Assessing the bloom dynamics and the ecological role of the dinoflagellate Noctiluca scintillans

This thesis investigates the ecological role, bloom dynamics, and spatiotemporal distribution of the harmful algal bloom-forming species Noctiluca scintillans, which adversely impacts food webs, ecosystem productivity, aquaculture, and tourism. With an extensive literature review (1857–2023), the potential global expansion of N. scintillans and its associated environmental factors are described. The analysis shows that N. scintillans has increased over time in coastal regions of Australia, China, and Europe, including the North Sea, with eutrophication, ocean warming, and deoxygenation identified as potential key drivers of these intensifications.
To validate these findings and identify drivers as well as spatiotemporal hotspots of N. scintillans in the rapidly warming North Sea, two long-term, high-resolution time series (Helgoland Roads and Continuous Plankton Recorder survey) were analysed. Results reveal that N. scintillans abundances and bloom durations increased significantly after the 1990s, with hotspots identified near the Elbe and Rhine river estuaries.
Following the identification of these hotspots, sampling campaigns were carried out in summer 2022 near Helgoland to study an entire N. scintillans bloom. Innovative imaging tools provided novel insights into its ecological role. Heavy predation on diatoms and its function as a significant recycler of phosphate and ammonium, followed by an increase in nutrient-affine dinoflagellates, suggest an important role for N. scintillans within plankton communities.
This thesis underscores the value of long-term time series and advanced methodologies in understanding and predicting N. scintillans trends. The research contributes to mitigating the economic and societal impacts of these blooms on aquaculture, fisheries, and tourism in rapidly changing ecosystems.

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