The current and future risk of harmful algal blooms in the North Sea = Het huidig en toekomstig risico voor schadelijke algenbloei in de Noordzee

The growing need to feed the world’s population has led to significant advances in agricultural practices over the last 50 years. The Haber-Bosch process, in particular, enabled an intensification of the global fertilizer use, resulting in higher crop yields across the world. Yet, due to the inefficient incorporation of fertilizers into agricultural products, these nutrients led to significant environmental pollution and eutrophication. Fuelled by nutrient enriched runoff, rivers upset the biogeochemical balance of the marine environment, leading to a global increase in size, frequency and distribution of harmful algal blooms (HABs). During these events, a phytoplankton species is able to proliferate at the expense of others, causing severe harm to the environment through hypoxia, shading, physical disruption and the release of potent toxins. As a result, HABs are a severe threat to marine biodiversity as well as the safety and security of seafood. Now, with climate change looming over the horizon, scientists fear that HABs could become more prevalent in our future oceans. To date, however, there is a lack of experimental evidence that global change will affect HABs. Despite the implications for human health and ecosystem health, the link between eutrophication and HAB development is still not fully understood. Yet, while scientists are discussing the fundamental ecological importance of biological features such as toxicity, mixotrophy and allelopathy, we are slowly missing our opportunity to prevent a major escalation of HABs. For this reason, we urgently need to identify and employ model species in cross-validated, long-term multifactorial studies with co-occurring species to rapidly progress our understanding of HABs, and quantify the impact of HABs on the socioeconomic well-being of our species to persuade policy makers. During this PhD research, we tried to achieve both. Given our lack of knowledge on the occurrence of HABs in our own regional sea, the main objective of this thesis was to assess whether the ongoing changes in the environmental state of the Belgian Part of the North Sea (BPNS) have enhanced the risk of HAB development. In addition to regular sampling campaigns, we identified several knowledge gaps in the available literature that we first had to resolve in order to answer this main question. In particular, we needed to determine whether changes in the total or relative nutrient availability, both effects of eutrophication, affect the competitive traits of potentially harmful dinoflagellates. Next, we needed to verify that climate-change driven temperature increases would not affect these traits either.Lastly, we wondered if toxic dinoflagellates could have unknown mixed toxicity effects – by themselves or in together with marine pathogens - on keystone bivalve species like Mytilus edulis