The importance of the relationship between scale and process in understanding long-term DOC dynamics
Introduction
There have been widespread observations of increased dissolved organic carbon (DOC) concentrations in surface waters across parts of Europe and North America over the last two decades (Driscoll et al., 2003, Worrall et al., 2004, Evans et al., 2005, Skjelkvale et al., 2005). This has raised concerns about drinking water treatment and the production of carcinogenic byproducts (Gallard and von Gunten, 2002, Holden et al., 2007), and the further possibility that climate change is causing degradation of soil carbon stores (Freeman et al., 2001a, Bellamy et al., 2005). In both cases there is a common perception that DOC increases are likely to be environmentally detrimental, and increasingly land managers are seeking guidance from the scientific community with respect to practical methods to control or even reverse these trends.
Several hypotheses have been put forward to explain increasing DOC trends (Table 1). One hypothesized driver for increasing DOC trends is a long-term change in the chemistry of atmospheric deposition that has been recorded across many of these areas as a result of reductions in anthropogenic sulphur and, in some locations, seasalt deposition (Evans et al., 2006, Vuorenmaa et al., 2006, de Wit et al., 2007, Monteith et al., 2007, Dawson et al., 2009, Hruska et al., 2009, Oulehle and Hruska, 2009). However, others have rejected this hypothesis, arguing that DOC trends are more consistent with changes in rainfall, temperature and/or atmospheric carbon dioxide (CO2) than declining atmospheric sulphur deposition (Worrall and Burt, 2007a, Eimers et al., 2008c, Lepisto et al., 2008, Sarkkola et al., 2009), building on earlier studies suggesting relationships between these drivers and increased DOC (Freeman et al., 2001a, Freeman et al., 2004, Hongve et al., 2004, Fenner et al., 2007). Some reject the deposition hypothesis outright as DOC concentrations have decreased in some areas where acid deposition has declined (Clair et al., 2008). Other drivers have also been suggested; these include changing nitrogen deposition (Findlay, 2005), solar radiation in boreal lakes (Hudson et al., 2003), and land management practices (Yallop and Clutterbuck, 2009). In the UK uplands, an increase in heather burning has coincided with an increase in DOC concentrations (Yallop et al., 2006, Yallop and Clutterbuck, 2009) and restoration practices such as blocking of artificial drainage ditches are being carried out with the intention of reducing DOC concentration at source prior to drinking water treatment (Wallage et al., 2006, Armstrong et al., 2009).
Long-term trends in DOC concentrations (typically representing change over the last two decades) can be orders of magnitude smaller than spatial and seasonal variation (Table 2), making it difficult to detect weaker trends (Fig. 1). Several studies have shown that much of the larger seasonal and spatial variability can be explained by processes and catchment characteristics controlling the availability of soluble organic matter and subsequent hydrologic transport (e.g. McDowell and Likens, 1988, Hope et al., 1997, Laudon et al., 2003, Clark et al., 2007b). It is clearly necessary to distinguish between cause-effect relationships influencing the spatial and seasonal variability in DOC from those influencing long-term changes if drivers of long-term trends are to be correctly identified. Any factors invoked to explain long-term change must also show (either individually or in combination) an appropriate long-term trend. Otherwise, there is a risk that analysis will confuse variables explaining larger spatial and seasonal variance with those responsible for smaller changes in the annual level (i.e. mean annual concentration).
Section snippets
Conceptual model of DOC dynamics in surface waters
DOC concentrations in surface waters at any one site can be expressed using a time series model. In a general model, the variation in the data can be decomposed in to a series of components describing the level or overall mean value (α), long-term trend (T), seasonal component (S) and random noise (N) (Chatfield, 1984). For a specific DOC time series model, an additional term accounting for inter-annual variation (IAV) is added, such that the generic model for DOC concentrations in surface
Process controls on DOC: biology, chemistry and hydrology
DOC production in terrestrial systems is a biological process. Low-molecular weight highly degradable DOC is released by plant roots, soil and aquatic microorganisms, whereas high-molecular weight coloured aromatic and refractory DOC is released during decomposition of organic material (Thurman, 1985). Release of labile DOC in plant root exudates is thought to stimulate decomposition and release of high-molecular weight DOC from soil organic matter (Kuzyakov, 2002, Freeman et al., 2004).
Scale controls on DOC: spatial and temporal (seasonal, short-term inter-annual and long-term trend)
Variation in the spatial and temporal scale of the drivers of the processes of DOC production, solubility and transport are summarized in Table 3. Catchment factors determine whether conditions are appropriate for DOC production and subsequent export to surface waters. Fundamentally, DOC cannot be exported to surface waters if it is not produced, therefore, biological production provide a primary control on freshwater DOC. Since DOC is produced through incomplete decomposition of organic
Confusion of spatial and temporal scale
As noted above, DOC dynamics within soil and stream waters are controlled by a number of different factors that influence the production, solubility and transport at a range of spatial and temporal scales (Table 3). As many of these drivers vary over both spatial and temporal scales, it is not easy to attribute change in a particular driver to the appropriate components of the DOC time series (Eq. 1). Therefore, correct identification of the factors that account for the majority of the
Conclusion
Given the spatial heterogeneity between catchments and the difference between factors driving seasonal, short-term inter-annual and longer-term temporal variation, most of the explanations of DOC dynamics are potentially compatible with each other. In acid impacted regions, we believe that long-term trends in DOC over the last two decades have been driven principally by declining atmospheric deposition; but this explanation of long-term trends does not exclude other spatial and temporal factors
Acknowledgements
This research was funded by NERC (NE/D00599X/1). We thank the Acid Waters Monitoring Network (AWMN) and Environmental Change Network (ECN) for use of their data; and two anonymous reviewers for their constructive comments that have helped to improve the manuscript.
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