Herbivory by an invasive snail increases nitrogen fixation in a nitrogen-limited stream

Despite anthropogenic nitrogen contributions, nitrogen fixation contributes half of biosphere inputs but has rarely been quantified in streams. Herbivory controls algal biomass and productivity in streams, and we hypothesized that herbivory could also control nitrogen fixation. We released periphyton from herbivory in nitrogen-limited Polecat Creek, Wyoming, where heavy grazing by the invasive New Zealand mudsnail (Potamopyrgus antipodarum) dominates nitrogen cycling. One and two weeks after releasing periphyton, we found higher rates of nitrogen fixation on heavily grazed rocks (two-way analysis of variance (ANOVA), p = 0.012). Time elapsed after snail manipulation had no effect (two-way ANOVA, p = 0.24). Grazing changed periphyton composition by reducing the proportion of green algae and increasing the proportion of nitrogen-fixing diatoms (multivariate ANOVA, p = 0.001). Nitrogen fixation rates increased disproportionately to nitrogen-fixing algal cells, indicating that snails increased nitrogenase efficiency, probably by improving light and (or) nutrient availability to nitrogen fixers. We incorporated our nitrogen fixation rates into a published nitrogen budget for Polecat Creek and found that nitrogen flux into the periphyton was 50% higher when we included nitrogen fixation. Herbivory can increase nitrogen fixation in streams, and future studies should measure nitrogen fixation for a more thorough understanding of stream nitrogen cycling.