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Nutrient-diffusing substrates were used to investigate nutrient limitation of attached algal assemblages in a shaded stream and an unshaded stream in northern California. Water front both streams contained low levels of nitrogen (<14 μg · L[SUP-1] and very low N:P ratios (<2). After 31 days of colonization and growth, attached algal biomass on nitrate-diffusing substrates was significantly greater than on control substrates in the unshaded stream. Nitrate-diffusing substrates also supported larger numbers of grazing insects in the unshaded stream. The prostrate diatoms. Achnanthes lanceolata Bech. and Cocconeis placentula Ehr. displayed the most consistent positive responses to nitrate enrichment. Nutrient enrichment did not increase the accrual of algal biomass in the shaded stream, but algal biomass was significantly greater at sites located under openings in the tree canopy, implicating light as a limiting factor in this stream. Several Navicula and Nitzschia species, and one unidentified Gomphonema species, were positively associated with higher light levels in the shaded stream. Shade appears to be the primary factor limiting algal growth in small northern California streams, but when its effect is reduced by logging, the inherently low levels of nitrogen in these streams can become limiting. [ABSTRACT FROM AUTHOR]