Integrating microbial source tracking with fecal indicator bacteria monitoring improves water quality assessment in tropical island watersheds.

This study assessed 50 soil and 41 stream water samples across five watersheds for FIB (enterococci, Escherichia coli, Clostridium perfringens) and four MST markers (human CrAssphage and HF183, canine BacCan-UCD, and porcine Pig-2-Bac) in American Samoa (AS). AS microbiological recreational water quality standards for enterococci and E. coli were not met by 80.48% and 65.9% of the stream samples, respectively. 81.8% and 77.8% of the samples that did not meet the standards contained no detectable human-associated markers or animal-associated markers. While elevated FIB levels were detected across all watersheds, regardless of classification, human-associated markers were detected only in intermediate and extensive watersheds. Enterococci were detected in 94% and E. coli in 92.0% of soil samples, averaging 954 MPN/g and 1375 MPN/g, respectively. No human-associated markers were detected in soil samples, while animal-associated markers were detected in 4% of samples. Based on the analyses of 37 animal fecal samples, all MST markers exhibited high sensitivity and specificity, except the canine marker, which showed low specificity (41%). Elevated FIB levels and limited MST marker detection in soil and water samples indicate that naturalized soil communities are most likely the primary source of FIB in American Samoa streams. Integrating FIB with MST markers provides a more robust framework for tropical water quality assessment, enabling managers to distinguish naturalized background FIB from true fecal contamination and to align regulatory and infrastructure management actions with public health risk.