Caffeine pollution disrupts freshwater
A 2026 study in Biology Letters finds that caffeine pollution in freshwater makes the amphipod Gammarus pulex more active and less likely to avoid light
Traces of caffeine from coffee and tea are passing through wastewater treatment and entering rivers and streams. A new study suggests this pollution could be making a common freshwater crustacean more vulnerable to predators.
Researchers from the University of Bonn and Manchester Metropolitan University published their findings in the journal Biology Letters on September 16, 2026. They investigated how environmentally relevant concentrations of caffeine affect the behavior of Gammarus pulex, a small shrimp-like amphipod key to freshwater ecosystems.
"Caffeine is an emerging environmental contaminant in freshwater ecosystems that is primarily introduced through wastewater," wrote lead authors Elias Fritzsche and Hannah Otto. They noted that even low concentrations can affect aquatic organisms, though the ecological impact is poorly understood.
The experiment with infected and healthy crustaceans
The team studied 120 individual amphipods. They placed the animals in tanks with either a low or a high concentration of caffeine, levels comparable to those detected in polluted freshwater environments. Each tank held five individuals infected with the parasitic worm Polymorphus minutus and five uninfected individuals. All crustaceans were exposed to the caffeine for 20 hours.
This parasite is known to manipulate its host's behavior. "This acanthocephalan parasite manipulates the antipredator behavior of its intermediate host, the gammarid, by altering its photophobia, thereby increasing its transmission to the final host, water birds," the authors explained. The experiment aimed to see if caffeine exposure produced a similar behavioral shift.
Caffeine's impact mirrors a parasitic manipulation
After exposure, the researchers assessed the crustaceans' phototactic behavior, or their tendency to avoid light. They found that amphipods exposed to the high caffeine concentration were more active and displayed significantly less photophobia than those in the low-concentration tanks. They were less inclined to hide from light.
Notably, this reduction in light-avoidance behavior was comparable to the change seen in amphipods infected with the P. Minutus parasite. Since the parasite's manipulation makes the crustaceans more likely to be eaten by water birds, the study suggests caffeine pollution could have a similar effect, increasing predation risk.
"These results suggest that human-derived chemicals can shift antipredator behavior in a direction similar to an evolved parasite-mediated manipulation," the authors concluded. This shift could ultimately alter freshwater ecosystem dynamics.
Limitations and future research directions
The study's design did not include a caffeine-free control group. Therefore, the findings demonstrate concentration-dependent effects rather than proving caffeine exposure itself causes the change compared to a clean environment. The a high concentration has a stronger effect than a low one.
Future studies are needed to test the real-world ecological implications. Researchers could determine if predators actually catch caffeine-exposed amphipods more frequently. Long-term monitoring of Gammarus pulex populations in contaminated waterways could also reveal population-level impacts.
The amphipod Gammarus pulex, sometimes called a freshwater shrimp or scud, is a grayish-brown crustacean that can grow up to 21 mm. It plays a central role in breaking down leaf litter and is a primary food source for fish, birds, and other aquatic predators. The species is native to European freshwaters, but related amphipods fill similar ecological niches globally.
The work adds to a growing understanding of how pharmaceutical and personal care product pollution affects wildlife. It may eventually inform new wastewater treatment technologies or policies aimed at reducing caffeine and similar contaminants entering freshwater habitats.
