Influence of phytoplankton functional traits on microzooplankton community composition during 2021 north sea spring bloom.
Supervisor: Cedric Meunier (Alfred-Wegener-Institut)
Phytoplankton growth rate is expected to increase with currently ongoing climate change (e.g., rising sea surface temperatures and increasing pCO2). As master trait, growth rate is related to various other functional traits like size, metabolic rate, and complexity of the cell surface (e.g., spine formation). Therefore, these traits are expected to change along with the change in growth rate and by doing so, affecting the food quality of phytoplankton as prey for a next trophic level, microzooplankton. Microzooplankton (20-200 μm), with the major groups being dinoflagellates and ciliates, are an important link between primary production and mesozooplankton as the main grazers of phytoplankton. This study aimed to investigate the natural succession of the microzooplankton community in the northeastern North Sea in the field during spring bloom 2021 and whether the microzooplankton community was affected by the elemental composition and morphological traits of phytoplankton. From the field, I observed that the microzooplankton community composition changed, with a shift of dominance by dinoflagellates to ciliates, with the spring bloom succession as the biotic and abiotic environment developed over time. The growth rate of the phytoplankton, cryptomonad Pyrenomonas helgolandii was manipulated to 0.1, 0.3 and 0.7 d-1 leading to contrasting treatments of cells with differences in biochemical composition (elemental stoichiometry and pigment content) and morphological characteristics (cell size and granularity). Even though the difference between the growth rate treatments was significant but minor, the community composition changed in response to the treatments. This was however not visible in the diversity of the community nor the growth rate. This was the first study using the approach of testing contrasting trait features of a single phytoplankton species on a complete microzooplankton community during a spring bloom event. Studying the trophic

interaction between phytoplankton and microzooplankton is of importance as minor changes in the microzooplankton community can amplify towards higher trophic levels and ultimately alter the complete marine food web.

Keywords: growth rate; elemental stoichiometry; trophic interaction; microalgae; grazing; field study; flow cytometry; chemostats