Seasonal Bioavailability of Dissolved Organic Carbon Within Eight Urbanized Watersheds Across the Portland, OR, Metropolitan Area (Oregon, USA)

The bioavailability of dissolved organic carbon (DOC) measures how readily organic carbon compounds are utilized by heterotrophic microorganisms and depends on the size and molecular composition. DOC in aquatic systems are known to vary seasonally and across urbanization intensity; however, the effects of urbanization and seasonality on bioavailable DOC (BDOC) are not well understood. This study examines seasonal patterns in BDOC across a range of urbanization intensity within the Portland, OR, metropolitan area. We hypothesized that BDOC concentrations would decrease from the summer (dry season) through the spring (wet season). Water samples were collected seasonally from 20 stream sites located within eight watersheds between 2023 and 2024. Samples were filtered, inoculated with unfiltered water and incubated in the dark for 28 days. Significant differences in both concentrations and the bioavailable proportion of DOC were observed across seasons, with the direction of the change differing by watershed. For example, from fall 2023 to winter 2024, most watersheds decreased in BDOC concentration and in BDOC/DOC while the Willamette River and Lake Oswego increased in both BDOC concentration and BDOC/DOC. Between winter and spring 2024, Lake Oswego decreased in both BDOC concentration and BDOC/DOC, the Willamette increased in BDOC concentrations but decreased in BDOC/DOC, and the rest of the watersheds increased in both concentrations BDOC and BDOC/DOC. Overall, variability in seasonal changes in BDOC indicates that factors beyond seasonality influenced BDOC. Additional analysis of urban and hydrological factors will provide insight into the influence of urbanization on BDOC.