Stormwater and organic matter in the urban stream continuum
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.
Ecology
Limnology
dissolved organic matter
impervious surface
stream ecology
urban stream
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