HILO — Those who frolic in Hilo Bay might want to take note of new research conducted by the University of Hawaii at Hilo.
In a paper published this month in the Journal of Environmental Quality, scientists from the university examine how rainfall runoff affects the amount of harmful bacteria making its way into the bay.
The researchers measured the presence of Staphylococcus aureus, or staph, methicillin-resistant Staphylococcus aureus, or MRSA, and “fecal indicator bacteria” — bacteria used to estimate the level of fecal contamination — in the bay and in soil, sand, river, wastewater and storm water within the Hilo watershed.
Those results were 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,” according to a post on the university’s website. “In terms of water quality, high turbidity (water cloudiness) was associated with higher bacteria concentrations, and high salinity with lower bacteria concentrations.”
Authors of the published paper are Louise Economy, an alumna of UH-Hilo’s tropical conservation and environment science graduate program who is currently employed by the state Department of Health; Tracy Wiegner, professor of marine science at UH-Hilo; Ayron Strauch, a hydrologist with the state Department of Land and Natural Resources; Jonathan Awaya, professor of biology at UH-Hilo; and Tyler Gerken, a UH-Hilo alumnus who is a graduate research assistant at the University of Washington.
Wiegner said the project began when she had a student interested in seeing if staph or MRSA were present in near-shore waters.
That student, Economy, was an undergraduate student at the time, Wiegner said. The work began as a senior thesis research project in 2015 and was developed into part of a master’s thesis over another two years.
Wiegner said they discovered the concentration of pathogens — bacteria, viruses or other microorganism that can cause disease — would “substantially increase following rain events.”
A large portion of the funding for the project came from the Pacific Islands Climate Adaptation Science Center, so they also looked at how the amount of pathogens flowing into the bay could vary with rainfall, she said.
They found that there were higher concentrations of bacteria in the bay after storms following dry conditions, compared to storms that followed wet conditions.
According to Wiegner, when it’s dry, bacteria in the soil accumulates until it rains, whereas continuous rain dilutes the amount of bacteria in the soil, so when a major rain event occurs, not as much runs off.
Climate, she said, is predicted to become dryer but with more intense and with more frequent storm events, so there is potential for greater amounts of pathogens in near-shore waters due to predicted changes in future rainfall patterns.
Gerken has done research to determine where in the watershed the bacterial might be coming from, Wiegner said.
Wiegner said she wasn’t surprised by the research findings.
“We have been doing research on other bacteria coming into Hilo Bay and their concentration always increased after rain events,” Wiegner said.
The results also fit with people self-reporting staph infections, she said.
When it comes to swimming, Wiegner said “if it’s brown, turn around,” serves as a good rule of thumb — meaning, if the water is brown after a storm event, it’s probably better to wait because bacterial concentration is higher, and recreational water users will have the greatest health risks in those conditions.
Those who do enter the water should rinse off after and be cautious going in with an open cut, she said.