Tracking Catchment-Scale Shifts In The Magnitude And Partitioning Of Carbon Export In Response To Changing Climatic Conditions
Recent trends towards warmer and drier summers in the Great Lakes-St. Lawrence Forest Region is leading to fundamental changes in the magnitude of and partitioning between atmospheric and aquatic carbon export from forested catchments. In this study, a distributed network of sampling stations was established to assess carbon pools and fluxes within a forested catchment. Samples were collected to estimate the size of the soil carbon pool, the ability of the soils to sorb dissolved organic carbon (DOC), the DOC and carbon dioxide (CO2) efflux along hillslope transects (with crest, backslope, footslope, toeslope, and wetland) as well as DOC export to the stream over a six year period (2003 to 2008). Topographically controlled hydrologic flows were found to influence the distribution of carbon substrates and their fate. When the continuum of upland- wetland-stream was hydrologically disconnected, DOC was effectively trapped and subsequently emitted to the atmosphere. DOC that flowed down the backslope accumulated at the footslope and created "mobile" DOC and a CO2 hotspot. As DOC continued to move down to the toeslope, it sorbed onto iron and aluminum oxyhydroxides and created "immobile" DOC and another CO2 hotspot. However, when the continuum of upland-wetland-stream or wetland-stream was hydrologically reconnected, the remaining DOC was mobilized and exported to the stream. The recent trend towards warmer and drier summers is resulting in increased total annual carbon export from this forested catchment, with rates of increase higher in atmospheric export (CO2) compared to aquatic export (DOC), but with reduced DOC export during summers which may have significant effects on downstream aquatic ecosystems dependent on terrestrial carbon subsidies.
The Importance of Hydroscape Complexity in Regional Aquatic Carbon Dynamics
While past research has emphasized the importance of watershed composition, very little is known regarding how the configuration of different aquatic elements affects the biogeochemical signatures of surface waters. We investigated variation in watershed biogeochemistry in a lake-rich region to determine if relationships existed between hydroscape complexity (i.e., composition and configuration of the aquatic network) and stream biogeochemical signatures. To address this question, we quantified the biogeochemical attributes of 52 streams representing 3 distinct hydroscape categories in the Northern Highlands Lake District, Wisconsin, USA. Hydroscape categories were based on the presence/absence of lakes and the position of lakes within the watershed. Headwater stream watersheds (HSW) contained no lakes within the stream network while the other two categories both contained lakes. Headwater lake watersheds (HLW) contained a single headwater lake (i.e., no stream inlets and a single stream outlet), and drainage lake watersheds (DLW) contained at least one drainage lake (i.e., a lake with a stream inlet and single outlet) at the base of the watershed. At the regional scale, wetlands exerted a strong influence on watershed dissolved organic carbon (DOC) concentrations. The amount of wetlands in the watershed explained 50% of the variation in watershed DOC concentrations. However, variables that characterized hydroscape complexity appeared to have little or no influence on watershed DOC concentrations. While prior studies have observed that lakes can decrease watershed DOC concentrations, at the regional scale, our results indicate that the presence and location of lakes within the hydrologic networks had not influence on DOC concentrations. Taken together, these results suggest that although aquatic ecosystems are important factors in both lake and global carbon budgets, they do not have a significant influence on aquatic DOC dynamics in the NHLD region. Consequently, we suggest that within this region and potentially at the regional scale, understanding other factors such as wetland composition and hydrologic connectivity are likely more important drivers of aquatic DOC than processes occurring within the hydrologic network.
Export of Dissolved Organic Carbon From the Penobscot River Basin in North-Central Maine
Climatic, hydrologic, and landscape controls on the export of dissolved organic carbon (DOC) from the Penobscot River and its major tributaries in Maine are evident from data collected during 2004 through 2007. DOC flux was determined using continuous discharge measurements, discrete water sampling, and the LOADEST regression model. The average daily flux during 2004 through 2007 was 392 Mt C per day (7.11 Mt C per sq km per yr) which is higher than most northern temperate and boreal rivers that have been studied. Distinct seasonal variation was observed in the relation between concentration and discharge (C—Q). During June through December (summer/fall) there was a relatively steep positive C—Q where concentration increased by a factor of 2 to 3 over the range of observed stream discharge for the Penobscot River near Eddington. In contrast, during January through May (winter/spring) concentration did not increase with increasing discharge. In addition, we observed a major shift in the C—Q between 2004-2005 and 2006-2007, apparently resulting from unprecedented rainfall, runoff, and soil flushing beginning the late fall of 2005. The relative contribution to the total Penobscot River basin DOC flux from each tributary varied dramatically by season reflecting the role of large regulated reservoirs in certain basins. DOC concentration and flux were highest in tributaries containing the largest areas in palustrine wetlands. DOC concentration was positively correlated to percentage wetland area and negatively correlated to total forested area and average slope.
Natural Variability in Dissolved Organic Carbon and Dissolved Organic Nitrogen Transport in Artificially Drained Landscapes of the U.S. Midwest.
Nutrient inputs into coastal ecosystems have become a major issue with serious consequences for water quality; nonetheless, few studies focus on N and C transport during storms in Midwestern tile-drained fields in spite of the known importance of N and C export from artificially drained landscapes of the U.S. Midwest on the development of the "Dead Zone" in the Gulf of Mexico in the summer. Monitoring tile-drain flow at a high temporal resolution during storms is difficult and expensive and little information is therefore available on natural spatial and temporal variability in dissolved organic nitrogen (DON) and carbon (DOC) concentration dynamics in tile drains during storms. This lack of information on DON and DOC export patterns from tile drains hinders our ability to accurately and precisely estimate total nutrient loads and hydrological processes regulating the movement of these nutrients from the watershed to streams via tile flow. The objective of this research is therefore to characterize natural variability in DOC and DON export in two seemingly identical tile drains (length, contributing area) in Leary Weber Ditch; a small (7.6 km2) artificially drained agricultural watershed near Indianapolis, IN. Four storms with various characteristics (intensity-duration) were sampled in May and June 2008. The two studied tile-drains showed different flow and discharge patterns but similar spatial and temporal patterns of DOC and DON concentrations. Large inter-storm variability in DOC and DON concentration patterns were attributed to different storm intensities and soil pre-event moisture conditions. Multiple other storms are currently being monitored to further characterize natural variability in nitrogen and carbon exports in the studied tile drains and identify primary hydrological controls (season, vegetation development stage, antecedent moisture conditions…) regulating N and C export in tile drains over a 12-month period. This work will inform model development and help better characterize processes regulating N and C exports in artificially drained landscapes of the U.S. Midwest and other artificially drained agricultural lands around the world.
Predicting the Fate of Dissolved Organic Carbon Within Forest Mineral Soils
The soil-solution partitioning of dissolved organic carbon (DOC) within mineral soil horizons is primarily controlled by processes of adsorption and desorption. These abiotic processes largely occur within a short equilibration time of seconds to minutes, which generally occur faster than microbial processes. To characterise the adsorption of DOC to mineral soils, we used the Langmuir adsorption isotherm, which holds several advantages to the commonly used linear initial mass (IM) isotherm. One advantage to using the Langmuir isotherm is an estimation of the maximum DOC adsorption capacity (Qmax). The Qmax estimates the number of remaining DOC binding sites available on the mineral soil surface. We also modified the traditional Langmuir isotherm in order to estimate the DOC desorption potential of native soil organic matter. Sorption characteristics were derived for a broad range of 52 mineral soils collected from 17 soil profiles spanning across Canada from British Columbia to Québec. Mineral horizons with the greatest Qmax included the Fe-enriched B horizons of Podzols and Volcanic soils, followed by B horizons not enriched in Fe, followed by A and C horizons. Podzol B horizons were distinct from all other horizons due to significantly higher desorption potential. Soil properties predicting the adsorption characteristics of DOC also predicted the adsorption characteristics of dissolved organic nitrogen (DON). Adsorption of DOC and DON were tightly coupled (R2 = 0.86), however the ratio of DOC:DON in the final equilibrium solution lowered for 48 out of 52 minerals horizons. A short-term (32 day) incubation was performed to establish the fate of indigenous SOC relative to newly adsorbed SOC to four mineral soils with distinct adsorption characteristics. Soil columns were leached periodically and sampled for DOC and CO2 production. Two Fe-enriched mineral horizons with high adsorption capacity released low amounts of old SOC, yet released almost all of the newly adsorbed SOC. In contrast, two B horizons without Fe-enrichment released greater amounts of old SOC, and retained a greater fraction of the newly adsorbed SOC than the Fe-enriched horizons. These results identify a contrast between the fate of indigenous and newly adsorbed SOC on mineral soils with differing Qmax.
Carbon Cycling in Alpine and Arctic watersheds affected by permafrost degradation: An insight from Sweden
Linking the processes and dynamics acting within and between terrestrial and aquatic ecosystems is crucial in order to understand the impacts of environmental change on the re-distribution and transformation of energy within watersheds. Nearly 1300 Pg of carbon are stored in permafrost soils in boreal and arctic ecosystems. Permafrost degradation can result in the loss of significant amounts of terrestrial carbon, both through the release to the atmosphere in the form of carbon dioxide and methane, or through export downstream to lakes and rivers. The fate and effects of this carbon in lake ecosystems is poorly understood. We investigated the capacity of lake bacteria to utilize carbon from different adjacent mire soils in a discontinuous permafrost region of northern Sweden. We, additionally, studied other lake ecosystems by using organic matter quality as a proxy for the state of permafrost degradation within the watershed. Finally, we propose simple predictive models for the bioavailability of soils to aquatic bacteria. Our study identified three distinctive time sensitive pools of bacterial respiration whose carbon availability varied according to chemical characteristics. Soil dissolved organic carbon (DOC) was rapidly consumed by lake bacteria with nearly 85% consumed within the first 24 hours. Bacterial production was higher in the soil bioassays and increased in a lag fashion relative to bacterial respiration, resulting in increasing bacterial growth efficiencies over time as a function of C pool and soil type. The mean DOC consumption by lake bacteria was 0.087 mg C L-1 d-1 and varied between 0.382 mg L-1 d-1 and 0.491 mg L-1 d-1 when supplied with terrestrial DOC. The lake water bacterial respiration could explain a varying degree of pCO2 saturation in lakes as a function of both carbon quality and course. Carbon quality and end members can be used as proxies for the degree of permafrost degradation within the watershed. The data clearly show that export of DOC from degrading permafrost mires can have pronounced effects on the energy mobilization of receiving lakes and on the land- atmosphere carbon balance.
Landscape Change, Organic Carbon Mineralization, and Carbon Export in Northern River Basins
Climate warming, reservoir construction, and other landscape changes alter hydrologic flow paths of
watersheds and affect the amount and chemical composition of aquatic carbon exported from them.
Permafrost thaw changes the routing and the timing of water flow to rivers in subarctic and arctic watersheds,
allowing for increased infiltration and increased ground water contribution to stream flow. This quantitatively
enhances terrestrial mineralization of organic material in soil and along groundwater flow paths, alters the
chemical and isotopic composition of the exported organic carbon, and decreases the ratio of dissolved
organic to inorganic carbon exports. Using the Yukon River basin of northwest Canada and central Alaska as
an example, I will discuss measured and expected basin-scale changes in carbon export in response to
climate warming and relate them to changes observed in contributing watersheds. I will also explore potential
consequences of climate warming and land use change on carbon exports across the Arctic Ocean basin.