Human activities have a major impact on ecosystems worldwide. Emission of greenhouse gasses, such as carbon dioxide, methane and nitrous oxide has resulted in global warming. Warming and nutrient rich conditions can stimulate mass development of phytoplankton. Understanding these impacts of global change on phytoplankton bloom development is complex, since biomass accumulation is a net process of many growth and mortality related factors. Indeed, blooms are not only a result of enhanced growth, they can also be a product of reduced mortality caused by a lack of top down control. Algal blooms often consist of relatively large and inedible species, which are generally less easily ingested by zooplankton. This may benefit bloom development and result in trophic bottlenecks. Such a constraint may be alleviated by fungal parasite infections on large-sized phytoplankton taxa like diatoms and filamentous cyanobacteria. Chytridiomycota, often referred to as chytrids, are virulent fungal parasites which can kill their host and thereby delay or suppress phytoplankton blooms. Chytrids occur across all climatic regions in both freshwater and marine systems. They are characterized by their motile flagellated zoospores that can swim through the water to find suitable hosts. These zoospores have been found to constitute a highly nutritional food source to zooplankton. Consequently, carbon and nutrients from inedible host cells can be transferred to zooplankton via the zoospores of chytrids, thereby forming the so-called ‘mycoloop’. However, little is known about how global and land use change will influence host-parasite dynamics and the role of chytrid parasites in the food web of future waters.
The overall aim of this thesis was to investigate how climate change factors affect bottom-up and top-down control of phytoplankton disease. Specifically, this research focuses on direct and interactive effects of warming, nutrient availability and grazing on chytrid infections. In this thesis, I show that warming promoted chytrid infections and thereby accelerated the termination of a spring bloom. Furthermore, chytrid zoospores may support zooplankton growth by providing them with an alternative food source. Fungal parasites thereby re-established trophic links between phytoplankton and zooplankton, if the latter are food limited during presence of large inedible cyanobacteria or diatoms. Changes in the nutrient supply will alter phytoplankton elemental composition, and I have shown that this has consequences for reproduction and stoichiometry of their parasites. Specifically, during low P conditions chytrids can maintain a higher zoospore production rate and efficiency, while these decreased upon low N conditions. Furthermore, the chytrids revealed a putative trade-off between zoospore size, and thereby possibly survival, and production rate. The observed changes in chytrid production and stoichiometry may have consequences for higher trophic levels, and thereby alter food web functioning. In conclusion, this thesis contributes to the work existing on the frontier of phytoplankton chytridiomycosis research, providing a better understanding on the role of parasites in plankton food webs and the potential impacts of global change on parasite infection dynamics. With this work, I highlight the importance of integrating fungal parasites into plankton ecology.