Levels of dissolved organic carbon in British streams and lakes have risen over the past two decades. It might be a downstream effect of decreased acid rain — but isolating single factors is notoriously difficult.
It may be small, but dissolved organic carbon (DOC) — operationally defined as organic compounds in water that can pass through a 0.45-μm filter — is of great interest. The export of DOC from land to aquatic ecosystems and the oceans is a significant mover of carbon through continents and local landscapes (Fig. 1). In some ecosystems, loss of DOC can be as great as long-term accumulation of carbon in the form of dead organic material, for example as peat in peatlands. Then there is its role in the metabolism of lake ecosystems and the protection of aquatic organisms: DOC-rich waters are light brown, and so absorb ultraviolet radiation. On the other hand, DOC-rich water can produce carcinogens when chlorinated. Last — but for the bibulous not least — DOC content is even believed to help determine differences in the flavours of malt whiskies.
This is the context in which Evans et al., writing in Global Change Biology1, supply evidence that median DOC concentrations in eight streams and ten lakes in the United Kingdom have almost doubled from 1988 to 2003. It is important to distinguish between DOC export and concentration here: whereas increased export can result from an increase in runoff with no change in concentration, increased concentrations can occur with no change in hydrology, but with altered production or retention of DOC in the landscape. Increased export of DOC through streams and rivers in the Northern Hemisphere has been observed before, specifically in western Siberia as a result of increased release of DOC from peatlands2. But decreased DOC export has also been observed, in the Yukon river system of northwestern Canada and Alaska3. Although increased DOC concentrations in hydrological systems have been observed before4,5, there has been no clear agreement on whether this constitutes a trend.
Where clear evidence of temporal changes in the concentration and/or export of DOC does exist, it is often difficult to provide convincing explanations. Many factors can influence its production and transport, and Evans and colleagues1 review a range of hypotheses to explain their measured increases. First, they consider increased rates of decomposition in organic litter, and especially in peat soils, associated with changes in moisture and temperature6. This process could trigger an enzymatic 'latch' that would release more DOC7. Second, there is the possible stimulation of plant primary production by elevated atmospheric carbon dioxide8. Lastly, the authors examine the effect of changes in hydrological pathways.
After concluding that no proposed hypothesis is particularly convincing, Evans et al. posit a new one1: that the increase in DOC concentration has been caused by the decrease in sulphur deposition that has occurred in northern Europe and North America over the past 20 years, as clean-air legislation has taken effect. The anthropogenic sulphur deposition is estimated to have declined by 41% between 1998 and 2003 in the stream and lake catchments that Evans and colleagues have studied9.
The mechanisms through which sulphur might influence DOC concentrations in this scheme are primarily chemical, rather than biological. Increased sulphate concentrations and raised pH in soil pore water have been shown experimentally to decrease the mobilization of DOC in soils. To test this sulphur hypothesis, the authors present a correlation analysis of the relationship between the temporal trends in DOC concentrations and sulphate deposition.
But the origin and interactions of DOC in an entire landscape is a complex issue, and what is observed at the outflow of hydrological catchment areas is the combined result of many processes that occur upstream. In nine small boreal lakes in Canada, for example, variations in ice-free DOC concentrations were found to be most strongly correlated with solar radiation and winter precipitation5. Increases in sulphate deposition and lake acidity were not correlated with DOC concentration.
The problem is that in these types of analysis there is very little control on the experiments that nature is providing. Studies are plagued by the influence of extraneous variables, and the cause of changes in some of these variables is unknown. What is certain is that, at the heart of any DOC increase, there has to be an increase either in net DOC production in terrestrial ecosystems or in the leaching of DOC from them. Evans et al. demonstrate that in their catchment areas there has been no increase in runoff, thus focusing attention on the second option.
It is true that, during the period of their study, temperatures were increasing and sulphate deposition was decreasing, both in North America10 and in western Europe11. But the same sources show that deposition of nitrates was increasing at the same time. This point is significant because nitrogen production can improve ecosystem productivity, resulting in increased litter production, and can at the same time influence the rate of carbon mineralization in litter and soil. Elevated DOC concentrations in the Hudson River in the northeastern United States have been attributed to increased DOC production from forest soils subjected to higher nitrogen deposition, as well as to decreased degradability of DOC once in the river system12. Similarly, it has been estimated that soils in England and Wales have lost 4 million tonnes of carbon each year from 1978 to 2003 because of climate change. At least part of this loss might be as increased DOC transport to aquatic ecosystems.
The sulphur hypothesis could be tested. There are several regions in the developing world where sulphate deposition is rapidly increasing. If Evans and colleagues' hypothesis is correct, this should lead to a decrease in DOC stream concentration, once changes in the volume and pathways of water flow are taken into account.
Although the increase in DOC export in some regions is clear, studies showing that the trend is universal, and establishing its causes, are still wanting. Chemical analyses of the DOC in rivers and lakes or analysis of its 14C and 13C isotopic signatures might help in pinpointing its age and origin. An issue of more global import is whether there will be large changes in the concentration and export of DOC in rivers undergoing major changes as a result of the changing climate, such as the melting of permafrost. This might release large amounts of DOC2, or alternatively allow greater water movement through mineral soils, and thus increase DOC retention through sorption3. Studies such as that of Evans and colleagues1 are valuable contributions to the debate on rising DOC, but we are far from reaching definitive answers.
About this article
Environmental feedbacks in temperate aquatic ecosystems under global change: why do we need to consider chemical stressors?
Regional Environmental Change (2017)