STABLE-CARBON-ISOTOPE RATIOS OF RIVER BIOTA:IMPLICATIONS FOR ENERGY FLOW IN LOTIC FOOD WEBS

Abstract

Stable-isotope ratios of carbon (13C/12C or δ13C) have been widely used to determine the energy base of stream food webs, but such use is controversial due to unexplained variability in algal δ13C. I used published δ13C data from temperate headwater streams through medium-sized rivers (0.2–4000 km2 watershed area) collected during summer baseflows and original data from streams in northern California to analyze energy pathways through river food webs. The analyses showed three important results. First, epilithic algal δ13C and watershed area are positively related, suggesting that effects of carbon limitation on algal carbon uptake result in 13C enrichment of algal δ13C in larger, more productive rivers. Second, epilithic algae and terrestrial detritus δ13C values are often distinct in small shaded streams but overlap in some larger unshaded streams and rivers. Measurements of δ13C values may be most useful in distinguishing algal and terrestrial energy sources in unproductive streams with supersaturated dissolved CO2 concentrations, and some productive rivers where CO2 concentrations are low relative to photosynthetic rates. Finally, consumer δ13C values are more strongly related to algal δ13C than terrestrial δ13C. The relative contribution of terrestrial and algal carbon sources often varied by functional feeding group within and between sites. However, with the exception of shredders and scrapers, which respectively relied on terrestrial and algal carbon sources, patterns of consumer δ13C clearly show a transition from terrestrial to algal carbon sources for many lotic food webs in streams with ≥10 km2 watershed area. The observed transition to algal carbon sources is likely related to increasing primary production rates as forest canopy cover declines in larger streams, although decreasing retention or quality of terrestrial carbon may also play a role. Improved analyses of algal δ13C and δ15N combined with quantitative study of organic matter dynamics and food web structure should allow the relative importance of these factors to be distinguished in future food web studies.