, Volume 94, Issue 2, pp 95-110

Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems

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Abstract

We performed surveys of nitrogen (N2)-fixation in three oligotrophic lake-stream systems in the Sawtooth Mountains of central Idaho to address two questions: (1) Which habitat types within linked lake-stream systems (lake pelagic, lake benthic, and stream) exhibit the highest rates of N2 fixation?, and (2) How does N2 fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N2 fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N2-fixation rates by epiphytes were highest at mid-lake depths, where their percent cover was highest, while rates by epipelon were greatest at shallow lake depths. Pelagic N2 fixation was below detection. Stream N2-fixation rates were greatest on rock substrates and in the lake outlet stream. These patterns were supported by a baseflow survey (late July) in three lake-stream ecosystems which confirmed that N2-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N2-fixation rates to whole lake and stream areas revealed that N2 fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N2-fixation contributions (kg/day) from lakes were greater because they had far larger surface areas than the stream environments. Fixed nitrogen contributions from stream outlets were also relatively high because of high N2-fixation rates and despite low surface areas. This study suggests that N2 fixation could be a seasonally important nitrogen source to nutrient deficient subalpine lake-stream ecosystems. In addition, the frequency and location of lakes could control N2-fixation contributions to watersheds by providing a large area for within-lake N2 fixation, and creating conditions favorable for N2 fixation in outlet streams.