Impervious cover is associated with culture-based morphotype diversity in urban stormwater runoff

Urban stormwater runoff transports diverse biological material into receiving waters, but whether storm-by-storm culture-based diversity is associated with simple environmental variables is not well quantified. This study examined whether culture-based morphotype diversity in urban runoff was associated with impervious cover percentages and basic weather variables. Runoff was sampled during 25 storms at three Chapel Hill, NC, catchments representing green, residential, and commercial urban land-use patterns to obtain 75 site-storm samples. For each sample, colonies grown on nutrient agar were segmented into morphotypes from overhead images, and Shannon, Gini-Simpson, and Pielou evenness indices were computed from morphotype area fractions. Shannon diversity and Simpson diversity were consistently highest for the green space site, intermediate at the residential space site, and lowest for the commercial space site, while evenness showed the opposite pattern. Linear mixed-effect models with storm as a random effect identified land-use type, used here as a proxy for impervious cover, as the dominant predictor, whereas 24-h rainfall and daily mean air temperature showed weak associations. These results show a strong and repeatable association between impervious cover and culture-based runoff morphotype diversity. Because the assay relies on nutrient agar culturing and visible colony morphotypes and includes no non-storm controls, it should not be interpreted as a measure of total microbial diversity.

By sampling 25 storms in three contrasting urban catchments and using culture-based morphotypes on nutrient agar, this study shows that a simple plate-based assay can detect a strong and repeatable land-cover signal in stormwater runoff. Shannon and Gini-Simpson morphotype diversity were consistently highest at the least impervious site and lowest at the most impervious site, whereas rainfall and temperature showed weak associations within the sampled range. The study does not capture the full runoff microbiome, but it does provide a clear culture-based observation that impervious cover is associated with runoff morphotype diversity across repeated storms.