1. The Effects of Urbanization on Nitrogen Processing in Urban Streams
Peter Francissen, Kevin Geedey, Michael Reisner
Augustana College, Rock Island, IL
Introduction
Results
Discussion
Due to the global shift towards urbanization, research efforts focused on urban ecology have increased in recent decades (Picket et. al 2011). In light of this, scientists have observed an ecological phenomenon called the “urban stream syndrome”, which describes the ecological degradation of streams draining urban land (Walsh et. al 2005). This degradation is significant because it inhibits proper stream function , which consequently allows excess nutrient loads downstream. Since nutrient loads have profound ecological impacts, the study of nutrient export and retention in streams has become critical (Alexander et. al 2000). Nutrient uptake is the process by which free-flowing inorganic nutrients in a stream are absorbed and converted into organic form by microorganisms. Rapid nutrient uptake indicates that a stream is functioning properly and regulating the nutrient export downstream. This research sought to determine if the urban streams of Rock Island, IL were performing nitrogen uptake, and if so, how variation in N uptake across the watershed could be explained. Prior studies have found that the nutrient uptake in urban streams is significantly lower than that of unaltered streams in intact ecosystems (Grimm et. al 2005, Meyer et. al 2005), indicating that an urbanized watershed reduces the stream’s ability to process inorganic nitrogen. Therefore, we hypothesized that many of the streams in highly urban/residential areas would not display measurable N uptake. However, if these urban/residential streams did exhibit N uptake, we predicted that their uptake lengths would increase with higher background nutrient/ contaminant concentrations.  
Our results showed that N uptake was present in seven of the nine streams tested, indicating that these streams are still functioning biologically despite their urban environments. These results are interesting because the Sw of streams 3,4 and 22 are lower (faster uptake of N) than that of all streams tested by Grimm et. al 2005 , even the unaltered ones. This contrasts with our hypothesis, as sites 3 and 4 have higher levels of imperviousness and nutrient/contaminant concentrations than those of sites 7 and 9, yet they showed significantly lower uptake lengths (Table 1). We were unable to explain variation in uptake length between sites, although there was a non- significant trend with chloride (Figure 1). In future studies, more sites with high background chloride levels (>160 mg/L) and high discharge must be incorporated to see if site 13 represents a pollution impact or an outlier. The finding of <100m Sw in four of the streams is still significant, as it suggests that there is high biotic N demand in those streams. This information carries implications for the management and restoration of urban stream networks. Traditionally, it is thought that implementation of filter strips and riparian buffers will greatly improve stream functioning by reducing solute inputs. However, in this study, some of the more urbanized/residential streams with higher nutrient levels exhibited more N uptake than the heavily forested streams with lower nutrient levels, indicating there are other factors besides land use and solute inputs that affect nutrient uptake.  
We found evidence of nitrate uptake occurring along seven of the nine streams tested (Figure 1). There were no significant correlations between uptake length and the other stream variables displayed on Table 1. Background Cl-, NO3-, Discharge (Q), Total Dissolved Solids (TDS), and Specific Conductivity (SpC) were all determined from the mean of four sampling events. % Imperviousness was measured using raster data. The solute that appeared to have the most influence on Sw was chloride (linear regression, r2=0.412, F(1,5)=3.506, p=0.12, Figure 2), although this correlation was statistically insignificant. Multiple regression analysis of Sw versus background chloride and discharge (Q) suggested that background chloride had a larger influence on Sw than Q, although both were insignificant with p-values of and 0.838, respectively . All regressions were highly influenced by two outlier cases (sites 3 and 13).  
Methods
Site Selection
The Rock Island watershed has four main stream branches that converge into one 4th order stream before draining into the Rock River. We selected sites among all four branches and included headwater, 2nd order and 3rd order streams. Sites were chosen to represent different environments within the watershed, including streams with mostly forested, concrete and residential catchments.
Pulse Addition Technique
Uptake length was assessed using the pulse addition technique (Tank et. al 2008). This technique involves releasing a solution containing reactive solute (NO3-) and conservative tracer (Cl-) into the stream, and then measuring the concentrations of each using a calibrated YSI professional plus meter as they pass downstream. The solution was prepared by mixing 8-25 grams of KNO3 or NaNO3, with a bit higher amount of NaCl, into a one-liter solution. The amount of solute put in the solution depended on the discharge of the stream; for streams with higher discharge, more solute was needed to produce a measurable pulse. The solution was mixed with liters of stream water before being released into the stream. For each run, only the first measuring point along the stream was pre-determined (usually meters downstream of the release point), after that, sites were chosen based on the location of the pulse. At each measurement point, [NO3-] and Specific Conductivity readings were recorded every 30 seconds, one minute or two minutes, depending on how quickly they were changing. Specific Conductivity (SpC) measures the water’s capability of conducting electricity, which indicates the concentration of dissolved ions in the water. It is measured in microsiemens per centimeter (μS/cm) and we used it as a proxy to measure change in [Cl-].
Calculations
We calculated nitrate uptake using nutrient spiraling theory, which quantifies nutrient transport and removal in flowing waters (Tank et. al 2008). Uptake length (Sw) measures the average distance a solute molecule travels before being removed from the water column (Tank et. al 2008). Short uptake length, then, indicates more rapid uptake. To control for physical stream factors that influence Sw, we calculated the mass of reactive solute passing through each point, relative to the mass of conservative tracer. The first measurement point is used as the reference, after which any reduction in the mass of NO3- relative to SpC is assumed to be caused by biological uptake between points. For each measurement point along the stream, we calculated the background-corrected mass of nitrate divided by the background-corrected mass of SpC, and plotted the natural log of this fraction against distance downstream. Sw is the absolute value of the inverse of the slope (Stream Solute Workshop 1990).
Figure 2) Uptake Length versus Background Chloride Concentration
Figure 1) All stream sites tested within our study watershed. Shorter uptake length (Sw) indicates better stream quality as it means nutrient molecules are being incorporated into the system faster.
References
Stream Site
Uptake
Length
NO3- Sw
(m)
Discharge Q
(L/s)
Background
NO3-
(mg/L)
Cl-
Specific Conductivity
(μS/cm)
TDS 
Imperviousness within 5m buffer
(%) 4
52.08
45.67
2.69
110.42
879.11
590.94
38.30 22
56.18
2.82
1.24
107.02
781.56
29.70  3
59.52
38.36
3.05
176.62
759.20
46.80 9
81.30
2.15
1.33
79.75
701.06
0.40 6
163.93
22.82
2.76
101.92
896.27
702.24
33.40 7
243.90
6.12
2.23
102.45
103.63
675.19
15.30 13
769.23
59.71
2.29
196.98
917.31
29.20 11 NA
34.12
2.26
154.47
725.40
26.80 21
3.94
1.50
126.29
849.86
38.50
Alexander, Richard B., Richard A. Smith, and Gregory E. Schwarz. "Effect of stream channel size on the delivery of nitrogen to the Gulf of Mexico." Nature (2000):
Grimm, Nancy B., et al. "N retention and transformation in urban streams." Journal of the North American Benthological Society 24.3 (2005):
Meyer, Judy L., Michael J. Paul, and W. Keith Taulbee. "Stream ecosystem function in urbanizing landscapes." Journal of the North American Benthological Society 24.3 (2005):
Pickett, Steward TA, et al. "Urban ecological systems: Scientific foundations and a decade of progress." Journal of Environmental Management 92.3 (2011):
Tank, Jennifer L., et al. "Are rivers just big streams? A pulse method to quantify nitrogen demand in a large river." Ecology (2008):
Walsh, Christopher J., et al. "The urban stream syndrome: current knowledge and the search for a cure." Journal of the North American Benthological Society 24.3 (2005):
Workshop, Stream Solute. "Concepts and methods for assessing solute dynamics in stream ecosystems." Journal of the North American Benthological Society 9.2 (1990):
Table 1) Comparison of background nutrients/pollutants, physical characteristics and nitrate uptake lengths (Sw) of the stream sites sampled. NA signifies that no measurable uptake was observed, Sw, uptake length; Q, stream discharge; TDS, Total Dissolved Solids.
Acknowledgments:
We would like to thank all of the interns that worked at the Upper Mississippi Center for the summer of 2016.