Denitrification rates in a Lake Superior coastal wetland

Abstract

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 NO3−, in this context we measured DNF in organic-rich sediments of Lost Creek wetland on the south shore of Lake Superior, where NO3− concentrations have increased dramatically over the last century. The concentrations of N2, O2, and Ar were determined on intact water-sediment cores. Denitrification and respiration rates were determined using membrane inlet mass spectrometry and N2:Ar and O2: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.