Patterns and controls of lotic algal stable carbon isotope ratios

Spatial and temporal variations in stable carbon isotope ratios (i.e., delta(13)C) of primary producers are common but poorly understood features of isotopic characterizations of aquatic food webs. I investigated factors that control delta(13)C of algae (concentration and delta(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 delta(13)C of the green alga Cladophora glomerata, microalgal-influenced epilithic biofilms, and their herbivores. Algal and herbivore delta(13)C were depleted in C-13 in small, unproductive tributary streams (-44parts per thousand to -30parts per thousand) compared with more productive sites downstream (-31parts per thousand to -23parts per thousand). The majority of variation in algal delta(13)C of Cladophora and epilithic biofilms was determined by dissolved CO2 (CO2aq) via effects on delta(13)C of CO2aq and photosynthetic fractionation. In contrast, two other taxa (the cyanobacterium Nostoc pruniforme and the red alga Lemanea sp.) showed little variation in delta(13)C or fractionation in response to varied inorganic carbon availability because of their distinct modes of inorganic carbon acquisition. Although variation in algal delta(13)C might complicate use of delta(13)C to resolve consumer diet sources under some circumstances, better understanding of such variation should improve the use of delta(13)C techniques in aquatic food web studies.