Eutrophication is a type of water pollution caused by human activities, such as intensive agriculture or waste discharges, which notably stimulates the growth of microalgae and makes the water murky. Nowadays, this disturbance constitutes a major threat to most water bodies because it causes direct changes in aquatic ecosystems, including excessive algal growth, health problems linked to the toxicity of the introduced pollutants (various forms of nutrients like nitrates and phosphates) and harmful algal blooms, as well as more complex and indirect changes, such as modifications of the food chain, loss of large plants, increased turbidity and oxygen depletion. It is commonly assumed that wildlife health declines as water condition deteriorates, favouring parasite outbreaks in eutrophied areas. However, parasites themselves can also be affected directly by eutrophication. Accordingly, eutrophication can both enhance and prevent parasite transmission, inducing both positive and negative effects on the health of aquatic organisms. Care should thus be taken when predicting the ultimate consequences of eutrophication on parasites. In this synthesis, I discuss how the different changes induced by eutrophication in water bodies can influence infections by presenting studies conducted on the topic for a variety of organisms, ranging from viruses infecting microalgae to worms that travel via the food chain to successively infect snails, frogs and birds. In spite of the large number of publications currently available in the scientific literature, much remains unknown concerning the effects of eutrophication on parasitic diseases, as illustrated by the knowledge gaps identified in this review article. Future directions for research should notably include investigations on the possible synergies between the different changes caused by eutrophication in aquatic ecosystems, the behavioural responses of wildlife that might influence parasite transmission, and large-scale monitoring of parasites. Stronger collaborations between aquatic ecologists and disease ecologists are necessary to predict outbreaks in eutrophied areas and, hence, improve wildlife management.
Image caption: Large-scale phytoplankton bloom is a sign of eutrophication. The photography was taken from a plane over the Baltic Sea in August 2015. The trail left by a cargo ship that passed through the bloom (left side) attests of the impressive scale of the phenomenon. Credit: Alexandre Budria.
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