Food chain effects of sublethal ultraviolet radiation on subarctic Daphnia pulex – a field and laboratory study

In shallow subarctic ponds Daphnia and their algal food are often exposed to solar ultraviolet radiation (UVR). It was previously demonstrated that direct exposure of Daphnia to UVR may result in sublethal damage to Daphnia‘s intestinal system. This led to the hypothesis that, as consequences of such exposures, Daphnia may be experiencing a situation similar to starvation. We examined, in controlled experiments, the indirect effect of UVR-treated food algae on Daphnia pulex which, themselves, were also exposed to UVR. We specifically tested whether exposure of D. pulex to solar UVR affects food transport and enzymatic digestion. As expected, the UVR-exposed food and UVR-treated Daphnia combination produced the strongest effects on intestinal damage, and mortality. Some of these effects, as well as the grazerrelated effects, were similar to those observed during starvation. The total activities of digestive enzymes (amylase and cellulase) were somewhat reduced in UVR treated D. pulex, but the function of enzymes is not seriously damaged as is clear from the increasing trend of protein-specific activities. We conclude that, in some ways, Daphnia undergoing sublethal UVR exposures may be experiencing a condition similar to what they would experience under very low food or even fasting conditions.