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 an in situ experiment 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 which darken waters, including lake browning associated with climate change and eutrophication.

中文翻译：

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揭示环境传播的寄生虫：湖泊内较轻的条件限制了流行规模

寄生虫健康取决于从一个宿主到另一个宿主的成功旅程。对于环境传播的寄生虫，非生物条件可能会影响这一旅程的成功。在这里，我们评估了光（一个关键的非生物因素）如何影响浮游动物疾病的时空模式，其中光随季节、湖泊和湖泊深度而变化。在使用这三种变异来源的原位实验中，我们测试了两种寄生虫的孢子对环境光的敏感性。与暴露于黑暗中其他相似条件的寄生虫相比，暴露于环境光下的两种寄生虫的传染性都较低。在深秋（11 月），即使在较低的环境光线下，更敏感的寄生虫（真菌，Metschnikowia）也会受到损害。建立这种差异灵敏度后，我们评估了光环境与湖泊自然爆发之间的联系。与孵化一致，较不敏感的寄生虫（细菌，巴斯德氏菌）的流行在秋季较早开始（在较高的环境光下），并且两种寄生虫在更透明的湖泊中的爆发规模较小。总体而言，光照环境可能会影响疾病爆发的时间和规模。因此，人类活动使水域变暗，包括与气候变化和富营养化相关的湖泊褐变，可能会加剧爆发。光照环境可能会影响疾病爆发的时间和规模。因此，人类活动使水域变暗，包括与气候变化和富营养化相关的湖泊褐变，可能会加剧爆发。光照环境可能会影响疾病爆发的时间和规模。因此，人类活动使水域变暗，包括与气候变化和富营养化相关的湖泊褐变，可能会加剧爆发。