Samenvatting

Diatoms are essential in the aquatic ecosystem, especially in estuary systems like the Scheldt Estuary, in which centric diatoms are the dominant phytoplankton and serve as an important food resource for upper trophic levels. The understanding of the population dynamics is therefore an essential element of the management of the aquatic ecosystem. However, the diatom lifecycle in natural populations remains understudied. Most diatoms need sexual reproduction to restore their cell size, thereby completing their life cycle and escaping death. This requires a minimum threshold cell size and an environmental cue, which is not yet known in natural populations. Year-long surface samples were taken along the Scheldt Estuary, accompanied by a salinity experiment on two-day freshwater diatom populations, to assess these environmental cues in the natural population. All samples were subsequently analysed via the imaging flow cytometer. Only auxospores have been observed in the salinity experiment. Cell size was also used as a proxy to asses potential sexual reproduction. Key physiological processes have been found during May and June, in which the cell size increased significantly, and the auxospores were observed in the salinity experiment. The study also indicates multiple environmental conditions needed for higher chances of sexual reproduction. The answer to the question: Under which environmental conditions can centric diatoms sexually reproduce in the Scheldt estuary remains to be determined. But this has been another example of the difficulty of finding auxospores in the natural population, but will provide a groundwork for further research regarding the population dynamics of centric diatoms and the relation between the environmental conditions as a cue for sexual reproduction.