Fixing the nitrogen budget in oligotrophic arctic lakes: Reassessing the importance of benthic and pelagic nitrogen fixation

Dinitrogen (N₂) fixation may be important to the N budgets of oligotrophic lakes in the Arctic because N inputs from other sources are very low.  However, N₂ fixation has not been considered thus far in arctic lake N budgets, and few measurements exist.  We performed a survey of benthic and pelagic N₂ fixation measurements in 12 lakes across the landscape near Toolik Field Station, Alaska and examined the importance of N₂ fixation to the N budget of a headwater lake (Lake Fog 2).  Benthic N₂ fixation ranged from 12 to 90 mg N m^-2 year^-1 in the survey lakes.  On an areal basis, this is an important input similar to or greater than input from precipitation, which is 25 mg N m^-2 year^-1_.  On a whole-lake basis, benthic N₂ fixation contributed only 1% of the inputs to Fog 2, but accounted for a significant portion of the N flux from sediments to the water column (5.5 – 11%).  Depth-integrated water-column N₂ fixation ranged from 129 to 626 mg N m^-2 year^-1 in the survey lakes, which is much higher than has been previously measured for other oligotrophic lakes and is similar to oligotrophic oceanic regions.  Water-column N₂ fixation was by far the most important source of N to Lake Fog 2 and accounted for nearly 75% of annual N inputs. Total N₂ fixation including both benthic and pelagic habitats exceeds measured rates in terrestrial arctic ecosystems (672 mg N m^-2 year^-1 vs. 255 mg N m^-2 year^-1 respectively).  Thus, lake ecosystems can be considered hot spots for N inputs via fixation in the arctic landscape.