Introduction 

Dissolved organic carbon (DOC) is a broad classification for organic molecules of varied origin and composition in aquatic systems. The size of these organic compounds is defined as the fraction that can pass through a 0.45µm filter. The main source of DOC is leaching of decomposed organic matter from soils into freshwater systems. In soil and water, DOC is an important source of carbon and energy for microorganisms and thus plays an important role in many chemical and photochemical reactions and transformations. These complex mechanisms make both aquatic ecosystems and the global carbon cycle sensitive to changes in DOC concentrations. Long-term observations over the last few decades show a steady increase in DOC in freshwaters. During a 12-year period (1988-2000), Freeman et al. ([10]) found an increase of 65% for a series of streams and lake catchments in the United Kingdom. Increases in DOC concentration are also evident in large areas of Europe and North America ([7], [14], [23], [26], [27], [28]). Using time series (1990-2004) of 522 remote streams and lakes in northern Europe and North America, Monteith et al. ([23]) found upward slopes for 70% of their analyzed time series. Some studies, however, report stagnating or negative trends ([9], [13], [17]). Whether there really is a widespread positive trend in DOC concentrations has been the subject of heated debates ([25], [30]).

Understanding the trends in DOC concentrations in surface water is important not only because of its role in the global carbon cycle ([1]), but also because changes can affect surface water pH and acid neutralizing capacity ([20]). In addition, high DOC concentrations reduce light penetration in water by giving it a brownish hue. This affects the aquatic productivity, which has implications for the aquatic food chain and lake stratification ([19]). An increase in DOC is generally beneficial for aquatic biota. Because organic molecules are strong complexing agents for toxic aluminium for a given level of total aluminium content, fish have a better chance of survival at higher DOC concentrations ([16]). On the other hand, an increased leaching of DOC from the soil is coupled with an increased leaching of metal ions and their transport into surface water ([22]). The resulting decrease in water quality and transparency increases the cost of water treatment. In order to protect surface water quality, it is essential to clarify the processes that lead to the dissolution and transfer of dissolved organic matter.

  Possible drivers of increased DOC 

Many researchers have tried to identify and evaluate the factors that govern both DOC production in the soil and its transport to the drainage network. In the following, we provide an overview of possible drivers that lead to increased DOC concentrations in freshwater.

Changes in air temperature

Changes in air temperature may directly influence DOC export from peat soils by altering decomposition and mineralization of organic matter, both sensible to changes in moisture and temperature ([6], [10], [30], [32]). Hence, greater soil aeration under warmer and dryer conditions will increase decomposition through greater enzymatic activity. Although these authors argue that climate change leads to greater DOC concentrations in freshwater systems, Evans et al. ([5]) noted that no quantitative process-based data are available to prove that this mechanism affects DOC production. Other field and laboratory studies indicate that DOC concentrations are not responsive to warming ([11]). Experimental tests suggest that more than 10 °C of additional warming would be required to yield increase in DOC, a level of change not anticipated by even the most pessimistic IPCC scenario. Monteith et al. ([23]) also found no correlation between regional temperature patterns and DOC trends.

Increased precipitations

Increased precipitation alters the water budget and discharge, which then increases DOC concentrations ([15]). This can affect the mass of DOC export independently of any changes in DOC concentrations. However, various analyses of stream water suggest no consistent hydrologic trends in recent decades ([7], [21]).

Land use changes

Land use changes, such as altered forestry practices ([29]), draining of peatlands ([32]), or changes in grazing or burning of grassland and peatlands ([12]), can also influence the retention and the export of organic carbon from catchments. However, Monteith et al. ([23]) did not find any consistent land use changes in northern Europe and North America where increasing DOC trends had been observed.

Increased atmospheric carbon dioxide

Increased atmospheric carbon dioxide (CO₂) stimulates primary plant production, as shown in laboratory experiments ([11]). Under elevated CO₂ levels, the proportion of DOC in the soil solution derived from recently assimilated CO₂ was ten times higher compared to the control ([11]). However, the magnitude of the ambient CO₂ increase measured at global monitoring stations since 1990 amounts to only 8% of the concentration used in the experiments, and even laboratory experiments failed to reproduce the magnitude of DOC increase ([22]).

Decreased atmospheric sulphur deposition

Decreased atmospheric sulphur deposition is subject of several recent studies suggesting to influence patterns and trends in DOC concentrations in freshwaters ([2], [3], [31]). Increases in both acidity of soils and ionic strength of soil solution (associated with a high sulfate loading) reduce soil solution DOC concentrations ([18]). Monteith et al. ([23]) showed that DOC concentrations have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined. In contrast, Hudson et al. ([16]) observed no correlation of DOC with sulphate deposition.

Accumulation of atmospherically deposited nitrogen

Accumulation of atmospherically deposited nitrogen is proposed to increase DOC concentrations because of higher rates of soil microbial decomposition ([8]). Nitrogen retention can improve ecosystem productivity, resulting in increased litter production. It can influence the rate of carbon mineralization in litter and soil. However, as many regions in northern Europe and North America are already at a nitrogen saturation level, further nitrogen deposition does not necessarily lead to increasing DOC leaching ([23]).

  State of current research 

Based on the work of Evans et al. ([5]), Roulet & Moore ([25]) concluded that the origin and interactions of DOC in hydrological catchments are very difficult to determine because many of the processes occurring are still unknown. Tools that may help to determine the age and origin of DOC in rivers and lakes are analyses of its chemical composition or its¹⁴ C and¹³ C isotopic signature ([25]). Eimers et al. ([4]) note an important issue about differences in reporting methods. Some studies consider changes in average measured DOC whereas others compute discharge weighted concentrations. These differences in reporting methods and varying record length complicate the comparison of the results among studies and regional generalization. Due to contradictory findings reported in literature, Porcal et al. ([24]) point out that descriptive studies have their limitations and that detailed modelling studies that integrate key mechanisms are considered necessary to allow testing of various scenarios. They also mention the need for more studies investigating how run-off and temperature related changes in DOC affect metal and nutrient export to rivers and lakes.

  Conclusions 

Increasing DOC concentrations have been observed in lakes and streams throughout large areas of Europe and North America. Although several hypotheses have been proposed to explain this widespread phenomenon, Evans et al. ([5]) argue that none is particularly convincing. For these authors, the most realistic mechanism is a complex interaction of changing atmospheric deposition and rising temperature. This short review of recent literature shows that it is not possible to explain the increase in DOC by one single driver for all sites.

  Acknowledgements 

We would like to thank the organizers of the 5^th NFZ Summer School, who gave us the opportunity to write this short paper, especially Marcus Schaub. We would also like to acknowledge the valuable commentaries provided by two anonymous reviewers.

  References