Summary1. Daphnia was collected from five subarctic ponds which differed greatly in their DOC contents and, consequently, their underwater light (UV) climates. Irrespective of which Daphnia species was present the ponds with the lowest DOC concentrations contained Daphnia with the highest concentrations of eicosapentaenoic acid (EPA). In addition, EPA concentrations in these Daphnia generally decreased in concert with seasonally increasing DOC concentrations.2. Daphnia from three of the ponds was also tested for its tolerance to solar ultraviolet radiation (UVR) with respect to survival. D. pulex from the clear water pond showed, by far, the best UV-tolerance, followed by D. longispina from the moderately humic and D. longispina from the very humic pond.3. In addition, we measured sublethal parameters related to UV-damage such as the degree to which the gut of Daphnia appeared green (as a measure of their ability to digest algae), and whether their guts appeared damaged. We developed a simple, non-invasive scoring system to quantify the gut proportion in which digestive processes were presumably active. This method allowed repeated measurement of the same animals over the course of the experiment. We demonstrated, for the first time, that sublethal damage of the gut precedes mortality caused by exposure to UVR.4. In a parallel set of experiments we fed UV-exposed and non-exposed algae to UV-exposed and non-exposed daphnids. UVR pre-treatment of algae enhanced the negative effects of exposure to natural solar UV-irradiation in Daphnia.5. The UV-related effects we observed in Daphnia were generally not species-specific.