Abstract
We examined how nitrogen-stable isotopic signatures of food web components (basal resources, primary and lower consumers, and omnivores) in rivers change with increasing levels of human population density (HPD) in their watersheds. Samples were collected from 22 rivers flowing in the Lake Biwa basin, Japan. Among three potential resources at the base of food webs (epilithon, benthic and suspended particulate organic matter), the mean isotopic values (δ15N) of the epilithon (4.5–7.8%) were consistently higher than those of other items (1.9–4.2%) and displayed the most pronounced elevation (by 3.3%) with increasing HPD. The mean δ15N values of the individual taxa of lower consumers (bivalve, snail and caddisfly) tended to increase with increasing HPD, although the pattern and the extent of the elevation were highly variable among the taxa. These results suggest a taxon-specific feature in the N source (or sources) of lower consumers. Our data suggested that human activities (e.g. nutrient loading) potentially induce changes in the N baselines of river food webs. The major N source of bivalves appeared to be shifted from suspended particulate organic matter to other items with increasing HPD. Trophic levels of goby fish (Rhinogobius sp. OR) and shrimp (Palaemon paucidens), being estimated to be at 2.4–3.8 and 2.1–3.4, respectively, did not differ significantly among rivers with different HPD levels.
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Acknowledgments
We thank N. Okuda and T. Miyajima for their useful comments on the manuscript. This study was supported by the Research Project 3-1 of the Research Institute for Humanity and Nature and the Basic Research Program (CREST-type strategic sector, R and D of Hydrological Modelling and Water Resources System) of the Japan Science and Technology Agency.
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An erratum to this article can be found at http://dx.doi.org/10.1007/s11284-008-0517-x
Appendix
Appendix
The data gathered from the 22 study rivers flowing in the Lake Biwa basin, Japan are shown in Table 4.
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Kohzu, A., Tayasu, I., Yoshimizu, C. et al. Nitrogen-stable isotopic signatures of basal food items, primary consumers and omnivores in rivers with different levels of human impact. Ecol Res 24, 127–136 (2009). https://doi.org/10.1007/s11284-008-0489-x
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DOI: https://doi.org/10.1007/s11284-008-0489-x