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Spatial and Temporal Variation in Benthic Macroinvertebrate Communities In an Urban Watershed

Susan F. Cushman1, Shannon M. Beston, Joan L. Hilton and Matthew R. Paufve 1Finger Lakes Institute, Hobart & William Smith Colleges, Geneva, NY

Introduction Urbanization is known to have profound impacts on stream ecosystems. The “urban stream syndrome” is a term coined for the many symptoms observed in streams due to anthropogenic changes to the land (Meyer et al. 2005). Flashy hydrographs due to storm drains and increased impervious surface result in quick delivery of pollutants and erosive forces to the stream channel. Benthic macroinvertebrates vary in their pollution tolerance and therefore are good indicators of stream health and urbanization impacts. While urbanization has historically occurred in the downstream reaches watersheds, many times agricultural land use dominates the upstream rural regions of watersheds, particularly in central NY. Castle Creek is the most urbanized watershed draining into the Seneca Lake, the largest of the Finger Lakes (Figure 1), but is dominated by agricultural (row crop) land used in the upper half of the watershed. Changes in a stream community are expected from headwaters to the mouth of a system, similar to the River Continuum Concept introduced by Vannote et al. (1980). However, multiple land use effects in differing regions of a watershed creates complexity in understanding the outcomes & potential for restoration. In addition, while many studies capture biological communities during one period of time, it is expected that the stream community is dynamic both spatially and temporally, potentially throughout this matrix of impacts. We designed a long-term study to monitor and better understand changes that occur along the rural –urban gradient throughout the year in a small urban watershed. The goal was to determine 1) where communities showed signs of impact, 2) how this changed seasonally and interannually.

Methods Two sites in rural, suburban, and urban locations were selected along Castle Creek based on surrounding and upstream land use (Figure 1). Sites were generally upstream of the nearest road crossing, except R1 which was below a low-traffic gravel road through the New York State Agricultural Experiment Station run by Cornell University. The upstream watershed from rural sites (R1 & R2) as well as S2 are dominated by agriculture (~70%). Developed land in R2 was only 6%, while it was 28% of the upstream watershed at U1. Forested land cover is primarily found in riparian zones upstream from R1 (Figure 1). Benthic macroinvertebrates (BMI) samples and water quality measurements were conducted at each site on a monthly basis from March to December 2012-2013. BMI were collected using a D-net held downstream while the substrate was disturbed along a 5m diagonal transect in riffle habitat for 5 min following the NYDEC protocol. The sample was preserved in 95% EtOH and sorted in the lab to family level taxonomic ID. Water quality parameters (temperature, DO, pH, and conductivity) were measured instream using a YSI 556 handheld multiprobe meter. Chloride levels were measured using LaMotte kit #4503-DR-01. BMI were analyzed by calculating the Percent Model Affinity (PMA; Novak & Bode 1992) to detect changes in community composition related to pollution. Since urban watersheds exhibit flashy flows and reduced habitat quality, a substrate (channel) stability index was also calculated by the proportion of BMI scrapers & collectors to shredders & collectors. Finally, to assess the impact urbanization has on foodwebs, the predator control potential was calculated by percentage of predators to all other functional feeding groups (Merritt &Cummins, 1996).

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Figure 5. Seasonal changes in benthic macroinvertebrate community across the rural to urban gradient in Castle Creek. Each land use category incorporates three monthly samples at two sites (n=6; error bars represent SEM). PMA is calculated by comparing the sample community to a NY model community (20% Chironomidae, 10% Trichoptera, 40% Ephemeroptera, 5% Plecoptera, 10% Coleoptera, 5% Oligochaeta, and 5% other). Spring = March-May; Summer = June-August; Fall = October-November. The asterisk above bars denote significant differences within each time period (p <0.05).

Discussion Our intensive study of Castle Creek over the last few years indicates that these communities are dynamic in response to land use impacts, seasonal and climatic (interannual) variation. The initial assessment of BMI using the PMA showed that most sites were considered moderately impacted (35-49% affinity), while suburban sites were only slightly impacted (50% affinity, in spring) and rural sites were severely impacted (<35% affinity) in fall. Interestingly, there was little difference between sites in summer, but there was a significant difference between rural, suburban, and urban sites in spring (Figure 5). Suburban BMI were significantly more healthy than rural and urban communities. As expected, urbanization but also agricultural land use in the watershed may play a role in degrading the stream community. Channel substrate was much more stable at R1 throughout 2012, however there were a few outliers that increased the variation in the data (Figure 6). Since climatic difference from year to year can effect the water level and therefore a myriad of other parameters, we expected to see more of a difference between 2012 (dry) and 2013 (wet; Figure 7). Future analysis of other seasons may elucidate how climate combined with various land uses effects stream communities and overall health. The presence of predators was also slightly higher at rural sites than suburban and urban which indicate the impact of pollution, including sedimentation downstream, of the survivorship of sensitive species (Figure 8 & 9). Predators are much more sensitive than generalists and therefore are a good indicator of water quality. Water quality itself was also significantly better at upstream, rural sites than downstream. Conductivity is known to be elevated in urban areas (Cushman, 2006) and is correlated to chloride concentration in Castle Creek (Figures 10 & 11). In conclusion, although the rural, agriculturally dominated sites in Castle Creek show signs of impact using the PMA, they indicated a slightly healthier stream community using functional feeding group analysis. They also show the most variation across the year. Urban sites are impacted in spring (PMA) but also show fewer sensitive species (predators) and poor water quality. Continued analysis of monthly samples across both 2012, 2013 and 2014 may better demonstrate the complexities of structural and functional dynamics in urban watersheds with agricultural headwaters. It is our hope to suggest better management and future restoration of Castle Creek, as well as understand recovery in multistressor systems.

References Cushman, SF. 2006. Fish movement, habitat selection and stream habitat complexity in small urban streams.

Doctoral dissertation, University of Maryland, College Park. Merritt, GW and KW Cummins, edited by. 1996. An introduction to the aquatic insects of North America.

Kendall/Hunt Publishing Co., Dubuque, IA. Meyer, JL, MJ Paul, and WK Taulbee. 2005. Stream ecosystem function in urbanizing landscapes. Journal of the North

American Benthological Society, 24:602-612. Novak MA & RW Bode. 1992. Percent model affinity: A new measure of macroinvertebrate community composition.

Journal of the North American Benthological Society, 11:80-85. Vannote, RL, GW Minshall, KW Cummins, JR Sedell, and CE Cushing. 1980. The river continuum concept. Canadian

Journal of Fisheries and Aquatic Sciences, 37:130-137.

Acknowledgements We thank field assistant Laura Carver Dionne (WS’13) for her help in Fall 2012. The land use watershed map (Figure 1) was made as a part of a course at HWS by S.M. Beston. The other maps were made by Rob Beutner (now at RIT). This project was funded by the HWS Biology Department and the FLI Endowment and through the NYS DOS in support of the Seneca Lake Watershed Management Plan.

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Figure 2. Rural site (R2) in summer. Figure 3. Suburban site (S2) in spring. Looking downstream, the right bank is eroded, but somewhat stabilized by woody debris.

Figure 4. Urban site (U2) in fall. Looking downstream, the right bank is channelized. Trash as well as a mattress that was dumped in the stream are pictured here.

Figure 10. Conductivity in Castle Creek increases in suburban and urban sites. Rural sites (1&2) are significantly lower than the downstream sites (* p< 0.001).

Figure 11. Chloride concentration in Castle Creek increases in suburban and urban sites. Rural sites (1&2) are significantly lower than the downstream sites (* p < 0.001).

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Figure 6. Channel substrate stability across the rural-urban gradient from March – December 2012. Substrate stability is a measure of the proportion of benthic macroinvertebrate scrapers and collectors to shredders and collectors. Stable substrate is considered plentiful above 0.5. Bars represent X±SEM (n = 51).

Figure 8. Interannual variability in channel substrate stability across the rural-urban gradient in spring. Stable substrate is considered plentiful above 0.5. Bars represent X±SEM of April and May comparing 2012 to 2013 (n=24).

Figure 7. Top-down predator control potential across the rural-urban gradient from March – December 2012. Percent predators is a measure of the predator to prey balance, and values above 0.15 indicate a healthy balance. Bars represent X±SEM (n = 51).

Figure 9. Interannual variability in top-down predator control across the rural-urban gradient in spring. Values above 0.15 indicate a healthy predator-prey balance. Bars represent X±SEM of April and May comparing 2012 to 2013 (n=24).

Figure 1. Map of Castle Creek watershed as an inset of the NY state and the Finger Lakes region. Study site locations are labelled by upstream land use category and order (moving upstream; U1 = Urban 1, U2 = Urban 2, S1 = Suburban 1, S2 = Suburban 2, R1 = Rural 1, R2 = Rural 2. Castle Creek flows directly in Seneca Lake ~100 m downstream of U1.

Figure 12. Satellite image of the lower portion of the watershed and stream reach with accompanying pictures looking upstream (a) and downstream (b & c) towards the confluence with Seneca Lake in spring. This culvert is the last of 5 underground tunnels that Castle Creek flows through (total ~500m). Pipes draining streets and unknown sources constantly flow. U1 site is indicated on the satellite map by the red triangle.

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