Carbon and microbes from thawing palsas and peats do not reach a wider fluvial system

Climate change is altering the biogeochemical cycling of carbon and nutrients in the northern peatland and permafrost regions, which provide one of the largest terrestrial carbon storages. Lateral transfer of carbon needs to be more widely studied, especially in smaller streams and catchments, as they receive high loading of organic matter and are hotspots of carbon degradation. In this study we combined measurements of dissolved organic matter (DOM) quality and quantity with microbial community data from a small Arctic catchment. Our aim was to understand how the catchment is affected by two sub-catchments, degrading palsa permafrost mire and peatland thawing in spring. The small thaw ponds in the palsa mire were clearly distinct from the rest of the catchment and ponds in the peatland. Palsa ponds had higher DOM quantity, more aromatic DOM and distinct microbial communities. Thus, our results point towards analogous and low export rates of organic matter from the permafrost mire and peatland to the receiving small streams. Instead, the DOM is being retained and metabolised within the small ponds in both palsa mire and peatland. In our study we were also able to detect high abundances of ultra-small Patescibacteria. Patescibacteria dominated the microbial community composition in all the sampled waters.