Abstract
Laguna Lake is an economically important resource in the Philippines, with reports of declining water quality due to fecal pollution. Currently, monitoring methods rely on counting fecal indicator bacteria, which does not supply information on potential sources of contamination. In this study, we predicted sources of Escherichia coli in lake stations and tributaries by establishing a fecal source library composed of rep-PCR DNA fingerprints of human, cattle, swine, poultry, and sewage samples (n = 1,408). We also evaluated three statistical methods for predicting fecal contamination sources in surface waters. Random forest (RF) outperformed k-nearest neighbors and discriminant analysis of principal components in terms of average rates of correct classification in two- (84.85%), three- (82.45%), and five-way (74.77%) categorical splits. Overall, RF exhibited the most balanced prediction, which is crucial for disproportionate libraries. Source tracking of environmental isolates (n = 332) revealed the dominance of sewage (47.59%) followed by human sources (29.22%), poultry (12.65%), swine (7.23%), and cattle (3.31%) using RF. This study demonstrates the promising utility of a library-dependent method in augmenting current monitoring systems for source attribution of fecal contamination in Laguna Lake. This is also the first known report of microbial source tracking using rep-PCR conducted in surface waters of the Laguna Lake watershed.
Highlights
Laguna Lake (Laguna de Bay) is the largest freshwater lake in the Philippines and is considered as the most economically important lake in the country (Santos-Borja & Nepomuceno 2006)
Library performance The reference library consisted of 1,408 thermotolerant E. coli isolates, 444 of which were from agricultural sources while 964 were from domestic sources (Table 1)
microbial source tracking (MST) of fecal contamination is an important aspect of water quality management
Summary
Laguna Lake (Laguna de Bay) is the largest freshwater lake in the Philippines and is considered as the most economically important lake in the country (Santos-Borja & Nepomuceno 2006). Despite the lake’s relatively good water quality, the majority of its 28 tributary rivers failed to conform to the water quality guidelines for Classes A–D in terms of fecal coliform (Laguna Lake Development Authority 2019). Pollution brought about by population growth, industrialization, and land use may cause the lake’s deterioration, especially when polluted water from its tributary rivers flows directly into the lake. This poses risks to the estimated 16 million residents within its vicinity (Santos-Borja & Nepomuceno 2006). Strategies to mitigate the lake’s degradation include regular monitoring of the water quality in terms of physicochemical properties as well as fecal coliform count, which is performed by the Laguna Lake Development Authority (LLDA). To efficiently prevent the water bodies’ further degradation, identifying the origin of contamination is likewise essential
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