.3 95.8 22.3 103 20.3 91.0 nd nd
41.8 45.3 47.2 54.1 42.6 47.8 49.7 44.9 47.4 45.7 53.7 51.3 42.1 40.9 38.0 49.4 43.8 60.8 44.5 47.2 48.9 40.2
39.5 45.5 46.5 52.0 41.5 44.0 48.0 45.0 46.5 42.0 54.0 52.5 43.5 42.0 38.0 51.0 46.0 61.0 42.0 46.5 48.0 39.5
6.21 6.41 6.37 6.29 6.23 6.32 6.33 6.22 6.23 6.29 6.27 6.34 6.35 6.26 6.30 6.37 6.27 6.49 6.36 6.48 6.27 6.23
6.47 6.51 6.52 6.61 6.49 6.52 6.55 6.56 6.54 6.46 6.43 6.58 6.50 6.52 6.43 6.58 6.52 6.69 6.54 6.63 6.60 6.44
0.66 0.54 0.46 0.69 0.42 0.33 0.42 0.49 0.52 0.31 0.73 0.35 0.40 0.20 0.17 0.27 0.29 0.19 0.17 0.19 0.20 0.16
0.37 0.33 0.28 0.36 0.31 0.20 0.28 0.27 0.31 0.22 0.21 0.21 0.25 0.14 0.14 0.17 0.20 0.08 0.16 0.14 0.12 0.17
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
4.43 5.13 5.99 5.41 5.75 5.58 6.04 5.45 5.38 4.48 4.00 4.42 4.63 4.45 4.36 4.52 4.32 4.02 5.42 4.39 3.62 4.14
6.91 6.78 8.82 8.41 8.54 8.34 7.86 7.30 7.76 8.54 8.24 9.47 8.70 8.21 8.45 7.55 8.07 7.98 8.69 9.24 8.58 9.08
13.0 12.8 16.5 15.7 15.9 15.9 14.7 13.7 14.1 18.5 16.3 16.8 16.2 14.7 14.9 14.1 14.1 14.9 16.2 17.2 16.0 16.9
10.9 10.7 13.4 12.9 13.1 13.4 12.2 11.4 11.3 14.1 12.9 14.0 13.3 11.9 12.5 12.2 12.0 12.6 13.3 14.0 13.1 13.8
3057 2102 2720 2763 3118 2484 2946 2632 2927 2501 2182 2527 2831 2962 2870 2099 2923 2130 2386 2749 2633 2256
nd nd 2.17 3.26 4.35 2.17 1.09 2.17 2.61 1.09 1.09 1.44 2.27 2.21 2.17 3.32 4.25 2.75 4.35 2.17 nd nd
(b) Loch Ard - Burn 2 nd nd nd nd 27.2 208 20.7 218 23.9 224 17.4 190 29.3 239 20.1 317 33.2 276 19.0 264 17.4 207 15.0 187 29.9 224 22.6 206 21.6 176 16.3 139 29.1 219 15.9 148 21.6 197 21.7 255 nd nd nd nd
1.78 13.1 4.18 3.75 1.04 11.7 6.74 9.19 4.76 16.8 10.2 3.56 2.77 1.98 5.98 16.7 4.52 14.4 9.60 12.8 9.08 13.9
-5.0 10.0 2.0 0.0 -4.5 4.0 -0.5 4.5 0.0 6.0 -1.0 -2.0 -1.0 0.0 2.0 8.0 -1.0 8.5 4.0 8.0 1.0 16.0
5.05 5.39 5.22 5.11 5.12 5.27 5.25 5.20 5.19 5.29 5.14 5.15 5.17 5.18 5.21 5.35 5.12 5.42 5.36 5.19 5.21 5.50
5.13 5.79 5.32 5.28 5.12 5.40 5.24 5.47 5.31 5.62 5.26 5.21 5.23 5.23 5.32 5.42 5.20 5.54 5.62 5.63 5.32 5.86
0.83 1.00 0.92 0.63 0.77 0.86 0.68 0.83 0.62 0.52 0.85 0.70 0.44 0.42 0.37 0.41 0.39 0.52 0.37 0.32 0.29 0.26
1.45 1.40 1.72 1.21 1.43 1.08 1.22 1.32 1.26 0.98 1.13 0.99 0.86 0.79 0.70 0.66 0.72 0.74 0.63 0.62 0.59 0.42
a
nd ) not determined, no available data; see Supporting Information for full definitions of column headings.
with contrasting climatic and depositional characteristics (16). The drier and colder Allt a’Mharcaidh catchment is situated on the western edge of the Cairngorm Mountains in NE Scotland and forms a tributary of the River Spey. The wetter Loch Ard (Burn 2) site is part of a monitored network of streams in Central Scotland draining into the River Forth. All samples were passed through a 0.45 µm filter and stored refrigerated at 4 °C prior to analysis. All DOC analyses were performed under the UK Acid Water Monitoring Network analytical quality control program providing a quantitative assessment of laboratory performance (16). Absorbance was determined using a UV spectrophotometer at 250 nm and SUVA250 values were also calculated for each sample (11, 12). Other parameters covering the time period under investigation were used to determine potential explanatory variables influencing the long-term relationship between Abs250 and DOC. Rainfall and air temperature data were obtained from weather stations situated in the vicinity of each catchment. Atmospheric nonmarine sulfate deposition data were calculated from precipitation chemistry (18). Stream water pH and alkalinity were determined from
samples obtained at the same time of sampling as DOC data (19). Discharge was measured at a continuously gauged station at the Allt a’Mharcaidh and at Loch Ard, Burn 2 discharge (1988-2005) was based on data from a nearby gauged catchment applying a proportional area approach (16). More detailed descriptions for data collection of the above parameters are given in Supporting Information (SI). Annual metrics of these explanatory variables (Table 1) were then collated for subsequent statistical analysis. Statistical Analysis. Previous studies have used SUVA to describe the relationship between Abs250 and DOC, and to infer quality and degradability (8-12). As such, SUVA (Abs250/ DOC) was calculated and plotted (Figure 1) in this study to provide a ready link to previous studies. However, the use of SUVA (a simple ratio) is not necessarily the best way to represent the DOC-Abs250 relationship. Consequently all statistical analyses in this study assumed a linear relationship between raw Abs250 and DOC data which potentially uses both an intercept and a slope to describe the relationship. Statistical models were constructed with DOC as the response variable and a range of other explanatory variables including VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Results of Likelihood (L) Ratio Tests Used to Assess the Significance of the Four Explanatory Metrics in Controlling Observed Changes in the Long-Term Abs250-DOC Relationship (Only the SO42--S Load (italics) Falls below the Critical Level of Significance (P = 0.0125) at Both Sites) annual metric a
total sulfate-S load stream alkalinity (mean) temperature (mean) total rainfall
Allt a’Mharcaidh
Loch Ard (Burn 2)
L ratio
P-value
L ratio
P-value
11.47 0.75 4.53 0.94
200 km2) Swedish catchments (1987-2004). In contrast to the present study, the SUVA420 (Abs420/TOC) actually increased in 19 of the rivers. It was concluded that the trend in SUVA420 was negatively correlated to aquatic sulfate concentrations and interannual variability was positively correlated to discharge (33). As well as different land uses with inherently contrasting soil characteristics and processes, alternative drivers may control the Abs or DOC properties depending on the spatial scale, as previously noted with temporal controls. Moreover, a recent study in a peat-dominated catchment in NE England (covering the period from March 1999 to November 2006) investigated changes in coagulant (ferric chloride) dose required to remove color from incoming waters at a water treatment works. This study suggested that DOC is becoming increasingly hydrophilic with time, which is in agreement with the work undertaken at these Scottish catchments (30). Implications of This Long-Term Study. The mechanisms controlling trends in both DOC concentration and composition in acidic upland streams are potentially linked. As this long-term study indicates, the hydrophobic proportion of DOC shows a decreasing trend and the changing nonmarine sulfate-S load is the most likely causal factor through impacts on organic matter decomposition and solubility. It also suggests that there is an increasing proportion of the rising DOC exported in surface waters that is more easily degradable. However, this relationship is not unambiguous; aliphatic organic compounds can also fluctuate to some extent in their biodegradability (8, 10). Moreover, Ågren et al. have suggested that the bioavailability of the DOC may be more related to molecular weight (indicted by Abs254/Abs365 ratios), encompassing both aliphatic and aromatic components (34), than simply to its aromatic content (as suggested with the sole measurement of SUVA at 250 nm). Unfortunately, we were unable to determine if there was a concomitant decreasing trend in molecular weight as well as aromaticity due to a lack of available absorbance data at the higher wavelength. An increasingly hydrophilic DOC composition has implications for further mineralization of DOC to CO2 as more readily assimilated carbon (energy) substrates increase, potentially enhancing downstream biotic productivity in fluvial ecosystems as well as affecting the subsequent fate of DOC within soil-water-atmosphere carbon cycling (1, 35, 36).
Acknowledgments We thank David Moore (FRS) for searching out relevant databases for both the Allt a’Mharcaidh and Loch Ard (Burn 2). We acknowledge the support of the Scottish Government, the Department for Environment Food and Rural Affairs (DEFRA), and University College London in funding and supporting water quality analysis at Allt a’Mharcaidh as part of the UK Acid Waters Monitoring Network, and the Scottish Government for funding data collection and water quality analysis at Loch Ard. We also thank Mike Hutchins (CEH) and the ECN (http://www.ecn.ac.uk/) for discharge, rainfall, and air temperature data at the Allt a’Mharcaidh; and the Scottish Environmental Protection Agency for discharge data at Loch Ard. Finally, the British Atmospheric Data Centre (http://www.badc.nerc.ac.uk/home) is thanked for rainfall 7752
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and air temperature data from sites surrounding both study catchments.
Supporting Information Available Full methodology description; Figure S1, principal component analysis for the initial 13 explanatory metrics; Figure S2, temporal variability (1986-2007) of annual DOC concentrations and absorbance values; Figure S3, temporal variability (1986-2007) of annual sulfate-S loads. This material is available free of charge via the Internet at http:// pubs.acs.org.
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