| This dissertation examines the role of small streams in controlling nitrogen export from a suburbanizing watershed in Maryland. Results from several field and modeling studies are presented, addressing aspects of spatial and temporal variability. To estimate nitrogen uptake, a transport-based nutrient addition approach was developed, which accounts for the effect of nutrient saturation. A saturation-limited uptake function was incorporated into an existing solute transfer model and the field experiments were specifically designed for this modified model. Cross-method evaluations indicated that the new method produced accurate results. The new method was applied to assess spatial aspects of nitrogen uptake. Short-term nutrient addition experiments were conducted to examine the effect of stream size, transient storage and organic debris dams on ammonium uptake, and two distinct seasons were compared (fall 2003 and summer 2004). Results showed that the effect of stream size was most evident in fall (because of high leaf litter), and was less defined in summer; the effect of transient storage was confounded in fall by high leaf litter, but was positive in summer; and the effect of organic debris dams was prominent in summer. An additional set of short-term nutrient additions were conducted in summer 2004 to examine how ammonium and nitrate uptake changed as a function of stream size and concentration. Results showed distinct longitudinal trends for ammonium and nitrate uptake rate, which was partly attributed to differences in longitudinal concentration profiles and differences in biological uptake capacity. To examine the overall importance of nitrogen removal, a mass balance approach was applied over a 2.4 km stream section. The results showed that nitrate loss was small and relatively constant across flows, and proportional nitrate loss was important only at low flows. This suggests that for urbanizing watersheds, subject to high nitrogen inputs, in-stream processing has only a limited potential for reducing watershed nitrogen export. Results also showed evidence of saturation kinetics, which suggests that the common use of first-order process models is not valid in streams subject to high nitrogen inputs. |
