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Subterranean estuaries (STEs) under high‐energy conditions function as effective biogeochemical reactors, where waterbodies of varying physico‐chemical properties mix. Chemical reactions within the STE modulate the solute composition of groundwater discharging to the sea providing for example nutrients for plankton growth to coastal ecosystems. Groundwater residence times are fundamental to understanding groundwater flow and transport regimes, and thus to interpreting (bio) geochemical data. The present study aims to exemplarily show for the barrier island Spiekeroog exposed to high energy conditions, how morphological changes of the beach surface imprint on subsurface flow, groundwater age and mixing patterns. To this end, we have combined multiple environmental tracers, including electrical conductivity, apparent tritium‐helium groundwater ages, temperature (T) and dissolved silica (Si), obtained over a period of 1.5 years from different depths and locations along a 200 m long cross‐shore transect. The results of apparent groundwater ages, travel times and seawater mixing fractions allowed delineating zones where the different endmembers were prevailing. Recirculating seawater of the upper saline plume (USP), infiltrating near the high water line, was youngest with travel times of days to weeks. Near the dune base, freshwater entering the beach from the islands interior was several years old, subsequently forming the freshwater discharge tube below the USP, and directed toward the discharge zone where decade old brackish groundwater was encountered at depths below 18 m. The results support the interpretation of geochemical data as well as the underlying processes and advance the understanding of the functioning of the biogeochemical reactor in high energy beach systems.

Coastal observatories are key to improve the understanding of processes within the coastal area and their interactions with regional and global environmental changes. The land-sea transition zone is an essential area that allows research on unique scientific questions under anthropogenic and natural influences. Amid the Wadden Sea UNESCO world natural heritage site – the largest tidal flat region worldwide – the barrier island Spiekeroog is an excellent location for an observatory studying land-sea interactions. The integrated Spiekeroog Coastal Observatory (SCO) operated by the Institute for Chemistry and Biology of the Marine Environment (ICBM, University of Oldenburg) is dedicated to interdisciplinary marine and terrestrial ecosystem research. Its position within the tidal area and the multitude of research-field addressed establishes the SCO as a unique coastal observatory with the potential to identify patterns in long-term variability and simultaneously understanding short-term changes. The establishment of the Time-Series Station (TSS) Spiekeroog in a tidal channel west of Spiekeroog back in 2002 laid the foundation of the SCO. Since then, the observatory is expanding continuously and is now representing a valuable asset supporting education, industry, government, and environmental conservation efforts in the area. Summing up the infrastructure and technical components, the importance of the SCO is evident, and individual projects greatly benefit from the collaboration with the partners in and the elements of the SCO. Harmonizing the infrastructure and competences of contributing partners will be a next step to further consolidate the SCO. A challenge poses the maintenance of the SCO based on projects, which is focused on the addition of new facilities, not maintaining, refurbishing, or (if necessary) deconstructing existing infrastructure. Therefore, structural support and funding opportunities not linked to projects but aiming to sustain observational capacities are required.

Subterranean estuaries are connective zones between inland aquifers and the open sea where terrestrial freshwater and circulating seawater mix and undergo major biogeochemical changes. They are biogeochemical reactors that modify groundwater chemistry prior to discharge into the sea. We propose that subterranean estuaries of high-energy beaches are particularly dynamic environments, where the effect of the dynamic boundary conditions propagates tens of meters into the subsurface, leading to strong spatio-temporal variability of geochemical conditions. We hypothesize that they form a unique habitat with an adapted microbial community unlike other typically more stable subsurface environments. So far, however, studies concerning subterranean estuaries of high-energy beaches have been rare and therefore their functioning, and their importance for coastal ecosystems, as well as for carbon, nutrient and trace element cycling, is little understood. We are addressing this knowledge gap within the interdisciplinary research project DynaDeep by studying the combined effect of surface (hydro- and morphodynamics) on subsurface processes (groundwater flow and transport, biogeochemical reactions, microbiology). A unique subterranean estuary observatory was established on the northern beach of the island of Spiekeroog facing the North Sea, serving as an exemplary high-energy research site and model system. It consists of fixed and permanent infrastructure such as a pole with measuring devices, multi-level groundwater wells and an electrode chain. This forms the base for autonomous measurements, regular repeated sampling, interdisciplinary field campaigns and experimental work, all of which are integrated via mathematical modelling to understand and quantify the functioning of the biogeochemical reactor. First results show that the DynaDeep observatory is collecting the intended spatially and temporally resolved morphological, sedimentological and biogeochemical data. Samples and data are further processed ex-situ and combined with experiments and modelling. Ultimately, DynaDeep aims at elucidating the global relevance of these common but overlooked environments.

Coastal lagoons are important for ecological, cultural, economic and recreational reasons. Globally, they are subject to significant anthropogenic pressures. Our understanding of the importance of groundwater discharge into coastal lagoons for water and solute budgets is evolving, yet key gaps remain. This study resolves sources of groundwater seepage and estimates nutrient loads from direct groundwater discharge into a large hypertrophic coastal lagoon in New Zealand. We analysed major ions, stable water isotopes and nutrients in lagoon surface water, porewater, groundwater wells and springs. Groundwater and porewater samples split into two distinct groups: (1) inland samples that were MgHCO3 dominated with more negative δ2H:δ18O ratios and lower ion concentrations, and (2) permeable barrier samples that were NaCl dominated with more positive δ2H:δ18O ratios and higher ion concentrations. Porewater entering the lagoon is sourced from alpine river and rainfall recharge on the plains. Barrier porewater appears to be sourced from infiltration from the lagoon through the barrier and local rainfall. Despite higher nitrate in deeper groundwater wells, low nitrate in shallow porewater indicates potential denitrification before groundwater discharges to the lagoon. Our observations support efforts to restore and construct wetlands around the lagoon to remove nutrients. However, wetland restoration will need to be carried out by maintaining a balance between enhancing denitrifying conditions while preventing phosphorus release from sediments. Nutrient load calculations revealed that direct groundwater seepage to the lagoon provides ~ 3% of dissolved inorganic nitrogen and ~ 30% of dissolved reactive phosphorus compared to river inputs, indicating that groundwater discharge may play an important role in phosphorus transport to the lagoon.

The ecological tidal model simulates the cycling of carbon, nitrogen, phosphate, and silicate and describes the tidal, diurnal, and annual dynamics of the back barrier area of the island Spiekeroog in the German Bight. The region is characterized by strong tidal currents and extensive tidal flats. It is strongly influenced by the conditions in the southern part of the German Bight. This model study investigates the dependence of the model behavior on the boundary conditions and the forcing. The effect of short- and long-term sea level rise on nutrient and plankton dynamics is analyzed. As the model is set up as semi-Lagrangian with only a coarse approximation of the hydrodynamics, the seasonal and intratidal variability in the biogeochemical cycling can only be reproduced qualitatively. By varying the boundary conditions, the intrinsic dynamics of the back barrier area can be separated from boundary condition effects. This study shows that any agreement between model results and field data cannot be expected without correct boundary conditions. The seasonal variability is of major importance, while higher-frequency variability only plays a minor role.