Humics in the City: Dissolved organic matter (DOM) as a signal of altered biogeochemistry in urban waterways

Abstract:

Streams in urban systems are subjected to a range of physical and chemical stresses. Stream burial, channelization and stormwater management alters flowpaths, changing the natural hydrology of a watershed. Surface runoff and storm culverts increase the load of chemicals from sources such as fertilizers and road salt and legacy chemicals and nutrients can enter through groundwater. These alterations to the physical and chemical environment, along with the introduction of non-native species, results in significant impacts to the ecosystem and biogeochemical processes. Understanding these urban alterations to stream biogeochemistry can help explain urban impacts to stream environments and provide an indicator for which streams are important targets for restoration.

We focused on dissolved organic matter (DOM), an important base of the carbon cycle in aquatic ecosystems, as a potential signal of urban alterations to stream biogeochemistry. We collected water samples from a range of urban aquatic systems, including headwater streams and higher order rivers as well as rivers impacted by wastewater effluent and streams with varying degrees of urbanization as measured by impervious cover. DOM was analyzed for DOC and DON concentration and chemical characterization was performed by UV-Vis and fluorescence spectroscopy. Additional hydrologic and chemical measurements were also used to support the DOM data. We found distinct alterations in DOM chemistry corresponding with urbanization, including changes in the C:N ratio of the organic matter, changes in the aromaticity, and apparent changes in dominant source material for the DOM. These results help quantify how urbanization impacts carbon-related biogeochemical processes in aquatic environments and suggest a possible metric for degree of urban impacts which could be used to guide restoration efforts.