The influence of nitrate and pathogen dose on infection dynamics and host traits in a Daphnia-microparasite system

Background/Question/Methods

Nutrient pollution has the potential to alter many ecological interactions, including host-parasite relationships. One of the largest sources of nutrient pollution comes from anthropogenic alteration of the nitrogen cycle, specifically the increased rate of nitrate deposition to aquatic environments, potentially altering host parasite relationships. The current study aims to assess the mechanisms through which nitrate may impact host-pathogen relationships using a fungal pathogen (Metschnikowia bicuspidata) parasitic to a zooplankton (Daphnia dentifera) as a tractable model system. First, the influence of nitrate on host populations was assessed along a spectrum of nitrate concentrations. Second, the influence of nitrate on parasite fitness was assessed at the individual host level by examining the dose response relationship at control and intermediate (12 mg NO₃ -N * L^-1) levels of nitrate. These two approaches combined address the influence of nitrate on hosts through alteration of life history traits and the influence of nitrate on the parasite through alteration of infection dynamics.

Results/Conclusions

In the first experiment, nitrate concentration reduced host population size and increased disease prevalence, both exhibiting asymptotic relationships. In the second experiment, pathogen dose negatively influenced host fecundity and infection intensity, though infection intensity was significantly enhanced by nitrate. Nitrate had no effect on host growth rate. Taken together, this study provides evidence that nitrate both directly influences host (reduced population size through increased mortality) and pathogen (increased infection intensity) populations by acting as a stressor. While the secondary effects of nitrate (enhanced primary productivity) may counter the reduction in host survival in elevated nitrate conditions, increased infection intensity and/or prevalence may increase free-living pathogen spores and alter epidemic dynamics.