Patterns of Pore Water Chemistry which produced non-uniform groundwater flow conditions. We conclude that on mesotidal 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.

During seawater circulation in permeable intertidal sands, organic matter degradation alters the composition of percolating fluids and remineralization products discharge into surficial waters. Concurrently, coastal seawater nutrient and organic matter composition change seasonally due to variations in pelagic productivity. To assess seasonal changes in organic matter degradation in the intertidal zone of a high energy beach (Spiekeroog Island, southern North Sea, Germany), we analyzed shallow pore waters for major redox constituents (oxygen [O2], manganese [Mn], and iron [Fe]) and inorganic nitrogen species (nitrite [NO2−], nitrate [NO3−], and ammonium [NH4+]) in March, August, and October. Surface water samples from a local time series station were used to monitor seasonal changes in pelagic productivity. O2 and NO3− were the dominating pore water constituents in March and October. Dissolved Mn, Fe, and NH4+ were more widely distributed in August. Seasonal changes in seawater temperature as well as organic matter and nitrate supply by seawater were assumed to affect microbial rates and degradation pathways. Pore water and seawater variability led to seasonally changing constituent effluxes to surface waters. Mn, Fe, and NH4+ effluxes are minimal in March and reached their maximum in August. Furthermore, the intertidal sands switched from a net dissolved inorganic nitrogen sink in March to a net source in August. In conclusion, seasonal effects on intertidal pore water biogeochemistry affect constituent fluxes across the sediment‐water interface. The seasonality of the beach bioreactor must be considered when fluxes are extrapolated to annual timescales.

Sandy beaches have received increasing attention due to their global ubiquity and ecological services they provide. Previous investigations on their microbial diversity mostly targeted low energy beaches, lacked seasonality or geochemical information. We now have used a multidisciplinary approach to study the microbial diversity within sediments of a sandy, high energy beach that additionally exhibits submarine groundwater discharge (SGD) in the intertidal. We sampled two transects at three seasons down to a depth of one meter for 16S rRNA gene amplicon sequencing and subsequent correlation of microbial diversity to physico-chemical parameters. We found that advection driven transport of porewater constituents and constant physical reworking of the sediments prevented a distinct formation of vertically stratified redox zones. Consequently, a uniform microbial core community of generalists established independently of sampling site and depth. During spring, the community structure was disturbed by a "subsurface bloom" of Bacteroidetes and Firmicutes, probably due to deeper oxygen and nitrate penetration depths. In summer, the community returned to its equilibrium state with imprints of the subsurface bloom still visible. In our study, we did not detect a clear impact of SGD on microbial diversity, but found an unexpected homogeneous composition and resilience of the microbial community structure.

Submarine groundwater discharge (SGD) is an important source of nutrients and metals to the coastal ocean, affects coastal ecosystems, and is gaining recognition as a relevant water resource. SGD is usually quantified using geochemical tracers such as radon or radium. However, a few studies have also used dissolved silicon (DSi) as a tracer for SGD, as DSi is usually enriched in groundwater when compared to surface waters. In this study, we discuss the potential of DSi as a tracer in SGD studies based on a literature review and two case studies from contrasting environments. In the first case study, DSi is used to calculate SGD fluxes in a tropical volcanic-carbonate karstic region (southern Java, Indonesia), where SGD is dominated by terrestrial groundwater discharge. The second case study discusses DSi as a tracer for marine SGD (i.e., recirculated seawater) in the tidal flat area of Spiekeroog (southern North Sea), where SGD is dominantly driven by tidal pumping through beach sands. Our results indicate that DSi is a useful tracer for SGD in various lithologies (e.g., karstic, volcanic, complex) to quantify terrestrial and marine SGD fluxes. DSi can also be used to trace groundwater transport processes in the sediment and the coastal aquifer. Care has to be taken that all sources and sinks of DSi are known and can be quantified or neglected. One major limitation is that DSi is used by siliceous phytoplankton and therefore limits its applicability to times of the year when primary production of siliceous phytoplankton is low. In general, DSi is a powerful tracer for SGD in many environments. We recommend that DSi should be used to complement other conventionally used tracers, such as radon or radium, to help account for their own shortcomings.

Advective flows of seawater and fresh groundwater through coastal aquifers form a unique ecohydrological interface, the subterranean estuary (STE). Here, freshly produced marine organic matter and oxygen mix with groundwater, which is low in oxygen and contains aged organic carbon (OC) from terrestrial sources. Along the groundwater flow paths, dissolved organic matter (DOM) is degraded and inorganic electron acceptors are successively used up. Because of the different DOM sources and ages, exact degradation pathways are often difficult to disentangle, especially in high-energy environments with dynamic changes in beach morphology, source composition, and hydraulic gradients. From a case study site on a barrier island in the German North Sea, we present detailed biogeochemical data from freshwater lens groundwater, seawater, and beach porewater samples collected over different seasons. The samples were analyzed for physico-chemistry (e.g., salinity, temperature, dissolved silicate), (reduced) electron acceptors (e.g., oxygen, nitrate, and iron), and dissolved organic carbon (DOC). DOM was isolated and molecularly characterized via soft-ionization ultra-high-resolution mass spectrometry, and molecular formulae were identified in each sample. We found that the islands’ freshwater lens harbors a surprisingly high DOM molecular diversity and heterogeneity, possibly due to patchy distributions of buried peat lenses. Furthermore, a comparison of DOM composition of the endmembers indicated that the Spiekeroog high-energy beach STE conveys chemically modified, terrestrial DOM from the inland freshwater lens to the coastal ocean. In the beach intertidal zone, porewater DOC concentrations, lability of DOM and oxygen concentrations, decreased while dissolved (reduced) iron and dissolved silicate concentrations increased. This observation is consistent with the assumption of a continuous degradation of labile DOM along a cross-shore gradient, even in this dynamic environment. Accordingly, molecular properties of DOM indicated enhanced degradation, and “humic-like” fluorescent DOM fraction increased along the flow paths, likely through accumulation of compounds less susceptible to microbial consumption. Our data indicate that the high-energy beach STE is likely a net sink of OC from the terrestrial and marine realm, and that barrier islands such as Spiekeroog may act as efficient “digestors” of organic matter.