Wednesday, June 6, 2007 - 9:30 AM
196

Effects of fine sediments on nitrate removal via denitrification in streams

Jennifer L. Tank1, Fleur E. Matheson2, John M. Quinn2, James P. Sukias2, and Kay Vopel2. (1) Department of Biological Sciences, University of Notre Dame, Notre Dame, IN 46556, (2) National Institute of Water & Atmospheric Research, Gate 10 Silverdale Road, PO Box 11-115, Hamilton, New Zealand

Sediment accumulation has been shown to influence stream ecosystem function, including the potential to alter stream nutrient cycling. We investigated the effect of fine sediments (<2mm) on nutrient uptake, algal biomass, denitrification, and benthic respiration using artificial stream channels.  Experiments were conducted during late summer under both ambient light and shaded conditions to distinguish the relative role of autotrophs and heterotrophs in nutrient uptake in the presence and absence of fine sediments.  We found that net nutrient uptake and algal biomass were variable and apparently not affected by fine sediment level.  However, adding fine sediments increased denitrification rates and denitrification was co-limitation by both dissolved carbon and nitrate. In contrast, only nitrate limited denitrification when sediments were absent or at ambient levels. Microbial respiration rates were comparable among sediment treatments under light conditions, but when shaded, rates were significantly higher with added fine sediment. Microelectrode profiles showed that oxygen concentrations decreased with depth into the sediments creating anoxia only 5mm below the sediment surface, which would potentially enhance denitrification, particularly during low light conditions when algal primary production is low. Increased loading of fine sediments to streams may alter benthic nutrient dynamics through effects on oxygen, nitrate, and carbon availability.

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