The effects of solar radiation on organisms are complex and wavelength dependent, with organisms sometimes being damaged by high-energy ultraviolet (UV) wavelengths. Bacterial and fungal parasites in lakes may be particularly susceptible to UV because they cannot escape by swimming deeper (as their hosts do). Lakes differ in the extent to which UV can penetrate into the water column: in lakes with more dissolved organic matter (DOM), UV attenuates more quickly. We sought to understand how dominant parasites of Daphnia hosts tolerate ambient light conditions, and if these conditions influence parasite dynamics. We exposed spores of two Daphnia parasites, Metschnikowia bicuspidata (a fungus) and Pasteuria ramosa (a bacterium) to ambient light conditions in lakes of differing clarity in July, August, and October in order to understand 1) how parasites are affected by light conditions in lakes across a DOM spectrum and 2) how parasites are affected by light over the course of an epidemic season as incoming light declines throughout the fall. Daphnia were exposed to incubated spores in the lab to compare the effects of ambient light on the spores in controlled conditions. We also analyzed field data of Daphnia infection prevalence in lakes in Michigan and Indiana from 2009-2016.
Both parasites were sensitive to ambient light in July (infectivity was on average 3.0 times higher for the bacterium and 3.2 times higher for the fungus in the dark controls; P<0.001 for each). For the fungus but not the bacterium, this sensitivity persisted into the fall (P<0.001), indicating greater sensitivity of the fungus to light. The effect of light was strongest in surface waters: spores suspended 2m deep in the water column were more infective than those suspended 0.5m deep; deeper bacterial spores were 1.8 times more infective (P=0.05), and deeper fungal spores were 6.8 times more infective (P=0.02). In addition, lakes with more DOM protected parasites from damaging light: infection levels were 3 times higher for bacterial spores incubated in the darkest lake compared to the clearest lake in July (P=0.009). Field data showed that bacterial epidemics start on average 23 days earlier in the fall than fungal epidemics in Indiana lakes (P=0.03) for the four lake-years where both parasites co-occurred. Epidemics of both parasites start significantly earlier in Michigan than they do in Indiana where higher light levels persist later into the fall. Light may therefore be a factor controlling the timing of Daphnia parasite epidemics.