The studied 253 ponds spanned a wide gradient of environmental conditions, from transparent oligotrophic waterbodies in areas not affected by thawing permafrost, to humic‐ and nutrient‐rich ponds exposed to thermal erosion (Fig. 2 ). Nutrients in bedrock and tundra ponds were typical of oligotrophic freshwater systems, and comparable to values usually found in clear‐water high‐latitude ponds (Rautio et al. 2011 ). In thaw ponds, they showed values more characteristic of mesotrophic and eutrophic systems, and likely originated from the eroding catchment (Larsen et al. 2017 ). Following the low nutrient concentrations in the water column, Chl a values in bedrock and tundra ponds indicated low phytoplankton biomass. Consequently, most primary production in clear‐water ponds is produced by the benthic mat and biofilm communities, with phytoplankton often representing less than 2% of the total photosynthetic biomass in these systems (Bonilla et al. 2005 ; Rautio et al. 2011 ). In thaw ponds, more elevated Chl a values suggested a higher planktonic primary production, more probably supported by the higher nutrient concentrations (Vonk et al. 2015 ). However, the higher CDOM concentrations and suspended solids in thaw ponds efficiently attenuate the solar radiation (Watanabe et al. 2011 ), limiting benthic and therefore overall primary production (Vadeboncoeur et al. 2008 ). Increased terrestrial DOM in circumpolar surface waters could therefore lead to a considerable decrease in the light availability for photosynthesis, resulting in a shift toward a heterotrophic production‐based food web as has been documented in experimental conditions (Forsström et al. 2015 ), and a high production of CO 2 and CH 4 (Roiha et al. 2015 ).

DOC and CDOM properties

Highest DOC and CDOM values were observed in the thaw ponds, with several CDOM proxies indicating higher terrestrial inputs from the catchment (Fig. 2). Similar accumulation of DOM has also been reported previously at circumpolar sites with high terrestrial inputs (Vonk et al. 2013; Abbott et al. 2014; Roiha et al. 2015). The elevated values of a 320 and a 440 in thaw ponds indicate high concentrations of CDOM, inducing more light attenuation in the water column. Consistent with earlier studies, S R , S 289 , and SUVA 254 indicated fresher aromatic compounds of higher molecular weight and a large proportion of terrestrial vs. algal carbon sources in thaw ponds (e.g., Roiha et al. 2015). This high degree of DOM allochthony in thaw ponds was also supported by low values of FI, an indicator of large inputs of fulvic acids from terrestrial sources and a function of carbon storage in the catchment (Rantala et al. 2016).

The association of [C1] and [C2] with SUVA 254 and other CDOM proxies in ponds influenced by thawing permafrost also indicates strong DOM allochthony in these waters (Fig. 3A), with high inputs of DOM from the catchment. C3 also had fluorescence characteristics of humic materials from a terrestrial origin, but showed an opposite association with C1 and C2. This suggests that C3 could be the product of biological transformation of C1 and C2 in the water column (Jørgensen et al. 2011). C4 matched well with humic materials of microbial origin, and therefore can be linked to the degradation of both algal and terrestrial sources. Finally, the amino acid‐ or protein‐like algal component C5 showed higher values in bedrock and tundra ponds, and negative relationships with SUVA 254 and thawing permafrost, indicating a greater algal origin of DOM in non‐thaw ponds. This algal signature likely reflects the benthic primary production in these ponds, given its dominant contribution to overall algal biomass in clear‐water circumpolar ponds (Rautio et al. 2011).

It is important to note that we focused on thermokarst ponds, with permafrost thaw and degradation along pond banks, as is commonly found across the North. There are other modes of permafrost thaw that can have different consequences for surface‐water DOM concentrations and composition. For instance, in certain hydrological conditions, catchment‐scale permafrost thaw via active layer thickening or permafrost loss can reduce DOC concentrations or SUVA 254 values (Cory et al. 2013; O'Donnell et al. 2014). The highly variable organic carbon content of thawing permafrost soils (Vincent et al. 2017) may also influence the DOM properties in the receiving waterbodies. In this study, we did not measure the organic carbon content in the watershed, but the existing information from the study regions (Bouchard et al. 2015; Vincent et al. 2017) as well as our observations of the bank morphology and benthic substrates indicate that the studied thaw ponds were predominantly located in organic‐rich sites. Therefore, substantial impacts of permafrost thaw on pond DOM were expected.

Thawing of ice‐rich permafrost appears to have a strong effect on the ratio of allochthonous to autochthonous DOM in surface waters, resulting from direct inputs of allochthonous DOM from eroding permafrost soils, and from its effect on DOM age (O'Donnell et al. 2014), in situ transformations and respiration (Laurion and Mladenov 2013; Cory et al. 2014). In particular, the level of bacterial and photochemical transformation of terrigenous DOM is likely to vary between the turbid and bacteria‐rich thaw ponds as compared to clear and oligotrophic waters of non‐thaw ponds, with important consequences on DOM allochthony. Moreover, drivers that are independent of permafrost thaw could potentially explain part of the differences observed between thaw and non‐thaw ponds. These drivers include the composition, size and slopes of the catchment (Olefeldt et al. 2014; Vonk et al. 2015), influence of groundwater and precipitation (Olefeldt et al. 2013), water retention time (Catalán et al. 2016) and temperature (Porcal et al. 2015). However, our sampling covered a great variety of environments, mitigating the influence of these other factors as drivers of the observed DOM differences among pond types. Furthermore, although some of these other factors likely differed considerably between bedrock and tundra ponds, the overall water chemistry as well as the DOM concentrations and optical characteristics differed less between these two pond types than for the thaw ponds. Additionally, the DOM sourcing by stable isotope analysis for ponds in the Kuujjuarapik region indicated that these other drivers were less likely relative to allochthony. Overall, our results point to the importance of thawing permafrost for the biogeochemistry of circumpolar surface waters.