B24A-01 16:00h
Alternative Explanations for Increased Dissolved Organic Carbon Concentrations in UK Surface Waters
Since 1988, there has been a 91% increase in dissolved organic carbon (DOC) concentrations of UK lakes and streams in the Acid Waters Monitoring Network (AWMN). Similar increases in DOC have been observed in surface waters across Europe and North America. This has generated great debate about the factors driving DOC rises. The role of global warming has been a topic of great attention, as DOC production is known to increase with temperature. Although recent research has suggested changes in temperature cannot sufficiently explain this trend, data from our own warming experiment on a peat soil has shown higher rates of DOC release under both anaerobic and aerobic conditions. These data suggest that temperature is quantitatively significant, but may not explain the full magnitude of DOC increases. Additional monitoring and experimental work has identified a clear, inverse link between DOC and sulphate (SO4), such that doubling the SO4 concentration decreased DOC by a factor of 1.5. Increased concentrations of SO4 are associated with a reduction in pH and increase in ionic strength, with both factors known to control DOC solubility. Further statistical analysis of 11 AWMN lakes has revealed that rising temperature and declining sulphur deposition can account for the majority (average 66%) of the observed DOC trend. Therefore a key driver of DOC increase observed at AWMN sites across the UK appears to have been the major (50%) decrease in sulphur (S) deposition, and associated recovery from acidification, which has occurred in the UK (and other parts of Europe and North America) since the mid 1980s, in addition to rising temperatures.
B24A-02 16:15h
A Thirty Year Budget of DOC in a Boreal Lake and Watershed
Since 1971 we have monitored the hydrologic inflows and outflows and the DOC concentrations to a headwater lake at the Experimental Lakes Area on the Canadian Shield on a weekly basis. In addition to variation due to short term weather we were fortunate to have had near average precipitation for a decade in the 70's, a dry warm decade in the 80's and a wet decade in the 90's. Along with measurements of carbon sedimentation and gas exchange this record has allowed us to interpret changes in the fluxes of DOC from the watershed and the impact of the changes in DOC on the characteristics of the lake. DOC makes up by far the largest part of the carbon flux to the lake and supplies most of the carbon for long term burial in the sediments. About 1/3 of the DOC is removed from the lake each year by decomposition and sedimentation so the decreased fluxes during the dry decade resulted in considerable decrease in DOC concentrations in the lake. This had the consequence of substantially increased light penetration and increased heating of the deep waters of the lake, important factors for the biological community. Volume weighted mean concentrations of DOC are about 50% higher in the 90's than in the 70's and fluxes to the lake in the 90's were about 50% higher. Fires in part of the drainage basin in 1974 and in most of the drainage basin in 1981 had little effect on the DOC concentrations or fluxes to the lake.
B24A-03 INVITED 16:30h
Dissolved Organic Matter in the Yukon River Basin
Source materials, watershed geochemistry, oxidative processes and hydrology exert strong influences on the nature and reactivity of dissolved organic matter (DOM) in aquatic systems. At present, a critical question in carbon cycling is how climate change could alter the fate and chemical nature of dissolved organic carbon (DOC) released from watersheds, particularly those underlain by permafrost, and transported to rivers, lakes, estuaries and coastal waters. The spatial and temporal variability of DOM in surface waters associated with the Yukon River are being studied to better define processes controlling DOM in this system. Like many northern ecosystems, the Yukon River Basin is experiencing melting permafrost, drying of upland soils and changing wetland environments. Study results indicate that the transport of DOM in the river and its major tributaries is strongly seasonally dependent. Specific ultraviolet absorbance (SUVA) data, an excellent indicator of aromatic carbon content of DOM, also indicate a large variation in the chemical nature of the organic matter transported by the river. Lowest DOC concentrations and SUVA values were observed for samples collected in the winter under low flow conditions and for tributaries dominated by ground water inputs. Greatest DOC concentrations and SUVA values were measured on samples collected during the spring on the leading part of the hydrograph. High SUVA values are indicative of greater amounts of organic material originating from higher plants that are present in upper soil horizons and wetlands of the watershed. Aquatic humic substances collected from the Yukon River during the snowmelt period were found to have low nitrogen contents and greater amounts of aromatic C relative to samples from other aquatic environments. Low N content and high aromaticity are indicative of humic substances evolved from higher plant sources with little alteration resulting from microbial degradation or soil interactions. In addition, 14C data indicate that the DOM in the river is comprised of recent C and suggests that, at this stage of permafrost melting, the C signal is dominated by young organic matter. Continued permafrost melting can be expected to result in significant changes in hydrologic flow paths and soil microbial processes in the Yukon River Basin watersheds that will alter both the amount and reactivity of DOM transported by the river.
B24A-04 16:50h
Flood Pulse Influence on Export of Terrestrial Organic Matter
While much attention has been placed on characterizing Terrestrial Organic Matter (TOM) export from large rivers, recent research has shown that in-stream processing of TOM in smaller streams and rivers over shorter time scales can be an important upland component of regional carbon budgets not detected at the outlets of large rivers. With predictions of climate change accompanied by more intense rainfall patterns in some areas, it is important to understand the linkage between flood events and watershed export of TOM. To this end, we have collected water samples from Big Pine Creek watershed, an 850km2 watershed located in west central Indiana. Organic carbon in dissolved, colloidal, and particulate size fractions has been described with molecular and stable carbon isotope techniques to track source, quantity, and compositional changes of TOM over changing flow conditions. Results from these samples show that flood conditions export dramatically more TOM; not only from increases in discharge, but also from increases in concentration of terrestrial organic carbon to all size fractions. While molecular biomarkers show increases in terrestrial organic matter, bulk stable carbon isotope values show that the sources of TOM do not remain constant. Rather, relative contributions from C4 plants (corn in this study area) increase during flood conditions by up to 40 percent. Finally, increases in rainfall intensity are likely to disproportionately increase organic carbon export from terrestrial systems, especially from smaller watersheds where short duration and high intensity flow events dominate annual discharge.
B24A-05 17:05h
High-mountain lakes as a hotspot of dissolved organic matter production in a changing climate
Changes in climate may adversely affect mountain environments before downstream ecosystems are affected. Steep topography, thin soils with limited extent, sparse vegetation, short growing seasons, and climatic extremes (heavy snowfalls, cold temperatures, high winds), all contribute to the sensitivity of high mountain environments to perturbations. Here we evaluate the role of oligatrophic high-elevation lakes as "hot spots" of aquatic production that may respond to changes in temperature, precipitation amount, and pollution deposition faster and more directly than co-located terrestrial ecosystems. Our research was conducted in the Rocky Mountains, USA. Water samples were collected for dissolved organic carbon (DOC), other solutes, and water isotopes over the course of the runoff season along a longitudinal transect of North Boulder Creek in the Colorado Front Range from the continental divide and alpine areas to downstream forested systems. Sources of DOC were evaluated using chemical fractionation with XAD-8 resins and fluorescence spectroscopy. There was net DOC production in the two alpine lakes but not for the forested subalpine lake. Oxygen-18 values showed that water residence times in lakes increased dramatically in late summer compared to snowmelt. Chemical fractionation of DOC showed there was a increase in the non-humic acid content across the summer of 2003 at all elevations, with alpine waters showing greater increases than subalpine waters. The fluorescence properties of DOC and water isotopes suggested that DOC in aquatic systems was primarily derived from terrestrial precursor material during snowmelt. However, fluorescence properties of DOC in high-elevation lakes on the recession limb of the hydrograph suggest DOC derived from algal and microbial biomass in the lakes was a more important source of DOC in late summer and fall. Alpine lakes produced 14 times more DOC on unit area basis compared to the surrounding terrestrial ecosystems. We hypothesize that much of the authochthonous production is a result of algal growth in alpine lakes caused by the increases in nitrogen deposition from wetfall.
B24A-06 INVITED 17:20h
Quantifying Sources and Fluxes of Aquatic Carbon in U.S. Streams and Reservoirs Using Spatially Referenced Regression Models
Organic carbon (OC) is a critical water quality characteristic in riverine systems that is an important component of the aquatic carbon cycle and energy balance. Examples of processes controlled by OC interactions are complexation of trace metals; enhancement of the solubility of hydrophobic organic contaminants; formation of trihalomethanes in drinking water; and absorption of visible and UV radiation. Organic carbon also can have indirect effects on water quality by influencing internal processes of aquatic ecosystems (e.g. photosynthesis and autotrophic and heterotrophic activity). The importance of organic matter dynamics on water quality has been recognized, but challenges remain in quantitatively addressing OC processes over broad spatial scales in a hydrological context. In this study, we apply spatially referenced watershed models (SPARROW) to statistically estimate long-term mean-annual rates of dissolved- and total- organic carbon export in streams and reservoirs across the conterminous United States. We make use of a GIS framework for the analysis, describing sources, transport, and transformations of organic matter from spatial databases providing characterizations of climate, land use, primary productivity, topography, soils, and geology. This approach is useful because it illustrates spatial patterns of organic carbon fluxes in streamflow, highlighting hot spots (e.g., organic-rich environments in the southeastern coastal plain). Further, our simulations provide estimates of the relative contributions to streams from allochthonous and autochthonous sources. We quantify surface water fluxes of OC with estimates of uncertainty in relation to the overall US carbon budget; our simulations highlight that aquatic sources and sinks of OC may be a more significant component of regional carbon cycling than was previously thought. Further, we are using our simulations to explore the potential role of climate and other changes in the terrestrial environment on OC fluxes in aquatic systems.
B24A-07 17:40h
Six year carbon budget of a northern peatland: no evidence for substantive increase in DOC loss due to `biogeochemical drought'
Northern peatlands store between 250 and 500 Gt C, or 10 to 20% of all terrestrial carbon. Accumulation of peat, which comprises partially decomposed dead plant material, results from plant production persistently exceeding decomposition. Decomposition is inhibited by the presence of saturated conditions. Since the hydrology of peatlands is in part of function of their climatic setting (precipitation and temperature) questions have been raised about the sensitivity of this large carbon store to climate change. Recent studies, mainly in British peatlands have suggested an increase in carbon export from peatlands driven by either an increased frequency of what some authors refer to as `biogeochemical drought' and/or increased productivity due to elevated CO$_{2}$. For the last six years we have continuously monitored the carbon input, outputs and change in storage in a northern peatland. The study site is the Mer Bleue bog of the Eastern Peatland Station in the Fluxnet Canada Research Network, located in the transition zone between the cool temperate and boreal ecoclimatic regions. We find net ecosystem production ranges from a sink of approximately 60 g m$^{-2}$ yr$^{-1}$ to a very small carbon source of less than 10 g m$^{-2}$ yr$^{-1}$ . Over the last six years, Mer Bleue has experienced most combinations of warm/cold, wet/dry growing and winter seasons. However, Mer Bleue in 2001 and 2002 was very dry (i.e. far in excess of what is term a 'biogeochemical' drought), as was much of northeastern North America. Even under these conditions Mer Bleue became only a small source of CO$_{2}$ and we saw no substantial increase in the mass loss of carbon as DOC. DOC is an important loss of carbon for the calculation of the net carbon balance since it can be equivalent to between 15 and 100% of the change in the annual carbon store, but DOC loss represents a fairly small mass flux of carbon (e.g. $<$ 10%) compared to the gross annual atmospheric inputs and output. Based on our calculations of the complete carbon balance we find no evidence for a large sustained increase in ecosystem respiration during prolonged dry periods which would be required for increased export of DOC. These results contrast with the hypothesis that increased `biogeochemical droughts' in British peatlands are driving the observed secular increase in DOC in rivers and streams over the last few decades.
B24A-08 17:55h
Controls on DOC Concentrations in Peatlands During Droughts
The role of droughts in controlling the production of dissolved organic carbon (DOC) from peatlands has received much attention recently. Although some experimental work suggests drying should increase DOC production, several field studies have shown reduced concentrations of DOC in stream water during drought years. Several theories have been suggested about the factors controlling DOC under these extreme conditions. These include (i) low hydrologic flows, (ii) physio-chemical changes in peat structure delaying/inhibiting peat re-wetting and (iii) enhanced biological degradation of DOC to carbon dioxide (CO$_{2}$). Few have considered the role of chemical changes in peat soil water as a mechanism for DOC suppression, even though (i) acidity and ionic strength increase in peat waters during droughts in response to the oxidation of reduced sulphur and (ii) both pH and ionic strength affect DOC solubility. Analysis of a 10-year time series (1993-2002), collected as part of the UK Environmental Change Network (ECN) programme, has shown a strong link between sulphate (SO$_{4}$) and DOC dynamics under drought conditions. A statistical model including a function to account for DOC suppression due to drought-induced acidification explained more of the variability of DOC in soil solution (R$^{2}$ = 0.81) than temperature alone (R$^{2}$ = 0.58). This model of peat soil solution DOC at 10 cm depth was extended to explain 74 % of the variation in stream water DOC. As future climate change scenarios suggest an increase in the magnitude and frequency of drought events, these results suggest there is potential for a similar increase in DOC suppression resulting from episodic drought-induced acidification.