An integrated approach to coupled nutrient and microbial source tracking in an agricultural watershed

Estuaries are important and sensitive habitats that are often faced with multiple anthropogenic water quality impairments including nitrogen enrichment and elevated fecal pollution. These pollutant sources are often linked and increasingly hard to characterize, especially in multiple use watersheds, making it difficult to identify effective mitigation steps. Tillamook Bay, located on the Northwest coast in Oregon (USA), is situated in a mixed-use watershed and impacted by many potential nutrient and fecal bacteria sources due to agricultural activities, human waste management practices, and local wildlife populations. In this study, microbial source tracking, Spatially Referenced Regressions on Watershed attributes (SPARROW) modeling, and stable isotope analysis were combined to understand sources and delivery of watershed nitrogen and fecal bacteria to receiving waters. To examine delivery of nitrogen and bacteria, tributaries of Tillamook Bay were sampled approximately monthly from June 2016 to May 2017 at 16 sites. Paired measurements of host-associated qPCR-based genetic markers targeting human (HF183/BacR287 and HumM2), ruminant (Rum2Bac), cattle (CowM2 and CowM3), canine (DG3), and avian (GFD) fecal pollution sources and nitrate stable isotope (δ15N-NO3) were compared to each other and to SPARROW model outputs. Cattle and ruminant genetic marker concentrations were found to increase along a downstream gradient in four out of five tributaries, mirroring δ15N-NO3 spatial trends during the wet season, suggesting a similar source and watershed delivery for these co-pollutants. Results support the utility of using a combined pollutant tracking approach when evaluating nutrient and fecal pollution in agriculturally intensive watersheds.

Impact/Purpose

Estuaries are often faced with multiple water quality impairments including nitrogen enrichment and elevated fecal pollution. These pollutant sources are often linked and increasingly hard to characterize, especially in multiple use watersheds, making it difficult to identify effective mitigation steps. Tillamook Bay, located on the Northwest coast in Oregon (USA), is situated in a mixed-use watershed and impacted by many potential nutrient and fecal bacteria sources due to agricultural activities, human waste management practices, and local wildlife populations. In this study, microbial and nutrient source tracking methods (host specific genetic markers, nitrate stable isotope, and SPARROW watershed model) were applied in parallel to investigate sources and delivery of fecal pollution and nutrients to receiving waters.  Our integrated monitoring approach demonstrated that agricultural inputs as the primary driver of both elevated nitrate and fecal contamination levels at most sites within the Tillamook watershed suggesting that efforts to mitigate these sources will have the largest impact on water quality improvement. Our integrated monitoring approach which tracks nutrients and bacteria as co-pollutants may be useful to identify pollutant sources in other coastal systems and develop effective management strategies to improve water quality.

Citation

Zimmer-Faust, A., C. Brown, O. Shanks, W. Rugh, T Chris Mochon Collura, AND H. Stecher. An integrated approach to coupled nutrient and microbial source tracking in an agricultural watershed. Elsevier Science Ltd, New York, NY, (272):122981, (2025). [DOI: 10.1016/j.watres.2024.122981]

DOI: An integrated approach to coupled nutrient and microbial source tracking in an agricultural watershed