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

Estuaries often experience multiple water quality impairments including nitrogen enrichment and elevated fecal pollution. These pollutant sources are often linked and difficult to characterize, especially in multiple use watersheds, hindering the identification of effective mitigation steps. Tillamook Bay (Oregon, USA) has a mixed-use watershed including many potential nutrient and fecal bacteria sources due to agricultural activities, human development, and local wildlife populations. In this study, microbial source tracking, watershed modeling, and stable isotope analysis were combined to understand sources of watershed nitrogen and fecal bacteria to receiving waters. 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 watershed modeled contributions. Ruminant and cattle-associated genetic markers were detected at a high frequency across sites, with the Rum2Bac marker detected in 94 % of samples collected across sites and concentrations significantly correlated with E. coli levels. Cattle and ruminant genetic marker concentrations increased downstream in four out of five tributaries, mirroring δ¹⁵N-NO₃ spatial trends during the wet season, suggesting a similar source and delivery for these co-pollutants. Although agricultural inputs are the dominant source of both fecal contamination and nitrogen to this system, human-associated genetic markers and elevated nutrient levels (NH₄⁺ and PO₄^-3) were observed at two sites in proximity to a wastewater treatment facility on the Trask River. Elevated δ¹⁵N-NO₃ and HF183/BacR287 levels in the same samples further corroborated a wastewater impact at these sites. Results support the utility of using a combined pollutant tracking approach when evaluating nutrient and fecal pollution in agriculturally intensive watersheds.