Sources and export fluxes of inorganic and organic carbon and nutrient species from the seasonally ice-covered Yukon River

Abstract

Climate and environmental changes are having profound impacts on Arctic river basins, but the biogeochemical response remains poorly understood. To examine the effect of ice formation on temporal variations in composition and fluxes of carbon and nutrient species, monthly water and particulate samples collected from the lower Yukon River between July 2004 and September 2005 were measured for concentrations of organic and inorganic C, N, and P, dissolved silicate (Si(OH) 4 ), and stable isotope composition (dD and d 18 O). All organic carbon and nutrient species had the highest concentration during spring freshet and the lowest during the winter season under the ice, indicating dominant sources from snowmelt and flushing of soils in the drainage basin. In contrast, inorganic species such as dissolved inorganic carbon (DIC) and Si(OH) 4 had the highest concentrations in winter and the lowest during spring freshet, suggesting dilution during snowmelt and sources from groundwater and leaching/weathering of mineral layer. The contrasting relation with discharge between organic, such as dissolved organic carbon (DOC), and inorganic, such as DIC and Si(OH) 4 , indicates hydrological control of solute concentration but different sources and transport mechanisms for organic and inorganic carbon and nutrient species. Concentration of DOC also shows an inter-annual variability with higher DOC in 2005 (higher stream flow) than 2004 (lower stream flow). Average inorganic N/P molar ratio was 110 ± 124, with up to 442 under the ice and 38-70 during the ice-open season. While dissolved organic matter had a higher C/N ratio under the ice (45-62), the particulate C/N ratio was lower during winter (21-26) and spring freshet (19). Apparent fractionation factors of C, N, P, Si and dD and d 18 O between ice and river water varied considerably, with high values for inorganic species such as DIC and Si(OH) 4 (45 and 9550, respectively) but lower values for DOC (4.7). River ice formation may result in fractionation of inorganic and organic solutes and the repartitioning of seasonal flux of carbon and nutrient species. estimation without spring freshet sampling results in considerable underestimation for organic species but significant overestimation for inorganic species regardless of the flux estimation methods used. Without time-series sampling that includes frozen season, an over-or under-estimation in carbon and nutrient fluxes will occur depending on chemical species. Large differences in carbon export fluxes between studies and sampling years indicate that intensive sampling together with long-term observations are needed to determine the response of the Yukon River to a changing climate.

Sources and sinks of carbon in boreal ecosystems of interior Alaska: A review

“…Of particular note, nutrient exports are rapidly f lushed from surface soils during large discharge periods such as snow melt runoff or major summer precipitation events. This is consistent with results from studies in other subarctic and arctic rivers (Striegl et al, 2005Raymond et al, 2007;Walvoord and Striegl, 2007;McNamara et al, 2008;Frey and McClelland, 2009;Guo et al, 2012). The DOC mobilized during high f low events is generally believed to be of modern age (Guo and Macdonald, 2006;Neff et al, 2006) and part of the DOC transported during spring melt may be labile (Holmes et al, 2008).…”

Insights and issues with simulating terrestrial DOC loading of Arctic river networks

“…While our overall estimates of terrestrial DOC loading of pan-Arctic rivers are about the same magnitude as riverine DOC export to the Arctic Ocean determined by other studies, whose empirical estimates also have substantial uncertainties (e.g., Guo et al 2012), discrepancies between model results and observations at several major pan-Arctic rivers indicate that a number of issues still need to be resolved before a terrestrial biogeochemistry model like TEM can reliably estimate the transfer of DOC between land and the Arctic Ocean.…”

Seasonal Variations of CDOM Optical Properties in Rivers Across the Liaohe Delta

“…Sobek et al (2007) found that the catchment, soil, and climate significantly affected DOC concentrations, and mean annual runoff and precipitation were negatively correlated with DOC concentrations in large lake data set from 6 continents. Strong positive relationships between DOC concentrations and river discharge in the arctic river basin were found in previous investigations (Neff et al 2006;Guo et al 2012). In this study, DOC concentrations measured in April were significantly higher than those previously observed in January and September, as soils began to thaw in the spring and drained into nearby streams through surface runoff.…”