Dissolved organic carbon (DOC) concentrations in UK rivers have been rising significantly, with the largest increases occurring in peat dominated catchments. These catchments are major sources of potable water in the UK and DOC compounds have adverse effects on many aspects of water quality, from serving as precursors for the formation of disinfection by-products and causing low residual chlorine (lin-ýiting its protection against biological contamination), to reducing aesthetic value. The potential effects of some major climate change predictions for northern regions on the quantity and quality of DOC produced in peatland catchments have therefore been examined. Such predictions include elevated atmospheric C02 concentrations (eC02). warmer temperatures (eTemp), an increased frequency of summer droughts and increased rainfall. Both eCO2 and eTemp increased leachate DOC concentrations with selective enrichment of recalcitrant phenolic compounds. Under eCO2, such increases were associated with the stimulation of plant inputs (biomass and exudation), reduced extracellular enzyme activities and suppressed DOC decomposition. Warmer conditions provided evidence for enzymic mobilization of DOC and phenolic compounds from the peat matrix. These treatments in combination (eCO2/eTemp) apparently interact to produce the highest concentrations of DOC and phenolic enrichment. Stable isotope (13C) labelling studies revealed that increased exudation of recently synthesized DOC potentially has a crucial role in this response. Successive droughts seemingly induced an increasing trend for DOC concentrations, due to an increased diversity of aerobic, aromatic degrading bacteria and enhanced enzyme activities stimulating mobilization of the peat matrix. Conversely, increased rainfall simulations showed reduced bacterial diversity and enzymic inhibition, conducive to the accumulation of high molecular weight DOC. Potential effects of climatic changes on biofilm communities in the recipient waters were also studied. Elevated C02 and eTemp in combination were found to increase DOC and phenolic compound production when photoautotrophs were present, but also to compromise the removal of these increased inputs by the biofilm heterotrophs. It is therefore proposed that increased DOC production (in the peatland system and by the biofilm) coupled with reduced biodegradation of this material could account for the rising DOC concentrations in UK rivers.