The capacity of a lake to remove reactive nitrogen (N) through denitrification has important implications both for the lake and for downstream ecosystems. In large oligotropic lakes such as Lake Superior, where nitrate (NO3 -) concentrations have increased steadily over the past century, deep oxygen penetration into sediments may limit the denitrification rates. We tested the hypothesis that the position of the redox gradient in lake sediments affects denitrification by measuring net N-fluxes across the sediment-water interface for intact sediment cores collected across a range of sediment oxycline values from nearshore and offshore sites in Lake Superior, as well as sites in Lake Huron and Lake Erie. Across this redox gradient, as the thickness of the oxygenated sediment layer increased from Lake Erie to Lake Superior, fluxes of NH4 + and N2 out of the sediment decreased, and sediments shifted from a net sink to a net source of NO3 -. Denitrification of NO3 - from overlying water decreased with thickness of the oxygenated sediment layer. Our results indicate that, unlike sediments from Lake Erie and Lake Huron, Lake Superior sediments do not remove significant amounts of water column NO3 - through denitrification, likely as a result of the thick oxygenated sediment layer.
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Acknowledgments We thank the crew of the R/V Blue Heron for their support during this study. This research was supported by the National Science Foundation under NSF grant OCE-0927512. Sediment O2 profiles from 2010 were measured by Sergei Katsev and Jiying Li. Kurt Spokas analyzed N2O and CO2 samples. Holly Meier contributed to the potential nitrification measurements. We acknowledge the suggestions of two anonymous reviewers in helping to improve the manuscript.
- Laurentian Great Lakes