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Lead-PI: Ulrike Feudel

Involved: Stefanie Moorthi, Helmut Hillebrand, Michaela Busch, Anneke Kristin Purz

Collaboration: Holger Kantz (Max-Planck-Institute for Physics of Complex Systems, Dresden), Jürgen Kurths (Potsdam-Institute for Climate Impact Research, Potsdam), Klaus Lehnertz (Department of Epileptology and Interdisciplinary Center for Complex Systems, University of Bonn), Dr. D. Caron and Dr. A. Schnetzer, University of Southern California (USC), Los Angeles.

Duration: 2012-2018

Funded by: Volkswagen Foundation

Recurrent Extreme Events in Spatially Extended Excitable Systems: Mechanisms of their Generation and Termination

Although extreme events (ExEvs) occur in a wide variety of contexts, there is no common definition of an ExEv, though everybody has a certain understanding of such phenomena depending on the specific context. In general, ExEvs are considered to be rare events characterized by a large impact on a particular system which is measured in terms of very different quantities. In this proposed project we will study phenomena in excitable systems which appear recurrently in rather large, irregularly spaced time intervals. Particularly we will focus on two such ExEvs, namely the recurrent occurrence of harmful algal blooms (HAB) in the ocean and the recurrent emergence of epileptic seizures (ES) in the human brain. Our part project will conduct time-series and experimental work on HAB, which appear when a toxic species (e.g. dinoflagellates or cyanobacteria) becomes extremely abundant and dominant in a plankton community. The emergence of such blooms can not only be dangerous for direct competitors or consumers of the toxic algae, but also for higher organisms such as shellfish, fish, birds, and mammals as well as for human beings, either by direct toxin effects or indirectly by propagating effects up the food web (e.g., algal toxins accumulate in fish or shellfish that are consumed). Therefore, these blooms can have dramatic effects on human health and industry, affecting for instance coastal fish- and shellfish farms as well as tourism by prohibiting swimming in endangered areas. At first glance, both phenomena appear to have nothing in common. However, both HAB and ES occur in spatially extended systems where the dynamics in a localized region of the ocean or in the brain can be modeled as an excitable system. For the plankton blooms this offers the opportunity to study the emergence or termination of a HAB by an extreme weather situation. Since different noises can be realized in plankton experiments, our hypotheses about possible triggers of ExEvs can be directly tested. We will manipulate factors proposed to trigger HAB. The concise treatments will depend on the output of the modeling and the time series analysis. However, the chemostat system allows a flexible array of manipulating the mean and temporal variance of abiotic (e.g., temperature and nutrient supply) and biotic conditions (competitor and grazer concentrations). The temporal variance can comprise different colors of noise as well as anomalously large fluctuations. Such fluctuations have been shown to influence population dynamics, a.o. in plankton , and represent the conceptual basis of altered bloom dynamics through climatic variability. We will also manipulate the absolute magnitude and temporal fluctuation of resource supply. Grazing and the supply of resources are often considered as major triggers of harmful algal blooms.

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