Responses of algae, bacteria, Daphnia and natural parasite fauna of Daphnia to nutrient enrichment in mesocosms
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Understanding responses of parasites to changes in nutrient regimes is necessary for prediction of their role in aquatic ecosystems under global change in nutrient loading. We studied the response of the natural parasite fauna of Daphnia longispina to nutrient enrichment in mesocosms in a small humic lake. We measured the concentrations of inorganic phosphorus and nitrogen in the water, total nutrients in the seston, algal and bacterial biomass, Daphnia population dynamics, Daphnia stoichiometry, Daphnia stable isotope values and the presence and abundance of parasites in treated mesocosms as compared to three control ones. Incorporation of the nutrient enrichment in the food web was seen as increased nutrient concentrations in the epilimnion and as a decrease in carbon:nutrient ratios and δ15N values in Daphnia. Nutrient enrichment did not significantly influence algal, bacterial or Daphnia biomass. One of the four parasite species observed, unidentified small gut parasite, had a higher prevalence (percentage of Daphnia infected) in treated mesocosms, but its intensity (number of parasites per infected host) remained the same among treatments. Our results suggest that the effect of nutrient enrichment on host–parasite dynamics is dependent on complex interactions within food webs and on the epidemiological traits of parasites.
KeywordsDaphnia longispina Host–parasite interaction Enclosure Stoichiometry Food web dynamics
We thank the staff of Lammi Biological Station, who contributed to field and laboratory work. We are grateful to A. Ojala for providing the float and R.I. Jones and P. Kankaala for providing the mesocosms material used in the study. We also thank P. Kankaala, R. I. Jones, D.O. Hessen and two anonymous referees for comments, which helped us to improve the MS. The study was financed by the Academy of Finland project 116782 awarded to KS.
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