Michael F. Billett

A review of the export of carbon in river water: Fluxes and processes

This review summarizes data on exports of carbon from a large number of temperate and boreal catchments in North America, Europe and New Zealand. Organic carbon losses, usually dominated by dissolved organic matter, show relatively little variation, most catchments exporting between 10 and 100 kg C ha(-1) yr(-1). Inorganic carbon exports occur at a similar rate. However, a lack of information on the flux of particulate organic carbon and dissolved CO2 is highlighted, particularly for rivers in Europe. Processes regulating the flux of organic carbon to streams and its subsequent fate in-stream are reviewed, along with the effects of land use and acidification on these processes. The size of the global riverine flux of carbon in relation to the global carbon cycle and the possible effects of environmental change on the export of carbon in rivers are considered.

The relationship between dissolved organic carbon in stream water and soil organic carbon pools at different spatial scales

The relationship between stream water DOC concentrations and soil organic C pools was investigated at a range of spatial scales in subcatchments of the River Dee system in north-east Scotland. Catchment percentage peat cover and soil C pools, calculated using local, national and international soils databases, were related to mean DOC concentrations in streams draining small- ( 700 m) catchments, suggesting that disturbance and land use may have a small effect on DOC concentration. Our results therefore suggest that the relationship between stream water DOC concentration and catchment soil C pools exists at a range of spatial scales and this relationship appears to be sufficiently robust to be used to predict the effects of changes in catchment soil C storage on stream water DOC concentration. Copyright

Linking land-atmosphere-stream carbon fluxes in a lowland peatland system

Any change in the ability of northern peatlands to act as a sink for atmospheric CO2 will play a crucial part in the response of the Earth system to global warming. We argue that a true assessment of the sink-source relationships of peatland ecosystems requires that losses of C in drainage waters be included when determining annual net C uptake, thus connecting measurements of stream C fluxes with those made at the land surface-atmosphere interface. This was done by combining estimates of net ecosystem exchange (NEE) with stream water measurements of TOC, DIC, and gaseous C loss, in a 335-ha lowland temperate peatland catchment (55°48.80′N, 03°14.40′W) in central Scotland over a 2-year period (1996–1998). Mean annual downstream C flux was 304 (±62) kg C ha−1 yr−1, of which total organic carbon (TOC) contributed 93%, the remainder being dissolved inorganic carbon (DIC) and free CO2. At the catchment outlet evasion loss of CO2 from the stream surface was estimated to be an additional 46 kg C ha−1 yr−1. Over the study period, NEE of CO2-C resulted in a flux from the atmosphere to the land surface of 278 (±25) kg C ha−1 yr−1. Net C loss in drainage water, including both the downstream flux and CO2 evasion from the stream surface to the atmosphere, was therefore greater or equal to the net annual C uptake as a result of photosynthesis/respiration at the land surface. By combining these and other flux terms, the overall C mass balance suggests that this system was either acting as a terrestrial C source or was C neutral.

EXPORTS OF ORGANIC CARBON IN BRITISH RIVERS

This study provides the first detailed estimate of riverine organic carbon fluxes in British rivers, as well as highlighting major gaps in organic carbon data in national archives. Existing data on organic carbon and suspended solids concentrations collected between 1989 and 1993, during routine monitoring by the River Purification Boards (RPBs) in Scotland and the National River Authorities (NRAs) in England and Wales, were used with annual mean flows to estimate fluxes of dissolved and particulate organic carbon (DOC and POC) in British rivers. Riverine DOC exports during 1993 varied from 7·7–103·5 kg ha−1 year−1, with a median flux of 31·9 kg ha−1 year−1 in the 85 rivers for which data were available. There was a trend for DOC fluxes to increase from the south and east to the north and west. A predictive model based on mean soil carbon storage in 17 catchments, together with regional precipitation totals, explained 94% of the variation in the riverine DOC exports in 1993. This model was used to predict riverine DOC fluxes in regions where no organic carbon data were available. Calculated and predicted fluxes were combined to produce an estimate for exports of DOC to tidal waters in British rivers during 1993 of 0·68±0·07 Mt. Of this total, rivers in Scotland accounted for 53%, England 38% and Wales 9%. Scottish blanket peats would appear to be the largest single source of DOC exports in British rivers. An additional 0·20 Mt of organic carbon were estimated to have been exported in particulate form in 1993, approximately two–thirds of which was contributed by English rivers. It is suggested that riverine losses of organic carbon have the potential to affect the long-term dynamics of terrestrial organic carbon pools in Britain and that rivers may regulate increases in soil carbon pools brought about by climate change.

Sources of organic and inorganic carbon in a headwater stream: evidence from carbon isotope studies

A combination of stable isotope studies and 14Cdating were used to identify the main sources andprocesses controlling streamwater DOC and TIC in atemperate non-forested watershed. δ13Cvalues for terrestrial (−24.9 to −29.1‰) and aquatic(−30.5 to −33.5‰) plants were similar to valuesreported in the literature for similar ecosystems.δ13C values for DOC in soil solution andstreamwater were consistent with soil and terrestrialvegetation, indicating that the terrestrial ecosystemis the dominant source of aquatic DOC in thiswatershed. δ13C values of soil atmosphereCO2 (−17.2 to −25.2‰) were slightly lessnegative than would be expected for production viaaerobic soil microbial decomposition and rootrespiration. There was a close correspondence betweenδ13C values (−15.5 to −21.5‰) forstreamwater TIC and soil atmospheric CO2 in thecentral part of the catchment where the stream drainsCO2-rich peats. 14C dating showed thatalthough peat has been accumulating in the watershedfor at least 2700 years, DOC in soil pore water andstreamwater contains carbon of predominantly recentorigin (post-AD 1955).

Is in-stream processing an important control on spatial changes in carbon fluxes in headwater catchments?

Data on small-scale spatial variations in instantaneous fluxes and concentrations of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and free carbon dioxide (CO2) are presented for a small acidic headwater stream in NE Scotland. Chloride is used as a conservative element to estimate additional, diffuse inputs of water into the main stem of the stream, other than those from tributaries. Downstream changes in instantaneous carbon fluxes were calculated and then used to estimate losses and gains of carbon within the stream system. Dissolved organic carbon concentrations in the stream ranged from 1.19-6.06 mg l(-1) at its source to a maximum of 10.0-25.3 mg l(-1) as the stream passed through deep peats; DOC concentrations then declined in the lower part of the catchment. DIC concentrations were initially low, increased to 1.5-3.0 mg l(-1) and then decreased to 0.1-1.65 mg l(-1) at the lowest site. Free CO2 concentrations increased from 0.35 mg l(-1) at the stream source to 3.30 mg l(-1) as the stream passed through the peat dominated area. Continually high inputs of CO2-rich water (> 6.0 mg l(-1)) from tributaries maintained these high concentrations in the main stem, until approximately 1.74 km downstream, when there was a rapid decline in concentration. Significant changes in DOC, DIC and CO2 fluxes occur over a distance of 2.7 km downstream from the stream source to the catchment outlet. Between 5.64-41.5 mg C s(-1) as DOC and 2.52-16.2 mg C s(-1) as DIC are removed from the water column. Between 6.81 and 19.0 mg C s(-1) as CO2 is lost along the stream length as progressive equilibration with the atmosphere occurs. We estimate that 11.6-17.6% of the total DOC flux is removed from streamwater by in-stream processes. Dissolved inorganic carbon (HCO3- and free CO2) losses are in excess of nine times its measured flux at the outlet of the catchment. These results suggest that in-stream processing of DOC and DIC and outgassing of CO2 are important controls on the spatial variability of carbon fluxes within headwater streams in upland catchments dominated by organic-rich soils.

A method for measuring free CO2 in upland streamwater using headspace analysis

Abstract Existing titration-based methods for the measurement of dissolved free CO 2 are indirect and require the measurement of a number of other determinands (e.g. pH); they may underestimate free CO 2 concentrations, because analysis is carried out frequently in an open vessel from which some free CO 2 may be lost prior to measurement. Here, a method of headspace analysis is described; this minimises CO 2 loss and provides a more direct technique for determining free CO 2 in low ionic strength, organic-rich upland streamwaters. A sample of streamwater is collected in a sealed flask and a headspace is created by pumping out a known volume of sample, replacing it with CO 2 -scrubbed air. After equilibration of CO 2 between the remaining water and the headspace, the concentration of CO 2 in the headspace is measured using an Infra Red Gas Analyser. The concentration of free CO 2 in the original sample is then calculated using Henrys law. This method measured free CO 2 in standard solutions containing 1–10 mg l −1 free CO 2 to within 0.1 mg l −1 . The method was used to measure free CO 2 in streamwater from 19 sites on the River Dee in north-east Scotland and the results were compared with those reported for streams elsewhere. Free CO 2 concentrations measured by headspace analysis were significantly higher than those found using acidimetric titration.

Riparian zone influence on stream water chemistry at different spatial scales: a GIS-based modelling approach, an example for the Dee, NE Scotland

A geographical information system (GIS-ARC/INFO) was used to collate existing spatial data sets on catchment characteristics to predict stream water quality using simple empirical models. The study, based on the river Dee catchment in NE Scotland, found that geological maps and associated geochemical information provided a suitable framework for predicting chemical parameters associated with acidification sensitivity (including alkalinity and base cation concentrations). In particular, it was found that in relatively undisturbed catchments, the parent material and geochemistry of the riparian zone, when combined with a simple hydrological flow path model, could be used to accurately predict stream water chemistry at a range of flows (Q95 to > Q5) and spatial scales (1-1000 km2). This probably reflects the importance of the riparian zone as an area where hydrological inputs to stream systems occur via flow paths in the soil and groundwater zones. Thus, evolution of drainage water chemistry appears to retain the geochemical characteristics of the riparian area as it enters the channel network. In more intensively managed catchments, riparian land use is a further influential factor, which can be incorporated into models to improve predictions for certain base cations. The utility in providing simple hydrochemical models, based on readily available data sets, to assist environmental managers in planning land use in catchment systems is discussed.

UK peatland streams release old carbon dioxide to the atmosphere and young dissolved organic carbon to rivers

Since the end of the last ice age, sequestration and storage of CO2 from the atmosphere by peatlands in the northern hemisphere has produced a terrestrial C pool of comparable magnitude to that of the global atmosphere. Destabilisation of the peatland C pool will have significant positive climate change feedbacks both directly (via the atmospheric pathway) and indirectly (via the aquatic pathway). Streams and rivers draining peatlands are supersaturated with CO2 and contain high concentrations of dissolved organic carbon (DOC); these are often associated with large lateral (downstream) and vertical (evasion) fluxes, which may produce significant changes in the sink/source relationships of individual peatlands. Here we present isotopic evidence from four UK peatlands to suggest that whilst the age of DOC released in the drainage system of peatlands is consistently young (modern to 202 years BP), the age of CO2 lost by evasion from the water surface is much older, varying from modern to 1449 years BP. 13C data suggest that the sources of DOC and CO2 are different. Whilst antecedent moisture conditions affect within- and between-site differences in the isotopic signature of DOC and CO2, we suggest that the release of CO2 (in contrast to DOC) into the aquatic system is related to a significantly older C pool. The source of this CO2 is likely to be both geogenic (carbonate weathering) and biogenic (decomposition of soil organic matter).

The prediction and management of water quality in a relatively unpolluted major Scottish catchment: current issues and experimental approaches

Abstract The potential impacts of diffuse pollution from atmospheric deposition and land use on the water quantity and quality of the river Dee in N.E. Scotland are currently being assessed. The importance of headwater regions for supplying a large proportion of catchment runoff with water of a high quality is clearly demonstrated. However, the quality of this water is threatened by the impact of acid deposition in a number of sub-catchments. In some of the more agriculturally developed lowland sub-catchments, there are increasing levels of nitrogen runoff. The catchment attributes, together with hydrochemical data, are being considered in terms of an ongoing research programme established to predict the impact of future environmental and land-use change scenarios.