Nutrient composition degradation of Daphnia pulicaria by a highly prevalent chytridiomycete fungal pathogen (Polycaryum laeve) during naturally occurring lake-wide epidemics

Despite evidence illustrating that chytridiomycete fungal infection can be highly prevalent in Daphnia (>80%) and that infected individuals are preferentially consumed by fish, no studies have measured the nutritional consequences of using chytrid-infected Daphnia as a food source. We tested the hypothesis that Daphnia pulicaria infected with a pathogenic chytrid (Polycaryum laeve) is diminished in food quality relative to uninfected Daphnia during two naturally occurring lake-wide epidemics. We found that, compared to healthy adults, infected adults were smaller, contained less nitrogen and phosphorus, and lacked essential fatty acids.  Infected zooplankton were significantly shorter (8%) than gravid females and lighter (8 to 20%) than both non-gravid and gravid healthy individuals.  Per gram dry weight, the infected animals contained significantly less phosphorus (16 to 18%) and significantly less nitrogen (4 to 6%) compared to healthy individuals. Per gram dry weight, infected individuals averaged 28 to 30% less of the saturated fatty acid 18:0 (an indicator of total lipid content) and 35 to 45% less of docosahexaenoic acid, an essential fatty acid critical to consumer growth (both are significant differences). Our results suggest that naturally occurring levels of chytrid fungal infections in D. pulicaria populations can reduce the quality of food available to secondary consumers, such as planktivorous fishes, with potentially significant effects for lake food webs.