Impact of lateral intrusions and mixing on the biogeochemistry and microbiology of pelagic redoxclines

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In coastal and marginal seas with stable stratification and reduced water exchange, hypoxic deep-water conditions are a frequently observed phenomenon. The so-called redoxcline, developing under such conditions at the top of the anoxic layer, is characterized by strong biogeochemical gradients, which are the location of some important microbially mediated element transformations. Here, our project hypothesis is that the rapid and intermittent modification of the local biogeochemical and microbiological conditions associated with lateral intrusions, in particular due to turbulent mixing between intrusions and ambient waters, constitutes an essential component of the redoxcline system. We plan to study this link between hydrography, turbulent mixing, biogeochemical processes, and microbial activities, taking the central Baltic Sea with its well-defined redoxcline as an example. A recently established autonomous profiling system in the Central Baltic Sea will provide high-resolution data, allowing us to determine the long-term variability of hydrographic, biogeochemical, and mixing parameters associated with the intrusions. The impact of mixing for the microbial activities and communities will be investigated during ship cruises, combining observations of mixing parameters, and representative biogeochemical and microbial transformation processes.