Subterranean estuaries are hidden connective zones between inland aquifers and the open sea where meteoric freshwater and circulating seawater mix and undergo major biogeochemical changes. Hence, they are considered powerful biogeochemical reactors affecting elemental net fluxes to the sea. DynaDeep is motivated by the fact that the functioning of subterranean estuaries below high-energy beaches is currently poorly understood, as field studies have mostly focussed on low- or moderate-energy sites. In particular the effect of hydro- and morphodynamics on subsurface flow and transport is unclear and related consequences on biogeochemical reactions and microbial habitat characteristics have not been investigated. We propose the existence of a so far unexplored biogeochemical reactor within the dynamic deep subsurface of high-energy beaches where dynamic boundary conditions propagate deep (tens of meters) into the subsurface leading to strong spatio-temporal variabilities of geochemical conditions. Subterranean estuaries below high-energy beaches may thus form a unique microbial habitat with an adapted microbial community unlike other typically more stable subsurface environments. DynaDeep will study groundwater flow patterns as a function of hydro- and morphodynamics. It will deliver rates of redox processes and related abiotic transformations. Furthermore, it will assess speciation and transformation of trace metals and metal isotopes. Microbial interactions with organic matter will be investigated as well as the diversity and metabolic functioning of microbial communities. Six subprojects will cooperate in joint field campaigns, shared sampling approaches and experimental work and use mathematical models in an integrative approach. In the first phase, the subprojects will jointly establish a Subterranean Estuary Online Observatory on the island of Spiekeroog to investigate one core site in detail, before verifying observations at other high-energy sites in a second phase. Ultimately, DynaDeep will elucidate the global relevance of the dynamic deep subsurface of high-energy beaches for coastal ecosystems as well as for carbon, nutrient and trace element cycling.

uol.de/en/icbm/dynadeep

Alfred Wegener Institute, Bremerhaven

Federal Institute for Geosciences and Natural Resources, Hannover

Institute of Biology and Environmental Sciences, University of Oldenburg

Institute of Chemsitry and Biology of the Marine Environment, University of Oldenburg

Institute for Geosciences, University of Kiel

Leibnitz Institute for Applied Geophysics, Hannover

Max Planck Institute for Marine Microbiology Bremen

National Park Authority, National Park Wadden Sea of Lower Saxony, Wilhelmshaven

Niedersächsische Landesbetrieb für Wasserwirtschaft, Küsten- und Naturschutz, Norden

Deutsche Forschungsgemeinschaft (DFG)

April 2021 - March 2025

Dr. Rena Meyer

Dr. Stephan Seibert

Dr. Nele Grünenbaum (Koordination)

Meyer, R., Greskowiak, J., Seibert, S.L., Post, V.E., Massmann, G. (2025). Effects of boundary conditions and aquifer parameters on salinity distribution and mixing controlled reactions in high-energy beach aquifers. Hydrol. Earth Syst. Sci. Disc. 1–27. doi.org/10.5194/hess-2024-196.

Seibert, S. L., Massmann, G., Meyer, R., Post, V. E. A., Greskowiak, J. (2025). Reactive transport modeling to reveal the impacts of beach morphodynamics, storm floods and seasonal groundwater recharge on the biogeochemistry of sandy subterranean estuaries. Advances in Water Resources, Volume 196, 104884. doi.org/10.1016/j.advwatres.2024.104884

Reckhardt, A., Meyer, R., Seibert, S.L., Greskowiak, J., Roberts, M., Brick, S., Abarike, G., Amoako, K., Waska, H., Schwalfenberg, K., Schmiedinger, I., Wurl, O., Böttcher, M.E., Massmann, G., Pahnke, K. (2024). Spatial and temporal dynamics of groundwater biogeochemistry in the deep subsurface of a high-energy beach. Marine Chemistry 267, 104461, doi.org/10.1016/j.marchem.2024.104461

Reckhardt, A., Beck, M., Greskowiak, J., Waska, H., Ahrens, J., Grünenbaum, N., Massmann, G., Brumsack, H.-J. (2024). Zone-specific longshore sampling as a strategy to reduce uncertainties of SGD-driven solute fluxes from high-energy beaches. Estuarine, Coastal and Shelf Science 301, 108733, doi.org/10.1016/j.ecss.2024.108733

Seibert, S-L., Massmann, G., Meyer, R., Post, V.E.A., Greskowiak, J. (2024). Impact of mineral reactions and surface complexation on the transport of dissolved species in a subterranean estuary: Application of a comprehensive reactive transport modeling approach. Advances in Water Resources 191, 104763, doi.org/10.1016/j.advwatres.2024.104763.
Greskowiak, J., Seibert, S. L., Post, V. E. A., & Massmann, G. (2023). Redox-zoning in high-energy subterranean estuaries as a function of storm floods, temperatures, seasonal groundwater recharge and morphodynamics. Estuarine, Coastal and Shelf Science, 290, 108418. doi.org/10.1016/j.ecss.2023.108418

Grünenbaum, N., Günther, T., Greskowiak, J., Vienken, T., Müller-Petke, M., Massmann, G. (2023). Salinity distribution in the subterranean estuary of a meso-tidal high-energy beach characterized by Electrical Resistivity Tomography and Direct Push technology. Journal of Hydrology  617, 129074, https://doi.org/10.1016/j.jhydrol.2023.129074

Massmann, G., Abarike, G., Amoako, K., Auer, F., Badewien, T.H., Berkenbrink, C., Böttcher, M.E., Brick, S., Medina Cordova, I.V., Cueto, J., Dittmar, T., Engelen, B., Freund, H., Greskowiak, J., Günther, T., Herbst, G., Holtappels, M., Marchant, H.K., Meyer, R., Müller-Petke, M., Niggemann, J., Pahnke, K., Pommerin, D., Post, V., Reckhardt, A., Roberts, M., Schwalfenberg, K., Seibert, S.L., Siebert, C., Skibbe, N., Waska, H., Winter, C., Zielinski, O. (2023). The DynaDeep observatory – a unique approach to study high-energy subterranean estuaries. Hypothesis and Theory, Frontiers in Marine Science, 110:1189281.
doi: 10.3389/fmars.2023.1189281 https://www.frontiersin.org/articles/10.3389/fmars.2023.1189281/full

Seibert, S. L., Greskowiak, J., Bungenstock, F., Freund, H., Karle, M., Meyer, R., et al. (2023). Paleo-hydrogeological modeling to understand present-day groundwater salinities in a low-lying coastal groundwater system (Northwestern Germany). Water Resources Research, 59, e2022WR033151. doi.org/10.1029/2022WR033151

Greskowiak, J. and Massmann, G. (2021). The impact of morphodynamics and storm floods on pore water flow and transport in the subterranean estuary. Hydrological Processes, e14050, doi.org/10.1002/hyp.14050