Long-term increases in surface water dissolved organic carbon: Observations, possible causes and environmental impacts
Introduction
Dissolved organic matter (DOM) is a ubiquitous component of natural waters, operationally defined as comprising any organic compound passing through a 0.45 μm filter. The number of such compounds is effectively limitless, and it is thus impossible to provide a general chemical description of DOM. However, in general it includes a small proportion of identifiable, low-molecular weight compounds such as carbohydrates and amino acids, and a larger proportion of complex, high-molecular weight compounds collectively termed humic substances. Humic substances have a medium to high molecular weight, and are a complex mixture of aromatic and aliphatic hydrocarbon structures with attached amide, carboxyl, ketone and other functional groups (Leenheer and Croué, 2003). Humic substances absorb visible light, most strongly at the blue end of the spectrum, giving high-DOM water a characteristic brown colouration.
DOM is generated by the partial decomposition of, or exudation from, living organisms including plants, animals, and soil microorganisms. The organic matter generated by these processes may be stored in the soil for a varying length of time (e.g. as peat) before decomposition processes render a part of this material soluble. The compounds comprising DOM in natural waters may therefore range in age from relatively recent to thousands of years (Raymond and Bauer, 2001). DOM affects the functioning of aquatic ecosystems through its influence on acidity (Eshleman and Hemond, 1985), trace metal transport (Lawlor and Tipping, 2003), light absorbance and photochemistry (Schindler, 1971, Zafariou et al., 1984), energy supply (Wetzel, 1992), and nutrient supply (Stewart and Wetzel, 1981). It also affects water treatment processes (Alarconherrera et al., 1994) and, as a transfer of carbon from terrestrial to aquatic and ultimately marine ecosystems, forms a significant component of the global carbon cycle (Hope et al., 1994).
In most studies concerning DOM, the DOC component is quantified. The organic nutrients, DON and DOP, are also widely measured. Concentrations of DOC in natural waters vary widely, from <1 to >50 mg l−1 (Thurman, 1985), with the lowest values observed in the oceans, groundwater, and “clearwater” lakes and rivers draining bare rock or thin, organic-poor soils. Concentrations are highest in organic soil porewaters, and freshwaters draining wetlands and peatlands, especially where runoff is low. In a number of studies (e.g. Curtis, 1998, Xenopoulos et al., 2003) positive spatial relationships have been demonstrated between DOC export and wetland area. In the UK, Hope et al. (1997) demonstrated a correlation between riverine DOC flux and the amount of organic matter (predominantly peat) in catchment soils. While controls on spatial DOC variations among freshwaters can be considered reasonably well understood, the causes of temporal change in DOC at a particular site remain more uncertain. The aim of this paper is to: (i) evaluate the changes in DOC concentration observed over the last 15 years among the lakes and streams of the UK Acid Waters Monitoring Network; (ii) consider these trends in the context of other observations in the UK, Europe and North America; (iii) identify the possible drivers of observed trends; (iv) analyse the monitoring data relative to these potential drivers; and (v) assess the significance of observed trends in terms of local biological impacts and larger-scale environmental change.
Section snippets
Trends in the UK Acid Waters Monitoring Network
The UK Acid Waters Monitoring Network (AWMN) comprises 11 lakes and 11 streams, at which coordinated chemical and biological monitoring has been undertaken according to consistent measurement protocols since 1988 (Patrick et al., 1991). Lakes are sampled quarterly, whilst streams, due to their greater short-term variability, are sampled monthly. Catchments are located across the main acid-sensitive regions of the UK, and mostly comprise moorland, with land-use limited to rough grazing by sheep,
Recovery from acidification
It has been suggested that, in response to increasing inputs of mineral acids, soils may release lower quantities of organic acids, thereby buffering the impact of acid deposition on runoff acidity (Rosenqvist, 1978, Krug and Frink, 1983). Surface water DOC concentrations would thus be depressed when acid anion concentrations are high, and would increase as acid anion concentrations are reduced. Palaeolimnological evidence has been used to support this hypothesis, with lake cores showing diatom
Analysis of AWMN data
Previous research, and environmental changes over the last 15 years, suggest several drivers with the potential to explain observed UK DOC increases. Those considered here are (i) decreasing soil acidity; (ii) decreasing soil solution ionic strength; (iii) increasing temperature; and (iv) dry–wet cycles. The relative role of these drivers is examined firstly with regard to spatial patterns in observed chemical trends. Secondly, a stepwise regression analysis of raw DOC data is undertaken for
Environmental impacts of rising DOC
The lakes and streams of the AWMN have clearly undergone major chemical change during the last 15 years, and increased concentrations of DOC are likely to have significantly affected other chemical variables. In particular, metal transport may have increased due to increased complexation by organic compounds. Organic Al concentrations have increased widely (significant at eight sites) whereas concentrations of toxic inorganic Al have decreased (significant at ten sites). Total iron
Conclusions
There is now overwhelming evidence that DOC concentrations have increased during the last two decades at the AWMN sites, and other UK upland surface waters. There is evidence of similar changes at other monitoring sites across Europe and North America. These observations suggest a systematic response to one or more external drivers across a large spatial scale. At the AWMN sites, concentrations have now almost doubled, and it seems likely that changes of this magnitude will have significant
Acknowledgements
This work was supported by the UK Department of the Environment, Food and Rural Affairs (Contract No. RMP 2036), the European Union Framework Programme 6 Eurolimpacs project (GOCE-CT-2003-505540) and the Scottish Executive Environment and Rural Affairs Department/Welsh Assembly Government (Contract No. FF/03/08). We are grateful to Jo Clark and two reviewers for comments on the manuscript.
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