Nitrogen fixation in subarctic streams influenced by beaver (Castor canadensis)

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.