Microbial Source Tracking Reveals Nutrient Pollution Sources in a Mixed-Use Watershed

Microbial source tracking (MST) is typically used to identify pathogen sources, but can also trace nutrient pollution. We sampled eight Cayuga Lake (NY, US) tributaries over three years and quantified MST markers for human, ruminant, avian, and canine sources, as well as biologically relevant forms of N and P to test whether marker presence and concentration track nutrient concentrations and how these relationships vary over time. Ruminant markers were positively associated with all forms of P, ammonia, total nitrogen, and total suspended solids (TSS), but not consistently across year or season. In contrast, human and avian markers increased with dissolved organic phosphorus, but only in the growing season. Relative to ruminant markers, soluble reactive, total, total dissolved, dissolved organic, and biologically available phosphorus increased in the early to mid-growing season, possibly indicating a lag in the transport of DNA markers, particulate phosphorus, and TSS. We conclude that ruminant sources (e.g., field application of manure) likely contribute most of the P entering into some of the lake’s most nutrient-rich tributaries. This study also demonstrates that MST can inform the type, timing, and locations of management actions needed to reduce nutrient pollution.