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We examined the individual and interactive effects of light and leaf-litter-derived leachate from red alder (Alnus rubra) on growth and elemental composition of stream biofilm. We manipulated the quantity of alder leaf litter used to produce leachate (control: no addition; low: 26.7 g d-1; high: 106.7 g d -1 ) and light (shaded/ unshaded) in experimental channels to assess responses of biofilms colonizing tiles. The greatest changes in stream- water chemistry were observed in channels receiving the high leachate treatment, where streamwater ammonium increased by ~1.4×, phosphate increased by ~2×, and dissolved organic carbon was ~1.5× higher than the control. Effects of light, leachate and their interaction significantly increased biofilm ash-free dry mass and chlorophyll-a. We observed weaker, non-significant effects of leachate in shaded conditions, suggesting that effects of leachate on biofilms were mostly autotrophic. Results indicate that the individual effect of leaf-litter leachates on heterotrophic components of biofilm is minor, but light interactively stimulates the effects of leaf-litter leachates on autotrophic components of biofilms in low-order streams.

We tested experimentally the hypotheses that leaves are colonised for their value as food and not as microhabitat, and that colonisation of leaves would be faster in a clearcut reach due to food limitation. During weekly trials in a forested and a clearcut reach of a small stream in the continental interior of British Columbia, Can- ada, we provided colonisation cages with one of three leaf treatments (no leaves, artifi- cial, red alder). Shredder-detritivores preferentially colonised the alder leaf packs in both reaches (densities 3.9 times greater in alder leaf treatments), but there was no dif- ference between the artificial and no leaf treatments. Preferential colonisation of alder leaves suggests they were being colonised for their value as food and not microhabitat. Colonisation was faster in the forested reach for all treatments (cage: benthos ratio for densities greater than 2.3 times those of the clearcut reach), contrary to predictions. Faster colonisation in the forested reach may be a response to lower UV radiation, a more patchy food source (leaf litter vs. algae) or greater habitat heterogeneity. These results suggest forest harvesting can affect community dynamics of streams, primarily through shifts in community composition.

Coniferous forests predominate in the boreal and western North American regions, yet most studies of litter breakdown in streams have focussed on litter from deciduous trees. We studied breakdown rates and invertebrate colonization of leaf litter from two common conifer species (western red cedar and western hemlock), along with red alder (a common riparian, deciduous species) in a small, coastal rainforest stream of British Columbia during two seasons (summer, autumn). Alder lost mass 40- 100% more quickly than the two conifer species. During summer, hemlock lost mass significantly more quickly than cedar, but this trend was reversed in autumn. Measures of relative lignin concentration and carbon content did not differ signifi- cantly between litter species. Nitrogen content was nearly twice as high in alder as the two coniferous species. Decomposition rates were positively related to initial nitrogen content of the litter and negatively related to C:N ratio. Invertebrate species associated with particular types of leaf litter were clearly distinct during the autumn, but less so during summer. In the autumn, invertebrate assemblages on alder and cedar were dis- tinguished from those on hemlock by the high numbers of detritivores on the former litter types. Cedar had more mayflies and fewer orthoclad midges than alder. There were no significant differences between litter types in densities of any invertebrate taxon per gram of leaf tissue, although alder always had higher numbers.

To characterize organic matter dynamics in headwater streams of southwestern British Columbia, Canada, we monitored epilithic biomass and fluvial transport of organic matter. Fluvial organic matter was collected once to twice monthly from 7 streams: samples were processed as coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and dissolved organic matter (DOM). Epilithon was collected monthly from unglazed, ceramic tiles in 12 stream reaches and measured for ash-free dry mass (AFDM) and chlorophyll-a. CPOM and FPOM concentrations were higher during the wet season (1 October-30 April) than the dry season (1 May-30 September). Averaged across sites, CPOM and FPOM concentrations were positively related to mean daily discharge averaged over the 7 days before sample collection. In contrast, discharge and DOM concentration were inversely related. DOM concentration was positively correlated to chlorophyll-a accrual. Seasonal variation in epilithic chlorophyll-a and AFDM accumulation was high. AFDM accumulation rates ranged from a low in January 1997 of 0.31 to a high of 3.6 μg cm -2 d -1 in May 1997. As with organic matter in stream water, discharge influenced the temporal dynamics of epilithic biomass accumulation; there was a negative correlation between discharge and epilithic AFDM.