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Analyzing the Impacts of Beach Drive on the Water Quality and Abundance of Vegetation in Rock Creek Park

Principles and Methods of Ecology Independent Research Project

Mike Stoever

ABSTRACT This research project sought to assess the impacts and effects of anthropogenic disturbance in Rock Creek Park, which is located in Washington, D.C. The focal point of this research was a comparison of a 1.5-mile long segment of Beach Drive, which bisects the park and is adjacent to Rock Creek, and two areas of natural habitat that contain tributaries which feed into the creek. It was hypothesized that the presence of Beach Drive alongside Rock Creek would result in higher pH, nitrate, and phosphate levels, and decreased diversity and abundance of vegetation than would be found in the tributaries feeding into the creek and the areas of natural habitat that house them. Over the course of multiple weekend trips into the park, water samples were collected and photographic evidence of vegetation was recorded. Analysis found that the average pH, nitrate, and phosphate levels in Rock Creek adjacent to Beach Drive were in fact higher than the tributaries. This difference was attributed to road’s role in facilitating urban stormwater runoff that often results in higher levels of pH, nitrate, and phosphate. It was also found that while the species richness did not vary widely between Beach Drive and the natural areas found further within the park, their abundance did. The difference in richness was attributed to the presence of a pervasive non-native invasive species and in abundance to higher levels of disturbance found along Beach Drive; namely, the weekend users of this stretch of road (which is closed to traffic on those days) who occasionally like to walk and run on the softer soil that would otherwise be home to vegetation.

INTRODUCTION

Established in 1890, Rock Creek Park is the oldest and largest urban national park in the United States and one of the oldest and largest natural parks in the world, covering over 2,800 acres and comprising 7% of Washington, D.C. (NatureServe and NPS NCR, 2016; Carruthers et al., 2009). It serves as a natural oasis in the middle of the city that provides a multitude of recreational and ecological benefits to millions of visitors each year (Carruthers et al., 2009). The park is home to numerous hiking and horse-riding trails that wind and weave their way throughout its footprint, affording visitors close access to its unique flora, fauna, and landscape.

This close proximity to anthropogenic activity has resulted in an increase of stressors to the park, however. Its urban setting has left it vulnerable to high levels of ozone and atmospheric deposition due to air pollution; increased pollutant and nutrient loading via stormwater runoff; habitat fragmentation due to the construction of roads around and within the park; and increased levels of disturbance due to its high visitation rate (Carruthers et al., 2009). As Forman and Alexander (1998) noted, runoff facilitated

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by the presence of roads often adversely affects stream ecosystems, where pollutants are dispersed and diluted over considerable distances. Further, Trombulak and Frissell (2000) found that while not all species and ecosystems are equally affected by roads, their presence adds nutrients to roadside environments and is highly correlated with species composition, population sizes, and hydrologic and geomorphic processes that shape aquatic and riparian systems.

This project set out to assess if Beach Drive, which is part of the heavily trafficked Rock Creek Parkway that bisects the park from north to south, had an impact on the water quality and vegetative abundance of the segment of Rock Creek and its banks immediately adjacent to it. The hypothesis was two-fold: 1) that the presence of Beach Drive would result in higher pH, nitrate, and phosphate levels than would be found in the tributaries within the park away from any roads; and 2) that the road’s presence would result in a lower richness and abundance of plant species alongside it than would be found alongside the tributaries within the park away from any roads. This hypothesis was tested by first collecting samples of water from the segment of Rock Creek adjacent to Beach Drive and from two tributaries found further within the park. These samples were tested for their pH, nitrate, and phosphate levels and their averages then analyzed to assess whether any statistically significant difference was present. Plant species richness and abundance were also observed in both the road-adjacent and road-free habitats, comparing the two for any of the hypothesized differences.

MATERIALS AND METHODS

Water Quality Analysis

For the water quality analyses, the study area was narrowed from the entirety of Rock Creek alongside to the 1.5-mile length that runs along the section of Beach Drive that is closed to traffic on weekends (the road is highlighted in yellow in Figure 1). This allowed for easy and safe access to the stretch of Rock Creek that runs directly alongside it. This formed the northern and southern borders of the study area, with the eastern border being the Valley Trail (shown in pink on Figure 1). The western border of the study area was the Western Ridge Trail (shown in green on Figure 1). This stretch of Rock Creek was then divided up into six grid cells of equal size/length (1320 ft each). Over the course of four weekend trips to the park, five sets of samples (with one sample each of pH, nitrate, and phosphate constituting a set) were randomly taken from each grid, for a total of 30 samples. For the comparison tributaries, the two most prominent were selected. These were divided into three grid cells each, with five sets of samples being taken from each grid, again creating a total of 30 samples for the two tributaries.

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FIGURE 1: Map of study area. Beach Drive is highlighted in yellow; the Western Ridge Trail is highlighted in green; and the Valley Trail is highlighted in pink. The two tributaries are shown in blue and labeled T1 and T2, and the six grids are displayed in red. SOURCE: Adapted from NPS, 2016. pH Testing

For pH, a HealthyWiser Digital pH Meter with a 0.00-14.00 pH range purchased from Amazon.com was used. After the initial calibration, the digital pH meter was inserted into Rock Creek and the two tributaries at the selected locations and held there for 60 seconds, allowing for a full accounting of the pH level(s) and to set a standard across all samples. This field research took place during the daytime and the data was recorded onsite and later entered into a spreadsheet for analysis. Analysis included computing the mean, standard variation, and variance for the two data sets (Rock Creek was one, the two tributaries was the other) and then running a two-tailed T-Test to ascertain whether or not any observed differences were statistically significant at a 95% confidence level. Nitrate and Phosphate Testing

To test the levels of nitrate in Rock Creek and the two tributaries, an API Nitrate Test Kit that tested nitrate levels from 0-160 ppm was purchased from Amazon.com and used. A clean test tube was filled with 5mL of water from Rock Creek and the two

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tributaries at the selected sites. Two test solutions were then added to the test tube, with a five-minute waiting period following. Once the waiting period was complete, the color of the water in test tube was compared to seven color-coded comparison levels on a “Freshwater Nitrate Color Card” (shown in Figure 2a). These seven color-coded comparison levels were for 0, 5, 10, 20, 40, 80, and 160 ppm. The closest level was chosen, with an averaged level used for when the sample was between two levels. This field research took place during the daytime and the data was recorded onsite and later entered into a spreadsheet for analysis. Analysis included computing the mean, standard variation, and variance for the two data sets (Rock Creek was one, the two tributaries was the other) and then running a two-tailed T-Test to ascertain whether or not any observed differences were statistically significant at a 95% confidence level.

To test the levels of phosphate in Rock Creek and the two tributaries, an API Phosphate Test Kit that tested phosphate levels from 0-10 ppm was purchased from Amazon.com and used. Similar to the nitrate testing, a clean test tube was filled with 5mL of water from Rock Creek and the two tributaries at the selected sites. Two test solutions were then added to the test tube, with a three-minute waiting period following. Once the waiting period was complete, the color of the water in test tube was compared to seven color-coded comparison levels on a “Freshwater Phosphate Color Card” (shown in Figure 2b). These seven color-coded comparison levels were for 0.0, 0.25, 0.5, 1.0, 2.0, 5.0, and 10.0 ppm. Again, the closest level was chosen, with an averaged level used for when the sample was between two levels. This field research took place during the daytime and the data was recorded onsite and later entered into a spreadsheet for analysis. Analysis included computing the mean, standard variation, and variance for the two data sets (Rock Creek was one, the two tributaries was the other) and then running a two-tailed T-Test to ascertain whether or not any observed differences were statistically significant at a 95% confidence level.

a b FIGURE 2: The color-coded comparison cards for nitrate (a) and phosphate (b) levels.

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Vegetation Analysis To compare the abundance of vegetation alongside Beach Drive to the areas alongside the two tributaries, a simple quadrat composed of PVC pipe was constructed. Lengths were measured, selected, and purchased from a local hardware store that when inserted into angular connection pieces formed a one square meter quadrat. Due to the winter season, sampling for this analysis was restricted to one day in late April in order to allow for seasonal vegetation to make an appearance as the weather warmed. One sample was taken at random in each of five locations alongside the two tributaries at a distance of 0-5 meters from the water, and ten taken along the 1.5-mile length of Beach Drive that comprised the northern and southern boundaries of the study area at distances of 0-5 meters from the road. This field research took place during the daytime and photographs were taken of each sampling site, allowing for further analysis and comparison.

RESULTS

Water Quality Analysis pH Testing

Upon analysis, the mean pH for Rock Creek was found to be 8.37 with a standard deviation of 0.28, while the mean pH for the two tributaries was found to be 7.29 with a standard deviation of 0.46 (Figure 3). When a two-tailed T-Test was run comparing the two sets of data, a p-value of 9.02E-15 was given. This p-value is less than 0.05, which indicates that the null hypothesis that the mean pH levels for Rock Creek and the two tributaries are statistically the same should be rejected.

FIGURE 3: Comparison of average mean pH levels for Rock Creek and two tributaries.

6.6  6.8  7  

7.2  7.4  7.6  7.8  8  

8.2  8.4  8.6  

Mean  pH

 level  

Rock  Creek  

Tributaries  

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Nitrate and Phosphate Testing Upon analysis, the mean nitrate level for Rock Creek was found to be 4.58 with a standard deviation of 0.95, while the mean pH for the two tributaries was found to be 2.17 with a standard deviation of 3.20 (Figure 4). When a two-tailed T-Test was run comparing the two sets of data, a p-value of 3.65E-04 was given. This p-value is less than 0.05, which indicates that the null hypothesis that the mean nitrate levels for Rock Creek and the two tributaries are statistically the same should be rejected.

FIGURE 4: Comparison of average mean nitrate levels for Rock Creek and two tributaries.

For phosphate, upon analysis the mean level for Rock Creek was found to be

0.121 with a standard deviation of 0.02, while the mean pH for the two tributaries was found to be 0.083 with a standard deviation of 0.06 (Figure 5). When a two-tailed T-Test was run comparing the two sets of data, a p-value of 0.002 was given. This p-value is less than 0.05, which indicates that the null hypothesis that the mean phosphate levels for Rock Creek and the two tributaries are statistically the same should be rejected.

0  

0.5  

1  

1.5  

2  

2.5  

3  

3.5  

4  

4.5  

5  

Mean  nitrate  level  (in  ppm

)  

Rock  Creek  

Tributaries  

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FIGURE 5: Comparison of average mean phosphate levels for Rock Creek and two tributaries.

Vegetation Analysis

As a whole, a much higher abundance of vegetation was observed along the tributaries to Rock Creek (Figure 6a) than along the section of the creek adjacent to Beach Drive (Figure 6b). Vegetation abundance was relatively high in all sites observed along the tributaries. For Rock Creek, the biggest determinant in vegetation abundance was the distance between the creek and Beach Drive. Where the distance between the two was less than 10 m, low vegetation abundance was observed (Figure 7a) but where the distance was greater than 10 m, higher vegetation abundance was observed (yet was still less than was found at any point along the two tributaries; Figure 7b). Species richness was relatively similar in both sections of the study area; common plants found were the American hornbeam (Carpinus caroliniana), the New York fern (Thelypteris noveboracensis), and the lesser celandine (Ranunculus ficaria), an invasive species also known as the fig buttercup (Ficaria verna). The lesser celandine was widely established in and appeared to dominate the sites where it was found, behavior consistent with literature on the species (NPS, 2010).

0  

0.05  

0.1  

0.15  

0.2  

0.25  

Mean  phosphate  level  (in  

ppm)   Rock  Creek  

Tributaries  

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a        b   FIGURE 6: Observed vegetation abundance along tributaries (a) and Beach Drive (b).

a b FIGURE 7: Observed vegetation abundance where distance between Rock Creek and Beach Drive was <10 m (a) and >10 m (b).

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DISCUSSION AND CONCLUSIONS The hypothesis that this research project tested was two-fold: 1) that the presence of Beach Drive would result in higher pH, nitrate, and phosphate levels than would be found in the tributaries within the park away from any roads; and 2) that the road’s presence would result in a lower richness and abundance of plant species alongside it than would be found alongside the tributaries within the park away from any roads. With regards to the water quality analysis, the p-values given by all three two-tailed T-Tests were less than 0.05, which indicated that the null hypothesis that the mean pH, nitrate, and phosphate levels for Rock Creek and the two tributaries were statistically the same should be rejected. There is a <1% chance that, by random sampling error, the estimated means would appear different when the true means are really not different. In other words, there is a >99% chance that any difference between the two means is not due to a random sampling error and that they are in fact likely statistically different. Therefore, it is concluded that the pH, nitrate, and phosphate levels are in fact higher in the segment of Rock Creek adjacent to Beach Drive than they are in the two tributaries located within the park, away from the road. According to criteria developed by the EPA and others, the average pH of 8.37 for the segment of Rock Creek alongside Beach Drive that was observed is considered slightly high; the average level of nitrate of 4.58 that was observed is considered severely impaired; and the average level of phosphate of 0.121 is considered not impaired (Vandervoort, 2007). It is acknowledged that these results, and thus conclusions and speculations, would benefit from replication utilizing more sophisticated tools. While impressive for a layman’s device, the pH tester used would likely not pass muster in a professional laboratory that analyzed water quality. Further, the accuracy and precision of the testing kits for nitrate and phosphate left something to be desired. They relied upon the human eye to match the colored water in test tubes to a color-coded reference chart, which itself had a rather large range of values covered between colors. This left a lot of room for interpretation in the results and thus, they cannot be said to be exact in any way. It must also be noted that while a statistically significant difference was found between pH, nitrates, and phosphates in Rock Creek along Beach Drive than in the tributaries feeding into it, this difference may be due to runoff from further upstream and not necessarily from the adjacent road. While the impact of Beach Drive cannot be ruled out as the lack of vegetation cover allows easier transit for runoff, it cannot be definitively stated how much of an impact the road’s presence has on water quality. I am confident in stating that it does have an impact, however, as the results are in line with Carruthers et al.’s (1999) finding that automobile traffic is one of three equally significant internal threats to the natural resource condition of the park. Interestingly, the impact of invasive exotic species is one of the other two threats, which will be discussed shortly (Carruthers et al., 1999).

Finally, an interesting effect on water quality was observed on the final day of sampling. After a morning of steady rain, the samples taken that afternoon all displayed lower pH levels than any of samples that were taken on previous days. This was attributed to the fact that rainwater is slightly acidic, which would have the resulting effect of lowering the pH of the water in both Rock Creek and the tributaries, and thus for

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all of the samples taken that afternoon. Further research could be done to assess whether the difference between samples taken during dry conditions and those taken during rainy conditions is statistically significant.

With regards to the vegetative analysis, the observations led to the conclusion that while the richness between the two locations did not vary widely, the abundance was noticeably greater along the two tributaries than along Beach Drive. This difference was attributed to two main factors: 1) anthropogenic disturbance in the form of runners, walkers, and automobiles; and 2) the sandier soils found along Beach Drive. The first factor appears to be a manifestation of Connell’s intermediate abundance hypothesis, where diversity declines at higher disturbance levels due to increased mortality rates (Connell, 1978; Cain, 2014). High volumes of runners and walkers utilize the softer ground of the creek bank on the weekends when Beach Drive is closed, and it would make sense for this increased disturbance to result in high rates of mortality among established and juvenile plant life. Additionally, the location of the creek bank results in it being the first hit by any stormwater or automobile runoff. The second attributed factor, sandier soils, may also play a role in diminished vegetative abundance, as coarse-grained sandy soils hold less of the water and nutrients necessary for plant life (Gonzaga College High School, 2007).

It should also be noted that the greater abundance of vegetation found alongside the two tributaries found within the park is not necessarily indicative of great habitat health. In fact, the most widely seen and abundant species, the lesser celandine (Ranunculus ficaria; also known as the fig buttercup or Ficaria verna), is a non-native invasive species that has been show to exert a negative impact on ecosystem biodiversity (NatureServe and NPS NCR, 2016). This vigorously growing species forms large, dense patches in floodplain forests (where soils are more fertile and moist), displacing many native plant species, especially those with the similar spring flowering life cycle (NPS, 2010). The lesser celandine also holds a developmental advantage over its native competitors in that it emerges well in advance of them, allowing it to establish and overtake areas rapidly (NPS, 2010). The species germinates and spreads easily and is most prolific along trails and edge habitats, which it prefers (Gonzaga College High School, 2007). Their abundant presence poses a great threat to the ecological integrity of the natural community within the park (Gonzaga College High School, 2007). By shading out native spring ephemerals that normally provide food and shelter to butterflies and other insects, birds, and other animal species, the lesser celandine interrupts a whole web of natural interdependencies (NatureServe and NPS NCR, 2016). Rock Creek Park is a valuable natural oasis in the urban environment of Washington, D.C. It has been determined to have a very high regional ecological value, supporting many communities of flora and fauna and housing increasingly rare seeps and springs, some of which serve as the primary habitat for a federally endangered species endemic to the park, the Hay’s springs amphipod (Carruthers et al., 1999). If Rock Creek Park is to maintain this status, it is the opinion of this author that the high levels of pH, nitrate, and phosphate found in Rock Creek be addressed via additional research and the support of further work focused on lessening the impact of stormwater runoff. The decreased vegetative abundance observed along Beach Drive in this study is likely an acceptable lost cause of sorts, as the resulting trade-off is 1.5-miles of open, car-free road

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for pedestrians to safely utilize on weekends. Additionally, while the increased vegetative abundance observed along the two tributaries was nice to see, the fact that an invasive species comprises the majority of that vegetation is troubling. Further research and control measures are suggested to address this invasive species and lessen its impact.

REFERENCES Cain, M. L., W. D. Bowman, and S. D. Hacker. 2014. Ecology: Third Edition.

Sunderland, MA: Sinauer Associates, Inc. Carruthers, T., S. Carter, L. Florkowski, J. Runde, and W. Dennison. 2009. Rock Creek

Park natural resource condition assessment, National Capital Region Network. Natural Resource Report NPS/ NCRN/NRR—2009/109. Fort Collins, CO: National Park Service. Available online at: http://nature.nps.gov/water/nrca/assets/docs/ROCR_NRCA.pdf. Accessed May 6, 2016.

Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199:1302-1310.

Forman, R. T. T. and L. E. Alexander. 1998. Roads and their major ecological effects. Annual Review of Ecology and Systematics 29:207-231.

Gonzaga College High School. 2007. A Technical Guide to the Natural Communities of the National Capital Region – Rock Creek Park: An Overview. Available online at: http://www.gonzaga.org/NetCommunity/Document.Doc?id=2696. Accessed February 23, 2016.

NatureServe and NPS NCR (National Park Service National Capital Region). 2016. Explore Natural Communities: Rock Creek Park. Available online at: http://www.explorenaturalcommunities.org/parks-places/rock-creek-park. Accessed May 6, 2016.

NPS (National Park Service). 2010. Fact Sheet: Fig Buttercup. Available online at: https://www.nps.gov/plants/alien/fact/pdf/rafi1.pdf. Accessed May 6, 2016.

NPS. 2016. Rock Creek Park: Maps. Available online at: https://www.nps.gov/rocr/planyourvisit/maps.htm. Accessed May 6, 2016.

Trombulak, S. C. and C. A. Frissell. 2000. Review of Ecological Effects of Roads on Terrestrial and Aquatic Communities. Conservation Biology 14(1):18-30.

Vandervoort, C. 2007. The Status of Water Quality in the Rivers and Tributaries of the Shenandoah River Watershed. Available online at: http://fosr.org/wp-content/uploads/2013/07/WQ-Shenandoah2007.pdf. Accessed May 6, 2016.