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nitrogen fixation, terrestrial-aquatic linkages, benthic primary production, nutrients, Kenai Peninsula, Alaska

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Alder (Alnus spp.) is a woody plant with bacterial symbionts that fix atmospheric N2into bioavailable N. We studied 12 North American boreal headwater streams spanning a steep gradient of catchment alder cover (0–27%) to test the hypothesis that increasing inputs of inorganic N associated with increasing alder cover would reduce or eliminate in-stream benthic N2 fixation. We measured N2 fixation rates, chlorophyll a, and ash-free dry mass (AFDM) of periphyton in early (May) and late (August) summer 2011. Dissolved inorganic N (DIN) concentrations, composed almost entirely of NO3/NO2-N, ranged from below detection limits to nearly 2 mg/L and were strongly predicted by catchment alder cover in both months. Higher N2 fixation rates were observed in August than in May. N2 fixation rates declined sharply when alder cover exceeded ~2% of catchment cover, corresponding to 20 to 40 µg/L DIN. This pattern also was evident among 3 streams with contrasting catchment alder cover sampled approximately every 2 wk during the study. The stream with no catchment alder cover exhibited a steady increase in N2 fixation rates over the summer, whereas the streams with low and high alder in their catchments had very low N2 fixation rates that did not vary over time. The influence of alder cover on periphyton biomass was not clear. The strong regulation of alder cover on in-stream N availability leads us to suggest that alder presence may be an important terrestrial regulator of stream N cycling.

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Freshwater Science, v. 36, issue 3, p. 523-532

© 2017 by The Society for Freshwater Science