Description: For millennia humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectives on future research directions to better understand land-ocean connectivity through groundwater, including the spatial distributions of the essential hydrogeological parameters, highlighting technical and scientific developments, and briefly discussing its societal relevance in rapidly changing coastal oceans.

Description: The vast majority of freshly produced oceanic dissolved organic carbon (DOC) is derived from marine phytoplankton, then rapidly recycled by heterotrophic microbes. A small fraction of this DOC survives long enough to be routed to the interior ocean, which houses the largest and oldest DOC reservoir. DOC reactivity depends upon its intrinsic chemical composition and extrinsic environmental conditions. Therefore, recalcitrance is an emergent property of DOC that is analytically difficult to constrain. New isotopic techniques that track the flow of carbon through individual organic molecules show promise in unveiling specific biosynthetic or degradation pathways that control the metabolic turnover of DOC and its accumulation in the deep ocean. However, a multivariate approach is required to constrain current carbon fluxes so that we may better predict how the cycling of oceanic DOC will be altered with continued climate change. Ocean warming, acidification, and oxygen depletion may upset the balance between the primary production and heterotrophic reworking of DOC, thus modifying the amount and/or composition of recalcitrant DOC. Climate change and anthropogenic activities may enhance mobilization of terrestrial DOC and/or stimulate DOC production in coastal waters, but it is unclear how this would affect the flux of DOC to the open ocean. Here, we assess current knowledge on the oceanic DOC cycle and identify research gaps that must be addressed to successfully implement its use in global scale carbon models.

Description: For millennia, humans have gravitated towards coastlines for their resource potential and as geopolitical centres for global trade. A basic requirement ensuring water security for coastal communities relies on a delicate balance between the supply and demand of potable water. The interaction between freshwater and saltwater in coastal settings is, therefore, complicated by both natural and human-driven environmental changes at the land-sea interface. In particular, ongoing sea level rise, warming and deoxygenation might exacerbate such perturbations. In this context, an improved understanding of the nature and variability of groundwater fluxes across the land-sea continuum is timely, yet remains out of reach. The flow of terrestrial groundwater across the coastal transition zone as well as the extent of freshened groundwater below the present-day seafloor are receiving increased attention in marine and coastal sciences because they likely represent a significant, yet highly uncertain component of (bio)geochemical budgets, and because of the emerging interest in the potential use of offshore freshened groundwater as a resource. At the same time, “reverse” groundwater flux from offshore to onshore is of prevalent socio-economic interest as terrestrial groundwater resources are continuously pressured by overpumping and seawater intrusion in many coastal regions worldwide. An accurate assessment of the land-ocean connectivity through groundwater and its potential responses to future anthropogenic activities and climate change will require a multidisciplinary approach combining the expertise of geophysicists, hydrogeologists, (bio)geochemists and modellers. Such joint activities will lay the scientific basis for better understanding the role of groundwater in societal-relevant issues such as climate change, pollution and the environmental status of the coastal oceans within the framework of the United Nations Sustainable Development Goals. Here, we present our perspectives on future research directions to better understand land-ocean connectivity through groundwater, including the spatial distributions of the essential hydrogeological parameters, highlighting technical and scientific developments, and briefly discussing its societal relevance in rapidly changing coastal oceans.

Description: 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.

Description: Data from pore water (subterranean estuary) and seawater from Spiekeroog south (near ICBM time series station and campsite) and west beach ("Sturmeck"). South beach data were collected in August 2012, and west beach data were collected in November 2012. Pore water (event labels: DUNE, MIX, LTWL) sample collection was conducted at different sediment depths (50, 100, 150 cm below sediment surface). Stainless steel push-point lancets were insetred into the sediment, and pore water was withdrawn via vacuum (hand pumps) into nalgene polycarbonate bottles. Filtration was done using inline PES cartridge filters. The vacuum bottles were argon gas-flushed to avoid oxygen contamination. Sea water (event label SW) was collected with polycarbonate bottles and from LDPE seepage meter bags (event label SP) and filtered upon return to the laboratory (same day, PES filter cartridges). Sample collection was trace organic and metal clean (soaking and rinsing of bottles, tubing, and filters with diluted HNO3 and HCl suprapur), with sample materials consisting of polyethylene, polypropylene, and polycarbonate. Solid-phase extraction was done with BOND Elut PPL cartridges and elution with Methanol Optima grade. Measurements dissolved organic matter (DOM) were done with FT-ICR-MS. The crosstables describe the molecular composition of DOM and associated Fe and Cu. They contain characteristic properties and classifications of molecular sum formulas as well as FT-ICR-MS signal intensities of each sum formula for each sample (see event table of https://doi.pangaea.de/10.1594/PANGAEA.902704). File name description of “Spiekeroog beach STE_ESI_x_Crosstab_x": pos & neg = positive and negative ionization mode of electrospray ionization (ESI). BSA & NWA = basic/strong acidic and neutral/weak acidic DOM fraction (solid phase extracted)

Description: Data from pore water (subterranean estuary) and seawater from Spiekeroog south (near ICBM time series station and campsite, 53°45'13.5"N 7°40'22.5"E) and west beach ("Sturmeck", 53°46'10.0"N 7°40'26.2"E). South beach data were collected in August 2012, and west beach data were collected in November 2012. Sample abbreviations: SB=South Beach, WB=West Beach. DUNE=most landward station near dunes, MIX=mid-way station between dune base and low water line, LTWL=low tide water line. Pore water (DUNE, MIX, LTWL) sample collection was conducted at different sediment depths (50, 100, 150 cm below sediment surface). Stainless steel push-point lancets were insetred into the sediment, and pore water was withdrawn via vacuum (hand pumps) into nalgene polycarbonate bottles. Filtration was done using inline PES cartridge filters. The vacuum bottles were argon gas-flushed to avoid oxygen contamination. Sea water (SW) was collected with polycarbonate bottles and from LDPE seepage meter bags (SP) and filtered upon return to the laboratory (same day, PES filter cartridges). Sample collection was trace organic and metal clean (soaking and rinsing of bottles, tubing, and filters with diluted HNO3 and HCl suprapur), with sample materials consisting of polyethylene, polypropylene, and polycarbonate. Solid-phase extraction was done with BOND Elut PPL cartridges and elution with Methanol Optima grade. Measurements were done with VA Computrace 757 (Cu ligand concentrations and stability constants), HR-ICP-MS (Cu, Fe, and Mn concentrations), FT-ICR-MS (DOM), spectrophotometry (nutrients), and TOC analyzer (DOC and TDN). Trace metal concentrations (Fe, Cu, Mn) and speciation (oxidation state, size fractions, and organic association), as well as nitrogen species and concentrations: "Spiekeroog beach subterranean estuary environmental data". Concentrations (µM or nM) are denoted in the headers. CuL1=Cu-binding ligands concentrations (nM), logK1=Cu-binding ligand stability constant. Molecular composition of dissolved organic matter and associated Fe and Cu: "Spiekeroog beach STE_ESI_x_Crosstab_x". pos & neg = positive and negative ionization mode. BSA & NWA = basic/strong acidic and neutral/weak acidic, refer to the extracted DOM fraction. Processed figures and tables based on the original data are published here: https://doi.org/10.1016/j.gca.2019.06.004

Description: Data from pore water (subterranean estuary) and seawater from Spiekeroog south (near ICBM time series station and campsite) and west beach ("Sturmeck"). South beach data were collected in August 2012, and west beach data were collected in November 2012. Pore water (event labels: DUNE, MIX, LTWL) sample collection was conducted at different sediment depths (50, 100, 150 cm below sediment surface). Stainless steel push-point lancets were insetred into the sediment, and pore water was withdrawn via vacuum (hand pumps) into nalgene polycarbonate bottles. Filtration was done using inline PES cartridge filters. The vacuum bottles were argon gas-flushed to avoid oxygen contamination. Sea water (event label SW) was collected with polycarbonate bottles and from LDPE seepage meter bags (event label SP) and filtered upon return to the laboratory (same day, PES filter cartridges). Sample collection was trace organic and metal clean (soaking and rinsing of bottles, tubing, and filters with diluted HNO3 and HCl suprapur), with sample materials consisting of polyethylene, polypropylene, and polycarbonate. Solid-phase extraction was done with BOND Elut PPL cartridges and elution with Methanol Optima grade. Measurements were done with VA Computrace 757 (Cu ligand concentrations and stability constants), HR-ICP-MS (Cu, Fe, and Mn concentrations), spectrophotometry (nutrients), and TOC analyzer (DOC and TDN).

Description: These data were compiled from original and published datasets of coastal groundwater / subterranean estuary research efforts along global coastline (sites within 1km of shoreline). The dataset includes sampling site names, locations, original sample information, sample depth, temperature, salinity, dissolved nitrogen concentrations, and dissolved phosphorus concentrations. The data source or curator is also included in the dataset.

Description: This dataset was acquired to investigate the sources and sinks of dissolved organic matter in a high-energy beach subterranean estuary on Spiekeroog Island, German North Sea. The North Beach sampling location is approximately at 53°46'45N, 7°42'40E. Data were collected during a total of five sampling campaigns in the period of September 2016 - September 2019, spanning over all four seasons. The samples collected were from beach porewaters, seawater, and groundwater from the islands' freshwater lens. Pore water samples were collected with push-point lances (10-100 cm depth), seawater samples with bottles in the surf zone, and groundwater samples from inland monitoring wells (4-40 m depth). Samples were filtered onsite and preserved for storage (acidification for dissolved organic carbon, dissolved organic nitrogen, and trace metals, and poisoning with HgCl2 for dissolved nutrients). In situ data were collected, including temperature, salinity, oxygen concentrations (instrument: WTW Multi 3430), and humic-like fluorescent DOM (instrument: Turner Aquafluor). Laboratory analyses were conducted within a week of sampling and included spectrophotometry for nutrients, ICP-OES for trace metals, and TOC-VCPH analysis for DOC and TDN. Acidified subsamples were desalted and concentrated by solid-phase extraction, then analyzed on via ultra-high resolution Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS on a 15T Bruker solariX XR.

Description: Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes.