Spiekeroog Island Research Articles

Spatial and temporal dynamics of groundwater biogeochemistry in the deep subsurface of a high-energy beach

Intertidal sandy beach systems are considered complex biogeochemical reactors. At beach sites that are subject to high tidal and wave energy, seawater circulation can reach tens of meters deep into the subsurface and changing environmental conditions are assumed to lead to dynamic groundwater flow paths, saltwater-freshwater mixing zones, and a spatio-temporally variable groundwater biogeochemistry. Previous studies mainly focused on the upper meters of subterranean estuaries (STE), while the deep subsurface remained a black box. This study presents spatial (cross-shore) and temporal (∼ six-weekly, over 1.5 years) dynamics of the groundwater biogeochemistry that were observed down to 24 m below the ground surface (mbgs) of a sandy high-energy beach on Spiekeroog Island (Germany).In addition to redox conditions along a cross-shore transect ranging from oxic to Fe oxide reducing/slightly sulfidic, we found a previously unknown, distinct vertical redox zonation as well. Temporal variations of the biogeochemistry within low salinity groundwater at the most landward station close to the dune base were mainly driven by storm flood related seawater infiltration. Around the high water line, the extent of the upper saline plume (USP) varied over time. Furthermore, temporal dynamics of the O2 saturation at 6 mbgs indicated a seasonally shifting depth of the oxycline at this location. In the lower intertidal zone, groundwater solute concentrations displayed a temporally variable zone of deep freshwater discharge.Regarding the impact of the deep STE on the groundwater biogeochemistry of the discharge zone, our data revealed that nutrient, Mn, and Fe release along the deep flow paths through the USP towards the discharge zone was limited, likely due decreasing availability of labile organic matter and subsequent slowing down of metabolic processes with depth. High concentrations of metabolites in the upper ∼ 2 mbgs of the discharge zone were, therefore, rather attributed to the incorporation of labile organic matter during continuous and storm flood related sediment relocation and/or the contribution of older waters, e.g., the subtidal saltwater wedge.

Comparison of methods measuring electrical conductivity in coastal aquifers

Coastal aquifers, the transition zone between freshwater and saltwater, show large salinity contrasts in the subsurface. Salinity is a key parameter to understand coastal groundwater flow dynamics and consequently also geochemical and microbial processes. For mapping porewater salinity, a variety of methods exists, mainly using electrical conductivity as a proxy. We investigate methods including hydrological/geochemical (well sampling, fluid logger) as well as geophysical method (direct push, geoelectrics) utilizing measurements near the high-water line of a high-energy beach at the North Sea island of Spiekeroog. We compare the methods, discuss their benefits and limitations and assess their spatial and temporal resolution. One key to enable a comparison is the estimation of formation factors transforming bulk conductivity measured by geophysical tools in to fluid conductivities obtained from direct measurements. We derive depth-dependent formation factors derived from time-series measurements of fluid loggers and a vertical electrode installation. Using these formation factors, the vertical electrode chain proves to provide reliable salinities at high spatial and temporal dimension. Direct-push profiling data provide the highest vertical resolution. However, a careful calibration is needed to allow for salinity quantification. On the other hand, electrical resistivity tomography (ERT) exhibits the lowest spatial resolution, but can image two-dimensional salinity distributions. We found ERT to fit very well to all other methods, but the data analysis should be aimed at salinities instead of bulk conductivities, i.e. including formation factors and temperature models into the inversion process.

Simple and affordable colorimetric sensing strips for quantitative determination of total manganese in porewater samples

AbstractSimple and economical colorimetric strips for measuring manganese (Mn) in natural waters are described. For their construction, leucomalachite green (LMG) embedded in a Nafion® polymeric matrix was immobilized on a polyvinylchloride surface. Upon immersing the strips in the sample, any soluble manganese(II/III) present catalyzed the oxidation of the LMG base to malachite green by adding sodium periodate. The observed color change is related to the manganese concentration and can be quantified using a field device constructed using commercial red–green–blue sensors and an Arduino® board. A linear response from 0.1 to 0.6 μmol L−1 manganese (limit of detection: 0.1 μmol L−1) was observed. The signal for strips prepared on 7 d gave a relative standard deviation equal to 13%. The strips showed good agreement with results obtained by ICP‐MS in porewater collected on Spiekeroog Island (North Sea). Therefore, the method is a tool for rapid measurements of manganese in porewater samples offering new possibilities in understanding biogeochemistry for high temporal and spatial resolution manganese surveys and providing an analytical technique that provides field results comparable to laboratory systems.

From seasonal field study to surrogate modeling: Investigating the biomechanical dynamics of Elymus sp. in salt marshes

AbstractSalt marshes have been studied in the context of ecosystem services they can provide for coastal protection. In this study, monthly field campaigns focusing on Elymus spp. and its biomechanical properties were conducted from December 2021 to December 2022 on the German Barrier Island Spiekeroog. A total of 1390 specimens were investigated to determine their growth length, out of which 418 specimens were investigated mechanically with three‐point bending tests to determine their biomechanical properties. To evaluate the interaction of hydraulic loads and vegetation, the challenge of modeling biomechanical plant properties to scale is addressed by using resin 3D printing with flexible material, while focusing on the materials mechanical properties. Based on the field data acquired and additional literature (adding up to 1959 measurements), a cylindrical plant model with an outer diameter of (scale 1 : 1) was developed. It was manufactured mixing two resin components with varying volume ratios resulting in surrogates with different flexural stiffnesses. The surrogates were characterized using three‐point bending tests and image analysis of their bending behavior when subjected to currents between 0.4 and 1.2 m/s. With the average Young's modulus ranging from 8.45 to 1708.42 MPa, the bending angle varies from 0° to 77.4° displaying the influence of material stiffness and flow velocity. Applying the Cauchy scaling law, this study shows that resin 3D printing can be used to model Elymus sp. with respect to its biomechanical properties allowing for seasonally independent physical laboratory experiments with plant models.

Extensive dune grasslands largely lacking human disturbance are an important refuge for a vole-dependent raptor

Agricultural intensification and abandonment have led to a dramatic decrease of semi-natural grasslands such as low-intensity pastures and hay meadows. The Short-eared owl (Asio flammeus) is a ground-nesting raptor of open grasslands that has severely suffered from these changes. We studied the habitat preferences of this umbrella species of open grasslands in its last permanent breeding area in Germany (East Frisian Islands, southern North Sea). We analysed the breeding-territory preferences based on 576 territories on six of the islands. Moreover, we assessed nest-site preferences of 13 breeding pairs on the German abundance hot spot, the island of Spiekeroog. Our investigation revealed that the Short-eared owl strongly preferred open dunes for breeding, especially dune grasslands. By contrast, built-up areas and small stands of trees were avoided. For nest-building, microhabitats with a high cover of the herb layer and litter resulting in tall vegetation were favoured. By contrast, the vegetation in the wider surrounding of the nest was characterised by more bare ground and shorter vegetation but still a high cover of the herb layer and litter. In conclusion, our study highlights the prime importance of extensive open grasslands with a pronounced litter layer and largely lacking human disturbance as breeding habitats for the Short-eared owl. At the nesting site, we suggest that tall and dense vegetation with a high cover of litter (i) might enhance concealment and (ii) causes a favourable microclimate by protecting fledglings against adverse weather conditions. In the wider surrounding of the nest, shorter vegetation with a pronounced litter layer (i) improves fledgling mobility, (ii) fosters vole abundance and (iii) increases prey accessibility.

Efficient representation of transient tidal overheight in a coastal groundwater flow model using a phase-averaged tidal boundary condition

Ocean tides cause water table overheight near the seaward boundary of coastal aquifers which can have a large impact on groundwater flows and saltwater intrusion. Despite this, regional-scale coastal groundwater flow models often neglect the effects of ocean tides or alternatively assume that the influences of ocean tides and sea level are constant over time. These simplifications are often required because simulation of the phase-resolved tidal signal at a coastal boundary is computationally demanding. Our objective was to derive a phase-averaged tidal boundary condition for application along gently sloping coastlines that enables simulation of real, complex tidal signals combined with sea-level changes due to meteorological effects. The boundary condition extends an existing analytical solution of tidal overheight by including an empirical correction function for conditions where the assumptions of the existing solution are violated. The correction function was developed by conducting a parametric study on an idealized two-dimensional cross-sectional coastal aquifer model and considering the effects of parameters including horizontal hydraulic conductivity, vertical anisotropy, specific yield, aquifer thickness, and beach slope. The performance of the new phase-averaged tidal boundary condition was assessed using a three-dimensional groundwater flow model of the island of Spiekeroog, Northwest Germany, which comprises complex coastal morphology with non-planar beach slopes. Simulations applying the new boundary condition were compared to simulations that adopted a phase-resolved tidal boundary condition and to observed groundwater levels. Accounting for time-varying changes in the tidal signal and sea level was found to be critical to simulate observed data and to adequately reproduce the transient tidal overheight simulated by the phase-resolved model. The performance of the new phase-averaged boundary condition in the regional-scale model varied depending on how the coastal morphology was represented in the boundary condition with local morphological features increasingly important for lower intertidal vertical infiltration capacity (low isotropic hydraulic conductivity or high vertical anisotropy) or specific yield. In conclusion, the new boundary condition presented overcomes current limitations in simulating transient tidal overheight in regional-scale groundwater models.

The DynaDeep observatory – a unique approach to study high-energy subterranean estuaries

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.

Weather conditions determine reproductive success of a ground-nesting bird of prey in natural dune grasslands

The Short-eared Owl (Asioflammeus) inhabits open grasslands and belongs to the ground-breeding birds that have experienced the most severe declines during recent decades. Here, we studied nest and fledgling survival of the owl species in relation to (i) habitat composition, (ii) vegetation structure, (iii) weather conditions and (iv) vole abundance. The study was conducted on the East Frisian Island of Spiekeroog (southern North Sea, Lower Saxony, Germany), which harbours one of the last remaining permanent populations of the species in Central Europe. With a mean hatching success of 5.6 young per nest (N = 34) and an average probability of nest survival of 0.9 (N = 28), values ascertained in this study exceeded those reported in previous research. We attribute this to the special environmental conditions on the island, i.e. (i) the absence of mammalian mesopredators such as the Red Fox (Vulpesvulpes), (ii) nearly no disturbance through agricultural measures and (iii) low level of human disturbance due to legal regulations of the National Park. By contrast, the survival of fledglings was lower than survival of nests and varied considerably between the investigated years. Weather conditions were the key driver of fledgling survival. Maximum wind speed and sunshine duration had a negative effect on the probability that chicks successfully fledged. Both lead to reduced hunting success, and the former is also associated with increased costs for thermoregulation. Consequently, increasing frequency of extreme weather events caused by climate change could negatively impact the breeding success of vole-dependent raptors, such as the Short-eared Owl.

Trophic structure and origin of resources of soil macrofauna in the salt marsh of the Wadden Sea: a stable isotope (15N, 13C) study

Salt marshes exist along the gradient of the marine mudflat to the terrestrial dunes, with a gradient of shore height and associated plant zonation. The lower salt marsh (LSM) extends from the mean high tidal level to 35 cm above that level and is followed by the upper salt marsh (USM). Despite changes in the amount of allochthonous marine input and in abiotic conditions, little is known about changes in the trophic structure and used of basal resources by the soil macrofauna along marine—terrestrial boundaries. Natural variations in carbon stable isotope ratios (δ13C signatures) allow insight into basal resources of consumers such as marine algae, terrestrial C3 and C4 photosynthesising plants. Furthermore, variations in nitrogen stable isotope ratios (δ15N signatures) allow insight into the trophic position of consumers. We investigated spatial and temporal changes in stable isotope signatures in salt marsh soil macrofauna of the island of Spiekeroog, German Wadden Sea. The range of δ15N signatures indicated no changes in food chain length across salt marsh zones with consumers in both zones comprising primary decomposers, secondary decomposers and first order predators. However, the trophic position of individual species changed between zones, but in particular with season. Contrasting δ15N signatures, the range in δ13C signatures in the LSM was twice that in the USM indicating a wider range of resources consumed. Bayesian mixing models indicated predominant autochthonous resource use in both the LSM and USM, with the use of marine allochthonous resources never exceeding 29.6%. However, the models also indicate an increase in the use of marine resources in certain species in the LSM with no use in the USM. Overall, the results indicate that the resource use of salt marsh macrofauna varies more in space than in time, with the food web being generally based on autochthonous rather than allochthonous resources. However, there also is trophic plasticity in certain species across both temporal and spatial scales including variations in the use of allochthonous resources. Generally, however, marine input contributes little to the nutrition of salt marsh soil macroinvertebrates.

Seasonal and Zonal Succession of Bacterial Communities in North Sea Salt Marsh Sediments.

Benthic microbial communities of intertidal zones perform important biogeochemical processes and provide accessible nutrients for higher organisms. To unravel the ecosystem services of salt marsh microbial communities, we analyzed bacterial diversity and metabolic potential along the land–sea transition zone on seasonal scales on the German North Sea Island of Spiekeroog. Analysis of bacterial community was based on amplicon sequencing of 16S rRNA genes and –transcripts. Insights into potential community function were obtained by applying the gene prediction tool tax4fun2. We found that spatial variation of community composition was greater than seasonal variations. Alphaproteobacteria (15%), Gammaproteobacteria (17%) and Planctomycetes (11%) were the most abundant phyla across all samples. Differences between the DNA-based resident and RNA-based active communities were most pronounced within the Planctomycetes (17% and 5%) and Cyanobacteriia (3% and 12%). Seasonal differences were seen in higher abundance of Gammaproteobacteria in March 2015 (25%) and a cyanobacterial summer bloom, accounting for up to 70% of the active community. Taxonomy-based prediction of function showed increasing potentials for nitrification, assimilatory nitrate and sulfate reduction from sea to land, while the denitrification and dissimilatory sulfate reduction increased towards the sea. In conclusion, seasonal differences mainly occurred by blooming of individual taxa, while the overall community composition strongly corresponded to locations. Shifts in their metabolism could drive the salt marsh’s function, e.g., as a potential nitrogen sink.

The Spiekeroog Coastal Observatory: A Scientific Infrastructure at the Land-Sea Transition Zone (Southern North Sea)

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.

3D ground-penetrating radar attributes to generate classified facies models: A case study from a dune island

Ground-penetrating radar (GPR) is a standard geophysical technique used to image near-surface structures in sedimentary environments. In such environments, GPR data acquisition and processing are increasingly following 3D strategies. However, the processed GPR data volumes are typically still interpreted using selected 2D slices and manual concepts such as GPR facies analyses. In seismic volume interpretation, the application of (semi-)automated and reproducible approaches such as 3D attribute analyses as well as the production of attribute-based facies models are common practices today. In contrast, the field of 3D GPR attribute analyses and corresponding facies models is largely untapped. We have developed and applied a workflow to produce 3D attribute-based GPR facies models comprising the dominant sedimentary reflection patterns in a GPR volume, which images complex sandy structures on the dune island of Spiekeroog (Northern Germany). After presenting our field site and details regarding our data acquisition and processing, we calculate and filter 3D texture attributes to generate a database comprising the dominant texture features of our GPR data. Then, we perform a dimensionality reduction of this database to obtain meta texture attributes, which we analyze and integrate using composite imaging and (also considering additional geometric information) fuzzy c-means cluster analysis resulting in a classified GPR facies model. Considering our facies model and a corresponding GPR facies chart, we interpret our GPR data set in terms of near-surface sedimentary units, the corresponding depositional environments, and the recent formation history at our field site. Thus, we demonstrate the potential of our workflow, which represents a novel and clear strategy to perform a more objective and consistent interpretation of 3D GPR data collected across different sedimentary environments.

EVALUATING DIFFERENT WATER-LAND-BOUNDARY APPROXIMATIONS TO IMPROVE SAR-DERIVED DIGITAL ELEVATION MODELS

Abstract. The coastline of the German Wadden Sea is constantly subjected to the tides and the tidal-induced environmental changes like erosion and accumulation of sediments need to be monitored constantly. This task requires digital elevation models (DEMs), which are derived from remote sensing data. To model those DEMs, a separation of data collected over landmasses and water bodies is required. In the GeoWAM project the potential of airborne SAR-data (F-SAR) is investigated for monitoring purposes in the Wadden Sea. As part of the project, this paper focuses on the suitability of F-SAR data regarding the derivation of water-land-boundaries (WLBs). Therefore, water-land-boundaries based on independent data sets are compared and evaluated. Analyzed data sets include data collected via F-SAR, airborne laserscanning (ALS), on site GNSS measured WLB points and sea-level data from two acoustic gauges. The algorithms were tested on a study site on Spiekeroog island. Our results show, that the accuracies of the derived WLBs mostly depend on the on-site topography and sediments. The spatial deviation between the reference data and the approximated WLBs is mostly less than 2 m horizontally and 0.15 m vertically. Identified challenges to overcome are mostly related to processing of F-SAR data in areas with highly water saturated sediments. Our results suggest, that F-SAR data in tidal flats is not necessarily dependent on further supplementing surveys, as one of the main advantages of the F-SAR data is the potential to derive DEMs and WLBs from the same data set.

The Three Domains of Life Within the Discharge Area of a Shallow Subterranean Estuary at a High Energy Beach

Subterranean estuaries (STEs) play an important role in linking nutrient cycling between marine and terrestrial systems. As being the primary drivers of nutrient cycling, the composition of microbial communities and their adaptation toward both, terrestrial and marine conditions are of special interest. While bacterial communities of STEs have received increasing scientific attention, archaeal and meiofaunal diversity was mostly neglected. Previous studies at the investigated sampling site, the STE of a mesotidal beach at the German North Sea island of Spiekeroog, focused on spatial and seasonal patterns of geochemical and bacterial diversity. By additionally investigating the archaeal and meiofaunal diversity and distribution, we now aimed to fill this gap of knowledge to understand the microbial response to submarine groundwater discharge (SGD). The topography of Spiekeroog beach and associated geochemical gradients in porewater displayed a distinct cross-shore zonation, with seawater infiltration on the upper beach at the high water line (HWL), and saline and brackish porewater exfiltration (SGD) at the ridge-runnel structure and the low water line (LWL) on the lower beach. This led to a higher evenness of prokaryotic communities in lower beach areas impacted by SGD compared to unimpacted areas. Archaea contributed 1–4% to the 16S rRNA gene sequence dataset. Those were dominated by Nitrosopumilaceae, corresponding well to higher concentrations of NH4+ in the discharge area of the STE. The unimpacted sites had elevated abundances of Wosearchaeia, which were also detected previously in impacted areas of an STE at Mobile Bay (Gulf of Mexico). While a large proportion of prokaryotes were present in the entire intertidal area, meiofaunal community compositions were site specific and dominated by nematodes. Nematode communities of the high-water line differed distinctively from the other sites. Overall, our data indicates that the three domains of life display distinctly different adaptations when facing the same conditions within the STE. Therefore, distribution patterns of any domain can only be understood if all of them, together with basic environmental information are investigated in an integrated context.

Thallium cycling in pore waters of intertidal beach sediments

Pore water data of thallium (Tl) in marine environments are scarce. It has been suggested that anoxic sediments are a sink for dissolved Tl likely due to its incorporation into Fe sulfides. However, recent studies indicated that Tl is removed from solution prior to the onset of sulfidic conditions and that Tl is involved in biological cycles in coastal seawater. To assess the Tl behavior in marine sediments, pore waters of a high energy beach (Spiekeroog Island, Germany) were sampled during different seasons and on different spatial scales. The study site provides a perfect set of samples to assess trace metal behavior under various redox conditions.Samples were analyzed for the dissolved trace metals Tl, molybdenum (Mo), uranium (U), and rhenium (Re). Additionally, sediments were incubated to monitor trace metal removal or release under controlled conditions. On-site redox conditions were characterized using dissolved oxygen (O2), manganese (Mn), and iron (Fe) concentrations.Our results indicated high enrichments of Tl in pore waters concurrently to aerobic organic matter (OM) degradation, exceeding seawater concentrations up to 6-fold. In contrast, Tl was completely removed in weakly reducing pore water characterized by more than 1 µM Mn or Fe. The spatial Tl distribution in intertidal pore waters responded to the present dissolved Mn and Fe level, which varied in space and with season. We suggest, Tl release to be attributed to OM degradation, particle desorption, or reoxidation, whereas Tl removal is likely related to the presence of trace amounts of dissolved sulfide.In conclusion, the fast reaction kinetics of Tl led to fast and complete removal in comparison to Mo, U, and Re and we suggest the following order of removal: Tl > Re > U > Mo. The removal behavior as well as the high enrichments of Tl in pore waters implicate that sinks and sources in the marine environment are not yet fully understood. Although the intertidal sediments form an overall sink for Tl (U and Re), especially under mostly anoxic conditions in summer, sediments turn into a Tl net source in winter, when sediments are more oxic.For global marine Tl balances, Tl fluxes have been estimated based on pore water Tl/Mn ratios. Contrastingly, Tl removal from pore waters at low threshold concentrations of Mn at our site indicated that these Tl flux estimations may not be applicable in permeable sandy beach sediments and in many other types of sediments, which host mildly sulfidic conditions.

Young soils of a temperate barrier island under the impact of formation and resetting by tides and wind

The island Spiekeroog (Germany) at the southern North Sea coast exhibits an island tail (Ostplate) of which large parts formed after 1860 by gradual eastward deposition of marine sediments. This study aimed to (i) provide a first overview of soils on the Ostplate, (ii) verify whether soil formation stages in different landscape elements (dune chain, dune slacks, saltmarsh, pioneer zone) are linked to the island’s gradual development and (iii) to assess the impact of tides and winds on soil formation. Standard physicochemical properties of dune -and dune slack topsoils and 19 pedons (i.a. pH, total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), and mean weight diameter (MWD) of water stable aggregates) were analyzed. Soils of the dune slacks, the saltmarsh and the pioneer zone classified as Gleysols which showed no trend towards differences along the Ostplate. The dune chain exhibited Protic Arenosols that were initially acidified in lee positions (min. pH = 5.1). Carbon (C) and nitrogen (N) isotopes (δ13C, δ15N) signatures of saltmarsh sediments were used as a proxy for the tidal influence on soil formation. In addition, sulfur (S) isotope (δ34S) signatures revealed insights on sulfur sources in the saltmarsh. The δ13C signatures of organic matter (OM) decreased and TOC/TN -and TOC/total sulfur (TS) ratios increased with increasing elevation and decreasing flooding frequency. At higher flooding frequency, δ15N resembled the dominant input of OM from the southern North Sea, at a lower interaction level, mixing of a wider range of sources was indicated. S isotopes indicated dissolved sulfate and reduced species as sulfur sources. It was concluded that an ongoing impact of wind-induced sand movement at the dunes, and flooding-induced sediment movement in the saltmarsh and the pioneer zone, repealed any potential disparities in soil formation stages, which might have existed along the Ostplate.

Tidal creeks as hot-spots for hydrological exchange in a coastal landscape

Coastal ecosystem health and sustainability is tightly coupled to submarine groundwater discharge (SGD) and associated nutrient, carbon and pollutant fluxes. However, there are few studies that systematically analyse the interaction between the terrestrial aquifer system, catchment morphology and coastal SGD. The objective of this study was to evaluate the role of catchment morphology and how this influences the spatial distribution, timing and volume of the SGD flux to a branched tidal creek system, on the barrier island Spiekeroog, Germany. The subsurface salinity was mapped using electrical resistivity tomography (ERT) and a hydrogeochemical survey of the shallow groundwater. Temporal and spatial analysis of geochemical tracers (radon-222, chloride) in the tidal creek water was integrated into a transient 222Rn mass balance model for quantification of SGD rates during two field campaigns that encompassed spring and neap tides. The ERT mapping indicated that fresh groundwater dominated under the dune ridges down to about 15 m depth, but became progressively more brackish seawards. This is likely due to frequent tidal and storm flooding of low-lying areas. The highest groundwater fluxes into the creek were indicated by high 222Rn activities (average 468 Bq m−3) towards the dune ridge. Chloride concentrations (up to 17.3 g L−1) increased seawards showing the progressive salinization of water in the creek. The freshwater component of SGD was highly variable in time but was highest at low tide, while the total SGD flux (saline + fresh) was highest when tides changed from inflow to outflow as the rapid pressure release on the local aquifer caused a large hydraulic gradient towards the creek. Comparing the freshwater component of mean daily SGD to the creek with estimated groundwater recharge rates in the catchment (665 m3 d−1) shows that the fresh groundwater discharge exceeds fresh recharge during spring tides (~120%) but is was lower than recharge during neap tides (~27%). In this study we show that tidal creeks and their relation to catchment morphology are relevant for understanding the spatial and temporal exchange of fresh and saline water between the catchment and coastal zone.

Hydrogeochemistry of near-surface groundwater on a developing barrier island (Spiekeroog, Germany): The role of inundation, season and vegetation

Low-lying barrier islands present ideal locations to investigate hydrogeochemical processes in sandy coastal aquifers, as they (i) host typical geographic features of the coast, including beaches, dunes, as well as salt marshes, (ii) are susceptible to the impacts of a global climate change, because of their low surface elevations, and (iii) are ubiquitous (>2,100) amongst the global coastlines. The aim of this study was to investigate the evolution of hydrogeochemical patterns in near-surface groundwater on developing barrier islands, using the Ostplate on Spiekeroog Island, Germany, as an example. The impact of inundation, season and vegetation on the hydrogeochemical processes were of particular interest. Six groundwater monitoring wells were installed for this purpose, covering main geographic features of the Ostplate, i.e., supratidal beaches, dunes and salt marshes. Groundwater samples were collected bimonthly from April 2016 to March 2017. Measured parameters included major ions, nutrients, dissolved organic carbon/matter (DOC/DOM), δ13C signatures of dissolved inorganic carbon, as well as δ34S signatures of dissolved sulfate and sulfide. We found major ion compositions to be a function of the annual flooding frequency, although cation concentrations were further impacted by calcite dissolution and cation exchange. The redox chemistry varied at the different sites and through seasons. Aerobic respiration was the dominant pathway for carbon oxidation at the dune locations, whereas organic matter mineralization in the salt marsh was also mediated by anaerobic processes. Regarding electron donors, the presence of labile DOM was linked to salinity, and terrestrial DOM signatures as well as gross inputs of organic carbon agreed with vegetation patterns. A physico-chemical interpretation of the pore water compositions revealed a potential for the authigenic formation of carbonate and phosphate minerals in the investigated sediments, triggered by the redox processes.

Post‐fledging migration and wintering strategies of individual juvenile Lesser Black‐backed Gulls (Larus fuscus)

Research into the patterns and drivers of juvenile migration is important for understanding the development of individual migration strategies. Although several recent studies have tracked adult large gulls throughout the annual cycle, the movements of juveniles remain poorly understood. We fitted Global Positioning System (GPS) devices that transmit locations through the Global System for Mobile Communications (GSM) to ten juvenile Lesser Black‐backed Gulls Larus fuscus prior to fledging on the island of Spiekeroog, Germany, to study their first autumn migration and wintering behaviour. The tracked birds initially departed on similar compass bearings south to south‐west, after which migration routes diverged. Individuals took 38–107 days to reach their wintering sites in Algeria, Morocco or Spain. Birds visited 6.7 ± 3.7 (mean ± sd) stopover sites en route and spent substantial time in northern Europe, with some individuals converging at the same sites over small spatial scales (<1 km), but not over the same time periods. Birds increased travel speeds in the second half of their migration. They showed relatively high site‐fidelity after arrival at wintering sites, and there was no evidence that the size of foraging areas increased over time, suggesting limited exploratory behaviour. Individuals used the same predominant habitats – cropland, open water and built‐up areas – to varying degrees, but showed limited variation in habitat use over time. Overall, the migration routes and timings of juveniles broadly resembled those of previously tracked adults. Given their similar initial bearings and abundance of large gulls on the flyway, we suggest that social learning is likely to be an important process shaping the individual migration and wintering strategies of juvenile gulls.

Cycling of redox-sensitive trace metals in barrier island freshwater lenses

Freshwater lenses connect the terrestrial and marine realm via groundwater discharge at the edges of islands and serve as drinking water resources. We studied the redox-sensitive metals U, Mo, V, and Tl along the redox gradient of fresh groundwater lenses on Spiekeroog Island, northern Germany. Groundwater solute concentrations were linked to groundwater age and redox characteristics. We further quantified the contribution of precipitation, sea spray, and aquifer matrix to the groundwater metal concentrations and evaluated the sink and source function of the aquifer under oxic and reducing conditions.We found that biogeochemical processes altered the concentrations of the trace metals. In young, oxygen to nitrate reducing zones, the aquifer matrix represented the major metal source to the groundwater. For Tl, rain was an additional important (anthropogenic) source. Under manganese and iron oxide to sulfate reducing conditions, U and Tl were sensitive to redox dependent removal, whereas Mo and V were less affected by reductive precipitation/adsorption. In detail, 99% of dissolved Tl, 88% of U, 66% of Mo, and 44% of V were removed to the solid phase in comparison to values from less reducing zones. Large parts of the western freshwater lens on Spiekeroog were anoxic. For this reason, the delivery of aquifer derived metals to the ocean via fresh groundwater discharge appeared to be limited. Higher U, Mo, V, and Tl concentrations were observed in the presently developing young freshwater lens in the east of Spiekeroog Island. This suggests that less reducing groundwater lenses may be a source of these metals to the adjacent beach/coastal seawater. Especially for V, freshwater discharge from sandy coastal aquifers may be important, as groundwater concentrations exceeded seawater concentration under oxic as well as anoxic conditions. Regarding the suitability of the freshwater as drinking water, all measured trace metal concentrations were classified as uncritical.