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

Abstract

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 (dD) could provide a robust tracer to distinguish contributions of aquatic and terrestrial production in food webs, but variation in dD of consumers and their organic food sources are poorly known. To explore the utility of the stable hydrogen isotope approach, we examined variation in dD in stream food webs in a forested catchment where variation in d 13 C has been described previously. 3. Although algal dD varied by taxa and, to a small degree, between sites, we found consistent and clear separation (by an average of 67&) from terrestrial carbon sources. Environmental conditions known to affect algal d 13 C, such as water velocity and stream productivity did not greatly influence algal dD, and there was no evidence of seasonal variation. In contrast, algal d 13 C was strongly affected by environmental factors both within and across sites, was seasonally variable at all sites, and partially overlapped with terrestrial d 13 C in all streams with catchment areas larger than 10 km 2 . 4. While knowledge of isotopic exchange with water and trophic fractionation of dD for aquatic consumers is limited, consistent source separation in streams suggests that dD may provide a complementary food web tracer to d 13 C in aquatic food webs. Lack of significant seasonal or spatial variation in dD is a distinct advantage over d 13 C for applications in many aquatic ecosystems.

Terrestrial support of detritivorous fish populations decreases with watershed size

“…Our approach using the single regression of seston dD vs. %NVSS yielded an algal dD value of À226% in the baseline scenario, and a range among lakes of À287 to À192% in the two residuals scenarios. These values are similar to the range observed in other field studies (Doucett et al 2007, Caraco et al 2010, Finlay et al 2010. One downside to our regression approach is the relatively large error term associated with our estimate of phytoplankton dD.…”

“…dD, the ratio of deuterium ( 2 H) to hydrogen ( 1 H) expressed relative to a known standard, has emerged recently as an effective means of separating organic matter produced via aquatic vs. terrestrial photosynthesis (Doucett et al 2007, Solomon et al 2009, Caraco et al 2010, Finlay et al 2010. These studies show that dD is more effective than other naturally occurring stable isotopes, such as d 13 C, in distinguishing terrestrial and autochthonous energy sources.…”

“…The large separation in dD of terrestrial vs. aquatic energy sources suggests that hydrogen isotopes are useful for quantifying allochthony (Doucett et al 2007, Solomon et al 2009, Caraco et al 2010, Finlay et al 2010. However, there are still several sources of uncertainty associated with this new technique, including the importance of dietary water and the true dD value of phytoplankton.…”

Strong evidence for terrestrial support of zooplankton in small lakes based on stable isotopes of carbon, nitrogen, and hydrogen

“…We assumed that POM is comprised of a mixture of phytoplankton and terrestrial material and solved for the 13 C and 15 N of phytoplankton algebraically (Table S2 for details). The consistency of ε H in these lakes and in the literature (5,15,39,60) justifies this approach.…”

Unexpected hydrogen isotope variation in oceanic pelagic seabirds

“…Further, hydrogen exchangeability of muscle and collagen appears to be comparable to that of keratin (Wassenaar and hobson 2000;chesson et al 2009;sauer et al 2009;soto et al 2011). thus, we analyzed lipid-extracted muscle and collagen samples in the same fashion as feathers, including normalization to keratin standards (Doucett et al 2007;Finlay et al 2010;cole et al 2011;Wiley et al 2012). …”