Response of aquatic hyphomycetes to reach-level alder leaf additions in headwater streams of the Pacific Southwest of Canada

J.D. Sleeman and J.S. Richardson. Centre for Applied Conservation Research, Department of Forest Sciences, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4

Aquatic hyphomycetes are important for detritus processing in headwater streams. Their response to differences in detrital standing stocks was tested by supplementing Alnus rubra leaves to streams in conifer dominated headwater basins of coastal British Columbia. We sampled conidia bi-weekly from September to November 2002 from flows upstream and downstream of 75-m reaches on four streams flowing through undisturbed mature forest and three in a cutblock clear-felled in 1997. In mid-October, we distributed alder leaves throughout one of the undisturbed reaches and in two clear-cut reaches for an input of 100 to 200-gm^-2. Leaves largely remained within reaches near where released. After litter additions, detrital standing stocks were higher in manipulated reaches than in control reaches. Membrane filtration showed conidial concentrations were higher in the clear-cuts before and after manipulation as compared to the undisturbed forest. Although concentrations increased in all streams, conidial fungi appear to have responded to litter additions as indicated by higher increases in manipulated reaches. The results of this study suggest that changes in detritus inputs following logging can have significant effects on stream community structure and subsequent detrital processing, which can have important consequences on organic matter processing throughout coastal stream networks.