Environmental causes of increasing Dissolved Organic Carbon (DOC) in Arctic tundra ponds over the past 40 years

Background/Question/Methods

Active layer is used to describe the upper most layer of soil that thaws each summer in the Arctic. With a warming Arctic, permafrost is expected to thaw and active layer depth to increase, thus releasing organic material and nutrients into aquatic environments; however, there are few long-term datasets with which to test this prediction in aquatic ecosystems. The Arctic tundra ponds at the International Biological Program (IBP) site in Barrow, Alaska, studied for the first time in the 1970s, represent one of the very few locations in the Arctic where long-term data are available on freshwater ecosystem structure and function. The objective of this study was to determine whether Dissolved Organic Carbon (DOC) concentrations in Arctic tundra ponds had changed over time, and how thaw depth, temperature and UV radiation have impacted DOC quantity (mg/L) and quality (SUVA254).  Over the summers of 2010, 11 and 12, we collected water samples and measured thaw depth from 5 IBP ponds and compared these with 1970’s data and to thermokarst ponds, which have experienced increased permafrost thaw.

Results/Conclusions

On average, ponds were 2°C warmer in the 2000’s, compared to the 1970’s, and there were fewer Accumulated Degree Days of thaw in the 1970’s. Maximum thaw depth was 13-19 cm deeper in 2011-2, which likely impacted both the infiltration of surface water into the ground and the release of organic (e.g. DOC) and inorganic compounds (e.g. phosphorus, nitrogen) from formerly frozen organic ground into the ponds. For example, DOC was significantly higher in 2009-2011 compared to the 1970s; this was most notable later in the growing season. This study will add to our understanding of the changes that warmer temperatures and altered aquatic environments bring to the Artic and the consequences for carbon budgets in northern latitudes.