Better fed host does not necessarily imply more resources for parasites

Increases in phytoplankton biomass following nutrient enrichment can impact host-parasite interactions by altering the amount of resources that parasites can acquire during an infection. As parasite fitness is primarily affected by the quality of the host as a resource (i.e. the nutritional quality of the host and, by extension, host body condition), a positive relationship between host body condition and parasite fitness was evidenced in a number of systems impacted by anthropogenic eutrophication such as parasite-induced malformations in amphibians (Johnson et al. 2007), whirling disease and proliferative kidney disease in salmonids (Hartikainen etal. 2009) and swimmer’s itch (Soldánová etal. 2013).

Male threespine stickleback in eutrophied waters during breeding season

However, it should be noted that better fed host does not necessarily imply more resources for parasites because higher food resources for hosts as a result of eutrophication can also limit parasite proliferation by promoting host resistance to infection (Aalto, Decaestecker & Pulkkinen 2015). Such a negative influence of nutrient enrichment on parasite transmission is poorly documented in the literature. An exception is the study by Anaya‐Rojas et al.(2016). Using mesocosms, the authors investigated the connections between nutrient loading, parasitism and host condition using the monogenean parasite Gyrodactylus spp. and the threespine stickleback Gasterosteus aculeatus. The parasite reproduces on the host skill and gills and transmission is directly connected to host density (Bakke, Cable & Harris 2007). By controlling fish density in mesocosms and by manipulating nitrogenous nutrient (NaNO₃ and HNa₂PO₄) loading and exposure to parasites, the authors found fish assigned the low nutrient treatment to be in worse condition, with lower stomach fullness compared to fish assigned to high nutrient treatment (Anaya‐Rojas et al. 2016). For sticklebacks originating from the lake ecotype, where Gyrodactylus spp. load is high, nutrient enrichment was negatively correlated to parasite load, but this was not true for fish originating from the stream ecotype, where prevalence is lower. Additionally, more sticklebacks from the lake ecotype died after being exposure to parasites during the experiment, indicating a higher sensitivity to Gyrodactylus infection compared to fish from the stream ecotype. This led the authors to hypothesize that lake sticklebacks might alter their diet or feeding rate in order to better cope with Gyrodactylus spp. (Anaya‐Rojas et al. 2016).