Elemental changes along a temporal and spatial gradient: Does it impact Daphnia-parasite interactions?

Background/Question/Methods .Urbanisation and eutrophication have played a crucial role as drivers of local, regional and global changes in N and P biochemistry of aquatic ecosystems. Moreover, as the rate of N and P-inflow can differ considerably, we expect the aquatic N:P ratio to vary. As many aquatic grazers perform best under a fixed N:P ratio, this altered food quantity and quality can impact their life history traits directly (growth, mortality) or indirectly (predation, immune defences). Although often neglected, defences against parasites are important as they regulate host density or susceptibility to predation. Here, we focus on how temporal and spatial gradients in nutrient availability impact the interaction between Daphnia and its parasites. In a laboratory experiment, we used two Daphnia subpopulations that were hatched from different sediment layers in a pond from which we characterized the changes in nutrient household over time.

Results/Conclusions .Our results show that a higher nutrient availability in time induces a higher sensitivity of Daphnia with respect to changing N:P ratios, which in turn resulted in a higher parasite virulence for these sensitive clones. Secondly, we monitored changes in phytoplankton N- and P-concentration and ratio of 81 ponds. Our results indicate that on a local scale (within 9 km²), phytoplankton N-concentration and N:P ratio increased with the percentage of build-up area around the pond. This altered elemental context impacted the interaction of three different zooplankton species (Daphnia magna, D. pulex and Simocephalus vetulus) with their parasites.