Nutrient enrichment and altered habitat structure increase disease in the plankton

Background/Question/Methods

Two determinants of epidemic size are food abundance and habitat structure. Food affects disease spread through effects on host density, foraging behavior, and internal resources fueling parasite growth. Habitat structure influences disease processes by affecting spatial distributions and contact rates of hosts and parasites, as well as distributions of other species and materials (e.g., food resources). In many systems, effects of habitat structure on host-parasite contact rates may be confounded with effects of habitat structure on food supply. We used a zooplankton-host, yeast-parasite system to untangle roles of resource quantity and habitat structure in shaping epidemics. Epidemics occur in Daphnia populations in lakes where thermal stratification structures habitat vertically. Daphnia become infected by ingesting parasite spores released from dead hosts; therefore, stratification should inhibit host-parasite contact if dead hosts sink to the lake bottom, where spores cannot mix into host habitat. Stratification also inhibits mixing of nutrient-rich water near sediments with surface waters, which may limit abundance of algal food for hosts. We manipulated nutrient levels, frequency of water column mixing, and parasite exposure in mesocosm enclosures (6000 L each) in a lake, and sampled the mesocosms twice-weekly for 7 weeks to measure effects on population and disease dynamics.

Results/Conclusions

We hypothesized that both adding nutrients (nitrogen and phosphorus) and decreasing habitat structure (mixing the water column) would yield larger epidemics. Maximum infection prevalence did not differ across the parasite-exposed treatments. However, both nutrient enrichment and mixing increased host population sizes, such that densities of infected Daphnia were significantly greater in these treatments. Infected host density is a key measure of epidemic size, partly because it indicates how many spores will be released to seed future epidemics. Densities of infected Daphnia were lowest in the low nutrient, unmixed treatment and highest in the high nutrient, mixed treatment. Infected host densities were equivalent between the low nutrient, mixed and high nutrient, unmixed treatments. Our results suggest that both nutrient enrichment and water column mixing stimulated algal growth, which fueled larger host populations. The effect of decreasing habitat structure was similar to that of adding nutrients; mixing brought deep nutrient-rich water into contact with algae in upper water layers. This nutrient resuspension was more important than resuspension of spores, since differences in disease dynamics were driven by host densities rather than infection prevalences. Our results suggest that alterations of habitat and productivity have the potential to increase disease in natural systems.