Abstract
Spatial and temporal variations in stable carbon isotope ratios (i.e., δ 13 C) of primary producers are common but poorly understood features of isotopic characterizations of aquatic food webs. I investigated factors that control δ 13 C of algae (concentration and δ 13 C of inorganic carbon, algal fractionation, and growth rates) in riffle habitats across a gradient in stream size and productivity in northern California. There was considerable seasonal and spatial variation in δ 13 C of the green alga Cladophora glomerata , microalgal‐influenced epilithic biofilms, and their herbivores. Algal and herbivore δ 13 C were depleted in 13 C in small, unproductive tributary streams (−44‰ to −30‰) compared with more productive sites downstream (−31‰ to −23‰). The majority of variation in algal δ 13 C of Cladophora and epilithic biofilms was determined by dissolved CO 2 (CO 2aq ) via effects on δ 13 C of CO 2aq and photosynthetic fractionation. In contrast, two other taxa (the cyanobacterium Nostoc pruniforme and the red alga Lemanea sp.) showed little variation in δ 13 C or fractionation in response to varied inorganic carbon availability because of their distinct modes of inorganic carbon acquisition. Although variation in algal δ 13 C might complicate use of δ 13 C to resolve consumer diet sources under some circumstances, better understanding of such variation should improve the use of δ 13 C techniques in aquatic food web studies.