Waterborne transmission largely contributes to the epidemiology of a plankton parasite

Understanding how parasites spread is crucial for preventing infections and predicting epidemic dynamics. These efforts are challenging for environment-borne parasites because transmission is host density-independent. We monitored the dynamics of Pasteuria ramosa, a common bacterial parasite of Daphnia magna with prolonged free-living transmission stages (spores). Spores are released into the sediment from decaying host cadavers. Transmission is believed to happen from sediment to host, though recent work has challenged this assumption, suggesting that transmission may also happen in the water column. We collected water samples during an epidemic and found infectious spore levels in the water, thus confirming this transmission pathway. Water column infectivity correlated with epidemic dynamics, suggesting that this pathway contributes to disease dynamics. We demonstrated experimentally that spores have a low sedimentation rate, suggesting that once suspended in the water, they remain in suspension for extended periods. We excluded that spores are released in the free water through macroinvertebrate predation or decomposition of sinking dead Daphnia, thus suggesting that perturbation transfers spores from the sediment to the water, potentially by water birds and large zooplankton. This research contributes insights into the transmission of environment-borne parasites, emphasizing the importance of considering the physical environment and ecological community.