Abstract
An urban stream was examined from June to August in 2014 to determine if the effluence exiting a stormwater drainage pipe affected the benthic and drift macroinvertebrate assemblages above and below the pipe. The distribution of macroinvertebrates was also compared above and below the pipe to determine the difference in composition. Statistical means were run to explore the data and to identify any significant correlations between the assemblages in relation to the pipe, month, and location in the stream. Physical parameters, including water temperature, pH, dissolved oxygen, turbidity, and flow rate were collected from the river on each sampling date. The Shannon-Wiener and Simpson Indices were used to assess the richness and evenness distribution on a month-to-month basis. The macroinvertebrate assemblage varied with each month with respect to position above and below the drainage pipe. This was true for both benthic and drift macroinvertebrates.
Keywords: aquatic invertebrates; benthic invertebrates; ecology; diversity; macroinvertebrate distributions
Introduction
Worldwide urbanization has resulted in detrimental effects in its corresponding urban streams. One of the aspects of urbanization is the replacement of natural land by asphalt concrete, which allows for more waste to wash into streams and increases the chance of local flooding1. Local flooding intensifies the possibility of potential destruction of local infrastructure, while also initiating an influx of chemicals among other hazardous substances into waterways of the region. The introduction of contaminated runoff from storm drains emptying into such streams often causes a shift in the stability of a river system2. Of the five hundred pounds of pesticides used yearly in the United States, 52% are dispatched through storm drains. These pesticides contain the insecticides fipronil and dichlorvos, among others, which are concentrated at levels high enough to pose a threat to the water-dwelling organisms in urban waterways3. Though monitoring projects are used extensively throughout the country to monitor physical and chemical factors of streams, these projects do not necessarily assess the overall health of the streams. Many of these projects are costly, both in terms of money and time. It has been proposed that a more efficient method is to evaluate these stressors on the freshwater macroinvertebrate community4. Because of their ability to combine effects of short-term changes on a stream over long periods of time, macroinvertebrates are considered to be indicator species; a species used to monitor the stream’s stability and evaluate the impacts of environmental disturbances on an ecosystem5. Restoration and management of these sorts of ecosystems are driven by knowledge of factors affecting the macroinvertebrates inhabiting these river systems6. In such urban streams, macroinvertebrate communities have become progressively disrupted downstream from storm drains7. In this study, the effects of the existing storm drain in Macy Park on both the benthic and drift macroinvertebrate assemblages were examined, above and below the drainage pipe.
This study tests the hypothesis that there will be a difference in the macroinvertebrate communities, both benthic and drift, above and below the drainage pipe at the study site in Macy Park of the Saw Mill River in southwestern Westchester County, New York. The Saw Mill River, an urban stream flowing from Chappaqua to Yonkers in New York, accounts for a wide range of animals and plants. Heavy land use, population density, and miles of roadway have presented serious challenges to the river, these including flooding, pollution, and habitat loss. Increasing population density as a result of increasing urbanization has placed nearly 110,000 people in the watershed of the river. This represents around 12 percent of Westchester’s population. Other areas such as the upper reaches contain around 1,000 people per square mile, up to the river’s mouth, with close to 10,000 people per square mile8. The study area has extensive recreational use, including ball fields, a playground, picnic areas, and an excellent boating and fishing area which have existed for many years since the parks opening in 1926. Direct financial benefits resulting from boating and sport fishing may provide employment, profit, and dollar savings for commercial fishers and wholesalers9. However, as a result of runoff due to urban flooding, pollutants reach levels so high in that the area may become unsuitable for human and animal recreational use (Clean Water Education Partnership, no date given). The introduction of storm water pollution and runoff thereby poses a threat not only to the organisms living in the urban stream, but to the local communities as well. The macroinvertebrate community, including caddisflies, chironomids, insect larvae, snails, and molluscs, are affected by any alteration to the river. Among the organisms found in this study were the net-spinning caddisflies. Belonging to the family Hydropsychidae, the net-spinning caddisfly is known for its abundance in stable freshwater systems and its sensitivity to pollutants and certain contaminants, deeming it an overall good indicator species. As an indicator species, the caddisfly could assess any potential problems the drainage pipe and its outflow may be posing in this current study. Solutions to such problems are often not possible because those who are held accountable for all changes in local waterways: stormwater managers, public works departments, and local citizens, most often hold conflicting opinions about the placement and location of storm drains with hardly any regard to the stability of each portion of the river. To understand how storm drainage may affect the organisms in urban streams, sampling was conducted both above and below the drainage pipe for both benthic and drift organisms.
Materials and Methods
Study Site (Site Description)
The Saw Mill River drains into the Hudson River in downtown Yonkers. The river compromises a 26.5 square mile watershed basin consisting of surrounding residential communities along with forested recreational areas8. Being an urban stream, the river accommodates several drainage pipes and therefore accounts for all the urban runoff during flooding. The river flows approximately 20 miles long from suburban to urban areas in Westchester County8. A general decline in environmental quality is seen from north to south. Part of the river located in New Castle is said to be “relatively healthy”, with plants filtering pollutants in the riparian zone along with growing tree canopy to keep the water cool and rich in oxygen. Moving farther south through Yonkers, the Saw Mill River has been brought to daylight by removing any concrete that had previously covered it, however providing very few environmental functions8.
Data
Data was collected above and below the drainage pipe for both benthic and drift macroinvertebrates in the study area. The sampling site was located in the Saw Mill River of Macy Park, in Ardsley, New York. The most efficient method to obtaining quantitative data is through the use of a Surber sampler (500 ?m mesh), which was used to collect samples both above and below the drainage pipe. To collect drift samples, the Surber sampler was held approximately 4 feet into the water in the direction of the current, which flowed through the net and into the attached Surber jar at the end. To collect benthic samples, the Surber sampler was dug into certain areas of substrate and allowed to collect materials that would fall to the bottom of the jar. Along with macroinvertebrates for both, the Surber jar for the benthic samples contained large rocks, small pebbles and vegetation, while the Surber jar for the drift samples usually contained water and occasional vegetation. On a monthly basis, the samples were generally collected between 10:00 h and 13:00 h on June 22nd, July 29th, and August 8th in 2014. Using the Surber sampler, 4 replicates, 2 above and 2 below the drainage pipe were done in June and July, and 6 replicates, 3 above and 3 below the drainage pipe were done in August in order to ensure a fair sample size.
Physical variables were measured with Vernier probes and sensors, including dissolved oxygen, turbidity, and flow rate. A Hanna pH meter and a Hanna water temperature thermometer were also used throughout the study. Dissolved oxygen was measured by immersing the calibrated tip of the probe into the water and gently stirring while collecting data on an electronic device. Turbidity was measured by injecting a sample of the water in the appropriate chamber of the calibrated turbidity sensor. Flow rate was measured by placing the sensor’s propeller in the direction of the current to measure the velocity of the flowing water. The temperature and pH thermometers were placed into the water, and values were read. Samples were consistently taken 10 feet downstream (below) the pipe and 15 feet upstream (above) the pipe. Data was taken in the same order each time sampling occurred, with the area below the drainage pipe sampled first, starting with water temperature and pH, followed by dissolved oxygen, turbidity, and flow rate. A similar procedure was completed above the drainage pipe and data was recorded for both.
For each month, the weather conditions for each site had scattered clouds but were primarily warm, except for a thunderstorm that occurred the night before sampling in July, which increased the volume and rate of flow of the water. The fast riffle area of the river above the drainage pipe was 24 cm deep and exhibited a rocky substrate. The slow riffle area of the river below the drainage pipe was approximately 71 cm deeper than that of the area above, and had more of a coarse sand substrate. It was observed that the water levels in both July and August were lower than in June, and in August, water levels were low enough in that areas with rocky substrate were exposed.
June seemed to be consistent with the norm of the river, while July had a remarkably low water level along with August displaying a very rocky substrate.
Data Analysis
All organisms collected were transferred from the Surber sampler into separate jars that were clearly labeled with pencil on parchment paper listing the date, site, and time of collection. A preservative made up of 75% ethanol and 0.025% rose bengal was added into each jar at the sampling site in order to preserve and stain the living macroinvertebrates. Organisms were later sorted in the laboratory and keyed to the lowest practicable taxon using reference texts10. Shannon Diversity and Simpson Diversity indices were also incorporated into this study in order to assess the richness and evenness distribution on a month-to-month basis. Data was analyzed using the Statistical Package PAST V2.1711.
Results and Discussion
Table 1. Macroinvertebrate distributions with respect to assemblage position above and below the drainage pipe.
Macroinvertebrate Data
The benthic and drift fauna above and below the drainage pipe showed a significant difference in the numbers and types. Among Insecta, twenty-eight organisms of Hydropsychidae were found above while nine species below, 3 species of Chironomidae were found above while zero species were found below, and one species of Elmidae, the Riffle Beetle, was found above while zero species below. Among Gastropoda, one species of Physidae was identified with thirteen organisms of Physid Snails found above while two organisms below. One species of Bivalvia, the Bithynia Snail, was identified, along with one species of Arachnida, the Hydrachnidia, and one species of Clitellata, the Aquatic Earthworm.
Statistical Analysis of Macroinvertebrate Data
Since the assemblage values were different above and below the drainage pipe, the means were statically evaluated using the specific package PAST. The macroinvertebrate occurrence was measured above and below for the species obtained during experimentation in the months of June, July, and August. In June, the sample size was 11 and the variance was 32.618, which showed to be seemingly high. The correlation coefficient, “R”, output a very strong and accurate value of 0.927, indicating more organisms will be found above than found below for a majority of the species. With a p-value of .043, the null hypothesis stating that the frequency of species found above is no more than the frequency of the species found below is rejected and the alternate hypothesis is accepted that there are more species above than below seen by a p value less than .05. A similar significance is seen in July with a p-value of .047, only slightly higher than that of June, showing values that are very consistent. The alternate hypothesis is supported once again, proving there are more species above than below. The R value dropped to around 0.565, which overall is much smaller than that of June, indicating that the number of organisms will not always be larger above than it may be below for each particular species. In August, however, there was no significant difference in the assemblages above and below the drainage pipe, with a p-value of 0.136, indicating a very high similarity in the distribution of organisms.
Species richness and evenness measured with the Simpson and Shannon-Weiner Indices were calculated for each month. Diversity t-tests were run for the two indices to evaluate the level of diversity and evenness seen between the two locations in regard to the pipe. The Simpson Index output a p-value of 0.429, which showed a very insignificant diversity above versus below the pipe. The Shannon Index further showed a very insignificant diversity above versus below the pipe with a p-value of 0.11, indicating very little difference in the evenness when comparing the two locations in reference to the pipe. Similar results were seen with diversity t-tests run above versus below the pipe for the months of July and August, indicating further similarity in the diversity and evenness in the experimental study area.
Table 2. Physical data collected at the site above and below drainage pipe at the dates specified.
Physical Data
Physical data was collected at Macy Park above and below drainage pipe on a monthly basis from June to August. The time each sampling occurred was recorded at arrival and departure of the station. Several replicates were taken, 2 above and 2 below for June and July, and 3 above and 3 below for August. These replicates were done in order to ensure that there would be enough of a sample set to examine between the three months. An extra replicate, both above and below, was taken in August due to an exceptionally low water level in the river.
In July, the study site experienced a heavy thunderstorm the night before sampling. This rainfall could signify the immensely high flow rate found above the drainage pipe for that month. One would assume that in neutral pH’s with a high flow rate, high dissolved oxygen levels are expected, which should provide a better habitat for macroinvertebrates. Rapidly moving water tends to contain a lot of dissolved oxygen, whereas stagnant water contains less. Higher flow rates found above versus below suggests greater values of dissolved oxygen above and thereby should support a greater population of macroinvertebrates that need dissolved oxygen to survive. In each month, June, July and August, the flow rates and dissolved oxygen levels were greater above than below. A statistical analysis was run to explore whether this was significantly significant in terms of the data collected in this study. The p-value found for flow rate was .086612, while R, or the correlation coefficient, was 0.25546. For dissolved oxygen, the p-value outputted was .082299 and the R-value was 0.57373. Although the data suggest that the numbers above are greater than below, this is not statistically significant, most likely due to too small of a sample size. However, according to the R-values, there was a weak positive correlation for the values found for flow rate, meaning there was some sort of correlation between the flow rate and the month. The R-value for the dissolved oxygen data showed a moderate positive correlation between the dissolved oxygen and the month of the year, meaning the amount of dissolved oxygen produced was in some way dependent on the month, June, July, or August. Being that there was a moderate correlation between the two, it follows that an increase in flow rate as the months progressed would also allow an increase in dissolved oxygen levels as well. With an increase in dissolved oxygen, more organisms are thus able to survive.
Conclusion
The evaluation of the benthic and drift macroinvertebrates clearly demonstrates their value in assessing water quality and the differences above and below the site’s drainage pipe. The macroinvertebrate community indicated that there was a difference above and below the drainage pipe, and was consistent with what was found in the literature. The macroinvertebrate assemblages varied with the month of the year. Except for the month of August, there was a difference in the macroinvertebrate communities, both benthic and drift, above and below the drainage pipe at the study site, sustaining the alternate hypothesis. Given the fact that there was a difference would be consistent with the notion that substances coming from the drainage pipe may be affecting the macroinvertebrate distribution. The gap in research pertaining to the lack of information known about the drainage pipe calls for future studies to examine the nature of what comes through the pipe. In August, the null hypothesis was sustained, stating that there wouldn’t be a significant difference in the macroinvertebrates communities, both benthic and drift, above and below the drainage pipe at the study site. In both June and July, the macroinvertebrate distributions above and below show a significant difference, with the difference seemingly larger in July. In regard to species, June had a sample size of 11 different kinds of species while July had 10. August had a population size of 9, which considered them smaller. As opposed to June and July, data collected in August indicated that there was not a significant difference in the numbers and types of macroinvertebrates above and below the drainage pipe. Although the sample sizes were close in value, August had a much smaller population size of organisms overall than both June and July. This could account for the scarcity of organisms found in that particular data set, thereby leading to a lesser amount of organisms to examine and statistically analyze. The reproductive cycle of macroinvertebrates could account for an explanation of the sudden decrease in population size for many of the organisms found in the sample sets. The most abundant organism found in every month, both above and below and both benthic and drift, was the net-spinning caddisfly. These species generally represent a stable ecosystem, and depending on which species that caddisfly belongs to will determine what kind of case they will make for themselves; in this case, the net-spinner spinning organic debris into a thin net. In terms of the caddisfly development pattern, the life cycle shown for these organisms begins with larval feeding and growth in the autumn, winter, and spring. Not until late spring and early fall do these organisms emerge as flying adults. However while living in a freshwater ecosystem, larvae are active in very cold water and prefer this environment to ensure that most caddisflies will find a member of the opposite sex to reproduce12. The net-spinning caddisfly population in this specific research for June, July, and August are 37, 19, and 3, respectively. The decline in population size as each month proceeds could be a result of the net-spinning caddisflies stamina to reproduce in colder seasons, emitting larvae in the early spring, and setting off as adults in late spring and so forth. These were the results of a preliminary study, and based on this data a more extensive study is wanted. In order to further such research, the net-spinning caddisflies, among other freshwater organisms, should be put forward to monitor the life and health not only in the Saw Mill River but all freshwater ecosystems.
Acknowledgements
I would like to thank both Joseph W. Rachlin (Lehman College of CUNY) and Barbara E. Warkentine (SUNY Maritime College) for their continued help throughout my research and on the subject of Freshwater Ecology. I would also like to thank Diana Evangelista for her guidance and support along with the Ardsley PTA for materials used in the field.
Additional References
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