Land use practices within stream catchments greatly influence fundamental stream ecosystem properties such as light and nutrient availability as well as the resource base of aquatic systems. We used stable isotope analyses to investigate the effects of agriculture versus forest land use on stream food web structure in the Finger Lakes region of New York State. We compared the δ15N, δ13C and δ2H signatures of samples of common food web compartments from two agricultural and two forested streams, and constructed isotopic food webs based on δ15N vs. δ13C isotopic signatures for each stream. We predicted increased algal biomass in agricultural versus forested streams, and consequently increased incorporation of autochthonous primary productivity into the tissues of consumers in agricultural streams. Furthermore, we expected that increased algal productivity would support larger herbivore populations, enabling consumers to rely more on animal than plant tissue, resulting in a greater range of trophic positions and a greater maximum δ 15N in agricultural food webs. We predicted that the greater range in δ15N, as well as increased incorporation of algal derived carbon, would lead to a larger trophic web, possibly indicating an increase in available trophic niches within agricultural streams.
Results/Conclusions
Because many consumer species displayed δ13C values outside the range of either tree leaf or algal end-member, we were unable to ascertain whether autochthonous carbon contributed more to food webs in agricultural than forested streams. However, the δ2H of nearly every consumer was between the expected algal and detrital end members, suggesting that δ2H may be more reliable than δ13C for indicating the origin of basal carbon resources within stream food webs. δ15N analysis revealed that all trophic compartments collected from agricultural sites were enriched in 15N compared to their forest derived counterparts. Moreover, the range for δ15N was greater in agricultural compared to forested sites, consistent with our predictions. The results of this study suggest that comparisons of trophic niche metrics based on stable isotopes may provide important insights into the impact of environmental change on stream food webs.
