Shedding light on environmentally transmitted parasites: lighter conditions within lakes restrict epidemic size.

Parasite fitness depends on a successful journey from one host to another. For parasites that are transmitted environmentally, abiotic conditions might modulate the success of this journey. Here we evaluate how light, a key abiotic factor, influences spatiotemporal patterns of zooplankton disease where light varies seasonally, across lakes, and with depth in a lake. In anin situexperiment using those three sources of variation, we tested sensitivity of spores of two parasites to ambient light. Infectivity of both parasites was lower when exposed to ambient light in comparison to parasites exposed to otherwise similar conditions in the dark. The more sensitive parasite (the fungus,Metschnikowia) was damaged even under lower ambient light during late fall (November). With this differential sensitivity established, we evaluated links between light environment and natural outbreaks in lakes. Consistent with the incubations, epidemics of the less sensitive parasite (the bacterium,Pasteuria) started earlier in the fall (under higher ambient light), and both parasites had smaller outbreaks in more transparent lakes. Overall, light environment may impact the timing and size of disease outbreaks. Outbreaks could thus become exacerbated by human activities that darken waters, including lake browning associated with climate change and eutrophication.