Rainfall Brings Harmful Bacterias to Hilo Bay, Study Shows

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A new study conducted by a team of scientists at University of Hawai‘i at Hilo has found staph and fecal bacteria in Hilo Bay increase with rainfall and river discharge.

The study was a collaboration of state agencies along with UH Hilo faculty and alumni now working in health and science fields. The team published its findings in the Journal of Environmental Quality, which shows how rainfall-driven runoff increases concentrations of harmful bacteria in Hilo Bay.

Photo: Chris Yoakum.

The scientists hope their work can be used to predict water quality conditions based on rainfall patterns and to help assess the health risks faced by swimmers, surfers and other recreational water users in the bay.

A University of Hawai‘i at Hilo press release explained how scientists used culture-based methods to quantify the presence of Staphylococcus aureus (known informally as “staph”), methicillin-resistant Staphylococcus aureus (MRSA) and fecal indicator bacteria (FIB) 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 FIB concentrations increased with rainfall and river discharge. In terms of water quality, high turbidity (water cloudiness) was associated with higher bacteria concentrations, and high salinity with lower bacteria concentrations.

One of the authors on the paper is Tracy Wiegner, a UH Hilo professor of marine science. She noted that Hawaiʻi has the highest level of community acquired staph infections in the country.

“It’s two times the rate of the rest of the U.S.,” she said. “That may be because it’s warmer here or because people are in the water more.”


One of the other authors is Louise Economy, an alumna of UH Hilo’s tropical conservation and environment science graduate program who is currently employed by the Hawaiʻi Department of Health. She said staph is an opportunistic pathogenic bacterium. Given the right conditions, it can cause disease.

“It can invade wounds and cause boils, rashes and even flesh-eating disease,” Economy said. “These infections are becoming more and more common in the community and affecting people who were previously healthy.”

Wiegner said they are trying to develop real-time models using the water quality buoys, river discharge gauges and rainfall data to be able to make real-time predictions.


“The idea is that you could look at your phone and see what your risk is before going in the water,” Wiegner said.

Until then, scientists advise swimmers and surfers to stay home after a heavy rainfall, since rainfall and turbidity are associated with higher pathogen concentrations.

“A good rule of thumb for recreational water users is if the water is brown, turn around,” Wiegner said. “You don’t want to go in with open cuts, and if you do go in, you should always rinse off.”

To view the entire paper, visit

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