B41D-0751
A Multi-scale, Multi-method Approach to Examining the Effects of Landscape Age on Dissolved Organic Carbon and Nitrogen Movement in Arctic Alaska
Studies of arctic terrestrial carbon cycling have emphasized gaseous carbon fluxes, with less attention to dissolved carbon fluxes. However, recent work suggests that dissolved fluxes could be significant components of arctic terrestrial carbon budgets. Nitrogen availability exerts a strong control on terrestrial C balance through effects on plant productivity. Although most nitrogen in arctic systems is found in the organic form, little is known about the controls on dissolved organic nitrogen (DON) production and transport at high latitudes. We examined regional patterns of dissolved organic carbon (DOC) and DON production and concentrations over a glacial age chronosequence in northern Alaska. We used soil incubations, lysimeters, and stream sampling to study the relative importance of glacial age and associated differences in geochemistry at the plot, hillslope and watershed scale. Landscape age significantly affected DOC and DON production in soil incubations and DOC and DON concentrations in soil water at the plot scale. The importance of landscape age diminished at the more integrative hillslope and watershed scales, perhaps due to the influence of hillslope vegetation or riparian soils. DOC and DON concentrations were more correlated in laboratory incubations without vegetation and than at the hillslope and watershed scale. Two possible influences on dissolved carbon production that vary with landscape age are differences in exchangeable calcium and pH, which decrease over time as a result of weathering. In a factorial soil manipulation experiment with samples from multiple sites across the glacial age chronosequence, both decreasing pH and increasing exchangeable calcium concentrations significantly reduced the production of dissolved organic carbon. Our results suggest that landscape age is an important control on dissolved fluxes of carbon and nitrogen, but the effect is dependent on the scale of measurement.
B41D-0752
Where does boreal stream DOC come from? – Quantifying the contribution from different landscape compartments using stable C isotope ratios.
Stable carbon isotope (δ13C) ratios are frequently used as a source tracer of e.g. organic matter (OM) produced in terrestrial versus aquatic environments. To our knowledge there has been no previous attempt to quantify the relative contribution of dissolved organic carbon (DOC) from various landscape compartments in catchments of different sizes. Here, we test to what extent δ13C values can be used also to quantify the relative contribution of DOC from wetlands/riparian zones along streams, and off stream forest habitats, respectively. We present data on spatial and temporal variability of DOC concentrations and δ13C-DOC values, during the year of 2005 in Krycklan catchment, a boreal stream network in northern Sweden. Ten stream sites, ranging from order 1 to 4, were monitored in sub catchments with different wetland coverage. Spatial variation of DOC concentration showed a weak but statistically significant relationship with wetland area, with higher concentration with increasing percent of wetland in the drainage area. During base flow the difference in δ13C-DOC values was significantly different between forest (-27.5‰) and wetland (-28.1‰). This spatial pattern disappears during spring peak flow when higher discharge flushing upper soil layer and the riparian zone on DOC in the catchments. A simple mixing model using DOC and δ13C-DOC showed that stream water DOC could be describe as a mixture of DOC coming from forest (deep) groundwater and wetland/riparian zone water. The result indicates that during spring peak flow almost all stream DOC (84-100%) is derived from wetlands and riparian zones. The wetland/riparian water dominates the stream DOC flux at all hydrological events, except for two sites, one forest dominated and one mixed catchment, where the forest groundwater dominated the DOC transport during base flow. Although the total wetland area in Krycklan catchment only represent 8.3%, it contributed, together with riparian zones, to as much as 83% of the yearly DOC transport. This study shows that there is a great potential in using stable carbon isotopes to quantify the relative contribution of DOC from various landscape compartments in catchments. Quantitative patterns are crucial for several reasons. It is for example necessary in predicting the response to global warming which will result in a changed hydrology and shifts in the relative area of the landscape compartments in boreal environments. KEY WORDS carbon isotopes; dissolved organic carbon; streams; boreal; landscape compartments; wetland; groundwater
B41D-0753
Chemical Character and Biodegradability of Dissolved Organic Carbon in Streams and Rivers of Interior Alaska
The amount and chemical character of dissolved organic carbon (DOC) in streams and rivers in subarctic Alaska vary according to watershed type and season. In general, DOC concentrations and aromaticity are greatest in streams draining wetland-dominated catchments, while streams that are fed by groundwater or glacial meltwater have lower DOC concentrations and aromaticity. It is not well known, however, whether there are correspondingly wide ranges in the proportions of biodegradable DOC (BDOC), which is important to our understanding of carbon cycling in high latitude systems. We sampled three large rivers and five streams draining various catchment types in the Yukon River basin in Alaska in late winter (under ice), mid-summer, and autumn 2007 for DOC concentration, chemical character, and BDOC. Chemical character of the DOC was determined by fractionation, specific ultraviolet absorbance (SUVA), and fluorescence. BDOC was determined by 3-month incubations, with and without nutrient additions. The concentration and chemical character of DOC varied widely with season, such that DOC concentration, aromaticity (measured as SUVA), and hydrophobic acid content are at a minimum in winter (~ 2-3 mg C L-1, SUVA < 3.0), peak during spring snowmelt (> 7 mg C L-1, SUVA > 3.0) and remain relatively high through summer and autumn. BDOC varied from 10% to 30% of total DOC, and was typically greater when nutrients were added. In addition to characterizing the DOC analyses and incubations, we will explore potential links among watershed type, season, and BDOC.
B41D-0754
Regional DOC Fluxes for the Tongass National Forest in Southeast Alaska
The Tongass National Forest (Tongass) is the largest national forest in the United States and contains most of the remaining high value Coastal Temperate Rainforest in North America. The Tongass covers 68,748 km2 of southeast Alaska with 42,000 km of coastline. Annual rainfall ranges from 1.5 to 8 meters. The Tongass contains about 8% as much total carbon as forests in the lower 48 states and 90% of the Tongass lies within 5 km of saltwater, creating a high potential for terrestrial influence on the near-shore environment. Riverine carbon from the Tongass flows into a web of estuaries and channels within the Alexander Archipelago. In contrast to riverine inputs to the open ocean, Tongass carbon is likely retained near the coastline for longer duration and may exert a larger influence on coastal productivity. An important impact of climate change on the carbon cycle is in the transfer of dissolved organic carbon (DOC) from terrestrial to aquatic ecosystems. We have combined a regional survey of riverine DOC concentrations with estimates of climatic and hydrologic factors to estimate the present DOC flux to the marine ecosystem of southeast Alaska. The preliminary flux estimate yields an annual runoff of 960 gigagrams DOC y-1 from the Tongass (140 kg C ha-1 y-1). Streamwater dissolved organic carbon concentrations vary widely with catchment properties such as wetland extent and slope, with values ranging from <0.1 mg l-1 to 20 mg l-1. Wetland soils cover 30% of the Tongass and are the primary source of DOC and associated variability in concentration in coastal watersheds. The large variation in DOC flux among watersheds with differing geographic composition suggests that the pattern of terrestrial influences on marine process will also vary as will the sensitivity of exports to changes in soil retention and processing caused by climate change.
B41D-0755
Breaking Down the Yukon River: Analysis of Solute Chemistry in Yukon River Tributaries
The Yukon River (YR), one of the largest unregulated rivers in the northern hemisphere, drains a diverse assemblage of terrain from glacierized mountains to lowland peatlands underlain by permafrost. Tributaries to the YR can be characterized by the nature of the suspended sediment, inorganic solutes, and dissolved organic matter they carry depending on the topography, geology, and ecology of the individual watersheds. In order to understand how these landscape characteristics affect solute flux from the tributaries we examine surface water data from 43 tributaries to the YR in Alaska and Canada. Principle component analysis (PCA) based on 9 dissolved constituents has been applied to the major tributaries of the YR and has successfully reduced the dimensionality of the data set through non-subjective groupings of variables. Five principle components explain approximately 90% of the total variance in dissolved constituents among rivers within the YR basin that represent 85% of the total Yukon River area. The first principle component (PC 1) shows strong negative (<-0.6) correlations with the major dissolved ions (Ca2+, Mg2+, SO42-, HCO3-) and Sr. PC 2 shows strong positive (>0.6) correlations with Na+, Cl- and SiO2. PC 3 is represented by inverse loadings of SiO2 (-0.6) and Cl- (0.6). PC 4 is a nearly unique loading for K+ (-0.7), with additional weak loadings for SiO2 (0.3) and Mg2+ (0.2). PC 5 shows weak positive loadings for Ca2+ (0.2) and HCO3- (0.4) and weak negative loadings for Sr (-0.2) and SO42- (-0.3). This study demonstrates the usefulness of PCA for simplifying a set of surface water solute data for a major river system in order to establish the important factors controlling river chemistry. It is expected that further analysis will resolve specific correlations between the five PC's and subbasin characteristics such as percent of land covered by wetlands, glacier extent, permafrost distribution, and type of underlying bedrock in YR tributaries.