Phytoplankton play a vital role in aquatic ecosystems and the global carbon cycle. Fungal parasites of phytoplankton can impact how phytoplankton grow and how they cycle nutrients. Asterionella formosa Hassall, 1850 is a large stellate colonial bloom-forming freshwater diatom widely found in temperate lake ecosystems. Due to its large size, density, and peculiar shape, A. formosa may have a high potential for sedimentation. A. formosa is also parasitized upon by chytrid fungal parasites, which can lyse the host cell and fragment colonies, leading to a potential redistribution of nutrients. While the nature of host-parasite interactions has been studied in the past 50+ years, the implications of parasitism on carbon fluxes to lake sediment have yet to be addressed. In this thesis, I show that, in a year of low bloom density, infections lead to smaller A. formosa colony sizes (i.e. fewer cells per colony). This impact of infections is especially pronounced in the sediment trap where the colony size is half of that observed in the surface layer. My research shows that colony shape influences sedimentation, contrary to Stoke’s law which suggests that larger colonies will sink faster or more often into the sediment trap. Despite reduced sedimentation and carbon accumulation potential, I observed higher carbon-to-nitrogen (C: N) ratios in the sediment trap compared to the productive layer. My thesis shows how the A. formosa and chytrid host-parasite dynamics function throughout a spring bloom and the implications on carbon fluxes in Lake Maarsseveen. As lake ecosystems are one of the most efficient carbon sinks, understanding how the processes of parasitism and sedimentation of dominant phytoplankton species like A. formosa can provide insights into carbon fluxes, which are imperative within the context of the global carbon budget.