Urban Ecohydrology and the Watershed Microbial Continuum

A rapidly urbanizing world is adding unprecedented stress to water resources. Urbanization results in high rates of stream impairment, including but not limited to ecological stress, pollution, and flooding. A fully integrated and holistic perspective is necessary for effective remediation strategies for these wicked problems. This monitoring study from a heterogeneous urban watershed investigated the geospatial and seasonal interactions between urban hydrology and biological processes using stable isotope compositions, anions, and microbial.

This research found that nitrate concentrations in an urban watershed were ecologically driven. NO₃^- concentrations ranged from 0.4 to 12.7 mg/L throughout the watershed. Nitrate concentrations were responsive to seasonal, land use, and wet weather drivers. Average dissolved NO₃^- concentrations in the baseflow water column were the highest in the fall/winter (8.46 and 9.41 mg/L, respectively) and the lowest in the spring/summer (4.844 and 5 mg/L, respectively). Geospatially, the urbanized, downstream region of the watershed had the highest NO₃^- concentrations in the fall/winter seasons, while the forested uplands dominated NO₃^- concentrations in the spring/summer. The stable isotope compositions of δ¹⁵N and δ¹⁸O in the dissolved NO₃^- support for the hypothesis that these variations in NO₃^- concentrations across the watershed are driven by tree canopy cover, decomposition of particulate organic matter, and soil nitrification/denitrification processes.

The microbial community diversity and assembly processes were significantly correlated with changes in land use, though the effect of changes in land use was small. Assembly processes and diversity were more strongly responsive to seasonal and precipitation events. For this reason, it seemed plausible to test a microbial source tracking technique to partition between different landscapes, such as roadways, grasses, and forest. The study found that MST has potential for accurate portioning of the hydrograph and giving keen insights into the complex landscape response to rainfall intensity, but more sampling efforts are needed.

Overall, this body of work investigated the poorly understood interactions between ecohydrology, the microbiome, and the urban landscape. The complexity of watershed hydrological and ecological response to urbanization must be considered for effective pollutant mitigation and regulation. It also highlighted the need for more studies on the urban microbiome. The literature is lacking consensus among scientists on indicator species or assemblies for implications of urbanization of stream health.