Patterns and controls of lotic algal stable carbon isotope ratios
Finlay, Jacques C.
Limnol. Oceanogr., 49(3), 2004, 850–861

Spatial and temporal variations in stable carbon isotope ratios (i.e., ¹³C) of primary producers are common but poorly understood features of isotopic characterizations of aquatic food webs. I investigated factors that control ¹³C of algae (concentration and ¹³C 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 ¹³C of the green alga Cladophora glomerata, microalgal-influenced epilithic biofilms, and their herbivores. Algal and herbivore ¹³C 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 d13C of Cladophora and epilithic biofilms was determined by dissolved CO₂ (CO_2aq) via effects on ¹³C 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 ¹³C or fractionation in response to varied inorganic carbon availability because of their distinct modes of inorganic carbon acquisition. Although variation in algal ¹³C might complicate use of ¹³C to resolve consumer diet sources under some circumstances, better understanding of such variation should improve the use of ¹³C techniques in aquatic food web studies.