Application of SourceTracker for Accurate Identification of Fecal Pollution in Recreational Freshwater: A Double-Blinded Study

The
efficacy of SourceTracker software to attribute contamination
from a variety of fecal sources spiked into ambient freshwater samples
was investigated. Double-blinded samples spiked with ≤5 different
sources (0.025–10% vol/vol) were evaluated against fecal taxon
libraries characterized by next-generation amplicon sequencing. Three
libraries, including an initial library (17 nonlocal sources), a blinded
source library (5 local sources), and a composite library (local and
nonlocal sources), were used with SourceTracker. SourceTracker’s
predictions of fecal compositions in samples were made, in part, based
on distributions of taxa within abundant genera identified as discriminatory
by discriminant analyses but also using a large percentage of low
abundance taxa. The initial library showed poor ability to characterize
blinded samples, but, using local sources, SourceTracker showed 91%
accuracy (31/34) at identifying the presence of source contamination,
with two false positives for sewage and one for horse. Furthermore,
sink predictions of source contamination were positively correlated
(Spearman’s ρ ≥ 0.88, P <
0.001) with spiked source volumes. Using the composite library did
not significantly affect sink predictions (P >
0.79)
compared to those made using the local sources alone. Results of this
study indicate that geographically associated fecal samples are required
for SourceTracker to assign host sources accurately.