Patterns and controls of lotic algal stable carbon isotope ratios

Spatial and temporal variations in stable carbon isotope ratios (i.e., δ¹³C) of primary producers are common but poorly understood features of isotopic characterizations of aquatic food webs. I investigated factors that control δ¹³C of algae (concentration and δ¹³C 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 δ¹³C of the green alga Cladophora glomerata, microalgal-influenced epilithic biofilms, and their herbivores. Algal and herbivore δ¹³C were depleted in ¹³C in small, unproductive tributary streams (-44‰ to -30‰) compared with more productive sites downstream (-31‰ to -23‰). The majority of variation in algal δ¹³C of Cladophora and epilithic biofilms was determined by dissolved CO₂ (CO_2aq,) via effects on δ¹³C 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 δ¹³C or fractionation in response to varied inorganic carbon availability because of their distinct modes of inorganic carbon acquisition. Although variation in algal δ¹³C might complicate use of δ¹³C to resolve consumer diet sources under some circumstances, better understanding of such variation should improve the use of δ¹³C techniques in aquatic food web studies.