FOR 5094:  The Dynamic Deep Subsurface of High-Energy Beaches (DynaDeep)

• Subject Area: Geosciences
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431491505

DynaDeep studies subterranean estuaries (STEs), the hidden connective zones between inland aquifers and the open sea where meteoric freshwater and circulating seawater undergo major biogeochemical changes prior to discharge. DynaDeep specifically targets high-energy beaches with high tidal and wave energy that were so far rarely studied. At the start of Phase 1, we proposed that these are effective biogeochemical reactors, where dynamic boundary conditions propagate tens of meters into the subsurface, leading to strong spatio-temporal variability of geochemical conditions, thus forming a unique microbial habitat. In Phase 1, we installed an unparalleled STE observatory on Spiekeroog, where we collected a novel data set in interdisciplinary field campaigns that were supported by laboratory work and modelling. Our results highlight the transient nature of high-energy beaches. While beach morphodynamics and stormfloods lead to spatio-temporal salinity changes at the fringes of the large tidal upper saline plume (USP), its core remains comparatively stable with regard to salinity. The USP acts as a high-throughput reactor, whose surplus of oxygen supply processes large amounts of organic carbon while keeping the reactor in a carbon-limited state. The presence of genes for autotrophic processes in metagenomes from the USP indicates a community response to carbon limitation. The seasonal carbon input leads to moving redox interfaces within the USP, triggering secondary reactions like dissolution/precipitation of minerals, further causing corresponding shifts in microbial community composition. In Phase 2, we will address new research questions regarding the role of reactive zones (‘hot spots’), events (‘hot moments’), and compounds (‘hot stuff’) for the overall turnover of the biogeochemical reactor that evolved during Phase 1 at our core field site Spiekeroog. We will further challenge the transferability and generalisability of our results by conducting research at additional high-energy field sites (De Panne, Belgium and Truc Vert, France) and with different global model approaches. Ultimately, DynaDeep will elucidate the global relevance of the dynamic deep subsurface of high-energy beaches for carbon, nutrient and trace element cycling.

• DFG Programme: Research Units
• International Connection: Norway, USA

Projects

• Coordination Funds (Applicant Massmann, Gudrun )
• Global variability of groundwater flow and transport through STEs (Applicants Meyer, Ph.D., Rena ;  Moosdorf, Nils )
• Metabolic rates: From heterotrophic to autotrophic processes (Applicants Holtappels, Moritz ;  Könneke, Martin )
• Microbial communities: Diversity and metabolic function (Applicants Degenhardt, Julius ;  Engelen, Bert )
• Morphodynamics, subsurface flow and transport (Applicants Massmann, Gudrun ;  Müller-Petke, Mike ;  Winter, Christian )
• Organic matter: Abiotic transformations and microbial interactions (Applicants Dittmar, Thorsten ;  Niggemann, Jutta ;  Waska, Hannelore )
• Reactive transport (Applicants Greskowiak, Janek ;  Seibert, Stephan Ludger )
• Trace elements and metal isotopes: Transformation and fractionation (Applicants Pahnke-May, Katharina ;  Reckhardt, Anja )

• Spokesperson: Professorin Dr. Gudrun Massmann