Tons of Fishing
Web Directories And resources!

The watershed, as defined by its topographic boundaries, overlies and is well connected to a productive system of fractured rock aquifers. The hydrogeology has been previously well characterized, as reported by Sloto (1990). The aquifer system is phreatic, which is reflected by water levels that respond to evapotranspiration and recharge, rising in spring and dropping in late summer. Ground water divides do not precisely coincide with surface water divides on the western and eastern ends of the watershed, with the southwest/northeast ground water flow direction being dominated by the geologic structure of the Chester Valley carbonate rocks. Springs discharge where the ground water potentiometric surface is at or above the land surface. Springs occur in and above the stream's channel, and in times of low precipitation, Valley Creek relies largely on springs to supply base flow. One hundred and seventy two springs were found within this watershed, 109 of which could be sampled (Figure 1). Several portions of Valley Creek do not maintain a continuous base flow and the streambeds dry out intermittently. The reaches with no surface flow lie within carbonate rock regions where the stream is flowing only in the subsurface. Consequently, the influence of the geology can contribute to the patchy pattern of fish distribution in Valley Creek in some areas, but does not explain varying fish diversity at adjacent stations having continuous flow in between.

Fish assemblages in nonurbanized streams typically follow a continuous pattern of distribution from headwater reaches to mouth (Vannote et al., 1980). As stream width, depth, and habitat complexity increase downstream, fish species diversity and richness tend to increase as well. In urban streams there can be a patchier pattern of fish distribution related to local land use and water quality than is normally found in more pristine systems. In urban systems, species diversity for the watershed as a whole can be much higher than at any individual sampling location and community similarity between upstream stations is often low (Steffy, 2003). Fish that have strict temperature and substrate requirements for survival and reproduction such as brown trout (Salmo trutta) are typically not present in urban streams (Carlander, 1969; Spotila et al., 1979). Brown trout were stocked in Valley Creek beginning in the early 190Os to promote recreational fishing in the area. Although stocking was halted in 1985, there remains a naturally reproducing population of brown trout. Before the intense pressures of urbanization in the watershed, Valley Creek was a high quality, spring-fed cold water stream that supported the naturally reproducing brown trout population. However, because of the high amount of degradation which has taken place in the stream as a direct result of urbanization and land use changes, present day Valley Creek fish assemblages more closely resemble a warm water stream system. It is now dominated by warm water, eurythermal species. This is typical of highly urbanized, highly degraded cold water streams (Wang et al., 2003). The population of brown trout continues to decline owing to increased urbanization, land use changes, and increased stream temperatures (Kemp and Spotila, 1997; Steffy, 2003). Some of the eurythermal species include white sucker, creek chub, northern hognose, blacknose dace, bluegill, and pumpkinseed.

Impervious surface cover (ISC) can be used as a measure of urban land use and there is a definitive link between fish assemblages and ISC (Schueler and Galli, 1992). Increased imperviousness often results in increased stream water temperatures (Pluhowski, 1970). During storm events, surface water runoff from impervious surfaces (., rooftops and parking lots) is generally warmer and accounts for much of the flow in the stream. Base flow stream temperatures can be elevated from the absence of riparian canopy cover, which is often associated with developed areas. Increased stream temperatures and high annual temperature fluctuation have a negative impact on fish communities, particularly for fish that thrive at cooler water temperatures.

Valley Creek watershed has greater than 17 percent ISC by area. Generally, it has been observed that between 10 to 12 percent ISC there is a decline is fish communities and above 25 percent ISC fish are usually absent (Paul and Meyer, 2001). More specifically in Maryland, fish diversity decreased dramatically in warm water streams having 12 to 15 percent ISC, with fish being absent above 30 to 50 percent impervious area (Klein, 1979). At another site in Maryland, fish diversity decreased above 10 to 12 percent impervious area (Schueler and Galli, 1992). Increases in ISC also affect species richness. Schueler and Galli (1992) report a significant reduction in species richness above 12 percent ISC. In Wisconsin, fish cold water index of biotic integrity (IBI) decreased rapidly at 10 percent ISC (Wang et al, 1997) and 8 to 12 percent connected ISC resulted in major changes in stream condition (Wang et al., 2001). In Canada, fish IBI decreased sharply above 10 percent ISC but streams with high riparian cover were less affected (Steedman, 1988).

Knowing and understanding land use patterns is helpful in interpreting fish community data. Valley Creek has an unusual pattern of urbanization in that traditionally the most intense urbanization has been in the western, upstream end of the watershed. The most downstream three kilometers of Valley Creek lie in VFNHP and are relatively undeveloped. Upstream of VFNHP, the land use is primarily residential, including some dense housing areas and some large lot residential properties. Even further upstream, the watershed contains most of the heavy industrial land uses along with dense commercial properties and less residential area.

Very little research has been published documenting the connections between the quantity and quality of ground water flowing into a stream and the distribution and survival of local fish assemblages. Traditionally, ecologists and hydrologists do not work closely together and therefore the connection between stream base flow and fish assemblages is not well documented. Some older studies indicating a positive relationship between ground water and trout populations were carried out (Benson, 1953; Boussu, 1954); however, those studies did not quantify fish communities and the influence of ground water. This paper quantitatively connects ground water influxes to fish species diversity by using a standard geostatistical method, cross covariance analysis. The cross covariance reveals the spatial correlation between fish species diversity and spring flow rate as a function of the average distance between a fish station and an upstream spring.

METHODS

One hundred and nine springs were sampled for temperature, and their flow rates were measured in one sampling round. Most of the springs found in this watershed had an opening less than four inches in diameter. The springs were most often located within a few feet of the stream channel and could be seen flowing directly into the stream. Temperature was measured when the flow rates were taken in the field with a Horiba Conductivity Meter ES-12. The flows of the springs were measured at their source with a bucket, stopwatch, and graduated cylinder (McGinty, 2003). Thirteen stream stations were sampled for fish assemblage and species composition. Stations 1 and 2 were located in crystalline formations, while Stations 3 through 13 were located in carbonate rock formations (Figure 1).

Fish were sampled by electroshock fishing (SmithRoot 110 V AC backpack electroshocker at 70 Hz every two ms) and dip netting at each of the 13 stations in Valley Creek watershed in July 2001. Stations were intentionally spread throughout the entire watershed (Figure 1) in order to include a variety of land uses and geology formations. Three passes were made through each sampling reach (mean length 35 m), which consisted of two pool and two riffle reaches. Captured fish were placed in buckets filled with stream water until all electroshocking was completed at a station. Fish were identified, weighed, and measured in the field, and a majority were returned to the stream. The remaining fish were taken to Drexel University laboratories for other analyses. Stream temperature was measured seasonally for two years at all 15 station locations.

Fish species diversity was calculated using Simpson's species diversity index (Odum, 1971) at each of the 13 stations (Steffy, 2003). Using a diversity index to quantify fish assemblages and to characterize a particular stream reach allows for better comparison than abundance or species richness alone. Two stations (Stations 13 and 14) were dry during the sampling period and thus had no fish and no diversity index value.

In order to quantitatively assess the influence of the location and flow rate of springs on fish diversity, the spatial cross covariance function was calculated for all fish station/spring pairs, for springs upstream of each fish station. In situations where there was no base flow, the springs upstream of the dried out reaches were not included in the cross covariance calculation. The purpose of this calculation was to test the hypothesis that the species diversity at a sampled fish station was generally higher when the station was downstream of a productive nearby spring. This qualitative observation had been made from comparing field notes on the spring attributes to the fish diversity data, and a more quantitative measure was desired. The shortest distance between a spring and closest fish sampling station ranged from 20 to 1,600 m, with an average of 600 m.

In the case presented here, the coordinate of the "tail" of the lag separation vector is defined as the fish station location, having Simpson's diversity index as its attribute; the "head" coordinate of the separation vector is defined as the spring location, having spring flow rate (L/s) as its attribute. The distances between each fish station and all springs upstream of each station were measured along the stream using Arcview . Because Simpson's diversity index is dimensionless and the flow rate is reported in L/s, the resulting units on the cross covariance function are L/s.

Impervious surface cover data for the entire watershed were derived from a geographic information system (GIS) impervious layer (Cahill and Associates, West Chester, Pennsylvania, 2000, unpublished data) from 1995 and updated using aerial photographs from 2000. The subbasins for each sampling station were derived using the HEC-GeoHMS (. Army Corps of Engineers, 2000) model running in conjunction with ArcView . The input data were from the USGS Digital Elevation Model at a 30 m resolution (clay Emerson, Drexel University, February 1, 2002, personal communication). Species diversity was then plotted as a function of percent impervious surface cover for each subbasin to show the general trend of decreasing species diversity with increasing ISC (Figure 2).

RESULTS

The average temperature for all of the springs did not vary much between the crystalline and carbonate rock types. The crystalline formation showed an average temperature of °C and the carbonate showed the highest average temperature at °C. The flow rates did vary largely between the two rock types. The mean flow rate for the crystalline rocks was L/s, or gallons per minute (gpm). A larger average flow rate occurred in the carbonate rocks and was found to be L/s ( gpm). Temperature and flow rates of the springs were therefore both highest in the carbonate rock areas (McGinty, 2003).

The stream temperature in Valley Creek watershed is highly seasonally variable. The mean summer temperature is 20°C while the mean winter temperature is 5°C. Some instream temperatures were found to be affected by ground water inputs. Stations located near large springs were 5 to 10°C cooler during the summer and had a smaller annual fluctuation in stream temperature. For example, Station 10, located 200 m downstream of multiple springs, had a mean summer temperature of 13°C with a 5°C difference between summer and winter stream temperature. Other stations much further downstream (on the order of 1,000 to 2,000 m) of significant spring flow inputs (., Stations 1 and 2) fluctuated up to 20°C throughout the year (Steffy, 2003). Figure 1 contains station location information.

Impervious surface cover in the entire watershed was greater than 17 percent by area at the time of this study. Subbasins for each station ranged from just less than 14 to 32 percent ISC. The station with the highest impervious area (Station 7 at 32 percent ISC) was located in Little Valley Creek in a large industrial area. Evaluating diversity as a function of impervious cover (Figure 2), the fish diversity generally declined with increasing impervious surface between 15 and 17 percent ISC. The exceptions to this were three stations (Stations 5, 7, and 15) with high ISC on Little Valley Creek that also had higher fish species diversity than would be predicted based in the literature. The main branch of Little Valley Creek lies in the carbonate rock, where some of the highest flow rates in springs were recorded. Excluding Stations 5, 7, and 15, there was a significant pattern of decreasing fish diversity with increasing impervious surface. Simpson's diversity index was significantly negatively correlated with increasing impervious surface (r^sup 2^ = ; p = ) when those three stations were excluded (Figure 2). This relationship between percent ISC and fish species diversity superimposed on spring sampling results led to the idea that ground water inputs were important in maintaining the fish assemblages in Valley Creek watershed. Because the subbasins in Valley Creek were less impervious than those in Little Valley Creek, the effect of the ground water on the fish communities was more pronounced in the three stations in Little Valley Creek having the highest ISC. The influx of ground water allowed for fish species diversity to be similar at sampling locations with 15 percent ISC and 30 percent ISC. Without this ground water flowing into the stream, these stations would likely have had very different fish assemblages made up of primarily eurythermal fish. Two of these three stations also had fish assemblages that included brown trout, which were not present at all of the 13 stations owing to the species' strict temperature requirements. Without the large ground water inputs upstream of these stations, which keeps the stream temperature within an acceptable range, there would likely have been no brown trout at the stations with such highly impervious drainage basins.

This relationship between ground water inputs and fish species diversity was further quantified using a cross covariance geostatistical analysis. The distance between a fish sampling station and each spring location upstream of the fish sampling station was calculated as a one-dimensional curvilinear function of stream length using the GIS coverage of the watershed. This resulted in 342 lag pairs of fish stations and spring locations ranging from 19 m to km apart. The sample cross covariance function (Equation 1) was calculated for groupings of fish-spring lag pairs and plotted as a function of average lag distance in a lag class, as shown in Figure 3. This figure illustrates that the correlation between Simpson's diversity index and spring flow rate is strongest at small separation distances (up to 400 m) and that this correlation falls off sharply by about 800 m. This statistical illustration is consistent with what was generally observed from field notes.

DISCUSSION

Temperature is one of the major organizing factors in stream fish assemblages and is important to consider because it affects the growth and respiration of individual organisms and the productivity of ecosystems through its many influences upon metabolic processes (Allan, 1995). Optimal stream temperature ranges are vital for fish to survive and reproduce effectively (Spotila et al., 1979). Stream temperatures above this range can be immediately detrimental and ultimately lethal to cold water species. Brown trout are the only cold water species remaining in the Valley Creek system and are also the most sensitive to temperature changes. Historically, in Valley Creek, the inputs of ground water into the stream have facilitated a relatively constant stream temperature throughout the Valley Creek watershed. Temperature is a good indicator of ground water inputs because it remains constant and helps keep the stream temperature constant. Therefore, in urban systems where high stream temperature is often correlated with high ISC, any significant input of ground water into the stream should be reflected in the fish assemblages the stream can maintain at these locations.

The spatial pattern of species diversity is critical in understanding the nature of an urban stream. The number of species that occur within a region is the upper limit on the number of species that make up any particular aquatic community. Local environmental conditions largely determine which subsets of species are represented in any given stream (Allan, 1995). In urban streams, these localized effects can be even more magnified due to the patchiness of the land use and the noncontinuous nature of the riparian corridor (Steffy, 2003).

While literature reports a decrease in fish species diversity at 10 to 12 percent ISC, this decline was not evident in Valley Creek watershed until higher ISC was reached (15 to 17 percent). This pattern is unusual for urban streams especially due to the cumulative effects of urbanization in stream systems. However, the large volumes of ground water entering the stream in some locations act as a buffer and allow fish assemblages to remain intact for longer than a system with lower ground water inputs. The three stations in Little Valley Creek that were over 20 percent impervious area and yet maintained diverse fish assemblages are very unusual because often at about 20 to 25 percent ISC, fish are few or absent (Paul and Meyer, 2001). The diversity at these three stations was much higher than would be predicted from the general relationship between species diversity and ISC. Each of these three stations was located downstream of points of high ground water input, which seems to have compensated for the higher impervious cover resulting in relatively high fish species diversity even at levels as high as 32 percent impervious cover.

The cross covariance function between spring flow rate and species diversity showed quantitatively what had been observed in Valley Creek. In urban stream systems, fish assemblages, and patterns of spatial diversity cannot be fully understood without considering the hydrology of the region. Collaborations and data sharing between ecologiste and hydrologiste are vital in urban watershed research but are often overlooked. This paper demonstrates the important role of interdisciplinary efforts in understanding the complexities of urban watersheds.

ACKNOWLEDGMENTS

Financial support was provided by the National Science Foundation, under the 1999 EPA/NSF/USDA Water and Watersheds Competition grant, "An Acre an Hour: Documenting the Effects of Urban Sprawl on a Model Watershed in Philadelphia, Pennsylvania" (EAR-00018884). The authors thank clay Emerson for his assistance in the GIS analysis. The authors are also very grateful to the many Drexel students who contributed to all aspects of field sampling and laboratory analysis throughout this project.