Frozen tundra soils hold one of the Earths largest pools of organic carbon. With global warming, permafrost is thawing at an accelerated rate, promoting the formation of thermokarst ponds. These ponds have become increasingly abundant in high latitude areas, representing up to 90% of all lakes in some regions. They act as recipients of the huge amount of carbon stored in permafrost that, due to thawing, is transported to aquatic ecosystems. Originally transparent ponds with high oxygen production by benthic algae and general net autotrophy are receiving increasing amounts of terrestrial organic material from the changing watershed with consequences for the ecosystem metabolism. Increasing reliance to terrestrial carbon and nutrients stimulate bacteria production and respiration and decrease the light availability to benthic primary production, contributing to making an increasing number of circumpolar freshwaters an important source of greenhouse gases to atmosphere. The combined effect of increasing humic compounds from the watershed and the modifications in the pond metabolism contribute to changes in the composition of the carbon pool in water. The extent and rate of this change are however unknown although shallow ponds are the major freshwater ecosystem type in the circumpolar North and constitute an integral component in the carbon cycling at high latitudes. In order to investigate the influence of permafrost thaw to the stocks and composition of dissolved organic material (DOM) in circumpolar North freshwater systems, we used a suite of chemical (DOC), biological (chlorophyll a), optical (spectrophotometric indexes, EEMs, PARAFAC) and stable isotopic (d13C and d2H) indicators. During the summer periods from 2002 to 2016, we sampled a total of 253 ponds distributed in 14 circumpolar regions, for a total of 356 samples, a fraction of ponds being sampled more than one time during these 15 years. The regions span over a very wide geographic area, covering ~200° in longitude (from Alaska to Russia) and ~30° in latitude (from Subarctic to High Arctic), and resulting in a large range of thawing permafrost influence. We divided the ponds into three categories according to their exposure to permafrost thaw: (1) bedrock ponds, characterized by a bedrock catchment and not directly affected by thawing permafrost; (2) tundra, not impacted by permafrost thaw, but characterized by forest, shrub or desert tundra watersheds, depending on the region; and (3) thaw ponds, directly affected by thawing permafrost.
