Tracing energy flow in stream food webs using stable isotopes of hydrogen

1. Use of the natural ratios of carbon and nitrogen stable isotopes as tracers of trophic interactions has some clear advantages over alternative methods for food web analyses, yet is limited to situations where organic materials of interest have adequate isotopic separation between potential sources. This constrains the use of natural abundance stable isotope approaches to a subset of ecosystems with biogeochemical conditions favourable to source separation. 2. Recent studies suggest that stable hydrogen isotopes (delta D) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in delta D of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in delta D in stream food webs in a forested catchment where variation in delta C-13 has been described previously. 3. Although algal delta D varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67 parts per thousand) from terrestrial carbon sources. Environmental conditions known to affect algal delta C-13, such as water velocity and stream productivity did not greatly influence algal delta D, and there was no evidence of seasonal variation. In contrast, algal delta C-13 was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial delta C-13 in all streams with catchment areas larger than 10 km(2). 4. While knowledge of isotopic exchange with water and trophic fractionation of delta D for aquatic consumers is limited, consistent source separation in streams suggests that delta D may provide a complementary food web tracer to delta C-13 in aquatic food webs. Lack of significant seasonal or spatial variation in delta D is a distinct advantage over delta C-13 for applications in many aquatic ecosystems.