UH-Hilo Study IDs Sources of Harmful Bacteria Washing Into Hilo Bay

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Hilo Bay (Photos courtesy of the University of Hawaiʻi at Hilo)
Lead researcher of the study, Tyler Gerken, who is a Keaʻau native, collects soil in native forests.

What lurks in the microscopic world beneath the surface of waters in Hilo Bay and where it comes from is the focus of a new study by University of Hawaiʻi at Hilo researchers recently published in the Journal of Environmental Quality.

The study identifies the origins of high levels of harmful bacteria that wash into the bay following heavy rainfall. Researchers collected soil samples from urban, agricultural and native forest lands within the Hilo Bay watershed and found Staphylococcus aureus, informally called staph; methicillin-resistant Staphylococcus aureus, or MRSA; and fecal indicator bacteria present in soil samples from all of the areas.

The highest concentrations of the bacteria were detected in urban soils on residential properties and farm lots.


MRSA and other fecal indicators increase in Hawai‘i’s streams and estuaries following storms, posing a health threat to recreational water users. Researchers say identifying watershed bacteria sources is critical to implementing management actions to reduce risks.

“This research resulted from community members always talking about staph or MRSA infections that are acquired while swimming,” Tracy Wiegner, a UH-Hilo marine science professor who co-authored the study, said in a press release.

Lead researcher and Keaʻau native Tyler Gerken, a UH-Hilo alumnus, spearheaded the study in 2018 while he was an undergraduate. He is a descendant of kahuna lā‘au lapa‘au, or masters of traditional Native Hawaiian healing, which fuels his dedication to environmental justice. Louise Economy, an alumna of UH-Hilo’s tropical conservation and environment science graduate program currently employed by the state Department of Health, also co-authored the study.


In 2019, Economy worked with this same team of researchers to publish a study that confirmed rainfall-driven runoff increases concentrations of harmful bacteria in Hilo Bay. The scientists used culture-based methods to quantify the presence of staph, MRSA and fecal indicator bacteria in Hilo Bay and in soils, sands, rivers, wastewater and storm water within the Hilo watershed.

These pathogen concentrations were then compared with rainfall and river discharge levels and water quality data. The results showed that staph and fecal indicator bacteria concentrations increased with rainfall and river discharge.

“One way to reduce the amount of bacteria going into water bodies is to employ green infrastructure technology, which includes things like maintaining riparian (or streamside) wetlands, constructing wetlands for stormwater retention and beach grooming,” Wiegner explained in the press release. “These techniques have been successfully used in the Great Lakes region of the United States to improve water quality and make it safer for swimmers.”

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