Browsing by Subject "stream ecology"
- Results Per Page
- Sort Options
Item Open Access Effects of urbanization on stream ecosystem functions(2011) Sudduth, ElizabethAs the human population continues to increase, the effects of land use change on streams and their watersheds will be one of the central problems facing humanity, as we strive to find ways to preserve important ecosystem services, such as drinking water, irrigation, and wastewater processing. This dissertation explores the effects of land use change on watershed nitrate concentrations, and on several biogeochemical ecosystem functions in streams, including nitrate uptake, ecosystem metabolism, and heterotrophic carbon processing.
In a literature synthesis, I was able to conclude that nitrate concentrations in streams in forested watersheds tend to be correlated with soil solution and shallow groundwater nitrate concentrations in those watersheds. Watershed disturbances, such as ice storms or clear-cutting, did not alter this relationship. However both urban and agricultural land use change increased the nitrate concentrations in streams, soil solution, and groundwater, and altered the correlation between them, increasing the slope and intercept of the regression line. I conclude that although the correlation between these concentrations allows for predictions to be made, further research is needed to better understand the importance of dilution, removal, and transformation along the flowpaths from uplands to streams.
From a multi-site comparison of forested, urban, and urban restored streams, I demonstrated that ecosystem functions like nitrate uptake and ecosystem metabolism do not change in a linear unidirectional way with increasing urbanization. I also showed that Natural Channel Design stream restoration as practiced at my study sites had no net effect on ecosystem function, except those effects that came from clearing the riparian vegetation for restoration construction. This study suggested further consideration is needed of the ecosystem effects of stream restoration as it was practiced at these sites. It also suggested that more study was needed of the effects of urbanization on ecosystem metabolism and heterotrophic processes in streams.
In a 16-month study of ecosystem metabolism at four sites along an urbanization gradient, I demonstrated that ecosystem metabolism in urban streams may be controlled by multiple separate effects of urbanization, including eutrophication, light, temperature, hydrology, and geomorphology. One site, with high nutrients, high light, and stable substrate for periphyton growth but flashy hydrology, demonstrated a boom-bust cycle of gross primary production. At another site, high benthic organic matter standing stocks combined with low velocities and high depths to create hypoxic conditions when temperature increased. I propose a new conceptual framework representing different trajectories of these effects based on the balance of increases in scour, thermal energy and light, eutrophication, and carbon loading.
Finally, in a study of 50 watersheds across a landscape urbanization gradient, I show that urbanization is correlated with a decrease in particulate carbon stocks. I suggest that an increase in dissolved organic matter quality may serve to compensate for the loss of particulate carbon as fuel for heterotrophic microbial activity. Although I saw no differences among watershed landuses in microbial activity per gram of sediment, there was a strong increase in the efficiency of microbial activity per unit organic sediment with increasing watershed urbanization. Ultimately, I hope that this research contributes to our understanding of stream ecosystem functions and the way land use change can alter these functions, with the possibility of better environmental management of urban streams in the future.
Item Open Access Energetic Opportunities and Chemical Risks in Urban and Forested Stream Ecosystems(2024) Behrens, Jonathan RichardUrban watersheds are dynamic ecological systems shaped by social, chemical, physical, and ecological forces. Urban stressor and disturbance regimes alter the availability and incorporation of energetic resources and chemical risks into stream ecosystems, and their ultimate transport and fate to downstream or paired terrestrial ecosystems. In this dissertation, I investigate how human-driven stressors (land-use, organic and inorganic chemical contaminants, heat, and hydrologic "flashiness") alter the energetic and chemical regimes of each system. Study systems include stream ecosystems that drain urban (Ellerbe Creek) and forested (New Hope Creek) watersheds in the Piedmont of North Carolina. In Chapter 1, I quantified the concentrations of chemical indicators in a novel approach to characterize the timing and spatial distribution of three common mixtures of contaminants (and their chemical indicator) in our urban watershed (Ellerbe Creek): treated and untreated sewage (sucralose, artificial sweetener), lawncare runoff (DPG and 6PPD-Q, automobile tire additives), and road runoff (Glyphosate and AMPA, active ingredient of herbicide RoundUp® and its major degradation product). We demonstrate that the concentrations of these chemical indicators, and by extension the mixtures of contaminants they represent across the heterogenous urban watershed, are highly variable across the landscape, seasons, and discharge regimes. In Chapter 2, I measured rates of ecosystem metabolism (gross primary production and ecosystem respiration), macroinvertebrate secondary production, and aquatic insect emergence to assess the efficiency at which energy moves through two types of urban stream ecosystems (wastewater and stormwater dominated) in Ellerbe Creek and our forested watershed in New Hope Creek. Urban disturbance regimes alter the composition, magnitude, and timing of energy availability. In turn, the efficiency at which energetic inputs were converted into primary production did not readily predict macroinvertebrate secondary production and emergence in our urban sites. This is the first known investigation of primary and secondary production in an urbanized watershed, and it provides compelling evidence that energetic regimes in two common types of urban watersheds (wastewater and stormwater dominated) are notably distinct from one another. Finally, in Chapter 3, emergent insects collected for Chapter 2 were assessed for trace metals to investigate how distinct communities of emergent winged insects altered the timing, magnitude, and composition of metals flux. I demonstrated that the transport of positive (energy) and negative (trace metals) subsidies is driven by distinct communities of organisms, which are simplified and constrained in our wastewater and stormwater dominated sites, rather than total emergent biomass or metals loading alone. Families of aquatic insects were differentially efficient at moving metals (Zn, Cu, Se) out of the stream food web and into the terrestrial food web. The biological community dynamics—composition, diversity of traits, and timing—play an equally if not more important role in the magnitude, timing, and composition of metals flux relative to metals inputs alone. The availability, assimilation, and transformation of energetic and contaminant inputs in-stream will ultimately enable (or suppress) export into paired riparian and terrestrial ecosystems. In human impacted systems, such as urban watersheds, analysis requires methods, models, and concepts in ecosystem and community ecology, urban ecology, environmental chemistry, and biogeochemistry. By further exploring the heterogeneity of urban watersheds, as demonstrated in this dissertation, stakeholders can better manage these ecosystems to support wildlife communities and human societies.
Item Open Access Stormwater and organic matter in the urban stream continuum(2017) Fork, Megan L.Dissolved organic matter (DOM) is present in all natural waters and modulates
aquatic ecosystems by absorbing light and heat, and because it comprises a complex
mixture of organic molecules including amino acids, sugars, fulvic acids, and humic
material. DOM is derived from dissolution of organic matter and can be altered by
both biotic and abiotic processes that may its structure or mineralize it to CO2.
Urbanization is a widespread agent of landscape change that can alter DOM
regimes by changing the amount and types of organic matter in the catchment and
by changing the way that water moves through the landscape (transporting DOM
from land to stream). This dissertation examines DOM in urban stream networks,
exploring its sources, bioavailability, and broad patterns throughout the continental
United States.
We determined the role of impervious infrastructure as a proximate source of
DOM to stormwater by a) constructing an annual carbon budget for the roof of a
house as a small catchment nested within the 60 h catchment of an urban headwater
stream and b) comparing the estimated fluxes of solutes and stormwater from imper
vious infrastructure in the catchment. We found that roofs convert nearly one-third
of the leaf litter carbon they receive into dissolved organic carbon (DOC), which
leaves through downspouts. On the event scale, we estimated fluxes of DOC and
total dissolved nitrogen from impervious surfaces that generally exceed the fluxes
that leave the catchment in stream stormflow.
When we compared the chemical composition of runoff from impervious surfaces
to stream stormflow, we found them to be distinct, despite the fact that the we
estimated a volume of runoff from impervious surfaces that generally matched the
volume of water flowing through the stream during storms. Our findings suggest
that a water source other than baseflow and impervious runoff contributes to stream
stormflow, and that a considerable proportion of impervious runoff is lost before it
reaches the catchment pour point.
An experimental incubation of potential DOM sources in the urban landscape and
DOM in stormwater showed that urban DOM is highly bioavailable. The composition
of DOM also became more homogeneous over the course of processing.
Finally, we examined continental-scale patterns and long-term trends in riverine
DOC. Unlike the widespread ’browning’ trends observed in far northern aquatic
systems, we did not find evidence for long-term increases in DOC throughout most
of the U.S. Instead, we both decreases and increases in long-term DOC concentrations
that differed among regions and generally seemed to be driven by changes in weltand
cover. We also found evidence for a marginal effect of impervious surfaces that
increases DOC concentrations at high canopy cover, consistent with our observations
that urban infrastructure can contribute considerable DOM loads in storm runoff.
Together, this research shows that urban stormwater infrastructure functions
as the ephemeral headwaters of the urban stream network. In catchments with
significant canopy cover, these ’engineered headwaters’ collect and transform organic
matter between storms and transport DOM during stormflow.
Item Open Access Thermal Pollution in Urban Streams of the North Carolina Piedmont(2013) Somers, KayleighCurrently, cities comprise 52% of the Earth's land surface, with this number expected to continue to grow, as most of the predicted 2.3 billion increase in population over the next 40 years is expected to occur in urban areas (United Nations Population Division 2012). Urban areas necessarily concentrate food, energy, and construction materials, and as a result tend to be hotter and more polluted than the surrounding landscape. All urban ecosystems are thus quite altered from their pre-urban state, but urban streams are particularly impacted. As low lying points on the landscape, streams are subject to the degradation caused by urban stormwaters, which are transmitted rapidly from the surfaces of pavements, roofs, and lawns through stormwater infrastructure to streams.
The systematic changes seen in many urban streams have been described as the "Urban Stream Syndrome" (USS) and serve as an organizing conceptual framework for urban stream research (Walsh et al. 2005b). A primary symptom of USS is increased flashiness in hydrographs, as stormwater in urban areas is routed efficiently into streams (Booth and Jackson 1997, Konrad and Booth 2005). With this stormwater runoff comes intense scour leading to deeply incised channels, large amounts of contaminants and nutrients, and, as will be discussed in this thesis, heat surges (Booth 1990, Tsihrintzis and Hamid 1997, Walsh et al. 2005a, Nelson and Palmer 2007, Bernhardt et al. 2008). At baseflow, urban streams are contaminated by sanitary sewage leakages, are unable to exchange water with their floodplains due to incision and with groundwater due to lower water tables, and are warmer due to canopy loss and urban heat island effects (Paul and Meyer 2001, Pickett et al. 2001, Groffman et al. 2002, 2003). These baseflow and stormflow changes lead to the loss of sensitive taxa and increase in tolerant biota, as well as changes in ecosystem function, including carbon and nitrogen processing (Paul and Meyer 2001, Meyer et al. 2005, Imberger et al. 2008, Cuffney et al. 2010).
The urban heat island effect can increase air temperatures up to 10°C above those in surrounding, non-urban areas, while impervious surfaces can reach temperatures up to 60°C (Asaeda et al. 1996, Pickett et al. 2001, Kalnay and Cai 2003, Diefenderfer 2006). These changes are particularly troublesome, as research has shown that temperature is a controlling factor in aquatic systems for both stream biota and ecosystem processes (Allen 1995, Kingsolver and Huey 2008). Thermal changes control and can alter basic morphological features of biota, such as size and growth rates (Gibbons 1970, Kingsolver and Huey 2008). USS synthesis reports have called for further research into the processes by which urban areas influence the temperature of streams and the resulting effects on the ecosystems, but until recently have largely been ignored (Paul and Meyer 2001, Wenger et al. 2009). This dissertation explores the timing, magnitude, and pattern of thermal pollution for streams within urban heat islands, with the goal of understanding what aspects of watershed development most strongly influence the thermal regimes of streams. In order to explore thermal pollution in urban streams, I asked three overarching questions:
1) How much hotter are highly urban streams than streams in less developed watersheds?
2) How far do urban heat pulses propagate downstream of urban inputs?
3) How can development configuration mitigate or exacerbate development amount in mediating urban thermal pulses?
In Chapter 2, I explore the differences in baseflow and stormflow temperatures in 60 watersheds across the North Carolina Piedmont that ranged across a gradient of urbanization. I asked:
1) How do maximum temperatures at baseflow and maximum temperature surges at stormflow differ across watersheds with varying development intensity?
2) What reach- and watershed-scale variables are most correlated with these 2 aspects of stream thermal regimes?
3) Do stream management approaches (riparian buffers, channel restoration) address the links between these variables and stream temperature?
I found that the 5 most urban streams were on average 0.6°C hotter at baseflow than the 4 most forested streams. During a single storm event, urban streams showed an increase over five minutes of up to 4°C, while forested streams showed little or no thermal increase. Reach-scale characteristics, specifically canopy closure and width, primarily controlled baseflow temperatures. These local factors were not important drivers of stormflow temperature changes, which were best explained by watershed-scale development and road density. Management that focuses on baseflow temperatures, such as riparian buffers and reach-scale restoration, ignores the intense urban impacts that occur regularly during storm events.
Next, in Chapter 3, I explore longitudinal temperature patterns in a single stream, Mud Creek, in Durham, North Carolina. Mud Creek's headwaters are suburban, and the stream travels through a number of housing developments before entering a 100-year-old forest. I placed 62 temperature loggers over a 1.5 km reach of this stream. To explore the mechanisms by which stormflow heat pulses dissipate along this stream reach, I asked:
1) What is the range of heat pulse magnitudes that occur over a year?
2) What is the maximum distance that a heat pulse travels downstream of urban inputs?
3) How do the magnitude and distance vary with storm characteristics, including antecedent air temperature and amount and intensity of precipitation?
I found that heat pulses with amplitude of greater than 1°C traveled more than 1 km downstream of urban inputs in 11 storm events over one year. This long dissipation distance, even in a best-case management scenario of mature and protected forest, implies that urban impacts across a developing landscape travel far downstream of the impacts themselves and into protected areas. Heat pulses greater than 1°C occurred in storms with greater intensity of and total precipitation and greater time of elevated storm flow. Air temperature, flow intensity, maximum flow, and total precipitation controlled the magnitude of the heat pulse, while the distance of dissipation was controlled by the magnitude of the heat pulses and total precipitation. The importance of air temperature, flow, and precipitation metrics imply that both magnitude and distance of dissipation of heat pulses are likely to increase with climate change, as air temperatures increase and sudden, intense storms become more frequent. This translates to even greater ecological impacts in urban landscapes like Durham municipality, where the 98.9% of streams less than 1 km downstream of a stormwater outfall will become even more likely to be impacted by urban stormwaters.
In Chapter 4, I examine which aspects about development best explain thermal differences observed at baseflow and stormflow. To do this, I selected 15 similarly sized watersheds in the North Carolina Piedmont region within 45 to 55% development that varied in other development characteristics, specifically density of stormwater infrastructure and aggregation of development patches. I asked two questions:
1) How does the configuration and connectivity of development within a watershed influence baseflow and stormflow temperatures in receiving streams?
2) How do baseflow and stormflow temperatures vary with development characteristics?
I found that aspects of development varied greatly within this urban intensity subset, with ranges for some metrics nearly equal to the variation observed across all watersheds in the landscape. Longer pipe lengths, shading from incised channels, and shaded impervious surfaces resulted in cooler baseflow temperatures. As in Mud Creek, stormflow metrics were influenced through two physical pathways: air temperature and either flow intensity, to explain overall thermal change, or antecedent flow, to explain intensity of thermal change. Greater sub-surface connectivity of development to the stream network increased thermal responsiveness to storms through faster delivery and greater amount of heated runoff. Greater proportions of forest in a watershed decreased the amount and temperature of runoff delivered to the stream, while development within the riparian zone throughout a watershed led to warm baseflow temperatures and lack of response to stormflow heat surges. By decreasing the connectivity of development to the stream network, thermal regimes of streams can be less impacted even in relatively urban watersheds.
Thermal pollution in urban streams is a problem that will only be exacerbated by predicted climate change and urban expansion. These findings imply that thermal pollution is a problem throughout urban landscapes, even far downstream of urban inputs and within protected areas, and must be managed as an important component of the USS. Future research should focus on the transferability of these findings to regions outside of the southeastern United States and to the movement of other urban pollutants, and on exploring the potential to manage these systems by decreasing sub-surface connectivity.