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

We performed surveys of nitrogen (N-2)-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 N-2 fixation?, and (2) How does N-2 fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N-2 fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N-2-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 N-2 fixation was below detection. Stream N-2-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 N-2-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N-2-fixation rates to whole lake and stream areas revealed that N-2 fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N-2-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 N-2-fixation rates and despite low surface areas. This study suggests that N-2 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 N-2-fixation contributions to watersheds by providing a large area for within-lake N-2 fixation, and creating conditions favorable for N-2 fixation in outlet streams.