The amount of excess fixed nitrogen removed from the freshwater aquatic nitrogen cycle, particularly by freshwater wetlands, through denitrification (DNF) is largely unknown. Typically, DNF rates increase within sediments that have higher organic content and a source of sufficient NO 3 -, in this context we measured DNF in organic-rich sediments of Lost Creek wetland on the south shore of Lake Superior, where NO 3 - concentrations have increased dramatically over the last century. The concentrations of N 2, O 2, and Ar were determined on intact water-sediment cores. Denitrification and respiration rates were determined using membrane inlet mass spectrometry and N 2:Ar and O 2:Ar ratios. Nitrogen flux rates measured in August 2000 and 2001 using overlying ambient wetland water, Lake Superior water, and nitrate augmented wetland water ranged from <10 to 78 μmol N m -2 h -1. These rates are low compared to those published for a variety of wetland and aquatic ecosystems. Nonetheless these are the first DNF measurements we know of to assess natural rates in the Lake Superior Basin and they help quantify a missing piece of wetland and lake nitrogen transformations.
Bibliographical noteFunding Information:
We thank Todd Kana and staff at Horn Point Laboratory, University of Maryland, for advice, in- struction, and analysis of core water samples for dissolved gasses; Anne Cotter for the nutrient analysis of core water and wetland samples; and Corlis West for the sediment characterization. Thanks to Brian Hill and MaryAnn Starus for reviewing and commenting on this manuscript. This work was wholly funded by the U.S. Environmental Protection Agency and has been approved for publication after review by EPA’s National Health and Environmental Effects Research laboratory. The contents do not necessarily reflect the views of the agency, nor does mention of commercial products constitute endorsement or recommendation for use.
Copyright 2012 Elsevier B.V., All rights reserved.
- sediment respiration rate