Bioreactivity of dissolved organic matter in ponds of the ice-wedge polygonal tundra

Abstract. The role of ponds in transforming dissolved organic matter (DOM) within landscapes affected by permafrost thawing remains poorly understood, despite their potential importance in carbon cycling. Ice-wedge degradation in polygonal landscapes creates diverse pond types through various hydro-morphological processes. We hypothesized that this pond diversity generates DOM of varying bioreactivity. To test this, we conducted a 188 d bioassay using water from 15 ponds in northeastern Canada, representing the main geomorphological types characteristic of this widespread arctic landscape. Using optical spectroscopy and carbon quantification, we examined the relationship between DOM properties and bioreactivity, and conducted a parallel nutrient-addition bioassay to assess potential inorganic nitrogen and phosphorus limitations. Results show that a significant proportion of dissolved organic carbon (DOC) was available to bacterioplankton during summer (33 % decomposed after 97 d). Contrary to our hypothesis, and despite variations in DOM composition, no difference in DOC loss was observed among the three pond categories studied (erosive ice-wedge trough ponds, stable ice-wedge trough ponds, and coalescent polygon ponds), indicating comparable bioavailable DOC pools. Moreover, nutrient addition did not significantly enhance DOC loss or decay coefficients, suggesting that bacterial decomposition depends primarily on the bioavailability of organic carbon. This was further supported by a positive correlation between DOC loss and tryptophan-like fluorophores, a marker of bioavailable DOM, suggesting that DOM released by cyanobacterial mats and other autochthonous producers may be more readily utilized by bacteria than DOM derived from peaty soils. These findings highlight the importance of freshly produced organic matter in regulating carbon cycling in polygonal tundra ponds, with implications for the fate of carbon released from thawing permafrost soils.