Zooplankton—tiny aquatic animals known to graze on bacteria—are ineffective at removing fecal microorganisms from sewage-contaminated water, according to a new study.
The findings challenged the assumptions of the researchers that these tiny animals could act as natural cleaners by inactivating harmful pathogens in freshwater and saltwater environments.
The hypothesis was that zooplankton would consume or neutralize fecal microorganisms, potentially reducing the risk to human health after water contamination. But the results told a different story.
"I was expecting to see a negative effect associated with the plankton present, but if anything, we saw a very small positive effect from the presence of plankton for a few locations sampled, which suggests that the pathogens in some cases are actually protected while associated with the plankton," the study's corresponding author, Lauren Kennedy, told Newsweek. "That surprised me the most."
The team's research comes at a crucial time. A 2017 U.S. water quality report found that over 50 percent of rivers, bays and estuaries were unsafe for at least one use, with fecal contamination being a major factor.
This poses serious health risks when humans are exposed to untreated water, which can occur due to sewage leaks, inadequate water treatment or corroded infrastructure.
In their experiment, the researchers added two common proxies for fecal contamination—E. coli bacteria and MS2, a virus commonly found in sewage—to water samples from the San Francisco Bay area.
These samples contained both large particles, such as zooplankton, and smaller, dissolved materials like salt.
Surprisingly, they found that the zooplankton had little effect on reducing the presence of the contaminants. Instead, higher salinity levels were linked to greater inactivation of the harmful microorganisms. Ocean water taken from San Pedro Beach, for example, showed that the pathogens were inactivated.
However, as the study wasn't designed to specifically test the effects of salinity, it's not possible to attribute the trends with absolute certainty.
"Regardless, I think implications are that salinity is a complex but important factor that needs to be assessed, for example, when trying to predict how long after a contamination event there will be an elevated risk of swimming in the water," Kennedy said.
The findings are particularly significant for water quality management. Many areas across the U.S. face frequent sewage spills, and local governments rely on natural processes to treat water contamination when resources for intervention are limited. The assumption that zooplankton might help in this process, however, may no longer hold water.
Kennedy added, "In the U.S., we handle the issue of pathogen pollution by allowing people who put wastewater into natural water bodies a certain level of pollutant that can be detected in the water body per day to ensure a lower human health risk.
"These thresholds are difficult to calculate and heavily dependent on the environmental conditions in the water body. This study provides an example for how to assess the effect of large particles, including plankton, when attempting to calculate these values and demonstrates the importance of particle characterization in environmental water bodies."
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References
Kennedya, L. C., Mattisa, A. M., & Boehma, A. B. (2024). You can bring plankton to fecal indicator organisms, but you cannot make the plankton graze: particle contribution to E. coli and MS2 inactivation in surface waters. mSphere. https://journals.asm.org/doi/10.1128/msphere.00656-24
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