Factors controlling periphyton accrual during summer in headwater streams of southwestern British Columbia, Canada

In headwater coastal streams of southwestern British Columbia, previous res earch suggested that light limited periphyton growth and abundance of grazi ng invertebrates. Logging along a reach of stream allowed us to further exa mine the importance of light, as well as other abiotic factors, in regulati ng stream periphyton and grazers. We placed unglazed ceramic tiles in three watershed, two of which served as controls. In the third watershed, we pla ced tiles in one reach that was newly harvested, as well as in an upstream, forested reach. Tires were placed in streams in late June and removed week ly over a six-week period for determination of periphyton chlorophyll a and ash-free dry mass. We also measured discharge, dissolved nitrate and phosp hate, and counted the number of invertebrate grazers on each removed the we ekly. Peak biomass, as chlorophyll a was reached on day 29 with alga biomas s at the logged site (19 mu g m(-2)) seven to fourteen times higher than at the control sites. Stepwise, multiple Linear regression suggested that Lig ht was the single best predictor explaining 64% of the variation in peak bi omass of chlorophyll a. Although periphyton biomass on tiles was much highe r in the clearcut reach, so was fine sediment. Inorganic mass entrapped in the periphyton mat was two to four times higher in the clearcut stream than at other sites. Grazer abundance was not related to periphyton biomass, bu t was negatively related to sediment levels. Our results are consistent wit h the hypothesis that grazer abundance was determined by sediment levels ra ther than alga biomass.