Small-scale distribution of interstitial nitrite in freshwater sediment microcosms: the role of nitrate and oxygen availability, and sediment permeability.
The spatial distribution of interstitial NO2(-) concentrations was studied in NO3(-)-exposed freshwater sediment microcosms, using pore water extractions as well as ion-selective microsensors. Porewater extractions revealed ecotoxicologically critical NO2(-) concentrations in hypoxic and anoxic sediment layers in which significant NO3(-) consumption took place. In contrast, the use of ion-selective microsensors demonstrated the high capacity of the thin oxic surface layer of the sediments to consume NO2(-) and to produce NO3(-). Two modes of NO3(-) supply to the sediments were compared: In treatments with NO3(-) supply to the overlying water, a subsurface maximum of NO2(-) concentration was observed, coinciding with the site of maximum NO3(-) consumption. When NO3(-) was perfused up through the sediment cores, however, NO2(-) accumulated throughout the entire sediment column. Such spatially extensive NO2(-) accumulations were only observed in sediments poor in organic matter with a relatively high permeability. By manipulating the O2 content of the overlying water, the release of NO2(-) from the sediments could be influenced: In treatments with air-saturated overlying water, the sediments did not release detectable amounts of NO2(-) into the water phase. When kept hypoxic (25% air saturation) instead, significant NO2(-) accumulations were recorded in the overlying water. These findings suggest that in treatments with air-saturated overlying water, NO2(-) that was produced in deeper sediment layers (denitrifying conditions) was completely consumed at the oxic sediment surface (nitrifying conditions) before it could reach the overlying water.
Stief
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P
Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany. pstief@mpi-bremen.de
De Beer
D
D
Neumann
D
D
D001682:Biological Availability; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000737:chemistry*; D009566:Nitrates / Q000032:analysis; D009573:Nitrites / Q000032:analysis*; D009584:Nitrogen / Q000378:metabolism*; D009930:Organic Chemicals / Q000378:metabolism; D010100:Oxygen / Q000032:analysis; D010539:Permeability; D014871:Water Microbiology
D009566:Nitrates; D009573:Nitrites; D009930:Organic Chemicals; D009584:Nitrogen; D010100:Oxygen