Periphyton Biomass and Species Composition in a Coastal Rainforest Stream in British Columbia: Effects of Environmental Changes Caused by Logging

Periphyton biomass, species composition, and accumulation rates on Plexiglas substrates were determined in a 6-yr study from 1974 to 1979 in Carnation Creek, Vancouver Island, B.C. During the first 2 yr of the study, the watershed was unlogged. Clearcut logging during the remainder of the study resulted in increases in light intensity of over 100% at some sites, slight increases in stream temperature (2–3 °C in summer) and increases in some dissolved ion concentrations. Phosphorus concentrations were similar throughout the study. Diatoms were the most common class of algae, and Achnanthes minutissima and Synedra rumpens were the most common diatoms. Filamentous chlorophytes (predominantly Mougeotia sp. with some Draparnaldia sp., Spirogyra sp., and Zygnema sp.) occurred sporadically throughout the study but were more common after logging, primarily as a result of increased light intensity. Although highest recorded periphyton biomass and accumulation rates occurred after logging, post-logging values were generally similar to those recorded prior to logging. We attributed this to the lack of increase in phosphorus concentrations after logging.