Future increase in temperature may stimulate litter decomposition in temperate mountain streams: evidence from a stream manipulation experiment

1. Small woodland streams constitute the majority of water courses in many catchments. Given their generally low water temperature, high surface:volume ratio and primarily heterotrophic nature, such streams can be strongly affected by increases in temperature. It is therefore important to assess how stream communities and processes respond to the global warming projected for this century. We assessed the effects of a c.3 degrees C experimental warming of stream water on decomposition of Quercus robur leaf litter and on the associated fungal biomass (ergosterol concentration), sporulation rates of aquatic hyphomycetes, and total macroinvertebrate and shredder abundance in spring, autumn and winter. A mountain stream reach in central Portugal was divided longitudinally over 22m with local stones. The study followed a before-after control-impact design, with both stream halves at ambient temperature during 1year and one stream half being experimentally warmed in the second year. Experimental warming of stream water stimulated litter decomposition only in winter, probably because at that time, the low natural temperature limited microbial activities. The effect of experimental warming did not depend on the presence of macroinvertebrates. Contrary to expectations, no significant effect of experimental warming was found on fungal biomass accrual, sporulation rate of aquatic hyphomycetes or macroinvertebrate abundance on decomposing litter. Although the stimulation of litter decomposition in winter could lead to food depletion, this is unlikely when streams are subsidised by more recalcitrant leaves such as oak, which enter the stream in later winter and decompose slowly.