Diatoms are responsible for +20% of global primary production and represent important drivers of nutrient cycling, with several clades, including thalassiosiroid taxa, known for their boom and bust life strategy. The wax and wane of such blooms is mostly studied in the context of abiotic drivers and zooplankton grazing, while the potential impact of pathogens is expected to be substantial, yet is often overlooked. The main objective of this PhD project is to investigate how and to which extent abiotic stress and chytrid parasitism drive diatom species succession and seasonal bloom dynamics. As a model ecosystem I will use the Belgian Part of the Schelde Estuary (BPSE), characterised by strong gradients in light and salinity stress and the regular occurrence of chytrid outbreaks. I will combine both field and experimental studies, using state-of-the-art, high throughput imaging techniques, and molecular and bioinformatic approaches. Based on the characterisation of dominant diatom and chytrid species in culture, and results from laboratory experiments mimicking processes occurring in the field, I will develop novel tools to assess the physiological state and spatio-temporal dynamics of thalassiosiroid diatoms at a high taxonomic resolution in the wild. My results will be highly relevant to understand the role of disease in phytoplankton in the context of climate change and (de-)eutrophication.