RATES, IMPORTANCE, AND CONTROLS OF NITROGEN FIXATION IN OLIGOTROPHIC ARCTIC LAKES, TOOLIK, ALASKA

Gettel, Gretchen

RATES, IMPORTANCE, AND CONTROLS OF NITROGEN FIXATION IN OLIGOTROPHIC ARCTIC LAKES, TOOLIK, ALASKA

Abstract

Biological nitrogen (N) fixation of atmospheric N2 by free-living cyanobacteria in aquatic environments is common, and in many ecosystems, it can account for a significant portion of the biologically available N inputs. Although N fixation can compensate for N limitation, N limitation is maintained over relatively long time scales in many oligotrophic lake ecosystems. This dissertation examines the importance of benthic and pelagic N fixation in the N economies of oligotrophic lakes in arctic Alaska (Chapter 1) and examines nutrient, light, and grazer controls on benthic N fixation (Chapters 2 and 3). Both benthic and pelagic N fixation are prevalent in many lakes across the Alaskan arctic landscape, ranging from 0.12 ? 1.5 mg N m-2 day-1 and 0 ? 2.56 mg N m-2 day-1 respectively. Pelagic N fixation is much higher than has been reported elsewhere for oligotrophic lakes, and is more important than previously thought, comprising ~ 75% of N inputs to one lake. Benthic N fixation is lower than has been reported for other oligotrophic systems, and is roughly equivalent to N inputs from atmospheric deposition on an areal basis (~25 mg N m-2 year-1). On the landscape scale, N fixation in lakes roughly equal that in terrestrial ecosystems in this Arctic region. Benthic N fixation generally appears to have a saturating response to light availability within individual lakes, but light does not explain variation in benthic N fixation across lakes or years. Whole-lake fertilization and laboratory experiments indicate that P input stimulates benthic N fixation while N input suppresses N fixation when N is added either alone or in conjunction with P in Redfield proportion. Snails at ambient density cause a small decline in benthic N fixation (0.85 ? 1.8% reduction over the summer). These patterns are corroborated in the landscape: lakes on younger surfaces have higher P, more snails, and higher rates of N-fixation than lakes on older surfaces.

Date Issued

2006-08-03T12:51:59Z

Types

dissertation or thesis