Nitrogen fixation in subarctic streams influenced by beaver (Castor canadensis) |
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Authors: | Margaret M Francis Robert J Naiman Jerry M Melillo |
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Institution: | (1) Woods Hole Oceanographic Institution, 02543 Woods Hole, MA, U.S.A.;(2) The Ecosystems Center, Marine Biological Laboratory, 02543 Woods Hole, MA, U.S.A. |
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Abstract: | Nitrogen fixation was measured in four subarctic streams substantially modified by beaver (Castor canadensis) in Quebec. Acetylene-ethylene (C2H2 C2H4) reduction techniques were used during the 1982 ice-free period (May–October) to estimate nitrogen fixation by microorganisms colonizing wood and sediment. Mean seasonal fixation rates were low and patchy, ranging from zero to 2.3 × 10–3 µmol C2H4 · cm–2 · h–1 for wood, and from zero to 7.0 × 10–3 µmol C2H4 · g AFDM–1 · h–1 for sediment; 77% of all wood and 63% of all sediment measurements showed no C2H2 reduction. Nonparametric statistical tests were unable to show a significant difference (p > 0.05) in C2H2 reduction rates between or within sites for wood species or by sediment depth.Nitrogen contributed by microorganisms colonizing wood in riffles of beaver influenced watersheds was small (e.g., 0.207 g N · m–2 · y–1) but greater than that for wood in beaver ponds (e.g., 0.008 g N · m–2 · y–1) or for streams without beaver (e.g., 0.003 g N · m–2 · y–1). Although mass specific nitrogen fixation rates did not change significantly as beaver transform riffles into ponds, the nitrogen fixed by organisms colonizing sediment in pond areas (e.g., 5.1 g N · m–2 · y–1) was greater than that in riffles (e.g., 0.42 g N · m–2 · y–1). The annual nitrogen contribution is proportional to the amount of sediment available for microbial colonization. We estimate that total nitrogen accumulation in sediment, per unit area, is enhanced 9 to 44 fold by beaver damming a section of stream. |
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Keywords: | acetylene reduction beaver Castor canadensis nitrogen fixation streams subarctic |
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