Increasing Dissolved Organic Carbon (DOC) in Arctic tundra ponds over the past 40 years

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 these predictions 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, how changes in thaw depth and temperature have impacted DOC quantity, and what the consequences of any changes have been for aquatic ecosystems. Over the summers of 2010-13 we collected water samples and measured thaw depth from 5 IBP ponds and compared these with data from the 1970s. Ponds were 2°C warmer and thaw depth was up to 19cm deeper in the 2000s as compared to the 1970s. Release of organic (e.g. DOC) and inorganic compounds (e.g.nitrogen) from formerly frozen organic ground into the ponds was associated with the increased thaw depth. For example, DOC was significantly higher in 2009-2012 compared to the 1970s; this was most notable later in the growing season. Similarly, incubations experiments indicate a steady rise in DOC released from permafrost with warming. DOC is likely a relatively conservative tracer of permafrost thaw in aquatic systems, and thus was a better predictor of increased phytoplankton levels than inorganic nutrients. This study will add to our understanding of the changes that warmer temperatures and altered aquatic environments bring to the Arctic and the consequences for carbon budgets in northern latitudes.