Natural mires and forested peatlands are known to be very significant sources of dissolved organic carbon (DOC) to aquatic ecosystems. Peatland management operations (e.g., forestry operations, restoration of drained mires and peat mining) and extreme hydrological events may increase the DOC runoff. We hypothesized that an increase in phosphorus (P) leaching, together with near-neutral conditions in recipient lakes will accelerate decomposition of DOC that originates from acidic, nutrient-poor mire waters. The efficiency of DOC utilization was evaluated by measuring microbial respiration and bacterial production (BP) in short-term laboratory experiments with runoff waters from six boreal mire sites. Mere inorganic phosphorus (PO 4 -P) addition did not affect the rate of respiration or the proportion of decayed DOC. However, in the nutrient-poor bog waters, P addition slightly promoted BP and bacterial growth efficiency (BGE). In contrast, the elevation of pH alone, and the elevation of pH and PO 4 -P level together, caused a significant increase in respiration and in the proportion of decayed DOC, but did not affect net BP. Elevated pH alone, however, depressed BGE when compared to that under the combined elevation of pH and PO 4 -P. These results suggest that the increased P availability, e.g., after mire restoration, would slightly benefit bacterial net growth in P-limited waters. However, in near-neutral recipient lakes, the increased microbial decomposition of mire-originated DOC contributes more to carbon dioxide (CO 2 ) supersaturation than potentially supporting detritusbased food chains.

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A considerable proportion of boreal mires have been drained for soil amelioration purposes. In response to drainageinduced degradation, restoration practices have been implemented in recent decades. Restoration by raising the water level is often followed by changes in the quality of runoff waters, especially in concentrations of dissolved organic carbon (DOC), nitrogen (N) and phosphorus (total P, PO4-P). We studied how mire restoration affected bacterial production (BP), bacterial growth efficiency (BGE%) and respiration (R) in mire runoff waters from spruce swamps and Sphagnum pine bogs in south-central Finland. The quality of runoff water was monitored for eight years (2008-2015) and bacterial activity was measured during three years (2010-2012) at runoff weir sites, including two pristine controls, one drained control and four treatment sites. The concentrations of DOC, N and P increased for 3-5 years after restoration. The increased availability of nutrients was followed by doubled BP (from ca. 0.34 to 0.88 µmol C L-1 d-1 , averages of restored sites) and BGE% (from ca. 2.7% to 9.2%), whereas microbial respiration was only slightly increased. However, bacterial activity in mire waters was low compared with those generally measured in river and lake waters. This was presumably related to the recalcitrant quality of the mire-originated DOC, which was not clearly influenced by restoration. Dissolved organic matter (DOM) of low bioavailability contributes to browning of headwaters. As our study was focused only on short-term (1-5 years) effects, more research is needed for evaluating long-term impacts of peatland origin DOM on carbon fluxes, microbial activity and food webs of recipient aquatic ecosystems.