Exploring the factors behind the expansion of the harmful dinoflagellate Alexandrium pseudogonyaulax in Northern European waters

This Phd-thesis investigated factors driving the expansion of the harmful dinoflagellate Alexandrium pseudogonyaulax in Northern European waters. It includes an ecophysiological study investigating the influence of dissolved nitrogen and light availability on growth and toxin content of A. pseudogonyaulax, a food web study addressing the effects of this HAB species on multiple trophic levels ranging from phytoplankton to fish, as well as a time series analysis analysing the apparent expansion of A. pseudogonyaulax across Northern European waters. The first study, published in Limnology & Oceanography, indicates that urea is not an important driving factor of the expansion of A. pseudogonyaulax and shows that this species has high intraspecific variability and resilience to light. The second study, published in Harmful Algae, demonstrates deleterious effects of A. pseudogonyaulax on other planktonic organisms which suffer adverse fitness effects ultimately resulting in lysis and mortality. This study also demonstrates for the first time ichthyotoxic characteristics of A. pseudogonyaulax, which are likely driven by uncharacterized bioactive extracellular compounds (BECs) and not by goniodomins. The thesis contains a third first-author manuscript, envisioned for future publication, which demonstrates that the frequency of A. pseudogonyaulax occurrences has increased in Northern Europe. In addition, it indicates that temperature may have partly driven increasing occurrences and that A. pseudogonyaulax benefits from high carbon-to-chlorophyll a ratios.