Influence of permafrost extent on photochemical reactivity, functional group composition, and geochemical cycling of a subarctic discontinuous permafrost Alaskan watershed

Abstract

Sub-Arctic Alaskan boreal forests are currently extremely susceptible to permafrost thaw caused by increases in atmospheric temperatures in the region. Upon thaw, permafrost soil organic matter can leach out organic matter, nitrogen, and metals. It is important to observe the effects the leaching of permafrost may have on photoreactivity, functional group composition, and metal introduction. Photoproduced reactive oxygen species may affect metal fate and transport through mechanisms such as the photo-Fenton reaction. Functional group analysis allows for differences in natural organic matter source and ability to complex metals throughout a watershed. Additionally, permafrost soils may have the ability to leach in metals through lateral flow of surface waters as observed in other studies. These metals could then complex to organic matter and alter the geochemical cycling within the watershed. Organic matter is a nutrient source, and metals (e.g., As) may increase the toxicity of surface waters through the thaw of permafrost. The influx of sequestered organic matter and metals to surface waters has the potential to drastically alter ecosystem processes. This study observes how permafrost leaching affects water composition, including its overall photoreactivity and functional group composition. The data obtained was then used to observe and deduce conclusions on how permafrost thaw influences surface water photoreactivity and functional group composition. Finally, trace metal analysis was conducted on a whole watershed scale over three years to observe how permafrost influences the geochemical composition of three main thermokarst surface waters with varying degrees of permafrost degradation. Overall, permafrost was determined to be heterogeneous and highly photoreactive both inter- and intra- watershed. Additionally, the functional group composition of surface waters influenced by permafrost thaw was different between summer and winter, indicating that winter is an important period to sample. Due to this change in functional group composition, the photoreactivity of winter samples was higher than summer with regard to the production of reactive oxygen species. Metal concentrations also increased during the winter for lakes identified to be undergoing active permafrost thaw. Finally, this case study found that metal concentration data combined with optical indices provided important information for resolving the possible extent of permafrost beneath thermokarst lakes.