Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays. 2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP7; mu g C L(-1)day(-1)) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP7 can be used as a measure of export of terrestrial organic carbon that is highly bioavailable. 3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha(-1) and was negatively correlated to forest cover (%). However, the export of BP7 carbon was positively correlated to forest cover and varied from about 0.1 kg ha(-1) in mire-dominated streams to about 0.2 kg ha(-1) in forest-dominated streams. 4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.