UH Hilo scientists document how rainfall brings harmful bacteria into Hilo Bay

Wednesday, October 23, 2019, 6:07pm by

  • The study was a collaboration of state agencies along with UH Hilo faculty and alumni now working in health and science fields.
  • Findings: Staph and fecal indicator bacteria in Hilo Bay increase with rainfall and river discharge. Cloudy water is associated with higher bacteria concentrations, and high salinity with lower bacteria concentrations.

By Leah Sherwood.

Aerial of Hilo Bay
Hilo Bay. Photo credit: Hollyn Johnson for UH Hilo.

A team of scientists from the University of Hawai‘i at Hilo has published a paper in the prestigious Journal of Environmental Quality on how rainfall-driven runoff increases concentrations of harmful bacteria in Hilo Bay.

The paper is titled, “Rainfall and Streamflow Effects on Estuarine Staphylococcus aureus and Fecal Indicator Bacteria Concentrations.” The authors are Louise Economy, an alumna of UH Hilo’s tropical conservation and environment science graduate program who is currently employed by the Hawai‘i Department of Health; Tracy Wiegner, professor of marine science at UH Hilo; Ayron Strauch, a hydrologist with the Department of Land and Natural Resources; Jonathan Awaya, professor of biology at UH Hilo; and Tyler Gerken, a UH Hilo alumnus who is currently a graduate research assistant at the University of Washington.

The scientists used culture-based methods to quantify the presence of Staphylococcus aureus (known informally as “staph”), methicillin-resistant Staphylococcus aureus (abbreviated 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.

The project is based on Economy’s undergraduate and graduate work at UH Hilo, supervised by Wiegner, as well as work done by Gerken, also supervised by Wiegner, while he was at UH Hilo earning his baccalaureate degree in environmental science.

“Staph is an opportunistic pathogenic bacterium, meaning that given the right conditions it can cause disease,” explains Economy. “It can invade wounds and cause boils, rashes, and even flesh-eating disease. These infections are becoming more and more common in the community and affecting people who were previously healthy.”

Wiegner notes that Hawai‘i 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 says. “That may be because it’s warmer here or because people are in the water more.”

Traditionally, scientists focused on the transmission of bacterial pathogens to the water from the skin of recreational water users. “Two out of five people have staph on their skin at any given time,” explains Economy. “These people can be carriers without getting infected. However, our work showed that staph and MRSA can persist on land, and can be moved into our ocean waters through mauka to makai connections driven by rainfall.”

Swimmers beware

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

Until then, she advises 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,” she says. “You don’t want to go in with open cuts, and if you do go in, you should always rinse off.”

Wiegner worries that climate change could make the potential risk higher. “What’s predicted by climate change is that the climate will be drier, but when we do have rain, it will be much more intense,” she says. “When you have dry periods followed by more intense rain events, you get higher pathogen concentrations.”


The project was funded through the Hau‘oli Mau Loa Foundation and the U.S. Geological Survey Pacific Islands Climate Adaptation Science Center (PI-CASC). PI-CASC contributed through the Manager Climate Corps, a program that focuses on supporting research teams comprised of UH Hilo faculty, students, agencies and community members to address the island’s climate adaptation challenges. Undergraduate student research was supported by the National Science Foundation through UH Hilo’s Pacific Internship Programs for Exploring Science (PIPES) and UH Mānoa’s Center for Microbial Oceanography, and by the National Institutes of Health through the Students of Hawai‘i Advanced Research Program (SHARP).

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