Biogeochemistry of “pristine” freshwater stream and lake systems in the western Canadian Arctic

Climate change poses a substantial threat to the stability of the Arctic terrestrial carbon (C) pool as warmer air temperatures thaw permafrost and deepen the seasonally-thawed active layer of soils and sediments. Enhanced water flow through this layer may accelerate the transport of C and major cations and anions to streams and lakes. These act as important conduits and reactors for dissolved C within the terrestrial C cycle. It is important for studies to consider these processes in small headwater catchments, which have been identified as hotspots of rapid mineralisation of C sourced from ancient permafrost thaw. In order to better understand the role of inland waters in terrestrial C cycling we characterised the biogeochemistry of the freshwater systems in a c. 14 km2 study area in the western Canadian Arctic. Sampling took place during the snow-free seasons of 2013 and 2014 for major inorganic solutes, dissolved organic and inorganic C (DOC and DIC, respectively), carbon dioxide (CO2) and methane (CH4) concentrations from three water type groups: lakes, polygonal pools and streams. These groups displayed differing biogeochemical signatures, indicative of contrasting biogeochemical controls. However, none of the groups showed strong signals of enhanced permafrost thaw during the study seasons. The mean annual air temperature in the region has increased by more than 2.5 °C since 1970, and continued warming will likely affect the aquatic biogeochemistry. This study provides important baseline data for comparison with future studies in a warming Arctic

Aquatic carbon and GHG export from a permafrost catchment; identifying source areas and primary flow paths

The aquatic pathway is increasingly being recognized as an important component of landscape scale greenhouse gas (GHG) budgets. Due to low temperatures and short residence times limiting in-stream production in northern headwater catchments, much of the exported carbon is likely to be allochthonous, transported via throughflow to the surface drainage system. Identifying sources and primary flow pathways is therefore essential in understanding and predicting changes in the aquatic flux magnitude. Arctic landscapes are now widely recognised as being particularly vulnerable to climate driven changes. The HYDRA project (“Permafrost catchments in transition: hydrological controls on carbon cycling and greenhouse gas budgets”) aims to understand the fundamental role that hydrological processes play in regulating landscape-scale carbon fluxes, and predict how changes in vegetation and active layer depth in permafrost environments influence the delivery and export of aquatic carbon. In this study we present aquatic concentrations and fluxes of carbon and GHG species collected across two field seasons (2013, 2014) from an arctic headwater catchment in northern Canada. Measured species include dissolved organic (DOC) and inorganic carbon (DIC), CO2, CH4 and N2O. Measurements were made across a range of freshwater types within the tundra landscape, including lakes, ice-wedge polygons, and the ‘Siksik’ stream which drains the (c.a. 1 km2) primary study catchment. A nested sub-catchment approach was used along the ‘Siksik’ stream; ‘snapshot’ sampling of eight points along the stream length allowed specific vegetation communities to be targeted to assess individually their contribution to aquatic export. A combination of stable isotopes and major ion concentrations measured at each sampling point provide additional information to trace source areas and flow paths within the main study catchment. Catchment scale evasion and downstream export were calculated and an initial comparison between the relative importance of different water body types presented

Utilising conservative tracers and spatial surveys to identify controls on pathways and DOC exports in an Arctic catchment

Dissolved organic carbon (DOC) is typically the predominant form of carbon exported from headwater streams, it therefore represents a major carbon export from Arctic catchments. The projected deepening of thaw depth in permafrost regions, due to an increase in air temperature, may have a significant effect on the amount of DOC exported from these systems. However, quantification of the impacts of climate driven changes on DOC export are still highly uncertain. Understanding the processes controlling DOC export is therefore crucial in predicting the potential impact of projected environmental changes. The controls of DOC production and transport are heavily influenced by soil and vegetation, which are highly variable across the landscape. To completely understand these systems information regarding spatial variability of plants, soils and thaw depths must be taken into account. In this study sub-weekly sampling of DOC was undertaken throughout 2014 in a headwater (<1 km2) catchment in the Northwest Territories, Canada. Spatial surveys of soil properties, active thaw depth and normalised difference vegetation index (NDVI) were collected and used in conjunction with conservative stable water isotopes tracers and major ions to understand sources, flow pathways and timing of DOC exports from the catchment. Stable isotope tracers act as fingerprints of water allowing sources and pathways to be assessed. Observations reveal changing DOC concentrations throughout the season as the active layer deepens and the connectivity of the soils to the stream network throughout the catchment increases. Linking the DOC data with the conservative tracer response improves the identification of carbon pathways and fluxes from the soils; preliminary analysis indicates DOC is being delivered via deeper more mineral soils later in the season. The results indicate that the active layer depth has a strong influence on the amount of DOC exported from the system, independent of the amount of carbon stored in these deeper soils

The age of CO2 released from contrasting ecosystems during the arctic winter

In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2 release, yet the source of this C is not clear. We analysed the 14C content of C released from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2 was found to be a similar age (tundra) or older (forest) than C released during the previous autumn. Overall, our study demonstrates that the decomposition of older C can continue during the winter, in these two contrasting arctic ecosystems

Abundant pre-industrial carbon detected in Canadian Arctic headwaters: implications for the permafrost carbon feedback

Mobilization of soil/sediment organic carbon into inland waters constitutes a substantial, but poorly-constrained, component of the global carbon cycle. Radiocarbon (14C) analysis has proven a valuable tool in tracing the sources and fate of mobilized carbon, but aquatic 14C studies in permafrost regions rarely detect 'old' carbon (assimilated from the atmosphere into plants and soil prior to AD1950). The emission of greenhouse gases derived from old carbon by aquatic systems may indicate that carbon sequestered prior to AD1950 is being destabilized, thus contributing to the 'permafrost carbon feedback' (PCF). Here, we measure directly the 14C content of aquatic CO2, alongside dissolved organic carbon, in headwater systems of the western Canadian Arctic—the first such concurrent measurements in the Arctic. Age distribution analysis indicates that the age of mobilized aquatic carbon increased significantly during the 2014 snow-free season as the active layer deepened. This increase in age was more pronounced in DOC, rising from 101–228 years before sampling date (a 120%–125% increase) compared to CO2, which rose from 92–151 years before sampling date (a 59%–63% increase). 'Pre-industrial' aged carbon (assimilated prior to ~AD1750) comprised 15%–40% of the total aquatic carbon fluxes, demonstrating the prevalence of old carbon to Arctic headwaters. Although the presence of this old carbon is not necessarily indicative of a net positive PCF, we provide an approach and baseline data which can be used for future assessment of the PCF