Research topic 2: Groundwater composition

The subterranean estuary modulates the composition of submarine groundwater discharge (SGD) constituents such as macronutrients (N, P, Si), micronutrients (Fe), and dissolved organic matter (DOM). Due to the mixing of recirculating seawater and fresh groundwater, precipitation/dissolution reactions can occur, and new electron donors (labile marine DOM) are introduced to electron acceptors (Fe oxides, nitrate). We study the biogeochemical reactions occurring at the recharge zone where seawater is introduced into the beach sediment and at discharge sites where groundwater is released to nearshore waters. The heart of the topic is a thorough assembly of physical (temperature, salinity) and chemical (redox-sensitive trace metals, nutrients, DOM) field data throughout the shallow subterranean estuary. These data are collected in unprecedented analytical detail (Fe isotope analyses and ultra-high-resolution mass spectrometry analyses of DOM). Our results on SGD composition and modification are fed into research topic 1 (SGD hydrology) to determine SGD constituent fluxes to the coastal water column, and research topic 3 (microbial metabolism in the STE) to assess the boundary conditions for biotic transformations of SGD constituents.

Project Biogeochemistry

Prof. Dr. Hans-Jürgen Brumsack (Microbiogeochemistry Group)
Dr. Katharina Pahnke (Max-Planck Research Group Marine Isotope Geochemistry)

PhD student
Janis Ahrens (Microbiogeochemistry Group)

Research scientist
Dr. Melanie Beck (Microbiogeochemistry Group)
Dr. Claudia Ehlert (Max-Planck Research Group Marine Isotope Geochemistry)
Dr. Philipp Böning (Max-Planck Research Group Marine Isotope Geochemistry)

Knowledge about biogeochemical processes in the subterranean estuary is crucial to evaluate the impact of submarine groundwater discharge (SGD) on nutrient and trace element budgets of the coastal North Sea. Most nutrients and many trace elements are transformed during transport through the subterranean estuary (STE) by microbial activity and chemical reactions. Our objective is to investigate these biogeochemical reactions in the STE, which are affecting the composition of pore waters discharging into the North Sea.
To date many studies focusing on SGD and biogeochemical transformations within the STE were carried out at sheltered beach sites like bays or at sites influenced by microtidal regimes. Since exposed high-energy beaches like those on Spiekeroog Island are covering large parts of the global coastline, the findings of our study site will be essential when trying to assess the global importance of these land-sea-interfaces.

Liquid-solid interactions
Biogeochemical transformations may especially occur at sharp redox and salinity gradients within the coastal aquifer. Such gradients develop, for example, when recirculating seawater and fresh groundwater of different chemical composition mix in the STE. Furthermore, the interaction with organic carbon may influence trace element concentrations (research topic 2).
The overarching question is whether the STE acts as source or sink for nutrients and trace elements. For example, redox changes (oxygen gradients; research topic 3) in surface sediments may either lead to precipitation/fixation of trace elements or to the dissolution of solid phases. Furthermore, the microbial community does affect liquid-solid transformations, for example by using trace metals as electron acceptors (research topic 3). The dissolved species are transported by groundwater flow (research topic 1) and may finally discharge into the open water column.

The fate of iron
The absence of dissolved iron, which is essential for photosynthesis, may limit algal productivity in the open ocean. Pore waters of Spiekeroog beach may contain dissolved iron concentrations 3-4 orders of magnitude above open ocean values. Although iron may be reoxidized and fixed within the sediment, its actual fate is yet unknown, especially taking into account the short groundwater residence times.

Spatial heterogeneity
To capture the dynamic changes in topography and groundwater flow (research topic 1), we are applying high resolution sampling strategies during different seasons using up-to-date analytical facilities such as photometric analyses, different mass spectrometry techniques (ICP-MS, MC-ICP-MS) and radioisotope methods.

Master thesis
Michael Kossack: Coupling short-term pore water residence times with biogeochemical cycles in a tidal subterranean estuary: A tracer study using 222Rn and 223,224Ra.

Presentations at national and international symposia
Ahrens J, Beck M, Waska H, Schnetger B, Brumsack H-J (2017): Spatial variability in pore water biogeochemistry and trace metal cycling of a subterranean estuary. Goldschmidt Conference, Paris, France.

Waska H, Ahrens J, Grünenbaum N, Massmann G, Meier D, Schwalfenberg K, Simon H, Zielinski O, Dittmar T (2018): Beach morphology impacts dissolved organic matter (DOM) dynamics in the subterranean estuary. ASLO Ocean Science Meeting, Portland (Oregon), USA.



Project DOM

Prof. Dr. Thorsten Dittmar (ICBM-MPI Bridging Group for Marine Geochemistry)
Dr. Hannelore Waska (ICBM-MPI Bridging Group for Marine Geochemistry)

PhD student
Roger Carvalho (CAPES fellow, ICBM-MPI Bridging Group for Marine Geochemistry)

Research Scientist
Heike Simon (ICBM-MPI Bridging Group for Marine Geochemistry)


The role of DOM on microbial processes in the subterranean estuary, the modulation of SGD-derived DOM at the sediment-water interface, especially due to microphytobenthic primary production, and the interaction of DOM with trace metals such as Fe, are investigated.


DOM as fuel for the microbial bioreactor
Biological transformations of DOM are studied over different seasons, in three main zones of the subterranean estuary: The seawater infiltration zone at the high water line, the dynamic in- and exfiltration zones of the intertidal ridge and runnel structure, and the exfiltration zone at the low water line. In addition, flow-core experiments and time series investigations help elucidate pathways of DOM processing. By combining targeted amino acid and carbohydrate analyses with ultra-high resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) we can track release and respiration of labile DOM compounds from terrestrial and marine sources.


The iron curtain: A transient trap for terrestrial DOM
At redox gradients in the subterranean estuary, dissolved, reduced Fe(II) precipitates as iron(III) oxides, scavenging chemical species such as manganese, phosphate, and arsenic. The so-called “iron curtain” also acts as a molecular filter which selectively retains aromatic, carboxyl-rich DOM fractions commonly stemming from terrestrial biomass. In the scope of BIME, this so far unknown DOM sequestration zone is investigated with regards to its distribution, temporal stability, and quantity. The scavenging and release of this “humic-like” terrestrial DOM fraction is investigated with hand-held sensors for fluorescent DOM (FDOM) and via ultra-high resolution FT-ICR-MS of batch samples from natural pore water and experimental setups.

Presentations at national and international symposia
Waska H, Ahrens J, Grünenbaum N, Massmann G, Meier D, Schwalfenberg K, Simon H, Zielinski O, Dittmar T (2018): Beach morphology impacts dissolved organic matter (DOM) dynamics in the subterranean estuary. ASLO Ocean Science Meeting, Portland (Oregon), USA.

Waska H, Ahrens J, Maier D, Schwalfenberg K, Simon H, Zielinski O, Dittmar, T (2017): Lifting the iron curtain: Release of dissolved organic matter (DOM) in an iron(II) reduction zone of the subterranean estuary. Goldschmidt 2017, Paris, France