An Examination of the Impact of Urbanization on Stream Biodiversity and Ecosystem Function
The "Urban Stream Syndrome" is a term that refers to a group of predictable negative impacts to stream ecosystems due to the alteration of the natural hydrologic regime that is associated with urbanization. These negative impacts include increases in the volume and intensity of stormwater inputs to streams, channel erosion, streambed sedimentation, and nutrient and pollutant concentrations. The impacts of urbanization ultimately degrade the habitat available to the aquatic biota in streams including fish and benthic macroinvertebrate communities. The decline in benthic macroinvertebrate taxa richness due to urbanization has been well documented. However, the impact of the stressors associated with the increased stormwater flashiness to the composition of the benthic macroinvertebrate assemblages’ taxa and trait richness and diversity is not well known. For my dissertation, I proposed three research studies designed to improve the understanding of how the increased stormwater volume and intensity due to unmitigated runoff from urban areas impacts the benthic macroinvertebrate assemblages’ taxa and trait richness and diversity. To do this, I first examined a 26-year data set to study the impact of land use changes on biodiversity and ecosystem function in stream ecosystems in watersheds that span a gradient of impervious cover (IC) and stream habitat conditions. Next, to better understand the impact of urbanization on biodiversity and ecosystem function, I examined the relationship between stream habitat diversity and aquatic insect taxa and trait richness and diversity at the watershed scale. Finally, to better understand stormwater impacts on macroinvertebrates assemblages, I took advantage of a natural field experiment to compare macroinvertebrate taxa and trait richness and diversity in 2 adjacent headwater tributaries that received stormwater runoff through different processes (via stormwater infrastructure versus natural overland and subsurface processes).Study 1) I investigated: How do EPT Taxa and Trait Richness and Diversity change with increases in percent IC? I hypothesized that EPT taxa richness and diversity would decline greater than EPT Trait Richness and diversity with increasing percent IC. I also investigated: Which Indicators of Hydrologic Alterations (IHA) have greater impacts on EPT taxa and trait richness and diversity? I hypothesized that the IHA metrics closely associated with storm runoff flashiness would have greater impacts on taxa and trait richness and diversity. EPT taxa richness and diversity significantly declined with increases in percent IC. Trait richness and diversity declined with increases in percent IC, but not to the same extent as taxa richness and diversity. Threshold Indicator Taxa ANalysis (TITAN) identified distinct percent IC thresholds for sensitive EPT taxa between 5 to 10% IC while tolerant EPT taxa increased in abundance beginning around 30% IC. TITAN analysis identified similar thresholds for traits associated with sensitive EPT taxa between 5% to 10% IC while traits associated with tolerant EPT taxa increased in abundance starting at 30% IC. Stream habitat condition declined with increases in percent IC which was positively correlated with the R-B Flashiness Index. Higher shear stress associated with flashy stormwater contributed to the less stable and more homogenous habitats found in urban streams in watersheds with impervious cover greater than 25%. TITAN analysis found distinct thresholds for sensitive EPT taxa between MHAP scores of 120 and 140 (lower end of the partially supporting habitat condition classification) while tolerant EPT taxa began increasing in abundance at MHAP conditions below 100 which are typical habitat scores in urban streams. TITAN analysis showed distinct declines in traits associated with sensitive EPT taxa at MHAP scores at 120 and a second threshold at MHAP scores of 80. Several sensitive EPT taxa were found to be indicators of streams with percent IC <10%. Several of the more tolerant EPT taxa were found to be indicators of streams with watersheds with percent IC greater than 25%. Study 2) I investigated: How do taxa and trait richness and diversity respond to decreases in stream habitat condition? I hypothesized that the decrease in total taxa richness and diversity would be greater than the decrease in total trait richness and diversity with decreasing habitat diversity. I also investigated: How are taxa and traits distributed within and between the microhabitats found in streams? I hypothesized that similar traits are found among the aquatic insect taxa residing in similar microhabitats within the same stream. Streams with fully supporting habitat conditions were more similar to each other than to partially supporting and impaired streams when described by taxa abundance; however, fully supporting and partially supporting streams were more similar when described by trait abundance. As MHAP scores declined, habitat diversity variability increased in the partially supporting and impaired sites. I found that both taxa richness and diversity declined at a greater rate than trait richness and diversity along an in-stream habitat diversity gradient. Trait richness and diversity changed very little with changes in habitat condition. The impaired streams generally had lower habitat diversity than both the partially supporting and supporting streams. Taxa richness was positively correlated with pools, runs, backwater, leaf packs, and riffles, while trait richness was positively correlated with runs, small wood, and riffles. Taxa diversity was positively correlated with pools and leaf packs while trait diversity was positively correlated with runs, leaf packs, small wood, and riffles. Riffles, leaf pack, large wood, and runs were important microhabitats for taxa richness while undercut banks were important for taxa diversity and habitat diversity and small wood were important for trait richness. The taxa and traits found in riffles and leaf packs were more similar than taxa and traits found in undercut banks and root wads. Study 3) I investigated: Do the patterns of taxa and trait richness and diversity differ between the 2 tributaries receiving stormwater from different sources? and Do the carbon sources available to benthic macroinvertebrate assemblages differ between these 2 tributaries? I hypothesized that both taxa and trait richness and diversity would be lower in the stormwater impacted (TI) tributary than in the forested (TF) tributary due to the increased stormwater runoff. I hypothesized that the carbon sources available to benthic macroinvertebrate food webs would be altered in the TI tributary with changes in food sources. The habitat condition in the TI tributary, which had higher predicted shear stress than the TF tributary for the same storm event, was impaired and had EMAHP scores significantly lower than in the TF tributary. Taxa richness and diversity were significantly higher in the TF tributary than in the TI tributary. Trait richness was also significantly higher in the TF tributary. Trait diversity was not significantly different between tributaries. The summer carbon δ13C values of the periphyton and leaf pack food sources were distinctly different in the TF tributary while they were very similar and closer to allochthonous source δ13C values in the TI tributary. The carbon δ13C values of the collector-gatherers, herbivore-scrapers, predators, and shedders were closer to the carbon δ13C values of the periphyton in both tributaries. In the winter, the carbon δ13C values of the periphyton and leaf pack food sources were distinctly different in both tributaries. All the FFG trait richness except for the collector-gatherer were similar between tributaries. The collector-gatherer richness was significantly greater in the TF tributary. Collector-gatherers have been found to be the most abundant benthic macroinvertebrate feeding groups in impaired urban streams like TI. Improving restoration design is necessary to restore the benthic macroinvertebrate community in urban streams. This study’s results show that unmitigated stormwater will have significant negative effects on the benthic macroinvertebrate assemblages. Addressing the sources of hydrologic alterations as part of a stream restoration plan may lead to more successful restoration of an urban stream aquatic ecosystem. Another improvement to stream restoration design to improve the benthic macroinvertebrate community would be to expand the habitat improvement designs to increase the diversity of habitats available to aquatic biota.