Stable-isotope ratios of carbon (C-13/C-12 or delta C-13) have been widely
used to determine the energy base of stream food webs, but such use is cont
roversial due to unexplained variability in algal delta C-13. I used publis
hed delta C-13 data from temperate headwater streams through medium-sized r
ivers (0.2-4000 km(2) watershed area) collected during summer baseflows and
original data from streams in northern California to analyze energy pathwa
ys through river food webs. The analyses showed three important results. Fi
rst, epilithic algal delta C-13 and watershed area are positively related,
suggesting that effects of carbon Limitation on algal carbon uptake result
in C-13 enrichment of algal delta C-13 in larger, more productive rivers. S
econd, epilithic algae and terrestrial detritus delta C-13 values are often
distinct in small shaded streams but overlap in some larger unshaded strea
ms and rivers. Measurements of delta C-13 values may be most useful in dist
inguishing algal and terrestrial energy sources in unproductive streams wit
h supersaturated dissolved CO, concentrations, and some productive rivers w
here CO, concentrations are low relative to photosynthetic rates. Finally,
consumer delta C-13 values are more strongly related to algal delta C-13 th
an terrestrial delta C-13. The relative contribution of terrestrial and alg
al carbon sources often varied by functional feeding group within and betwe
en sites. However, with the exception of shredders and scrapers, which resp
ectively relied on terrestrial and algal carbon sources, patterns of consum
er delta C-13 clearly show a transition from terrestrial to algal carbon so
urces for many lotic food webs in streams with greater than or equal to 10
km(2) watershed area. The observed transition to algal carbon sources is li
kely related to increasing primary production rates as forest canopy cover
declines in larger streams, although decreasing retention or quality of ter
restrial carbon may also play a role. Improved analyses of algal delta C-13
and delta N-15 combined with quantitative study of organic matter dynamics
and food web structure should allow the relative importance of these facto
rs to be distinguished in future food web studies.