Phytoplankton Blooms "Like It Colorful"(BLIC) - The relative influence of biodiversity and biotic interactions on the early stages of phytoplankton bloom formation

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The global loss of biodiversity is also visible in aquatic ecosystems. How biodiversity affects ecosystem functions is mechanistically only partially understood. Such biodiversity - ecosystem function relationships are already documented at the level of primary producers; phytoplankton in pelagic systems. Phytoplankton is involved in about 50% of the global primary production, essentially controls global cycles of important elements and is the basis of important ecosystem services such as fisheries. Phytoplankton is characterized by a special dynamic, the ability to quickly accumulate large amounts of biomass. These so-called phytoplankton blooms can often occur on a regular basis (e.g., in spring, so-called spring blooms) and provide important food for higher trophic levels, or can be triggered by external factors (e.g., external nutrient supply, eutrophication). Phytoplankton blooms are usually supported by one or a few species; in the case of toxic or poorly edible species, blooms can have significant negative effects on the transfer of energy and matter in food webs. A question that has been studied essentially only theoretically is whether the diversity of phytoplankton communities has an impact on the formation of phytoplankton blooms. With increasing diversity, the probability that there is a species in the community that can monopolize resources and form blooms is higher (selection effect). At the same time, as diversity increases, complementarity and efficiency of resource use may increase, and it becomes increasingly difficult to monopolize resources (complementarity effect). BLIC is investigating this issue using a mechanistic up-scaling approach. In controlled laboratory experiments, increasingly diverse communities are exposed to nutrient pulses and the strength of selection effects and complementarity are quantified. In a next step, diversity-manipulated field communities will be exposed to nutrient pulses and investigated to what extent the change in diversity makes blooms more likely or not. Finally, mesocosm experiments are planned with natural phytoplankton communities along natural diversity gradients. All experiments will take place in the same way with marine and freshwater laboratory and field communities. Besides experimental approaches, the second pillar of BLIC is the theoretical model analysis of these research questions. So-called "trait" based multidimensional mathematical models will analyse in detail the early stages of phytoplankton blooms. The models will receive important parameterizations from the experiments and at the same time have an important influence on the execution of individual experiments, which can then investigate particularly interesting environmental combinations for bloom formation arising from the model in detail.